Compression strength of concrete
Concrete possesses an excellent compressive strength. The compressive strength depends on many factors such as water-cement ratio, aggregate gradation, chemical additives, pozzolans, consolidating, and curing of casted concrete. Compressive strength of concrete usually tested by crushing cube or cylinder sample. Cubes dimension are 150mmx150mmx150mm while the cylinder dimension is 150mmx300mm. usually, the cylinder compressive strength is lower than the cube compressive strength. Cylinder compressive strength estimated to be 80% of cube compressive strength.
Usually, 6 samples are taken from each concrete pour. 3 samples will be tested at 7 days, and the remaining samples will be tested at 28 days. Concrete is estimated to reach most of it is strength at 28 days. At 7 days, concrete is expected to reach 65-75% of 28 days compressive strength. The testing of concrete samples at 7 days will give an indication of concrete quality and the expected final compressive strength. Therefore if 7 days of compression test results are less than the expected, the concrete quality should be improved to ensure that the concrete meeting the requirements.
The reason for selecting a 150mmx150mmx150mm cube dimension could be the capacity of the testing machine. Let say the machine can supply enough compressive force to crush concrete with compressive strength less than 50 Mpa, for example. If the strength is more than the machine capacity. The size of the cube will be reduced so the machine can develop sufficient strength to crush the cube.