Increasing the Allowable Temperature Differential in Mass Concrete Elements
Why Using Maturity to Determine the Allowable Variation in Temperature is Important
Measuring the temperature differential in mass concrete elements is essential. Because of the mass effect, the concrete core can have a really high temperature while the surface, which is greatly affected by environmental conditions, tends to be cooler. If the difference in temperature between the core and the surface is too large, it can cause internal thermal stress. If the tensile strength of the concrete is not high enough to withstand the thermal stress it can create significant cracking.
The ACI 207- Mass Concrete guideline states that the difference in temperature between the center of the element and the surface must remain smaller than 20ºC (68 ºF) during curing. In the majority of cases this approach is very conservative, in other cases it can be an overestimation of the allowable gradient.
As concrete hardens the tensile strength increases, which means that the concrete is actually able to withstand higher temperature gradient differential as it cures. In the past, obtaining the actual in-place strength was a challenge, but with recent developments in technology, using maturity to determine the in-place strength as become a lot easier. By measuring the in-place strength based on maturity, it is possible to determine the actual temperature differential allowed in order to prevent cracking. The following temperature difference limit equation can be used:
Using maturity to determine the allowable variation in temperature in your mass pour can reduce the amount of heating or the cooling required as well as provide the appropriate length of time for curing.