Plastic shrinkage cracking (as shown below) is caused by rapid drying of the concrete surface. They can occur at any time of the year, from cooler low-humidity weather conditions to, in particular, hot weather conditions involving any one or a combination of high temperature, low humidity and wind.
These types of cracks occur while the concrete is bleeding and during the finishing operation. Fresh concrete may be exposed to the elements for a considerable time during these stages, causing the concrete at the surface to dry and shrink before it has any strength to resist the shrinkage forces.
Concrete cracks in much the same way as clay soils.
Methods of Preventing Plastic Shrinkage Cracking of Concrete
1. Erecting Temporary Wind Breaks
Higher wind velocities can greatly affect the rate of evaporation of water from the concrete being placed. This in turn will result is plastic shrinkage cracking of concrete. Therefore erect temporary wind breaks to reduce the wind velocity over the surface of the concrete.
2. Providing Sunshades
In case of construction of concrete slabs, it is very effective to provide sunshades over the concrete slabs and thereby controlling the surface temperature of the slab.
3. Proper Placement Timing
In very hot weather conditions, concrete should not be placed in the afternoon, because it can lead to plastic shrinkage cracks on concrete due to loss of water. Therefore it is a good practice to schedule concrete placement in early morning or early evening. This way we can have better control over concrete temperature.
4. Using Fog Sprays
In very hot and dry periods the wind is very hot. Therefore use fog sprays to spray fog into the air over the concrete surface and in the direction opposite to the direction the wind is blowing. Using of fog sprays reduce the rate of evaporation from the concrete surface.
5. Dampening the Subgrade
During construction of concrete pavement in hot and dry weather, dampen the subgrade which is liable to water absorption from concrete. But remember the subgrade should not be over dampened. The formwork and the reinforcement should also be dampened.
6. Using Evaporation Retarder
Spray an evaporation retarder such as aliphatic alcohol over the freshly screeded concrete surface. Surfaces can dry prematurely when the rate of evaporation is greater than the rate of bleeding. These products form a film on the surface that reduces the rate of evaporation of water, thus allowing time for all the bleed water to rise to the surface. In extreme conditions, re-application of the evaporation retarder may be necessary both before and during trowelling. These products do not affect the strength or performance of the concrete and can act as a finishing aid.
7. Start Curing Immediately
Commence curing as soon as possible after finishing. Spray the surface with liquid membrane curing compound or cover the surface with wet burlap or start water curing after concrete placement.
8. Accelerating Setting Time of Concrete
Use some form of chemical admixture in concrete to accelerate the setting time of concrete and avoid large temperature differences between concrete and air temperature.
9. Adding Polypropylene Fibres in Concrete Mix
Polypropylene fibres can be added to the concrete mix to reduce the incidence of plastic shrinkage cracking. Fibres tend to bind the surface of the concrete together and provide some strength to the plastic concrete to resist the shrinkage forces.
10. Be Ready
Have proper manpower, equipment and supplies on hand so that the concrete can be placed and finished promptly.