Two developments have made possible the new generation of high-strength concrete now available for routine construction: superplasticizers, and silica fume. The advent of this high-strength concrete has put requirements for strength and durability in conflict. Because entrained air reduces the strength of concrete, builders seek to eliminate or limit its use.
AIR CONTENT VERSUS STRENGTH
Previous findings have shown that in the high-strength range (8000 psi and above) there is a net loss in strength when air is substituted for water. However, a more recent study says superplasticizers have changed that condition and that the reduction of water now made possible by a superplasticizer permits the addition of air without sacrificing strength.
ROLE OF THE SUPERPLASTICIZER
The principal effect of superplasticizers is to extend concrete technology into a range of low water-cement ratios never before possible. In freshly mixed concrete where high strength is not sought, they make possible high-slump, flowing concretes that may be placed with little vibration and little segregation.
INFLUENCE OF SILICA FUME
Because of its fineness, silica fume increases the water requirement of the concrete mix and is therefore usually used with a superplasticizer. By its extreme fineness and its pozzolanic action, silica fume produces a finer, more discontinuous pore system than that in concrete without it. The extremely low permeability makes the probability of creating concrete with no freezable water greater.