Controlling Thermal Properties of Asphalt Concrete and its Multifunctional Applications
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Controlling infrastructure temperature, especially flexible pavement, has attracted attention in both industrial and academic societies because: 1) material properties of asphalt, and corresponding structural responses and distresses are temperature dependent and 2) pavement surface temperature directly relates to various environmental or safety problems. This study investigates the feasibility of mitigating the temperature-related problems of civil infrastructures (especially asphalt pavement) by controlling thermal properties of the construction materials. To change thermal properties of asphalt concrete, expanded polypropylene (EPP) pellet and graphite were selected as the additives and mixed into asphalt concrete. Experimental tests are classified into two categories: 1) physical and thermal characterizations of raw materials including Scanning Electron Microscope and heat susceptibility tests, and 2) mechanical and thermal properties of the modified asphalt mixtures via indirect tensile test and hot disk test, respectively. The heat susceptibility test results show that use of EPP as an aggregate replacement is a better choice than the use of the melted-EPP as a binder modifier because it has a good heat susceptibility and is hard to melt at the HMA working temperature. The mechanical performances and thermal properties evaluation results show that by replacing the aggregate with EPP to have 18% by volume of total mixture, the indirect tensile strength was reduced by 17%, and the thermal conductivity and volumetric heat capacity decreased by 32% and 27%, respectively. By adding 4.8 vol. % of graphite, the indirect tensile strength decreased by 20%, and an increase of 43% in thermal conductivity was obtained. To simulate the effect of the thermally modified asphalt mixtures on the surface temperature of pavements and bridges, a series of heat transfer analysis were conducted using the finite difference heat transfer model. In addition, a case study of a building using EPP modified cement concrete was carried out to investigate the benefits of EPP modified concrete as a wall insulation. From the simulation results, it is concluded that adding graphite into asphalt mixture mitigates the urban heat island effect during summer by dropping the maximum surface temperatures of both pavement and bridge (3.1?C and 1.9?C, respectively, with 4.8% graphite), and the graphite modified asphalt concrete can reduce the use of deicing agents during winter by increasing the minimum surface temperature by 0.5?C for pavement and 0.2?C for bridge. On the other hand, adding EPP increases maximum surface temperature by 0.8?C for pavement and 1.0?C for bridge during winter, which show the potential for snow and ice removal application. In addition, the simulation shows that the EPP modified concrete can serve as a wall insulator.