Estimation of low-temperature performance of recycled asphalt mixtures through relaxation modulus analysis

In cold climates, pavements undergo extreme thermal conditions which cause premature cracking and failure. The inclusion of Reclaimed Asphalt Pavement (RAP) could worsen this tendency due to the reclaimed bitumen stiffening effects. However, nowadays strict economic and environmental sustainability requirements strongly encourage the use of high RAP content in asphalt mixes. In this context, this paper discusses the low temperature performance of mixtures designed with 40% RAP according to the Bailey method. Different bitumen contents and polymer modification levels were employed. A mixture with 25% of unfractioned RAP was employed as the reference mixture. Laboratory tests were performed through Asphalt Thermal Cracking Analyzer (ATCA) by applying thermal loadings on restrained and unrestrained asphalt beams. Based on experimental data the relaxation modulus was evaluated to rank the materials. Its accurate determination is fundamental for designing long lasting pavements with proper performance at low temperature. The analysis was performed through a new analytical methodology for obtaining the relaxation modulus master curve by measuring thermally induced stress and strain. The solution, based on Boltzmann's equation and pseudo-variables concepts, accounts for time and temperature dependency of bituminous materials, avoiding complex integral transformations. The proposed solution successfully integrates the current ATCA analysis providing reliable estimations of relaxation properties fundamental in modelling of pavement behavior. Mixtures containing 40% RAP demonstrated enhanced relaxation capabilities at low temperature showing significant improved behaviours than the reference mixture. Although high amount of reclaimed material, proper selection of RAP and type and quantity of virgin bitumen can improve low temperature performance.