The compressive strength of Concrete is the most common performance measure to determine the strength of concrete for designing.This is measured by breaking cubical of cylindrical concrete specimen in Compression Testing Machine.
AIM
To determine the compressive strength of concrete specimens as per IS: 516 - 1959.
APPARATUS
AGE AT TEST
Tests should be done at recognized ages of the test specimens, usually being 7 and 28 days. The ages should be calculated from the time of the addition of water to the drying of ingredients.
NUMBER OF SPECIMENS
At least three specimens, preferably from different batches, should be taken for testing at each selected age.
PROCEDURE
i) The specimens, prepared according to IS: 516 - 1959 and stored in water, should be tested immediately on removal from the water and while still in wet condition. Specimens when received dry should be kept in water for 24hrs. before they are taken for testing. The dimensions of the specimens, to the nearest 0.2mm and their weight should be noted before testing.
ii) The bearing surfaces of the compression testing machine should be wiped clean and any loose sand or other material removed from the surfaces of the specimen, which would be in contact with the compression platens.
iii) In the case a of cubical specimen, the specimen should be placed in the machine in such a manner that the load could be applied to the opposite sides of the cubes, not to the top and the bottom. The axis of the specimen should be carefully aligned with the centre of thrust of the spherically seated platen. No packing should be used between the faces of the test specimen and the steel platen of the testing machine. As the spherically seated block is brought to rest on the specimen, the movable portion should be rotated gently by hand so that uniform seating is obtained.
iv) The load should be applied without shock and increased continuously at a rate of approximately 140kg/sq.cm/minute 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 should then be recorded and the appearance of the concrete and any unusual features in the type of failure should be noted.
CALCULATION
The measured compressive strength of the specimen should be calculated by dividing the maximum load applied to the specimen during the test by the cross - sectional area, calculated from the mean dimensions of the section and should be expressed to the nearest kg/sq.cm. An average of three values should be taken as the representative of the batch, provided the individual variation is not more than ±15% of the average. Otherwise repeat tests should be done.
A correction factor according to the height/diameter ratio of the specimen after capping should be obtained from the curve given below:-
The product of this correction factor and the measured compressive strength is known as the corrected compressive strength, this being the equivalent strength of a cylinder having a height/diameter ratio of two. The equivalent cube strength of the concrete should be determined by multiplying the corrected cylinder strength by 1.25.
REPORTING OF RESULTS
The following information should be included in the report on each test specimen:
i) Identification mark
ii) Date of test
iii) Age of specimen
iv) Curing conditions, including date of manufacture of specimen
v) Weight of specimen
vi) Dimensions of specimen
vii) Cross-sectional area
viii) Maximum load
ix) Compressive strength
x) Appearance of fractured faces of concrete and type of fracture, if unusual.
