Plastic shrinkage cracks appear on the surface of a freshly placed concrete slab during finishing operations or soon after.Be sure to distinguish plastic cracks from other early or pre-hardening cracks caused by settlement. Plastic cracks are usually parallel to each other, between 1 and 3 feet apart, and do not cross the perimeter.
Common Causes
• High winds produce rapid surface evaporation- an increase in wind speed from 0 to 10 mph will increase the rate of evaporation 4 times.
• High slump concrete increases shrinkage. Excess water can be expected to increase shrinkage approximately in proportion to its percentage of the total mix water.
• Vapor barriers are a key contributor to plastic shrinkage cracking. All bleed water must migrate to the surface, which seriously affects timing, and surface set control methods.
• High air and concrete temperatures increase rate of evaporation and set-a 20 degree temperature increase can double the rate of evaporation.
• Low relative humidity can increase rate of evaporation by five times.
• Rapid surface evaporation caused by all the above, can also happen inside with heated concrete in cold weather.
• Dry subgrade in hot weather can absorb too much water from the slab.
• Unsaturated absorptive aggregates in the mix decreases set time.

Prevention
To reduce plastic shrinkage cracking it is important to recognize ahead of time, before placement, when weather conditions may occur that are conducive to plastic shrinkage cracking. Precautions taken prior to and during construction will minimize its occurrence:

• Thoroughly dampen the subgrade just prior to placement.
• Schedule placement in late afternoon or early morning, if possible.
• If vapor barriers are required, cover them with a 2- inch layer of compacted damp sand.
• Have proper manpower, equipment and supplies on hand so that the concrete can be placed immediately and finished properly.
• Erect sunshades to control the surface temperature of the slab.
• Erect windbreaks to reduce wind velocity over the surface.
• In hot weather, it may be necessary to spray evaporation retardant on the surface. This product can be used after each finishing operation.
• Start curing the concrete immediately after finishing is complete. The longer the cure is in place the higher the strength.
• Avoid using water reducing admixtures with high amounts (in excess of 2%) of calcium chloride-they may actually increase shrinkage at early ages.
• Other options - Sprinkle the aggregate piles, add ice or liquid nitrogen, retarders, fly ash or type II cement to the concrete.