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.
One of the methods for controlling the heat of hydration of mass concrete structures is to insulate the poured concrete. Currently, no method can provide adequate insulation to prevent early-age cracking for mass concrete footings. This study developed a method for determining the required insulation for rectangular footings. The study included isothermal calorimetry testing of cementitious materials, field monitoring of temperature in footings, and finite element modeling. A fully insulated bridge pier footing constructed in the field in Florida was monitored for temperature development and for assessing the efficiency of the insulation used. A parametric study consisting of 63 finite element analyses was conducted on three footing shape—cubic footing, 4:4:1 footing (length–width–depth ratio of 4:4:1), and 4:2:1 footing (length–width–depth ratio of 4:2:1)—to determine the required insulation for footings with a volume-to-surface area ratio ranging from 1.1 to 13.1 ft. The obtained results suggest that the Styrofoam insulation used for the specific monitored footing might be excessive; therefore, the actual insulation thickness of 2 in. should be reduced to 1 in. to reduce the construction cost. In general, with a volume-to-surface area ratio of less than 4.0 ft, under the same insulation condition and with the same concrete mix, larger footings require a greater thickness of insulation. However, with a volume-to-surface area ratio of 4.0 ft or greater, larger footings only require a similar thickness of insulation to prevent cracking. The developed method for determining the required insulation for footings presented in this paper would be a practice ready for implementing in the field.
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