AIM
To determine the compressive strength of concrete specimens
as per IS: 516 - 1959.
APPARATUS
COMPRESSION TESTING MACHINE
i) Compression testing machine conforming to IS: 516 - 1959
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
continuosly 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.