TEST FOR COMPRESSIVE STRENGTH OF CEMENT CONCRETE
(IS: 516 - 1959)
OBJECT:
Determination of the compressive strength of cement concrete specimens.
APPARATUS:
Testing Machine, two steel bearing platens with hardened faces (As per IS: 516- 1959).
THEORY:
Tests shall be made at recognized ages of the test specimens, the most usual being 7 and 28 days, ages of 13 weeks and one year are recommended if tests at greater ages are required. Where it may be necessary to obtain the early strength, test may be made at the ages of 24 hours + 1/2 hour and 12 hours + 2 hours. The ages shall be calculated from the time of the addition of water to the dry ingredients. Number of Specimens At least three specimens, preferably from different batches, shall be made for testing at each selected age.
PROCEDURE:
Specimens stored in water shall be tested immediately on removal from water and while they are still in the wet condition. Surface water and grit shall be wiped off the specimens and any projecting fines removed. 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. Placing the specimen in the Testing Machine The bearing surfaces of the testing machine shall be wiped clean and any loose sand or other material removed from the surfaces of the specimens which are to be in contact with the compression platens. In the case of the cubes, 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 is, not to the top and bottom. The axis of the specimen shall be carefully aligned with the center of thrust of the spherically seated platten. No packing shall be used between the faces of the test specimen and the steel platten of the testing machine. As the spherically seated block is brought to bear on the specimen, that movable portion shall be rotated gently by hand so that uniform section may be obtained. The load shall be applied without shock and increased continuously at a rate of approximately 140 Kg/sq cm/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, calculated from the mean dimensions of section and shall be expressed to the nearest Kg/sq.cm. Average of three values shall be taken as the representative of the batch provided the individual variation is not more than + 15% of the average. Otherwise, repeat tests shall be made. In case of cylinders, a correction factor according to the height to diameter ratio of the specimen after capping shall be obtained from the curve shown in fig. 1 of IS: 516-1959. The product of this correction factor and the measured compressive strength shall be 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 shall be determined by multiplying the corrected cylinder strength by 5/4.
REPORTING OF RESULTS:
The following information shall be included in the report on each test specimen:
a) Identification mark
b) Date of test
c) Age of specimen
d) Curing conditions including date of manufacture of specimen in the field
e) Weight of specimen
f) Dimensions of specimen
g) Compressive strength
h) Maximum load and
i) Appearance of fractured faces of concrete and type of fractures if these are unusual.
RESULT: Compressive strength of Concrete --------------.
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