COMPRESSIVE
STRENGTH TEST
OBJECT: To determine the characteristic compressive strength of given sample of concrete cubes of size 150mm x 150mm x 150mm
APPARATUS:
(i) Testing Machine: - The testing machine may be of any reliable type, of sufficient capacity for the tests. The permissible error shall be not greater than ± 2 percent of the maximum load.
(ii) Tests shall be made at recognized ages of the test specimens, the most usual being 7 and 28 days. The ages shall be calculated from the time of the addition of water to the dry ingredients
(iii) Number of Specimen - At least three specimens, preferably from different batches, shall be made for testing at each selected age.
PROCEDURE:
(i) Specimens stored in water shall be tested immediately on removal from the water and while they are still in the wet condition. Specimens when received dry shall be kept in water for 24 hours before they are taken for testing. The dimensions of the specimens to the nearest 0-2 mm and their weight shall be noted before testing.
(ii) Placing tile Specimen in the testing machine: the specimen shall be placed in the machine in such a manner that the load shall be applied to opposite sides of the cubes as cast, that I.S not to the top and bottom I he axis of the specimen shall be carefully aligned with the center of thrust of the spherically seated platen. The load shall be applied without shock and Increased continuously at a rate of approximately 140 kg/sq-mt/min until the resistance of the specimen to the Increasing load breaks down and no greater load can be sustained. The maximum load applied to the specimen shall then be recorded and the appearance of the concrete and any unusual features in the type of failure shall be noted.
CALCULATION:
The measured compressive
strength of the specimen shall be calculated by dividing the maximum load
applied to the specimen during the test by the cross-sectional area. Average of
three values shall be taken as the representative of the batch provided the
individual variation is not more than ± 15 percent of the average. Otherwise
repeat tests shall be made.
