Plastic Shrinkage Cracks appear in the surface of fresh concrete soon after it is placed and while it is still plastic. These cracks appear mostly on horizontal surfaces. They are usually parallel to each other on the order of 1 to 3 feet apart, relatively shallow, and generally do not intersect the perimeter of the slab.

 

Plastic shrinkage cracks are caused by a rapid loss of water from the surface (evaporation rates) which are causing the surface to dry faster than the bottom and before initial set.  

 

• Wind velocity in excess of 5 mph
• Low relative humidity
• High ambient and/or concrete temperatures

 

The critical condition exists when the rates of evaporation of surface moisture exceeds the rate at which rising bleed water can replace it. Water receding below the concrete surface forms menisci between the fine particles of cement and aggregate causing a tensile force to develop in the surface layers. If the concrete surface has started to set and developed sufficient tensile strength to resist the tensile forces, cracks do not form. If the surface dries very rapidly, the concrete may still be plastic, and cracks do not develop at that time; but plastic cracks will surely form as soon as the concrete stiffens a little more.

To reduce the potential for plastic shrinkage cracking, it is important to recognize ahead of time, before the placement, when weather conditions conducive to plastic shrinkage cracking will exist. Precautions can then be taken to minimize its occurrence.

 

• Early morning placements can afford better control on temperatures.
• Use fog sprays to discharge a fine mist upwind and into the air above the concrete.
• Dampen the sub-grade, form work and reinforcement but not to a point that there is free standing water prior to placement.
• The use of vapor retarders under a slab on grade greatly increases the risk of plastic shrinkage.
• Consider using synthetic fibers to resist plastic shrinkage.
• Start curing methods as soon as possible.
• Accelerate the setting time of concrete and avoid large temperature differences between concrete and air temperatures.

 

If plastic shrinkage cracks should appear the finisher may be able to close them by refinishing. However, when this occurs precautions should be taken as discussed above to avoid further cracking.