Influence of the Type of HMA and Free Space Content and the Thermal Conductivity Coefficient λ

Abstract

The surface temperature is diversified in cross-section and varies with external conditions. The dynamics of these changes depend on the possibility of taking over heat through the top layer and from its further flow to the lower layers of the structure. The heat transfer through pavements takes place due to convective heat exchange with air and radiation. Further heat transfer through the surface layers is due to conduction and is defined by the Fourier equation. The solution to the equation is the method of separating variables in the form of the product of two functions: time and space. The parameter decisive for the heat flow rate into the surface is the thermal conduction coefficient λ. For the purpose or type of HMA the value of this coefficient is not constant, assigned to the layer or type of asphalt mixture. It depends on the type of aggregate (its thermal properties), the volume shares of individual components (asphalt, aggregate), the structure of the material and the content of free spaces. The value of the thermal conductivity coefficient is also influenced by the degree of material moisture and temperature. The article presents the results of tests of the thermal conductivity coefficient of sixteen HMA. They were different due to the type of mixture (asphalt concrete, stone mastic asphalt, porous asphalt, macadam mix), the type of aggregate used (granite, diabase), the volume of free space and the temperature of the designation. The choice of aggregates was dictated by their diversity in terms of origin, acidity and thermal and physical properties. In both cases, they are igneous rocks, where the diabase belongs to the vein group (with a composition similar to basalts), and the granite to the deep-sea ones. They differ from each other in terms of acidity (diabase is a basic rock - alkaline, whereas granite is an example of acidic rock - salic), thermal parameters (thermal conductivity, specific heat and emissivity of radiation) and physical (density). The choice of the type of binder was conditioned by the intended use of the mixture. 50/70 asphalt was used for the abrasive layer, while the binding layer and foundation as well as the 35/50 asphalt macadam mix were used. The content of free spaces in the mixes ranged from 2.0% to 33.8%. The test was carried out at 20 ° C and 50 ° C. The obtained results indicate that concrete-type mixes (AC) are characterized by higher thermal conductivity coefficients than mixtures with an increased grit fraction (SMA, PA, Macadam). The basic parameter influencing the value of the thermal conductivity coefficient was the content of free spaces. With its increase, the heat conductivity decreased.