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X Concrete is supposed to be tested for uniaxial
compression with the line of action of load on a cube specimen at right angles to the axis of cube
about which it is cast. However, cube specimens of concrete tested are under nonuniform
triaxial compression. Aslo, in actual structure, the concrete at any point is in a complex stress
condition and not in a uniaxial compression which forms a ground for the recommendation of
cube specimen. Concrete under triaxial state offers more resistance and fails only after
considerable deformations giving higher crushing strength than the actual, which justifies
uniaxial compression testing and the use of cylindrical specimen.
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X The test specimen, as regards the
size and shape, is different in different countries. Generally 150 mm cubes are specified
irrespective of size of aggregate. However, for aggregate less than 19 mm in size 100 mm cube
is also allowed.
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2 2 X Strength of concrete depends upon moisture content at the
time of testing. The dry cubes may have drying shrinkage and bond failure leading to smaller
strength. The moisture content in concrete provides lubrication effect and reduces the strength.
Due to dilation of the cement gel by the absorbed water the force of cohesion between the
cement particles is decreased. Probably the decrease in strength on account of this reduction is
more than that of the loss of strength due to rupture of gel bond and the dry cubes give higher
strength. Also, the specimen should be tested immediately after taking it out of curing tank to
give uniformity to results as compared to testing of samples which may have dried to different
degrees.
Strength of dry sample = (1.1 to 1.2) × strength of saturated sample
e 2 X These are formed because of the evaporation of the water used in making concrete
and by entrained air. If the water used is more than the optimum water required for maximum
strength the concrete becomes permeable and susceptible to deterioration. The reduction in
strength with an increase in percentage air voids is shown in Fig. 10.21.
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