Among the many unique features of the Wilshire Grand mat foundation pour was the thermal control plan. Occasionally we need to make provisions for the heat that is generated in mat foundations. In mass concrete structures there are two thermal properties that must be considered: the maximum temperature and the temperature difference between the center of the structure and surface.
There are multiple ways to control the overall thermal properties of mass concrete. For the Wilshire Grand pour, one of the first things Conco addressed was the concrete mix design. There are a few common adjustments that can be made to the actual mix, but they all have some impact on the production rate when the concrete is being batched. In this case, working with our supplier Catalina Pacific, we increased the proportion of fly-ash which normally results in reduced maximum temperatures.
The next step was to mechanically control the temperatures. Pre-chilling the concrete was not an option due to the rate that we needed to batch and place the concrete. Nor was there a way to feasibly pre-chill 21,200 cubic yards of material ahead of time so we had to consider post-chilling by means of cooling pipes to mechanically remove heat.
For this, we turned to John Gajda of the CTL Group. CTL developed a thermal control plan specific to the mix design, the geometry of the foundation, and the configuration of the rebar. With help from Couts Heating and Cooling, we implemented CTL’s thermal control plan. Ultimately the system we installed to post-chill the concrete was composed of 18 miles of cooling pipes, over 660 tons of chiller capacity, 40,000 gallons of chilled water, multiple pumps, 3 x 500kva generators, and over 2,200 valves. This includes a large amount of contingency and redundancy for reacting to unanticipated thermal phenomena and providing backup during the critical early stages of the thermal control plan.
Subsequently we installed 24 thermal couplers to digitally monitor the thermal properties of the concrete. For assistance, we teamed up with Twining Laboratories who were able to provide hourly readings of the foundation utilizing Intelirock equipment. By monitoring temperatures closely, we could respond by increasing chiller capacity and/or adding insulation if required. Furthermore, monitoring allowed us to determine when the thermal control system could be removed and the concrete core started.
In the end the thermal control system was a success. The maximum temperature was kept below ACI limits and the temperature difference was well within the ranges required to avoid thermal cracking.