Research On Calculation Method About Superficial Heat Preservation Of Massive Concrete

With the extensive application of massive concrete structure on modern engineering construction, the concrete crack caused by temperature stress has drawn more and more attention. In winter that temperature is sub-zero or in early spring and late autumn that air temperature is sudden change and cold wave is frequent, if there isn't any heat preservation measures on the surface of the concrete, the concrete will be liable to crack at surface due to the temperature stress caused by diurnal amplitude and cold wave. So in engineering construction it is urgent to carry out exact numerical simulation on the heat preservation effect of insulation layer before design and construction.In this paper, the calculation method about superficial heat preservation of massive concrete has been researched: based on the principle of heat balance, the computing equation of temperature field which used the thermal coefficient, surface coefficient of heat transfer and quantity of heat as the basic parameters of the heat conduction equation (the thermal coefficient method) is derived, and thereafter the FEM computation procedure about temperature field named RCTS which used the thermal diffusivity as basic parameter is modified. The modified procedure can reasonably reflect the temperature field of composite construction which is made of a great variety of material with different thermal property. According to the equivalent surface coefficient of heat transfer method, the original procedure RCTS has been modified, and the modified procedure can simulate the effect of surface heat preservation at arbitrary periods and arbitrary parts (such as over-wintering level) of the bulk concrete structure. According to the calculation example, the similarities and differences of the heat preservation effect in the heated board calculation by using the method: the thermal diffusivity method, the thermal coefficient method, the equivalent surface coefficient of heat transfer method, the equivalent thickness method are analyzed. The calculation results showed that the computing results of the thermal coefficient method are closed to the result of the equivalent surface coefficient of heat transfer method, the result of the equivalent thickness method has certain difference but not quite because it is the approximation process of the equivalent surface coefficient of heat transfer method, and the reasonable result often missed by using the thermal diffusivity method. The workload required in establishing model, splitting mesh and pre-processing is great and the computational efficiency is low by using the thermal coefficient method and the equivalent thickness method in which the model should be rebuilt when the thickness of heated board changes. The heat preservation on the over-wintering level can not be simulated by using the thermal coefficient method and the equivalent thickness method. The equivalent surface coefficient of heat transfer method can not only carry out the insulation calculation on upstream face and downstream face, but also can carry out the insulation calculation at given period on over-wintering level, that the computational accuracy is high and the workload is mini.In addition, combined with some roller compacted concrete gravity dam in severe cold area, the temperature field and temperature stress field of the dam that has no heated board and the dam that has heated board has been calculated by equivalent surface coefficient of heat transfer method and compared. The results show that the superficial heat preservation using foamed polystyrene board with 5cm thick at upstream face and downstream face of dam can weaken heat transmission between concrete and environment, reduce the effect of environment on the surface of concrete and decrease the temperature fluctuation on the surface of concrete, Consequently, the temperature stress of concrete has been reduced effectively. Simultaneity, the decreased degree of the maximum stress has reached up to 18.3%～60% when foamed polystyrene board with 5cm thick is used at over-wintering level during wintering period.