Explain the procedure to determine compressive strength of concrete in lab.
Procedure to determine compressive strength of concrete in lab: The following steps are essential to determine the compressive strength of concrete in the laboratory. 1. Take three cube moulds of 15 cm side and apply oil to its inner surface. Prepare the concrete mixture of required grade and fill it in each mould in three layers. 2. Compact each layer evenly spaced 25 times strokes with 16 mm. diameter standard tamping rod. Compaction of concrete is done by using table vibrator to remove air completely from concrete. 3. Keep all the moulds at room temperature for 24 hrs for initial hardening and at relative humidity 90%. 4. Remove cube moulds and keep concrete cubes under fresh water for curing for 7, 14, 21, 28 days. 5. Remove cube from water after curing period and keep it under compression testing machine (CTM) for testing. 6. Apply compressive load at a rate of 4 tones / min for 10 minutes or till failure of cube. Note down the failure load in kN shown by red pointer of dial gauge. 7. Calculate compressive strength of each cube. Take failure load in N and cross sectional area of cube in mm2 . 8. Calculate average compressive strength of three test cubes in N/mm2 .