Feasibility of using lightweight aggregate to mitigate frost damage in flexible pavements

Frost associated distresses in flexible pavements are commonly encountered in Canada and other cold regions of the world. Road agencies in these regions are mostly confronted with increased maintenance costs due to frost problem. Because of the scarcity of annual maintenance funding, there is a need to develop a cost-effective method of reducing or eliminating frost penetration into the pavement subgrade to mitigate frost associated distresses. The existing methods of mitigating frost damage in flexible pavements are costly and involve increased construction efforts. Also, some are not fully effective and raise safety concerns. This research explores an alternative approach for mitigating frost damage by engineering a pavement structure with reduced heat conductivity using lightweight aggregate (LWA).;Volumetric properties of a number of asphalt concrete (AC) mixes containing various proportions of LWA were studied in order to achieve an optimized LWA-asphalt mix that can meet the technical requirements for paving material with respect to Superpave mix design standards. Thermal and engineering properties of the optimized LWA-asphalt mix were compared with those of a conventional asphalt mix. The LWA-asphalt mix showed superior insulating behaviour compared to the conventional asphalt mix. It was also found that LWA-asphalt mix has better Marshall stability compared to the conventional asphalt mix. The indirect tensile strength and tensile strain at failure of both the mixes were almost the same. The dynamic modulus of the LWA-asphalt mix was lower than that of the conventional asphalt mix by an amount ranging from 9.2 -- 35.3%. California bearing ratio (CBR) of LWA was also determined to evaluate its potential use in subbase and/or base course construction, and was found to be 21 which is acceptable for subbase and base course construction. However, such a number corresponds to the lower limit of CBR values for granular material that range from 20 to 100; a well graded standard crushed gravel has a value of 100.;A finite element thermal response model was used to estimate frost penetration into the subgrade of seven different typical pavement structures consisting of different layer combinations of LWA and conventional materials. Based on the simulation results, it was concluded that the use of LWA-asphalt surface course and/or a LWA base and subbase course in a pavement will completely eliminate frost penetration into the subgrade. The fatigue and rutting service lives of an 850 mm conventional pavement were comparable to LWA-pavements of equal layer thicknesses.;Keywords: Lightweight aggregate; Frost damage; Thermal properties; Asphalt mix; Flexible pavements.;The research investigates a novel application of LWA in flexible pavements for achieving a cost-effective insulative pavement structure that can act as a buffer zone between the freezing frost line and the subgrade soil. The purpose of developing this new approach is to reduce frost associated maintenance cost. Such an insulative pavement structure has to be properly designed for ease of construction, fatigue resistance and rut resistance.