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PLASTIC SHRINKAGE CRACKS & ITS PREVENTION IN CONCRETE
Plastic Shrinkage Cracks and Its Prevention in Concrete
Cracking caused by plastic
shrinkage in concrete occurs most
commonly on the exposed surfaces
of freshly placed floors and slabs or
other elements with large surface
areas when they are subjected to a
very rapid loss of moisture caused
by low humidity and wind or high
temperature or both.
Plastic shrinkage usually occurs prior to final finishing, before curing starts. When
moisture evaporates from the surface of freshly placed concrete faster than it is
placed by curing water, the surface concrete shrinks. Due to the restraint provided
by the concrete on the drying surface layer, tensile stresses develop in the weak,
stiffening plastic concrete, resulting in shallow cracks that are usually not short and
run in all directions. In most cases, these cracks are wide at the surface. They range
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from a few millimeters to many meters in length and are spaced from a few
centimeters to as much as 3 m apart.
Preventing Plastic Shrinkage Cracks in Concrete
Plastic shrinkage cracks may extend the full depth of elevated structural slabs.
Since cracking because of plastic shrinkage is due to a differential volume change
in the plastic concrete, successful control measures require a reduction in the
relative volume change between the surface and other portions of the concrete.
There are many methods and techniques to prevent this type of crack in case of
rapid loss of moisture due to hot weather and dry winds. These methods include
the use of fog nozzles to saturate the air above the surface and using plastic
sheeting to cover the surface between the final finishing operations. In many
cases, during construction it is preferable to use wind breakers to reduce the wind
velocity; sunshades to reduce the surface temperature are also helpful.
Additionally, it is good practice to schedule flat work after the walls have been
erected.
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