Study On Oriented Heat-Induced Mechanisim And Structures Of Asphalt Pavement

Black asphalt pavement tends to strongly absorb solar radiation, resulting in excessive accumulated heat and high pavement temperature. The high-temperature pavement subsequently releases heat to the atmosphere by two means of heat convection and effective radiation. Simultaneously, the heat accumulation in asphalt layers transfers to embankment and then increases embankment temperature.Several heat-induced structures were designed by adding graphite and floating beads into asphalt mixture, implanting high-thermal-conductivity rods and graphite-filled cavity structures in the pavement. On this base, the oriented heat-induced mechanism of asphalt pavement was systematically studied by means of finite element simulation from two aspects of reducing the downward heat conduction efficiency in asphalt pavement and accelerating the heat transfer in the pavement to the embankment. The simulation results were validated by using indoor irradiation tests.First, the thermal properties of graphite-and floating beads-modified asphalt mixtures were measured, and the performances of floating beads-modified asphalt mixture were experimentally investigated. The results showed that graphite could increase the thermal conductivity of asphalt mixture while floating beads could reduce the thermal conductivity of asphalt mixture. Besides, floating beads could improve the high-temperature performance and water resistance of asphalt mixture, and reduce its low-temperature performance.A principle of preventing solar radiation entering pavement and reducing downward heat transfer efficiency was proposed to design two highly oriented heat-induced structures, G-OHIS and G+R-OHIS. The results showed that the heat accumulation in asphalt layers decreased by 14.2% for the G-OHIS and 34.0% for the G+R-OHIS. The maximum temperature reduction was 2.5℃ for the G-OHIS and 7.1 ℃ for the G+R-OHIS. The highly oriented heat transfer in asphalt pavement could also be used to decrease the heat accumulation in the embankment. The summertime daily heat absorption of embankment decreased by 9.9% for the G-OHIS and 23.2% for the G+R-OHIS. The annual net heat absorption of embankment decreased by 6.2% for the G-OHIS and 37.9% for the G+R-OHIS. Two integrative heat-dissipating structures, G-IHDS and G+R-IHDS, were designed by using the characteristics of the highly oriented heat induction of asphalt pavement and the wintertime heat convection of crushed-stone embankment. The daily heat absorption of embankment in summer decreased by 11.9% for the G-IHDS and 24.8% for the G+R-IHDS. The annual net heat absorption of embankment decreased by 16.7% for the G-IHDS and 34.9% for the G+R-IHDS.Another principle of preventing solar radiation entering pavement and increasing downward heat transfer efficiency was proposed to design two bidirectional heat-induced structures, G-BHIS and G+R-BHIS. The results showed that the heat accumulation in asphalt layers decreased by 15.9% for the G-BHIS and 37.6% for the G+R-BHIS. The maximum temperature reductions reached 2.4℃ for the G-BHIS and 7.8 ℃ for the G+R-BHIS. Based on the research on the G-BHIS, high-thermal-conductivity rods (e.g., steel rods) were vertically implanted in middle and bottom layers to form thermal channels and accelerate the oriented heat transfer from asphalt pavement to the base layer and embankment. The results showed that steel rods absorbed heat from asphalt mixtures in the middle and bottom layers and then fast transferred the heat downwards. Accordingly, pavement temperature reduced significantly, e.g., the temperature at the depth of 4cm reduced up to 3.6～6.5℃. The study of the heat transfer of asphalt pavement induced by gradient steel rods showed that the upward convex gradient steel rods could expand the cooling range and further reduce the temperature on the top of bottom layer, while the downward convex gradient steel rods could narrow the cooling range and further reduce the temperature on the top of middle layer. The influences of implanting forms of steel rods on the heat transfer of asphalt pavement were also analyzed. It was found that asphalt pavement could absorb more external heat and had lower temperatures in the top the and middle layers, when steel rods were implanted into the top layer by 2cm and then the middle and bottom layers. The daily heat absorption increased by an average of 30.6%. The surface temperature reduced by 1.7～3.5℃. The temperature at the depth of 2cm received the most significant reduction, reaching 2.4～6.4℃.Graphite-filled cavity structures were tried to form continuous powder columns and accelerate the downward heat transfer of asphalt pavement. It was proved that the cavity structures did not affect the mechanical properties of asphalt pavement. Due to higher thermal conductivity of graphite than that of steel rod in numerical models, the temperatures of the top and middle layers further reduced, e.g., by 3.7～6.9℃ at the depth of 4cm.Due to the difficulty in performing rutting tests for specimens with gradient temperature distribution, the influencs of cool asphalt pavements on rutting resistance were numerically simulated. For example, the SR-BHIS could reduce the maximum rutting depth by up to 50.8%, compared with control asphalt pavement.The oriented heat-induced mechanism and structures proposed in this paper are helpful to further understand the heat transfer characteristic of asphalt pavement. It is hopeful to design other more suitable heat-induced structures according to the application environments and structural features of asphalt pavement, and apply these structures to reduce high-temperature rutting, mitigate urban heat island effect or protect permafrost embankment in the future.