The Mechanism Of Thermal Effect And Its Control Methods For Asphalt Pavement

Temperature of artificial underlying surface in cities,which has smaller heat capacity rises more rapid than that of natural ones.So temperature of artificial underlying surface is higher than that of natural ones.As the city's main form of underlying surface,asphalt pavement occupies 30%of surface area of a city.In summer its surface temperature can reach 60-70?.It is like a huge heat source,baking the surrounding atmosphere,rising environmental temperature and increasing urban heat island effect.In order to reduce the negative effects of asphalt pavement,foreign countries are researching lower heat-absorbing pavement and the focuses of researches are materials of it.In China,the research of lower heat-absorbing pavement is in the initial stage.Currently,foreign researchers have done some testing work on the heat output from asphalt to environment.However,there is no effective quantitative index evaluating the negative effect of asphalt pavement on environment.People cannot deeply understand the heat exchange mechanism between asphalt pavement and environment.In this paper,the thermal effect and its influence on environment temperature were researched,the technical methods of reducing asphalt temperature in summer.These methods can improve city thermal environment and provide solution of reducing urban heat island.Basis on the basic principles of heat transfer,the heat exchange between asphalt pavement and the environment were studied,and the value ranges of key parameters were recommended.Then the effects of key parameters on the pavement were analyzed.Based on the finite element method,considering the heating boundary conditions,a prediction model of asphalt pavement temperature was established basis on the finite element method.Pavement temperature was accurately simulated.The simulated results showed that reducing the thermal conductivity of surface material can reduce the asphalt pavement temperature at night.It is also showed that increasing the heat transfer coefficient between pavement and environment and increasing surface reflectivity of pavement can reduce the asphalt pavement temperature of,and it's helpful for urban heat island.Various factors affecting the thermal effects were analyzed,the results showed that change of thermal conductivity of pavement materials has few influence on the whole day thermal effect of asphalt pavement.However,thermal effect at night reduces along with the reduction of thermal conductivity,which is benefit for alleviating the heat island.The negative thermal effect of pavement decreases with the increase of convective heat transfer coefficient and pavement albedo.The albedo has more affection than convective heat transfer coefficient.Combination meteorological research,the concept of atmosphere layer above pavement was proposed;based on the finite element software ADINA,a temperature prediction model of atmosphere layer above pavement was established.Simulation results showed that the temperature of atmosphere layer above pavement is much higher than the ambient air temperature because of the existence of asphalt pavement and the temperature of atmosphere layer above pavement decreased with the height increasing.The results also showed that the maximum affection on atmosphere layer above pavement of asphalt pavement is 1.5m vertical and 2.0m transverse.Influence on temperature atmosphere layer above pavement of various factors was analyzed.The results showed that thermal conductivity has little effect on temperature of atmosphere layer above pavement,surface reflectivity of pavement has more impact on temperature of atmosphere layer above pavement,the temperature reduced along with the increase of surface reflectivity,and convective heat transfer coefficient between pavement and environment had the most impact in temperature of atmosphere layer above pavement.Atmosphere layer above pavement temperature decreased rapidly with the increase of convective heat transfer coefficient.Based on the thermal effect of asphalt pavement and the heating of atmosphere layer above pavement,lower heat-absorbing pavement which were named heat shading pavement,water retention pavement and thermal resistance pavement were proposed.The materials of lower heat-absorbing pavement were researched and recommended by lots of testing.Tests results showed that the cooling effect of solar reflectance pavement,water retention pavement and thermal resistance pavement is pretty well.Then the road performance of lower heat-absorbing pavement was researched by experimental studies.According the cooling effect experimental data the thermal effect of three types of lower heat-absorbing pavements were calculated and the results showed that they all have smaller thermal effect than general asphalt pavement.The suitable application places of three types of lower heat-absorbing pavement were recommended according to their characteristics.