Q: I'm familiar with what causes plastic shrinkage cracks the evaporation of moisture from the surface. I also know some ways to prevent their occurrence. What I haven't been able to figure out is when they will occur. Can you give me some guidelines so that I can be prepared with the manpower and materials needed?

A.: Plastic shrinkage cracking is more likely to occur on days when one or more of the following weather conditions are present: high temperature, low humidity, or wind. These conditions all lead to a high rate of evaporation of moisture from the surface. Use the following chart to estimate the rate of evaporation. Precautions against plastic shrinkage cracking should be taken when the evaporation approaches:

0.20 lb/ft

2/hr for portland cement concrete

0.15 lb/ft

2/hr for portland cement concrete containing more than 15% pozzolan

0.10 lb/ft

2/hr for portland cement concrete containing more than 5% silica fume

To use this chart:

  1. Enter with air temperature and move up to relative humidity.
  2. Move right to concrete temperature.
  3. Move down to wind velocity.
  4. Move left; read approximate rate of evaporation.