The age at which concrete must develop its high strength is an important factor in the design of a mix. If high early strength is required, it can often be achieved by modifying an existing mix design to incorporate: (1) Type III rather than Type I or Type II portland cement; (2) additional cement; (3) an accelerator, such as calcium chloride; and/or (4) a water-reducing admixture.
If high ultimate strength- strength at 28 days or later- is the aim, the steps available to achieve it include using: (1) a high cement content; (2) a low water/ cement ratio and (3) a water-reducing admixture. The use of a high cement content will ordinarily produce higher ultimate strength. In general, however, no more cement should be used then necessary for required workability.
Water-reducing admixtures will permit a reduction in unit water content without loss of workability. The effectiveness of water-reducing admixtures will vary significantly with the brand of admixture and the brand of portland cement with which it is used.
Extensive studies made by the Research and Technology Division of the Air Force on development of mixtures for high compressive strength concrete led to the following conclusions: (1)_ workable concrete mixtures with compressive strengths in excess of 10,000 psi at 90 days are feasible; (2) the use of crushed limestone aggregate, graded to a maximum size of one-half inch, will aid in producing high-compressive strength concrete; (3) continuous moist-curing of the concrete for at least 28 days is necessary to develop compressive strengths in excess of 10,000 psi at 90 days; (4) at least 10 percent, by weight, of the portland cement used in high compressive strength concrete can be replaced by pozzolans such a fly ash or calcined shale; and (5) the tensile and flexural strengths of high compressive strength concrete are approximately 770 psi and 1,050 psi respectively.