PORTLAND CEMENT - Materials for Civil and Construction Engineers - page 224

2. The strength of the mix can be increased by using the water reducer
by lowering the quantity of water and keeping the cement content
constant.
3. The cost of the mix, which is primarily determined by the amount of
cement, can be reduced. In this case, the water reducer allows a de-
crease in the amount of water. The amount of cement is then reduced
to keep the water-cementitious materials ratio equal to the original
mix. Thus, the quality of the mix, as measured by compressive
strength, is kept constant, although the amount of cement is decreased.

Superplasticizers Superplasticizers, or high-range water reducers, can ei-
ther greatly increase the flow of the fresh concrete or reduce the amount of
water required for a given consistency. For example, adding a superplasti-
cizer to a concrete with a 75-mm (3 in.) slump can increase the slump to 230
mm (9 in.), or the original slump can be maintained by reducing the water
content 12% to 30%. Reducing the amount of mixing water reduces the
water-cementitious materials ratio, which in turn, increases the strength of
hardened concrete. In fact, the use of superplasticizers has resulted in a
major breakthrough in the concrete industry. Now, high-strength concrete in
the order of 70-80 MPa (10,000-12,000 psi) compressive strength or more
can be produced when superplasticizers are used. Superplasticizers can be
used in the following cases:
1. a low water-cementitious materials ratio is beneficial (e.g., high-strength
concrete, early strength gain, and reduced porosity)
2. placing thin sections
3. placing concrete around tightly spaced reinforcing steel
4. placing cement underwater
5. placing concrete by pumping
6. consolidating the concrete is difficult
When superplasticizers are used, the fresh concrete stays workable for a
short time, 30 min to 60 min, and is followed by rapid loss in workability.
Superplasticizers are usually added at the plant to ensure consistency of the
concrete. In critical situations, they can be added at the jobsite, but the con-
crete should be thoroughly mixed following the addition of the admixture.
The setting time varies with the type of agents, the amount used, and the in-
teractions with other admixtures used in the concrete.

■ 6.11.3 Retarders

Some construction conditions require that the time between mixing and
placing or finishing the concrete be increased. In such cases, retarders can
be used to delay the initial set of concrete. Retarders are used for several rea-
sons, such as the following:

1. offsetting the effect of hot weather
2. allowing for unusual placement or long haul distances
3. providing time for special finishes (e.g., exposed aggregate)