SPECIALCONCRETES[PART 03]

Fahim Al-Neshawy & Esko Sistonen20.11.2015

Concretetypes

HPC &HSC

SCC

Fast dryingconcrete

Fiberreinforcedconcrete

Lightweightconcrete

ColoredconcretePolymer

concrete

Massconcrete

Self-Healing

concrete

Shotcrete

Underwaterconcrete

Normalconcrete

MASS CONCRETE

DefinitionMass concrete – is defined as

“Any volume of concrete with dimensions

large enough to require that measures be

taken to cope with (resist) the generation of

heat from hydration of cement and attendant

volume change to minimize cracking.”

Why study of mass concrete?

Why study of mass concrete?

generation of heat from hydration of cement

Why study of mass concrete?

• When the temperature in the core of the concrete increases, the surface ofconcrete is cooled in comparison with the core, due to thermal expansion

• The respective volume changes in the concrete causes compressive forces todevelop in the core, and tension forces to develop at the surface

Factors affecting temperature rise1. Geometry

− Pours (cast) with a large volume: surface arearatio are more susceptible to thermal cracking!

− Research has been performed to produce moreaccurate dimensions for when temperature riseshould be considered.

2. Cement Composition− Cements used for Mass Concrete should have a

low C3S and C3A content to reduce excessiveheat during hydration.

− Most Mass Concrete structure do not require earlystrength, so slower hydration is usually notharmful to construction.

Factors affecting temperature rise3. Cement Fineness

− Cement with a lower fineness with slow hydration,and reduce temperature rise.

4. Cement Content− Mass Concrete mixtures should contain as low of

a cement content as possible to achieve thedesired strength.

− This lowers the heat of hydration and subsequenttemperature rise.

− Can be as low as 100 kg/m³

Factors affecting temperature rise5. Aggregate Content

− Coarse Aggregate should be have an MSA (max.size of agg.) of 15 cm if possible.

− A higher coarse aggregate content (70-85%) canbe used to lower the cement content, reducingtemperature rise.

Factors affecting temperature rise5. Coarse Aggregate Coefficient of Thermal Expansion

− The CTE of the coarse aggregate is the maininfluence on the CTE of the concrete.

− Choosing an aggregate with a low CTE can cutthermal stresses in half.

Factors affecting temperature rise6. Supplementary Cementitious Materials (SCMs)

− SCMs such as Fly Ash, and Slag can greatly reduce the heat ofhydration.

− Pozzolans such as FA (class F is best > slower hydration) andSlag will produce between 15-50% of the heat of normalPortland Cement.

− Highly reactive SCMs such as Silica Fume and Metakaolin donot substantially lower the heat of hydration.

Factors affecting temperature rise6. Supplementary Cementitious Materials (SCMs)

− Use of SCMs such as FA and GGBFS is commonly up to 60-75% cement replacement.

Factors affecting temperature rise7. Placement Temperature

− Casting at lower temperatures will reduce the thermal stresses;o Slows hydration > lowers heat of hydrationo Lowers temperature differential between the core and the

outer surface

Factors affecting temperature rise8. Water cement ratio

− W/C has a large effect on temperature rise− W/C = 0.25-0.4 is common

9. Workabilityo Most Mass Concrete mixtures have a ~0-60 mm slumpo Water reducer WRs or Superplasticizers may have to be

used to retain workability

Materials and Mix Proportions• The heat of hydration of a cement is a function

of its compound composition and fineness− cement contents as low as 100 kg/m3

o substitution of 20 percent pozzolan− 4 to 8% entrained air− water-reducing admixtures− the extent of reduction in water content by

the use of entrained air and the largestpossible size of aggregateo At a given water-cement ratio and consistency,

as the maximum aggregate size is increased,both the water and the cement contents arereduced.

Applications

SELF-HEALING

CONCRETE

Self healing (bacterial) concrete• What is S.H.B.C?

− A new concrete technology that autonomously repairscracks.

• How does it work?− Alkaliphilic bacteria added to concrete matrix.− Bacteria react to the water and metabolize crystals, which

close the crack and protect the steel within.

Self healing bacterial concrete

Bacterias used

Bacillus cohnii Bacillus filla Bacillus parturii

• Cement and water have a pH value of up to 13 when mixed together, usually ahostile environment for life, most organisms die in an environment with a pHvalue of 10 or above.

• Microbes that survive in alkaline environments can be found in naturalenvironments, for example:o alkali lakes in Russia,o carbonate-rich soils in desert areas of Spaino soda lakes in Egypt.

Preperation of bacterial concreteSelf healing bacterial concrete can be prepared in twoways.

− By direct application− By encapsulation in light weight concrete.

• By the method of direct application− bacterial spores and calcium lactate are added

directly while making the concrete and mixed.− Here when the crack occurs in the concrete

bacterial spores broke and bacteria comes to lifeand feed on the calcium lactate and limestone isproduced which fill the cracks.

Preperation of bacterial concrete• By encapsulation method

− the bacteria and its food, calcium lactate, are placed insidetreated clay pellets and concrete is made

− About 6% of the clay pellets are added for making bacterialconcrete

− When concrete structures are made with bacterial concrete,when the crack occurs in the structure and clay pellets arebroken and bacterial treatment occurs and hence the concreteis healed.

− Minor cracks about 0.5mm width can be treated by usingbacterial concrete

• Among theses two methods encapsulation method iscommonly used, even though it’s costlier than directapplication. Bacillus bacteria are harmless to human lifeand hence it can be used effectively.

2. The water is deliberately forced into the crack and theprecursor is activated

What is happening inside bio concrete(mechanism)

1. The cracks are formed on the surface of concrete dueto many reasons like shrinkage, Inadequate water forhydration …etc.

Ca(C3H5O2)2 + 7O2 → CaCO3 + 5CO2 + 5H2O

CO2 + Ca(OH)2 → CaCO3 + H2O(carbonation)

3. The activated precursor intern induces the bacteria to reactwith that precursor and form a base of calcium carbonatecalled as limestone, the chemical equation is given above.

What is happening inside bio concrete(mechanism)

BEFORE HEALING AFTER HRALING

20x

40x

Advantages• The self healing bacterial concrete helps in reduced

maintenance and repair costs of steel reinforced concretestructures.

• Oxygen is an agent that can induce corrosion, as bacteriafeeds on oxygen tendency for the corrosion ofreinforcement can be reduced.

• Self healing bacteria can be used in places where humansfind it difficult to reach for the maintenance of thestructures.− Hence it reduces risking of human life in dangerous

areas and also increases the durability of the structure.• Formation of crack will be healed in the initial stage itself

thereby increasing the service life of the structure thanexpected life.

Disadvantages• If the volume of self healing agents (bacteria

and calcium lactate) mixed becomes greaterthan 20%, the strength of the concrete isreduced.

• Preparation of self healing concrete needsbacteria and calcium lactate.− Preparation of calcium lactate from milk is

costlier.− Hence preparation of self healing concrete

costs double than conventional concrete.

ApplicationsSelf healing bacterial concrete can be used forsectors such as

− tunnel-lining,− structural basement walls,− highway bridges,− concrete floors− marine structures.

Tunnel liningMarine structure

Applications

Concrete flooring

Highway bridge

Undergroundretaining walls

SHOTCRETE

• A concrete placing processwhere concrete mixtures areconveyed through a hosethen - with the help ofpressure - projected at highvelocity onto a surface toachieve high quality in-placecompaction.

• It produces high qualitydense concrete, with a loww/cm ratio, low permeability,and a high cementitiousmaterial content.

Ø Shotcrete is today a term that describes sprayingconcrete or mortar with either a dry or wet mixprocess.

Ø Gunite is a trademarked name that is incorrectlyused to describe the dry-mix shotcrete process

Ø Shotcrete emerged as the only acceptableindustry term to correctly describe "pneumaticallyapplied concrete“.

+

Shotcrete, high performance productconsisting of …

aggregates waterCement admixture

non-alkaline accelerator

+ + +

Øwas invented in the early1900s by Americantaxidermist Carl Akeley.

Ø used to fill plaster modelsof animals.

Ø In 1911, he was granted apatent.

Ø Until the 1950’s, the wet-mix process was devised,only the dry-mix processwas used.

Ø Sprayed concrete isreinforced by conventionalsteel rods, steel mesh,and/or fibers.

Ø Fiber reinforcement (steelor synthetic) is also usedfor stabilization inapplications such as slopesor tunneling.

Ø conventional concrete is first placed and thencompacted in the second operation.

Ø shotcrete undergoes placement and compaction at thesame time.

Ø Shotcrete is more dense, homogeneous, strong, andwaterproof .

Ø It can be impacted onto any type or shape of surface,including vertical or overhead areas

1. Dry process 2. Wet process1. Dry process:Ø Step1: Pre blended, dry or semi-dampened

materials are placed into shotcrete equipment andmetered into a hose.

Ø Step2: Compressed air conveys materials at highvelocity to the nozzle where the water is added.

Ø Step3: Then the material is consolidated onreceiving surface by high impact velocity.

Advantages of Dry process:Ø Easy start up, shutdown and clean up.Ø Control of materials is on site.Ø Nozzle can be up to:

• 300 m horizontally or• 150 mm vertically from the gun.

Wet process:Ø Step 1: All ingredients, including water, are

thoroughly mixed and introduced into the shotcreteequipment.

Ø Step 2: Wet material is pumped to the nozzlewhere compressed air is introduced

Ø Step 3: Mostly wet-process shotcreting is donewith premixed mortar or small aggregateconcrete.

Wet process:

Advantages of Wet process:

Ø Little or no formwork is required.

Ø Cost effective method for placing concrete.

Ø Ideal for irregular surface applications

Ø Allows for easier material handling in areas withdifficult access

Rehabilitationof subwaytunnels

construction of domed roofs

lining constructionSwimming pool construction

UNDERWATER

CONCRETE - UWC

Introduction

There are often situations such as :− Port and harbor installations− Bridge piers in rivers− Water industry structures− Metro systems− Deep shafts in unstable ground

, in which concrete is to be placedunderwater.

� Performance requirements for UWC� Workability & self compaction� Cohesion against washout & segregation� Low heat of hydration� Controlled set time� Compressive strength� Adequate bond

� Problems faced� Segregation of fine aggregates from coarse

aggregates� Water pollution� Increased w/c ratio� washout

Materials used :

Composition (Example 0 – 32 mm aggregate):• Aggregate

− Use an aggregate suitable for pumped mixes− Fines including cement > 400 kg/m³

• Cement and Powder Additives− Minimum cement content 350 kg/m³− Limestone can be added to the fines content in the mix

design• Admixtures

− Superplasticizer for the reduction of free water in the mix− Mix stabilizer to minimize washout effect of fines and

cement (especially in running water conditions)

Materials used :

Components Description Example formula

AggregatesAny quality aggregatespossible

All aggregate sizes are possible

CementAny cement meeting localstandards

Target cement paste volumeaccording pumping concreterecommendations: > 350 kg/m³

Powderadditives

Limestone, fly ash or groundgranulated blast furnace slag

Sufficient fines content byadjustment of the binder content:Fines including cement > 400 kg/m³

Water contentFresh water and recyclingwater with requirementsregarding fines content

w/c-ratio according to standardswith regard to exposure class: < 0.48

Materials used :

Components Description Example formula

Concreteadmixtures

• SuperplasticizerType dependent onplacement and earlystrength requirements

• Stabilizer for stagnant waterStabilizer for running water

Superplasticizer 0.60 - 1.50% *Cement

Stabilizer: 0.20 - 2.00% * Cement

Installationrequirements

Most often used today is pumping a suitably modified mix througha standard concrete pump. The end of the delivery pipe must bekept deep enough in the fresh concrete.

Concrete laying techniques

• Tremie method

• Bucket Placing

• Pump method

� A Tremie is a watertight pipe

� Generally 250mm in dia.

� Funnel shaped hopper at itsupper end and a loose plugat the bottom.

� It is supported on a workingplatform above water level.

Tremie method

Tremie method

Specifications of concrete to be used in Tremie method:• Coarse Aggregate:

− Gravel of 20 mm max. size.− Use 50-55 % of the total aggregate by weight.

• Sand, 45-50% of the total aggregate by weight• Water/Cement Ratio: 0.42 (0.45 Maximum).• Water-Reducing Admixture (preferably it is also plasticizer):

− Do not use super plasticizers.• Air-Entrainment Admixtures:

− To give 6% total air.• Retarding Admixture:

− To increase setting time to 4-24 hours, as required.• Slump: 160 mm ± 25 mm• This mix will develop compressive strength in the range of 40

– 50 MPa at 28 days.

Bucket placing• The buckets used for underwater placement of

freshly mixed concrete should have drop-bottomor roller- gate openings.

• The gates should be able to be opened from abovewater.

• If air is used to open the bucket, the air shoulddischarge through a line to the surface to preventwater disturbance.

• The top of the bucket must be covered to preventwater from washing the surface of the freshlymixed concrete.

Bucket placing

drop-bottom opening roller- gate opening

Pump method:

ØPumping concrete directlyinto its final position,involving both horizontaland vertical delivery ofconcrete.

ØPumping concrete has theadvantage of operationalefficiency with potentialsavings of time and labour.

Pump method of layingconcrete

References1. ACI 207.1R-96 (1996). “Mass Concrete” Reported by ACI Committee 207

2. S. B. Abdul Wahab, (2014). Self healing bacterial concrete. Online at:http://www.slideshare.net/neenavahab/self-healing-bacterial-concrete

3. Jürg Schlumpf and Jürgen Höfler (2006). Shotcrete in TunnelConstruction - Introduction to the basic technology of sprayed concrete.Online at: https://www.sika.com/dms/getdocument.get/7850f7a9-8547-3441-bf68-599a2734334e/Fact%202_shotcrete%20handbook.pdf

4. Underwater concrete - mix design and construction practicesftp://dfi.org/OneMine/Marine%20Foundations%20Book%20-%20individual%20papers/29-5.4%20Underwater%20Concrete%20-%20Mix%20Design%20and%20Construction%20Practices.pdf

Next lecture: Mon. 23.11.2015• Permeability of concrete:

− Permeation defines the ease with whichfluids, both liquids and gases, can enterinto, or move through concrete.

• Durability of concrete:− Is the ability to resist

o weathering action,o chemical attack,o abrasion,o or any process of deterioration.