Strength tests

Compressive strength is the property used to specify structural concrete. In India and the UK the standard specimen is a 150 mm cube; in the USA, Canada, Japan and Australia it is usually a cylinder, typically 150 × 300 mm or 100 × 200 mm. Europe allows both. The same concrete gives a lower cylinder strength than cube strength because a taller specimen is less restrained by the platens, so grades are always quoted with the specimen type.

Specimens are cast in moulds, compacted, demoulded after about a day and cured in water until test, usually at 7 and 28 days. They are crushed in a compression machine at a steady loading rate. The load at failure divided by the loaded area gives the strength.

Strength from a single specimen means little. Results are judged in groups against acceptance criteria in the governing code, because concrete strength varies from batch to batch. The characteristic strength is the value below which only a small fraction of results is expected to fall, which is why mixes are designed for a higher target mean. In Indian practice that margin is the larger of a statistical allowance based on the standard deviation and a fixed allowance that depends on the grade.

Non-destructive tests such as the rebound hammer and ultrasonic pulse velocity are useful for surveys but must be calibrated against cores or cubes.