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gXCracks in concrete may originate from one or more of the following causes:
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X Water in excess of the requirement causes coarse aggregate to settle more and
the water bleeds to top and evaporate leaving voids. This porous, weak concrete will be unable
to bear shrinkage stresses without cracking. Concrete may also shrink down in the forms in the
cases of beams and walls.
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X results in shrinkage cracks during or soon after the finishing because of
moisture absorption by dry subgrade, dry forms, dry aggregates or hot sunny weather.
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X arising out of incorrect selection of coarse aggregate may lead to
cracks.
X The corrosion of steel bars may cause cracks and rust stains to appear in the concrete.
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2 2 X The cracks because of freeze and thaw are normally observed in concrete
with high w/c ratio, producing tiny crevices and voids around the aggregates, allowing the
rain water to penetrate into concrete. On freezing, the ice produces tremendous force causing
spalls and cracks.
g3 X of concrete products results from differences in shrinkage between the surface and the
interior. The cracks rarely exceed 12 mm or so in depth, and are therefore not serious, apart
from the unsightliness. The best method to overcome crazing is to be either to use an earth-
moist mix, or if a plastic mix is necessary use as low a w/c ratio as is practicable and remove the
cement skin to expose the aggregate. Trowelling should be avoided, as the surfaces are prone
to crazing.
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X Sulphate attack is mainly caused by the soil containing sulphates or by
sulphate water.
iX is the appearance of fluffy white patches on the surface of concrete members. It
is caused by poorly washed aggregate, salty water used in making concrete the salts being
leaching out to the surface by rain water afterwards. As the water evaporates white patches
appear on the surface. This defect can be controlled to some extent by coating the surface by a
water repellent.
X usually implies separation of: (a) coarse aggregate from fine aggregate, (b) paste
from coarse aggregate, or water from the mix and the ingredients of the fresh concrete no
longer remain uniformly distributed. It can be reduced by increasing small size coarse aggregate,
air entrainment, using dispersing agents and puzzolana.
The causes of segregation are dropping concrete from heights, badly designed mixes, concrete
carried over long distances—pumping, belt conveyor system etc. over vibrations, and during
concrete finishing extra floating and tamping. Segregation mainly occurs in dry non-sticky
concrete mixes.
f X defined as an autogeneous flow of mixing water within or emergence to the surface
from freshly placed concrete is usually due to excessive vibrations imparted to concrete to
achieve full compaction. However well concrete may have been compacted, the force of gravity