This procedure describes the method for determining the compressive strength of concrete cubes. The Concrete Strength is normally tested at an age of 7, 14, and 28 days.
Required equipment
- Compression Testing Machine.
- Auxiliary platens. When auxiliary platens are used, the top auxiliary platen shall rest on and be aligned with the cube.
- A balance with min. 10 kg capacity, readable and accurate to 1 g.
Test Procedure
- Weigh each specimen, as-received or saturated.
- Check the nominal dimensions and take the measured dimensions of each specimen.
- Determine the density of each specimen.
- Immerse in water, for a minimum of 5 minutes, those cubes which have not been cured in water or where the surfaces have been allowed to dry. Remove the cubes from the curing or density water tank and test them while they are still wet.
- Carefully centre the cube on the lower platen and ensure that the load will be applied to two opposite cast faces of the cube.
- Without shock, apply and increase the load continuously at a nominal rate within the range of 12 MPa/min to 24 MPa/min until no greater load can be sustained. Record the maximum load applied to the cube.
Type of Failure
Record any unusual features in the type of failure. Refer to the figures below for examples of satisfactory failures and unsatisfactory failures.
Calculations
1. Calculate the cross-sectional area of the cube face from the checked nominal dimensions or from measured dimensions. Calculate the compressive strength of each cube by dividing the maximum load by the cross-sectional area. Express the result to the nearest 0.5 MPa (N/mm2).
2. Calculate the average of the test results for the two specimens of the same size and dimension, made from the same sample of fresh concrete. This is reported as the Compressive Strength of the test sample.
