Building Materials, Third Edition

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Factors affecting the strength of concrete can be broadly grouped into those depending upon
the testing methods and the others independent of the testing methods. Factors depending on
testing methods are size of test specimen, size of specimen relative to maximum size of
aggregate, moisture condition of specimen, rate of loading adopted, and type of testing machine
used; and those independent of testing method are type of cement and age of cement, type and
size of aggregates, degree of compaction, water-cement ratio, aggregate-cement ratio, air-
voids, curing method and curing temperature, and type of stress situation that may exist
(uniaxial, biaxial and triaxial).

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2Ī X The form of specimen which is most commonly adopted is the cube.
The concrete cubes of side 150 mm, cured for 28 days are tested in the moist saturated condition
in compression. For brittle materials such as concrete which rupture on planes inclined more
than 45° with the horizontal, the cube is not suitable; since the strength is increased by frictional
restraint acting at the surfaces under pressure. Frictional restraint from the platens opposes the
lateral expansion of the specimen, and subjects its ends to inward compressive forces. These
diminish with distance from the ends, but a cube is so short that their effect extends throughout
its volume. A cube is therefore tested under nonuniform triaxial compression. Prisms or cylinders
with a height equal to twice the least lateral dimension are more suitable for such materials. End
effects have less influence in these specimens, and it is generally agreed that the cylinder strength
of concrete is a good estimate of the monoaxial compressive strength. A cube of concrete is
expected to have a strength 15 per cent greater than a cylindrical specimen. If height to side
ratio is changed the compressive strength of the prisms relative to cube strength changes and
is given in Table 10.4. Beyond a height to side ratio of 4 it stabilises. The effect of height to lateral
dimension ratio of specimen on compressive strength is shown in Fig. 10.20.

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If size of cube is decreased the compressive strength tends to increase (Table 10.5), whereas
modulus of elasticity decreases.

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