Explain the laboratory procedure to determine the compressive strength of concrete cubes as per IS-516-1959 with reference to following points: i. Preparation of test specimen ii. Procedure of testing iii. Interpretation of results

Explain the laboratory procedure to determine the compressive strength of concrete cubes as per IS-516-1959 with reference to following points: i. Preparation of test specimen ii. Procedure of testing iii. Interpretation of results
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Preparation of test specimen: i. Take three cubes of 15 cm sides and apply oil to its inner surface. ii. Prepare the concrete mixture of required grade and fill it in each mould in 3 layers. Compact each layer 25 times with 16 mm dia. steel rod. iii. Keep all the moulds at room temperature for 24 hrs for initial hardening and at relative humidity 90%. iv. Remove cube moulds and keep concrete cubes under fresh water for curing for 7, 14, 21, 28 days.

Procedure of testing: i. Remove cube from water after curing period and keep it under compression testing machine (CTM) for testing. ii. Apply load at a rate of 35 N/mm2/min for 10 minutes or till failure load in N by cross sectional area of cube in mm2. iii. Finally calculate compressive strength of cubes as failure load in N by cross sectional area of cube in mm2. iv. The average of three test cubes can be calculated as average compressive strength in MPa 

Interpretation of results: i. If the calculated compressive strength is less than the grade of concrete used, then concrete can be rejected at site. ii. When such strength is found more (say 23N/mm 2) than the specified grade M20, then that concrete is safe and good for construction.

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