Compressive strength at a specified age, usually 28 days, measured on a standard cube or cylinder specimens is used as the criteria for the acceptance of concrete. The compressive strengths used in structural applications vary from 20MPa to as high as 100MPa.
In India, the characteristic compressive strength (fck) is determined by testing to failure a 28 day old concrete cube specimen of size 150mm x 150mm x 150mm.
Preparation of Mould:
The concrete is poured in the cube or cylinder mould in layers of 50mm and compacted properly by either hand or a vibrator so that there are no voids. After leveling the top surface, the test specimens are then stored in moist air at room temperature for 24 hours. After this period the specimens are marked and removed from the moulds and kept submerged in clear fresh water until they are tested.
Testing of Specimens:
These specimens are tested by a compressive testing machine after 7 days of curing 28 days of curing. Load should be applied gradually at the rate of 140kg/cm² per minute. A minimum of three specimens should be tested at each selected age. If the strength of any specimen varies by 15%, then the results of the specimen should be rejected.
Factors affecting compressive strength:
The compressive strength of concrete is affected by important factors such as water/cement ratio, type of cement, use of supplementary cementitious materials, type of aggregates, quantity and quality of aggregates , quality and quantity of mixing water, moisture and temperature conditions during curing, age of concrete, rate of loading during cube or cylinder test and the size of specimens.