Quantification of inhomogeneity and anisotropy of bituminous mixtures using biaxial experimental data

Abstract

Most of the existing mechanical characterization test methods and the analytical models for bituminous mixtures are developed based on the assumption of homogenous and isotropic material properties. However, the extent to which such assumptions hold good for bituminous mixtures is not very clear. A biaxial testing scheme in indirect tension mode can come in handy to collect the experimental data at different test temperature regimes. In this study, a bituminous mixture with a nominal maximum aggregate size (NMAS) of 13.2 mm is used to fabricate cylindrical test specimens with 150 mm diameter and 63.5 mm thickness for conducting the repeated load indirect tension (IDT) test. The input load level is selected as 5 to 13 % of the indirect tensile (ITS) failure load in the form of a haversine pulse of 0.1 s loading and 0.9 s rest period. Such loading is applied on zero-degree and ninety-degree orientations of the specimen. The resulting deformations in different directions are measured using linear variable differential transducers (LVDT) attached to both diametral faces of the specimen. The response of the bituminous mixture specimen is captured at three different temperatures of 20, 25, and 30 °C using two different gauge length (half and quarter gauge of the total diameter). A new framework is introduced in this study to evaluate the extent of inhomogeneity and anisotropy of bituminous mixtures. The deformation response of the test specimen captured in the same orientation at different diametral faces is compared to evaluate the extent of inhomogeneity. The extent of anisotropy is evaluated by comparing the deformation response of the test specimen captured at different orientations within the same diametral face. A 15 % variation in the average peak deformation is chosen as the demarcation for determining the extent of inhomogeneity and anisotropy. From this investigation, it is observed that half gauge length experimental data exhibits homogenous and isotropic behavior for all three test temperatures. The quarter gauge length data exhibit similar behavior at 30 °C, whereas it is observed as homogenous and anisotropic at the other two test temperatures.