Comparison of tensile and compressive relaxation modulus of asphalt mixes under various testing conditions

Abstract

Mode of loading, i.e., tension and compression, is an important factor that affects the relaxation modulus of asphalt mixtures. A laboratory experiment was conducted to compare the relative variations of tensile and compressive relaxation modulus master curves of dense graded asphalt mixes under various testing conditions including mix characteristics, aging condition, and temperature. Crushed stone aggregates having two gradations, and 60/70 penetration asphalt binder with two binder contents were selected to fabricate the asphalt mixture specimens at two air void levels and three aging conditions with three replicates for each experimental combination. Direct tension/compression relaxation modulus tests were carried out on the specimens at four temperatures using the trapezoidal pattern at a low level of input strain. Tensile and compressive relaxation modulus master curves were constructed for all the experimental combinations using the sigmoidal fitting model together with the numerical temperature shifting technique. Sigmoidal model coefficients of α, β, and γ were chosen, as representatives of relaxation modulus master curves, to be compared between tension and compression. Based on the graphical representations as well as the overall mean values calculated for the ratios of tensile–compressive coefficients under various testing conditions, it was concluded that tensile and compressive behavior of asphalt mixes may be generally different to each other. Both the tensile α and γ coefficients may always be higher than those obtained in compression for all the testing conditions. In addition, the β coefficient obtained in tension may be lower than the compressive one only for the shorter aging times as well as the higher reference temperatures; while this coefficient in tension may be higher than the compressive one for the other testing conditions. In addition, analyses of variance showed that the factors of binder content and air void level may have the most significant effects on the ratios of tensile–compressive α and γ coefficients, respectively; while the reference temperature may be the most inflectional factor on the ratio of tensile–compressive β coefficients.