Civil Engineering Reference
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16.2 Experimental Program
16.2.1 Emerging Time, Size, Shape, and Character
of Dehydration Crack
As discussed above, concrete crack can be divided into several kinds. Among them,
dehydration crack refers to crack which is generated from plastic stage to a stage
around
final setting after concrete is casted into mold. At the moment when concrete
transforms from plastic stage to solid stage, we also call concrete shrinkage gen-
erated in this stage as plastic shrinkage, and call the crack as plastic crack. As is
known to all, process of concrete setting is the process of shrinkage. Right now,
most hydration of cement hydration has just begun and free water in concrete will
be consumed by hydration. If free water in concrete is consumed at a greater speed
or exceeds a critical value, concrete surface will generate crack as it fails to
withstand an overlarge tensile stress, which is the internal cause for generation of
dehydration crack; additionally, if concrete is in
uenced by external environment
factors, such as high temperature, strong wind, and low relative air humidity, free
water on its surface will evaporate massively in the same way, which also makes
concrete generate crack as failing to bear exaggerated tensile stress, which is the
external cause for generation of dehydration crack.
Photographs in Figs.
16.1
,
16.2
, and
16.3
are typical dehydration cracks taken by
the author at construction sites all over China. The small dehydration cracks are as
short as several centimeters, and their lengths are almost the same; the deepest one
is generally less than one-third of concrete slab thickness; large ones can as long as
one or two meters, which even penetrate the whole member and result in rupture.
Generating time of dehydration crack mainly concentrates on plastic stage of
concrete.
Fig. 16.1 Dehydration crack
of a certain airport in Hebei
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