17.2
Stress and Cracks
Cracks
are commonly induced from numerous magnitudes of stress occurring within the
concrete, which this can be represented in term of graph (17.2), clearly
showing the effects of stress that take place inside the concrete.
As
shown, time and volume differentiation of Stress, Strength and Creep will be subject
to horizontal and vertical axis, respectively. When time increases, concrete
will be dry, then cool down and eventually shrink (curve A – Free Contraction)
but when cylinder is secured at both ends, tensile stress is progressively soar
(curve B – Elastic Stress) while emerge of creep is coincident and
results a deduction of tensile stress in concrete (curve C – Net Tensile
Stress). It would be clearly shown that the presence of creep will relatively
lessen the Net Tensile Stress lower than the Tensile strength of concrete
(curve D – Tensile Strength). In accordance with further increase of
tensile stress, as when its magnitude reaches the optimum tensile strength,
concrete will start to fracture. On the contrary, cracking will be inactive if
tensile stress remains within the tensile strength of concrete.
From
the foregoing paragraph, the influential causes of cracking can be summarized
as follow
-
Shrinkage of concrete when it is hardened or
cool down
-
Concrete is being restrained at both ends
-
Elasticity of Concrete
-
Creep
-
Tensile Strength
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