Three-dimensional finite element (FE) thermal and stress models were created to compute the temperature evolution, thermal stress and potential of cracking in a segmental concrete box girder segment during construction. User-defined subroutines were developed in the ANSYS program to activate the degree of hydration-dependent heat rate and material properties, and creep behavior in the thermal and stress calculations. The developed FE model was verified with experimental measurements of a concrete cube. Adiabatic temperature rise, together with compressive strength and splitting tensile strength for a high-strength concrete mix typically used in construction of box girders were tested and incorporated in the subroutines. The effect of casting time and placement season (summer and winter), initial concrete temperature change, and insulation on the risk of cracking in a cast-in-situ box girder segment at early ages was investigated using the proposed model. The results indicate that the temperature difference between the segment’s middle and the gate corner is very large leading to a high cracking risk. Use of an insulation material such as blankets along with casting concrete at the nighttime would significantly lessen the thermal tensile stress and thus could reduce cracking risk in the segment.

Engineering infrastructure includes a large number of concrete structures. The main binder used in concrete is Portland cement, which reacts with water when mixing. During the chemical reaction (cement hydration) heat is released causing non-uniform temperature distribution and subsequent internal stresses that may crack the concrete. Therefore, accurate prediction of concrete temperature evolution in design and control of concrete temperature in construction are crucial tasks for engineers. This paper presents the development of a finite difference (FD) model for predicting thermal behavior of concrete structures during cement hydration. A formwork or insulation layer that covers the outer of the concrete was accounted for in the model. The temperature profiles calculated by the proposed model show close agreement with those measured from a concrete bridge footing and a concrete cube. The Particle Swarm Optimization (PSO) algorithm was adopted to compute an “equivalent convection coefficient” for the convection boundary condition in the simplified model for several common insulation materials. The results show that using the simplified model along with an equivalent convection coefficient can give a significant reduction in the computation time while the accuracy is still maintained. In addition, an equation for estimating an equivalent convection coefficient for a given insulation material was developed for design and practical purposes. The proposed method can be used in the design of concrete structures and control of concrete temperature to prevent early-age thermal cracking thus ensuring the expected durability and service life. From the obtained results, varying equivalent convection values are also recommended to be used for different insulation materials.

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.