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Drying Shrinkage: Drying
shrinkage refers to the reduction in concrete volume resulting from a loss of water
from concrete. As the water is lost the concrete will shrink. Initially, free
water escapes to the concrete surface as bleed water, as the heavier aggregate
particles settle. This bleed water can evaporate off the surface to the
surrounding environment.
Once
the bleed water has disappeared the concrete will still be subjected to drying
and excess water will be pulled from the interior of the concrete mass. The
most common situation resulting from drying shrinkage at early ages is the appearance
of surface cracking. As drying takes place, concrete shrinks. Where there is no
restraint, movement occurs freely and no stresses or cracks develop. If the tensile
stress that results from restrained drying shrinkage exceeds the tensile
strength of the concrete, cracks can develop. Random cracks may develop if
joints are not properly provided and the concrete element is restrained from
shortening.
During the early ages shrinkage is even more critical since the
concrete has still not gained much strength. Even the smallest stress during
the early age can have large resulting shrinkage strains.
Drying
shrinkage can continue for many years, though it is generally expected that
about 80% of the laboratory-measured shrinkage occurs within about 3 months.
The duration of shrinkage is dependent on the concrete’s size and shape since
they control the rate of moisture loss. The size and shape are often considered
together as the volume-to-surface area ratio. Larger specimens will shrink
for longer periods but the ultimate magnitude may be lower. A high
volume-to-surface ratio will usually result in lower shrinkage magnitudes.
Autogenous Shrinkage:
Autogenous
shrinkage is defined as a concrete
volume change occurring without moisture transfer to the surrounding environment.
It is merely a result of the internal chemical and structural reactions of the
concrete components.
The
contribution of autogenous shrinkage has previously been viewed as
“insignificant” in typical concrete mixtures due to the dominant role of drying
shrinkage. In recent years the increasing use of high performance
concretes has lead to the re-introduction of autogenous concerns as
the mixtures are using more “special” cements and multiple admixtures while
reducing water. Autogenous shrinkage is usually a concern in high
strength or high performance concrete (> 40 MPa or 6000 psi) where there is
a low water-to-cement (w/c) ratio (say less than 0.42).
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