Importance of Insulation at the Bottom of Mass Concrete Placed on Soil with High Groundwater

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Abstract
Frequently, when mass concrete is placed directly on top of a soil layer, an insulation layer is not used at the bottom of the concrete. The rationale for this practice is that the soil on which the concrete is placed is already an insulating material. This study investigated the question of whether the absence of an insulating layer between the mass concrete and the soil may cause a problem with cracking of the concrete at an early age. A three-dimensional finite element model was used for this investigation. The typical soil condition in Florida, where the ground-water level is high, was considered. The soil layer beneath the concrete was modeled to simulate realistic heat transfer between the concrete and the soil. To validate the developed model, temperature development in a bridge pier footing constructed in the field in Florida was compared with the computed temperature distribution from the finite element model. The results showed that the temperatures predicted by the model closely agreed with those measured in the field. A parametric analysis was also conducted to determine the effects of insulation on the temperature distribution, induced tensile stresses, and cracking risk in the concrete. From the thermal cracking analysis for the monitored footing, it is concluded that full insulation with adequate thickness (a 63-mm-thick blanket at the top, 63-mm plywood panels at the sides, and a 63-mm polystyrene foam board at the bottom) should be used to reduce the temperature differentials and prevent early-age cracking in concrete.