Plastic shrinkage cracking is a potential problem when latex modified concrete (LMC) is placed on a dry, windy day. LMC has a lower water content than conventional concrete so it bleeds less. Because plastic cracking is caused by evaporation rate exceeding bleeding rate, LMC's low bleeding rate makes plastic cracking more likely. The American Concrete Institute's Standard Practice for Curing Concrete (ACI 308-81) recommends taking precautions against plastic shrinkage cracking if the rate of evaporation approaches 0.2 pound/square foot/hour.
But other LMC data and experience show that precautions are necessary if the evaporation rate exceeds 0.1 pound/square foot/hour. Whenever temperature, relative humidity, and wind conditions are such that evaporation rate exceeds the critical 0.1 value, contractors should be especially careful. They should take precautions such as: building windbreaks; starting curing sooner; cooling the concrete; and using fog sprays.
