# A quality of concrete: its compressive strength

Test pieces are used to measure the **compressive strength** of concrete.

The test pieces generally consist of cylinders 16 cm in diameter and 32 cm in height that are manufactured in cardboard moulds.

The test pieces are subjected to a crushing force (generally 28 days after manufacture) and the force required to break the specimen is measured.

The ratio between the crushing force and the cross-sectional area of the test piece gives the compressive strength of the concrete.

The compressive strength is reduced if the water content of the concrete is increased and raised if the cement content is increased.

Example:

A concrete with a compressive strength of 30 MPa after 28 days will require a crushing force of 60 tonnes in order to break a cylindrical test piece with a diameter of 16 cm.

1 MPa = 10.2 kg/cmÂ²

The compressive strength of normal concrete varies between 25 and 50 MPa. Above this level, the term High Performance Concrete is used.