Q: We plan to place a 6-inch-thick slab on grade in cold weather and cover it with insulating blankets. How long after placement do we have to wait before removing the blankets?
A: The answer depends on the air temperature to which the concrete will be exposed. ACI 306.1-90, "Standard Specification for Cold Weather Concreting," requires structural elements with any dimension less than 12 inches to be maintained at a temperature of 55° F for at least 3 days unless otherwise specified. Thus, you should measure the temperature at the slab surface under the insulating blanket at regular time intervals. The specification permits reducing the protection period to 2 days if the concrete contains any of the following:
- n Type III portland cement meeting ASTM C 150 requirements
- A strength-accelerating admixture meeting ASTM C 494 requirements
- 100 pounds per cubic yard of additional cement
ACI 306.1-90 also stipulates that the maximum decrease in surface temperature in a 24-hour period can't exceed 50° F for sections less than 12 inches thick. So at the end of a 3-day protection period, if the concrete surface and air temperatures were 60° F and 0° F, respectively, the specification might prohibit removal of the insulating blankets because the surface temperature could drop too rapidly. The problem lies in predicting how rapidly surface temperature will drop. If you were using two layers of blankets, one approach might be to remove the top layer and monitor the surface temperature under the remaining blanket to see how fast it drops. If it appears to be dropping too fast, you could replace the blanket and wait for the air temperature to rise. Section 3.2.3 of ACI 306.1-90 allows all protection to be removed when the concrete surface temperature is within 20° F of the air temperature.