How is the compressive strength of concrete tested?

Compressive strength is tested by casting standard cube specimens (150mm) or cylinder specimens (150mm dia x 300mm), curing for 28 days in water at 27C, then crushing in a compression testing machine. Load is applied gradually until failure, and strength equals failure load divided by cross-sectional area. Three specimens are tested and average taken. Cylinder strength is about 0.8 times cube strength due to shape effects.