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

Page 1 of 3PLASTIC SHRINKAGE CRACKS & ITS PREVENTION IN CONCRETE

9/15/2015http://theconstructor.org/concrete/plastic-shrinkage-cracks-its-prevention-in-concrete/5...

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.

Page 2 of 3PLASTIC SHRINKAGE CRACKS & ITS PREVENTION IN CONCRETE

9/15/2015http://theconstructor.org/concrete/plastic-shrinkage-cracks-its-prevention-in-concrete/5...

Page 3 of 3PLASTIC SHRINKAGE CRACKS & ITS PREVENTION IN CONCRETE

9/15/2015http://theconstructor.org/concrete/plastic-shrinkage-cracks-its-prevention-in-concrete/5...