Types of Concrete
Various types of concrete have been developed for specialist application and have become known by these names.
Regular concrete
Regular concrete is the lay term describing concrete that is produced by following the mixing instructions that are commonly published on packets of cement, typically using sand or other common material as the aggregate, and often mixed in improvised containers. This concrete can be produced to yield a varying strength from about 10 MPa to about 40 MPa, depending on the purpose, ranging from blinding to structural concrete respectively. Also, many types of pre-mixed concrete are available which include powdered cement mixed with an aggregate, needing only water.
High-strength concrete
High-strength concrete is concrete with a compressive strength generally greater than 6,000 pounds/square inch (40 MPa). High-strength concrete is made by lowering the water-cement ratio to 0.35 or lower. Often, silica fume is added to prevent the formation of free calcium hydroxide crystals in the cement matrix, which might reduce the strength at the cement-aggregate bond.
Low w/c ratios and the use of silica fume make concrete mixes significantly less workable, which is particularly likely to be a problem in high-strength concrete applications, where dense rebar cages are likely. To compensate for the reduced workability, superplasticizers are commonly added to high-strength mixtures. Aggregate must be selected carefully for high-strength mixes, as weaker aggregates may not be strong enough to resist the loads imposed on the concrete, and cause failure to start in the aggregate rather than in the matrix or at a void, as normally occurs in regular concrete.
In some applications of high-strength concrete, the design criterion is the elastic modulus rather than the ultimate compressive strength.