- Water.Water for use in concrete should be free of undesirable chem-
ical contamination, including organic contaminants. A general stan-
dard is that the water should be fit for human consumption.
- Pozzolans.Pozzolans are siliceous–aluminous substances which react
chemically with calcium hydroxide from the cement to form addi-
tional cementitious compounds. PFA, an artificial pozzolan, is now
almost universally employed, if available, in partial replacement
(25–50%) of cement. The introduction of PFA reduces total heat of
hydration and delays the rate of strength gain for the concrete up to
about age 90–180 days. Long-term strength is generally slightly
enhanced and certain aspects of durability may be improved, but
strict quality control of the PFA is necessary. The use of PFA may
also assist in marginally reducing overall concrete costs.
- Admixtures.The most commonly used admixtures are air-entraining
agents (AEAs). They are employed to generate some 2–6% by
volume of minute air bubbles, significantly improving the long-term
freeze–thaw durability of the concrete. They also reduce the water
requirement of the fresh concrete and improve its handling qualities.
Water reducing admixtures (WRAs) are sometimes employed to cut
the water requirement, typically by 7–9%. They are also effective in
delaying setting time under conditions of high ambient temperatures.
3.6.3 Concrete mix parameters
The parameters which are principally responsible for controlling the prop-
erties of concretes manufactured with specific cement and aggregates are
as follows: cement content, C (kg m^3 ); water content, W (kg m^3 );
water:cement ratio (by weight). Some further influence can be exerted
through the addition of PFA and/or the use of other admixtures such as
AEA and WRA.
The desirable primary characteristics of density, adequate strength,
durability and impermeability are favourably influenced by increasing
cement content and/or reducing the water:cement ratio. Thermal
characteristics and volume stability, on the other hand, are improved by
restricting the cement content of the mix. Economy is similarly dependent
upon minimizing the cement content. A balance has therefore to be sought
between upper and lower bounds for each parameter, limits which are set
by contradictory requirements.
The dominant considerations in selecting a suitable concrete mix are
controlling thermal characteristics and attaining the minimum cost consis-
tent with adequate strength and durability.
Thein situproperties of the mature concrete are dependent upon
attaining maximum density through effective compaction. The ability to
172 CONCRETE DAM ENGINEERING
