Question

You are constructing a building, with a concrete mix which has been specified to have a 28-day strength of 50 MPa at a water/cement ratio of 0.40. The laboratory of the supplier has tested the strength of the concrete using 25 mm cubic specimens at 28 days of age, and achieved a result of 51 MPa. Explain in detail whether or not you would accept this material for use in your building, and describe the reasons for this decision.

Answer 1

Characteristics strength of concrete is checked at site by conducting destructive compressive strength test on 150 mm cubes after 28 day curing. Also it is experimentally well established that compressive strength determined using 100 mm cubes instead of 150 mm cubes gives higher results by about 5-6%. Thus the value of compressive strength obtained from 100 mm cubes must be corrected to obtain actual check on characteristics strength. The laboratory testing has been done on 25 mm cubes and thus the characteristics strength of 50 MPa would not be achieved at site as explained above and hence the concrete should not be used in building work.