Guide on

IS 3370 - 2020, (2nd Revision)

Code of Practice - Concrete Structures for

Retaining Aqueous Liquids :Part 1- General Requirements,

Part 2- Reinforced Concrete Structures.

November 2020.

by

Lalit Kumar JainConsulting Structural Engineer

Nagpur

lkjain.ngp@gmail.com

Published on Web Jointly by

Indian Concrete InstituteNagpur Centre

and

Indian Water Works AssociationNagpur Centre

November 2020

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CONTENTS

PREFACE p 3

Guide to IS 3370 Part 1 – 2020 R 0 INTRODUCTION p 4R 1 SCOPE p 4R 2 REFERENCES p 5R 3 TERMINOLOGIES (& definitions) p 5R 4 MATERIALS p 8R 5 EXPOSURE CONDITION p 9R 6 CONCRETE p 12R 7 DURABILITY p 16R 8 SITE CONDITIONS p 18R 9 CAUSES AND CONTROL OF CRACKING p 19R 10 STABILITY p 25R 11 DESIGN, DETAILING & WORKMANSHIP AT JOINTS p 25R 12 JOINTING MATERIALS p 35R 13 CONSTRUCTION p 37R 14 TEST OF STRUCTURE p 38R 15 LIGHTNING PROTECTION p 39R 16 VENTILATION p 39R 17 DESIGN REPORT AND DRAWINGS p 39 APPENDIX 1 p 40

Guide to IS 3370 Part 2 - 2020R 0 GENERAL p 41R 1 SCOPE p 41R 2 REFERENCES p 41R 3 GENERAL REQUIREMENTS p 41R 4 DESIGN p 41 R.4.2 Loads p 42 R 4.3 Method of Design p 44 R 4.4 Limit State Design p 45R 5 FLOOR p 50R 6 WALLS p 51R7 ROOFS p 51R 8 DETAILING p 51R ANNEX A p 60R ANNEX B p 60 ANNEX C: Concrete Finishes p 61

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Guide to IS 3370 Part 1 & 2 - 2020 (2nd revision) Code of practice -

Concrete Structures for Retaining Aqueous Liquids

PREFACE“Retaining aqueous liquid” should be taken synonymous to ‘storage of, or containing aqueous liquids or its

exclusion on one side’. In this guide use of terms ‘aqueous liquid’ and ‘water’ are synonymous. In the title word ‘storage’ is changed to ‘retaining’, and clarified that only ‘aqueous liquids’ are dealt and liquids not in general. Here after ‘Liquid Retaining Concrete’ is abbreviated to ‘LRC’. The code does not differentiate between “water contact” and “water retaining” members. All “water contact” members may not be “water retaining” members.

These standards are also applicable to the units of structure conveying e.g. channels, handling e.g. sump and pump-houses, and treating water and waste water (sewage), i.e. for environmental engineering structures, and water resource engineering structures, though not mentioned specifically. Code is mainly for aqueous retaining, and other concrete structures where water-tightness and durability are of prime importance. For structures dealing with waste water and sewage or storing liquids which may attack concrete, additional requirements may also be needed, and some guidelines are given at appropriate places. If likely chemical attack is slow (in relation to design life of structure in years), higher concrete grade is needed. With increase in potential of chemical attack, surface finishes, and protective coatings are needed. Linings are to be provided where chemical attack may be very severe or rapid.

For water conveying, or cross drainage structures in water resource engineering (e.g. aqueducts, canal syphon, sump and pump-house etc.), IS 3370 is being traditionally referred for liquid retaining members, till a separate code would be available for such structures. All the requirements for these types of structures are not covered in this code. For such structures limiting crackwidth of 0.2 mm is enough and tighter limits may not be required.

Those interacting with code revision are normally dealing with bigger size works. Large number of works are for small water supply schemes; and for these few common requirements have become unnecessarily little heavy.

Working stress method is to be applied for LRC designed as plain cement concrete (PCC). For reinforced concrete liquid retaining structures, the working stress design (WSD) method is deleted. These are to be designed by limit state design method only. Design approach is made more rationalized in present revision, while keeping issues simple as far as possible. Design by LSD (compared to WSD) gives economy. With LSD, present revisions have very little effect on the cost economy of the liquid retaining structures.

All four parts of IS 3370 are revised. A new part to deal with construction practices, quality management and maintenance is required. Part 3 for prestressed LRC, is revised specifying limit state design, working stress design deleted and it is in line with IS 1343. The prestressing in one direction only or partial prestress is also considered.

This guide is dealing with the subject in wider perspective, and some opinion may not be from the standards. In few situations code is silent, keeping subject brief, or not explicitly clear, and these are discussed. Views are not necessarily ‘word to word’ interpretation of code, but a guide for understanding for designer to take decisions. The provisions are explained to understand background information.

Reader is assumed to be well conversant with concrete technology and reinforced concrete design and that dealt in basic code (IS 456:2000)# and text books*. The aim is to give guidance to an average engineer for small and usual projects, and may not cover all the requirements for large projects.

In this guide water-cement (indicated as w:c or w/c) ratio, and water-cementitious or water-binder (w:b) ratio (as used in IS 456) are used as synonymous.

For further understanding of the design of LRC structures reference can be made to EN 1992-3: 2006 - Part 3; ACI 350:2019; New Zealand NZS 3106; British code BS 8007:1987. For more details refer to specialist literature. Details can be added as per reader’s demand or suggestions. A handbook or design aids may also be prepared if demand is indicated. Reader may communicate disagreement on a specific issue, or suggestions for giving more explanation, thus help to revise or improve the guide.

Clause number of the IS code is preceded by letter R, and subsequent text is guide, remark or commentary on the concerned clause. Remarks are not given on every clause. Additional remarks are also given in clause numbers which do not exist in the code. The information given is as per the opinion of the author.

As a sample, clauses from the standard (in blue) are added with prefix S, in the section 1 of part 1.For any contract, the recommendations given in this guide if in variance with IS code, shall not be applicable, unless

the contract also specifies this reference.For supporting structure for elevated tanks, refer “Guide for Design & Construction of RCC Elevated Water Tanks”.

For more details on concrete and for guide on construction aspects of LRC, refer “Guide on Construction of Concrete Structures for Retaining Aqueous Liquid”. These guides are by same author.# IS 456 -2000, Indian Standard Code of Practice for Plain and Reinforced Concrete, with 6 amendments, (standard under revision).$ IS 3370 part 1, 2, 3 & 4 -2020, Indian Standard Code of Practice – Concrete Structures for Storage of Liquid, Part 1 General requirements & part 2

Reinforced Concrete Structures.*Suggested books : 1. Properties of Concrete, A. M. Neville; 2. Concrete microstructures, properties and materials, P. K. Mehta & P.J.M. Monteiro,

Indian edition by Indian Concrete Institute ; 3. Concrete Technology Prof. M. S. Shetty, S. Chand publishers 2005.;

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Guide to IS 3370 Part 1- 2020 (2nd Revision), Code of Practice - Concrete Structures

for Retaining Aqueous Liquids : Part 1, General Requirements

R 0 INTRODUCTION ‘Terminologies’ are added (refer R3). ‘Exposer condition’ dealt in more details (refer R5). A concept of H/t

(hydraulic gradient a ratio) at the construction joint has been introduced (refer R3.17, R11.2.b(i)). Factor of safety against uplift is deal in little more details (refer R8.c). Information on ‘joints’ has been expanded (refer R11). More details about construction joint are added (refer R11.2.b). IS 456 is still the mother code, though in some of the areas, the provisions in that are not made applicable.

For LRC members’ minimum exposure taken is ‘severe’. For members in liquid contact i.e. surrounded on all sides by liquid, wherein liquid travel under hydraulic gradient does not take place through the member thickness over the major part of life; situation is not as severe as for a liquid retaining. For members in contact with liquid and not retaining, the provisions in IS 3370 part 2 can be bit relaxed except the clear cover which will be as per IS 456, the minimum concrete grade can be bit lower (M25 in place of M30) and the crackwidth requirement can be 0.2 mm and need not be lower. Water-tightness class consideration is not required. Column inside tank is a ‘water contact’ member, similar is a baffle wall in a treatment unit always having water on both faces.

Code now deals with the weakness at construction joint, leading to design action. Designer should check strength capacities in direct shear, and crackwidth at the construction joints. Location of construction joint is to be specified and checks for adequacy of strength and satisfactory performance, are to be applied. Detailed specifications for construction joints are given. Autogenous healing of cracks is mentioned.

Coated steel and stainless steel have been permitted for reinforcement. Bond strength reduction for coated bars, is recommended. Fibres are permitted for improving concrete performance.

For PCC design, details are not there, and designer has to develop understanding and design strategy. It can be designed by working stress method for very small components. For PCC, the permissible tension in concrete is reduced.

Requirement and desirability of concrete surface finish, plaster, lining, coating etc. on concrete surfaces is not dealt. Guidance on surface finishes and smoothness is not given, which in sewage treatment plants may become important.

Design and execution of works are to be done under of a qualified and experienced engineers.Importance of low concrete permeability is emphasized, however requirement of tests and limiting values of

permeability of concrete are not given. Prescriptive specifications and deemed-to-satisfy rules are given.IS 456 and IS 1343 to the extent applicable, are to be treated as part of IS 3370. Few provisions of IS 456 are over-

ruled by these codes, and few others are not applicable as specified in IS 3370.

1 SCOPE : R 1. Pollutants or water transportation through the concrete thickness may increase over the design life, and affect the durability and functional requirement of the member. With guidance given, the design life of 50 years can be considered for non-replaceable main structural components, and average life may be >80 years with maintenance and interventions. This life would be approximate in view of action level of environment remaining undefined. Planned maintenance should be envisages for items other than main structural components. Ideally structural components should perform over design life without any intervention or maintenance. Maintenance may be required for cleaning, colouring, movement joints, secondary items, finishes, non-structural items, and for defects in concrete. Service life of structure may reduce due to inadequate quality of construction, especially the variation in size and quality of clear concrete.

S 1.1 This standard (Part 1) lays down general requirements for the design and construction of plain, reinforced or prestressed concrete structures, intended for storage or retaining of aqueous liquids. A concrete structure or member may function as liquid retaining, when the amount of liquid permeating through its thickness, under hydraulic gradient, is practically negligible.

The recommendations are generally applicable to the storage/retaining of aqueous liquids having temperature not exceeding 50° C and no detrimental action on concrete and steel or where sufficient precautions have been taken to ensure protection of concrete and steel from damage due to action of such liquids.

The requirements applicable specifically to plain and reinforced concrete and prestressed concrete liquid retaining structures are covered in IS 3370 (Part 2), and IS 3370 (Part 3) respectively.

R 1.1 Aqueous liquids in temperature range 1C to about 40C are normal. Reactivity increases above 40C, requiring additional precautions. The code gives a limit of 50C, internationally it is 40C. Generally water temperature is lower by about 15C from the maximum ambient. The daily temperature variation of water is less than that of ambient air. The code is applicable to the retaining the aqueous liquids and solutions having no detrimental action on concrete and steel, or where sufficient precautions are taken to ensure protection of concrete and steel from damage due to actions such as in the case of sewage. Outside the above range of temperature, design will have additional considerations and provisions like lining or coatings etc. For ambient temperature below 1°C i.e. freezing condition, designer may require more design actions and precautions for durability, serviceability. Design for temperature gradient (of any range) across the thickness if persistent over long time, needs a design action.

S 1.2 This standard does not cover the requirements for concrete structures for storage/retaining of hot liquids, hazardous materials and liquids of low viscosity and high penetrating power such as petrol, diesel and oil. This standard

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also does not cover dams, pipes, pipelines, tunnels and damp-proofing of basements. This standard does not cover all the requirements of pressurised tanks, floating structures and tanks having the additional requirement of gas tightness. The selection and design of coatings and linings are not covered in this standard.

R 1.2 The code applies to all components of LRC and roof members enclosing the space above the aqueous liquid, excluding well ventilated (i.e. ventilation area >4% of the free liquid surface), and free height above liquid is > 1.5 m.

Parts of IS 3370 apply to the units of structure conveying (channels), handling (pump-houses), treating water and waste water (sewage) for environmental engineering structures, and may be applied to water resource engineering structures till separate standards are formulated for these.

Hot, cryogenic, low viscosity liquids (high penetrating power like petrol, diesel, oil, etc.), hazardous, or those susceptible for explosions are excluded from the code; and those would need additional requirements. Liquids at high temperature or pressure are not considered. For liquids detrimental to concrete, precautions and protections to ensure durability of concrete, are required. Special problems of shrinkage arising in the storage of non-aqueous liquids and the measures necessary where chemical attack is possible are also not dealt with.

This standard does not cover all the requirements of pressurised tanks, floating structures, and gas tightness. Requirements regarding coatings, linings, and retaining of chemically active or hazardous materials are also not dealt. The code also does not cover dams, pipes, pipelines, tunnels, lined structures and damp-proofing of basements.

For all types of liquid containments excluded in above, the guidelines from the code can be used, however additional criteria may also be needed. For all LRC waterproofing or damp-proofing treatment is not necessary, if required for a members, refer IS 6494. Water-tightness is necessary for LRC.

Tank to store potable water, shall be provided with roof and screens to prevent contamination and to avoid entry of vermin, birds, insects and mosquitos.

Junctions and joints between members shall be treated as element requiring design, detailing and proper construction for achieving reliable performance of the structure. Enough details are not covered in this document for connections of precast concrete for liquid retaining components.

S 1.3 The criteria for design of RCC staging for overhead water tanks are given in IS 11682.

R 1.3 IS 11682 is under revision. “Guide for Design & Construction of RCC Elevated Water Tanks” can be referred.

R 1.4 To ensure compatibility of the design assumptions (Ec, shrinkage etc.) as per the standard, the actual nominal maximum size of the aggregate (MSA) being used should be 16 mm or above, and normally 20 mm. Concrete with lower MSA might not support the design assumptions, e.g. aggregate interlock at construction joint, shear capacity, fracture energy, stiffness (Ec value), shrinkage catered for, etc. A small thickness (<30 mm) of concrete with lower MSA can be placed only at horizontal construction joint to avoid segregation due to free fall of concrete pour. Variation in effect of this low MSA concrete at horizontal construction joint can be neglected.

R 1.5 For long term performance, use of dense, nearly impermeable and durable concrete, adequate concrete cover without macro defects, proper detailing practices, control of cracking, effective quality assurance measures, and good construction practices particularly in relation to joints should be ensured. Consider the need for long term chemical resistance while dealing with aggressive liquids or sewage. Preventing contamination of retained liquid and groundwater are the considerations.

2 REFERENCES : R 2. List of standards referred is given. While referring to a standard its latest revision with up to date amendments should be used. This information is freely available at www.bis.org.in, the web site of Bureau of Indian Standards.

3 TERMINOLOGIES : Terms, definitions with comments & explanation.R 3. (In the following, few more terms are given compared to the standard, and the numbering has changed.)

R 3.1 Base of structure : Level at which the horizontal earthquake ground motions are assumed to be imparted to the structure. This level does not necessarily coincide with the ground level and generally is at foundations. R 3.2 Binder or Cementitious material : Powdery materials having cementing value in concrete in combination with Portland cement or blended cements, such materials like flyash, other raw or calcined pozzolanas, ground granulated blast-furnace slag (GGBS), micro-silica (silica fume), glass powder, lime stone powder etc., which can be used in different combinations of blending. These together with cement are called as cementitious materials or binder. Multiple blends with Portland cement can be used. Generally these cementitious materials have major portion and average particle size smaller than cement (20μm) and should conform to the specifications.R 3.3 Blinding Layer : A base concrete on which structural concrete can be laid. For laying LRC, it should not allow loss of cement paste from the fresh concrete being laid over and compacted. In many cases the foundation PCC has also to act as blinding layer. It is also called mud mat, lean concrete or PCC base.

To receive a structural concrete, if the ground is too soft or slushy or muddy, the base can be prepared in two layers. First layer can be a layer of suitable material or lean concrete which itself is not enough to totally seal off the mud from underlying material coming over. Over this sub-base, blinding layer is required.R 3.4 Capacity : It shall be the net useful volume of liquid, the structure can retain under normal operations, between the full supply level (FSL) and lowest supply level (LSL) i.e. the level of the lip of the outlet.

Due allowance shall be made for applying lining, coating or plastering to the surfaces from inside if any specified, while calculating the capacity. The capacity, also called as live capacity or useful capacity or designated capacity of a

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tank excludes dead storage, which is the quantity of liquid below LSL, and also exclude that possibly in freeboard zone. Gross capacity includes live capacity, dead storage, as well as the quantity of liquid which may occupy space above FSL (up to MTL as specified) if specified, for design consideration.R 3.5 Clearance above inlet pipe : Between the roof and the top end of vertical inlet pipe, the vertical clearance is needed. This clearance is governed by the exit velocity of liquid from the vertical pipe. This clearance can be minimum half the diameter of inlet pipe, and in most cases nominal freeboard provided is enough. R 3.6 Construction joint : It is an intentional joint introduced for convenience in construction, a partial discontinuity in the concrete and treated to ensure near monolithic behaviour under serviceability and ultimate limit states. Its example is between two successive wall lifts, and where special measures are taken to achieve continuity without further relative strains. Also see R 11.2 b & R 11.5.1.R 3.7 Contraction joint : It is an intentional joint introduced as partial discontinuity in concrete, or induced by a partial groove or cut in the concrete, thus creating a weak plane. Tensile strength across the joint is reduced to induce development of a crack thus relieving stresses due temperature-shrinkage restrains. The joint will open, as concrete on the two side will contract due to shrinkage and drop in temperature of concrete. It is a type of movement joint. It shall be sealed on liquid side. Also see R 11.2 a (2).R 3.8 Dead storage : It is the volume of liquid below normal outlet level (LSL) or below live or useful capacity. It is also expressed as depth of this liquid (in mm) at lowest part of container.

It may have provision for accumulation of grit, silt, sludge etc., which may have higher density than the liquid retained. Some engineers feel that a small (20 to 50 mm) dead storage may be considered for flat bottom tanks. For domed bottom tanks, minimum 300 mm dead storage is considered, which can be more depending on diameter of bell-mouth on outlet pipe, its location and fixing arrangement. Floor of ground tank (on grade), may be provided with slopes towards a pit, for draining out sludge or grit accumulation and for cleaning of tank. The liquid in the sludge pit, the suction pit or outlet pit is dead storage and not counted in live capacity.R 3.9 Design Life (or design service life) : It is the specified time in years achievable, for which the structure or structural element is designed to remain in purposeful use, as per intended performance with anticipated maintenance or conservation, without substantial rectification to be required. Normally it may be 50 years. Average service life of a well-designed and well- build structure will be more than the design life, and it may get reduced if quality of construction have some slips and not satisfactory at some places.

R 3.10 Designed Concrete : A concrete mix (composition) engineered and proportioned for the sample of materials (aggregates, cementitious blend, admixtures etc.) to be used, for achieving the specified characteristic strength, rheological properties, others as specified, additional useful requirements, and while keeping control over the variations in properties within a small range, with achieving reliability.R 3.11 Durability : It is the ability of a structure, its element and connections, to assure limited deterioration to a level not harmful in the relevant environment, for the required performance over the design life. It is also the capability of structures, to serve its material requirements for usage i.e. serviceability up to specified life.

R 3.12 Environmental Actions : These are chemical and physical actions to which the concrete is exposed and which result in effects on the concrete or reinforcement or embedded metal that are not considered as loads in structural design, and these actions govern the durability and service life of structure.

As individually or in combination, assembly of physical, chemical, or biological influences and actions resulting from the atmospheric conditions or characteristics of the surroundings to the structure, which may cause restraint effects or deterioration to the materials making up the structure (i.e. concrete or reinforcement or embedded metal), which in turn may adversely affect its serviceability, safety and durability of the structure. Some environmental actions can be considered as loads in structural design. Actions due to wind or waves effects are mechanical loads, and temperature actions give stress due to restrains. R 3.13 Force Actions : These include bending moments, torsion, shear forces, direct tension or compression caused externally i.e. direct or internally i.e. indirect. Indirect actions can be due to imposed deformations, environmental actions (temperature, shrinkage, moisture variations, creep etc.) or vibration or seismic etc. These are also simply called ‘actions’ or ‘forces’ in brief. R 3.14 Foundation level : It is the level of the founding soil stratum on which structure will be constructed. It is the bottom level of PCC (blinding / mud-mat) base (if being provided) on which structure is constructed. R 3.15 Freeboard : With normal overflow blocked, it indicates the available space above FSL, in which liquid can rise maximum; measured vertically (in mm) up to soffit of roof, and for open top tank up to wall top.

Freeboard accommodates the waves generated on the surface of liquid and prevents loss of liquid due to sloshing or splashing. If freeboard is less than sloshing height, near the wall, roof can be subject to upward pressure due to sloshing of liquid, and roof and its connection with wall are to be designed for such upward force.

Volume of space in freeboard zone, divided by area of liquid surface at FSL, is the average freeboard. Normally freeboard provision can be 150 to 300 mm for tanks with roof. At times it is measured up to the lowest point of soffit of slab or beam supporting the roof. For open top tanks, freeboard is higher (up to 300 to 500 mm); and amount is related to possibility of splashing, and the wave generation is related to the unobstructed maximum length at liquid surface. R 3.16 Gross capacity : It includes live capacity, dead storage, as well as the quantity of liquid which may occupy space above FSL (up to MTL) if specified for design consideration.

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R 3.17 Ratio H/t : It is the pressure gradient, a ratio of head (H) of liquid percolating through concrete, to thickness (t) of concrete. H is the difference of pressure on the two faces of concrete member. It is a non-dimensional parameter.

H/t influences the seepage through construction joint or cracks. The amount of leakage should be very small through a crack or a construction joint with good workmanship, and varies with this ratio. The limiting ratio is related to the workmanship- ordinary 20, average 25, good 30, and excellent 35. Above these limits, water-bars are required to reduce the leakage through construction joints. Limiting values of H/t would be a bit low if whole section is in tension. Also see R 11.2 b). At higher H/t, the quantity of steel required will higher, and may lead to uneconomical design.

R.3.18 Intervention : A general term relating to an action or series of activities taken to modify or preserve the future performance of a structure or its components. It encompasses rectification, repairs, restoration, rehabilitation, strengthening etc. R 3.19 Joint filler : A compressible, preformed material used to fill an expansion joint or an articulation, to support the sealants and to prevent the infiltration of debris. The filler should be fixed to any one side, to old or new concrete. Filler may not be resistant to liquid flow. Some fillers do resist permeation, if fixed (or adhered) to concrete on both sides.R 3.20 Joint Sealant : An impermeable elastomeric (i.e. ductile), normally synthetic material used to finish a joint and to exclude liquid and solid foreign materials from entering in or passing through the joint. It is fixed to liquid face of the joint with adhesion to concrete on both sides of the joint, not allowing liquid to cross the joint. It should sustain the pressure of liquid, with the range of movement imposed, over the temperature range, and shall not de-bond or degrade in the service environment, and have an acceptable life as per specifications.

For selecting sealant, consider the shape factor of sealant, surface preparation, and the contact bond strength between the sealant and the concrete. Need will be to inspect, maintain, repair or reseal joints with proper sealant at appropriate intervals, few times during the life of structure.R 3.21 Kicker : A small (75 mm to 150 mm) lift provided as first one at bottom of column or wall over a slab or foundation, to ensure the correct location and alignment of the member to be constructed on it and in some cases to accommodate the flange if water-bar. It may also be called as starter. See R 11.5.1.1 (end para) and 11.5.5.R 3.22 Leakage : It is the continuous flow of liquid through the concrete, as a very small stream. Appearance of only wet patch on concrete surface will not constitute as leakage. These are through joints, construction joint, holes, cracks, interconnected pores, honeycombs or macro flaws in concrete, etc. It means loss of liquid retained. In some situations, leakage can result in risk of contamination of liquid being retained or excluded. Amount of leakage is more than that to be called seepage, and is unacceptable.R 3.23 Lift : It is height of a concrete member, between two successive horizontal construction joints. Vertical concrete members e.g. columns or walls are constructed in lifts. R 3.24 Liquid depth : Liquid depth in a tank shall be the difference between the full supply level (FSL) or working top liquid level (WTL) of the tank, and the lowest supply level (LSL). In case of water, the term ‘water depth’ can be used. The ‘design liquid depth’ for a tank can be more than the ‘liquid depth’ in service condition due to dead storage and some rise of liquid to be accounted above FSL as part of freeboard, for strength design.R 3.25 Liquid Retaining Concrete (LRC) : Concrete having negligible permeation or loss of liquid through its thickness under hydraulic gradient, over the design life. The concrete should not have defects like segregation, honeycombs, macro-voids, and interconnected pores. The micro-structure (pore-structure) in the concrete is also improved to get low permeability. Subsequently if problem appears, it shall be grouted and treated adequately. Liquid may be flowing within a structure, but not through concrete. Example– tanks or units of treatment plants or channels in which liquid flows. Such concrete can be termed as liquid retaining.R 3.26 Liquid Retaining Structure: Structure having very low liquid seepage through its members, junctions and joints, near embedment e.g. pipes, others intersecting piercing concrete or passing through, such that the liquid loss is very small. All liquid storage structures shall be liquid retaining.R3.27 Maintainability : The ability of a structure to be maintained, to meet performance objectives with ease and a minimum expenditure for maintenance effort under service conditions. R 3.28 Maintenance : It is the set of planned activities, periodically performed during the design life of the structure, intended to prevent or correct the minor deterioration, degradation or mechanical wear of its components, for reliably keeping the performance level as anticipated in the design, without major rectifications. Due to poor workmanship, defects or flaws if service life of a structure is reduced, same can be extended by repairs, restoration or strengthening and these activities can be parallel to maintenance.R 3.29 Stability or Overall Stability : It is the state of stable equilibrium for the whole structure as a rigid body.

R 3.30 Screed Layer : Other than formwork or shuttering, it is sub-base concrete laid in required profile, to provide a firm base, shape and surface finish as required. For bringing the top of this concrete to the required profile, the thickness of screed layer may vary depending upon profile and tolerance of surface below it. In many cases the purpose of blinding and screed may be combined in to one layer. R 3.31 Sympathetic Cracking : Crack produced in a member, influenced or aligned by other adjacent member intimately in contact with sufficient friction, and which has a joint having movements. Two adjacent concrete being considered have a nominal separation, but with some frictional resistance. In the member considered the crack is produced at a location just adjacent to the movement joint or an opening crack in adjacent concrete. The crack induced

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in the concrete member under consideration is said to be sympathetic (i.e. in phase) to the joint or crack movements in adjacent concrete.

R 3.32 Water-Bar (Water-stop) : It is a continuous preformed strip of impermeable material like polyvinyl chloride (PVC), thermo-plastic, elastomeric rubber, metal (stainless steel or GI sheet) etc., anchored on the two sides of a joint in concrete, designed and constructed such that the passage of liquid through the joint is prevented and sustain the movements in joint without permanent deformations in water-bar, within the range of temperature changes and chemical environment met. These are water-bars having wings. Refer R 12.2 for more details.

Hydrophilic water-bars or swellable strips are without wings, and are outside the preceding definition, and distinctly different type.R 3.33 Water Path : The most probable, least resistant, and usually smallest path along which water can travel through pores, joints and cracks in concrete, under hydraulic gradient. At a joint the water-bar gives additional obstruction to seepage by increasing the gross length of the water path i.e. creep length for hydraulic considerations.

4 MATERIALS S 4.1 R 4.1 Requirements of materials are covered by section 5 of IS 456-2000 (with 6 amendments), and additional requirements for prestressed concrete work are covered by IS 1343 (with 2 amendments). Following are further additional requirements.

Use of blended cement is preferable, unless 7 days strength >25 N/mm² is the target. Blended cement can reduce thermal cracking, improve durability of concrete, and are also improve environmental sustainability. With use of blended cement or SCM’s, the threshold chloride concentration reduces, hence addition of corrosion inhibitor in concrete can be recommended. For roof of tank retaining chlorinated water, if blended cement is used or flyash or GGBS is added, corrosion inhibitors should be used in the concrete, or only OPC should be used without blending.

4.2 Aggregates : R 4.2 AGGREGATES:

Some engineers feel that water absorption of aggregates should not be more than 3% which appears to be very stringent limit. Porous aggregate increases the permeability of concrete. If satisfactory low level of concrete permeability can be achieved, absorption of aggregate will not affect the performance. However still there is no recommendation and specification of the permeability value permissible. To offset the possible effect of higher absorption of aggregate (>5%) one may adopt a little lower limit of water-binder ratio (or concrete grade higher) than that recommended.

Porous aggregates are normally not permitted for the components of structure retaining aqueous liquid or enclosing the space above liquid. Limits of porosity or absorption are not specified in the code. However for roofs of tanks, if higher grade concrete is used (≥ M40) some types of light weight aggregate may be used. For components enclosing the space above liquid, the percolation of liquid through concrete is not important, but the permeability influencing the deterioration mechanism of concrete is of importance. Aggregate of higher absorption (<10%) can be used for roof concrete. In most cases grade of concrete (strength) needed is higher.

Sand may contain shell, which are contributed by aquatic life form. These consist of mostly calcium carbonate, but being hollow or flaky, may hinder the complete compaction of concrete. Tolerance of the shell content in sand will depend upon total fines (sand + cement) in the concrete (higher shell with higher fines). In absence of trials, testing or experience, shell content up to 3% may be tolerated in sand, for concrete with nominal maximum size of aggregate as 20 mm or 16 mm.

Sand dredged from sea, estuaries or from salty water may contain high amount of salts. This type of sand if used should be washed with fresh (not salty) water and should be tested for the salt content for its suitability. Limits of total chlorides as given in Table 7 of IS 456, should be taken as guidance. For some components like roof of chlorine contact tank (part of water treatment plant) the limit may be suitably reduced (say by 33%).

Use of sulphate resisting cement is discouraged, when chlorides and sulphates both are present.Alkali-aggregate reactions can cause an expansive action when reactive aggregates come in contact with alkali

hydroxides in the hardened concrete. These reactions can result in long-term deterioration in the interior of the concrete. It is recommended to specify testing of aggregate, if not known for its potential of Alkali-aggregate reactions. Aggregates having known past history of no a potential for alkali reactivity for over 10 years or reactive constituents, can be used without testing. With aggregate having low level of reactivity, use of class F (low calcium) flyash is advantageous. S 4.3 Reinforcement :R 4.3 REINFORCEMENT

The grade of steel refers to the characteristic strength of bars, which is the guaranteed yield (or proof) strength. Use of corrosion resistant (CR) bars, give only a little extra protection, which is quite small relative to design life.

Where needed, for reducing the risk of corrosion of reinforcement, coated steel or stainless steel can be used. Fusion bonded epoxy coated bars (IS 13620) can be used, however the epoxy coating anywhere shall not be less than 180 μm (micron) as required by other international codes. For using these bars, procedures and precautions are necessary to avoid scratches during handling and fixing bars. Refer “Field handling techniques for epoxy coated rebar at job site” published by Concrete Reinforcing Steel Institute, USA.

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Coated bars cannot be handled, cut, bent etc. in normal way. Scratches are to be avoided at all stages before and after cutting, bending, during handling and fixing. For handling, surfaces of all contrivances likely to have contact with bars should have hard rubber (or other similar material) lining. Specific bar cutting and bending machines having liners are to be used. While placing or inserting the bars in position, scratches are to be avoided. Immediately after each operation like cutting, bending or any handling, the exposed or likely scratched steel surface is to be inspected. Cut ends of bars and scratches (or punches) must be covered by appropriate epoxy or polymer. Similarly scratches may be caused during further handling, placing, inserting bars, and tying the cage etc., and movement of workers on the bars before and during placing of concrete. With the use of coated bars at site, adoption of a reliable quality system is very necessary though extremely difficult, to detect every scratch and repair those. If these scratches go uncoated, the protection of coating against corrosion of steel will be effectively very small, thus the purpose of coating is defeated.

Fusion bonded epoxy coated bars are useful when exposure to chloride is much higher during construction also, e.g. bars at site, and in the concrete are in direct contact with saline environment (e.g. sea water, sea water spay, brackish water, de-icing salts, soil having salinity, air laden with salinity as in coastal area etc.) If coated bars are used, binding wire should also be coated.

Different types of stainless steel (say IS 16651) or steel containing high chromium (>9%) can be used. Galvanized bars can also be used. If galvanized bars are used, ensure that the zinc coating shall be sufficiently passive to avoid chemical reactions with the cement or concrete shall be made with cement that has no detrimental effect on the bond to the galvanised reinforcement. Natural passivation of zinc coating can be achieved by storing the galvanised bars outdoors for more than a month. Instant passivation can be achieved by dipping the zinc coated product in passivation solution. Epoxy coated galvanised bars are also being used in other countries where environment is very severe for corrosiveness.

The tie wire or any corrodible item shall not transgress the concrete cover. The type of binding wire shall not cause bi-metallic (galvanic) reaction with reinforcement. If feasible, coated / insulated binding wire should be used. Different grades of uncoated steel and different types of steel should not be permitted in a reinforced cement concrete (RCC) component, without electrically insulating from each other.

Use of protective coatings should normally not permit reduction in concrete cover. For stainless steel bars or dual coated (zinc & epoxy) nominal cover can be reduced by 10 mm.

Compared to un-coated reinforcement, for coated reinforcement the bond strength (at limited slip) will reduce, and crackwidth can be higher. Hence, their use shall be accounted in design.

As reinforcement, fiber (continuous) reinforcement products (FRP rods or mats) can be used. Such composites are of carbon, glass or aramid fibres in matrix resin. Refer ISO 14484:2019. These bars have low or negligible ductility, hence cannot be substituted on design force basis. These bars do not corrode, and hence can be used with small cover (say reduce by 15mm) and at lower stress limits suited for small members.

R 4.4 ADMIXTURESS 4.4.1 Mineral Admixtures :

R 4.4.1 Mineral admixtures, i.e. pozzolanic materials like flyash, GGBS, Metakaolin, silica-fume or micro-silica, etc. as supplementary cementitious materials (SCM’s) or additives, are used to improve micro structure and thus reduces the permeability of concrete. Use of flyash and GGBS also reduce early age cracking due to less heat of hydration in initial period. There may also be a small saving in cost. It is preferable to use mineral admixtures, being advantageous for many chemical exposures. While SCMs are used, addition (3to 5% of cementitious) of lime stone (>80% CaCO 2) powder (<5μm) does further improve concrete properties. A basic action of SCM’s is to improve particle packing in the range of 150 to 1 μm (micron) against the cement particle (45 to 10 μm). Additional advantage is the second stage chemical actions giving more hydrated paste further contributing to strength by consuming calcium hydroxide, and thus further refinement of pore structure, reducing permeability.S 4.4.2 Chemical Admixtures R 4.4.2 Use of chemical admixtures plasticising type help to achieve desired workability, while keeping w/c ratio low, and thus reducing porosity. Admixtures for compensating shrinkage, reducing permeability and inhibiting corrosion are useful in LRC, and can also be considered for use. Calcium chloride or others containing chlorides, shall not be used. Chlorides due to impurities can be in negligible quantity. Corrosion inhibitors may show erratic variations in chloride ion penetration, if water cement ratio is not low enough (say >0.45). Hence with their use, choose a concrete ≥M40.

S 4.5 Jointing Materials : R 4.5 JOINTING MATERIALS

Jointing materials are required at construction joint, and movement (contraction & expansion) joints. All materials used at present at the joints in LRC, are not covered by Indian Standards. For such materials specifications should be obtained from the manufacturer or the other standards (like BS or ASTM) can be referred. Use of bituminous preparations are not desirable for structures retaining potable water, and similarly some other materials may not also be compatible. Compatibility with liquid in contact needs to be checked for the relevant LRC. See 12 and also 11.5.

The life of most jointing material is much shorter than the design life of LRC. Hence for the design and selection of materials, consider maintainability and restorability of joints.

Some Indian Standards related to joints are given in R-Appendix 1, at the end of this part 1.

S 5 EXPOSURE CONDITIONS

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R 5 EXPOSURE CONDITIONClassification of exposure conditions is given in Table 3 of IS 456. Components of LRC should be assumed to be

exposed to not less than ‘severe’ condition on both faces for design. Outer face of roof may be taken as ‘medium’ exposure, or higher. Roof top and outer surface of a tank may have higher exposure condition in polluted industrial area, coastal area or sea-face. If conditions demand or chlorine attack could be significant, inner face of roof enclosing space above chlorinated liquid, is to be assumed to be exposed to ‘very severe’ exposure, else it could be ‘severe’.

A face of a component may be subjected to higher exposure like ‘very severe’ or ‘extreme’ if liquid in contact or environment demands so. Consequently the two faces of a component may have for different exposures for design, e.g. one severe and other very severe. The grade of concrete has to be chosen for higher class of exposure. From a concrete surface, clear cover over the bar and limiting crackwidth are functions of the design exposure condition on that face. Map indicating climatic zoning, and susceptibility to corrosion of reinforcement can also be considered.

Components which for most of the time during design life will be surrounded on all its side by non-injurious liquid can be treated as exposed to moderate condition, e.g. column inside tank. These are ‘liquid contact members’. (See R0 2nd para). In most cases such members may be small and it may not be worthwhile to reduce the grade of concrete for small quantity. Many members in structures of water resource engineering require this consideration.

Higher exposure conditions e.g. ‘very severe’ or ‘extreme’, calls for protective surface treatment. The code does not specify lower crackwidths for higher exposures. Crackwidths below 0.2 mm, do not have significant effect on corrosion of reinforcement. However, estimation of crackwidth has many approximations, hence at important locations for better reliability crackwidth may be specified less than 0.2 mm. Also for smaller crackwidths, the water-tightness improves which in turn affect the long term durability.

Take an example of filter house in a water treatment plant. There are three locations of concrete components to be distinguished for design.

(a) Floor slab and wall of filter boxes, troughs (launders/channels) are LRC. Adjoining to filter box is pipe gallery, where water due to leakages from joints and valves come. If pipe gallery floor is suspended (not directly supported on ground), it is also designed as liquid retaining member. At top of filter boxes cantilever walkways are provided, which are always above liquid surface, however are designed as LRC.

(b) Operating platform above (>2m) pipe gallery is provided. Space between pipe gallery & operating platform is well ventilated like typical building. Operating platform is designed for clear cover required for moderate exposure, and crackwidth limiting to 0.2 mm under serviceability limit state. These types of members are not dealt in the code, and designer has to take decisions. Usually grade of concrete is same as provided in other components at that level.

(c) Roof of filter house is usually ≥ 3 m above the top of filter box i.e. walkway & operating platform level. The space below roof is well ventilated like typical building. Though roof can said to be enclosing space above liquid in filter box, the space is large and well ventilated due to doors & windows of filter house. Roof of filter house is designed like any other building for mild or moderate exposure condition as the case may be.

Similarly situation occurs in chemical solution room, wherein solution tanks are treated as LRC and other parts as normal building work. Also consider an example of sump and pump house. Wall, floor & roof of sump are designed as LRC. Floor of pump house is LRC. Floor of the pump house has some openings for access to sump and for installation of pumps etc. Space in the pump house is well ventilated and treated like industrial building. Above floor of pump house all RCC is treated like a building only and not LRC.

The modern approach is to recognize the possible combination of mechanisms of deterioration of concrete component, and design aim should be to achieve an expected durability for the design life.

On the surface of steel embedded in concrete, a protective oxide film tightly held on the bars, by hydration product of cement, is formed by the highly alkaline (pH greater than 12.5) chemical environment present in concrete. This thin passive film protect the steel from further corrosion reaction. As the concentration of chloride ion (acid soluble other than chloride combined in cement reaction, free in pore water) increases, to a threshold (critical), it brakes the protective film and initiates the corrosion of steel. With further continuing penetration of chloride ions, corrosion rate increases.

Apart from chloride ions (radical of salt or acid), chlorine gas or nascent chlorine also reacts with the concrete (like acid attack), reducing hydrated cement to powder, loosing capacity to bind. For roof enclosing liquid with high chlorine, refer R 7.2. Chlorine reaction is less severe in saturated concrete. Underside of roof not saturated, may be affected much more. Whereas wall in freeboard zone is mostly saturated, does not experience the damage by chlorine.

Tanks having chlorine dissolved in water i.e. chlorine solution tank, chorine contact tanks, or tanks holding water having break-point (high dissolved concentration) chlorination, will have nascent chlorine temporarily in air above water, which is highly corrosive to concrete. The underside of roof shall be assumed to be exposed to very severe condition. Such roofs shall be in concrete minimum M40 grade and water-binder ratio ≤0.40. Anti-chlorine surface coating (e.g. epoxy) should also be applied. Note that the life of treatment like coating if applied will be much less than the design life of structure, and this coating will remain a maintenance item.

All tanks of water supply scheme contain water which is normally chlorinated. The dissolved chlorine may be less than break-point chlorination, after few hours of adding chlorine at treatment plant. In such cases, the quantity of chlorine evolved will be less, and corrosive action of chlorine could be slow. However, similar treatment should be given to the underside of roof, considering long life,

The grade of concrete has to be chosen for higher level of exposure condition on any one of its surfaces.The surface treatment, its smoothness and applications of coatings also depend upon the exposure condition.

Concrete in contact with the sewage, requires smooth surfaces.

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R 5.1 Detailed exposure classification related to environmental actions causing loss of durability, needs consideration. Select exposure classes based on the environmental conditions of LRC in service and its place. Considerations may include, coatings or lining, and other special treatments.

Different surfaces of a component at different times, may be subjected to different environmental actions. It may be subjected to more than one actions. Based on actions affecting durability, exposure classes are given in Table A. One may also refer to ICI TC/08-01 handbook on durability.

Table A - Exposure classes (based on ISO 22965-1:2006, EN 206-1 & ICI TC/08-01 duly modified)Designation

/ ClassDescription of environment related to concrete For information, examples of the exposure class, concrete would be

subjected to -1. Penetration resistance or resistance against permeability of waterP0 No risk of water contact Resistance against water permeability is not required e.g. interior building

elements remaining mostly dry & no condensationP1 Exposer to water Requiring low permeability e.g. water retaining concrete

or that exposed directly to very heavy rainfall2. No risk of corrosion or attack on reinforcement or embedded metalX0 (a) PCC (no reinforcement or embedded metal):

Exposures except freeze & thaw cycles, abrasion or chemical attack.(b) For concrete with reinforcement or embedded metal: Almost dry

Inside buildings with very low humidity in air say relative humidity RH <40%.

3. Corrosion induced by carbonation of concrete cover, concrete exposed to air & moistureMoist condition relates to concrete cover on steel bars or embedment, and also to surrounding environment, except if effective barrier between the concrete and its environment is provided.XC1 Mostly dry or saturated for service life,

Effect is very smallInside buildings - low humidity in air (RH<60%);OR Concrete permanently submerged in water.

XC2 Mostly wet, rarely dry, Long term water contact, & most FoundationsXC3 Moderate humidity or

Cyclic wetting and dryingInside buildings - high humidity in air (RH>60%);External concrete not sheltered from rain or washing action

XC4 Cyclic wet & dry, Water contact not qualifying XC24. Corrosion induced by chlorides other than from sea waterConcrete containing reinforcement or embedded metal, subject to contact with water containing chlorides, including de-icing salts, sources other than from sea water, the exposure classified as follows:XCl 1 Moderate humidity Concrete surfaces exposed to airborne chloridesXCl 2 Wet, rarely dry Swimming pools ; Concrete exposed to industrial waters

containing chlorides, or chlorinated waterXCl 3 Cyclic wet and dry Exposed to water with high chlorine concentration, parts of bridges exposed

to spray containing chlorides, pavement, car park slabs in cold countries5. Corrosion induced by chlorides from sea waterConcrete containing reinforcement or other embedded metal is subject to contact with chlorides from sea water or air carrying salt originating from sea water, the exposure should be classified as follows:XCs1 Exposed to airborne salt but not in direct contact

with sea waterXCs1.0XCs1.1XCs1.2

Structures near to or on the coast, further subdivide as per distance from sea coastBeyond 50 km from coast10 to 50 km from coastCoastal area up to 10 km

XCs2 Permanently submerged in sea water Parts of marine structures or coming in contact with sea water

XCs3 Tidal, splash and spray zones Parts of marine structures6. Sulphate attackConcrete is subject to chemical attack by sulphate from exhaust gases, industrial pollution or from ground waterXS0 No risk of sulphate SO3 < 0.2% (in soil), or SO3 < 300 ppm in waterXS1 Risk of mild sulphate attack SO3 0.2% to 0.5% (in soil), or SO3 300 to 1200 ppm in waterXS2 Risk of moderate sulphate attack SO3 0.5% to 1.0% (in soil), or SO3 1200 to 2500 ppm in waterXS3 Risk of severe sulphate attack SO3 1.0% to 2.0% (in soil), or SO3 2500 to 5000 ppm in waterXS4 Risk of very severe sulphate attack SO3 > 2.0% (in soil), or SO3 > 5000 ppm in water

7. Freezing and thawing attack on concreteExposed to significant attack by freeze/thaw cycles whilst wet, the exposure classified as follows:XF1 Moderate water saturation, without de-icing agent Vertical concrete surfaces exposed to rain and freezingXF2 Moderate water saturation, with de-icing agent Vertical concrete surfaces of structures

exposed to freezing and airborne de-icing agentsXF3 High water saturation, without de-icing agent Horizontal concrete surfaces exposed to rain and freezingXF4 High water saturation, with de-icing agent or sea

waterRoad and bridge decks exposed to de-icing agents, Concrete surfaces exposed to direct spray containing de-icing agents and freezing, Splash zone of marine structures exposed to freezing

8. Chemical attack on concreteExposed to chemical attack from natural soils & ground water as given in Table A1, the exposure classified as below. Classification of sea water depends on the geographical location, therefore the classification valid in the place of use of the concrete applies. Note: Special study needed to establish relevant exposure condition where there is - limits outside of Table A1; other aggressive chemicals; chemically polluted ground or water; high water velocity in combination with the chemicals in Table A1.XA1 Slightly aggressive chemical

environment according to Table A1XA2 Moderately aggressive chemical environment

according to Table A1XA3 Highly aggressive chemical

environment according to Table A1

Table A1 - Limiting values for exposure classes due to chemical attack from natural soil & ground water

Aggressive chemical environments class based on natural soil and ground water at water/soil temperature between 5°C to.30°C and a water velocity sufficiently slow to approximate to static conditions.

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The most onerous value for any single chemical characteristic determines the class.Where two or more aggressive characteristics lead to the same class, the environment should be classified into the

next higher class, unless a special study for this specific case proves that it is not necessary.Chemical

characteristicReference test

methodXA1 XA2 XA3

Ground WaterSO4

2- mg/l EN 196-2 ≥ 200 and ≤ 600 > 600 and ≤ 3000 > 300 and ≤ 6000pH ISO 4316 ≤ 6.5 and ≥ 5.5 < 5.5 and ≥ 4.5 < 4.5 and ≥ 4.0CO2 mg/l aggressive EN 13577 ≥ 15 and ≤ 40 > 40 and ≤ 100 saturatedNH4

+ mg/l ISO 7450-1/2 ≥ 15 and ≤ 30 > 30 and ≤ 60 > 60 and ≤ 100Mg2+ mg/l ISO 7980 ≥ 300 and ≤ 1000 > 1000 and ≤ 3000 > 3000 to saturationNatural SoilSO4

2- mg/kg a total EN 196-2 ≥ 2000 to ≤ 3000c > 3000c to ≤ 12000 >12000 to ≤ 24000Acidity ml/kg DIN 4030-2 >200 Beaumann Gully Not encountered in practicea. Clayey soils with a coefficient of permeability below 10-5 m/s may be moved into a lower class.b. The test method should prescribe the extraction of SO4

2 by hydrochloric acid; alternatively, water extraction may be used, if experience is available in the place of use of the concrete.

c The 3000 mg/kg limit should be reduced to 2000 mg/kg, where there is a risk of accumulation of sulphate ions in the concrete due to drying and wetting cycles or capillary suction.

5.1.1 For exposure classes given in Table A, the concrete parameters are recommended in Table B.Table B – Recommended Concrete Parameters for exposure class as per Table A.

Exposure Class Minimum cement content Maximum water-cement ratio Minimum concrete grade No risk X0 260 0.60 M20

Penetration resistance or resistance against permeability of waterP1 PCC 300 0.55 M20P1 RCC 350 0.50 M25

Carbonation induced corrosion in RCCXC1 300 0.55 M25XC2 320 0.50 M30XC3 330 0.48 M35XC4 340 0.45 M40

Chloride induced corrosion : chloride other than from sea waterXCl 1 320 0.48 M35XCl 2 340 0.45 M40XCl 3 360 0.42 M45

Chloride induced corrosion : sea water actionXCs1.0 330 0.45 M25XCs1.1 350 0.45 M35XCs1.2 360 0.40 M40XCs2 360 0.42 M40XCs3 380 0.40 M45

Aggressive chemical environmentXA1 330 0.48 M35XA2* 360 0.45 M40XA3* 400 0.41 M45

* When SO4 leads to exposure class XA2 or XA3, it is essential to use sulphate-resisting cement. If classified, high sulphate-resisting cement should be used for exposure class XA3

Freeze-thaw attackXF1 300 0.50 M30XF2 320 0.48 M35XF3 350 0.45 M40XF4 380 0.44 M40

Minimum entrain air content should be 4% for XF2 to XF4Note : Recommendations in Table B are not same as per ISO 22965-1 or EN 206-1.

R 5.2 In construction the minimum cement content and the minimum grade of concrete shall be higher of the values as recommended from Table 1 of IS 3370 part 1, Table 2 of IS 456 and Table B above. Similarly maximum water-cement ratio should be lower of the values as recommended in these tables. The concrete characteristics shall be enveloping the requirement from different considerations.

S 6 CONCRETEProvisions given in IS 456 and IS 1343 for concrete shall apply for reinforced concrete and prestressed members respectively subject to the following further requirements:a) The concrete shall conform to Table 1.b) The cementitious content excluding mineral admixtures, such as flyash and ground granulated blast furnace slag,

should not be used in excess of 400 kg/m3, unless special consideration has been given in design to the increased risk of cracking due to drying shrinkage in thin sections, or to early thermal cracking and to increased risk of damage due to alkali silica reactions.

c) Cement plaster if applied to internal surfaces of concrete, should not be treated as an alternative to impermeable concrete.

Table 1 – Minimum Cementitious Content, Maximum free Water-cementitious Ratio and Minimum Grade of Concrete

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Concrete Minimum Cementitious content

Maximum freeWater-cementitious ratio

Minimum Gradeof Concrete

Plain Concrete 250 Kg/m³ 0.50 M 20 Reinforced Concrete 350 Kg/m³ 0.45 M 30 Prestressed Concrete 380 Kg/m³ 0.40 M 40

d)

NOTES : 1 Cementitious content mentioned in this table is inclusive of mineral admixtures mentioned in IS 456 and is irrespective of the grades of cement.

2 For small tanks having gross capacity up to 50 m³ at locations where there is difficulty in providing M30 grade concrete, the minimum grade of concrete may be taken as M25 (with minimum cementitious content as 350 kg/m³). However, this exception shall not apply in coastal area, or the area where air pollution is high or liquid retained is aggressive like sewage.

R 6 CONCRETEPCC base (or called mud-mat concrete, lean concrete, foundation PCC or blinding layer) is a non-structural concrete

and not govern by the requirements specified in Table 1, and is excluded from the following discussion. PCC in foundation is discussed in R 3.3, R 3.30, R 9.2.8b, R 11.2a, R 11.4, R 13.1.1, R 13.1.2,

The concrete by itself should be watertight (i.e. low permeability), and plaster should not be relied for reducing leakages, but concrete should be grouted to reduce permeation, if required.R 6.1 Table 1 specifies minimum cementitious or binder (i.e. cement + pozzolanas) content, maximum free water to cementitious /binder ratio, and minimum grade of concrete.

Cementitious content given in Table 1 is irrespective of the grades of cement and it includes mineral admixtures such as flyash or GGBS and are taken into account with respect to the binder content and water-binder ratio. Do not exceed the limit of pozzolana and slag specified in IS 1489 Part 1, IS 455 and IS 16714 respectively. With maximum size of aggregate less than 20 mm, the concrete may require higher binder (cement + mineral admixture) content, and for higher MSA minimum binder content can be less (refer Table 6 of IS 456). As the clinker (OPC) content in binder reduces, the w/b ratio should also reduce.

For higher exposure conditions (very severe or extreme), the requirements of Table 5 of IS 456 will also govern the specification of concrete.R 6.1.1 If during construction there is a good control (small variation within narrow range) on aggregate grading, and standard deviation in compressive strength of concrete is less than 6% of the characteristic strength, i.e. quality control is very good, the minimum binder content in RCC can be taken as 320 kg/m³.

For RCC work, total binder content in concrete can be lower than the limit given in the Table 1, where aggregates and powder content are well grades and proportion arrived at by particle packing theory, wherein main role of cementitious (binder) material is to coat other particles and its action as filler (filling finer space) is very small. This approach will also require particles graded below 200 to about a micron size. OPC content of concrete can be much lower in these cases, such as 200 to 250 kg/m³. SCM’s, additives and filler materials can be used in addition to OPC. However, water-binder (w:b) ratio should be ≤ 0.4 for such concrete. It is also advisable that in a cubic metre of concrete total water content should not be more than 140 litres including free water on aggregates and the water in admixtures. Note that, thus total water and total paste in concrete will be very low and need of superplasticizer will be higher.R 6.2 Concrete should satisfy all the requirements of IS 456, and specifically those in Table 5 of IS 456. Grade of concrete is a main criterion for specifying concrete. Though permeability is an important parameter for LRC, specific recommendation is not given. To control permeability, in addition to minimum grade (strength), maximum water-binder ratio is also specified. For water-binder ratio, the equivalent weight of SCM’s should be accounted. For flyash 0.2 to 0.4, for GGBS 0.5, Metakaolin 0.7 etc. and for ultrafine SCM’s the factor is higher.R 6.2.1 Concrete as proportioned (mix designed) should have enough of workability for ease of working, in relation to the method of handling and compaction of concrete. For increasing the workability the dose of plasticizer (or superplasticizer) can be enhanced. Limit on water-binder ratio should always be maintained.R 6.2.2 For concrete mix production, the specified water-binder ratio should be taken 0.01 less than the limiting value specified in Table 1 or Table B or the value taken for mix trial in laboratory. (Ref. ISO 22965-1). This is to account for the field variations. Water from all sources including that in admixture and the surface water with aggregate shall be accounted for calculating the total water in the mix, and also for water-binder ratio.

Most works of liquid-tank are small, and may not conform to note 1 of Table 8 in IS 456 (amendment 4), hence target mean strength shall be fck +1.65×6 MPa, i.e. for M30 grade the target mean strength should be 40 MPa. This margin of strength is required to cover variations in quality of materials, grading, batching, mixing and transportation etc., till a lower standard deviation is obtained from the record of strength test on the concerned work. For mix design, after obtaining standard deviation from the actual work (field) test the value can be used for arriving at the target strength, which shall not be less than fck +1.65×s MPa. Here “s” is standard deviation, taken not less than 4.

For RMC supplies the variations in concrete productions are small. However, variations in transportation (involving more than one hour time and temperature variation), placing, compaction and curing can take place, and the standard deviation can be taken as minimum 4.2 MPa or more as confirmed by the records test record from site of work (and not at RMC plant). Thus for RMC supplies the target mean strength would be 37 MPa for M30 grade. RMC supplier does not take this in to account and the average target strength of concrete as supplied is lower than that required. RMC supplier should leave a margin for variations in strength due to field operations. Hence at the time of placing order to a RMC, for the acceptance average strength shall be fck + 5 MPa for tests on samples taken on delivery.

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R 6.2.3 With the modern cement as available (strength >53 N/mm² and as high as 70 N/mm²), f or conformance of limiting maximum water-binder ratio (related to exposure condition), the achievable grade of concrete may be significantly higher than that being specified, and by mix design trials, it can be determined in laboratory. Also for good grading of aggregates, the strength can be higher at the specified w/c ratio. In such cases, to conform to the requirement of maximum water-binder ratio, the grade of concrete to be adopted in construction shall be related to developable strength at the limiting water-binder ratio conformed by test in laboratory. The field strength of concrete shall be not less than the average developable strength minus 1.65× standard deviation adopted for mix proportioning. The specified compressive strength should be reasonably consistent with the w:b ratio required for durability, which should be low enough, and the specified strength high enough, to satisfy both the strength criteria and the durability requirements.

In other words this means that for cements of much higher strength and optimised better graded aggregates, the concrete grade in construction should be higher than that given in Table 1. And designer has the option of designing and specifying higher grade concrete. When high strength OPC with ≤10% flyash and GGBS are used, the grade of concrete in construction should be M35 or more.R 6.2.4 In the modern concrete practice, for enhancing the grade of concrete, cement content need not increase. It can be enhanced by lowering the w/c ratio and marginally increasing the plasticizer dose. Hence for enhancing the grade from M30 to M40, increase in cost is very marginal (say 2 to 4% only as cost of more plasticiser dose) provided the cement content (kg/m³) does not change. This can be easily verified by difference in quotations for the two grades of concrete from a RMC supplier. In general higher grade concretes are more durable and also economical in designs. Concrete grade as higher as practicable should be adopted, and still it can be economical. R 6.3 Minimum concrete grade for LRC work in RCC is M30. Because of history of constructing tanks in M20 & M25 grade and satisfactory performance of many the tanks already constructed; small tanks up to 50 m3 in the environment of medium exposure and with H/t within 25, can be designed and constructed in M25 grade concrete, except those in coastal areas, or where air-pollution is high, or liquid retained is aggressive like sewage. However, minimum cement content will remain 350 kg/m³. R 6.3.1 For LRC designed as PCC (see 9.2.1), M25 grade is permitted; however H/t is ≤20 and minimum reinforcement is as per IS 3370. Very small (<5m) tanks can be designed as PCC in M20, with H/t ≤15 and nominal reinforcement confirming to IS 456. The code gives wide options to design tanks in different grade of concrete. For concrete M20 minimum clear cover to a bar should be 50 mm, and for M25 it be 45 mm.

For small members (channels in treatment plants), if H/t is ≤10, and thickness is safe in tension as PCC member (as per R 9.2.1.2, para 2), the clear cover 35 mm can be provided.R 6.3.2 For LRC use of mineral admixtures (i.e. supplementary cementitious powders) are advantageous. Their use reduces permeability and are favourable for heat of hydration and durability. Use of flyash (pulverized fuel ash i.e. PFA) and/or GGBS (ground granulated blast furnace slag) in concrete or use of flyash blended cement (Portland pozzolana cement conforming to IS 1489 part1) or Portland slag cement (IS 455) are preferable. Multiple blending can give better performance of concrete, and also lower OPC content which is necessity for sustainability.R 6.3.3 Site mixing of mineral admixture requires very efficient and through mixing. Unless a batch mixing plant or highly efficient (pan or twin shaft) mixer is used to produce concrete, site mixing of mineral admixtures in concrete should be avoided.

It may be noted that the common tilting drum mixers (0.16 to 0.2 m³) used ordinarily on construction sites, have very low efficiency of mixing, and theses should not be used to mix required for LRC with mineral admixtures (Flyash/GGBS/Metakaolin). Refer 10.3 of IS 456. If a concrete delivery is segregated or not properly mixed, it must be remixed before transporting and placing in position.R 6.3.4 Cement content should be as small as possible for better performance, but not less than the minimum specified in Table 1. The minimum limit specified is a durability requirement, and assumed to include all cementitious material (i.e. SCM’s/binder including mineral admixtures and additives like lime stone powder), and excluding portion of flyash retained on 45µm sieve.

For the requirement of minimum cement content and the maximum water-binder ratio, binder means either OPC or PPC (blended cement as per IS 1489 or IS 455). However while mineral admixtures (SCM’s/additions) are used, the equivalent cement content is the sum of OPC & mineral admixtures for the requirement of minimum cement content and the maximum water-binder ratio, as per IS 456.

The maximum cement content shall be 400 kg/m³, which can exclude the supplementary cementitious materials (mineral admixtures), however it is preferable to have OPC (clinker) content as low as possible. This limit (maximum 400 kg/m³) is irrespective of the grade and type of cement. Even for blended cements, limit is same. In case of addition of mineral admixtures (pozzolanic materials like flyash, GGBS, Metakaolin, silica fume /micro-silica etc. as supplementary cementitious materials) at the concrete mixer, total binder (cement + SCM’s) content can be up to 450 Kg/m³ in which OPC is not more than 80% of cementitious content.

As per the international practice, the “cement content” can be replaced by “equivalent cement content” which is sum of cement plus k times the additive content per cubic meter of concrete. Here k has a value 0.2 to 0.4 for flyash, which can be based on past experience or the tests.

Maximum limit of cement content (excluding mineral admixtures) is specified to keep a control over cracking as a result of temperature built up due to heat of hydration, and that due to shrinkage. After the rise of temperature due to heat of hydration, the subsequent cooling to ambient, causes cracking of concrete, which need to be controlled. In code the recommendation is based on a cement content of about 350 to 400 kg/m³. If the cement content exceeds 400 kg/m³

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(or 450 kg/m³ as total cementitious), due to heat of hydration thermal cracking can be higher requiring temperature control in construction, as well the shrinkage coefficient of concrete will increase, and have to be accounted in design, by increasing temperature–shrinkage (i.e. minimum) reinforcement. R 6.3.5 For evolution of heat of hydration and the temperature built up, if the conditions appear to be significantly adverse, the heat evolution characterises of the cement could be obtained from tests. The actual peak temperature build up (& subsequent cooling,) and the concrete age at its occurrence should be estimated (usually between 24 to 72 hours for ambient temperature 40 to 20 °C respectively), taking account of environment during early life of the concrete (ambient temperature, humidity, curing regime), thickness of member, formwork type (heat dissipation characteristics) etc. Substituting cement with supplementary cementitious materials (mineral admixtures e.g. flyash or GGBS etc.) as much as possible, and reducing the concrete temperature < 30°C, are the means to limit heat of hydration in in first few days, which eventually reduces thermal cracks, and improve durability.R 6.3.6 If subjected to peak temperature (>70°C) in early life (1 to 3 days) of concrete, delayed ettringite formation (DEF) can occur (in later life) in certain mixes, with wetness for most part of its life. To avoid adverse effect of DEF on the performance of LRC in service, the peak temperature in early life should not be >70°C. To control this use recommendations given in R 6.3.5. R 6.3.7 In LRC, chemical admixtures (plasticizers) enhance workability, reduce water-binder ratio and permeability, therefor advantageous. Avoided those containing chlorides. A particular admixture shall be permitted only after the compatibility test with the cement sample from the specific source. The source of admixture or cement, if any one changes, the compatibility test shall be carried out again.

Total acid soluble chloride content shall be ≤0.6 kg/m³ in concrete. For very sever environment like roof on chlorinated water, the chlorides shall be ≤ 0.4 kg/m³, and also <0.10 % of cement content.

Permeability of a concrete member can reduce with extended moist curing, and to a small extent with the use of smooth forms or proper trowelling. Cement plaster should not be taken to compensate or reduce the permeability of concrete, as its permeability is many times more than concrete.

If the grade of concrete for a work is higher by 10 MPa than that required by standard, clear cover requirement can reduce by 5 mm.R 6.3.8 Durability is the function of concrete properties related to permeation (transportability) of agencies (oxygen, carbon dioxide, hydrogen sulphite, water vapour, water, solution, chlorides, ions etc.) causing deterioration. Tests for each of these coefficients (permeation, capillary suction, diffusion, adsorption, osmosis and electrical migration i.e. movements of ions etc.) are cumbersome and approximate for the test conditions, and are not related (equivalence), except indicating trends. In addition to specifying a grade of concrete, for better control some type of permeation / penetration test should be carried out. Most tests are to be done on concrete samples in laboratory as initial test for accepting the concrete mix (proportioned or designed) for the construction. In-situ tests are also developed. In spite of shortcomings, for important or large projects few tests should be carried out. Also as per international trend, in-situ test on finished structure, representing permeability should also be carried out. ‘Rapid Chloride-ion Penetration Test’ (RCPT) is a laboratory test, commonly performed for many projects. However the results of this test are influence by other factors also and at times it can give deceptive variations, hence modern trend is to substitute RCPT by other tests say chloride migration coefficient, etc. Use of mineral admixtures is preferable, and with them RCPT results can be erratic. For reliability, combination of tests should be done, which should also include water absorption test of concrete.R 6.3.9 Nominal mix (as per 9.3 of IS 456) shall not be permitted for LRC.

R 6.4 FIBRES : The fracture energy of a cement-based material generally increases in proportion to the amount of short fibres or polymer used. The fracture energy corresponds to the area below the tension softening curve, with data of the crackwidth and transferred tensile stress. For members where the occurrence and progress of cracking are dominant, giving consideration to the tension soften property may make a rational performance verification possible. The fracture energy of a cement-based material can be obtained through the test specified in ICI TC 01-01.

For enhancing the performance of concrete, addition of fibres is permitted in concrete. Fibres like steel or synthetic/polymeric can be added. Steel fibres shall conform to ICI TC 01-03, (ISO 13270, EN 14889-1, ASTM A 820), macro polymeric/synthetic to ICI TC 01-04, or micro synthetic fibres to IS 16481 or ICI TC 01-04.

For guidance on use of fibres refer ICI monograph ‘Guidelines on Selection, Specification & Acceptance of Fibre & Fibre Reinforced Concrete’. With the use of fibres the performance of concrete can be improved.

Fibre type, having established as alkali resistant (like polypropylene and steel) can be used in concrete to control plastic shrinkage cracks, control temperature shrinkage cracks, to improve post-cracking behaviour, toughness, and flexural strength of concrete. Structural fibres like steel or macro synthetic, can improve the dispersion of cracks due to loads and restraints in service life, thus gives better control on cracks and reduces crackwidths. For any other fibre, its long term chemical stability shall be established. R 6.4.1 Micro-synthetic /polymeric fibres are used to reduce crackwidths and cracking due to plastic shrinkage of concrete. These may be monofilament or fibrillated, and in typical dosages 1.3 to 2.0 kg/m³. Macro-synthetic fibres may be used in addition to resist shrinkage and temperature cracking. While macro-structural fibres are used, the dose of micro-synthetic fibre can be 1 kg/m³ only. With use of structural fibres, the minimum (temperature-shrinkage) reinforcement can reduce.

For control of plastic shrinkage crack, dose of fibres should be such as to get average residual strength 0.30 N/mm² tested as per ICI-TC FRC 01.1 (or EN 14651 part 1 or ASTM C1609); and not less than 1.3 kg/m³ for micro polymeric

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fibres. To account enhanced flexural strength and toughness the minimum fibre dose shall be such that an average residual strength 1.50 N/mm² is achieved.R 6.4.2 At present guidance is not available to utilize in design, the enhanced flexural strength, toughness and reduced crackwidth by fibre concrete. In these regard the designer should consult specialist literature. Basic information on design model is given in National Building Code -2016, Part 6, section 5A, Annex A, page 73-83.R 6.4.3 If structural fibres are used resulting in the average residual strength not less than 1.50 N/mm², in limit state of serviceability the crackwidth 0.1 mm can be deemed to be satisfied, if the design of the section confirms to all other requirements under ultimate limit states for reinforced concrete as per IS 3370 part 2, and stress in reinforcement is ≤ 230 N/mm² in serviceability limit state.R 6.4.4 Enough dosages of fibres can change tension-softening to strain hardening in concrete, whereby cracks are very fine, and have high probability of autogenous healing. Hence strain-hardened fibre concrete or ferrocement have a very good water-tightness for thin members and also durability with smaller clear cover.

R 6.5 FORMWORK : Depending upon type of finish specified, sheets of steel, aluminum, marine plywood or plastic coated plywood may be selected for shuttering. Joints in shuttering shall be made leak-proof by using foam or rubber strips to prevent leakage of cement slurry. Use of through tie rods shall be avoided. If unavoidable, ties may be provided with creep bolt (water passage obstructed and not through), part of that to be removed later. Holes left in place shall be filled with mortar or grout, preferably non-shrink, or epoxy mortar. If form-release agents are used in LRC for drinking water, such a coating shall be non-toxic after a specified period, usually 30 days. Formwork should be rigid enough, such that change in deflections due to movement of worker on freshly laid concrete should be very small say within 1mm (excluding concrete load & DL).

R 6.6 CURING : It should be in following phases.i. First phase is from placing of concrete to the time till it is finished (as needed) and arrangements made for next

phase of curing. In this phase fogging or misting is to be done to avoid plastic shrinkage cracks. If air temperature is low (<25°C) or humidity in air is high (>90%) this phase may become be neglected. Requirements of misting is governed by duration of this phase.

ii. Main phase of curing during which all concrete surfaces are maintained continuously near saturation (i.e. RH >96%), by spraying water or covering concrete to avoid evaporation of water from it, i.e. applying curing compound. This phase continues till desirable properties are developed in concrete.

iii. Till liquid is filled in tank, it should not be allowed to dry-up i.e. RH ≥55% should be maintained.R 6.6.1 Concrete members should be initially cured continuously (without intermittent drying time) for at least 14 days, and also till 80% of the specified strength is achieved. During summer the main phase of curing period should extent to 28 days and in other dry season to 21 days, during which concrete it kept moist (RH >70%) and may not be 100% saturated continuously after 14 days. Thereafter, LRC should be sprayed with water at least once a day for not allowing the concrete to dry out i.e. keeping RH>55%. Curing activity in the last phase may not be required if ambient temperature is below 10C or humidity in air is high (>70%). [ RH = relative humidity ]

Proper curing of concrete is vital for controlling temperature-shrinkage cracks, and gaining durability. The method shall ensure that the surfaces of all concrete remain continuously moist in the curing period. With curing, the temperature of concrete shall also be kept in control. Temperature shock i.e. sudden cooling of concrete surface say by continuous cold water spray immediately after de-shuttering should be avoided.

R 6.7 For pneumatically applied concrete, the designer should approve the specifications, material requirements, mix proportions, effective water-binder ratio, mixing, placing, equipment to be used, and curing before the construction starts.

7 DURABILITYS 7.1R 7.1 Durability is the ability of a structure or structural element to withstand cumulative deterioration, which may otherwise is harmful to the required performance in the relevant environment, and deterioration should be small as well as tolerable till the design life. Durability should be satisfactory in all situations of limit states of serviceability; and also for few parameters in ultimate limit state for LRC. On reaching an ultimate limit state, LRC may get damaged and excessive leakage may take place, but otherwise after loads get reduced the performance can continue for short duration, till intervention if required will takes place.

Durability is also a limit state. The structure shall be designed such that deterioration will not reach to a limit which can affect the serviceability during the design life with the determined plan of maintenance (conservation). Repairs as additional maintenance would be required for short falls in quality of structure. Before reaching its limit state, structure should give enough warning.

It is a prime consideration for LRC. Refer 8 in IS 456 for its requirements. For it, exposure of concrete structure to environmental and service conditions during design life is to be considered. Each mechanism causing loss of durability, need to be taken in to account and enough resistance for that has to be designed in the structure.R 7.1.1 Important agencies causing loss of durability are following.

Chloride ion penetration, and increase its concentration to a threshold value near the steel surface, which reduces the passivation for corrosion of steel.

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Carbon dioxide permeation convert calcium hydroxide to carbonate, thus pH reduces in the protective concrete cover over steel bar. While concrete is saturated, carbonation does not take place. Normally concrete has pH >12 (in new concrete up to 14). With reduction in pH the rate of corrosion of steel increases.

With increase in permeability, calcium hydroxide comes out of concrete with water in dissolved form and deposits on the external surface of concrete. This results in more porosity, more permeability, and reduction of pH etc., all helping in loss of durability.

Loss of durability is caused by the permeation of agencies like water, oxygen, carbon dioxide, other gases, chloride ions, etc. through concrete. The permeation of each of these are not directly controlled, but indirectly by water-binder ratio and grade of concrete. Water to binder ratio (usually indicated as w:c or w:b) should be as low as possible (between 0.45 to 0.40), while keeping the concrete workable using plasticisers (chemical admixtures).

Other actions like sulphate attack, acid attack, freeze-thaw effect, and reactive aggregates can also contribute to loss of durability and need considerations, case-wise.

Defects like honeycomb and segregation, are havoc for durability, and these must be eliminated. Next is the very low seepage through cracks and joints. Better control over microstructure of concrete and low permeability, are desirable durability over the design life of LRC.

While mineral admixtures (e.g. flyash, GGBS) are added the threshold limit for chloride ion concentration may reduce, which can be compensated by using suitable corrosion inhibitor.S 7.2

R 7.2 From experience of observing roof of tanks storing chlorinated water, the durability of bottom of roof is highly important. ‘Very severe’ exposure should be considered for underside of roof, hence concrete grade minimum M40, w:c ratio 0.4, and anti-chlorine surface treatment like epoxy coating are needed. See also R5 last 5 para’s. Presence of other gases, like hydrogen sulphide in sewage treatment, may also require suitable coatings. R 7.2.1 In environment prone to chemical attack, all jointing materials should be chosen such that they are resistant to the chemical exposure as conformed by tests performed by the manufacturer or the supplier.R 7.2.2 Concrete should be protected against damage by abrasion, erosion and cavitation. Following actions may be taken- reduce velocity of flow to <3 m/sec (if possible), dissipate energy, use curves conducive of smooth flow, smooth finishes, supply air at flow boundary, consider concrete ≥ M40 and w/c ratio ≤ 0.40 with hard aggregate and sand of higher siliceous content, avoid carbonate aggregates. Apply erosion resistant coating as additional protection. Of these, combination of few requirement can give protection.

R 7.2.3 Within service life, example of association of the time-dependent material degradation, are the initiation of reinforcement corrosion, cover concrete cracking, and spalling due to progress of corrosion.

S 7.3 Nominal Cover to Reinforcement R 7.3 CLEAR COVER : It is the next consideration to prevent loss of durability. Years taken for chloride ion concentration to reach a threshold, at the surface of steel is a function of actual concrete cover on bars. Hence, the life of LRC is related to the actual clear cover. Important are the variations in cover being achieved, which prominently affects durability and performance. Systems are required, to measure and maintain documentation during construction, and to ensure through good construction practices and quality assurance, that the variations in cover to be very small and within the permitted tolerance, say 5 mm.

See IS 456, Table 16, and note 2 for tolerance. Code does not allow negative tolerance (-0 +10). Note that for a nominal cover of 45 mm with tolerance of -0 +10, the specified cover would be between 45 to 55 mm i.e. 50 ±5 mm.

For durability design, consider the nominal clear cover (i.e. 45 mm), negative tolerance being zero; whereas in the structural design (also crackwidth check) the clear cover taken as nominal plus half tolerance may be considered (i.e. 45 + 5 = 50 mm).

For a reliable construction, amount of clear cover achieved, should be checked regularly using ‘cover meter’, and take corrective actions to reduce the variation of cover within the specifies limits and tolerances.R 7.3.1 For the exposure condition chosen for design, the nominal cover shall be referred from IS 456 Table 16. For ‘severe’ exposure the clear cover requirement is 45 mm, and 50 mm for ‘very severe’.

Where concrete is cast against the surface of a blinding concrete (PCC sub-base), clear cover should be increased by 5 to 10 mm depending upon the roughness of top surface of PCC. If blinding concrete is finished plain and smooth, increase of cover can be 5 mm only.

If the grade of concrete for a work is higher by 10 MPa (say M40) than that required by code (i.e. M30 for sever exposure), clear cover requirement can be reduced by 5 mm.

The clear cover provision should increase by half the thickness that may degrade during design life, due to chemical action or erosion or abrasion. And such extra thickness should not be accounted in strength calculations.R 7.3.2 Not in code, but nominal clear cover can reduce by 5 mm, if the actual variations in the clear cover achieved are assessed and ensured to be within a narrow range (say ± 5 mm).

Similarly, there is scope to reduce the nominal clear cover by 10 mm, if in addition the reinforcement cage and formwork are rigid (RF2 or RF3 as per appendix C.4 at the end of part 2), i.e. during concrete vibration, the movements are limited (amplitude is less than 1mm) and intense compaction effort is applied as in case of precast in industrial environment with high level of reliability of getting a good surface finish without air bubbles. Also the clear cover achievable should be in a narrow range of variation ± 3 mm. The reduction 5 mm suggested in the 1 st para, is not additive to 10 mm in this para.

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Where stainless steel, or FRP bars are used clear cover can be reduced by 10 mm; and by 5 mm for GI bars and fusion bonded epoxy coated (FBEC) bars. FBEC bars should have very high reliability for absence of scratches and pinholes (holidays) i.e. there shall not be any smallest uncovered portion on the bar surface.

If actual clear cover is found to be deficit by 5 to 10 mm. the surface of concrete should be treated, say applied with an epoxy or polymer coat to reduce the penetration in concrete.R 7.3.3 Clear cover and space between bars shall be compatible with (say 2×) the nominal maximum size of aggregates, to ensure ease of placing concrete, proper encasement of reinforcement and to minimize honeycombing.R 7.3.4 Strength of cover blocks should be more than the surrounding concrete. The cover block shall not break, split or crack, when placed below a bar, under the load of reinforcement cage and workers on job. The permeability of cover block should be less than the concrete surrounding it. It could be of cementitious material having rough surface to bond well with the concrete in contact. In cover block w:c ratio should be ≤0.4 and strength equivalent to M40 or more. Blocks of plastics or a material not having enough bond compared to concrete to concrete interface, should not be used. For plastics and other materials, along their interface with concrete, permeation can be higher.

8 SITE CONDITIONSS 8 R 8 Considerations given here have influence on layout of structure, structural requirements and performance.

Due to constraint on site selection, if within plan area of a structure, the soil strata changes, differential settlement may not be avoidable. Where softer soil is in foundation, differential settlement may be a result of softening of soil due to heavy leakage which may be only on one side or at some part of the structure. Proper structural configuration say dividing structure in parts each bearing on different soil strata, or each part with different susceptibility to differential settlement; and proper planning of drainage including that due to leakage, is required.

Structures should have enough side margins to reduce possibility of interference due to leakages and foundation behaviour of other structures.

S 8 a)

R 8 (a) Chemical properties of soil and groundwater may affect the grade of concrete and specification of the cement to be specified. If soil or groundwater is having sulphates, refer to the requirements in Table 4 of IS 456. In addition to selection of proper type of cement, higher grade of concrete and protective treatment may also be required. If groundwater is acidic, protective treatment is required.

S 8 (a) (i) Earth Pressure

R 8 (a) (i) Earth pressure (EP)- Normally design liquid pressure on a wall should not be reduced significantly on account of earth pressure in opposite direction. Some engineers interpret this as a condition having maximum liquid inside and no soil assumed on outside, which is not proper. Though the earth is present outside, only its inward relieving pressure is to be taken very small, while liquid pressure is acting outwards. For many reasons earth pressure may reduce substantially, and also for the issues of stiffness of soil and wall. This is due to cyclic loading and un-loading of liquid pressure, creep of soil, with adverse temperature shrinkage condition, and soil stiffness being very small compared to concrete wall. Also due to ground erosion or due to temporary excavation near wall, earth pressure may substantially reduce. Earth pressures can change due to change in moisture content and temporary stiffening of soil. Though soil pressure can be neglected as a simplification in design, in most situations some depth of soil fill is definitely needed to comply with the requirement of minimum depth of foundation for the vertical foundation pressure as may be produced on foundation soil. Designer may opt for half of active soil pressure (a minimum possible value) as pressure relieving liquid pressure, refer R 4.2.6 in part 2. It is debatable, that soil pressure at rest can be reliable for relieving liquid pressure is acting from opposite side. While designing wall for tank empty condition and earth pressure from outside, the state of soil should be taken a bit higher than active (i.e. between active and that at rest).

S 8 (a) (2) Settlement and R 8 (a) (2) Settlement and Subsidence – Subsoils of varying stiffness may contribute to differential settlement. Effects

of softening of soil due to leakages in some part of structure, should also be considered. Considerations should be given in the preparation of the layout and design, to the possible effect of settlement or movements of the foundation.

S 8 (b)R 8 (b) Injurious Chemicals – Concrete structure may suffer severe damage in contact with injurious groundwater or

soil. If it is likely to be exposed to sulphate attack, requirements specified in IS 456 shall be followed. An isolating coat of bituminous or other suitable materials may improve the protective measure.

S 8 (c) Extent of floatation at the site – R 8 (c) FLOATATION – It is also known as ‘uplift’. Stability check against floatation accounts buoyancy. It is for ground or underground tanks, or with bottom below the highest groundwater table or highest flood water level. S 8 (c) (1)R 8 (c) (1) Safety - The factor of safety (≥1.25) is the ratio of downward forces to that of uplift (upward buoyancy) force. Uplift force is the product of gross volume (concrete including air void) of structure (in m³) below design ground-

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water level and the density of water (9810 N/m³). To counteract the uplift, in addition to the weight of members of structure, other materials or methods are also used

to provide downward force, e.g. lean concrete fill, rock-chips/boulder filling, sand/soil-fill, anchor rods, anchor piles etc., and each one has different reliability. Hence, for calculating factor of safety against up-lift, a reduction factor to the force contribution by these may be applied as follows. Lean concrete (<M10) - 0.9; Rock/boulder filling - 0.8; Soil fill - 0.7; Anchors - 0.75, Piles - 0.8, soil weight in failure wedge – 0.7, soil friction 0.7 ;

Structures having unsymmetrical configuration or loading, require addition check by taking moments of all the forces for equilibrium with a factor of safety ≥1.4 (& 1.25 is not enough). Under the uplift condition each member shall also be designed for load combinations with groundwater pressure and earth pressure (EP), both for ultimate and serviceability limit state. See also R 10.1.

To resist uplift, the frictional resistance due to soil or weight of soil wedge may be considered with reduction factor applied. On the foundation projection from wall, if the soil weight in failure wedge and its friction force is neglected, the factor of safety should not be below unity. Thus friction and soil weight can be accounted to enhance the calculated factor of safety above unity. For soil, possible depth of erosion should be accounted.

Uplift may be temporary, caused by heavy rainwater entering at sides and bottom of ground tank. This may happen, if soil (clay) below tank has low permeability. In a small time (few hours) built-up (accumulation) of rain-water may take place if water being added is more than that draining out or seeping down through the soil, causing uplift. During construction such possibility exists. Failures of this type were observed in past, while water table was far below the tank bottom. Hence, drainage arrangements during construction and in service are necessary.

Floatation is applicable on tank partly or fully in ground, and not to tanks above ground or above ground water table or flood level. In the design consider the possibility of sudden change in groundwater table or pore pressure in soil, or sudden accumulation of water in ground even if for a small period (hours) such as due to heavy rains.

S 8 (c) (2) R 8 (c) (2) Drainage : Drainage at the site of ground liquid tank should be designed and provided effectively to avoid uplift. Excavated trench around the tank even if refilled, gives an easy entry to rainwater and may create temporary uplift condition, till water gets drained through the soil. During construction precautions should be taken in advance such that the rainwater is not allowed to flow near the excavation trench and its path to take it away is not blocked, thus avoid development of such a condition casing uplift. To avoid such condition during design life, surface drainage of rainwater should be planned and entry of rainwater in to the refilled trench should be prevented even during construction of ground tank. Also refer R 8 c (5).

In this regard a typical specification is as below“For safety from heavy rains during construction, rainwater should be diverted away from the excavation for the

tank, and it should not be allowed to enter the excavated trench around the tank and its foundation. In the side trench outside, just above level of foundation PCC of tank, 300 mm thick refilling shall be done with selected impervious soil (clay). Remaining refill above this will be of selected excavated material duly compacted. The impervious soil shall also be filled from 0.3 m below GL to GL. Further refilling should be done a little above the finally graded ground level. It should be sloping away, such that rainwater is diverted away from the refilled portion of soil.”

S 8 (c) (3) R 8 (c) (3) Pressure Relief Valves : At times, pressure relief valves are installed in the floor slab of the tank for safety against uplift. For the pressure relief valve to function, there has to be higher pressure outside compared to inside, i.e. when pressure relief valve will actuate, there would already be some driving pressure i.e. uplift pressure acting on structure and its components. With age of installation, the operating pressure (i.e. differential pressure inside and outside) on the valve increases. Hence structure and its components are to be designed for some amount (say about 1.5 m) of uplift, and whole effect of uplift cannot be relieved by pressure relief valves.

Pressure relief valves can be used only for raw water storage, where entering of groundwater in to the tank is not objectionable. Pressure relief valves are not reliable enough as per the experiences. For these valves to operate, drainage (with filter layers) is to be provided under the floor of the tank with filter system which should work over the design life of tank. This drainage system can get chocked up over the years (much < design life); hence efficiency and reliability of the relief system can reduce drastically. If the system works, along with the liquid inflow, fine particulate matter (from soil beneath tank) is expected to come in the tank. With few repetitions of this process, porosity of soil in foundation will increase, and in turn may lead to foundation settlement. Hence system has poor reliability and a possibility of settlement, therefore as far as possible the system of pressure relief valve with drainage below floor may not be provide if long-term satisfactory performance is required.

Hence the drainage system under the tank is either counterproductive or unproductive, and the author recommends not to provide it. Drainage around the tank is definitely needed rather under it.

S 8 (c) (4) R 8 (c) (4) Where water may be present on outside of the tank, there is possibility of leakage both ways. Hence the joint should be constructed to seal leakage from both the faces of wall.

S 8 (c) (5) R 8 (c) (5) Due to heavy leakage from tank or other sources or unusually heavy rains, water level around the tank may rise temporarily, and under such condition uplift pressure would exert on the empty tank for a small period (may be in hours) till the water around the tank gets drained. Though the groundwater table does not rise, but since water reaching below the tank is more than the capacity of the soil to drain it, water accumulates for few hours and pore pressure

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increases. Also for such a situation it is not wise to provide drainage system under the floor of the tank. Such drainage will help in developing uplift and not delaying it. Requirement is to provide drainage around the tank to take away the water due to rain or leakage and not under the tank.

9 CAUSES AND CONTROL OF CRACKING R 9 Through cracks liquid seepage can take place, hence crack control is needed to keep concrete watertight. Cracks are preferential pathways for the ingress of environmental agents which may induce loss of durability. It is a chain action of permeable cracks, seepage leading to degradation, result in more seepage through crack which can become wider. Chloride induced corrosion can be faster, initiated with the crack widening. Crack control is needed to maintain water-tightness, for durability and corrosion protection of reinforcement, and for aesthetics.

Autogenous (autogenic) healing reduces the leakage through cracks or construction joints by blocking the continuous chain of pores in concrete. Healing is mainly due to core of cement grain remaining un-hydrated during construction period of the tank which can hydrate at later life. And also due to late hydration of cementitious (SCM’s) grain which remained un-hydrated and will form gel at later age. The blocking can also be by the process of carbonation i.e. molecule getting bigger and occupying pore space (may not be in saturated LRC), or by filler action as free particles move in to pores. The rate of healing dependents inversely upon the average width of the crack. The smaller the width the faster the crack will seal. The effects are smaller due to other factors such as the type of liquid and the cementitious material in the concrete. The rate of healing can slightly increase if flow is smaller through crack, and also by reducing the differential pressure across the crack; i.e. higher pressure gradient (i.e. high H/t) slows down the healing. The average width of crackwidth is smaller than the crackwidth at the surface of concrete. Through cracks will take more time to heal. Healing can be faster if crack is not through i.e. with more of compression block at the section.

First time under test of the tank, ensuring slow water filling can significantly reduce the loss of water and allow healing of cracks. The acidic nature of the water may inhibit autogenous healing. This risk may be reduced by the use of lime, flyash or ground granulated blast-furnace slag in the concrete mix. Placing a pozzolanic material such as fly ash, in the retained aqueous liquid can improve the autogenous healing of the concrete. There is a practice of adding lime (CaO getting converted in to CaOH2) in the water during first fill, which improves the pH of water and also availability of calcium for autogenous healing. Alternate method requiring much less amount of lime (& also less efficient) is to lime-wash the inner surfaces of the concrete. In cracks, the process of healing reduces with pH of water leaking through it. Hence, for first test the water filled in the tank should have pH >7 for which lime can be added.

Normally cracks up to 0.2 mm wide and not live during healing, will autogenously seal within 21 to 28 days; cracks up to 0.1 mm wide will seal in 7 to 14 days. Live cracks (width not stable but varying) are difficult to heal. Crack opening can be very much slowed-down by slow filling of tank first time. Thus, nearly steady-state condition of crack prevails, and by healing continuity of pores can be blocked. During testing for water-tightness, the first application of liquid load should be very slow, to allow slow formation of cracks and also their autogenous healing.

For autogenous heal to be effective, at a position limiting crackwidth should reduce as H/t increases. The limiting crackwidth chosen can be smaller than (0.225 – 0.005 H/t), but between 0.2 & 0.05 mm. This is for direct tension or where compression block is less than 50 mm. This equation suggesting modification of limiting crackwidth shall replace the recommendations in part 2, R 4.4.3.4 and R 8.3.1.

It should be ensured that the crackwidth in concrete, during its design life, does not exceed that needed from durability and seepage requirements of reinforced concrete.

Width of transverse crack ≤ 0.20 mm have very little effect on durability, if seepage through crack is under control. However, longitudinal cracks are more damaging and cause corrosion of reinforcement, which must be avoided. Control of longitudinal cracks requires enough clear cover (about ≥2×φ), and enough transverse steel (or rings). As a rule of thumb, the transverse reinforcement could have capacity to take tension about 20% of the actual tension in longitudinal bar.

The activation of autonomic healing with additional materials or some type of microorganisms is the other option, but not a common practice.

S 9.1 CAUSES R 9.1 CAUSES OF CRACKING :There are various combinations of causes for cracking, mainly following. Due to structural deformations by applied forces, e.g. flexural tensile and direct tensile cracking in mature concrete. Cracking in immature concrete due to drying shrinkage and restrain on concrete. Thermal and shrinkage effects in concrete with some types of restrains. Corrosion of reinforcement in concrete, leading to spalling of the concrete eventually. Within the concrete chemical reactions causing expansive action.

R 9.1.A. Cracks in Plastic Concrete :R 9.1A.1 Plastic shrinkage cracks are due to very rapid loss of moisture from concrete surface soon after placement and before any strength can development, influenced by combination of temperature, relative humidity and wind speed. These are on sizeable surface exposed to air, like top surface of concrete slab or plastered surfaces of wall. When rate of evaporation from surface is more than the amount of water which can be replenished from inside, the plastic shrinkage cracks appears which are shallow, but weaken the near surface concrete. After placing and till curing starts, concrete is to be protected from moisture loss by covering or maintaining high moisture over surface by fogging or misting etc. Depending upon possibility of high rate of evaporation, the precautions and protection against plastic shrinkage cracks should be undertaken 30 to 60 minutes after laying the concrete.

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R 9.1A.2 After compaction, concrete has a tendency to continue consolidation and settling of particles, which is restrained by horizontal steel bars. This results in void below the restraint to settlement i.e. the bar, and adjoining concrete settles (& not that on the bar), causing a crack along and above the bar in concrete surface, called as plastic settlement crack. To avoid these, concrete should be deposited and vibrated in lifts of about 300 mm and not more, with some time gap (20 to 30 minutes) for next lift. Non bleeding concrete has lower chances to develop these cracks. Concrete having low w:c ratio i.e. non bleeding concrete has less chance for plastic settlement cracks, but more chance for plastic shrinkage cracks.

R 9.1.B. Cracks in Hardened Concrete : R 9.1.B.1 Drying shrinkage strain is due to loss of moisture from cement paste during hardening. Restrain on this drying shrinkage induces cracks. These cracks can be minimised by avoiding moisture loss ahead for sufficient strength development in concrete. The shrinkage strain can be higher for concrete having higher paste (binder + water). LRC should not be allowed to totally dry out. Due to cracking, concrete gets weakened, but on rewetting of concrete the drying shrinkage strain reduces also. Out of total shrinkage, the permanent (non-reversible) shrinkage can be taken as one third (33 to 40%), remaining is moisture dependent (reversible) shrinkage. Movement joints can reduce the shrinkage cracks due to reduction of restrain. Many factors influence the shrinkage strain, for which specialised literature should be referred. If concrete is not allowed to dry and remain continuously saturated over a long period, total shrinkage can be negligible. In such condition the drying shrinkage and that due to physiochemical reaction can also reduce. From the stage of setting of concrete to filling of tank by water, if the concrete is not allowed to even once partially dry, the total shrinkage will be very small.

R 9.1.B.2 Temperature changes in concrete causes strain, and with restrain cracks develop, if stress in more than the tensile strength. Temperature variations occur due to heat of hydration of cement or due to environment; and can be viewed as of two types. One, the linear i.e. change in average temperature of a member; and second is the gradient i.e. variation across thickness of the member.

R 9.1.B.3 Deleterious chemical reactions like alkali-silica gives expansive compounds, and cause cracking. When such reactivity is low, use of mineral admixture (like flyash etc.) reduces the susceptibility to such reaction. For more details refer to specialised literature. Aggregates known to be susceptible to alkali-silica reaction shall not be used.

R 9.1.B.4 If concrete is subjected to temperature above 70°C during hardening, in some mixes with combination of certain conditions, delayed ettringite formation (DEF) occurs, which will cause expansion with availability of moisture during service, thus causing cracks.

R 9.1.B.5 Weathering due to freeze & thaw can cause cracking. Refer to specialised literature.

R 9.1.B.6 Corrosion of reinforcement is an electro-chemical process, which results in iron compound having very high volume creating bursting force on concrete cover, causing cracking. Alkaline environment in concrete forms a protective film on the surface of steel. If pH (alkalinity) reduces, or the protective film gets damaged by chloride ion reaching threshold concentration on steel surface, the corrosion of steel starts. Fine cracks (<0.2 mm) transverse to bars do not influence the corrosion rate. Corrosion of bars induces crack along the bar, and in turn the rate of corrosion gets very much enhanced. Compared to transverse cracks, longitudinal cracks are potentially very much harmful.

S 9.1.1 Effect of Applied load R 9.1.1 Effect of Applied load – In concrete, cracks are caused by the stresses due to loads, and restrain on shrinkage and temperature gradient due to environment or the liquid. For crack control load combinations at service loads are considered.

Due to heat of hydration, the temperature of concrete increases, and a peak may occur in 1 st 3 days age. Subsequent cooling of concrete having self-restraint will cause tension at the surface and thus it will crack. In young age of concrete while hardening, shrinkage and temperature variations cause stress under restrained condition, resulting in formation of micro-cracks. At young age, concrete continues to bear increasing shrinkage and daily temperature variations, which contribute to micro crack development, while it is gaining strength. The micro cracks, reduces the tensile strength of hardened concrete, hence in hardened state, cracks may form at lower stress compared to tensile strength of concrete found by test on small samples. These cracks relieve the stresses to a significant extent and develop in to hair-cracks by repeated variations under restrain. Crackwidth for this condition is controlled by reinforcement called temperature-shrinkage reinforcement designed, or minimum reinforcement provided for this purpose. Under the condition of no external loads also, cracks are in the concrete. Crack control due to temperature-shrinkage effect in young concrete, without combination with other load conditions, can be checked as per the procedure given in code (part 2 Annex A).

In hardened concrete also, shrinkage and temperature variation causes existing cracks to open, and to a large extent effect is relieved, though not complete. The strain induced forces may get relieved to level below 20% (due to few cracks), and where steel ratio is very high, many finer cracks could form and having a much smaller crackwidth. However for high steel ratio, force relieved could be higher say to 40%.

Under serviceability state, with the restrain effects combined with loads, the allowable crackwidth (0.2 mm) can be exceeded slightly, say to within 0.3 mm. For daily variations of temperature, the extra temperature stress and effect on crackwidth is reversible (not long term), however effect is small due to relaxation by existing cracks. Hence, usually serviceability check is not applied for load combination with temperature-shrinkage superimposed, because lot of its effect is relieved but not totally. Hence, at serviceability stage some recommends (as in New Zealand and Australian codes) to superimpose the modified temperature-shrinkage effect, by multiplying by a reduction factor (RF say 0.20

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onwards). As a simplification value RF can be read from a graph.For ultimate limit state (or limit state of collapse) crackwidth check is not expected, however severe cracking and

spalling of concrete are controlled by detailing rules as a qualitative measures. In such load combinations, the effects of temperature-shrinkage get substantially relaxed due to creep, cracking (reducing stiffness for the restrains) and elasto-plastic (i.e. nonlinear) behaviour of concrete. The strength check at ultimate limit state is not influenced by temperature-shrinkage effects.

For temperature design Australian (AS 3735) and New Zealand (NZS 3106) standards gives the guidelines. Such design is important for roofs of large tanks, and to a little extent also for walls subject to sunshine.

The minimum reinforcement (temperature-shrinkage steel) as recommended in IS 3370, takes care of only chemical shrinkage which is about one-third of full shrinkage and is irreversible. When a tank is kept empty for a long time (months) over which moisture from concrete dries out completely and thus maximum (i.e. full) shrinkage will take place, with cracks formed wider than normally expected, which can give unacceptable seepages at subsequent filling. Hence take precaution that the tank should not be allowed to dry out. Alternately higher temperature-shrinkage steel is to be calculated (i.e. designed) for full shrinkage and provided accordingly. Refer Part 2 A-2.3, where in strain of 50 to 60% shrinkage (after relaxing to 20%) should be added for condition of full shrinkage. In existing tank if such wider cracks are developed, the leakage can be reduced by grouting these cracks.

Note that more often the concrete in roof members of large tank may dry-out before the 1st filling of tank, and may cause cracks due to full drying shrinkage. Hence roof members should also be kept moist till some water is filled in the tank, or designed for higher shrinkage, wherein minimum reinforcement may be 33% higher.S 9.1.2 Temperature and Moisture EffectsR 9.1.2 Temperature and Moisture Effects- Control cracking due to variation in temperature, moisture and shrinkage, by reducing temperature gradient, moisture changes and shrinkage, during construction. In design reduce the crackwidth by enough reinforcement, and further by introducing movement joints; combinations of these three methods do provide workable solutions. Also refer discussion in R 9.1.1.R 9.1.3 As sum-up : Control over cracking can be achieved by following-

- limiting of temperature rise (and thus its peak) due to heat of hydration,- reducing (or removing) restraints,- reducing concrete shrinkage,- use concrete having low coefficient of thermal expansion,- use concrete of high straining capacity (fibre concrete / ferro-cement).

R 9.1.4 Minimising cracking due to limit imposed on strains: It is desirable (as a better control) to minimise the development of cracks in LRC, due to restrict strains by temperature changes and shrinkage. One of the method for this is to limit the tensile stress in concrete to characteristic flexural strength (only 5% probability of being exceeded i.e. fctk,0.05 = 0.26 fck

2/3) of concrete. Concrete grade M15 M20 M25 M30 M35 M40

fctk0.05 1.64 1.99 2.29 2.53 2.74 2.94Allowable in PCC 1.09 1.33 1.52 1.68 1.82 1.96

For stress in direct tension multiply by 0.7 approximately.

S 9.2 Methods of Control R 9.2 METHODS OF CONTROL ON CRACKINGS 9.2.1 R 9.2.1 PCC Design : There is not enough clarity about PCC design. It can be designed with following approach. Design by limit state of ultimate load is not developed for PCC, thus design has to be by working stress. Though called as PCC design, some reinforcement is provided and its contribution is neglected in resisting stresses due to loads.

R 9.2.1.1 Plain concrete having no reinforcement : It may be feasible for very small units, tension may be very small, and seepage is not a consideration. Structural stability may be given by gravity action of retaining wall (combined with some tension in concrete) or by masonry backing. H/t can be ≤ 8, size of structure < 1 m. This condition is not normally considered. Small tanks can be constructed more economically in masonry.

R 9.2.1.2 Concrete designed for structural safety by accounting tensile strength of concrete, and providing nominal (as minimum) reinforcement as per IS 456. Due to history of temperature-shrinkage variations, the actual concrete in a structure has micro-cracks, and tensile (or flexural) strength of concrete vary widely. Modern standards (e.g. EN 1992-1) as an example specify tensile strength of C20 concrete from 1.5 to 2.9 MPa (at 5% & 95% reliability). If structural failure is dependent on tensile strength, safety margin is applied to such low value (1.5 & not 2.9 MPa). It should be noted that normally for concrete design the tensile strength of concrete cannot be relied upon for a primary purpose. Hence lower permissible tensile strength should be specified as given below for achieving the reliability.

Design as plain concrete liquid retaining members by allowing lower permissible direct tensile stress for M20 & M25 being 0.93 N/mm² & 1.06 N/mm², and allowable flexural stress 1.33 N/mm² & 1.52 N/mm² respectively for working stress design method (or under limit state of serviceability). The purpose of specifying the limiting tensile stress, primarily is to keep a control on tensile strain in concrete, and it may give very small safety for strength failure. Higher values of tensile stresses are not desirable, and should be substantially lower than the values as was practiced for

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RCC design. If design by limit state method is to be practiced, one has to arrive at characteristic tensile strength of concrete in structure, and also differentiate between stages of crack development and failure.

For PCC design (keeping tension in concrete within the allowable tensile strength), a limit on the capacity of tank can be 10 m³. In addition, the horizontal size could be maximum 2.5 m; and H/t should be limited to 10, and for minimum steel is as per IS 456.

R 9.2.1.3 The strength design as RCC. The design and minimum steel could be as per IS 456, and a check on tension in concrete is applied. Permissible direct tensile stress for M20 & M25 being 1.04 N/mm² & 1.20 N/mm², and allowable flexural stress 1.49 N/mm² & 1.72 N/mm² for limit state of serviceability. In addition, a limit on capacity of tank as 25 m³, H/t should be limited to 15 and a more appropriate criteria could be the horizontal size of structure 5 m.

In above PCC tank, if tank is part of continuous structure, the size refer to the length of continuous structure and not only the tank portion.

PCC design can be considered by working stress method. For RCC design the safety is basically derived from provision of steel to take tension, and tensile stain in concrete is kept in limit to control crackwidth. Crackwidth control is necessary for keeping leakages under control. Calling it PCC design, steel provision is nominal only. The tensile stress in concrete has to control both the strength failure as well the crackwidth. Hence limiting tensile stress in PCC should be less than that was permitted in RCC design under working stress method (or serviceability state). Thus tension in concrete should be kept low, while minimum reinforcement is to be provided as per IS 456.

Corollary is that, a member can be designed as per IS 456 only, and if tension in concrete is within specified values (for PCC), no further requirements of IS 3370 part 2 are to be checked (except concrete cover); and this will deemed to be a PCC design. Grade of concrete should conform to Table 1 for PCC, and no crackwidth check will be applicable.

S 9.2.2 R 9.2.2 Shrinkage depends upon amount of paste (i.e. binder plus water) per unit concrete, and not only water. Increase of the nominal maximum size of aggregate (MSA), reduces the paste volume, and therefore reduces shrinkage. Largest size of aggregate should be used as is compatible with the cover being specified and detailing of reinforcement (space between bars) etc. Usually 20 mm MSA is used, and 30/32 mm may also be used. To reduce shrinkage, total cementitious content should be as low as possible while meeting the strength requirement, and subject to minimum content specified in Table 1.S 9.2.3 R 9.2.3 In cases where structures under construction are exposed to high wind, high temperature and low humidity, adequate measures during the initial stages of construction shall be taken to protect surfaces of plastic concrete from drying, such as by covering the concrete by plastic, polyethylene or tarpaulin sheets, or a cover of fog or mist be maintained. Protection from direct sun and wind also reduces evaporation. During young age (1 hour or more), reduce moisture loss, and in turn plastic shrinkage cracks.

The risk of cracking due to overall temperature and shrinkage effects may be minimized by limiting the changes in moisture content in concrete and temperature to which the structure as a whole is subjected. Reducing temperature gradient in immature concrete can reduce associated cracking. Curing regime involves control over eliminating variations in moisture, and also reducing temperature gradient in concrete. Avoid fast drop (steep changes) of moisture content in concrete, as well as sudden temperature drop. If changes are slow, the effect is reduced due to creep, and autogenous healing.

Young concrete is more susceptible to cracking as strength is low, but creep is high which allows substantial relaxation, if changes are slow and not sudden. Restrain induces tension causing cracks, thus restrain reduction can control cracking; also by avoiding or reducing the gradient of steep changes in temperature and moisture of especially at the early age concrete. Type of shuttering, de-shuttering procedure and curing method may affect the changes in temperature and moisture. Tanks can remain moist. It will be advantageous, if during construction, thin sections below final water level are kept damp.

Till tanks are put in to service, avoidance of drying of concrete can reduce shrinkage and associated cracking. Hence, after curing period, concrete members should be moistened at least once a day such that concrete is not allowed to dry out i.e. RH is >55%.S 9.2.4 R 9.2.4 Movement Joints - For adequate control over cracking, if effective, reliable and economic measures cannot be taken, movement joints can be designed to relieve restrain on volumetric changes in concrete, which could reduce cracks in for large tanks. If effective and economic means cannot be taken to avoid unacceptable cracking, provide movement joints in long walls or slabs on ground, with provision of a separation cum sliding layer for which normally low density polyethylene (LDPE) sheet is used. To economised steel requirement, reduce spacing of movement joints.S 9.2.5 R 9.2.5 If development of cracks or overstressing of the concrete in tension cannot be avoided, the concrete section should be suitably reinforced. In making the calculations either for ascertaining the expected expansion or contraction or for strengthening the concrete section, the coefficient of expansion of concrete shall be in accordance with the provisions given in 6.2.6 in IS 456.S 9.2.6 R 9.2.6 For first filling of the tank, the rate of filling should be slow to avoid sudden change (shock) in stress, and allowing time for creep to adjust the strains. Also while progressive crack development should be slow, allowing autogenous healing of cracks to take place, and thus marginally reduce the possible crackwidth and leakages. Hence

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increase in the water head per day is specified. However this does not mean that water can rise 1.2 m within few hours in a day. During a day also per hour rise may not be more than 20 cm. Also see R 9 on autogenous healing.S 9.2.7 R 9.2.7 Reduce the crackwidth by diffusion of cracks (i.e. more numbers of cracks at smaller crack spacing) preferably using smaller spacing of small size (diameter) bars more in numbers, such that it will not cause congestion. It is also preferable to keep clear spacing between bars more than 2.5× nominal maximum size of aggregate, and spacing less than 5× bar diameter (or 70 mm) on a face, have no specific advantage. Code does not specify a minimum size of bar as main reinforcement in walls and slabs.S 9.2.8 R 9.2.8 For ground supported floor slab, crack control is by one of the possible two types of strategies- (a) continuous external restrain by ground (through sub-base) or internal by reinforcement, and second (b) no restrain.

External restrain can be continuous nearly uniform, or at few locations or at two end, and in between these restrains the member will develop tensile stress and crack. S 9.2.8 (a) R 9.2.8 (a) Cracks are due to volumetric changes (i.e. temperature, shrinkage, moisture movement etc.) if restrained. External restrain if continuous, cracking is controlled. For ground supported tanks, continuous external restraint is offered by the friction due to roughness between structural member and ground (via screed layer). However, continuous restrain may not be uniform reliably, or may vary slightly along length, and thus some stress can occur at different locations, which a smaller reinforcement can resist. Thus continuous restrain reduces the requirement of reinforcement to control crack due to temperature-shrinkage-moisture variations. Reinforcement can be taken as internal restrain diffusing cracks, and external restrain reduces the reinforcement. Restrain as friction on slab on ground can change due to load (vertical) perpendicular to slab. At the position of column supporting the roof of tank, the load is higher than that in the remaining portion of slab. Hence, slab will behave as restrained (as pinned down) at column locations.

For construction without joints, satisfactory control over cracking can be obtained by increasing the reinforcement, which provides internal restraint. Requirement of reinforcement also increases as the length increases more than 20 m. Minimum reinforcement specified in 8 of IS 3370 (Part 2) is for this combination. As the length of slab increases, the probability of restrain being not uniform also increases, requiring more reinforcement. S 9.2.8 (b)R 9.2.8 (b) If the restrain on member is reduced, it will be free to move, stress and hence crack will not develop. To accommodate movement, movement joints are provided in the floor slab of tank. Movement joint will function only if restrain on slab is reduced, such that parts of slab between movement joints can move (shrink or expand). To economize reinforcement, movement (or partial movement) joints are introduced. External restraint is reduced by providing a separation (de-bonding) layer between structural member and ground support which permits sliding between members and sub-base below. This sliding layer below slab, should be such that the panel should be free to move or shrink. To allow slip between structural slabs on PCC sub-base, its top surface (PCC) shall be flat, in-plane and smooth, which normally can be achieved by screeding, levelling and float finish. The deviations from plain surface, and roughness will resist movement and induce restrain, thus defeating the purpose.

For separation, polyethylene (LDPE) sheet is provided of enough thickness, which can allow free movement of slab above. The roughness of sub-base below slab can provide interlocking friction, hence sheet thickness is proportional to the roughness, which is usually 1 mm thick as per British specifications. The required thickness can be provided by using single sheet or two sheets. The sub-base i.e. PCC layer (below LRC slab) must have flat smooth finish and surface in a perfect plane without projections (vertically) which may otherwise act as key. However smooth surface is made, a small friction (restrain) may act, and some reinforcement is needed to control crack due to that reduced restrain.

Usually bottom surface of structural floor is not plane, and provided with slopes, pockets, and extra thickness at different places. All these can lock or restrict the possible movement, and their positions govern the locations of the movement joints. Portions of slopes and pockets shall be separated by providing movement joints. For each panel between successive movement joints, the bottom surface of structural floor slab should be plane, and panel could be smaller.

With introduction of joints, the reinforcement is to be designed as per 11.3 & Annex A of IS 3370 (Part 2).

S 9.2.9 Structural FibresR 9.2.9 Structural Fibres : Fibres control plastic shrinkage cracks, and also temperature-shrinkage cracks in young age of concrete. Structural fibres like macro polymeric (tensile strength >450 MPa) or steel can improve the dispersion of cracks due to loads or restraint in service life, reducing crackwidth and enhance the toughness, ductility and durability.

For control of plastic shrinkage crack, minimum dose of fibres should be such as to give average residual strength 0.30 N/mm², tested as per ICI-TC FRC 01.1 (or EN 14651 part 1 or ASTM C1609); and not be less than 1.3 kg/m³ for polymeric fibres. For structural purpose (i.e. to account enhanced flexural strength and toughness) the minimum fibre dose shall be such that an average residual strength 1.50 N/mm² is achieved.

The guide for part 2 (R 4.4.3.1) gives conservative recommendations for crackwidth estimate in fibre concrete. Actual benefit of fibres are much higher, which can be accounted if detailed procedure as per accepted methodology can be applied. At the construction joints, the strength contributions of fibres cannot be accounted.

R 9.2.10 Longitudinal crack is along the bar, and it is necessary to control it from developing. For that adequate concrete cover ≥2× diameter of the bar, and enough transverse reinforcement perpendicular to the bar shall be provided. Bars having poor bond, such as due to plastic settlement, are more likely to develop longitudinal crack.

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R 9.2.11 Unacceptable seepages noticed through cracks or construction joints etc. are to be grouted by cement based grouts commonly. Cement can be mixed with finer flyash and/or finer GGBS, and grouting additives to prepare grout. For fine cracks formulations or polymeric grouts can be used as per manufacturer’s advice.

Generally tank is to be almost emptied for grouting. However, there are the methods, expertise available and specific grout materials which can be used while leakage is taking place. Grouting is a cyclic process. The seepage follows the path of least resistance. Once a water travel path is grouted, probably leakage may appear from other nearby path, requiring grouting of this alternate path in next cycle. Grouting cycles may be more for higher H/t ratio. Normally two to 3 cycles of grouting may be required where H/t could be >25.

Depending upon severity of crack (or seepage) other grouting materials may be used. For selection of grouting material and method of grouting refer ‘Guide on Construction of Concrete Structures for Retaining Aqueous Liquid’ and ISO 16475-2011 ‘Guidelines for the repair of water leakage cracks in concrete structures’. Grouting of cracks by polyurethane foam can be done without emptying the tank, but may not give long term remedy and hence it is to be supplemented by other grouting materials and sealing.

S 10 STABILITYR 10 General principles as per IS 456 and guidelines given here are applicable. Underground structures should also be designed for floatation (uplift) action as applicable. See R 8 (c) (1). Design for uplift involves not only the stability check, but also the design of components for the force actions during uplift condition. Tanks which are not symmetrical will need overturning check, while subject to uplift.

The equilibrium and safety of structure and parts of it, against sliding and overturning, especially when the structure is founded on or adjacent to sloping ground, shall also be checked. Also with wind or earthquake induced forces, the resistance to sliding is important for structures on sloping terrain.

S 10.1 Overturning R 10.1 OVERTURNING :

The stable equilibrium of the structure as a whole against overturning shall be ensured by checking that the restoring moment shall not be less than the sum of, 1.4 times maximum overturning moment due to the characteristic loads (DL, and IL), and 1.2 times the maximum overturning moment due to the characteristic wind or seismic (WL/EL) action

During overturning check, for buoyancy (uplift/ floatation), 1.25 be the load factor. In this check, load factor for liquid will be 1.0. Liquid (FL) in tank may be any value between empty to full capacity as may be critical, and imposed loads can be neglected unless that contribute to overturning moments. Earth pressure when contributing to de-stabilizing forces, will have a higher load factor of 1.6 times. In cases where DL provides the restoring moment, only 0.9 times the characteristic DL shall be considered. Restoring moment due to imposed loads shall be ignored. If contributes to overturning moment, IL shall be accounted. Check may be critical for combination of WL or EL. Restoring action due to earth pressure should consider minimum possible, (refer R 8.a.1), with 1.0 load factor. In any stability check, if earth pressure is providing restoring force, its contribution more than at rest cannot be accounted in many cases.

For stability, the load combination: (1.2 or 0.9) DL + 1.0 FL + 1.2 WL (or EL) + 1.4 IL[As per IS 1904, factor of safety against overturning is 1.5, and 2.0 while only DL, IL & EP are considered.]

S 10.2 PROBABLE VARIATION IN LOADSR 10.2 PROBABLE VARIATION IN LOADS : To ensure stability at all times, probable variations in DL, FL and earth pressures during construction, repair or other temporary measures shall be taken into account. Load factor for DL may be taken as 0.9 or 1.2, as may be more critical. Similarly FL may be nil or in part or full with load factor 1, whichever gives more critical combination. IL and provisional DL may be neglected, if it helps in stabilizing. WL and EL will be treated as overturning or de-stabilizing loads. R 10.3 SLIDING : The structure shall have a safety factor against sliding of not less than 1.4 under the most adverse combinations of the applied characteristic forces. In this check only 0.9 times the characteristic DL shall be accounted. Structure can be subjected to maximum earth-pressure from one side, and minimum possible earth pressure from opposite direction. Load factors as in 10.1 can be used. The factor of safety against sliding shall not be less than 1.75 when earth pressure contributes to sliding force.

[As per IS 1904, factor of safety against sliding is 1.5, and 1.75 while only DL, IL & EP are considered.]

R 10.4 MOMENT CONNECTION : As a framework, design adequate moment connections or provide a system of diagonal bracings to effectively transmit all the destabilizing forces to the foundations, with enough factor of safety, such that as a whole it will act a rigid structure. All joints, junctions, and connections of members or elements shall be designed and detailed so as to avoid disproportionate deformations, wide cracks, and failures within junctions or connections. Monolithic junction between the two members shall be designed for all the force actions on connection including moment, shear and direct force.R 10.5 STRUCTURAL INTEGRITY: Design shall provide general structural integrity, directly or implicitly. Localized damage or deterioration shall not impair general structural integrity of the structure. Structural system shall be such that sequential failure should be avoided. R 10.6 BUOYANCY (uplift) shall be considered as per R 8 c.

R 10.7 ROBUSTNESS : It is the ability of the structural system to fulfil its function during events like small accidents or due to human errors. The structural system should be able to serve, without being damaged disproportionally to the

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cause of the damage. Structure shall be robust.

11 DESIGN, DETAILING & WORKMANSHIP AT JOINTS Joints break the structure in sections (or parts) convenient for construction, to control possible cracking resulting

from excessive stresses and strains due to shrinkage, temperature changes etc., and to comply with design assumptions. Design joints for satisfactory performance and maintainability over design life.S 11.1

R 11.1 As far as possible joints can be minimized or avoided in LRC. Joints are source of weakness, leakages and also positions of maintenance. Movement joints are planned to reduce the reinforcement requirement, and also cracking. The recommendations about movement joints (expansion and contraction type) are drafted basically for ground tanks. For elevated tanks, usually movement joints are avoided, because the restrains for temperature-shrinkage movements are far less compared to ground tanks, and joint in floor slab is difficult to construct and seal.

Positions of all the proposed joints in a structure, should be specified and detailed by the designer. For these joints structurally strength and crackwidth should be checked as applicable. The joints shall be placed at accessible locations to permit inspection and maintenance. Most of the joint materials and sealants have a smaller life compared to concrete structure. Hence replacement of materials at joint, sealants, and maintenance for leakage prevention should be possible.

Number of joints, including construction joints, should be as less as possible, and their total lengths should be minimised. Concreting shall be carried out continuously between the joints planned.

For a joints to be added or deleted, or alterations to be done in the detailing of joint, by the construction engineer or contractor, the proposal shall be reviewed by the design engineer after considerations of effects on the structure’s design, crack control, strength and performance of the joint and such alteration will be carried out only on approval by the designer. Details and location of the joints shall be as per instructions, drawings, and positions approved by designer or a competent engineer.

Joints should be designed and constructed to prevent concrete cracking, spalling, and corrosion of steel, prevent leakage and resist chemical attack over the design life. The design of movement (expansion and contraction) joints should take account of the ability of the filler, sealant, and water-bar materials to sustain cycles of deformations, hence these materials cannot be substituted without verifying the design with properties of the substituted material regarding deformability and life.

S 11.2 JOINT TYPESR 11.2 JOINT TYPES : Type of joint depends on the number of degrees of freedom for movements to be permitted across the joint, and restrains to be provided, as well as the design and detailing for preventing leakages through the joint. Common joints can be categorized as given in following and are dealt here. However joints can be many more types, configurations and details.

S 11.2 a) Movement Joints : R 11.2 a) Movement Joints : In the members, movement joints introduces structural weakness, more than that due to construction joint. Normally direct tension and bending moment are not transmitted through the joints. Dowels are provided to transfer shear force across the joints. The joints are to be sealed, such that while permitting movements, passage of liquid (as leakage) does not take place from one face of member to other, and vice-versa also. The joints must be sealed on liquid face, or on both faces for underground structures, by providing sealant in a small groove. Sealant shall be bonded to the concrete on only two opposite sides of joint; and on the third side of the groove it should not be bonded to concrete at back i.e. it must remain de-bonded from concrete. Considering volume changes in concrete, joint should be designed such that it should remain functional over the design life with the desirable amount of maintenance. Different types of movement joints provide partial structural continuity (few restrains) or non across the joint. In elevated tanks the restrain to linear expansion or contraction of the structure is very little and hence movement joints are not required. Mostly movement joints are provided in large ground tanks. At all movement joints, water-bars are required, which in most cases will be at middle of thickness. For horizontal slabs resting on grade, it is convenient to provide water-bar along the bottom surface of the slab. Type and detailing of movement joint depend upon number of degree of freedom for movements, some are free, some are restrained and others indirectly restrained.

Joints can be of following main types.

R 11.2 a) (1) Expansion joints are designed to accommodate both linear expansion and contraction. It is a discontinuity in the structures. All reinforcement shall be terminated at the joint, and none will pass through it. A designed gap is provided between the two parts across the joint. It may be of the order of 15 to 25 mm. In the gap a compressible preformed joint filler and a water-bar are to be provided. A sealant is bonded to the two sides. The gap is to be designed for the movement expected and the compressibility (& extensibility) of filler and sealing material. Due to movement of the structure the gap and the filler should either expand (i.e. open out) or contract. The joint may be provided with dowel to restrict relative movements parallel to the plane of the joint. Such dowels can resist shear across the joint and avoid faulting. Water-bar should have a central bulb to allow expansive or contractive in-plane movements. Joint shall be provided with sealant on the face of liquid retention. Joint is supposed to have no strength across it otherwise (except shear strength of dowel).

R 11.2 a) (2) Full contraction joints have discontinuity in concrete laying and also in reinforcement, and does not have

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any gap. It does not provide resistance to tension across the joint, and also flexure. Shear strength at the joint reduces significantly. When concrete contracts, the joint will open up forming a crack, being a weak and preferred place for crack to appear. The joint is created by discontinuity in the concrete laying operations on the two sides of the joint. If at the joint, the surface of concrete cast first is not made rough, loss of shear strength across the joint will also be almost total. To transfer shear force across the contraction joint, shear key or dowels will required.

All contraction joints shall be sealed on the face retaining liquid. For shear transfer dowels can be provided at full or partial joint. At full contractions joint with discontinuity of reinforcement water-bar is required.

Full joint, with partial continuity of reinforcement may also be provided. Continuity reinforcement will need epoxy coating as corrosion protection over small length say 150 mm on both sides of joint.

Full contraction joint (reinforcement not continuous) should not be provided, where pressures are high (H/t >20) or where joint is required to be gas/air-tight.Induced contraction joint with discontinuity of reinforcement: Across the joint reinforcement is not continuous, however concrete laying is continuous. In fraction of thickness of member say one third, a discontinuity is introduced either by inserting a smooth material strip (groove former) at the time of laying concrete to break bond within concrete, or by cutting (sawing) a groove in the concrete. The groove former (material strip etc.) may be removable or to be left in position, which may be of a type to also act as outer water-bar or sealant. Depth of groove depends upon amount of reinforcement continuing. While concrete contracts due to shrinkage strains and temperature fall, the section of member at the joint being relatively weaker will attract crack. At induced contractions joint, full continuity of reinforcement is usually not possible. Additional bars can be provided at middle of the thickness of member, which will cross over the joint, and allow the groove or the cut to be provided. Area of such continuing steel is normally half or less than the minimum steel in the member section. Water-bar may be avoided. Flexural and shear strength of concrete section through joint gets very much reduced say by 80 to 50%. This joint can transmit some shear force. For more and reliable shear transfer, dowels can also be provided. At the surface on liquid side the joint should be sealed by sealant for achieving low permeation through the joint.

On each side of the contraction joint (without water-bar), for a width not less than the thickness of member (or >150mm) waterproof coating may be applied. Induced contraction joint with continuous reinforcement : At the joint location concrete laying is continuous, and also the reinforcement. In fraction of thickness of member a discontinuity is introduced by either inserting a smooth material strip at the time of laying concrete, or cutting (sawing) a groove in the concrete. It is difficult to form this type of joint, because of continuity of bars, groove of enough depth cannot be provided. It is also difficult to provide water-bar in the joint. And with groove of smaller depth, crack may not develop at the joint. The normal reinforcement on one or both faces may be discontinued, and continuity reinforcement may be provided nearly at middle of the member thickness, which is adequately lapped to the main reinforcement which is discontinued.

For a length of about 150 mm on each side of the joint, all the reinforcement bars should be coated by epoxy as a protection from corrosion of bars continuing. At the surface, the joint should be sealed for achieving low permeation of water across the joint. On each side of the joint, for a width not less than the thickness of member (or >150mm) waterproof coating may be applied if water-bar is not provided. Flexural and shear strength of concrete section through joint reduces. It should be noted that behaviour at this type of joint tends to be similar to the construction joint in some regards.

R 11.2 a) (3) Sliding joint has no restrain for shear across the joint. In most of sliding joint restrain for moment transfer is also very small. Typically this type of joint is used at base of wall.

Above list is not exhaustive, and other types of movement joints are also possible, and are in use.

For contraction joint only one linear relative movement (perpendicular to the joint) is provided for. No planning (or design) is done to reduce restrains for other movements. To avoid faulting (shear movement), dowel bars or shear key may be designed. Dowels are not required in all cases.

Figures given in the code should be treated as examples only. Modifications and alterations are possible. Alternate materials are also available and can be used. For horizontal members like slab, water-bar at the middle of thickness may not be avoided. It is difficult to achieve proper workmanship for the concrete placed around the water-bar. Therefore good amount of details and sequence of construction operations are to be planned such that honeycomb and un-compacted concrete is not possible.

The water-bar (wide strip type) at middle of member less than 300 (preferably 350) mm thick, will pose lot of workmanship problems, as well as it will reduce strength of section. Hence for high H/t and with water-bar, concrete thickening the joint should be considered. If at the joint water-bar, dowels or shear key are also required, the thickness requirement will be ≥ 400 mm (preferably 450 mm).

Unplanned construction joint (cold joint) at the ends of water-bar shall be avoided. Water-bars (or water-stops) are needed where water head is high (H/t > 30 for good workmanship, & 25 for average). Without water-bar maximum H/t can be up to 35 with excellent workmanship and very much higher steel requirement. For H/t up to 30, the function of water-bar becomes the transfer of the responsibility for seepage from designer to constructor. The seepage may be due to inadequate workmanship and efforts for good construction practices.

No leakage at joint because of water-bar, but poor workmanship (say poor compaction & honeycombs) is not an acceptable situation. Hence as far as possible the use of water-bar should be avoided for medium hydraulic gradients, and seepage should be reduced by grouting the joint adequately. It is preferable to increase thickness of member at

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partial contraction joint, than considering the provision of water-bar. Refer R 11.5.1.1. For water-bars refer R 12.2. For movement joints designed in floor on grade (LRC slab on PCC base), it should be ensured that the bottom

surface of the slab shall be plane, flat and smooth, more specifically flatness and smoothness are more important near the joint (for about >2 m on each side), and it is without any projection or key, pockets stiffeners or foundation thickening at bottom face. The projection /local thickening /key will act as anchor and will lock the movement, thus defeating the purpose of providing movement joint. Restrain to sliding movement should be very small. Smoothness shall be of an order which can allow the two surfaces to move (slide) relative to one another, and any mechanical friction if develops (i.e. locking) will become a restrain. The slab should also be de-bonded from PCC base, to facilitate movement. Also see R 9.2.4 & R 9.2.8 (b).

As a simplification, in many cases it may be advisable to avoid movement joints by providing continuous restrain by grade (layer below say base PCC), and also provide enough reinforcement.

Each joint should be designed for the estimated movement, strains on the material used in joint, estimated life of jointing materials, and method of maintenance. Shear keys may not work efficiently and can be avoided in full contraction joint or expansion joint, however corrosion resistant smooth dowels can be designed to transfer shear. Alternately shear transfer devices, allowing expansion and contraction, can be used. Half the length of dowel on one side of joint shall be kept un-bonded to concrete allowing expansion or contraction of the joint.

A sliding wall base joint, include a bearing pad and flexible water-bar is provided in circular prestressed tank.

R 11.2 b) Construction Joints :Construction joint is introduced for giving a break in pouring of concrete, or for convenience in construction, at

which measures are taken to achieve some continuity without future provision for relative movement i.e. to act as monolithic member across the joint. It is also called as cold joint.

Construction joints are positions of structural weaknesses in the member, hence it is desirable to minimize their total length in the structure, or to avoid them to the extent possible. These should be located where the effect of weakness is least and does not affect the performance. In general, these can be located where shear stress and flexural cracking is small. In slabs and beams it can be within 0.2 to 0.33 times the clear span of member from support, i.e. nearby the point of contraflexure. Joint may be provided at one of these locations, if not specified by designer on drawings. Joint may also be avoided where bars are curtailed or stopped or curtailment of bars be done away from cold joint. As far as possible, construction joints should not be made at critical sections. Exception only is the junction at bottom most end of wall with floor slab, thus this joint needs liberal provisions (i.e. smaller stress in steel and lower crackwidth).

Along the joint, direct shear strength reduces and also liquid percolation may increases, depending upon the treatment, detailing and workmanship at the joint. The position, arrangements, and the treatment of the construction joints should be specified by the designer and indicated on the drawings. At the joint location the strength adequacy should be checked by the designer. In horizontal members like slabs or beams, the joint can be nearly vertical. For vertical members like walls or columns, the joint should preferably be horizontal. Designer may specify joint at an inclination if designed. The joint should continue along the same alignment in to the adjacent panel or member to avoid additional sympathetic cracks.

Time lag between the two concrete phases is an important parameter for the behaviour of the joint. It is to be measured from the instance of mixing of concrete of first phase, to the instance when second phase concrete is compacted against the first phase concrete already laid. When the time lag is less than that of 80% of the initial setting time of first phase concrete, and also while second phase concrete is still plastic, monolithic concrete will be achieved, and loss of strength at interface is significantly small. When this condition is not satisfied i.e. time delay is more, the construction joint is assumed to form.

The time lag may be more than two hour or may be in days. As the time lag increases, the behaviour of the two concrete are in different phases of setting, hardening and shrinkage; hence effects of temperature (due to heat of hydration) difference, and also differential shrinkage set in. The temperature-shrinkage strains in the two phases of concrete are different, and the difference increases as the time gap between the two the phases is more, thus differential strain are sets in, forming an interface at the joint. With increasing time gap (hours to days), the severity at the joint also increases. Much of the hardening of the first phase concrete has taken place, and it undergoes contraction due to temperature fall and shrinkage strain. Adjacent to that, the new concrete laid will undergo temperature-shrinkage strains, while its stiffness (E value i.e. modulus) is lower. Thus due to differential movement (strains), shear slip develops at the joint. The bond between the two phases of concrete will reduce, formation of crack and its development takes place at lower level of strain. Thus slip takes place under shear stresses during hardening of the concretes, and the interface tend to become un-bonded with high permeability, and permitting relative movement at the shear stress imposed (normally low shear strength), thus the crackwidth may be increased due to shear.

Hence the design and specifications at the joint will depend upon the probable maximum time lag at the joint. More the time lag, less will be the bond between concrete of two phases.

At construction joint, if shear is higher than friction capacity, or tension across joint is significant; and the time leg is more than the period in which >70% of hydration of old concrete has taken place, the interface should be applied with a bonding material. For bonding two types of adhesives are available - one the resin-based adhesives whose base material is epoxy or acrylic; and second cement-based adhesives such as polymeric hydraulic cement mortar.

The bonding strength of an adhesive differs depending on the state of moisture and roughness of the surfaces of the existing concrete to be bonded, as well as on the environmental conditions including temperature, humidity and wind. Therefore, these should be taken into consideration when selecting the material. The adhesive should shrink a little (minimum possible) while getting hardened, and be of high quality in terms of water tightness, heat resistance, chemical resistance etc., and also ensure enough application time is available. The integrity and durability of bonding material

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should be ensured through the design life of structure under the actions of load and environment. The bonding property should not changes much over time and being resistant to the actions of degradation.

The resin-based adhesives, are empirically known to excel in energy absorption until the deformation limit is reached, and can ensure integrity in longer term than those with higher strength properties. For bonding materials (including polymer hydraulic cement mortar), the characteristic values of the material need to be determined under the temperature condition appropriate for the usage environment. Elastic modulus of bonding or inter-joint material should be lower than the concrete it is bonding, but staining capacity should be higher. Thin (<2mm) bonding material has little influence on the stiffness of the member or the joint; whereas thicker has some.

Along the plane of construction joint the permeation of water, aqueous solutions, chemicals, gases or ions can be higher, which may cause loss of durability. Hence bars lying or continuing (within or parallel to joint) in this region can be more prone to corrosion and loss of durability. Any steel bar or a ring shall not lay in a construction joint, or in a plane parallel to it within 25 mm on either side of the joint or 2× diameter of bar whichever more. If a bar is in this region, it should be moved away from it, or the joint should be planned a little away from the bar or the joint be kept between the parallel bars. Bar in the plane of construction joint will also have poor bond and be more susceptible to longitudinal crack along the bar. The construction joint should be grouted, and on the liquid face should be sealed to reduce entry of fluids etc. unless H/t is low. Enough reinforcement should cross the joint, resisting its opening.

Performance of the joint will depend upon the condition of the existing concrete regarding cleanliness and roughness. If it is an old concrete surface, conditions regarding cracking, tensile strength, ingress of chlorides and other aggressive agents, carbonation depth, moisture content, and temperature should be considered. Such concrete should be removed up to a depth good concrete is available. Also the required conditions of fresh concrete, such as ambient temperature, humidity, wind force, precipitation, and any temporary protection should be considered. Concrete surface preparation deals with the various operations needed to fulfil the requirements.

It is more usual that construction joints are permitted where the stress due to direct shear (or punching shear) is quite low. Shear key or shear dowels can be provided where roughness is not enough for the required shear resistance. Dowels can be of steel or other suitable materials, and need not be smooth at construction joint. In old hardened concrete post-installed anchors can be provided to act as dowels. Normally construction joints should not be made at critical sections, and if proposed at a critical section detailed checks would be needed. At construction joint, concrete section should be checked under direct shear for shear friction resistance.

Construction joint at bottom end of wall, is most critical location where section is subjected to high moment, high shear and susceptible to high crackwidth, hence at this joint adequate design is to be worked out. This joint must be grouted and sealed on the liquid face.

Construction joints can be provided at a particular location, only if designer specify it on drawing. Designers should check (during process of design) the adequacy of the strength, stiffness and durability at the joint.

Construction joints definitely introduce weakness (in strength and stiffness) in the structure as is known to influence the performance of TG foundations. There is experience of significant reduction of stiffness (due to poor modulus of elasticity of concrete in the region of joint) of staging for elevated liquid tanks; and after thoroughly grouting all the construction joints in columns, the performance (natural frequency and amplitude) improved to desirable level. The data on influence of construction joint on strength capacity and stiffness is not available. This does not absolve the designer from the responsibility of dealing with the weakness introduced by the joint. In past, research has not always preceded the engineering wisdom, and later it improved the subject from understanding to knowledge.

The account of construction joint (and stiffness reduction due to that) is normally not taken in the structural analysis. Hence during construction the full structural rigidity and continuity should be realized by providing enough roughness, good quality concrete (low porosity and low water-binder ratio) and rectification by grouting. Depending upon its type and expected performance, the designer should check the strength and possible crackwidth at the construction joints and the structural integrity.

Strength Reduction : Shear strength (as diagonal tension) reduction may be from 33 to 50% related inversely to the roughness at the joint interface, detailing and workmanship at the joint, say for design 33% reduction for normal work; for direct shear reduction 40 to 70%, say for design 50% for normal work. Flexural strength reduction may be assumed as 15 to 33% in moment capacity, say for design 33% reduction for normal work. Hence at all specified positions of construction joint, shear strength must be checked. Reduction of tensile strengths (both direct & flexural) are substantial across the joint, thus the estimated crackwidth may be increase say 1.2 to 1.5 times. Hence if a joint is proposed, the possible weakness should be accounted in design.

In concrete members construction joint coinciding with section where shear is high, should be avoided, and if provided should be checked by the designer for adequate resistance to shear stress.

At a section where significant reinforcement is curtailed, the shear strength of the section reduces, as well the possible crackwidth is higher. Hence in general, bars should not be curtailed near to a construction joint, and bars be extended to cross the joint by about 12× bar diameter or half of effective depth, whichever is higher. Loss of strength (due to water-bar) in the region of joint should be accounted in design if provided with water-bars, and detailing should be prepared such that it does not interfere with the reinforcement. Construction joint should have offset of minimum two times the width of beam, from beam junction.

Before the placement of second phase concrete at the joint, inspecting engineer’s approval for preparation of joint interface shall be obtained.

In wrapped prestressed structures, horizontal construction joints shall not be permitted in the core wall between the base and the top, without a water-bar or sheet lining.

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The behaviour at this type of joint is supposed to be different than the induced (partial) contraction joint, however expected difference is small only if enough roughness is given to joint. The joint may also need sealing on liquid face, specifically where across the joint interface there is no compressive stress under the load combination, or liquid pressure is acting with high H/t >25, or at the joint of wall bottom (where moment, shear and direct tension are present) if H/t >20.

It is not necessary to incorporate water-bars in properly executed construction joint, unless H/t is more than that in given in R11.2 b) (i) depending upon possible workmanship.

R 11.2 b) (i) Limiting H/t for construction jointWorkmanship H/t Grouting needOrdinary 20 Occasionally one cycleAverage 25 1 or 2 cyclesGood 30 2 or 3 cyclesExcellent 35 3 or more may be needed

Note : Above limits will reduced if equivalent thickness (te) is considered in place of t. Where bending moment, shear force and direct tension are present, the above H/t limiting values can be reduced by 5, (in absence of reduced equivalent thickness for accounting tension). Example of such joint is at wall bottom with floor slab of elevated tank. At important joints (having H/t >30) having high stress (example wall bottom), the tensile stress in steel should not be >0.42fy in limit state of serviceability. [See also Part 2, R 6.4.2 & R 8.3.1.]

It should be noted that for keeping H/t >20, the limiting crackwidth shall reduce requiring high amount of steel provision and sealing of joint at liquid face. For a particular H/t value, stress (compression or tension) on the construction joint affects the seepage through joint (progressively increasing).

1. Whole section in compression, no tension.2. Tension on one face & compression block ≥ 50 mm.3. Tension on one face & compression block < 50 mm.4. Tension on whole section (i.e. on both faces).

For better understanding the performance affected by H/t, this ratio can be modified for framing better criteria. Leakage through crack is also related to width of crack. Stress (compression or tensile) perpendicular to the construction joint (or a crack) has an influence of leakage. Part or full section (interface) may be subjected to compression, in presence of which the leakage will reduce. Hence an equivalent thickness can be accounted which is enhanced for part of section in compression.

If N is the size of compression block (depth of neutral axis), tension part = t – N ,Equivalent thickness = te = t + N /4 = t + 0.25 N {constant 4 can reduce up to 2}.

The limiting hydraulic gradient (H/t) through monolithic member (away from any joint) can be much higher (say 3 times) where there is no structural crack.

Above the limiting H/t, water-bar is required to reduce the leakage through construction joints. It is preferable to provide higher thickness, compared to providing water-bar. Refer 11.5.1 for details at construction joint.

R 11.2 c) Temporary Open Joint (Gap Joint) :Along a member like wall or slab, a temporary gap can be kept during construction, which will be filled in by

structural concrete lately, and before the scheduled time for putting the structure in service. The width of gap could be of the order of 0.8 to 1 m. This gap facilitates to lap the reinforcement extending in to the joint from two sides. Usually there is no advantage of keeping it much wider or narrower gap. This temporary gap accommodates the contraction of adjoining concrete lengths on each side, due to temperature effect in young concrete and also partially shrinkage taking place till the time of filling the joint. Through this gap the reinforcement can act as continuous by lapping. If the length of concrete member on both side of gap is more than 14 m, it may be necessary to provide laps for all bars within the gap, to reduce the restrain on contraction of concrete. As all bars are lapped within this small region, lap length should be suitably enhanced, and distribution bars (perpendicular to lap) should about 1.3 times the minimum reinforcement. To allow the contraction of adjoining concrete to the maximum possible extent, the gap can be filled in as late as possible, and after few days of drying of the concrete on each side. The interfaces at the gap should also be treated as a set of two partial contraction joints.

R 11.3 DESIGN AND DETAILING OF JOINTS : This is with reference to movement joints for ground supported tanks.

R 11.4 SPACING OF MOVEMENT JOINTS :The movement joints are provided to divide a structure in parts, allowing movement of concrete panel in each part

independently, by substantially reducing the friction between the concerned RCC panel and its sub-base. For permitting expansion or contraction of panel between movement joints, the top of PCC sub-base must be perfectly in a plane, flat and smooth. Flatness and smoothness of the surface must be specified, controlled and achieved in the construction.

For ground tanks, the spacing of joints should form a pattern, taking in to account the positions of columns supporting the roof. Preferably column should be at centre of a panel, i.e. equidistant from movement joint. Load of column can mobilise enough friction preventing horizontal displacement of slab at column location.

Even if the surface which may appears to be smooth, has very small roughness (seen under a magnifying lens) of sub-base top, the concrete above will set in to ups and down of roughness and friction will develop, thus resistance will be offered to free movement, and the purpose of providing movement joint may be partially defeated. Hence a separation (or bond breaking) sheet is put in between, which will facilitate sliding. The thickness of the separation sheet should allow friction-less in-plane shear deformation i.e. relative displacement between bottom and top layers. Normally 1 mm thick polyethylene (LDPE) sheet is specified with smooth top sub-base. Thickness of sheet is directly

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related to the roughness (on finer scale) of the top surface of sub-base. It should be noted that projections of structural concrete below the plane interface shall lock the movement, and

induce unwanted cracks. The sub-base PCC below the RCC floor slab should have specification such that it can be finished smooth. For good finishing-ability of base PCC it should have enough paste and finer aggregates.

The options for movement joint are for the cases where restrain to expansion and contraction movements are present, which is mainly in ground supported tanks. Following are the design options. (a) Option 1- Within the area of continuity, intermediate movement joints are not planned. Design assumes restraint.

Cracking behaviour is controlled by provision of enough reinforcement (normally high amount) which has smaller spacing with minimum possible size of bar. Construction joints will induce crack pattern, and are to be sealed. This option is preferred for tank up to 22 m size. For higher size tanks reinforcement has to be designed more cautiously.

(b) Option 2- Movement joints are introduced, therefore the amount of strain to be controlled by reinforcement reduces, and thus the requirement of reinforcement can reduce. Crack pattern is induced by spacing of movement joints.

(c) Option 3- Cracking can be controlled by close interval of movement joint, thus significantly reducing the requirement of reinforcement. Cracks in between movement joints if develop will be significantly small. Total freedom for movement is very difficult to be achieved and hence some reinforcement should also be provided.

In Table 2 Column 4, option 1, the steel ratio can be significantly higher say >1.4 times ρcrit . Calculation for ρcrit will be as per Annex A Part 2.

R 11.3.1 The steel ratio ρcrit as calculated (for 415 grade) will be as below. This refers to ground tanks only.fck M20 M25 M30 M35 M40 M45 M50 M55 M60

fct(min) 1.00 1.15 1.30 1.45 1.60 1.70 1.80 1.85 1.90ρcrit 0.18 % 0.21 % 0.23 % 0.26 % 0.29 % 0.31 % 0.33 % 0.34 % 0.34 %

Option 1 0.25 % 0.29 % 0.33 % 0.37 % 0.40 % 0.43 % 0.46 % 0.47 % 0.48 %Option 2 0.18 % 0.21 % 0.23 % 0.26 % 0.29 % 0.31 % 0.33 % 0.34 % 0.34 %Option 3 0.12 % 0.14 % 0.16 % 0.17 % 0.19 % 0.20 % 0.22 % 0.23 % 0.23 %

For elevated tank, refer steel % as for option 2 in above table. (This table in not numerically part of code, but will help in understanding the effect of concrete grade.)

Notes : Even if full contraction joints are provided in option 2 or 3, certain situations demand higher temperature-shrinkage reinforcement; e.g. at lightly loaded columns (supporting roof), high vertical load loads prevent free movement of floor between two such columns, & needs more reinforcement to resist cracking.

R 11.4.1 Horizontal cracks in free standing wall.

R 11.4.2 Vertical restrain : Walls are considered to have reduced restraint in the vertical direction because their own weight helps to reduce shrinkage and temperature stresses. Hence for vertical steel (except in bottom portion) the requirement can be reduced significantly say by 33% (similar to option 3 irrespective of height of wall). Such reduction shall not be available, where parts of structure of different heights are connected monolithically. Similarly spherical dome can function without cracks with reduced % of steel.

R 11.5 MAKING OF JOINTS (i.e. Construction aspects)

R 11.5.1 Construction Joints (Also refer R 11.2 b.)At construction joints measures are taken to achieve subsequent continuity without provision for further relative

movement at the joint. Shear displacement is prevent by imparting enough roughness to the interface. Similarly enough of steel be provided across the joint to prevent the opening of joint (as crack opening or crackwidth). Measures are to be taken to minimise the weakness getting introduced at the joint affecting the strength and leakage.

Concreting operation should be carried out continuously up to the pre-planned construction joint. During construction, a joint at a specified location can be avoided. However, to introduce a joint or to shift it to a new location, approval of the designer is necessary. At the proposed location, the designer should check the strength.

In each phase fully compact the concrete, to remove mega and macro pores, reduce porosity without segregation, and get minimum possible permeability through the joint.

At the joint, two concreting phases are with a time gap. If the time gap is less than 80% of the initial setting time of first phase concrete (or 30 minutes before its setting, whichever is late), the concrete can become monolithic, and cold joint is not formed. Concrete in each phase should be fully compacted without segregation, avoiding loose aggregates, to give minimum permeability of the joint.

At horizontal joints, while ensuring proper compaction of first phase concrete, there is a possibility that due to a bit over vibration, top few millimetre thickness may have coarse aggregate sunk and only mortar remains in top portion. In top few millimetre (say up to 5mm) thickness, water-binder ratio may also be higher (i.e. low strength & stiffness), also is upcoming bleed water, excess mortar and absence of enough aggregates, may be presence honeycomb sometimes etc., and this weak layer shall be removed. At an earliest, surface laitance, mortar layer, small mortar cover over aggregates, portion having bubbles, portions of un-compacted concrete if any, any loose material / aggregate, and aggregates having cavities around them should be removed, exposing coarse aggregates projecting out from matrix.

For an old concrete surface, layer of concrete carbonated, or with high ingress of aggressive agents like chlorides, etc. shall be removed. Also consider the conditions for laying fresh concrete, such as ambient temperature, humidity, wind force, precipitation, and any temporary protection needed. Concrete surface preparation deals with the various operations needed to fulfil the requirements.

In addition to removing the coating if any, remove the unsound concrete with reduced mechanical integrity and/or

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contaminations, portion got affected by spalling, delamination, disintegration, with severely cracked due to corrosion of reinforcement etc., and also a bit of sound concrete can be removed as a precaution for doing a reliable job, for reaching adequately solid composition and rough geometry at the surface.

At the time of roughening if concrete is 2 to 8 hours old, its surface can be roughened by scarification and wire brushing without dislodging or disturbing the coarse aggregates. At higher age it can be done by abrading techniques using sand blasting, shot blasting, hydro-blasting, chiselling, milling tool, small pneumatic hammers, small bush hammer, jackhammering, or by any other established method.

Among the various methods, only breakers (chipping hammers and jackhammers) and high-pressure water jets are the options to remove hard concrete of significant depth. Other methods are intended to remove the skin concrete and/or for texturize the surface.

Damage by the impacting tools such as chipping hammers should be avoided, which may otherwise completely outweigh the benefits of an increased roughness. With equipment use, choose proper tool and precaution be taken. Use of excessive energy, causing dislodging or fracturing of aggregates, damage to concrete by or inducing cracks shall be avoided. Effectiveness of the concrete removal techniques may differ depending upon hardness of concrete or its mortar layer, also for portions unsound and sound concrete, and a combination of techniques may be necessary.

Small breakers (5.5 to 9 kg) can be used for partial removal of unsound concrete or concrete around reinforcing steel because those can cause very little damage to surrounding concrete and the damage can be avoided. Take care while selecting the size of breakers to minimize the damage to concrete and its bond to embedded reinforcing steel. Most concrete removal work is done with a pointed tool, although a relatively narrow (25mm) blade-type tool is sometimes used to remove cracked and deteriorated concrete.

Bruising can be minimized by exercising care in the removal process and, where possible, by avoiding the use of more detrimental techniques using high energy tools. Only hand held, or light tools should be used.

System employing a small jet of water at high velocities (pressures 70 to 310 MPa), can be used as a primary technique for concrete removal and cleaning of steel, and minimize damage to the concrete remaining in place, and leaving a rough profile. Care be taken not to punch through thin concrete. Trial will be needed to choose appropriate water-jetting speed, pressure, and number of overlapping passes, depending upon hardness of surface.

The effectiveness of various concrete removal techniques may differ for unsound and sound concrete, and a combination of techniques may be necessary. However, the methods used to remove the deteriorated or contaminated concrete and prepare the reinforcement, must not weaken the surrounding sound concrete and reinforcement.

For formed surface (cast against shuttering) at vertical joint, side form should be removed as soon as possible, and the joint surface should be roughened. Concrete must be removed if it is affected by spalling, delamination, disintegration, or portion unsound or if it is in an area with severe cracking due to active corrosion of reinforcing steel. In addition to unsound concrete with reduced mechanical integrity and/or contamination, a bit sound concrete can also be removed as needed to provide adequate solid surface, geometry and roughness.

Roughness could be obtained by applying a retarder to the concrete surface immediately after compacting the concrete. For vertical surface, apply retarder to the formwork. After few hours surface is washed to get roughness. However use of retarder is not preferred now-a-days.

The roughness is an important aspect of the interface. The roughness is be defined as half the average amplitude i.e. height between peaks and valleys; and 3 to 5 mm (for concrete of 20 mm MSA) can be taken as sufficient. This roughness locks the relative movement between the two sides of the joint. Amplitude of roughness can be measured by sand patch method. However for routine jobs, visual observation is enough and experimental determination may not be done. Average roughness more than one fourth of the maximum size of aggregate in concrete, is not normally proper.

Preparation of surface of concrete, is the process by which sound, clean, and suitably roughened surface is reached in the area to receive further fresh concrete. After removing material and roughening, the surface should be washed clean, preferably by water jet. It is essential to make sure that the old concrete surface is clean, free of contaminants, dust, loose material or a bruised concrete.

The mechanical integrity of the existing concrete is important. Adjacent to the interface, the quality of concrete should be good. If from existing substrate (old concrete) damaged or weak portion is removed, tensile bond of overlay can develop. The roughening and surface preparation is basically to get interlock to develop bond, and avoid shear slip along the interface, which will result in subsequent behaviour of concrete approaching towards monolithic. Bond strength of joint depends on many parameters. For adequately bonding concretes on the two sides of joints, the coarseness of interface increases the shear-friction. The prepared surface should be covered with new concrete within 24 hours usually.

In the first phase concrete, shear key or shear dowels can be provided if specified in the drawing. If the time gap at joint is very high, a bonding material can be used at the interface. Refer R11.2 b, 7 th para onwards. If specified or if existing concrete is >3 to 7 days old, a chemical bonding agent is applied as per the instructions of the manufacturer. The efficacy of bond can be tested by pull out test, determining the tensile strength of the interface.

Before placing fresh concrete, the old concrete should be cleaned and wetted, without leaving free water at the surface of joint i.e. it should be ‘saturated and surface dry’ (SSD) while placing new concreting. Any free water at the surface, should be removed by suction, air blow, or evaporation etc. SSD means that immediately under the surface portion (below 3 to 10 mm) the pores are damp (70 to 90% RH), and surface zone having saturation 50 to 70% RH. These values of RH (relative humidity) are not sacrosanct. The aim is, not to allow old concrete to suck significant amount of water from new concrete, whereas unsaturated pores of the dry surface zone (say up to about 3 mm depth) allows slight penetration of

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cement slurry in to the old concrete, which is beneficial for bond. This condition is typically achieved in practice by soaking the substrate (old concrete) may be a day in advance, and then allowing the surface to dry out or air blown few hours before new concrete placement. If epoxy or polymer-modified bonding materials are used, such wetting of existing concrete may not be required.

The quality of the concrete surface preparation cannot be neglected, based upon the false assumption that a poor surface quality can be compensated for by using a bonding agent. Use of bonding agent prior to the placement of overlay is to enhance the bond between the latter and the existing substrate profile.

A practice was to use thin cement slurry (say water:cement as 3:1) to wet the surface of old concrete, which does not serve any specific purpose for impermeable concrete in LRC. If the slurry is very thin which does not form a layer of any appreciable thickness, it is not harmful, provided the water from the slurry is absorbed by the old concrete. Thick (more viscous) slurry will form a paste layer on the surface with high water cement ratio, and such a layer or a mortar layer is undesirable. Any mortar if applied at the joint, should have very low w:c ratio (<0.4) for enough strength and small permeability. Only if old concrete is porous (<M25) it can absorb the slurry. If specified by designer or competent engineer, or if time gap between the two phases is more than 3 to 7 days (depending up on cement type and importance of joint), a bonding agent should be applied to the old concrete just before pouring fresh concrete.

With the technology available today, cement based overlay having the rheological characteristics to properly wet the existing concrete substrate can easily be designed, eliminating the need of a bonding agent. Furthermore, a bonding agent may act as a bond breaker when used inappropriately.

Application of polymeric or epoxy based bonding agent should be as per the guidelines of manufacturer. Bonding agent should be compatible with the wet concrete, and should have tested or demonstrated capacity to improve bond. Moreover, application of a bonding agent requires a meticulous management of time. It is indeed necessary to place new concrete on the applied bonding agent while it is fresh and sticky, and before it can set. If the bonding chemical sets, it will act as de-bonding agent between two concretes.

Concrete of second phase as placed should be fully compacted against old concrete, without leaving any air pocket, and without causing segregation.

After the concrete of second phase is hardened, grouting of the construction joint is a good practice. Vertical construction joints in wall must be grouted with cement slurry, after a time gap as late as possible allowing part shrinkage to take place. For any grouting, cement can be mixed with fine flyash or ultra-fine GGBS. Horizontal joints could also be grouted and more specifically where H/t is high (>20) or workmanship at joint is poor.

If the pressure gradient (H/t) can be <30, and tension across the joint is be very small or in some portion (≥50mm) compression is be present, it is not be necessary to provide water-bar in properly worked and treated rough construction joint. It need not be provided unless specified by designer in the drawing. If necessary, interface can be grouted to get a leakage free joint. The joints should be sealed on water face, specifically if H/t is >20.

11.5.1.1 It is very difficult to get proper compaction of concrete around water-bar, and workmanship is usually poor. Water-bar further introduces weakness at the joint, flexural and shear strength across the joint would reduce, even if concrete around the water-bar is properly compacted. It is strongly recommended that as far as possible, use of water-bar should be avoided. Without water-bar, well compacted concrete (& no honey-comb) should be obtained at the joint. If concrete at a joint is found to be porous or leaky, the joint should be grouted to make it leak-proof.

Importance of workmanship at the joint increases as H/t increases from 20 to 35. With excellent workmanship and grouting in 2 to 3 cycles for H/t up to 35, use of water-bar can be avoided.

For proper workmanship with water-bar at middle, the member thickness more than 300 mm is needed. Hence first option should be, to increase member thickness at the joint rather inserting a water-bar. Better sealing options at surface are also available and may be preferred over water-bar. If H/t is high (say >30 for good workmanship) water-bar will be necessary, and sufficient thickness of member should be provided to achieve proper workmanship at the position of water-bar. If water-bar and shear-key both are required, for proper workmanship, the thickness should be >400 mm.

If tension across the joint is high, sealing at the surface on liquid side, is also required.Provision of key is also associated with problems of workmanship. Provision of groove or shear key at construction

joint is not required unless such shear key is designed and specified in drawings. As additional operations required for formation of groove or key, workmanship at the interface can remain significantly poor (incomplete compaction, high porosity, local cracking in immature concrete etc.). Good job can be done with rough joint, without key, and without water-bar.

Normally kicker (starter) can be avoided at the base of wall having water-bar. A construction joint between the base of a wall and the slab below (or foundation) may indicate the use of a kicker of wall to avoid interference between the water-bar and top reinforcement in the slab or foundation. For positioning of a water-bar an upturned keyway is not recommended because of the potential of a crack forming through the keyway width or emanating from the top edge of the water-bar (ACI 350 commentary). However better method is avoid use of water-bar, and reduce H/t at the joint by increasing the width of wall with provision of a generous fillet. However, fillet provision will increase the complication in construction, formwork assembly and joints in formwork.

R 11.5.1.2 Height of free fall of concrete on hard surface and segregation :There is no limit on the height of formwork, and an indirect limit is the consideration of increase in the cost of

formwork. As the height increases the surface area, as well the design pressure on the formwork will also increases, thus requiring more cost. Technically the formwork should be as high as possible, to reduce number of construction

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joints to minimum.At a horizontal joint in wall, placing of the second phase concrete involves free fall, which is limited by method of

working and precautions for avoiding problems. During free fall, concrete is susceptible to segregation. Free fall height is excluding descend through chute or tremie pipe. From initial pour some mortar portion may get attached to steel cage and formwork. Larger stones try to move away from each other, resulting in the segregation; which can increase with the height of free fall. While the fresh concrete falls on the hard surface, the coarse aggregates rebound and collect near the surface of formwork, thus increasing segregation. Higher the free fall of concrete, more will be the rebound and more segregation. Thus just above the joint, honey-comb is formed as seen in the cover region of concrete. After a padding layer of fresh plastic concrete or micro concrete is deposited, the aggregate from the falling concrete gets embedded in the padding concrete and further segregation is not resulted. Hence at horizontal construction joint hone-comb is seen only for few cm height if padding layer is not given. The height of likely honey-comb depends up on the height of freefall of concrete.

The concrete placing for initial height should be done by chute /pipe without significant free-fall. This requires sufficient space between the reinforcement mesh for insertion of chute or pipe.

For members having insufficient space for insertion of chute/pipe, the alternate method can be as follows. For the first pour of concrete to be placed over hard surface at a joint, maximum size of aggregate (MSA) should be small related to the freefall height. If the height of freefall is about a metre the MSA can be 6 mm, and for 300 mm freefall it can be 12 mm. A concrete mix highly cohesive and designed with smaller MSA is needed, which should be batched and mixed. However, if the quantity the padding concrete needed is small, one can remove larger size aggregate from the concrete supply, and thus modified mix can be used. Bigger size aggregates can be removed by sieving or hand picking.

Sealing at the surface in contact with liquid is required, if H/t is >20, water-bar is not provided and tensile stress across the joint is high.

Similar to construction joint, condition may arise for load transfer across concrete to concrete interface where concrete is cast at different time, or for cases of precast elements etc., which may require shear key.

R 11.5.2 Movement Joint Joints shall be constructed in such a way, to prevent leakage, spalling, reinforcement corrosion, and to reduce

cracking due to restrained shrinkage and temperature strains, as applicable. The number, spacing, and details of joints should be taking account of the physical properties and ability of the filler, sealant, and water-bar to withstand cyclic deformations in service life.

Based on the design of joint, proper specification and material properties should be selected for joint materials e.g. filler, sealing compound etc. These should have good life, and compatible with the liquid retained. Movement joint will have water-bar. For each type of material, follow the manufacturer’s advice, instructions and the notes of designer.

Geometry of joint details, with the position of the reinforcement, the water-bar, and the dowel should be given in drawing. Consideration should be given to the clear distances between the water-bar, reinforcement and formwork, in view of maximum aggregate size to allow proper placing and compacting concrete.

R 11.5.2.1 Contraction Joint At full contraction joint interface, adhesion between the surfaces is reduced by applying a coating or placing a film /

sheet before pouring new concrete. However, full compaction of concrete on both sides of interface should be assured. For making partial contraction joint, placing a groove former or cutting a notch are common methods. Groove

former (a strip) is applied at one or both the faces of the slab, for enough reduction of thickness (by 1/3 rd). However, top reinforcement in slab should not interfere with the depth of groove, and bottom of groove should not reach to reinforcing bar and leave it without cover. In LRC slabs >200 mm thick, at the location of groove the top reinforcement will have to be terminated and additional lapped steel may cross the joint provided at a depth equal to groove plus clear cover on bar. Hence induce contraction joints with continuity of reinforcement, are very difficult to design, detail and construct. [Also acknowledged in ACI 350 commentary,]

Joints must be sealed at surface. It can be provided with water-bar if total contraction expected at joint is higher (say >0.2 mm). For water tightness class 2, water-bar should be provided at all full contraction joint. If specified, full contraction joint shall have dowels as designed.R 11.5.2.2 Expansion Joint : Designer should design and specify initial gap, which will depend upon the expected movement (i.e. shortening of gap) and the compressibility of filler material. Filler should be fixed or adhered to concrete on at least one side. Some types of fillers are force-fit i.e. fixed in compressed form and remain in position due to friction, and these also resist the permeation of water. Water-bars are needed at all expansion joints, and dowels should be provided if specified as designed.R 11.5.2.3 Sliding Joint : For reducing the friction between two surfaces, the surfaces should be flat and smooth. Coefficient of friction should be reduced by application of suitable coating or provision of one or two layers of HDPE (high density polyethylene) sheets.

R 11.5.3 Temporary Open Joint : [ Gap joint ]

R 11.5.4 Joints in Ground Slab : The slab may have been divided in panels by full and partial contraction joints as per design and drawing. Between two full contraction (or expansion) joint, one partial contraction joint can be provided i.e. alternate joint can be partial contraction joint. The options of type of joints are not to be exercised during construction, but are frozen by design consideration, and in field one should follow the drawings and instructions of designer. For induced contraction joint, the work of cutting or sawing of the joints should be completed before the final setting time of floor concrete, as well before the temperature peak can occur due to heat of hydration. The joint cutting can start at 5

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to 8 hours age and should be completed at about 10 to 16 hours of age approximately.

R 11.5.5 Joints in Wall : A kicker at bottom is not necessary. It has two main functions at bottom end of formwork- to avoid movements of form at the bottom end, and to reduce the possible leakage of slurry from fresh concrete during compaction. The negative aspect of kicker is to add one more step in the construction process and also add one more construction joint. Also, the quality of concrete in small quantity for kicker is normally below average, and thus construction is poor. Hence without kicker, one should get good concrete by making arrangements for fixing bottom of formwork in proper position and avoid leakage of slurry. If kicker is provided, concrete to be poured in it can be one grade (5 MPa) higher than the specified grade. Top of kicker should be matched with the central bulb of water-bar, if provided. Movement joints if provided should coincide with the joints the alignments of joints in floor. Joints should be sealed on one or both sides where water could be present.

When water-bar is at the bottom of a lift of wall to be concreted, within the formwork it is very difficult to pour concrete, vibrate it and avoid segregation in the bottom portion, simultaneously avoiding displacement or bending of top wing of water-bar. It will involve lot of arrangements to be made for keeping water-bar in position while placing, vibrating and achieving a good quality concrete, and all this will require sufficient space.

R 11.5.6 Joints in Roofs : Joints in roof can be aligned along the vertical joints if provided in walls, or else it may be constructed monolithic without joints. For ground tanks, roof may experience more temperature variations than walls. In large length roof should be designed for temperature variation and construction should be done such as to justify the assumptions made in design.

R 12 JOINTING MATERIALS Jointing materials normally used are following types :

a) Joint fillers,b) Water-bars, andc) Joint sealing compounds (including primers where required).

All Jointing materials, water-bars, fillers and sealants shall not contaminate liquid retained more specifically if water is potable. As well the retained liquid should not have adverse chemically reaction on the water-bars and sealants. Chemical curing compound if used should be checked for its compatibility with drinking water. Bituminous materials should not be used in structures for drinking water. Joint fillers having bituminous material shall not be used with thermosetting, chemical curing sealants such as polyurethane (ACI 350).

R 12.1 JOINT FILLERSJoint fillers are usually compressible sheet or strip materials used as spacers. Sponge filler consisting of closed-cell

neoprene or rubber can be used. These can be performed. They are fixed to the face of the concrete placed earlier, and against it the concrete of second phase is cast. Fillers are detailed to remain in position, accommodate maximum joint movement and prevent restrain. Some types of fillers are pre-compressed at initial installation. Fillers can be performed. To provide a desired geometry to sealant, Backer rod consisting of compressible closed-cell polyethylene sponge or other suitable material can be provided to support the sealant.

Joint fillers, may themselves function as watertight at expansion joints. These give support to joint sealing compound when applied in floor and roof joints. But these should only be relied upon as spacers to provide the gap in an expansion joint, the gap being bridged by a water-bar and sealant (see Fig. 6).

R 12.2 WATER-BARS (Water-stop)Water-bars are preformed strips of impermeable material which are wholly or partially embedded in the concrete

during construction so as to span across the joint and provide a permanent watertight seal during the whole range of joint movement. For example, water-bars may be strips with a central longitudinal corrugation (see Fig. 5A and Fig. 6A), Z shaped strips (see Fig. 6B) and a central longitudinal hollow tube (see Fig. 5B and Fig. 6C) with thin walls with stiff wings of 150 mm or more width. These are normally 150 mm to 250 mm wide, for expansion joint higher widths are required.

Material of water-bar shall have proven or demonstrated acceptable performance under conditions of its intended use. The material used for the water-bars are PVC (poly-vinyl chloride), thermoplastic elastomeric rubber (TPE-R), metal sheet, stainless steel etc. Galvanized iron sheets may also be used with the specific permission of the engineer-in-charge provided the liquids stored or the atmosphere around the liquid retaining structure is not excessively corrosive, like sewage. GI (or other metal) sheets preferably be coated by an epoxy or polymer for better durability. Stainless steel is used where exposure to ozone is possible. While selecting the materials for water-bar, the possible corrosion aspects may be kept in mind. Chemical resistance, joint movement capacity, and design temperature range are among the criteria that should be investigated when selecting water-bars. Joint movement capacity and design temperature range are among the items that should be investigated when selecting water-bars. PVC water-bars are more common in use and have longer life, and rubber is also used. With use of water-bar, sufficient thickness of concrete members should be provided so as to ensure proper placing and compaction of concrete adjacent to water-bar in order to achieve adequate structural strength.

Water-bar should not interfere with re-bars, dowels, and in between space for concrete, all of which lead to enough thickness of the member needed. It is important to ensure proper placement and compaction of the concrete around water-bar without air pockets. The bar should have shape and width, such that the water path through the concrete around the bar should offer resistance to flow and should not be unduly short.

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The holes, sometimes provided on the wings of water-bars to tie it in position or to increase bond, shorten the water path and may be disadvantageous. These holes should be filled in or treated. The water-bar should either be placed centrally in the thickness of the wall or its distance from either face of the wall should not be less than half the width of the bar. The specified concrete cover to all reinforcement should be maintained. There should be enough space between water bar and reinforcement for concrete to be placed.

The strip water-bars at present available in the newer materials need to be passed through the end shutter of the first-placed concrete. It can be appreciated, however, that the use of newer materials makes possible a variety of shapes or sections. Some of these designs, for example, those with several projections (see Fig. 6D), would not need to be passed through the end shutter and by occupying a bigger proportion of the thickness of the joint would also lengthen the shortest alternative water path through the concrete.

R 12.2.1 Small swellable strip (say 20×25 mm) or a bulb (without wings) hydrophilic water-bar can be placed at middle of thickness of concrete, which seals the space by swelling, thus reduce the leakage through a construction or contraction joints. When comes in contact with water it can swells over to 200% (in unconfined state) and seal the passage of water. Due to easy methodology of application and property to seal the joints completely, these are effective to make joint watertight. Such water-bar does not have flaps which otherwise hinder the proper workmanship. These have re-swelling capacity even where cyclic wetting & drying may take place. The swelling pressure is small & not more than 1 MPa. Swelling is slow, hence there is sufficient time for concrete weight to act on it. Small hydrophilic water-bar does not significantly reduce section strength as well does not require more width of concrete, member thickness can be 200 mm or more. These are not useful where moist condition may not be maintained. Design of reinforcement crossing the joint should take in to account the extra tension imposed on the joint due to swelling of the hydrophilic water-bar.

Injectable hose water-stops can also be used as per the manufacturer’s recommendations.

R 12.3 JOINT SEALANTS Sealants are impermeable ductile materials, which provide a watertight seal by adhesion to the concrete throughout

the range of joint movements. At a joint pressures, temperatures, movements, and chemical resistance required will govern the selection of sealants material. Surface preparation of concrete is important for bonding of sealant. Sealant will be exposed to thermal or moisture induced cyclic movements. During service life, joint movement imposes cyclic mechanical strain on the seal which, depending on the exposure conditions and the design. For choice of sealant consider that, it may be subject to environmental degradation, leading to seal failure.

RILEM TC 190-SBJ recommendation are for weathering test of sealant. Some test are also specified in ASTM C1589, ASTM D1435, ASTM G7 and ISO 877-2. There are differences in test procedures as per these standards. Also similar test for adhesion of sealant is required, which can be as per recommendation of RILEM technical committee. ISO 6927 gives vocabulary/ definitions about sealants.

The shape factor (maximum strain) of the sealant and its bond with concrete are important for design of joint. Sealants are filled in the small groove at the joint near surface. It should be bonded to the concrete on each side of the joint. At the bottom of groove, the sealant should be free to deform and kept un-bonded. A bond breaking tape is normally applied between filler and the sealant. Where depth of sealant needed is smaller than the depth of grove created, a Backer rod is placed to support the sealant. Between a filler (in expansion joint) or Backer rod, and sealant a bond breaking tape is applied usually. Backer rods are provided to support sealant and to give proper geometry to sealant. Backer rods are compressible closed-cell polyethylene or other suitable closed cell material compatible with environment. For sealant application, joint preparation should be done as per the manufacturer’s recommendations.

Polymer bases sealants like polyurethane (PU) or polysulfide (IS 11433 Part 1 & IS 12118 Part 1) are popular which may be single component or two component. Silicone sealant has longer life compared to others. Sealants should be non-sagging. Mostly Polyurethane sealants are better and preferred, especially where the contact is with waste-water. During curing of polysulfide sealant, moisture has to be excluded for satisfactory adhesion. Low modulus, moisture-insensitive, epoxy sealants can be used were relative movements are very small. Epoxy has good bond with steel also. Sealants do not have a life span matching to the life of structure, and during life resealing is required at proper interval, depending upon type of sealant and severity of exposure.

Other materials are based on asphalt, bitumen, or coal tar pitch with or without fillers (refer IS 1834), such as limestone or slate dust, asbestos fibre, chopped hemp, rubber or other suitable material. After construction, or just before the reservoir is put into service, these are applied by pouring in the hot or cold state, by trowelling or gunning or as preformed strips ironed into position. These may also be applied during construction such as by packing round the corrugation of a water-bar. A primer is often used to assist adhesion and some local drying of the concrete surface with the help of a blow lamp is advisable. The length of the shortest water path through the concrete should be extended by suitably painting the surface at the concrete on either side of the joint.

The main difficulties experienced with this class of material are in obtaining permanent adhesion to the concrete during movement of the joint whilst at the same time ensuring that the material does not slump or is not extruded from the joint. To avoid frequent renewals, sealant should be chosen to perform over long period. However best of sealants have limited life compared to life of concrete structure. Hence joint should be designed such the repairs and maintenance can be undertaken at specified interval. The geometry of sealant as applied in position is important for design and performance.

In floor joints, the sealing compound is usually applied in a chase formed in the surface of the concrete along the

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line of the joint (see Fig. 7A). Preparation of concrete surface to receive cleaning chemical, primer and sealant are to be worked out. To enhance the bond between concrete and sealant, primer is used. The actual minimum width will depend on the known characteristics of the material. In the case of an expansion joint, the lower part of the joint is occupied by a joint filler (see Fig. 7F). This type of joint is generally quite successful since retention of the material is assisted by gravity and, in many cases, sealing can be delayed unit just before the reservoir is put into service so that the amount of joint opening subsequently to be accommodated is quite small. The chase should not be too narrow and too deep to hinder complete filling and the length of the shortest water-path through the concrete on either side of the joint. Here, again a wider joint demands a smaller percentage distortion in the material.

An arrangement incorporating a cover slab, similar to that shown in Fig. 7G, may be advantageous in reducing dependence on the adhesion of the sealing compound in direct tension. During concreting operation cement paste should not be able to enter into the sleeve or its caped end.

R 12.4 DOWELS : To restrict shear displacement across a joints, dowels are required. Normally these are of steel having plane smooth surface with saw cut ends (having no burrs) and epoxy coated for longer durability. Bar should be cylindrical i.e. same section continuing for length, to allow the un-bonded length of bar to slip in the concrete. Cutting of bar in shear is not acceptable, as the ends portion will get distorted. These may be of galvanised steel, stainless steel (reinforcement grade) or steel bar encased in stainless steel tube, to provide high resistance to corrosion. For epoxy coated bars at expansion joints, the coating thickness should not be less than 250 micron (0.25 mm). Proprietary systems such as plate dowels can also be used if approved by designer. On one of the side of joint the dowel bar is provided with a tight fitting plastic sleeve in which bar can slide axially without rotational play. Arrangement like greasing should be done to minimise friction to axial sliding action. For initial axial slip of 0.25 mm, the friction force should be between 0.2 to 3 kN (about 0.02 to 0.3 kg force). The sleeve should not be loose on the dowel and it should be able to accommodate a movement > 5 mm.

During concrete laying and compaction, at a joint alignment of all dowel bars should be kept accurately perpendicular to the joint and all bars parallel to each other, to avoid locking of the movement.

R 12.5 BEARING PADS : Provided in sliding joints. These should be attached to the hardened concrete base to prevent uplift. Anchoring is not commonly done, unless designed to avoid restraint. Space or interface between bearing pads should be filled by material compatible for deformation and water tightness (say sealant).

R 13 CONSTRUCTION R 13.1 Provisions of IS 456 are applicable with modifications as per additions given in the code IS 3370. For prestressed concrete work provisions of IS 1343 are also applicable.

During construction and the initial life before liquid filling, LRC members shall not be allowed to dry below 50% relative humidity to prevent shrinkage due to moisture loss.

Wearing apron shall be provided to prevent abrasion of structural concrete due to fall of water from a height of >3 m. Or the concrete member should be provided with extra thickness of concrete cover which may wear-out. Such extra thickness should be protected from cracking excessively by provision of structural fibers.

R 13.1.1 In general PCC base in foundation (lean concrete / mud mat / blinding layer) can be in M10 /M15, and if the injurious soil or aggressive groundwater is expected ≥ M20 grade. Thickness of PCC should not be less than 75 mm.

For small tanks PCC base concrete may be M10 grade, but should not be porous and should have fairly plain surface. If with M10 concrete proper finishing ability is not possible, M15 could be considered for PCC. Below LRC floor slab, without a separation sheet, the PCC should be minimum M15 grade, and if the injurious soil or aggressive groundwater is expected ≥ M20 grade.

PCC should give a fairly plane, smooth and hard surface to lay further watertight concrete. Enough fines to impart ability to get finished fairly smooth, and the proportion of fine aggregate should be higher than obtained by classical mix proportioning.

The base concrete should not be treated as structural concrete i.e. it may not conform to Table 5 of IS 456. The top surface finish can be U1 class (see appendix at the end of Part 2).

R 13.1.2 A separation sheet between PCC base and RCC floor of a ground tank is required where movement joint are proposed in the floor. The experience of constructing large number of small tanks (< 22 m size) indicates that such a sheet and the movement joints are not required, as per option 1 in Table 2 (see 11.3). For small tanks without movement joint, continuous restrain due to roughness of PCC surface is an advantage, hence separation sheet is not required and PCC should be impervious enough, preventing loss of cement paste from floor concrete above it when laid and compacted. Separation sheet is required where movement joints are planned. Thinner sheet will break the bond, but may not prevent friction due to roughness, thus advantage of providing movement joint will not be harnessed.

If option 2 or 3 is adopted, usually polyethylene (LDPE) sheet is to be provided. Polyethylene sheet of 1 mm thickness will have a weight of 0.92 kg/m². A virgin quality LDPE sheet (grade 232 of IS 2508) has a tensile strength 20 to 26 N/mm², % elongation >200%, and density 0.92 to 0.93 kg/litre. Thickness of sheet cannot be less than the roughness of the top surface of sub-base PCC.

For tanks larger size if considered in design, separation sheet (LDPE) can be provided to allow sliding of RCC floor with respect to base. For this it is necessary to have PCC top plain, in level and smooth for facilitating siding to relieve friction and minimize restraints.

Due to construction requirement, PCC can be laid in two layers if needed for a particular work. First layer is mud mat / lean concrete for filling or levelling purpose and to cover the soft soil which may become slushy when exposed or

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if the excavated ground is uneven. Fill for levelling can be of lean concrete (M5 / M7.5 / M10). Second layer of PCC can be M15 grade, with good ability to be finished as smooth and flat (in a plane & level). One may choose to provide only one grade of PCC to reduce two stage construction operations to only one, if the need for extra thickness is small.

For large ground tanks with separation sheet specified for allowing sliding, PCC top should not have slopes or surface deformations etc., may be to accommodate local thickening of RCC floor slab as structural foundation. At positions where PCC top varies from being flat level surface or has pockets, free sliding will be restrained. Hence movement joint have to be located in the RCC floor at such positions. For top surface class of finish shall be U2 with very low tolerance (i.e. line level ± 2 mm, gradual irregularity ± 1 mm, abrupt irregularity < ± 0.3 mm.)R 13.1.3 At the time of placing, concrete temperature should be between 10 to 40°C, for thicker members (>400mm) preferably ≤30°C. During setting and hardening period, concrete temperature should not exceed 70°C, for which arrangements be made right from cooling of aggregates and water, keeping the temperature of concrete low while mixing, and heat of hydration in first three days be kept low enough. As a rule of thumb temperature of concrete mix can drop 1°C, by cooling aggregates by 2°C, or by cooling water by 4°C, or by cooling cement by 8°C. If aggregates are cooled by 10°C and water by 20°C, the temperature of mix can be lower by 10°C. It is difficult to cool cement, however its silo can be protected from direct sun shine, not allowing its temperature to rise. Similarly all material should be stored in shed, water can be sprayed on aggregates. All operation of batching, conveying and mixing should be done in shed to reduce the effect of sunshine in summer. Refer precautions for hot weather concrete (IS 7861-1). At the core of concrete a peak temperature can develop in first 24 hours of its life in hot season, and in cold environment it can be in 3 days. The high peak temperature should be controlled to avoid excessive cracking and should >70°C. Also during curing period temperature difference (gradient) from core to surface of concrete if more than 20°C, cracks can occur at surface. This temperature gradient should be as less as possible during first 7 days of concrete. With curing, simultaneously surface temperature of concrete should not be allowed to be much cooler than the core temperature.

Thermal modelling can predict the maximum placement temperature to be used. For control on temperature during construction, pre-estimates of likely temperature, and method s of controlled are to be pre-planned. Non insulating formwork or covers, dissipate the heat and reduces its build-up. Concrete surface shall also be protected from temperature shocks.

R 13.1.4 Binding wires for tying reinforcement, or any other item which can corrode, shall not lay in the concrete cover zone or have insufficient cover. After tying the bars, end of binding wire should be bent inside and should not project out in the cover zone. Any item in the clear cover zone should be non-corroding, Reinforcement placing aids like chairs of steel bars shall also have the minimum concrete cover as specified.

Non-corrodible items such as conduits, cover blocks or chairs for reinforcement may be embedded in the cover zone provided- the concrete placement is not hindered, reduction in strength of section due to embedment if any is accounted in design, and the embedment does not create a path to increase the permeation (affecting durability) of liquid through the member section or through cover zone.

R 13.1.5 Metallic items that protrude from the concrete shall be detailed so that the galvanic corrosion between the buried and exposed portions will not occur. Aluminium shall be isolated from any wet concrete by a moisture-proof coating, lining or gasket. Bi-metallic corrosion should be avoided by isolating contacts between metals or metal to reinforcement contact.

R 13.1.6 In walls, horizontal construction joint is less demanding compared to vertical. Larger vertical lifts of wall should not be at the cost of introducing more vertical joints. For cylindrical tanks it is good practice not to have vertical construction joints (or have only one joint of time gap not more than few hours). All vertical joints should be sealed by sealant applied on the liquid face in a small groove (say 3 mm wide 3 mm deep).

R 13.1.7 As far as possible through tie holes should be avoided. If provided, filling and patching of tie holes is essential for long-term durability. If to be left in concrete, form ties with creeping flange (collars) should be used for members intended to be watertight. Fabricated ties of a type, for which central portion is left in concrete, and bolts in cover region from both sides are recovered. Thus the hole left is not through and should be filled in from both side by polymer modified mortar. Through form ties should be avoided, and if used the hole should be tightly filled with suitable non shrink /shrinkage compensating mortar, and surface be sealed.

R 13.1.8 Due to fall of water from a height, its impact can cause cavitation and abrasion loss of concrete. Similarly abrasion loss may be where water with debris (grit or silt) has velocity more than 2 m/sec, or without debris water velocity more than 4 m/sec. To avoid damage to the structural concrete from abrasion loss, a wearing apron should be provided at such locations. Such aprons may be provided in steel fibre reinforced concrete to have high wear resistance and thus reduces the number of replacement of apron during service life. Apron should be bonded to the structural concrete below, by laying it while concrete below has not yet set, else a bonding agent shall be used.

R 13.1.9 Dense and smooth concrete finish should be obtained by smooth shuttering, or trowelling, with extended curing etc. which can reduces permeability.

R 13.1.10 Enough curing is vital to developing required durability of the concrete. The method of curing shall ensure that the surface layer of all concrete remain moist at all times during the curing period or till at least 80% of the specified concrete strength is developed. Later, LRC should not be allowed to dry-up below 55% relative humidity, to avoid drying shrinkage, till tank is filled with aqueous liquid.

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13.2 JOINTS : Joints shall be constructed in accordance with requirements of R 11.

13.3 CONSTRUCTION OF FLOORSR 13.4 CONSTRUCTION OF WALLSR 13.4.3 Wall thickness 200 or more is preferably. Minimum thickness can be 180 mm where reinforcement size is ≤ 10 mm. However, minimum thickness of wall can be 160 mm for small tanks (<3 m size or <2.5m water head).

In treatment plants walls of channels can be 120 mm thick if single layer bar mesh is required and width or depth of channel is less than 0.9 m. Partition wall & baffles not subjected to significant pressure can be 120 mm thick with single layer of reinforcement mesh (or bars).

13.5 PRESTRESSED TANK 13.6 FORMWORK 13.7 LINING OF TANK

R 14 TEST OF STRUCTURE : The tank for potable water should be cleaned or disinfected before it is put to use. This can be done before or after

the test for water tightness. Till test by water filling is undertaken, it is advisable to the keep the liquid retaining concrete components slightly moist, so as to avoid development of drying shrinkage cracks. The water tightness test is performed for new construction and also after major repairs are undertaken to reduce leakages.

For ground tanks, water tightness test can usually be performed before backfilling of soil on outside. This will help in locating the spots of leakages.

Water-tightness of the structure is necessary as a performance requirement, and it also influences the durability.

R 14.1 Test of structures specified in IS 456 (clauses 17.3 to 17.8) are not mandatory. The structural test of structures as per IS 456 can be carried out only if the need be. These tests are to be performed in case of doubt due to new technology or lapses or non-conformance to design parameters or construction, noticed during inspection or operation of quality system. However the water tightness test should be carried out necessarily in all cases. Tank shall be slowly filled for first filling. Refer R 9.2.6.

R 14.2 For water tightness test, loss of water is measured in terms of drop in water surface. Apart from seepage additional loss is due to evaporation and sometimes a gain due to rain for open top tanks, and such loss or gain is to be corrected. To the actual water drop correction should be applied for evaporation loss or gain due to rain etc. For covered tank evaporation loss can be taken negligible.

R 14.2.1 During and after the 1st filling of tank leakages in general reduce due to autogenous healing of cracks. Hence during test, a stabilizing period is allowed. If tank does not pass the water tightness test, there is an option to extend the stabilizing period and retest. It should be noted that only fine cracks where leakage is quite small, can get healed. The stabilizing period may be 7 days, which can be extended to 21 days if need be and where a liquid retaining component is designed for 0.2 mm limiting crackwidth (as per IS 3370 part 2). If tank fails in water tightness test, the portions of concrete responsible for leakages are to be recognized. Cracks or passages where leakage is high, are to be grouted to reduce the leakage.

R 14.2.2 For an elevated tanks if leakages are not visible on the sides and bottom, it can be deemed to be watertight. The level of the water shall be recorded again at subsequent intervals of 24 hours over the period of test. The total

drop in surface level over a period of last seven days shall be taken as an indication of the water tightness of the tank. The actual drop in the surface level shall be corrected by evaporation losses or rainfall for open tanks. In seven days, the permissible loss shall be 10 mm plus 0.002 times the average water depth in tank, or the limiting value as per specification. Some international codes provide 5 mm as the limit for drop in level in 24 hours.

For ground supported tank, having water depth less than 5m, if drop in level of water is less than 20 mm in 7 days, the tank is deemed to be watertight.

If wall of tank show leakage, these spots should be rectified by proper action.

R 14.3 Roofs should also be tested for water tightness, even if roof is sloping or having dome shape.

15 LIGHTNING PROTECTION

R 16 VENTILATION Area of ventilation in roof should not be less than the area of inlet or outlet pipes. For tanks storing chlorinated

water, to reduce the concentration of chlorine in the air of freeboard zone, the ventilation may also be not less than 0.8% of the free surface area of water. Ventilation area should be protected by mosquito-proof net.

R 17 DESIGN REPORT AND DRAWINGS : Documentation shall be prepared which shall contain all salient features of the work, engineering data and brief maintenance scheme of the work. It shall cover the following:R 17.1 Brief data and features like description of liquid to be retained, capacity of tank (in m³ or liters), height of freeboard (in mm), liquid depth in tank. Salient levels – average GL, foundation level, LSL and FSL etc.R 17.2 Foundation investigation report, soil data, description of founding stratum, type of foundation, probable depth of foundation, ultimate, net safe and net allowable bearing capacity of founding strata. Position of groundwater table- highest and lowest. Soil classification for seismic design (based on corrected standard penetration value N, refer IS 2131 and IS 1893 part 1). Data on liquefaction potential of founding soil in seismic event. R 17.3 Location of structure (e.g. polluted industrial area, sea front area, costal area, urban area, rural area etc.), and purpose of liquid retention (i.e. public water supply, fire-fighting, industrial, sewage treatment etc.). The information on pollutants, salts, sulfates if any in air, soil, groundwater and liquid retained or excluded, if of significance at the concerned location. Design exposure class for tank members, and roof.

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R 17.4 Brief specifications of concrete and its grade(s), type of cement(s) to be used, limits of maximum and minimum cement content. Brief specifications and limits of mineral admixtures (SCMs’), chemical admixtures to be used, and additives if any to be used in concrete. Give brief specification of reinforcement bars and grade to be used. In case of fibre concrete, the type of fibres, its size, aspect ratio, dose etc.R 17.5 Salient features of structure and construction, method of construction, height of column lifts, height of wall lifts, and guidance on release of form work. For specialized form-work, the design and drawing of the formwork shall be given. Any constraint on construction assumed in the design. Specify surfaces acceptable as form-finished. Specify finishes separately on different formed and unformed surfaces. R 17.6 Type of finishes to be given to different surfaces. Special protective coatings if being specified, and its brief specifications.R 17.7 Clear cover of concrete on reinforcement bars for various members at different locations. Guidance on locations of laps in reinforcement shall be given. Table of lap length in reinforcement of different members for each size of bars (diameter wise) shall be given. R 17.8 General locations, specifications and treatment of construction joints should be specified. R 17.9 The curing procedure in brief and its duration should be specified.R 17.10 References of codes, standards, and guidance for construction. R 17.11 Design loads : Density of concrete, liquid, soil, masonry etc.; provisional loads of rendering, finishing, flooring, coating, lining etc. as applicable, loads of railing, parapets, masonry wall etc.; imposed loads on roof, balcony, walkways, platform etc.; load of ladder, stair, pipelines, valves etc.; load of permanent equipment (/pumps) if any; provisional load of any process operation, other provisional loads, construction loads; any other loads. Magnitude and location of prestressing force, stressing sequence.

Seismic zone, zone factor, response reduction factor, importance factor, critical damping factor, soil factor (see IS 1893 part 1 & 2).Basic wind speed, terrain category, factors k1, k2, k3, k4, Kd, Ka, Kc, (see IS 875 part 3); Load of equipment if any etc.Blast or explosion load if to be considered, any other loads considered.

R 17.12 Design report containing basis of design, water tightness class (refer Table 4 in part 2), method of structural analysis, structural configuration and the assumptions made, detailed loading computations, structural analysis, design calculations with sizes of members and reinforcement, checks required as per codes. In some cases design is influenced by method of construction and formwork details. Such cases should be covered by design.R 17.12.1 Documented computer output is acceptable in lieu of sheet of calculations. The extent of input and output information required will vary, according to the specific requirements of individual proof checking engineer. When a well-known computer program has been used by the designer, only skeleton data may normally be required. This should consist of sufficient input and output data and other information to allow the checking engineer to perform a detailed review and make comparisons using another program or manual calculations. Input data should be identified as to member designation, applied loads, and span lengths. The related output data should include member designation and the shears, moments, and reactions at key points in the span. For column design, it is desirable to include moment magnification factors (or additional moments) in the output where applicable.R 17.13 Drawings with reinforcement detailing, junction detailing, brief specifications, instructions and notes. Notes on construction joints, and detailing of movement joint should be given in the drawing with their locations and treatment.R 17.14 Design drawings, typical details, and specifications for all concrete construction shall bear the dated signature and seal of a design professional.R 17.15 Guide for completion drawing, and completion report for record. This will help in compiling completion record.R 17.16 Prepare a design brief, which is to be shared with construction manager, for preparing construction brief, which is key to quality assurance.R 17.17 Framework of quality management, its manual to be prepared by agency supervising the construction. Completion record will also have Record of quality of construction to be compiled by the contractor or producer or supplier.R 17.18 Proposed scheme of condition survey and maintenance (conservation) of structure.

R- APPENDIX 1Some of the available Indian Standards related to joints and jointing materials are listed below. These standards are for building

& pavement work, and may not be relevant to liquid retaining structures. Some Indian Standards refer to dams & similar massive works of water resource engineering.IS 1580: 1991 Specification for bituminous compound for water proofing and caulking purposes.IS 1834:1984 Specification for hot applied sealing compound for joints in concrete. 1st Revision.IS 1838 part 1 -1983 Specification for preformed fillers for expansion joint in concrete pavement and structures (non

extruding and resilient type): part 1 Bitumen impregnated fibre (1st revision).IS 1838 part 2 -1983 Specification for preformed fillers for expansion joint in concrete pavement and structures (non

extruding and resilient type): part 2 CNSL Aldehyde resin and coconut pith.IS 1838 part 3 - 2011 Specification for preformed fillers for expansion joint in concrete pavements and

structures (non extruding and resilient type) Polymer basedIS 3414:1968 Code of practice for design & installation of joints in buildings. (Scope specifies that it does not cover water retaining structures)IS 4461:1998 Code of practice for joints in surface hydroelectric power station.IS 5256:1992 Code of practice for sealing expansion joints in concrete lining of canals.IS 6494:1988 Code of practice for water-proofing of underground water reservoirs and swimming pools.IS 6509:1985 Code of practice for installation of joints in concrete paving & structural construction (1st revision).IS 10566:1983 Method of test for preformed fillers for expansion joint in concrete paving and structural construction.IS 10957:1999 / ISO 2444:1988 Glossary of terms applicable for joints in buildings (1st revision)IS 10958:1999 / ISO 3447:1975 General check list of functions of joints in buildings.IS 10959:1984 / ISO 6927:1981 Glossary of terms for sealants for building purpose.IS 11433 part 1 : 1985 Specification for one part gun-grade polysulphide base- joint sealant, part 1 general requirements.IS 11433 part 2 – 1985 Specification for one part grade polysulphide base joint sealant, part 2 method of test.IS 11817:1986 / ISO 7727 Classification of joints in building for accommodation of dimensional deviation during

constructionIS 11818:1986 / ISO 6589 Method of test for laboratory determination of air permeability of joints in buildingIS 12118 part 1 : 1985 Specification for two parts polysulphide based sealant, part 1 general requirements.IS 12118 part 2 : 1985 Specification for two parts polysulphide based sealant, part 2 method of test.IS 12200:2001 Code of practice for water-stops at transverse contraction joints in masonry and concrete dams.IS 13055:1991 Method of sampling and test for anaerobic adhesives and sealants.

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IS 13143:1991 Joints in concrete lining of canals – sealing compound – specifications. IS 13184:1991 Mastic filler – epoxy based – specifications.IS 16354:2015 Metakaolin for Use in Cement, Cement Mortar and Concrete - Specification

BS:5212 – Polysulphide & Polyurethanes ASTM D 2628 – Preformed sealsASTM D 5893 – Silicon sealantJoint filler – MORTH specifications – 602.2.9There are many IRC and MORTH specification also.

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Guide on IS 3370 Part 2 – 2020 (2nd Revision),Code of Practice - Concrete Structures for Retaining

Aqueous Liquids : Part 2 - Reinforced Concrete Structures

R 0 GENERAL Working stress method is now deleted for reinforced concrete. Limit state design (LSD) approach is improved, and

all design shall be performed this method (LSD). The requirement of crackwidth calculation is amplified.For ground tanks, concept of designing temperature-shrinkage reinforcement related to spacing of movement joint

has been clarified, and that is not applicable to elevated tanks.Code is for retaining aqueous liquids. For retaining other (non- aqueous) liquids concrete shrinkage will be higher,

and most other liquid may require lining of the tanks.‘Loads’ are dealt in details, and ‘liquid load’ is defined separately, and does not behave as DL or LL. Limit state

design provisions are given in more details. Partial safety factors specified for materials are as per IS 456, though these can be enhances a bit without affecting the cost of works. Table of ‘load combinations’ is given independent of IS 456. Crackwidth control is linked to class of water-tightness. Importance of detailing is brought in, and reinforcement detailing of right angled junction is specified.

Also refer to the comments under ‘General’ & ‘Scope” on IS 3370 part 1. Some guidelines are given here, which are not dealt in the code.There is experience of designs with earlier code, which have behaved satisfactorily. Hence there is no need to

increase reinforcement by way of minimum % of steel for small tanks, except large (size >14 m) ground tanks.In general deflections are not taken important for LRC members except roof slab. However check for deflections as

per IS 456 is retained. Clause 23.2 (b) of IS 456 will not apply. Design base should be such that only limited periodic maintenance may be required for serviceability over the

design life. And also an uncontrolled, rapid loss of the liquid retained would not occur in extreme events such as a major earthquake. The structure should be safe, serviceable, and in extreme case repairable.

Following are the significant modifications incorporated in this revision: a) Liquid load has been defined separately. Refer R 4.2.8.b) For circular wall liquid pressure will be assumed to act at centre of wall. Refer R 4.2.8.1.c) Only limit state design method is specified; working stress method is deleted for RCC design.d) Table of load combinations has been specified. Table 1, R 4.4.1.4.e) For member subjected to direct tension, reduction of shear strength of concrete, is specified. Refer R 4.4.2.1.f) At construction joint, design actions are specified. Refer R 4.4.2.3.d, R 4.4.3.1, R 4.4.3.5, R 4.4.3.6.g) Detailing at junctions (connections) of walls is recommended. (Refer R 8.3.)

R 1 SCOPE : (Also refer to scope in part 1)The code is also referred for structures dealing with sewage. Components of sewage treatment having liquid of low

pH (<6) or materials which can attack concrete, will require additional protection in form of coatings or lining.The code does not deal with ‘ferro-cement’ or ‘fibre concrete’, for which specialized literature should be referred.

Use of synthetic/polymeric (say polypropylene) fibres at a dosages of 0.14 to 0.18 % by volume (1.3 to 2.0 kg/m³) of concrete, are useful in controlling plastic shrinkage cracks. As well fibres are useful in dispersion of cracks, thus reducing crackwidth.

R 2 REFERENCES : List of standards referred in the code is given. While referring to a standard its latest revision with up to date amendments should be used. This information is freely available at www.bis.org.in , the web site of Bureau of Indian Standards.

R 3 GENERAL REQUIREMENTSCommon general requirements are covered in part 1 of the code. The requirements of IS 456 will also govern the

design & construction of LRC, unless a requirement is overruled or in conflict to that in IS 3370.

R 4 DESIGNAim of design is the performance of structure over the design life, with minimum maintenance and avoiding repairs,

rehabilitation and intervention. Adequate foundation support, robustness, stability, durability, sustainability, constructability, maintainability and restorability of structure should also be considered in design. Design should be such that construction is traditional and easy, or guided by enough details to avoid shortfall in quality of construction. Details should also be such that the planned maintenance would be possible in future for retaining the serviceability of structure. In case of slip in quality, repairs and restoration should be conveniently possible.

Structure should be designed for durability, and serviceability limit state by keeping check on water tightness, deflections, crackwidths, spalling, and excessive vibrations. Construction joints and movement joints shall be designed such that these are restorable few times during service life of structure.

Structure should also be designed for ultimate limit state by keeping check on stress limit, and strength resistance capacity. At ultimate limit state LRC may be partially damaged, leakage may occur, but should be temporarily usable, even in design wind or seismic condition, and should remain repairable. It should not become a mechanism arriving at collapse stage. Ample warning in terms of time, deflection or local damage shall precede any possible failure.

For proper functioning and least maintenance, monolithic construction is preferred. Keep joints to the minimum. For

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monolithic structure, effects of continuity and restrains should be worked out for determining forces (or stresses) at all critical sections of members and also within junctions of members.

The design should consider social, environmental and economic sustainability. Low carbon footprint and low consumption of materials are the present day needs.

In future, design requirements will also need consideration of demolition (dismantling).R 4.1 In general the word “forces” mean ‘force actions’ such as direct force (tension or compression), shears, bending moments and torsion acting on member at a section/ position. Concentrated or distributed mechanical forces acting on /in a structure are direct actions. Deformations imposed on the structure or contained within it or environmental deformations imposed on the structure or contained within it are indirect actions or environmental actions.R 4.1.1 For structural reinforced concrete members and junctions (connections), design approach based on strut-and-tie models is permissible and on most cases appropriate.R.4.1.2 Sizes (geometrical values) of members to be taken for design should be the characteristic value, which should account variation possible in construction. The design, dimension can be modified by half the possible variation. Such correction is more important in cantilever members and thin slabs or wall (<160 mm).R 4.1.3 Where thin shells (e.g. floor & roof domes or shaft staging) are provided, failure by buckling mode in all load combinations (including vertical & horizontal seismic effects) shall be avoided.

R 4.2 LOADS : Loads are dealt by IS 456 as dead load (DL), imposed load (IL also known as live load), wind (WL), seismic or earthquake load (EL). Liquid (FL or fluid load/pressure) and earth pressure (EP) do not fall in the classification either as DL or IL.

Loads (actions) almost constant or monotonically approaching a limiting value during a reference period, are permanent actions. Load (action) which is not permanently acting, not constant or not monotonically changing during a reference period are variable actions.

R 4.2.1 Consideration should also be given to loads during construction. R 4.2.2 DL : Differentiate permanent DL, and provisional DL (DLp) which may or may not be present at all time.R 4.2.3 IL : Differentiation should be made for normal IL, ILs of temporary storage, and ILp provisional imposed or equipment load.

Imposed loads (live loads) for equipment and process area shall take into account weights of fixed equipment, loads of material stored, and normal live loads due to personnel and other transient loads. Imposed loads should account for installation, operation, and maintenance of equipment, and possible modifications or changes in use. With equipment, its allowance for impact or its action under dynamic condition shall also be accounted.

During installation or maintenance, portions of equipment may be laid down at various locations on the floor. For example, heavy equipment (pump-sets) may be temporarily placed near the centre span of a floor during installation or maintenance, even though its final location may be near support locations. Weights of concrete bases for equipment may also be included in the loads, and consideration should be given to weights of piping, valves, and other equipment accessories that may be supported by the floor slab and beams. Consequently, conservative uniform live loads may be applied as an alternate.

Information on equipment weights and their bases should be obtained for design and a margin be kept for variation in the load of equipment from different suppliers. During assembly and installation of large equipment, area adjoining to its final location may experience loads of pieces, and such load provision should be considered.

Where applicable snow load shall be considered.R 4.2.4 EL : Earthquake or seismic base shear shall be worked out for an inertial load combination -

1 DL + 1 FL + 0 IL + 1 ILs + 0.7 ILp

Where, ILs is imposed load due to storage, and ILp is provisional imposed or equipment load. In above, liquid load (FL) can be considered full or part or tank empty case (FL 0 to 1) separately as may be taken in a particular load combination with seismic load. The seismic base shear thus obtained shall be multiplied by an appropriate load factor for a load combination as given in Table 1. R 4.2.4.1 For seismic analysis, the liquid mass should be idealized by two-mass model, wherein a part of liquid mass functions as convective mass and other part as impulsive mass. The impulsive mass of liquid, with the mass of container and the equivalent mass of the staging (for ESR) together shall be accounted as total impulsive mass. The convective mass of liquid shall be separately accounted. Effect of both of these shall be combined as per details in IS 1893-2 (amendment 2020). For h/d up to 0.64, impulsive mass can be taken equal to total mass of liquid less convective mass calculated.

R 4.2.4.2 The two mass model is more appropriate compared one mass model, and in most cases the two mass model can gives a smaller design base shear in staging, but higher overturning moments. All containers must be designed or checked for the actions (base shear & over-turning moment) induced due to two mass model.

R 4.2.4.3 The container may have columns and braces inside tank or baffle walls, which provide obstruction to the convective liquid, thus reducing convective mass and increasing impulsive mass of liquid. For this approximate correction can be applied to increase impulsive mass for estimating seismic effect. The amount of impulsive mass thus increased should be deducted from convective mass.

R 4.2.4.4 As per present design approach, the response reduction factor Ri is taken from IS 1893 Part 2 Table 2 & 3 for

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impulsive modes. For convective mode Rc value should be lower or may be taken between 1 and 2.For seismic coefficient due to convective mode [(Ah)c] the critical damping for liquid shall be taken as 0.5% (against

5% for concrete). For 5% critical damping in concrete, Sa/g is obtained from 6.4.2 of IS 1893 part 1- 2016 (under revision). This value shall be multiplied by 1.75 to obtain Sa/g for 0.5% damping. It is further recommended here that (Ah)c should not be taken less than 0.04 .

R 4.2.4.5 For elevated tank, mass of staging (Ms) is taken as a uniformly distributed over the height hs (from top of structural foundation to bottom of container). Height hm is up to C (i.e. CG of liquid in container). Equivalent mass of staging (Mse) is the mass applied at C which will give same horizontal deflection at C, as the distributed mass over height hs will give. See Fig. 25. As an approximation Mse = Ms /3 may be taken for design.

R 4.2.4.6 For vertical seismic, Sa/g should be taken as 2.5, and value of Ri should be smaller than that for horizontal seismic. However at present, the value can be taken as 2.5 for ESR and 1.5 for GSR. For vertical seismic, convective mass may be neglected, and total liquid mass taken as impulsive. For ESR, Av = Z/2 ; for GSR, Av = Z/1.2 .

R 4.2.4.7 For design of staging of tank less than 50 m³ having maximum horizontal spread of liquid less than 7 m at FSL, and founding soil is not soft, simplification by considering one mass model wherein total liquid is treated as impulsive mass only, can be an option of designer. However such simplification cannot be binding on any designer.

R 4.2.4.8 For estimating the natural period of vibration of staging, its stiffness for horizontal deflection need not be reduced on account of cracking or creep. Stiffness of un-cracked members should be accounted i.e. taking gross concrete area in to account. Correction to deflections (sway) due to cracking of members and that due to P-δ should be neglected, for estimating natural period.

R 4.2.4.9 If specified by owner or decided by designer a lower value of R can be applied for the seismic design, than that given in IS 1893-2.

R 4.2.4.10 If imposed loads are other than live loads on roof, and of nature like a process or operations or equipment (ILp) or storage, an appropriate part (may be 0.5 to 0.7) of such ILp excluding impact allowance should be accounted for estimating base shear in R.4.2.4.

R 4.2.4.11 In any design, Sa/g should not be taken less than 0.40; nor should the seismic base shear be less than that given in table below.

Seismic zone ratio in %II 0.015 1.5 %III 0.020 2.0 %IV 0.027 2.7 %V 0.036 3.6 %

[ e.g. for zone III, base shear not less than 0.020 (i.e. 2.0%) of the total seismic weight. ]

R 4.2.4.13 If the tank or staging do not have main members as overhanging or cantilever, other than cantilever walkway, the effect of vertical seismic can be neglected for tanks in zone II and III. Otherwise, in all other cases, horizontal and vertical seismic effects should be combined.

All overhanging or cantilever members even in zone II or III, shall be designed for vertical seismic also. For the purpose of this clause cantilever walkway (≤ 1.2 m cantilever span) will not classify the tank as having cantilever members. However, the cantilever walkway or gallery shall be designed considering vertical seismic also.

R 4.2.4.14 Mass of pipeline, stair, ladder etc. should also be estimated for arriving at total horizontal seismic action for elevated tank.

R 4.2.4.15 For Industrial water tower of high importance or tower in highly populated urban area (where collapse of tower may damage many other buildings or structures), the importance factor may be taken higher than 1.5 (can be 1.75 to 2.0), if specified by the owner. Also refer IS 1893 part 4.

R 4.2.4.16 For tank having slab supported on ground, it (slab) acts as a diaphragm for transferring the seismic forces from tank to ground. Hence connection of the ‘slab on grade’ to vertical members shall be detailed for the seismic load path. Movement joints may interfere with the adequacy of action as diaphragm.

R 4.2.4.17 Members of tank participating in lateral load resisting frame (i.e. floor beams) shall be designed conforming to ductility requirements of IS 13920-2016 (under revision). If flat slab is provided for floor of an elevated tank it should be designed for high degree of ductility (norms are not available in the standards).

R 4.2.5 WL : Wind load. As per section 5 to 7 of IS 875-3, wind should be accounted as pseudo-static force.[ Note that wind speed map of India is revised in 2020 as per amendment 2, or refer NBC.]

R 4.2.6 Earth Pressure (EP): The lateral pressure from earth backfill, may be symmetrical or asymmetrical. Including those caused by unequal backfill or surcharge, net lateral loads shall be determined by rational methods of soil mechanics based on soil and foundation investigations.

The wall does not deflect enough as required for active state of earth pressure. Hence effective earth-pressure coefficient could be higher (say 1.3 to 1.5 times active), and in between at-rest and active state. The coefficient shall not be less than 0.50 where soft soil is present. Where excavation is in hard strata like weathered rock or soft rock, the refilled trench is small and active pressure can be very small, and this active pressure can be enhanced by 50%.

If EP is definitely to remain always, it may counteract the liquid pressure from opposite direction. Such relieving earth pressure should not be more than half of the active state. Also refer R 8 a (i) in part 1.

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Earth cover on roof may be taken as saturated dead load. Also take in to account construction loads of machinery and heaped earth which may exceed the force action at some sections compared to intended design load.

Allowance should be made for the effects of any adverse soil pressures (EP) on the walls, due to the surcharge and process of compaction of the soil and the condition of the structure during construction and in service. The lateral pressure from earth backfill, may be symmetrical or asymmetrical. Including those caused by unequal backfill, net lateral loads shall be determined by rational methods of soil mechanics based on foundation and soils investigations.

R 4.2.8 Liquid load (FL) : FL shall take in to account the actual density of the contained liquid. Density of plain water can be taken as 9810

N/m³. Aqueous solutions (alum solution), suspensions or sludge can have higher densities. Accumulated sludge, deposited silt, grit, lime etc. if any, will add to the load. Wherever exists, liquid load includes dead storage which may have higher density due to sludge and silt etc.

FL may be accounted at zero or partial or full liquid load as may make a load combination more critical. It should be such as to cause the most critical effects at a position of a member. Liquid load also includes, both static and dynamic effect of the liquid as pressure. Each tank shall be designed and checked also for tank-empty condition.

For serviceability condition FL is to be taken up to normal working top liquid level (WTL) or the overflow level. This level is usually referred to as full supply level (FSL) in tanks. For overflow to match the rate of incoming liquid, usually heading of liquid above WTL is of the order of 20 to 50 mm. Such a small heading of liquid can be neglected in design. To keep a control over maximum FL, overflow arrangement should be designed for a discharge rate not less than the maximum filling rate of tank; an overflow pipe or weir arrangement of adequate size shall be provided to prevent overfilling of the tank.

If over flow is chocked, or for any other reason liquid level rises above WTL (/FSL), liquid load will be higher under such an unusual condition. Provision of such rare and temporary rise may be estimated, which may be termed as maximum top liquid level (MTL). For design, above FSL liquid may be considered to rise by amount as specified (need not be up to full freeboard), and level may be taken as MTL. In many cases height of MTL above WTL (/ FSL) is specified between 150 (for smaller tanks <100 m³) to 300 mm (for tanks >1000 m³). With this rise (i.e. MTL), FL is accounted only in ultimate load combination with DL & IL (limit state of collapse). However for all other combinations with WL/EL, FL up to FSL/WTL only is considered. Where freeboard available is very high, a suitable MTL shall be decided from hydrostatic design considerations.

Where MTL above WTL is higher by more than 300 mm, the load factor for FL can be taken as 1.40 (in ultimate load combination), the condition being very rare.

Except for pressurised tanks, vent(s) shall be provided in roof to regulate the internal pressure in space above liquid, while tank is being filled or emptied. See R 16 in part 1.

R 4.2.8.1 Internal pressure of liquid shall be assumed to act at the centre of thickness of the liquid retaining circular wall. Same criterion can be applied to rectangular wall panels. If lining or impermeable treatment is applied to the inner surface, or the pressure is due to granular material or soil, the inner surface of impermeable lining or wall may be assumed to be the point of application of force. The external pressure of liquid will be assumed to act at outer surface of structure, which may be in combination with earth-pressure. Weight of liquid on a member (e.g. slabs) shall be up to the internal surface (i.e. top face) of member.

For calculation of hoop force as a simplification, radius should be taken as clear radius plus half the thickness of wall i.e. centre-line radius of wall. Increasing the radius by half the thickness is the maximum possible. As per first level approximation, for actual hoop force the factor to thickness could be one third (1/3 rd). Where wall thickness is higher (>200mm) the factor to thickness will be less than 1/3 rd. In most cases, it may is appropriate to account increase of radius by 1/3rd of wall thickness, though code has introduced the factor as half as a simplification.

R 4.2.9 Other load actions which are significant for serviceability, strength and stability of the structure or its members as applicable, including the following shall be taken into account.

(a) Construction load, (b) ground movement, (c) thermal effect on roof. The clause is mainly for ground tanks.

R 4.2.10 An underground (or partly) structure subject to groundwater pressures should be designed for floatation (uplift). Design of each member shall account the pressure due to groundwater in suitable critical load combinations. See IS 3370 part 1, R8(c).R 4.2.11 If concrete is allowed to dry, the moisture dependent drying shrinkage will take place. This can happen where tank is provided with impermeable lining. Normal recommendation in this standard (for temperature-shrinkage reinforcement) does not account this moisture related drying shrinkage. Hence this extra shrinkage if restrained will need more reinforcement to control crackwidth. See also R 9.1.B.1 & R 9.1.1 of part 1.R 4.2.12 Loads should be grouped as ‘permanent loads’, ‘provisional loads’, ‘variable loads’ and ‘construction loads’. Environmental actions (physical, chemical or biological) are also types of loads. R 4.2.13 Junctions (say connections) of members should be designed and detailed for giving rigidity and satisfactory crack control throughout the design life of structure. R 4.3 METHOD OF DESIGN

The level of accuracy of various physical parameters of design should be refined by devoting more efforts for analyses, so as to arrive at improved accuracy in the behaviour and strength provided by design. In regard to a specific criterion, inadequate performance is considered to be a failure in that regard. In addition to serviceability and strength,

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degree of water tightness, absence of leakages and avoidance corrosion of reinforcement are prominent performance requirements amongst others.

R 4.3.1 Components of structure adjoining to LRC may not be designed as per IS 3370. However, continuity of these with LRC may need considerations of restrains imposed on LRC. R 4.3.2 Significant plastic redistribution of moments (as in IS 456 - 22.7 & 37.1.1) is not permitted directly. Limited redistribution can be done, keeping a limit on deformations or maximum strain. [ACI 350 permits up to 20% of BM to be reduced on account of plastic redistribution with some limitations.]

Also simplified estimate by coefficients (as in 22.5 of IS 456) cannot be permitted.

R 4.3.2.1 For two way spanning slabs and wall panels, moment coefficients from Table 26 of IS 456 for uniformly distributed load, which are based on modified (multiplied by 4/3 times) yield line theory, with this readjustments are normally acceptable for design of liquid retaining concrete including crack control, as is proven by experience and also use in other countries. In such plates permitting the elasto-plastic behaviour of concrete and reduction of peak moments, in serviceability state for -ve BM tensile stress in steel should be ≤190 N/mm² or crackwidth be ≤ 0.15 mm (in place of 0.2 mm).

Use of elasto-plastic behaviour of structure can be made, for members with ductility and limiting plastic strain considered. Use of simplifying assumptions or elasto-plastic behaviour of structure can be made, with limit on plastic strain considered and by imparting enough ductility, having proved by long experience or technical literature or tests that it can give satisfactory performance at or near junctions (connections) of members, and in estimation of crackwidth. Because the control on possible crackwidth during service is required, adjustments on account of plasticity (say redistribution of moment) can be done only if crackwidth can be predicted for such situations within acceptable accuracy. Alternately, strain can be enhanced by plastic strain, for crackwidth estimation.

However it should be noted that tables for bending moment coefficients for triangular liquid pressure (say for walls) are generally based on elastic analysis or linear finite element analysis. For a loading configuration, uniform pressure and hydrostatic pressure (triangular) can be added. But using the coefficient tables, difference of the two cannot be permitted. In other words BM worked out by Table 26 of IS 456, for uniformly distributed load and BM for triangular load, cannot be deducted from each other, but can be superimposed as additive.

For rectangular wall plate subjected liquid load, elasto-plastic analysis can be done wherein peak moment is reduced up to ≤20% only. For moments in other plate shapes and loading, results of elastic analysis can be used. For flat slab design analysis for suspended floor of tank can be done by finite element stiffness method, and high ductility is to be imparted. Norms for achieve high ductility for column width of flat slab are not available at present in standards. Under combination of seismic action, the flat slab participates in the frame action, demanding very high ductility. Use literature for imparting high ductility.

Where flexural moments are calculated by a method of linear elastic analysis of frame, the redistribution of maximum -ve BM in a continuous span can be done to a limited extent. Follow procedure as per 37.1.1 of IS 456, except that limit of reduction shall be 20% in place of 30%. A reduction (on account of plasticity) of critical elastic moment (maximum –ve) up to 20% can be done (as permitted by ACI 350) if strain is substantial (i.e. moment is a peak or maximum value in the member) but limited, at the section where redistribution is permitted. Before readjusting the BM, the -ve BM section should under-reinforced. For under-reinforced sections the limiting strain limits are satisfactory. Such reduced BM shall be used for calculating redistributed moments at other sections of the member by principles of static equilibrium.

The sections designed in flexure shall be under-reinforced (i.e. less than 75% of balanced wherein compression governing failure is avoided), and should not be over-reinforced, where plastic redistribution moment is done.

For sway frames no redistribution on account of plasticity will be permitted.

R 4.3.2.2 Flat slab design as per 31.4 of IS 456 is based on plastic redistribution of moments, and cannot be permitted for suspended floor slab of elevated tanks, because of inadequate control on ductility and possible under estimate of crackwidth. Flat slab design as per 31.4 of IS 456 may be permitted for roof slab of tanks and also for floor slab on grade of ground tanks. Flat slab analysis done by finite element method will be acceptable for design in all cases. For floor of elevated tank it will be acceptable with provision of high degree of ductility imparted to it. [ACI 350 permits the flat slab design based on direct coefficient method. However minimum reinforcement requirement is also quite high.]

R 4.3.2.3 The floor member of a tank, which is part of lateral action resisting frame has to be conforming to IS 13920. For other LRC members, the ductility requirement is small, and need not conform to IS 13920. For flat slab LRC floor, the ductility demand may be higher than that considered in IS 13920.

R 4.4 LIMIT STATE DESIGN (LSD) Structural safety i.e. ultimate resistance to actions (force) bearing, serviceability (deflection & crackwidth) are

required. In modern design considerations are also required to other limits like water-tightness, durability, robustness, sustainability, constructability, maintainability, restorability, etc. R 4.4.1.1 Limit State of Collapse (Ultimate limit state) : Load combinations are given in Table 1.

R 4.4.1.2 Limit State of Serviceability : R 4.4.1.2 a) Deflection check is required as per 23.2a of IS 456, and b is not applicable. For deflection check only 70% of FL can be treated as long term load (accounting creep coefficient) and remaining 30% as short term (no creep). For tanks which may remain filled up for a long time (say the provisional storage for firefighting or units of treatment plants remaining full most of time) 100% FL should be treated as long term. Earth load shall also be a long term load.

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For deflection of long term loads, the short-term modulus of elasticity (Ec) divided by 1+θ will be the effective modulus. Creep coefficient θ be taken 2.2 for DL & FL, and zero for short term loads. (As per IS 456).

R 4.4.1.2 b) At the concrete surface, estimated crackwidth (by procedure specified) due to the restraining effects on temperature and shrinkage (length change) in young concrete should not exceed 0.2 mm. Also at the concrete surface crack-width is estimated (by procedure specified) for the serviceability loads (1 DL + 1 FL + 1 IL only). Here the effects of temperature & shrinkage is not taken additive to the loads. The temperature-shrinkage effect or load effect should be taken independently for crackwidth check, and will not be combined.

It should be noted that in IS 3370, while ‘severe’ exposure condition shall be considered, the limiting crackwidth is 0.2 mm, and not 0.1 mm as specified in IS 456 clause 35.3.2, and is over-ruled by the provision of IS 3370. For ‘very severe’ and ‘extreme’ exposure, a lower limiting crackwidth say 0.15 mm may be taken relevant.

However author recommends that, for the bottom face of roof of tanks storing chlorinated water, or for very severe or extreme exposure, or members in contact with sewage (in STP) crackwidth limit 0.10 mm may be adopted. For more guidance on reducing the limiting crackwidth, refer the requirement given in R 4.4.3.2, R 4.4.3.3, and R 4.4.3.4.

The international trend is to assume that there is no significant relationship between crackwidth below 0.2 mm and the durability of concrete member. However, in more severe environment, for better control on permeability, lower crackwidth can be preferred for higher reliability. Method of estimating crackwidth is an approximate one hence for reliability, in more important cases lower crackwidth limit is to be adopted.

Longitudinal cracks are more prone to durability problems, and shall be avoided. See R 9 last but one para.

R 4.4.1.2 c) In addition to crackwidth check, the general recommendation (internationally) is also to limit the tensile stress in steel and compressive stress in concrete for members requiring crackwidth ≤0.2 mm. Higher compressive stress can produce cracks transverse to direction of compression. For these limiting values refer the clauses and discussion at RB-1 for section in flexure, and RB-4 for member in direct tension. Also see R 6.4.2 and R 8.3.1

R 4.4.1.3 Partial Safety Factors : Recommended partial material factors (γm) are as per IS 456, 1.5 for concrete and 1.15 for steel. Designer may take higher γm for LRC for small works where quality systems are not complete and workman ship may not be at good level. Increase in these factors by 15 to 20% for concrete and 9 to 15% for steel, will enhance the reliability without any significant increase in cost, as crackwidth check governs the design. Partial material factors (γm) can be 1.65 to 1.80 for concrete and 1.2 to 1.33 for steel.

High compression in concrete can also induce tensile strain in direction perpendicular to compressions (Poisson’s ratio effect). Due to micro-cracks and the effect of temperature and shrinkage superimposed, secondary cracking may result. As per EN code, excessive compressive stress (>0.45fck) in the concrete under the service load may lead to higher than predicted level of creep, and promote the formation of longitudinal cracks, and also result in micro-cracking in concrete. To control such secondary cracking, the design compressive stress in concrete could be reduced by about 15%. This amounts to the reduction of the utilised strength of concrete by 15% for purpose of design. Such reduction is not specified in the standard. If applied, it normally will have very little effect on sizes of members of LRC.

R 4.4.1.4 Load Combinations :Load combinations with partial load factors are given in Table 1 (reproduced below).

Table 1 Load Combination and Load FactorsCase Ultimate Limit State Limit State of Serviceability

DL FL EP IL WL/EL* DL FL EP IL WL1 2 2 4 5 6 7 8 9 10 11

1 1.5 1.5 1.5 1.5 0 1.0 1.0 1.0 1.0 02 1.2 1.0 1.0 0 1.4 1.0 1.0 0.7 0 0.32a 0.9 1.0 1.0 0 1.42b 1.4 0 1.0 0 1.43 1.2 1.2 1.2 1.2 1.2

*Consider WL or EL (Seismic) each separately.Notes :1 WL and EL load should be considered one at a time and not simultaneously. 2 For any combination, the partial safety factor for liquid load (FL) may be further reduced if it is expected to give

more critical design action at a section of a member. Liquid load (FL) may vary from zero (tank empty) to the specified value (1/1.2/1.5) in a combination. Same holds true for EP and IL in load combinations.

3 Seismic base shear be worked as per R 4.2.4. The seismic base shear (or force action) as determined shall be multiplied by the load factor as given in Table 1, for further combination with other loads.

While applying the load combinations, care must also be taken to account loadings in design for transient, short duration or in service conditions that may arise during the construction or the operation of the a structural unit. Conditions to be considered may arise when the flow of the liquid may lead to unequal hydraulic pressures at different locations of the structure. Similarly differential earth pressures, with presence and absence of soil or other fill material or deposits, may act for some duration during construction, which may produce critical design actions at some locations, need to be considered. In combination with TS, crackwidth limit can be exceeded. Refer R 4.7 for TS.

Still ‘Temperature- Shrinkage’ combination is not mentioned, as it may not be required in many small tanks. In elevated tanks its linear effect can be neglected. For gradient effect (variation along thickness) can be accountable in all cases (ground or elevated). In serviceability limit state DL + FL + 0.2 TS can be evaluated for design.

R 4.4.1.5 Due to shear, crackwidth is not checked. If shear reinforcement is provided, for serviceability state the gross nominal shear stress τc shall not exceed τmax given in Table 24 of IS 456.

R 4.4.1.6 Limit state of ‘Maintainability or Restorability’ need considerations. Some functional requirements like

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performance of movement joints need maintenance and restorability. It is also called as conservation. It may not be possible to design these joints for design life of structure. These joints should be designed such that without structural damage or modification, joints can be restored to perform.

Normally limit states of fatigue strength and fire resistance etc. are not required in LRC structures.R 4.4.2 Basis of Design :

Junctions (connections) of members should be assumed rigid. For frame analysis, and for analysis of continuous plates (slabs or wall) centre to centre span should be considered. Frame analysis can be done on the basis of relative stiffness of members or frames can be analysed by stiffness method or moment distribution (or other established method).

R 4.4.2.1 For member subjected to direct tension, shear strength at section shall be multiplied by a reduction factor as follows. δ = [ 1 – 8 Pu / (Ag fck) ] [In place of 8 constant is 12 in Australian code.]

For members with direct tension, the critical section for design of shear force shall be taken at a distance from face of support as below (in lieu of ‘d’ in 22.6.2.1 & 40.5 of IS 456) . Distance = d {1- 0.12 Pu / (b d fck)} – 40, but ≥ -40 mm (i.e. between -40 mm to d. Cover is 40 mm, marked as negative) Where, d is effective depth, & Pu is axial tension, all units in N & mm. As a simplification, for members in direct tension, design shear force can be taken at support face.

R 4.4.2.2 For slabs and walls, if the shear stress calculated exceeds the permissible value, shear reinforcement should be provided. The shear reinforcement should be designed for a shear capacity Vu – 0.7 τc bd , (refer 40.4 of IS 456). For arriving at the value of τc, correction due to direct compression (40.2.2 of IS 456) or tension (4.4.2.a) as applicable shall be applied.

R 4.4.2.3 Permissible bond stress depends upon the permissible slip which has to be low for LRC. Hence permissible bond stress has to be little lower for LRC. However no change is recommended for deformed reinforcement. Bond strength shall be reduced for some type of bars, at least by a multiplying factor as follows: Fusion bonded epoxy coated deformed bars 0.80, Plain bars (not deformed) 0.625, Plain coated bars 0.50, Stainless steel 0.85 .

For reinforcement in direct tension, the bond stress or lap length should be modified as per IS 456 -26.2.5.1 c, which specifies lap length as 2 Ld .

Following are additional considerations for shear and tension design.(a)Shear strength reduction due to tension in member (ultimate or factored Pu) should be accounted.(b) Shear strength of a member at construction joint reduces compared to monolithic concrete, depends largely on

interface roughness and cleanliness of the surface at the time of placing second phase concrete.(c) Half the average height of valley to peak, can be measured as roughness. Roughness more than 3mm gives

satisfactory performance if shear is not high. For higher shear resistance, combination of diagonal reinforcement, dowels and shear key can be designed to meet the strength demand in shear.

(d) In absence of better estimate, shear strength of concrete (as diagonal tension) can be assumed to be two-third, and strength in direct shear can be assumed to be one half at the roughened construction joint. No reduction shall apply on the contribution by reinforcement. At construction joint permissible shear strength is reduced, to reduce slip (hence crackwidth).

(e) Area of reinforcement for hoop tension or direct tension shall be calculated assuming all tension to be taken by reinforcement and tension in concrete is neglected i.e. concrete is assumed cracked. Anywhere else also, for strength design the reinforcement in tension shall be designed assuming concrete cracked and does not take tension.

R 4.4.2.4 If for floor beams of tank, the depth to span (c/c) ratio is >0.40, it requires reduction of lever-arm for calculations of tensile steel.

For design of tensile steel in floor and roof beams near support, the distance of critical section from centre of support shall be taken to be 0.35× width of core (i.e. column size along centre line of the beam less clear cover).R 4.4.2.5 For wall to wall junction (if continuous) having tension at the inside corner (opening type junction), the design BM shall be at centre of junction of members. BM at centre of junction shall be considered for working out tensile steel as well for crackwidth check. For wall to floor slab junction, the bottom of wall i.e. top of slab shall be the critical section for design.R 4.4.2.6 For elastic stability and prevention of buckling of shells of container should be adequately assessed. Check : ( R / t ) < 120 √( Ec / p) ; here

R = radius of curvature of shell in m, t = thickness of shell in m, Ec = Elastic modulus of concrete in N/mm² ( Ec = 5000 √fck ), p = pressure normal to surface of shell (superimposed loading including the component due to self-weight) in N/m² (un-factored).

Note: Constant ‘120’ may change from 100 to 150. Higher value can be taken where tolerance in deviation of the middle surface from as required is very tight and measured. If there are no specifications applied for tolerance, the constant should be reduced to 100.

In case of roof dome, load normal to surface shall be due to DL + (IL or that due to WL).

R 4.4.2.7 For thin shells (e.g. dome), the direct compressive stress due to factored (in ultimate limit state) membrane compression shall not exceed 0.24 fck .R 4.4.3 Crackwidth :

Estimate of minimum reinforcement, crack spacing and crackwidth due to temperature and shrinkage effect in early

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age (immature) concrete is given in Annex A of the code. Estimate of crackwidth in mature concrete is dealt in Annex B of the code.

The calculated crackwidth is assumed to have an acceptable probability of not being exceeded. If little wider cracks (say up to 0.3 mm) are noticed in the completed structure the matter should be under observation and may be investigated. If crackwidth reduces with time and settles within permissible while liquid load is full, the situation can be treated as acceptable. Few occasional wider cracks (>0.2 mm) noticed in the structure will not make structure unacceptable, if the design calculations and construction are proper. However these wider cracks should be cement grouted and sealed as may be desirable for the situation of the cracks, if investigation does not indicate design or strength deficiency. Structure with wider crack may become unacceptable as a result of investigation if local damage, serious workmanship flaw, or excessive leakage is also noticed along with a tendency for the crackwidth to increase. Such cracks shall be structurally repaired, treated (grouted by low viscosity epoxy or polymer) and sealed.

Satisfactory behaviour with regard to crackwidth could be achieved by properly placing adequate reinforcement at suitable smaller spacing. The reinforcement required to control cracking in immature concrete may also be totally accounted for crack control for service loads. This means the requirement of minimum steel for crack control in immature concrete, and amount of steel required for service load, are not additive. Similarly effect of load and effect of temperature during service are also not taken as additive.

The underside of roof members remains 100% humid most of the time. Hence these shall also be checked for the crackwidth limit similar to the liquid retaining members.

Before applying crackwidth check, section of member is already designed, i.e. member size and amount of steel on each face of member is determined. Next the crackwidth is checked. If it is found more than the specified limit (0.2 mm or less as specified), designer has following options.

(a) Reduce the steel bar size and the spacing of bars, however it is advisable to have spacing not less than 70 mm c/c on a face, nor less than 5× diameter of bar for slabs & wall.

(b) Increase the area of steel, i.e. reduce design stress in steel, may be bar size increased.(c) Increase the section size say depth/ thickness of member and re-proportion the reinforcement.

After these modification, crackwidth check is to be applied again.

R 4.4.3.1 Crackwidth may be deemed to be satisfactory if stress in steel in limit state of serviceability does not exceed the enveloping values given in Table 2. This table is applicable for reinforcement in all members like wall, slab or beams. These limits are irrespective of strain in concrete and for spacing of bars being not more than 300 mm c/c. If stress in steel is more than these limits, crackwidth check shall to be applied by detailed calculations.

Table 2 - Maximum tensile stress in steel reinforcement under limit state of serviceabilityTension in steel in limit state of serviceability

Limiting crackwidth Plain round bars Deformed bars0.10 mm 85 N/mm² 100 N/mm²0.15 mm 95 N/mm² 110 N/mm²0.20 mm 115 N/mm² 130 N/mm²

Tensile stress higher than that in Table 2 (deemed to limits) is acceptable, if detailed check for crackwidth is carried out and found to be within limit.

Similarly Table 3 gives limiting stress in deformed bars and the spacing for ‘deemed to’ criteria for 0.2 mm crackwidth. If the criterion is fulfilled, no further detailed check for crackwidth is required.

If a member in combined bending and compression, has compression on the two extreme fibres (i.e. neutral axis is outside the section or eccentricity of compression is small), it can be said to be case of compression predominant. In such a case no tension develops, hence no cracking and crackwidth check is required.

For a member in combined bending and axial force (tension or compression), if tension develops on one face and compression on another (i.e. neutral axis is inside the section) it can be said to be case of bending predominant. In such a case crackwidth check is to be applied as a flexural member.

For a member in axial tension with or without combined bending, such that depth of neutral axis is less than 50 mm or it is outside the section (i.e. tension on both faces), the section will be termed as predominantly in direct tension, and calculations will be done as a member in direct tension.

Section size and reinforcement on each face as arrived at, is analysed for stresses under service load, which is same as working stress method. For this modular ratio shall be as per IS 456, annex B. The crackwidth formulae are adopted from British code, wherein higher value of modular ratio (m) is specified. Hence (recommended by author though not in code), that in place of ‘m’, a modified value of ‘1.5×m’ may be taken in to account. Thus the calculation involves grade of concrete, however its effect on crackwidth is very small. For cracking long term modular ratio should be considered which will be much higher, and can be enhanced by 100%, in place of increasing by 50%.

Section analysis for depth of neutral axis requires solution of cubic equation, which can be done by a computer programme. For a depth of neutral axis, the maximum compressive stress in extreme fibre and tensile stress on tension steel will be calculated, and further calculate crackwidth.

In serviceability state, it is prudent to limit both the compressive stress in concrete to 0.36 fck and tension in steel to 0.55 fy (228 N/mm² for 415 grade) for long term crack control at important locations. In British practice (refer Design of Liquid Retaining Concrete structures by R.D. Anchor) the stress in steel is also limited to a lower amount. Also recommends that in serviceability state, for in direct tension the steel stress should be limited to 0.50 fy (207 N/mm² for 415 grade steel). In highly critical locations, steel stress should preferably be reduced further. Also see R 6.42 & R 8.3.1.

From stress, strain in steel can be calculated, and reducing it for concrete stiffening, crackwidth can be calculated

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using equations given. The estimated tension stiffening is the maximum capacity possible, however this maximum value may not be mobilised in all the cases. The tension stiffening of concrete can further reduce at the section of construction joint, or at the section of curtailment of bars.

At a construction (or partial contraction) joint, the tension stiffening reduces (may be by 1/4 th of its value), hence crackwidth will be higher. At any section, the correction to strain in the bar due to tension stiffening (duly reduced at construction joint or due to curtailment in tensile steel) cannot be more than 2/3rd of the strain in steel.

It should be noted that as per the procedure estimated crackwidth value is almost unaffected by the grade of concrete, its modulus of elasticity and its tensile strength. While the grade of concrete increases, the modular ratio (m) decreases, which has a very small effect on the calculated crackwidth. Hence the equations will need corrections when high strength concrete (say grade >M35) or concrete with higher flexural strength (>4 MPa) is used. Also the procedure cannot be applied to polymer concrete, fibre concrete or ferrocement.

Tension stiffening can be related to flexural strength of the concrete. Up to M30 concrete, flexural strength can be taken as fcr = 0.7√fck . For grades above M30 the value of fcr may be taken as 4 N/mm² or a characteristic value determined by tests. For fibre or polymer concrete the flexural strength shall be determined by test.

If average residual strength (fem,150) is ≥ 1.0 N/mm², the tension stiffening can be assumed to be enhanced by 20% (factor 0.25 becoming 0.30), for calculation of crackwidth. For FRC having average residual strength (fem,150) ≥ 1.50 N/mm², the tension stiffening can be assumed to be enhanced by 60% (factor 0.25 becoming 0.40), for calculation of crackwidth.

Equations (8, 9, 11 & 12 in Annex B) for tension stiffening (ε2) could be multiplied by 0.25fcr (in numerator), thus making the equation non-dimensional. For fibre concrete having average residual strength ≥1.50 N/mm², equations be multiplied by 0.4fcr ; and if ≥ 1.0 N/mm², multiplied by 0.3fcr .

Actual benefit of fibres are much higher, which can be accounted if detailed procedure as per accepted methodology can be applied. At construction joints, the contributions of fibres shall be neglected for strength, and for crackwidth the tension stiffening shall reduce. Refer R 4.4.3.5.

It should also be noted that in some cases, if steel on compression side of section is accounted the estimated crackwidth will be slightly higher, but it may be permissible to neglect steel in compression in such case, if the section (neglecting compression steel) remains under-reinforced.

R 4.4.3.2 A liquid retaining member may be classified in to water tightness class, as susceptible to the possible leakage related to crackwidth recommended. Note that concrete always permits the passage of small quantities of aqueous liquids by permeation and diffusion.

In long run, water tightness is reduced due to washout of particles by flowing water, leaching of calcium compound, and degradation of hydrate by ion exchange. Autogenous healing takes place only in initial life (say about a year) of LRC if crackwidth are very small, beyond that period the autogenous healing may be insignificant. At an age of 15 to 30 years, the seepages through cracks may enhance a bit, for which initially crackwidth may be controlled conservatively.

Classification of Water-Tightness - Giving limiting crackwidth in mmTightness

CassRequirement for leakage{ H/t is between 20 to 30 }

[ # whichever is more ]

Cracks through, no compression block (direct tension)

Compression block < 0.2 t or 50mm #

Compression block ≥ 0.2 t or 50mm #

1 Leakage to be limited to a small amount. Some surface staining or damp patches acceptable 0.15 mm 0.20 mm 0.20 mm

2 Leakage to be minimal, Appearance not to be impaired by staining. 0.10 mm 0.15 mm 0.20 mm

3 No leakage permitted. Special measures such as Prestressing or impermeable liner required.

Crackwidth nil with prestressed. ORLeakage prevented by lining; and limiting crackwidth as for class 1, in concrete.

Notes : 1. No compression block means member section with neutral axis outside section (or eccentricity of tension small) or member in direct tension or hoop.

2. The tightness classes of the wall and floor of a tank can be different.

R 4.4.3.3 Recommendation about crackwidth related to water tightness class is given in table above. Most LRC can be assumed to be of class 1. Where aesthetics is important or the passage of pollution through concrete is important, the Tightness class 2 can be applied. Tightness class 3 is where no permeation of liquid through concrete or wetness is permitted. Where appearance of wet patches are not acceptable in addition lining is necessary.

R 4.4.3.4 The limiting crackwidth values as recommended in table above (related to water tightness class) may exceed by 0.05 mm, if H/t is ≤ 20. In no case crackwidth shall exceed 0.2 mm. If H/t is > 30, the limiting crackwidth shall reduce by 0.05 mm.

R 4.4.3.5 The crackwidth at construction joint can be calculated by reducing the tension stiffening by concrete by 25%. However, crackwidth shall be assumed to increase not more than 0.05 mm, nor more than 50% of the crackwidth without reducing tension stiffening i.e. the crackwidth estimate for monolithic section.

R 4.4.3.6 For crackwidth enhancement due to shear, at that section the flexural moment in slab shall be enhanced by equivalent moment due to shear force (Mes) for crackwidth check only. Mes = SF × (D/3),

D is overall depth of slab at the section considered. If shear reinforcement is provided, enhancement by the equivalent moment need not be considered.

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R 4.4.3.7 After accounting different reductions on limiting crackwidth if value is less than 0.05 mm, the requirement for limiting value can be taken as 0.05 mm.

R 4.4.3.8 The estimated crackwidth has a probability of not being exceeded. An occasional wider crack in a structure should not necessarily be regarded as evidence of local damage unless leakage is unacceptable. Reduce leakage by rectification say grouting. Even if crackwidth appear to be within permissible limit, but unacceptable leakage takes place which does not appear to reduce, it should be controlled by grouting.

R 4.4.3.9 Top or outer surface of roof member may be subjected to moderate exposure condition for which crackwidth limit need not be checked (0.3 mm is deemed to be safe), and for higher exposer crackwidth limit will be 0.2 mm.

R 4.5. STRESSES DUE TO MOISTURE OR TEMPERATURE CHANGESR 4.5.1 The clause is applicable if the tank is to be used only for the storage of water or aqueous liquids at or near ambient temperature and the concrete never dries out; and adequate precautions are taken to avoid drying and hence cracking of the concrete during the construction period and until the tank is put to use.

In cases of calculating stresses due to shrinkage, assume shrinkage coefficient as 300×10 -6 for concrete having cement (OPC) ≤ 400 kg/m³ or total cementitious content not more than 450 kg/m³. For higher cement content the shrinkage coefficient will be higher.R 4.5.2 If tank can remain empty for more than a month, or where impermeable coating or lining is applied, the concrete can dry out and total shrinkage can be much higher, and the requirement of temperature-shrinkage (i.e. minimum) reinforcement would be higher (by 25% to 40%).

R 4.6 Between various members (e.g. between wall & floor, or wall & wall) junctions are intended to be rigid. The junctions (connection) should be designed accordingly and effect of continuity should be analysed and accounted in design and detailing of junction and each member. For LRC members the capacity of a junction to resist force actions (moment, shear etc.) should not be less than the maximum estimated actions within the junction.R 4.7 TEMPERATURE AND SHRINKAGE EFFECTS (TS)

Experience has shown that minimum of 0.22% (434 mm² for 200 mm thickness) has given satisfactory performance without any movement joint for ground tanks up to 15 m size. For ground tanks of size about 22 m, above minimum steel is found inadequate. Hence for small tanks minimum steel should not be increased. Minimum steel should not be increased for vertical direction, unless tank height is more than 15 m or it is restrained vertically to other source.

Though not clarified in the code, it can be stated that horizontal minimum reinforcement may be higher depending up on the horizontal size of structure (continuous construction), or spacing of movement joints. For elevated tanks which are restrained by structural features or by other structure, minimum reinforcement will be similar to ground tanks. Otherwise elevated tanks normally have very little restrains against linear temperature, moisture & shrinkage movements, hence requirement of minimum reinforcement does not increase substantially with the size of structure. Alternately minimum steel shall be calculated as per A-1.2 in code. For a tank if roof is free to slide and is not rigid with the walls, % minimum steel in roof can be reduced to that in one lower range of size.

Continuous construction without movement joints (as per option 1) can be done for ground tanks normally up to 30 m size. Above 30 m size provision of expansion joint is a normal solution, though tanks of more length (say 50 m) can be designed without movement joints.

Designer has an option to calculate the minimum steel as per the provisions of the code, depending upon the spacing of movement joints. Where concrete grade is higher than M30, designer must calculate the minimum steel required as critical steel ratio.

To economize on the minimum steel for ground tanks, top of foundation PCC should be a flat and have smooth surface with bond breaking sheet (as per R 9.2.8.b IS 3370 part 1) to facilitate sliding and thus reducing the restrain. The thickness of such bond breaking sheet will depend upon the roughness of the top of PCC base. For PCC having flat in-plane and fairly smooth surface, about 1mm thick LDPE (polyethylene) sheet is recommended by British practice. Also see R 13.1.2.

Options (Table 2 part 1) may be used to design movement joints at closer interval and also design the temperature shrinkage steel. For the continuous construction (Option 1), PCC top may have slight slope and gradual thickening, and also bond braking layer is not required. Where tanks are small, the horizontal tank size can be treated as spacing of movement joint and minimum steel can be smaller (as per option in Table 2 in part 1).

Temperature-shrinkage effects are in two groups, one the linear effect (in the plane of member), and second the gradient across the thickness of section of the member. All these effects are relaxed by cracking. The effect can also be subdivided as long term and short term. Long term effect will also have further relaxation by creep of concrete.

R 4.7.1 Shrinkage Coefficient : In reinforced concrete (different from prestressed), the effect of temperature & shrinkage get relaxed (reduced) due to creep and cracking of concrete. While accounting temperature fall (from peak due to heat of hydration at about 1 to 3 days), shrinkage can be assumed to be negligible in the immature concrete. In the calculation method given in annex B, a reduction of strain (100×10–6) is suggested on account of creep.

Shrinkage has two components, one the irreversible (physio-chemical), other the reversible (i.e. moisture dependent), total shrinkage if not known can be assumed to be 300×10–6, in which relaxation due to cracking can be assumed to be included. For M30 grade concrete typical total free shrinkage is much higher compared to 300×10–6 (may be 800×100-6). It is reduced due to absence of moisture dependant component and relaxation due to creep and cracking.

While a component is in contact with aqueous liquid, only chemical shrinkage i.e. irreversible part (33 to 40 % of total) will take place, and this gets almost compensated by creep. Hence in combination with liquid load, shrinkage may be neglected.

It should be noted that if cementitious content increases (> 450 kg/m³) the shrinkage will be higher and higher minimum steel will be needed.

R 4.7.2 In tanks protected by internal impermeable lining, the design strain will be higher due to drying of concrete. Hence design has to consider higher strain by about 150×10–6 (say total 450×10–6), and permit higher crackwidth if crack bridging property of the lining can be assured.

R 4.8 Sustainability should be given consideration in the design and construction. Aim should be to minimise the use of OPC, and maximise use of flyash and GGBS, and optimise the material use. Consideration should be given to long life and minimise the maintenance requirements, and construction quality should avoid repairs during design life.

5 FLOOR R 5.1 Provision of movement joint is linked to the basis of minimum steel. This is explained in Part 1. R 5.2 Floor can be assumed to rest on ground if proper foundation conditions are met with. If subjected to uplift, it should be designed for bending due to net upward pressure,R 5.3 Floors not supported on grade are also called suspended slabs, as are required for elevated tanks. For floor slabs up to 200 mm thick, in the region of +ve bending moment, reinforcement can be provided at the bottom face of the slab

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only (no steel at top face); and in such a case the bottom reinforcement shall conform to the minimum steel requirement for the total slab thickness. For slab thickness more than 200 mm, at least one third of minimum steel should be placed at top face, and total steel of top and bottom face together should conform to minimum steel requirement. In the region of -ve bending moment, the top reinforcement should not be less than two third of the minimum for gross thickness. R 5.4 For suspended floor slab supported on beam or wall, the critical section for design of slab near support shall be

45 mm inside from the face of support. R 5.5 In most cases for floor beams of tank, the depth to span ratio is >0.40, which requires reduction of lever-arm for

calculations of tensile steel.

R 6 WALLS For the monolithic construction of RCC floor and wall, sympathetic vertical cracks can develop at the locations of

movement joints in the floor if provided. Hence walls should also be provided with movement joints similar to floor.Where a pipe passes through a wall or floor, the pipe should be embedded while member is cast. Alternately a box-

out may be kept with additional precautions. In all cases pipe should not be very near to a joint being provided in concrete or a joint be provided on all side and a little way from pipe (say about 2 to 4 ×thickness of member). The joint between member concrete and box-out should be treated as a construction joint and should be grouted adequately. At the position of pipe the thickness of concrete should be increased and extra reinforcement provided to take care of stress concentration and extra stresses due to force/ restrain due to movement and pressure on pipe. As well to give more hydraulic creep length for water seepage. R 6.4.1 For cylindrical wall of elevated tank if assumed to have no radial displacement in simplified analysis, the design hoop tension in bottom portion of wall can be increased by an amount equal to one third (0.33%) of the amount it is short from membrane hoop tension.R 6.4.2 Wall junction with floor of tanks shall be designed and detailed for rigidity and crackwidth. Note that crackwidth limit is governed by class of water tightness (R 4.4.3.2), size of compression block, adjustment due to H/t (R 4.4.3.4), and with crackwidth check moment enhancement due to shear (R 4.4.3.6). Tension in steel should be limited to < 0.50 fy (207 MPa in 415 grade) in limit state of serviceability. Also refer R 8.3.1.

The junction shall also be checked for direct shear on the interface of construction joint (for reduced shear capacity)

R 7 ROOFSRoofs should be designed with sufficient slope or camber to ensure adequate drainage accounting for any long-term

deflection of the roof due to the dead loads, or the loads should be increased to account for all likely accumulations of water due to long term deflection by accumulated water itself. If deflection of roof members may result in ponding of water accompanied by increased deflection (in long term) and additional ponding, the design must ensure that this process is self-limiting.

For roof slabs up to 200 mm thick, in the region of +ve bending moment, reinforcement can be provided at the bottom face of the slab only (no steel at top face); and in such a case the bottom reinforcement shall conform to the minimum steel requirement for the total slab thickness. For slab thickness more than 200 mm, at least one third of minimum steel should be placed at top face, and total steel of top and bottom face together should conform to minimum steel requirement. In the region of -ve bending moment, the top reinforcement should not be less than half the minimum for gross thickness.

For large (>20 m) roof slab having multiple panels, exposed directly to external environment (i.e. without and soil cover or insulating cover), At the support where slab is continuous, the top reinforcement near the support (minimum for 0.1× clear span) shall not be less than 0.2% of gross section of the slab; unless slab is specifically designed for temperature and shrinkage requirement (duly relaxed by creep & cracking) superimposed with DL.

For roof slab supported on beam or wall, the critical section for design of tension reinforcement of slab near support shall be 45 mm inside from the face of support.

R 8 DETAILING The dimensions of structural components in local areas of the structure, and specifying the structural details and

reinforcement are the parts of structural detailing. Detailing is to be done on drawings.R 8.1 Minimum Reinforcement

Minimum reinforcement is also called as temperature-shrinkage reinforcement which can take care of normal cracking due to same and avoid repeated calculations for normal effects of temperature and shrinkage. Minimum reinforcement is based on gross concrete area, and where steel is on both faces it shall be based on thickness of surface zone for each of the faces.

With the revision of codes this minimum reinforcement is being increased. In general the requirement of temperature-shrinkage steel increases with the size of structure and also with the degree of restrain. For large size tanks there are always some cases where need of steel may be more than the minimum specified.

However, the minimum steel recommended in standard is higher than the need for small works. In India there are very many small water tanks for rural water supply scheme. For the small tanks which have behaved satisfactorily for 6 decades, cannot be overburdened with higher minimum steel, due to higher need felt for larger work or bad experience of poor construction.

[ As per Table 2 in IS 3370 part 1, where joint are at smaller spacing (option 3) recommended steel is 2/3rd. Thus in small tanks ≤ 7 m minimum reinforcement recommended can be reduced. ]

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It should also be noted that, container of elevated tank has much smaller linear restrain and will need smaller temperature-shrinkage steel. Floor slab of elevated tank need more steel for BM due to load, compared to a floor slab of ground tank. That cannot be an argument for higher minimum steel for elevated tanks.

Hence the author recommends the minimum steel as per table below. {ACI requirement is >1.3× than below.}

Type of reinforcement Elevated Tank Ground supported Tank≤10 m 14m 22 m 30 m ≤7 m 10 m 14 m 22 m 30 m

Plain Mild Steel -Grade 250 0.33% 0.40% 0.50% 0.66% 0.33% 0.36% 0.40% 0.60% 0.80%

High yield strength deformed – grade 415 0.20% 0.24% 0.30% 0.40% 0.20% 0.22% 0.24% 0.36% 0.48%

NOTES: 1. For ground tanks, spacing of movement joints will control the minimum reinforcement.2. For intermediate size, interpolation can be done.3. Length is counted along the direction of steel provision.4. Designer has to take decision for lengths higher than 30 m.5. Above minimum is valid if tank does not dry-out completely, or else higher steel will be required.6. For bars of grade higher than 415, the amount of minimum steel can be taken inversely proportioned to the characteristic strength

(in N/mm²) of bars.7. If grade of concrete is different from M30, the % minimum steel will change in proportion to fct .

For slabs on grade (resting on PCC & in turn on ground without separation sheet) less than 300 thick, steel may not be provided at bottom face. On the assumption that sub-base and ground below will provide friction (as continuous retrain) and hence need of steel on bottom face is eliminated. This is for continuous construction without bond braking layer put on PCC. Where bond braking layer is provided and spacing of movement joints is designed, the reinforcement in bottom layer can be as required for D/3 mm bottom zone, and need not be zero. For slab <200 mm thick bottom reinforcement may not be provided.

Though not clarified in the code, it should be noted that in ground supported tanks, it is preferable to give expansion joints at 30 to 40 m spacing. However for such a large spacing of expansion joint, proper design of minimum (temperature/moisture/shrinkage) reinforcement be provided.

Minimum reinforcement in floor slab on grade (without separation layer) should be by % of the surface zone specified for each face as follows. D = thickness of slab.

Slab Thickness less than 300 mm 300 to 500 mm > 500 mm Top zone in mm D/2 D/2 250 only Bottom zone in mm Nil 100 only 100 only

Minimum % steel specified will apply to all members. For the floor slab on grade (i.e. continuously supported by ground), for which it will be in % of the surface zones thickness specified.

At bottom portion (0.6m or one eighth of height) of wall, the minimum % steel will be similar to that for slab below wall. For remaining height of wall, the minimum steel will be as per the vertical height of tank.

R 8.2.1 Size (i.e. diameter) of bar and spacing of bars (in slabs and walls) could be small as practically possible without causing congestion of steel or difficulty in placing and vibrating concrete. In walls and slab minimum preferable c/c spacing can be nearly equal to 5× diameter of bar or 75 mm whichever is higher; reducing spacing below such a limit has no significant advantage. Maximum permissible spacing is 300c/c. This limit on spacing is also applicable for minimum steel or distribution steel. For beams the minimum clear distance between bars can be smaller. However in beams one of the clear space between horizontal bars should not be less than 75 mm for pouring the concrete and for insertion of needle vibrator.

R 8.2.3 Condition of ‘spacing of bars not more than thickness of member’ should not be applicable to lightly loaded members provided with thickness in excess of the requirement. This spacing limit can be treated as a requirement for highly stressed members requiring much higher area of steel, and do not apply to roof slabs and members lightly loaded (service stress in steel <190 MPa). There is already a limit of 3× effective depth of member. For small tanks and small members with thickness up to 200 mm, this criterion unnecessary requires steel much more than the minimum. The amount of steel (in mm²/m) required would be more by this rule as the thickness of member decreases. This requirement is penalty on small works.

This limit on spacing may be complied only if bars are >10φ, or reinforcement required is >500 mm²/m on a face of a member. It is preferable to keep spacing small and taking 8φ bars. Where reinforcement on a face of member required is >500 mm²/m, choose the bar size less than thickness of slab or wall.

Spacing of bar more than thickness can be permitted, where calculated crackwidth is <0.167 mm. It can also be permitted for –ve BM steel on outer face of roof.

R 8.2.3.1 On the side face of a beam, in region of designed shear reinforcement (high shear near support), vertical space between longitudinal bars if more than 400 mm, minimum longitudinal 1× 8φ skin /surface reinforcement as should be provided in between. If two or more skin bars are provided on as face, their spacing shall not exceed 300 mm c/c.

R 8.2.4 As far as possible minimum size (i.e. diameter) of bar should be as below.Footing – 10φ; small footings (<1.2 m) – 8φ; Beams longitudinal anchor /corner bars 10φ; column longitudinal bars 12φ; slab, wall and shells 8φ. For small tanks (< 50 m³), or designed as PCC bars can be smaller. Bar size shall not be more than one twelfth of thickness of slab or wall. In load bearing wall (shaft staging), vertical

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bars can be up to one eighth of thickness of wall. Bar at centre of thickness can be of higher size.

R 8.2.5 Detailing is more important in liquid retaining structures because of –a. higher possibility of detrimental effects in aqueous environment, b. higher risk of reinforcement corrosion, andc. seepage of water, pollutants or impurities can affect the functional requirements.

Follow the detailing rules as given in design codes IS 456, IS 3370, IS 13920 etc.Additional guidance is given below.

R 8.2.6 Basic Principles to Guide Reinforcement DetailingOut of the many aspects listed below, some would need considerations in designing the structure in large projects. In

smaller projects considerations in designing will be few only. All remaining aspects shall be taken care of in detailing.(i) Structural and reinforcement detailing shall cover and reflect idealization made in structural analysis and design,

and hence the possible behaviour of the structure.(ii) Members are designed at different sections. Strength and other performance requirements are to be built up with

continuity in members and structure. Monolithic action shall be ensured from section to section and member to member at junctions.

(iii) Behaviour of junctions of members shall approach near to the assumptions made in analysis. In RCC structures, junctions of members are usually assumed monolithic and rigid. Detailing shall impart rigidity, control cracking and seepage at the junction.

(iv) Reinforcement placing should be practically easy for achieving reliable construction. If it is difficult or complicated, suitable notes on sequence of placing and on ‘how to do’ should be given. For deviations from normal practices, notes and details with clarity should be given. For difficult detailing a mock-up trial should be done.

(v) There should be sufficient gap between reinforcement bars for pouring of concrete and compaction, while also allowing for tolerances in clear cover, placing and tying of bars. Congested reinforcement shall be avoided, which otherwise can result in low quality concrete. Concrete placement should be easy and possible without segregation or screening of mix though reinforcement. Rodding and vibration should not become aid in placement.

(vi) Reinforcement cage should be stiff enough, to avoid chances of displacement or misalignment of bars, variation of clear cover, reduction in effective depth, reduction in bond strength etc. during placing and compaction of concrete. The cage should resist forces and movements during concrete placing and compaction, without any adverse movements. In the young concrete already compacted, due to operations going on in adjoining area, if a bar embedded moves or vibrates, an annular space could form around the bar, which will reduce bond strength and also adversely affect seepages, durability, and corrosion susceptibility of bars.

(vii) Detailing shall make stress transfer effective from reinforcement to concrete and vice-versa. So also transfer of force action and stresses with compatible strains, from a member to junction and junction to other members shall be feasible. Enough development length, anchorage, and bond strength shall be available with minimum slip. For rings adequate anchorage shall be provided at hook ends, to develop adequate stress in leg of the ring.

(viii) Redistribution of force actions (i.e. axial forces, shears, moments, torsion etc.) and stress patterns occur in members and structures. Redistribution may be due to nonlinear material behaviour (/elasto-plastic behaviour), non-linear structural behaviour (i.e. deformation dependent effects and conditions approaching towards instability), time dependent strain variations due to temperature, shrinkage, creep etc., locked in strain due to cyclic applications, history of loading and restraints.

Force actions and stress may also vary due to assumptions and simplifications in force analysis as well as in stress analysis or resistance capacities, computational tolerances and small errors. Due to these actual safety margin may reduce, however the structure should not become drastically unsafe. Shifting of contraflexure will determine positions of curtailment of bars. Detailing should take care of above variations and disproportionate increase in probability of failures shall be avoid.

(ix) Some small actions, effects and restraints, normally are not accounted in design. However that does not mean that such actions or effects are always negligible. Such effects shall be covered by detailing, utilizing some rules of thumb, simplified design approach or simple criteria e.g. minimum reinforcement, liberal anchorages etc. Actions such as- temperature variations in young concrete, temperature variations and gradient in matured concrete, shrinkage, moisture variations, creep etc., need to be given considerations in detailing.

(x) Due to small causes, disproportionately small consequences leading to damage, or small excitations which can result in significant adverse effect, such effects should be avoided. Probability of damage due to some unusual risks may be reduced, if by relatively spending a very little. Such risk reduction wherever possible should be done by adequate detailing.

(xi) Near surface of concrete, reinforcement should be available within a reasonable distance, in two almost perpendicular directions to give integrity to the member and avoid probable wide cracks. On the surfaces distance of reinforcement from any point should not be large say ≤300 to about 200 mm where cracking is highly important, i.e. reinforcement should be available within a reasonable distance from any location.

(xii) For members of large cross-section, the interior zone may be ignored. If interiors are not properly detailed, the reinforcement near surface may be over stressed. From any point in interior also, reinforcement should be available within ≤300 mm.

(xiii) Detailing shall adequately take care of regions of stress concentration, e.g. at openings, cut-outs, embedment, pipes passing through, etc. in the members, changes in section, junctions of members, concentrated force application (load or reaction), etc.

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(xiv) Detailing can help in control cracking, deflections (/movements) at service loads. Near ultimate load, spalling and disintegration of concrete could be minimised, and should remain localized.

(xv)Detailing shall imparting ductility to members and structure as a whole. Confining of concrete by reinforcement, achieves this and many objectives given above. Structures and members subjected to earthquake, dynamic loading, or fatigue loading require specific detailing. Detailing will be related to the level of ductility needed.

(xvi) For analysis and detailing of reinforcement, a simple idealization by truss analogy is an effective tool. Wherein compression is resisted by equivalent concrete strut and tension by reinforcement. The anchorage or continuity at the nodes of tension bar of truss is important and be cared in details, for tension development.

(xvii) Durability of concrete and reinforcement should not be impaired. Corrosion protection of reinforcement is an important attribute of detailing. Integrity and sufficient thickness of clear concrete cover on the reinforcement, are the basic requirements. Excess of cover gives higher crackwidth at surface. By proper detailing, development of longitudinal crack shall be avoided. Longitudinal cracks are highly damaging and causing corrosion of bars.

(xviii) As far as possible, steel bars (also nominal/minimum/ temperature-shrinkage /distribution) shall be distributed or well disperse, and reinforcing steel should not tend to concentrate.

(xix) Curtailment of bars in tension zone should be gradual, and not abrupt i.e. staggered as far as practicable. At sections near curtailment point, shear strength reduces, and these sections are susceptible to higher crackwidth. With higher % curtailment such disadvantage is more.

R 8.2.7 Bends in Bars : At concrete surfaces making an angles less than 180 (i.e. acute angle), the tension bar should not be bent without

proven detailing. At such location resultant outward thrust lead to the failure.Similarly bend in a compression bar making an angle more than 180 (obtuse angle) on nearest concrete faces will

also give outward thrust, which may spall concrete cover. Hence avoid such bend in the bars, or calculated effective tying or lateral reinforcement is required to take the outward thrust.

In a bar kink should be avoided, or if provided the slope of inclined portion should not be steeper than 1 in 10, and should have enough lateral ties (more than minimum).

Any bend in the stressed bar inside the concrete give rise to a thrust as bearing on concrete, and result in a splitting tension crack in the concrete. This crack is likely to be along the bar in the plane of bend. Such a bend should be confined in enough concrete (say cover more than 2 to 4 × bar size) depending upon severity of bend on all sides, to keep crack smaller. Provide adequate reinforcement to resist the spitting tension.

At the bends in the stressed bars, bearing stress shall be high when radius of curvature is small. At such bend a perpendicular bar as pin, distributes the bearing stress over more area and also reinforces the splitting tension crack. Confining rings shall keep control over such internal cracks.

Minimum radius of curvature of reinforcing bar is recommended to be K times the diameter of bar. Recommended values of K are as given below.

Mild steel plain bar up to 20φ K = 2 , > 20φ K = 3 ; Deformed bars, grade 415, bar up to 20φ K = 4 , >20φ K = 5, >28φ K = 6;

grade up to 500, bar up to 20φ K = 5 , >20φ K = 7, >28φ K = 8; grade up to 550, bar up to 20φ K = 6 , >20φ K = 8, >28φ K = 10;

Where possible the radius of curvature should be bit more than the minimum recommended. Bearing stress need not be checked in a hook or a bend near the end of a bar, as stress in bar will be small at end.However at hook or bend in stirrup which has to take shear force, stress in bar at bend will be high, hence a cross

bar (same or higher diameter) is necessary to give effective anchorage and to develop stress in stirrup. High strength deformed bar may have slightly reduced strength at the bend or re-bend (i.e. bending and

straightening or bending and reverse bending) position in bars and ductility may also reduce, more specifically under dynamic (/repeated reversal) loading. Bars should be checked before use under a re-bent test.R 8.2.8 Hooks :

In the hooks, curvature inside bend can be as sharp as permitted. Diameter of mandrill (i.e. diameter clear inside of bend) permitted is- the bar diameter multiplied by ‘b’ as given below.

Steel Type → MS Medium TS High strength deformed barsBar size↓ 250 300 415 500 550 Bar grade

≤ 20φ 4 6 8 10 12 Values of ‘b’>20φ 6 8 10 14 16

Beyond the curved portion of the hook, the straight length of bar at the end should not be less than the bar diameter multiplied by ‘a’ as given below.

Hook type 180 135 90 Circular ringStandard hook 4 6 8 8 Values

of‘a’

Hook in rings 6 (4) 8 12 10Rings for seismic detailing 8 (6) 10 (8) 16 (NP) 12

Value in bracket are as per IS. NP = not permitted.

R 8.2.9 Congestion of Steel :Reinforcement may be regarded as congested, if it poses difficulty in placing and compacting concrete. While

designing the details of reinforcement, it is to be assured that there is space with margin for inserting, placing in position and tying the bar in final position with ease and simplicity; or else the sequence of tying of reinforcement

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should evolved and specified. During designing, as far as possible congested steel should be avoided by providing increased concrete section or less number of bars of higher size. Normally the clear gap between horizontal bars should be ≥ 2× maximum size of aggregate.

If bars are bundled, the permissible bond stress should be reduced by 33 to 50%. Bars if congested, need not be provided at uniformly spacing. Bars may be arranged having minimum spacing or grouped, leaving few wider spaces. One gap should be ≥75 mm wide nearly vertical, for pouring of concrete without getting screened, and also for insertion of needle vibrator. Only in exceptional cases, it can be 60 mm assuring vibrator needle of smaller size to be used.

While estimating the space between bars, allowance should be made for permissible tolerance (for bar placing and cover), and for laps required in some bars.R 8.2.10 Development Length, Laps and Anchorage :

To develop stress in the bar at a section, minimum anchor length or an end anchorage or a combination of the two is required on each side of the section. Length of bar required to develop the desired stress level without end anchorage is the ‘Development Length’. The basic development length (Ld ) is given by: Ld = φ s / (4 bd) , where φ = diameter of bar, s = stress in bar, and bd = average bond strength between the bar and the concrete.

Bars in compression have higher bond. Development length should be increased by about 15% for two bars in contact, 33% for three bars in contact, and

50% for four bars in a group. Depending upon the value of end anchorage provided, the development length may be reduced up to 0.5 Ld.Following aspects affect the lap or development length.

a) Type, grade and quality of concrete affects the bond strength.b) Type and deformations on bar affect the ultimate bond strength.c) Surface finish and surface condition of bars.d) Stress gradient i.e. variation of stress (tension or compression) along the length of bar favourably reduces the

development length. For members in pure tension the double lap length or anchor length is recommended; and in zone of constant tension due to bending increase by up to 40%.

e) For reliability of lapping, percentage of steel being lapped simultaneously (i.e. not staggered) affect the lap length. See table below.

Between adjacent laps clear distance

From concrete surface clear distance of bar

% of lapped bars relative to total

a b 20% 25% 33% 50% >50%a < 6φ b < 3φ 1.2 1.4 1.6 1.8 2.0a > 10φ b > 5φ 1.0 1.1 1.2 1.3 1.4

Laps in different bars not simultaneous i.e. laps staggered means, in the direction of bar length, c/c distance of two laps being considered is >1.3× lap length. For direct tension or hoop bars, it should be >2.3× lap length. This is on account of poor bond in the portion of circumference of a bar, not being effective for bars in contact

If at a critical section all bars are lapped (not staggered), for reliability higher lap length should be provided. Similarly anchorage of bars in a junction should have a liberal (higher) anchor length for reliability, as all bars are being anchored.

f) Position of bar with respect to concreting operation, affects the bond strength, hence the development length and lap length. With respect to placing of concrete the bond conditions are considered to be good for bars having an inclination of 45 to 90 to horizontal. Below nearly horizontal bars if fresh concrete within one lift is >250 mm, the bar is assumed to have poor bond condition. Horizontal bars having <250 mm of fresh concrete in a lift will also be treated as bar in good bond condition.

Concrete placement in about an hour (or in initial setting time of concrete), in lift if more than 300 mm may be considered to be the high, within which bleeding of water continues from concrete. When concrete lifts are more than 250 mm depth below horizontal bars, slight settlement in upper portion of concrete takes place. Due to bleeding the water comes up and particles in the mortar matrix settle down. Some loss of water also take place from the concrete, allowing the concrete to settle. This results in a small water gap or a water film on the underside of the bar which is in top portion of concrete. This is called plastic settlement of concrete. The gap (as water film) is responsible for reduction in bond strength. The concrete above the bars tends to settle by sliding down from sides, thus forming a longitudinal crack over the bar in conjunction with plastic shrinkage, which is called as plastic settlement crack.

When bond condition is not good the bond stress is assumed to reduce by 30% or development length increased by 42%. This reduction in bond stress need not be combined with that due to other reasons. Top bars of beams more than 300 depth, require reduction in bond.

g) Stress transfer at the lap gives secondary stresses in concrete especially due to projections of deformed bars, and also micro-cracks i.e. local small cracks round the bars which have potential for further development in concrete. At a lap stress transfer between bars takes place through concrete between bars and that around them, subjecting shear in the concrete, this shear also give rise to fine cracks in concrete near lapped bars. Distribution or lateral (or transverse) steel can control further development of the micro-cracks. With increase in the diameter of bar being lapped the secondary stresses and micro-cracking also increases. Hence amount of distribution steel (in relation to the size of bar being lapped) to some extent controls the development or lap length. Even better control is obtained by confining the concrete in closely spaced lateral ties or spiral reinforcement. For laps in bars above 26 mm φ, it is necessary to provide spiral reinforcement round the laps. In general, for laps in bars up to 20 mm φ, the distribution, transverse or lateral steel specified in codes (except columns) is adequate. At laps in columns, lateral ties should not be more than

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150 mm c/c, and ≤100 mm for columns in frames requiring ductility.h) Importance of the situation of lap or anchorage also governs the length to be provided in practice for reliability.

Structural systems in which alternate path to take loads near ultimate (collapse) stage is limited, may suddenly fail at a certain critical sections due to failure of anchorage or lap (as brittle failure) in case of over loads or accidents. At such critical sections, liberal anchorage or lap lengths will improve the reliability near ultimate load and reduce the subsequent losses. Lap and anchor failure should not precede the yielding of bars. At the ultimate failure stage due to slip at anchorage or lap, the failure load would suddenly drops down to appreciably lower value compared to the ultimate loads. Because of this character it is a brittle failure. Hence a capacity reduction factor or a partial safety factor of higher order is relevant to anchorages and laps. For such members, liberal anchorage or lap lengths can provide enough reliability for ultimate load and ductility. As an example the laps and anchorage should be liberal in case of a cantilever members or hanger (ties or tension) or tension members and at junctions of members having high ductility demand.

R 8.2.11 Compression Lap :Force is transferred between bars in compression lap partly by bond, and by end-bearing of the bars on the

concrete. Both these effects exert bursting forces on the surrounding concrete. Ultimate strength in bond and in end bearing is dependent on the resistance available to counteract the bursting forces. The secondary reinforcement (lateral ties or hoop) within lap length influences the efficiency of lap at ultimate load and gives small ductility to the system. All compression bars can be allowed to be lapped at the same position (no staggering of laps) if the compression reinforcement is not more than 2% and provided with adequate lateral reinforcement. Member regions where ductility is in demand (under seismic) laps in bars are not permitted, as in case of columns of liquid towers.

In addition to lateral ties (/rings) as required by IS 456, more ties are needed in the end region of compression lap length. Place minimum three ties within 15% from each end of the lap length. Total number of these additional ties in the length of lap should be such that the steel area of extra links taken together (i.e. number of extra links × area of the bar of a ties) is equal to the area of a longitudinal bar being lapped (larger of the bar diameter). Diameter of the bar of tie (ring) may be increased up to 0.4 × diameter of longitudinal bar lapped. To avoid small spacing of the ties, those may be placed in pairs. The extra links suggested here does not take care of the kink (bend) in the lapped bar just beyond the lap length, for which more ties need to be provided in the region of kink. As per ductile detailing requirement, the spacing of lateral ties over the lap length shall not exceed 100 mm. Minimum lap length in compression should be 40φ + 150 mm.

If the grade of steel for lateral ties is less than the grade of longitudinal bars, amount of steel in ties (area of bar divided by spacing) required is to be increased in proportion to grade of main bars and ties, or refer specialized literature.

Laps may be kept almost at contraflexure in the column height (if >6× column size), which in most cases will be in mid height region between beams or braces. Laps shall not be given in column of small height or if contraflexure does not occur in the height.

R 8.3 JUNCTION (connections) : Junctions of members are designed to behave as monolithic and rigid. Rigid means rotational displacement of the junction should be limited, and within it the reinforcement should have minimum possible slip. For achieving the objective it needs to be analysed.

This applies to opening type right angled (‘L’) junctions of wall to wall, or wall to slab, and bending tension is on inside face. Note that the tension bars of a member shall be bent as hairpin and come out on other face for at least d/2, where d is effective depth. The fillet bars (at 45 to main tension bars) should have area of steel not be less than 40% of the average main tensile steel (in two members at the joint) requirement for strength or crackwidth whichever is more. See Fig 17.

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The two members meeting at a junction may not have same thickness, as well main steel also may not be same. Fillet bars can be omitted if main bars are 40% more than the requirement. The reinforcement design at the junction of walls shall consider the BM at the meeting of the centreline of two members, and not at face of junction.

Also the anchor length of tension bar should be enough from the centre of hairpin bend, where stress in steel is supposed to be maximum.

For design of tension steel at the junction, bending moment shall be taken at the meeting point of centre line of the members. This applies to junctions of wall to wall, and beam to outer column junction. In other cases the position of critical section for design is specified. {See R 4.4.2 (a), R 4.4.2.5, R 4.4.2.6, R 5.4, R 7 last para.}

At other positions for detailing rational models should be used. Truss analogy models are most useful.

R 8.3.1 At the inside face of wall at bottom end, rigid junction with floor slab subject to tension, bending and shear, in an elevated tank, in serviceability limit state steel stress shall be ≤ 0.50 fy if H/t is 20, to ≤ 0.42 fy MPa if H/t is ≥30. Linear interpretation can be done for any H/t >20. And the steel area shall not be less than 0.2% of the wall thickness for tanks >200 m³. Also see R 4.4.1.2.c & R 6.4.2.It should be noted that for H/t >20, the limiting crackwidth reduces and steel requirement shall increase very much.

R 8.4 Within ‘T’ or ‘L’ junction diagonal tension is produced. In these junctions the tension bars changes its direction by right angle. The tension reinforcement shall be well anchored beyond the position of maximum stress, which is nearly at middle of the curved portion of the bar. (See Fig 18, Fig 19A & 19B).

R 8.5 CONTAINER : Typical detailing for members of container are given in Fig 23, Fig 24A & 24B.

R 8.6 CURTAILMENT OF BARS : At the position of curtailment of bars, the crackwidth enhances depending up on the ratio of area of bars curtailed to total bars (including that continue). Hence bars should not be curtailed at or near construction joints, as well as at sections where crackwidth estimated are critical. Bars should be curtailed away from such critical sections. For many bars to be curtailed, curtailment position should be staggered.

R 8.7 Cover blocks shall not have corrodible material in it. These can in concrete of ≤8 mm maximum aggregate size, with water-binder ratio ≤0.4 and strength grade ≥M40. These should be made by casting in suitable moulds and compacting by vibration, pressed (under pressure) or hammered by wooden mallet, and cured in pond for >14 days. For adequate durability, uniformly the specified cover should be achieved in RCC at all places over the reinforcement. Plastic blocks shall not be used, as these do not have proper bond with concrete, and along its interface with concrete water or air penetration can be higher, which can enhance the risk of corrosion of bars.

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R. ANNEX AR A-1.2 Crackwidth check is required on a face of concrete member, hence the steel ratio ρcrit is based on the gross concrete area (i.e. no deduction of steel area, or effective cover) in the respective surface zone of member under consideration.

Below is a table for fct & critical steel ratio ρcrit, for steel grade 415 (= fy).fck M20 M25 M30 M35 M40 M45 M50 M55 M60fct 1.00 1.15 1.30 1.45 1.60 1.70 1.80 1.85 1.90

ρcrit 0.18 % 0.21 % 0.23 % 0.26 % 0.29 % 0.31 % 0.33 % 0.34 % 0.34 %Option 1 0.25 % 0.29 % 0.33 % 0.37 % 0.40 % 0.43 % 0.46 % 0.47 % 0.48 %Option 2 0.18 % 0.21 % 0.23 % 0.26 % 0.29 % 0.31 % 0.33 % 0.34 % 0.34 %Option 3 0.12 % 0.14 % 0.16 % 0.17 % 0.19 % 0.20 % 0.22 % 0.23 % 0.23 %

Options are as per Table 2 in IS 3370 part 1. Above table is to understand the trend.

R A-1.4 Estimated shrinkage εcs can be assumed as 300×10–6. In equation 2, ρ is steel ratio based on area of surface zone. [As per IS 456 the bond strength for deformed bar is 1.6 times that for plain bars. Hence, for deformed bars fct / fb ratio will be 0.625 in place of 2/3].

Equation 5 & 6 are for calculating crackwidth, as taken from BS 8007. BS code in figure A.3 gives locations where the calculated crackwidth can be further assumed to reduce due be reduction in actual restrain. Estimate of strains are for the case where concrete has restrains (e.g. ground tank restrained by foundation, & no sliding layer).

Corollary is that these calculations are not applicable for elevated tank having very low restrain.

R. ANNEX B R B-1 The method of estimation of crackwidth is based on the assumption that the strain level in steel and concrete are not high enough. Crackwidth check is a serviceability limit state. For the for serviceability limit state, the tensile stress in reinforcement should be limited to 0.58 fy (240 N/mm² for 415 grade), and concrete stress limited to 0.40 fck (12 N/mm² for M30 grade). At higher stress level, the calculated crackwidth estimate may deviate.

Code gives the guideline for crack due to flexure, and separately for crack due to direct tension. However in most cases members are subjected to combined bending moment and direct tension. While neutral axis is within the section (i.e. one face of member is in tension & other in compression), the calculation can be done as for the case of flexural. If the both the faces of a member are in tension (i.e. NA outside section or depth of neutral axis <50 mm), the calculation can be done as for the case of direct tension. (NA = neutral axis)

The code does dealing with crackwidth in monolithic section and silent for construction joint (or a partial movement joint). Compared to a section where member is monolithic, the section at a construction joint will develop a little larger crackwidth under similar conditions otherwise. Refer R 4.4.3.5.

R B-3 For calculating tension stiffening based on crackwidth, the multiplying constants are as follows: For crackwidth 0.2 mm – 1.0 ; 0.15 mm – 1.2 ; for 0.10 mm – 1.5 ; for the case of flexure.

Tension stiffening dealt by equation 8 & 9 are for deformed reinforcement (uncoated). For other bars the tension stiffening strain be multiplied by following factors: Fusion bonded epoxy coated bars 0.80 , plain bars 0.625 , plain coated bars 0.50 .

R B-4 For members in direct tension, the tensile stress in reinforcement should be limited to 0.50 fy (i.e. 208 N/mm² for 415 grade). Limiting crackwidth is specified in clause 4.4.1.2 b & 4.4.3.2 of IS 3370 part 2.

R B-6 For calculating tension stiffening based on crackwidth, the multiplying constants are as follows: For crackwidth 0.2 mm – 2 ; 0.15 mm – 2.4 ; for 0.10 mm – 3.0 , (replacing constant 2.) for direct tension. For 0.15 mm crackwidth equation will become – ε2 = 0.8 bt D / (Es As )R B-6.1 Where limiting crackwidth required is 0.1 mm or less, it is also desirable to keep the tension in concrete ≤ 0.27 fck2/3 = fctk0.05

Concrete grade M20 M25 M30 M35 M40fctk0.05 1.99 2.31 2.61 2.89 3.16

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Annex C: Concrete Finishes [Not a part of the standard]

C.1 Formed SurfacesSurface finish Type F1

The main requirement is that of dense well compacted concrete. No treatment is required except repair of defective areas, filling all form tie holes and cleaning up of loose or adhering debris. For surfaces below grade which will receive waterproofing treatment the concrete shall be free of surface irregularities which would interfere with proper and effective application of waterproofing material specified for use.Surface Finish Type F2

The appearance shall be that of a smooth, dense, well- compacted concrete, may show the slight marks of well fitted shuttering joints. The Contractor shall make good any blemishes.Surface Finish Type F3

This finish shall give an appearance of smooth, in plane, dense, well-compacted concrete, with shutter marks ordinarily not visible, stain free and with no discoloration, blemishes, arises, air holes etc. Only lined or coated plywood with very tight joints shall be used to achieve this finish. The panel size shall be uniform and as large as practicable. Any minor blemishes that might occur shall be made good by the Contractor.

C.2 Unformed Surfaces Finishes to unformed surfaces of concrete shall be classified as U1, U2, and U3, ‘spaded or bonded concrete’.

Where the class of finish is not specified the concrete shall be finished to Class U1.Class U1 finish

A spaded finish shall be a surface free from voids and brought to a reasonably uniform appearance by the use of shovels as it is placed in the Works.

It is the first stage for Class U2 and U3 finishes and for a bonded concrete surface. Class U1 finish shall be a screeded and levelled, uniform plain or ridged finish which (unless it is being converted to Class U2, U3, or bonded concrete) shall not be disturbed in any way after the initial set and during the period of curing, surplus concrete being struck off immediately after compaction.

Where a bonded concrete is specified over its surface, the laitance shall be removed from the Class U1 finished surface and the aggregate exposed while the concrete is still green.Class U2 finish

It shall be a wood float finish. Floating shall be done after the initial set of the concrete has taken place and the surface has hardened sufficiently to allow the floating operation. The concrete shall be worked no more than is necessary to produce a uniform surface free from screed marks.Class U3 finish

It shall be a hard smooth steel-trowel led finish. Trowelling shall not commence until the moisture film has disappeared and the concrete has hardened sufficiently to prevent excess laitance from being worked into the surface. The surfaces shall be trowelled under firm pressure and left free from trowel marks.

The addition of dry cement, mortar or water shall not be permitted during any of the above operations.

C.3 TOLERANCES IN CONCRETE SURFACES Concrete surfaces for the various classes of unformed and formed finishes specified in various clauses shall comply with the tolerances shown in Table 1 hereunder, except where different tolerances are expressly required by the Specification or shown on the Drawings. In Table 1 ‘line and level’ and ‘dimension’ shall mean the lines, levels and cross-sectional dimensions shown on the Drawings. Surface irregularities shall be classified as ‘abrupt ‘or ‘gradual’. Abrupt irregularities include, but shall not be limited to; off-sets and fins caused by displaced or misplaced formwork, loose knots and other defects in formwork materials, and shall be tested by direct measurement. Gradual irregularities shall be tested by means of a straight template for plane surfaces or its suitable equivalent for curved surfaces, the template being 3 m long for unformed surfaces and 1.5 m long for formed surfaces.

Table 1 Maximum tolerance (mm) in:Class of Finish Line and level Abrupt

irregularityGradual

irregularityDimension

U1 ± 12 ± 6 ± 6 -U2 ± 6 ± 3 ± 3 -U3 ± 5 ± 2 ± 2 -F1 ± 12 ± 5 ± 6 +12, -6F2 ± 6 ± 3 ± 4 +12, -6F3 ± 3 ± 1 ± 2 +6, -2

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C.4 RIGIDITY OF FORM WORK : (Rigid formwork are called moulds.)RF1 : Highly rigid mould (or forms) are of very high rigidity as required for casting concrete samples for testing, say mould for cube or beam. Under the pressure of concrete while it is vibrated in mould the surface deflection should not be more than 0.2 mm. Tolerance for dimensions of mould should be within ± 0.2 mm.RF2 : Very rigid form as are used in precast factory. Under the pressure of concrete while it is vibrated in mould the surface deflection should not be more than 0.5 mm. Tolerance for dimensions of mould should be within ± 0.5 mm or smaller if in the specification.RF3 : Rigid form as are used for precast products cast on site but sensitive to tolerance. Under the pressure of concrete while it is vibrated in mould the surface deflection should not be more than 1 mm. Tolerance for dimensions of formwork should be within ± 1 mm or smaller if in the specification.RF4 : Normal formworks as are used for in-situ works. Under the pressure of concrete while it is vibrated in mould the surface deflection should not be more than 3 mm. Tolerance for dimensions of mould should be within ± 3 mm or smaller if in the specification.

It is proposed that this document will be upgraded from time to time. Hence send your comments to -

Er. L. K. JAIN, Consulting Engineer,36 Old Sneh Nagar, Wardha Road,NAGPUR 440 015, IndiaEmail : lkjain.ngp@gmail.comPhone +91 712 228 4037 , M +91 9423101453

Other documents by the same author :

1. Guide on Construction of Concrete Structures for Retaining Aqueous Liquid.Free down load from following link –https://drive.google.com/file/d/1bJzD3oUxpAAAjOaqxhK2m2M8Ysm-zpfv/view?usp=sharing

2. Guide on Design & Construction of RCC Elevated Water Tanks