We have been experiencing a considerable amount of surface cracking. How can this be prevented?
This type of cracking often occurs in freshly placed concrete due to rapid evaporation of moisture from the concrete surface and is commonly referred to as plastic-shrinkage cracking. This can occur at any time due to an unfavorable combination of temperature, low humidity and wind. When the rate of evaporation is as high as 0.2 or 0.3 pounds per square foot per hour, precautionary measures must be taken. The Portland Cement Association and other agencies have studied this problem and suggest the following simple precautions to minimize the possibility of plastic-shrinkage cracking: 1. Dampen the subgrade and forms. 2. Dampen the aggregates if they are dry and absorptive. 3. Erect windbreaks to reduce wind velocity over the concrete surface. 4. Erect sunshades to reduce concrete surface temperature. 5. Lower concrete temperature during hot weather by using cool aggregates and mixing water. 6. Avoid overheating the fresh concrete during cold weather. 7. Protect the concrete with temporary wet coverings during any appreciable delay between placing and finishing. 8. Reduce time between placing and start of curing. 9. Protect the concrete during the first few hours after placing and finishing to minimize evaporation. This is most important to avoid checking and cracking. Application of moisture to the surface, using a fog-spray nozzle, is an effective means of preventing evaporation from the concrete. This should be used until a suitable curing material such as a curing compound, wet burlap or curing paper can be applied.