Durability of concrete

Quality Control in Concrete and Durability factors : An overview

by

byRAJESH PRASAD, CPM/M, RVNL. KOLKATA

INTRODUCTION

• Concrete the man made rock is the most widely used construction material.

• More than 80,000 crores cum of concrete are poured every year.

• India has an installed capacity of 200 million tonnes of cement.

• Some Structures are designed with life of 100 years.

So it is necessary that the concrete used is durable.

QualityQuality

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Definition : Conformance to requirements – expressed

& implied.

Objective : Total customer (both internal & external)

satisfaction.

Performance standard : ZERO DEFECTS.

System of causing QualitySystem of causing Quality

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DIRFT (DO IT RIGHT THE FIRST TIME)

Process oriented & Not Inspection oriented approach.

Prevention and not appraisal.

MBWA and not MBTA (Management by Wandering

Around and not Management by Talking Around.)

KAIZEN – slow but continuous improvement.

QualityQuality

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Measure : The measurement of quality is the “Price of

non-conformance”.

Responsible for non- quality : Seniors are the sinners.

QualityQuality

DurabilityDurabilityDefinition –

• A durable concrete is one that performs satisfactorily in the working environment during its anticipated exposure conditions during service (IS 456-2000)

• Durability of concrete is its ability to resist weathering action, chemical attack, abrasion or any other process of deterioration (American Concrete Institute).

• When exposed to environment durable concrete is likely to retain its original form, quality and

serviceability during its lifetime.• Durable Concrete envisage limits for maximum water

cement ratio, minimum cement content, cover thickness, type of cement used and presence of amount of chloride and sulphates in concrete. ( IS-SP-28)

• As Low permeability as possible under situation. (IS-SP-23)6

Consequences of Improper Quality and Consequences of Improper Quality and Inadequate DurabilityInadequate Durability

• Loss of strength of concrete• Concrete liable to be easily affected by deterrents• Corrosion of rebars • Loss of serviceability• Unpleasant appearance• Danger to persons and property• Expensive repair costs• Poor perception of concrete as a material• Poor perception of agencies involved• Reduction of service life• External agencies like weathering, attack by natural or Industrial

liquids, Gases, bacterial Growth etc.• Alkali- aggregate reaction.• Ingression of moisture/air facilitating corrosion of steel and cracking

concrete cover.7

Consequences of Consequences of Inadequate DurabilityInadequate Durability

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Factors Influencing Durability of Factors Influencing Durability of Concrete (IS: 456-2000)Concrete (IS: 456-2000)

• The Environment • Type and quality of constituent materials• Cement content and W/C ratio of concrete• Workmanship especially in compaction curing – it is

very important• Cover to embedded steel• Shape and size of the member

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Factors Affecting DurabilityFactors Affecting DurabilityDurability depends on two main factorsa) The concrete system &b) The service environment

a) Concrete system is based on • Quality and quantity of materials used and • Processes involved in manufacture of concrete.

b) Service environment affects concrete by way of • Physical actions and• Chemical actions on concrete.

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Factors Affecting DurabilityFactors Affecting Durability

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DURABILITY

The Concrete System Aggressiveness of the Environment

Materials Process Physical Chemical

• Binder type• Binder content• Aggregates• Admixture• Mix design

• Mixing • Transporting• Compaction• Curing• Temperature• workmanship

• Abrasion• Erosion• Cavitation• Freeze-thaw

• Dissolution• Leaching• Expansion• Alteration

MATERIALSMATERIALS

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Selection of Selection of Good Quality MaterialsGood Quality Materials

(Conforming to relevant IS codes)(Conforming to relevant IS codes)

• Cement (from Reputed Manufacturers)

• Sand (River / Crushed, Silt < 5%)

• Aggregates (Cubical in shape, Innocuous)

• Water (Tested) with PH value ranging 6to 8

• Admixture (From Reputed Manufacturer)

• Compatibility of cement and plasticizer (PC based or Naphtha

based) and 3rd generation superplasticiters of Polycarboxylates

base, Polyacrylates based or Monovinyl alcohols based 15

UNSOUND MATERIALSUNSOUND MATERIALS

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UNSOUND MATERIALSUNSOUND MATERIALS

• Cement or aggregate is considered unsound when they cause unacceptable volume change, hardened concrete or mortar which causes cracks and affects durability.

• Aggregates containing certain materials such as shale, clay lumps, coal, iron pyrites etc show unsoundness later when concrete undergoes wetting and drying or freezing and thawing.

• More moisture absorption in aggregate (CA1 or CA11) is often used as a rough index for unsoundness

• Cement parameters that impact soundness are – free lime, MgO & excess gypsum

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Sr. No.

Tested as per Permissible Limit, Max

i) Organic IS 3025 (Part 18) 200 mg/l

ii) Inorganic IS 3025 (Part 18) 3000 mg/l

iii) Sulphates (as SO3) IS 3025 (Part 24) 400 mg/l

iv) Chlorides (as CI) IS 3025 (Part 32) 2000 mg/l for concrete not containing embedded steel and 500 mg/l for reinforced concrete work

v) Suspended matter IS 3025 (Part 17) 2000 mg/l

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Water - Permissible Limits for SolidsWater - Permissible Limits for Solids. IS 456:2000 Table-1IS 456:2000 Table-1

Impact of W/C RatioImpact of W/C RatioOn DurabilityOn Durability

• Permeability is the contributory factor for volume change and higher W/C ratio is the fundamental cause of higher permeability.

• Use of higher W/C ratio – permeability – volume change – cracks – disintegration – failure of concrete is a cyclic process in concrete.

• For a durable concrete, use of lowest possible W/C ratio is the fundamental requirement to produce dense and impermeable concrete.

• Modern superplasticizers of Polymer base are so efficient that it is now possible to make flowing concrete with a W/C as low as 0.31 or even as low as 0.29 with increased slump more than 250mm.

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Permeability Vs W/C GraphPermeability Vs W/C Graph

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PROCESSESPROCESSES

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Workmanship for Workmanship for Durable ConcreteDurable Concrete

Batching

Mixing

Transportation

Placing

Compaction

Finishing

Protection

Curing22

Compaction of ConcreteCompaction of Concrete

• Ensuring suitable workability employing appropriate placing and compaction equipment

• Adequate compaction without segregation• 1% voids reduces strength by 5%

Criticality of CuringCriticality of Curing• Extremely important if the water-cement ratio

is low & cement content is high.

• If the cement has a high rate of strength development

• Ambient temperature during placing is high and day is windy

• If the concrete contains GGBFS or Pulverized Fly ash (PFA)

Recommended Moist Curing Recommended Moist Curing (IS 456: 2000)(IS 456: 2000)

• Normal Weather Conditions

• Harsh Weather Conditions (Hot and Dry)

* OPC Based Concretes - 7 Days* Blended Cements based concretes - 10 Days

* OPC Based Concretes - 10 Days* Blended Cements based concretes - 14 Days

Cracks on concrete surface Cracks on concrete surface due to inadequate curingdue to inadequate curing

ENVIRONMENTENVIRONMENT

Environment can be classified asEnvironment can be classified as

a) Physical – Temperature, Moisture, alternate wetting and drying, freezing and thawing

b) Chemical – Acidic, gaseous, alkaline, corrosive

PHYSICAL ENVIRONMENTS PHYSICAL ENVIRONMENTS ATTACK AFFECTING DURABILITYATTACK AFFECTING DURABILITY

Physical Environment impactsPhysical Environment impactsTemperature

Significantly affects rate of hydration of cement. Leads to Plastic shrinkage cracks in fresh concrete Volume changes and cracking especially in mass

concrete Spalling and disintegration of concrete at higher

temperatures > 250ºC (3000c as per A.M. Neville P-387)

Variation in ambient temperature causes secondary stresses in structures

Physical Environment impactsPhysical Environment impactsMoisture

Shrinkage on drying, consequent volume change and cracking

Induces corrosion of steel Acts as carrier of chemicals inside the body of concrete Causes efflorescence and deposition of Ca(Cao+2H2 0=

Ca (OH)2 +H2 0 OH)2 on surface Seepage / Leakages cause inconvenience to occupants

and deteriorates structures due to permeable concrete.

Alternate wetting and drying Causes secondary stresses in the structures Accelerates corrosion of steel and Chemical attack on concrete

Freezing and Thawing Leads to expansion of concrete and cracking Ice-melting salts cause erosion of concrete

Physical Environment impactsPhysical Environment impacts

CHEMICAL ENVIRONMENT CHEMICAL ENVIRONMENT ATTACK AFFECTING DURABILITYATTACK AFFECTING DURABILITY

Chemical ActionChemical ActionWhen we are dealing with durability, chemical attack which results in volume change, cracking and consequent deterioration of concrete become a major cause of concernTypes of Chemical attack•Sulphate attack •Alkali aggregate reaction•Chloride ion attack - Corrosion•Carbonation•Acid Attack•Effect on concrete in Seawater

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1. Sulphate attack1. Sulphate attack• Sulphate attack denotes an increase in the volume of

cement paste in concrete or mortar due to chemical action between the products of hydration of cement and solution containing sulphate, and also sodium, magnesium and Cholorides.

• In hardened concrete, calcium aluminate hydrate (C-A-H) can react with sulphate salt from outside, product of reaction is calcium sulphoaluminate, which can cause an increase in volume up to 227%

• Rate of sulphate attack increases with a saturated sulphate solution.

• A saturate solution of magnesium sulphate can cause serious damage to concrete with high w/c ratio.

Sulphate attackSulphate attack

Methods of controlling sulphate attack•Use SRC (sulphate resisting cement)•Quality concrete - low w/c ratio, well designed and compacted dense concrete•Use of air-entrainment•Use of puzzolana•High pressure steam curing•Use of high alumina cement

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2. Alkali - Aggregate Reaction2. Alkali - Aggregate Reaction• Alkali from cement, reacts with reactive silica (of

aggregates) to form alkali-silica gel of unlimited swelling type.

• The continuous growth of silica gel exerts osmotic pressure within the concrete.

• This manifests into cracking and bulging of concrete

Occurrence is due to :1. High alkali content in

cement (more than 0.6%)2. Reactive silica in

aggregate3. Availability of moisture

Alkali - Aggregate ReactionAlkali - Aggregate Reaction

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• Alkali-aggregate reaction (AAR) is basically a chemical reaction between the hydroxyl ions in the pore water within concrete and certain types of rock minerals

• Since reactive silica in the aggregate is involved in this chemical reaction it is often called alkali-silica- reaction (ASR).

• It is recognized as one of the major causes of cracking of concrete.

• The reaction produces what is called alkali-silica gel of unlimited swelling type under favorable conditions of moisture and temperature.

• The crack width can range from 0.1mm to as much as 10mm.


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• Alkali content (K2O and Na2O) or what is called soda equivalent.

• This is calculated as the actual Na2O content plus 0.658 times the K2O content of the clinker.

• It should be less than 0.6 percent by mass of cement.

• Alkalis from all these sources must be included in finding the total alkalis.

• British standard 5328 : part 1 : 1091 specifies a maximum of 3.0 kg of alkalis (expressed as soda equivalent) in 1 m3 of concrete in case of alkali reactive aggregates are used.


4. Use of pozzolana, slag or silica fume

5. Control on service condition, limiting degree of saturation of concrete

Remedial Measures:1. Use non-reactive

aggregates from alternate sources

2. Use low-alkali cement3. Reduce cement content in

concrete

3. Chlorides in Concrete3. Chlorides in Concrete

• Chlorides in concrete increases risk of corrosion of steel (Electrochemical reaction) (IS 456:2000)

• Higher Chloride content or exposure to warm moist conditions increase the risk of corrosion

• To minimize the chances of corrosion, the levels of chlorides in concrete should be limited

• Total amount of chloride content (as Cl) in concrete at the time of placing is provided by IS 456:2000

Methods of Controlling Methods of Controlling ChloridesChlorides

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• Chlorides in cement to be less than 0.1 % max ( or 0.05% max for prestressed works)

• Chlorides in water to be less than 2000 mg/ltr for PCC and below 500 mg/ltr for RCC

• Chlorides in aggregates are generally not encountered but, it’s a good practice to wash sand containing salt more than 3%

• Chloride traces are also found in chemical admixtures. Chloride free admixtures should be generally preferred.

CorrosionCorrosionCorrosion is an electro-chemical process Basic Mechanism :Different areas of the same steel bar become anode and cathode.

The electrical connection being maintained by pore water, which acts as electrolyte

At the anode, steel releases electron and takes the ionic form Fe → 2e- + Fe2+

At the cathode, water in presence of oxygen and the released electron forms hydroxyl ions (OH)-

2O2 + H2O + 2e- → 2(OH)-

Fe++ + 2(OH)- → Fe(OH)2 → Iron Hydroxide (Rust)2H2O+O24e-=4 (OH-)

Example of Delamination Example of Delamination of Concrete coverof Concrete cover

CorrosionCorrosion

Crack formed due to bursting pressure on Crack formed due to bursting pressure on account of rusting of reinforcementsaccount of rusting of reinforcements

Methods of Methods of Controlling CorrosionControlling Corrosion

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• Limit the chlorides in water, cement, superplastizers, etc to acceptable levels

• Provide proper cover as per IS norms• Cover blocks also need to be of good quality• Concrete should be properly compacted• Make dense, impermeable / waterproof concrete• Have protective coatings wherever suitable• Ensure proper & timely maintenance of structures

4. Carbonation of Concrete4. Carbonation of Concrete

• Mechanism : Carbon dioxide from the air reacts with calcium hydroxide to form calcium carbonates

• In the presence of moisture, carbonic acid is formed which reduces the alkalinity of concrete

• pH value of concrete reduces from 12.5 to 9, thus destroying the protective layer and exposing the steel to corrosion

• Rate of Carbonation depends upon relative humidity, grade of concrete, permeability of concrete, depth of cover and time

• Nearly 1 mm carbonation is reported per year in normal M-20 grade of concrete

Measurement of Measurement of Depth of CarbonationDepth of Carbonation

Pink color indicates that Ca(OH)2 is unaffected by carbonation. The uncolored portion indicates that concrete is carbonated

CarbonationCarbonation

• Carbonation of concrete is a process by which carbon dioxide from the air penetrates into concrete and reacts with calcium hydroxide to form calcium carbonates.

• In actual practice, CO2 present in atmosphere permeates into concrete and carbonates the concrete and reduces the alkalinity of concrete.

• When all the Ca(OH)2 has become carbonated, the pH value will reduce upto about 8.3. In such a low pH value, the protective layer gets destroyed and the steel is exposed to corrosion.

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CarbonationCarbonation

• The highest rate of carbonation occurs at a relative humidity between 50 and 70 percent.

• Protective coating is required to be given for long span bridge girders, flyovers, Industrial structures and chimneys. Such as plastic paints (Impermeable)

• Deep cover plays an important role in protecting the steel from carbonation.

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Steel in Passivity conditionSteel in Passivity condition

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• Concrete is under continuous attack by aggressive environmental agencies.

• Good concrete and sufficient cover is the answer for durability

Steel in passivity condition

5. Acid attack 5. Acid attack

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• Concrete is not fully resistant to acids depending upon the type and concentration of acid.

• Oxalic acid and phosphoric acids are harmless.

• The most vulnerable part of the cement hydrate Is Ca(OH)2, but C-S-H gel can also be attacked.

• Concrete can be attacked by liquids with pH value less than 6.5.

Acid attack Acid attack

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• But the attack is severe only at a pH value below 5.5.

• At a pH value below 4.5, the attack is very severe.

• Cement compounds are eventually broken down and leached away.

• If acids are able to reach the reinforcing steel through cracks corrosion can occur leading to further cracking

COVER TO REINFORCEMENTCOVER TO REINFORCEMENT

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Cover to ReinforcementCover to Reinforcement(IS 456 – 2000)(IS 456 – 2000)

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Exposure Condition

Min Nominal cover (mm)

Mild 20Moderate 30

Severe 45Very Severe 50

Extreme 75Note :1)For main reinforcement upto 12 mm dia bar for mild exposures, the nominal cover may be reduced by 5 mm2)Unless specified otherwise, actual concrete cover should not deviate from the required nominal cover by +10mm or 0mm

Effect of Other materialsEffect of Other materials

• Mineral Oil – usually effects only fresh concrete in their hardening process (petrol, petroleum distillates etc)

• Organic Acid – have corrosive effect

• Vegetable & Animal oils & Fats – causes deterioration of concrete surfaces due to their corrosive action

• Action of Sugar – has retarding effect on fresh concrete; has gradual corrosive effect on hardened concrete

• Action of Sewage – concrete sewers running full remain unaffected; but in partially filled sewers where hydrogen sulphide gas is evolved & sulphuric acid is formed, concrete above sewage level gets affected due to corrosive action of such acids

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Fire ResistanceFire Resistance

• The fire resistance is not only dependent upon the type of concrete but also on the thickness of cover to reinforcement.

• Effect of temperature on the strength of concrete is not much upto a temperature of about 250oC

• Above 300oC definite loss of strength takes place.• Portland blast furnance slag cement is found to be

more resistant to the action of fire.• Light weight concrete stand up better to fire than

ordinary concrete.

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Fire ResistanceFire Resistance

• Best fire resistant aggregates amongst the igneous rocks are, the basalts and dolerites less amount of silica.

• Since then decomposition takes place only at a very high temperature of 900oC , Limestone is considered as a good fire resistant aggregate.

• Serious reduction in strength occurs at a temperature of about 600oC.

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Constraints/Issues for Concrete in Constraints/Issues for Concrete in Aggressive EnvironmentAggressive Environment

• Contaminants in soil/water like sulphates, chlorides, effluents, etc. highly affect the durability of concrete (limitation of SRC)

• Heavy reduction in the headroom space due to large depths of the beam

• Limitation of OPC leading to weak micro -pore structure

• Large dimension of columns in particular, leads to space constraints.

Why is Quality necessary?• Intense Competitive Environment

• Entry of Foreign Goods and Services

• Increased Customer Consciousness

• Earning Profit instead of making profit

• Organization’s survival / progress depends on it

Safety vs. Operation

Quality vs. Progress

In our day to day work COM gets a lot of importance…….Safety comes after Operation.

Similarly Quality comes after Progress..

I am CSO I am COM

Both are PHODs/CHODs…

With different duties

and responsibilities

Operation vs safety

Good Physique

Regular Check ups

Preventive Measures & Medicines

Proper & Timely Cure & Restoration

Good Construction

Regular Inspections

Preventive Measures & Materials

Proper & Timely Repairs & Restoration

This ROB constructed 14years back, was partly closed in 2007…

Distress in Concrete Slab at Bagha jatin ROB

GHPR Swimming Pool

Poor Quality & incomplete work by RITES as like this.

After construction & commissioning

Construction Process Analysis

What we think it is

What it really is ( Hidden )

What it should be

What it could be

5 Gaps in Service Quality

• Gap 1: Consumer Expectation- Management Perception

• Gap 2: Management Perception-SQ Specification

• Gap 3: SQ Specification- Service Delivery

• Gap 4: Service Delivery- Perceived Service

• Gap 5: Perceived Service- Consumer Expectation

5-S……….Seiri - Remove unnecessary items

Seiton - Keep necessary things properly

Seiso - Clean workplace thoroughly

Seiketsu - Maintain high standards regularly

Shitsuke - Maintain self-discipline

• Quality work does not mean the work to stop or standstill…

Give wider publicity about Quality and consequences of poor quality…….

Quality consciousness

This is a quality project…..

Never disrespect contractors

Listen to them but ensure quality

I can straightway say there is a huge mistake… I don’t know

whether it’s human error or an error in our judgment but we did not follow QAP properly.

QualityAudit

Durability of Structures…

Blacklisted Contractors

Poor quality is always exposed at sometime.

RVNL OFFICE

RVNL PIU

Are You Lonely and Worried for quality and durability of Concrete??

Don’t like working on your own?Hate making decisions?

Then call A MEETING!!

You can…

SEE people DRAW flowchats FEEL important IMPRESS your colleagues

And must discuss on quality related issues for Project Implementation.

MEETINGSThe practical alternative to work.

Concluding RemarksConcluding RemarksStrength and durability are two important properties of concrete.Concrete is a long lasting material provided adequate care is taken in

Selection of ingredients - Right type of materials & right concrete for appropriate end applications

Mix design, batching, mixing, transporting, placing, compaction, finishing, protection & curing

Reinforcement detailing and adequate cover Good quality form work Maintenance and upkeep

Concrete is a dependable construction material and it should not be abused to the limits….

It should be used with understanding, love and care.

Quality of Food is important for him. Quality of Construction is important for us.

It was a presentation byRajesh Prasad

It was a presentation byRajesh Prasad

Durability of concrete

Education

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