Testing of Concrete

There are many different test procedures for determining the properties of concrete. The two common tests are the slump test, applied to wet or fresh concrete, and the compression test, applied to hardened concrete.

Slump Test

The slump test is suitable for establishing uniformity of mixes in subsequent batches or deliveries. It is not strictly a test for workability, but it can be used as a guide when constituents are known to be constant. Mixes of the same slump can vary with regard to their cement and grade of aggregates.

Procedure - the cone is one-quarter filled with concrete and tamped 25 times. A further three layers are applied, each layer tamped as described. Surplus concrete is struck from the surface, and the cone is raised immediately.

 

 

The slump test is suitable only for the concrete of high or medium workability.

Compression Test

Concrete test cubes are made from samples taken from site. Specimens are taken before and during the placing of concrete.

Procedure:

       Concrete is placed in the mould in 50mm layers; each layer is tamped 35 times for 150mm cubes or 25 times for 100mm cubes. Alternatively, the concrete may be compacted by vibration. Surplus concrete is struck off. Samples remain in the mould for 24 hours +/- 30 minutes, covered with a damp sack or similar. After this time specimens are marked, removed from the mould and submersed in water at a temperature between 10 and 21°C until required for testing.

      The cube strength is the stress failure after 7 days. If the strength specifications is not achieved at 7 days, a further test is undertaken at 28 days. If the specification is not achieved at 28 days, specimen cores may be taken from the placed concrete for laboratory analysis.

      Typical 28-day characteristic crushing strengths (that below which not move than 5 per cent of the test results are allowed to fall) are graded 7.5, 10, 15, 20, 25, 30, 35 and 40 N/mm²

    As a guide, shear stress of the concrete is taken at approximately one-month of the stress under compression.