تحلیل عددی تنش حرارتی در روسازی بتنی ساده درزدار

Temperature changes is one of the major factors that causes cracks in concrete pavements. Temperature changes causes expansion or contraction and thus creates a movement in the concrete slab. Friction between the concrete slab and the sub-base layer resists this movement, and induces thermal stress in the concrete. In this research study, the thermal stress created in concrete pavement by a drop in temperature and other intensifying factors (such as friction changes in layers interface), are investigated using numerical analysis method. In current research two finite element models are used to analyze the friction and temperature drops stresses. The first concrete slab modeled does not have a dowel bar and the second includes the dowel bar with or without misalignment. Finite element models using ABAQUS software are created for concrete pavement with and without dowel bars, and with various coefficient of friction between the concrete slab and the sub-base layer. The effects of coefficient of friction between the concrete slab and the sub-base layer and temperature changes on frictional and tensile stresses in the concrete slab are analyzed for various cases. The results obtained in this research study indicate that the high temperature drops (about 30 °C) during the 7 to 8 initial hours of construction of concrete pavement and a high coefficient of friction in the layers interface are the main causes of cracks in concrete pavements. The combined effects of these factors, high temperature drops and high coefficient of friction in the layers interface, worsen the conditions. The reason for increased chance of occurring cracks during the initial days of the concrete formation, is reduction of tensile strength of concrete during this period. In second part of the research, the thermal stress created in the concrete pavement with dowel bars deviating from the horizontal (a vertical tilt in the bars) is analyzed too. The research results show that the amount and the distribution of frictional and tensile stresses in the concrete slab when the dowel bars have no misalignment and are well lubricated, have conditions similar to the concrete pavements without dowel bars. Meanwhile numerical analysis results show that misalignment of the dowel bars and temperature drop, increases the tensile stress around the dowel bars and increases the threat of concrete disintegration. Ultimately, the results of numerical analysis are compared with the tensile strength of concrete during the initial hours of concrete construction. Results show that a 33˚C drop in temperature and a coefficient of friction about 3, creates a tensile stress of 124000 Pascal in the concrete slab, which can cause cracks in the concrete pavement during the initial 7-8 hours of concrete pavement construction. Furthermore, a vertical tilt in the dowel bars causes a considerable increase in tension stress around the dowel bars. In order to prevent cracks during the initial days and hours of forming concrete pavements, the coefficient of friction in the layers interface should be reduced using friction reduction factors, such as polyethylene sheets, concrete pavement should not be made in conditions with severe temperature drops, and dowel bars should be implemented with no or allowable misalignment.