Compressive strength of high strength concrete mix is usually greater than 6,000 pounds per square inch.
High strength concrete is made by lowering the water cement (W/C) ratio to 0.35 or lower.
Often silica fume is added to prevent the formation of free calcium hydroxide crystals in the cement, 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 be used. To compensate for the reduced workability in the high strength concrete mix, 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.