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

Placing and finishing qualities Workability Strength development ADMIXTURE MAY ALTER Placing and finishing qualities Workability Strength development Appearance (gets improved) Re-uses of form work (facilitated) Pumping quality.

CATEGORIES OF ADMIXTURES Water reducing admixtures (Plasticizers or Super Plasticizers) Retarding admixtures Air-entraining agents (AEA) Accelerating admixtures.

Fact about Plasticizers Plasticizers or any other type of admixture are not substitutes for badly designed mixes or bad practices. Should only be used when there is a valid reason to use it.

Water Reducing ADMIXTURES For full hydration w/c of 0.23 is sufficient but it is generally kept 0.50 or above. Excess water is used to overcome internal friction between solid particles & to facilitate mixing, placing, transportation & compaction. Water reducing admixtures can reduce this extra requirement of water. Super-Plasticizers are better than conventional Plasticizers. Over 25% water reduction is possible

RETARDING ADMIXTURES Used where setting time & strength development of concrete has to be delayed The retarder delays only the hydration process (but doesn’t affect the eventual process) Initial setting time can be delayed by more than 3 hours. Main uses – (1) To eliminate the cold joints (2) To control set during long transit periods and high temperature.

To minimise the problem of over rapid slump loss. USE OF RETARDER To minimise the problem of over rapid slump loss. But how to minimise the slump loss without reduced early strength gain ? Use appropriate supplementary cementing material & retarder be used only as a last resort.

AIR ENTRAINING AGENTS These are surface active agents that form stable air bubbles. Function of air bubbles: To break capillary structure within the concrete To act as roller which enable the particles in the mix to move freely thus providing plasticising effect. Agents: Alkali salts of wood resin,Synthetic detergents,Animal/ vegetable fats Commercial makes:Vinsol resin,Teepol, Darex etc

SUPER PLASTICIZERS 3 types of super Plasticizers Ligno Sulphate based Melamine sulphate based Nepthalene sulphate based Generally the manufacturers keep it secret. Therefore these behave differently with different cement.

DOSAGE OF SUPER PLASTICIZER If strength is the criteria Lowest w/c ratio and highest super Plasticizers dose. If workability is the criteria Highest w/c ratio for a particular strength & the super Plasticizers equal to required workability.

Retempring of CONCRETE Process of remixing of concrete, if necessary with addition of small quantity of water and cement is called retempering IS :457-1957 doesn’t permit it in partially hardened concrete. But many researchers feel that it can be permitted to obtain the desired slump provided the design w/c ratio is not exceeded.

SUPPLEMENTARY CEMENT MATERIALS Up to 98 MPa – fly-ash or ggbfs (ground granulated blast furnace slag) is used with cement But when strength > 100 MPa – silica fume is essential (It can be used for less strength also).

SILICA FUMES It is not a well defined material, because it is only a by-product (waste) in the production process of silicon and silicon alloys. Before using its alkali, silica & carbon content be identified along with specific surface area.

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

BLAST FURNACE SLAG (ggfs) It is a by product in manufacturers of steel from open hearth blast furnace Dosage – 25 to 70% by wt. Of cement It imparts useful sulphate resisting properties Caution : Fly-ash and ggfs reduce early strength gain upto 7 days. Therefore form work stripping be rescheduled.

COMPARISON OF EXPANSION FOR VARIOUS CONCRETE Age The expansion OPC 50%OPC + 50% GGBFS 25%OPC+ 75%GGBFS 28 days 0.64% 0.09% 0.04%

Diffusion of chloride at 250C in cement paste of w/c 0.5 SN TYPE OF CEMENT DIFFUSIVITY (X 10-9 CM2/S) 1 Sulphate Resistant Portland Cement 100.0 2 OPC 44.7 3 70% OPC +25%Flyash 14.7 4 30% OPC + 70%GGBFS 4.1

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.                                                                                              ACI 116R-90, Cement and Concrete Terminology.

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

Asper SHRP (Strategic Highway Research Programme) HPC should meet following criteria 4 hrs strength > 17.5 MPa 24 hrs strength > 35 MPa 28 days strength > 70 MPa

High Performance Concrete (contd.) High strength concrete can be considered as high performance concrete if other attributes are satisfactory for intended application Recent development in mineral & admixtures has made possible high strength concrete of 98-112 MPa. HSC shows superiority of other attributes also.

High Performance Concrete (contd.) IMPORTANCE OF AGGREGATES Any normal weight agg. are stronger than cement paste up to 42 MPa . In HSC : All the components i.e. Hardened cement paste Aggregates Interfacial zone between paste & agg. are pushed to their critical limit.

STEPS TO ACHIEVE HSC Selection of suitable Ingredients Proportioning Cement Supplementary cementing materials like fly-ash and silica fumes Aggregates Water Chemical admixture Proportioning Quality Control

AGGREGATES FOR HSC Strength of agg. is very important in HSC. Since w/c ratio is generally low, the grading of agg needs to be controlled Coarse Agg Should be strong & equi-dimensional Free from silt and clay Max. size 10-12 mm ( even 20-25 mm can also be used)

FINE AGG Smooth rounded particles to reduce water demand FM of 3.0 & above recommended In HSC – agg. strength also becomes problem apart for strength of cement paste.

COMPARISON Material Normal Concrete HSC Cement content kg/cum CA/FA 350-550 0.9-1.4 500-650 1.5 to 1.8