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2 w X Compressive strength tests of concrete are made in various machines
either of the lever-arm type or adoptions of lever presses as well as computerized. Considerable
discrepancies have been found to occur between the results of compressive strength test made
with different types of machines. It may be up to even 20 per cent. It may be because of errors
in centering the cubes, wear of the lower compression plate, friction in the spherical seating of
the upper compression plate, and inaccurate calibration of the machine itself.
2 2 e 2 2 g
X The effect of type of cement on the rate of strength gain is shown
in Fig. 10.23. With age the strength of cement reduces since it will set by absorption of moisture
from the atmosphere. The effect of storage of cement under different conditions with age is
shown in Fig. 10.24.
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2 2v X Strength of concrete increases with the increase in rate of loading. The
influence of rate of application of load on the compressive strength of concrete is shown in Fig.
10.22. Normally in a static loading test, rate is such that the test is completed in 2 to 3 minutes.
Within this range, the effect is found to be negligible. At low rates of loading there is more time
for creep to occur, so that the increase of strength with rate of loading provides evidence for the
theory that failure occurs at limiting values of strain, rather than stress. The relation between
observed strength and rate of loading is given by
S = S 1 (1+ k log R)
where S = strength at a given rate of loading
R = rate of loading in kg/cm^2 /sec
S 1 = strength of concrete at a rate of 1 kg/cm^2 /sec
k = a constant = 0.08 for 28 day test
The rate of loading of cube specimen is 14 N/cm^2 /min.