The effect of temperature rise and fall on concrete at early ages is rarely measured. Concrete cured at elevated temperature has reduced strength at later ages. The changes that occur in the hydration products have been measured in terms of the amount of calcium hydroxide [Ca(OH)²] present. Since permeability is, to some degree, a function of the water/cement ratio and strength, the measurement of permeability was necessary. Concrete is more permeable after it has undergone a temperature cycle similar to that found in practice. The use of Class F fly ash can effectively reduce the permeability of heat-affected concrete.