ADMIXTURESADMIXTURES

It is defined as a material other It is defined as a material other than water, aggregate and cement than water, aggregate and cement that is used as an ingredient of that is used as an ingredient of concrete to modify the properties concrete to modify the properties of fresh or hardened concrete.of fresh or hardened concrete.

ADMIXTURE MAY ALTERADMIXTURE MAY ALTER

Placing and finishing qualitiesPlacing and finishing qualities WorkabilityWorkability Strength developmentStrength development Appearance (gets improved)Appearance (gets improved) Re-uses of form work (facilitated)Re-uses of form work (facilitated) Pumping quality. Pumping quality.

CATEGORIES OF ADMIXTURESCATEGORIES OF ADMIXTURES

1.1. Water reducing admixtures Water reducing admixtures (Plasticizers or Super (Plasticizers or Super Plasticizers)Plasticizers)

2.2. Retarding admixturesRetarding admixtures

3.3. Air-entraining agents (AEA)Air-entraining agents (AEA)

4.4. Accelerating admixtures.Accelerating admixtures.

Fact about PlasticizersFact about Plasticizers

Plasticizers or any other type of Plasticizers or any other type of admixture are not substitutes for admixture are not substitutes for badly designed mixes or bad badly designed mixes or bad practices.practices.

Should only be used when there is Should only be used when there is a valid reason to use it.a valid reason to use it.

Water Reducing ADMIXTURESWater Reducing ADMIXTURES

For full hydration w/c of 0.23 is sufficient but it For full hydration w/c of 0.23 is sufficient but it is generally kept 0.50 or above. is generally kept 0.50 or above.

Excess water is used to overcome internal Excess water is used to overcome internal friction between solid particles & to facilitate friction between solid particles & to facilitate mixing, placing, transportation & compaction. mixing, placing, transportation & compaction. Water reducing admixtures can reduce this Water reducing admixtures can reduce this extra requirement of water.extra requirement of water.

Super-Plasticizers are better than conventional Super-Plasticizers are better than conventional Plasticizers. Plasticizers. Over 25% water reduction is Over 25% water reduction is possible possible

RETARDING ADMIXTURESRETARDING ADMIXTURES

Used where setting time & strength development Used where setting time & strength development of concrete has to be delayedof concrete has to be delayed

The retarder delays only the hydration process The retarder delays only the hydration process (but doesn’t affect the eventual process) (but doesn’t affect the eventual process)

Initial setting time can be delayed by more Initial setting time can be delayed by more than 3 hours. than 3 hours.

Main usesMain uses – (1) To eliminate the cold joints – (1) To eliminate the cold joints

(2)(2) To control set during long To control set during long transit periods and high transit periods and high

temperature. temperature.

USE OF RETARDERUSE OF RETARDER

To minimise the problem of over rapid To minimise the problem of over rapid slump loss. slump loss.

But how to minimise the slump loss But how to minimise the slump loss without reduced early strength gain ? without reduced early strength gain ?

Use appropriate supplementary Use appropriate supplementary cementing material & retarder be used cementing material & retarder be used only as a last resort.only as a last resort.

AIR ENTRAINING AGENTSAIR ENTRAINING AGENTS

These are surface active agents that form stable These are surface active agents that form stable air bubbles. air bubbles.

Function of air bubbles:Function of air bubbles:– To break capillary structure within the concreteTo break capillary structure within the concrete– To act as roller which enable the particles in the To act as roller which enable the particles in the

mix to move freely thus providing plasticising mix to move freely thus providing plasticising effect. effect.

Agents:Agents: Alkali salts of wood resin,Synthetic Alkali salts of wood resin,Synthetic detergents,Animal/ vegetable fatsdetergents,Animal/ vegetable fats

Commercial makes:Vinsol resin,Teepol, Darex etcCommercial makes:Vinsol resin,Teepol, Darex etc

SUPER PLASTICIZERSSUPER PLASTICIZERS

3 types of super Plasticizers3 types of super Plasticizers– Ligno Sulphate basedLigno Sulphate based– Melamine sulphate basedMelamine sulphate based– Nepthalene sulphate based Nepthalene sulphate based

Generally the manufacturers keep Generally the manufacturers keep it secret. Therefore these behave it secret. Therefore these behave differently with different cement.differently with different cement.

DOSAGE OF SUPER PLASTICIZERDOSAGE OF SUPER PLASTICIZER

If strength is the criteriaIf strength is the criteria– Lowest w/c ratio and highest super Plasticizers Lowest w/c ratio and highest super Plasticizers

dose. dose. If workability is the criteriaIf workability is the criteria

– Highest w/c ratio for a particular strength & the Highest w/c ratio for a particular strength & the super Plasticizers equal to required workability. super Plasticizers equal to required workability.

Retempring of CONCRETERetempring of CONCRETE

Process of remixing of concrete, if Process of remixing of concrete, if necessary with addition of small necessary with addition of small quantity of water and cement is called quantity of water and cement is called retemperingretempering

IS :457-1957 doesn’t permit it in IS :457-1957 doesn’t permit it in partially hardened concrete. But many partially hardened concrete. But many researchers feel that it can be permitted researchers feel that it can be permitted to obtain the desired slump provided to obtain the desired slump provided the design w/c ratio is not exceededthe design w/c ratio is not exceeded. .

SUPPLEMENTARY CEMENT SUPPLEMENTARY CEMENT MATERIALSMATERIALS

Up to 98 MPa – fly-ash or ggbfs Up to 98 MPa – fly-ash or ggbfs (ground granulated blast furnace (ground granulated blast furnace slag) is used with cementslag) is used with cement

But when strength > 100 MPa – But when strength > 100 MPa – silica fume is essential (It can be silica fume is essential (It can be used for less strength also).used for less strength also).

SILICA FUMESSILICA FUMES

It is not a well defined material, It is not a well defined material, because it is only a by-product because it is only a by-product (waste) in the production process (waste) in the production process of silicon and silicon alloys.of silicon and silicon alloys.

Before using its alkali, silica & Before using its alkali, silica & carbon content be identified along carbon content be identified along with specific surface area. with specific surface area.

FLY ASHFLY ASH

It is also a waste product from thermal It is also a waste product from thermal power plants during combustion of power plants during combustion of coal. The variation in quality has been coal. The variation in quality has been noticed even from a single source. noticed even from a single source.

BLAST FURNACE SLAG (ggfs)BLAST FURNACE SLAG (ggfs)

It is a by product in manufacturers of steel It is a by product in manufacturers of steel from open hearth blast furnacefrom open hearth blast furnace

Dosage – 25 to 70% by wt. Of cement Dosage – 25 to 70% by wt. Of cement It imparts useful sulphate resisting It imparts useful sulphate resisting

propertiesproperties

Caution :Caution : Fly-ash and ggfs reduce early Fly-ash and ggfs reduce early strength gain upto 7 days. Therefore form strength gain upto 7 days. Therefore form work stripping be rescheduled. work stripping be rescheduled.

COMPARISON OF EXPANSION FOR COMPARISON OF EXPANSION FOR VARIOUS CONCRETEVARIOUS CONCRETE

AgeAge The expansionThe expansion

OPCOPC 50%OPC + 50%OPC + 50% 50% GGBFSGGBFS

25%OPC+ 25%OPC+ 75%GGBFS75%GGBFS

28 days28 days 0.64%0.64% 0.09%0.09% 0.04%0.04%

Diffusion of chloride at 25Diffusion of chloride at 2500C in C in cement paste of w/c 0.5cement paste of w/c 0.5

SNSN TYPE OF CEMENT TYPE OF CEMENT DIFFUSIVITY DIFFUSIVITY (X 10(X 10-9-9 CM CM22/S)/S)

11 Sulphate Resistant Sulphate Resistant Portland CementPortland Cement

100.0100.0

22 OPCOPC 44.744.7

33 70% OPC +25%Flyash70% OPC +25%Flyash 14.714.7

44 30% OPC + 70%GGBFS30% OPC + 70%GGBFS 4.14.1

Slurried silica fume. Typically, the slurry consists of approximately 50 percent silica fume and 50 percent water, by mass. When first introduced to the market, slurried silica-fume products often contained chemical admixtures such as water reducers or high-range water reducers. Today, slurry is available without any such additions.

                                                                                            

Silica fume is a byproduct of producing silicon metal or ferrosilicon alloys in an electric-arc furnace. This photo, taken before environmental regulations were put into effect, shows silica fume being discharged from a smelter. Today, no silica fume is discharged to the environment in the United States.

                                                                                            

This slide presents a comparison of some of the physical and chemical characteristics of silica fume, fly ash, and portland cement. Note that the surface area measurements for silica fume are done using a nitrogen absorption method (BET) while those for fly ash and portland cement are typically done using an air permeability method (Blaine).

                                                                                            

Silica fume functions in concrete by two distinct mechanisms. As noted, the keys to its performance in these roles are its very small particle size and its high silicon dioxide content. Each of these effects is discussed in the following slides.

                                                                                            

Several researchers have looked into the improvement in concrete properties resulting from including particles smaller than portland cement grains. The first person to promote the use of silica fume in this role was Hans Bache of Aalborg Portland in Denmark. Two of his papers are referenced in subsequent slides.

                                                                                            

Detwiler and Mehta used carbon black particles to examine the filler effect. These particles are not pozzolanic and are approximately the same size as silica fume. Rachel J. Detwiler and P. Kumar Mehta, "Chemical and Physical Effects of Silica Fume on the Mechanical Behavior of Concrete," ACI Materials Journal, Vol. 86, No. 6, pp. 609-614, 1989.

                                                                                            

Schematic showing the dispersion of silica fume particles among cement grains. This figure shows the basic concept of particle packing -- filling the spaces between cement grains with silica fume particles. H. H. Bache, "Cement-bound materials with extremely high strength and durability," brochure form Aalborg Portland, Aalborg, Denmark, undated.

                                                                                            

                                                                                            

ACI 116R-90, Cement and Concrete Terminology.

HIGH PERFORMANCE CONCRETE HIGH PERFORMANCE CONCRETE

It is not a particular commodity, rather It is not a particular commodity, rather it is a range of product specifically it is a range of product specifically designed to satisfy the performance designed to satisfy the performance requirement for the intended requirement for the intended application in the most effective way. application in the most effective way.

Asper SHRP (Strategic Highway Asper SHRP (Strategic Highway Research Programme)Research Programme)

HPC should meet following criteriaHPC should meet following criteria4 hrs strength > 17.5 MPa4 hrs strength > 17.5 MPa24 hrs strength > 35 MPa24 hrs strength > 35 MPa28 days strength > 70 MPa28 days strength > 70 MPa

High Performance Concrete (contd.)High Performance Concrete (contd.)

High strength concrete can be considered as High strength concrete can be considered as high performance concrete if other high performance concrete if other attributes are satisfactory for intended attributes are satisfactory for intended applicationapplication

Recent development in mineral & Recent development in mineral & admixtures has made possible high strength admixtures has made possible high strength concrete of 98-112 MPa.concrete of 98-112 MPa.

HSC shows superiority of other attributes HSC shows superiority of other attributes also.also.

High Performance Concrete (contd.)High Performance Concrete (contd.)

IMPORTANCE OF AGGREGATESIMPORTANCE OF AGGREGATES

Any normal weight agg. are stronger than Any normal weight agg. are stronger than cement paste up to 42 MPa .cement paste up to 42 MPa .

In HSC : All the components i.e.In HSC : All the components i.e.– Hardened cement pasteHardened cement paste– Aggregates Aggregates – Interfacial zone between paste & agg.Interfacial zone between paste & agg.

are pushed to their critical limit. are pushed to their critical limit.

STEPS TO ACHIEVE HSC STEPS TO ACHIEVE HSC

Selection of suitable IngredientsSelection of suitable Ingredients– CementCement– Supplementary cementing materials like fly-ash Supplementary cementing materials like fly-ash

and silica fumesand silica fumes– AggregatesAggregates– WaterWater– Chemical admixtureChemical admixture

ProportioningProportioning Quality Control Quality Control

AGGREGATES FOR HSCAGGREGATES FOR HSC

Strength of agg. is very important in HSC. Strength of agg. is very important in HSC. Since w/c ratio is generally low, the grading Since w/c ratio is generally low, the grading of agg needs to be controlled of agg needs to be controlled

Coarse AggCoarse Agg– Should be strong & equi-dimensionalShould be strong & equi-dimensional– Free from silt and clayFree from silt and clay– Max. size 10-12 mm ( even 20-25 mm can also Max. size 10-12 mm ( even 20-25 mm can also

be used)be used)

FINE AGGFINE AGG

Smooth rounded particles to reduce Smooth rounded particles to reduce water demandwater demand

FM of 3.0 & above recommendedFM of 3.0 & above recommended In HSC – agg. strength also becomes In HSC – agg. strength also becomes

problem apart for strength of cement problem apart for strength of cement paste.paste.

COMPARISONCOMPARISON

MaterialMaterial Normal Normal ConcreteConcrete

HSCHSC

Cement content Cement content kg/cumkg/cum

CA/FACA/FA

350-550350-550

0.9-1.40.9-1.4

500-650500-650

1.5 to 1.8 1.5 to 1.8