High-strength concrete is not a large part of the production output of any ready mix plant. However, the ability to produce this type of concrete without difficulty requires a higher level of sophistication and skill than is considered necessary for more conventional mixes. High-strength concrete is defined by ACI 363 as any concrete with a specified compressive strength of 6000 psi or greater. The Canadian code currently defines high-strength concrete as any concrete with a compressive strength exceeding 10,000 psi. The author defines it as any mix with a compressive strength requirement at least 2000 psi higher than what the clients are accustomed to dealing with.
According to ACI 318 Chapter 5, a new mix, without any historical backup, needs to be overdesigned with a greater psi than its specified strength. This overdesign factor is either 1200 or 1400 depending on the specifications of the final mix. Combined with the already raised final specifications, these revisions will significantly affect the fresh and hardened properties of the concrete.
The key to properly designing high strength concrete mixes is successfully lowering the water-cement ratio while still maintaining workable, placeable concrete. There are two ways to lower the water-cement ratio: 1) reduce the amount of water or 2) increase the amount of cementitious material. Water demand in a mix is determined by the requirement for the individual mix constituents to achieve the desired degree of workability. Surface area and particle shape have a profound effect on water demand.
Other necessary ingredients for high-strength concrete include: well washed, rounded gravel; a consistent Type II cement with a lower C3A content; a good quality fly ash; and 7-10% silica fume. Admixtures used should come from the same supplier and should be checked for compatibility.