Admixtures are a material added to the concrete before or during it’s mixing with a view to modifying one or more of the properties of concrete in the Plastic or Hardened state. These are distinct from cement additive which are incorporated into cement clinkers during grinding e.g. grinding Aids, or integral waterproofing compounds which reduce voids in mortars/ concrete and increase permeability resistance against water, gas & corrosive external agents.

Concrete is by far the most widely used construction material having high bearing attributes (Compressive strength at 28 days ranges from 5 to 60 N/mm 2 or 50 to 600 kg/cm 2 ) made of cement, aggregate & water. Aggregates are subdivided as fine & course. Send (Silica) is a fine aggregates are most of which passes through 4.75 mm I. S. Sieve; Basalt, Quartz, Granite, Gravel/Shingle in their crushed from constitutes a host of course Natural aggregate. For concrete making, 10 MM & 20 MM sizes are very common choice.

The inorganic Mineral binder – the cement is composed of oxides and silicates of calcium, Aluminium, Iron & Magnesium and Sulphuric Acid Anydride (SO 3 ) in the form of gypsum, plays the pivotal role in the formation of concrete matrix. Atypical OPC has the following composition

• SIO 2 – 19-24 (Percent)
• AI 2 O 3 – 3-6
• Fe 2 O 3 – 1-4
• CaO – 59 – 64
• MgO – 0.5-4

OPC (Ordinary Portland Cement) is a Silicate cementations material containing four major compounds which hydration with water and subsequent interactions are converted into crystalline compounds which are constituents of a set concrete. It is the phenomenon of curing/ setting on which depends the ultimate properties viz. Compressive strength, resistance to cracking, resistance to abrvasion, durability, split Tensile Strength, dimensional stability (Shrinkage) etc.

TABLE: Chemical Compounds of OPC

For General Concrete Constructions, the following cements are used in our country.

a) Ordinary Portland Cement (OPS) also known as low heat Portland Cement (IS : 269-1976)

b) Rapid Hardening Portland Cement (IS: 8041-1976).

c) Portland Slag Cement (IS: 455-1976).

d) Portland – Pozzolana Cement (IS: 8112-1976).

e) High Strength Ordinary Portland Cement (IS: 8112-1976).

f) Hydrophobic (IS: 8043-1978).

In addition to above, high alumina Cement (Cement Fondue) conforming to IS:6452-1972 & Super Sulphated Cement (IS:6909-1973) can be used under special circumstances with necessary precautions.

ADMIXTURES : Present Day concrete often incorporates a fourth intergradient-admixture in addition to cement, aggregate & water. These are added in small quantity on the weight of cement, to concrete Mix, immediately before or during mixing. Indian standards (IS:9103-1979) cover four classes of admixtures :-

1) Accelerating Admixture

2) Retarding Admixture

3) Water Reducing Admixture

4) Air-Entraining Admixture

In addition, NS: 456-1978 permits the use of pozzolana such as Fly-Ash, Burnt Clay or Micro Silica as admixtures for concrete. They are used mainly to improve the workability of concrete & thereby reduce the water demand for a given workability. As lowering of W/C ration increases compressive strength, one can also save on quantity of cement consumed for a targeted compressive strength by the use of water-reducing concrete plasticizer. Admixtures impart yet reducing W/C ratio.

Accelerating Admixture or Accelerator is defined as a substance, when added to concrete, mortar or grout increases the rate of hydration of hydraulic cement & shortens the time of set. The chemicals that increase the rate of hardening of cement paste/concrete mixes include soluble chlorides, carbonates, silicates, Fluorosilicates and hydroxides and also some organic compounds. Accelerating admixtures are used when concrete is to be placed at low temperature and delay in setting would adversely effect the concreting schedule. The present day knowledge on the use of admixture for pre-stressed & reinforced concrete has restricted use of accelerator containing relatively large amount of chloride which may accelerate corrosion of embedded steels in pre-stressed/Reinforced structure. The limit of total permissible chloride in cement, aggregates, additives and in mixing water is limited to 0.1% (1000mg/L) by weight of cement for pre-stressed concrete and 0.15%(1500 mg/L) for other reinforced concrete. Chloride ions induce electro chemical corrosion leading to rusting/deterioration of Steel Reinforcements. Rusts are hydrated Iron oxides. Due to formation of rusts the steel increases in volume. At times, even up to eight times its original volume. It results in tensions in concrete causing disintegration and fall in load bearing attitudes.

Retarding Admixture: A delay in setting of concrete is achieved by use of retarding Admixture, It is used in hot weather when normal setting time of cement is reduced by rise in temperature. Use of retarding admixture ensures freedom from cold joints as the concrete can be vibrated, Placed & compacted to build up thick cross-section. The mix can be poured/self-leveled to cat intricate shapes before it gets hardened.

A major field of application is pumping able concrete/ready mixed concrete. A flowing concrete delayed with ‘Reversion Time’ (Retention of workability) is essential in RMC. The time for a plasticized mix to return to the flow condition that it would have had without the addition of admixture is called ‘Reversion Time’.

Water-Reducing Admixture: These admixtures the workability of concrete (in terms of compacting factor) by reducing the W/C ratio and as a result. Increasing the comprehensive strength of concrete.

According to Abraham’s theory on the relationship of water: Cement Ration (W/C) to comprehensive strength of concrete, the lower the W/C ratio. The higher is the comprehensive strength will be increased by 1%.

For full hydration to take place in a corporate a minimum W/C of 0.27 is required. In common concreting, W/C in the range of 0.50 to 0.70 is used. Some high-Range water-Reducing admixtures (popularly known as Super plasticizer) are capable of reducing W/C ratio to 0.35 or less. These are mainly water-soluble organic polymers. The first of its kinds adopted by the concrete industry were polymeric, high molecular weight, dispersants derived from Lignin – bye product of pulp & paper industry.

How super plasticizer works in Concrete:

Portland cement particles have a strong tendency is the result of several types of interactions between particles ; electrostatic interactions between site bearing, opposite charges; and strong inter action (or bridging) involving Hydrogen Bonding of water Molecules or Hydrates C 3 S, C 2 S, Ca 3 A etc.

The Flocculation process results Trapping of part of water of hydration inside the open net work voids. This followed stiffening or increase in apparent viscosity of the cementations system. Super plasticizers influence morphology of the hydration products in addition to their principal function of deflocculating. The negatively charged cement particles preventing them from coagulation. The properties of super Plasticizers can be summarized as under:-

Reduction of surface Tension of water. Induced electrostatic repulsion between cement particles. Lubricating Film between cement particles. Inhibition of surfaces hydration reaction of cement particles leaving more water to fluidity the mix.

As regards Mechanism, majority of authors believe than the super plasticizer molecules absorbed strongly on to the surface of the cement grains.

Air Entraining Admixtures: These admixtures cause air to be incorporated in the form of minute bubbles in the concrete during mixing usually to increase workability and resistance to freezing and thawing. They control the amount of air in fresh concrete & disperse properly sized Air Bubbles throughout the concretes. They are mostly surfactants including soaps of vegetable/Animal oils & fats; petroleum sulphonate, Hydro Peroxide, Resin Acid soaps etc.

The choice & selection of dosages require careful Lab/Site Trials. Since entertainment of small amount if air leads to concrete of insufficient durability. Whereas with large amount of Air Entrainment, their is an excessive strength reduction in concrete.

Dosage of Concrete Plasticizer/ Admixture: Since 1938 moderate dose level generally 1 to 3 L/M 3 (0.2 to 0.6 gal/yd 3 ) was selected to fluidity concrete just before placing. Today super plasticizers are often used as High range water reducers, to construct stronger concrete by lowering W/C Ratio as high as 30 percent. Dosages range between 5 and 20 L/M 3 (1 to 4 gal/yd 3 ) on weight of cement in the mix.

Future of concrete Admixture: Materials Engineer and scientists are continually engaged in developing new range of chemicals/ Polymers/ modified Natural Polymers to improve upon the properties of state-of-the art concretes. High rise concrete structure, e.g. one peach tree centre, a tower of strength of compressive strength 12,000lb/in 2 down town Atlanta, Georgia completed in august 1991 (257 M High) selected water reducers, reterders & super plasticizers with control of W/C ratio within maximum variation of .01.

In Japan, delayed action precursor polymer which are insoluble in water has been developed which maintains the slumps over a period of hours.

This high reversion Time super plasticizer is an example signifying growth of Admixtures. Apparently, concrete is a sat out monolithic giant of construction. But concrete is having voids (Porosity) & of low crack resistively, poor elastic modules, poor weather & microbial resistance & not as impervious as metal or plastic.

It is believed that days are not far when invention of new generation admixtures (can we call them magic chemicals?) would take care to improve upon properties on concrete/ mortar/ cement with advantment of technology in the first decade of 21 st Century .

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