Concrete admixtures/additives:water reducers

Concrete plasticizers are the mainstream products for readymix facilities. They are widely used all over the world and for the moment they are the number 1 admixture/additive as far as it regards the worldwide consumption.

Others name used for this category of additives are:

- normal water reducers

- water reducing additives/admixtures

- normal plasticizers etc.

Plasticizers are mostly based on lignosulfonates. Lignosulfonates or sulfonated lignins are by - products of the paper industry, recovered from the pulping liquids.

Lignosulfonates are usually modified with special polymers to mainly control air entrainment and setting times, or improve/regulate other parameters.

Other chemicals used are hydroxycarboxylic salts.

Plasticizers are considered to give water reductions at the region of 10%. A little more or a little less.

Usual dosages are between 0.25 – 0.8% by cement weight.

Example: a concrete mix with 350Kg cement/m3 can accept a plasticizer quantity between 0.25 x 350/100 and 0.8x350/100 Kg/m3 or 0.875 – 2.8 Kg/m3.

The commonest dosages are in the range of 0.3-0.4% by cement weight.

For each different project, product trials should be made to conclude to an optimal dosage.

HOW DO PLASTICIZERS FUNCTION?

Cement particles tend to flocculate and the role of plasticizers/water reducers is to induce deflocculation.

This is achieved by adsorption of their active constituents onto the cement particles. Thus the surface charges on their surface are neutralized and the cement flocs break up.

By this way the water that was tied up in the cement flocs is released and used to reduce the viscosity of the mixture.

WHY ARE PLASTICIZERS USED?

Plasticizers are used for the following reasons:

1. To reduce the water content of the concrete by about +/- 10%. This they achieve with their dispersing power and with practically no alteration of the slump.

The water reduction will lead of course to increased strengths.

2. To increase the slump/workability without changing the ratio W/C. This will facilitate the placement of the concrete.

3. To achieve a compromise between the above.

4. To achieve economies in the mix design by reducing the cement content without compromising the final strengths.

RETARDATION

Most of water reducers/plasticizers have a retardation effect on the setting time of the concrete. This is more pronounced when:

- the dosages are higher than those recommended (overdosing)

- the temperatures are lower

- the synthesis contains polysaccharides.

OVERDOSING

Overdosing will induce setting retardation.

In case of severe overdosing – double quantity or more – additional curing measures should be undertaken to prevent excessive drying on the surface of the concrete.

CONCRETE STRENGTH

When plasticizers are used as water reducers, an increase of about 7-10% of the compressive strength of 28 days can be expected.

Typically, early strength, will depend on water reduction.

SLUMP/CONSISTENCY

The ability of the concrete to flow is called its consistency and slump is a measure of consistency. If ratio W/C remains unaltered, the addition of a plasticizer will increase concrete consistency and slump.

To delay the slump loss, the initial consistency should be increased.

AIR ENTRAINING

Plasticizers/water reducers based on lignosulfonates tend to induce some air entrainment in contract with hydroxycarboxylic salts.

OVERALL DURABILITY

The W/C is reduced the overall durability of the concrete is increased.