PTV f
2w
y2US72r
Gel-space ratio = h 22
h 0 0.657 C 0.657 500 0.75
0.6423
0.319 C W 0.319 500 0.75 275
Theoretical strength of concrete = 240 (0.6423)^3 = 58.4 N/mm^2.IHT ixqr2yp2gyxgii
The most useful property of concrete is its compressive strength. However, it is weak in
tension. Till date no relation exists between compressive, tensile, bending, and shear strengths
of concrete. However, it may be mentioned here that typically the tensile and bending strength
of the concrete are of the order of 10 and 15 per cent, respectively of the compressive strength.
The shear is approximately 20 per cent of the uniaxial compressive strength.
2
2
2
2g
2@sX2SITA
Testing plays an important role in controlling the quality of cement concrete work. Systematic
testing of the raw materials, the fresh concrete and the hardened concrete is inseparable part of
any quality control programme for concrete which helps to achieve higher efficiency of the
materials used and greater assurance of the performance of the concrete in regards to both
strength and durability. Since the strength of concrete is related to the structure of hardened
cement paste, it assumes more importance. Indian Standard specifies compression test, flexure
and test tensile strength test for assessing the strength of concrete.
g
2
Cement, fine aggregate and coarse aggregate (up to 38 mm) to be used for making concrete are
brought to room temperature (preferably 27± 3°C) before commencing the test. The ingredients
are weighed in the ratio to be used in the field and are mixed by hand mixing or by machine
mixing. First, the cement and sand are mixed thoroughly till a uniform colour is achieved. The
coarse aggregate are then added and mixed till these are distributed uniformly throughout the
mix. The water is then added and the entire batch mixed untill the concrete appears to be
homogeneous and has the desired consistency.
The test specimens recommended are 150 × 150 × 150 mm cubes or cylinders of 150 mm
diameter and 300 mm height. The mixed concrete is filled into the moulds in layers of 50 mm
to achieve full compaction. Each layer of mix so placed is tamped with bar, 16 mm in diameter
and 600 mm long, 35 times or with a vibrator. The test specimens are stored at a temperature
of 27± 3°C and at 90 per cent humidity for 24 ±½ hour from the time of addition of water to the
dry ingredients. After this period the specimens are removed from the moulds and placed in
water and kept there until taken out just prior to test. Normally, the recognized age of test of
specimens is 7 and 28 days. At least three specimens, preferably from different batches, are
tested at each selected age. The specimens should be tested immediately after taking out them
from water with surface water wiped off.
The specimen is placed between the platens of the compression testing machine with the
care that the axis of specimen is aligned with the centre of thrust of the spherically seated
platen. The compression testing machine should be able to apply gradual load of 14 N/mm^2 /
minute. Until the specimen is crushed.