Laboratory Investigation of Anisotropic Behaviour of HMA

The anisotropy refers to the material properties that are direction dependant. In the past, the HMA has been typically modeled as an isotropic material due to a lack of sufficient data for detailed quantification of its direction-dependency. However, it is reasonable to assert that HMA may exhibit significant anisotropic behaviour due to the major compaction direction and the resulting preferred alignment of strong axis of the aggregate particles. In order to gain basic understanding of the anisotropy of HMA, a laboratory test program has been conducted on HMA specimens with or without polymer modifiers. The method of specimen preparation to achieve samples with different orientations between the specimen compaction direction and the axial load in the triaxial testing is described. The confined triaxial compression test is used to obtain mechanical properties of various HMA specimens with three orientation angles (0, 45, and 90 degrees) between the direction of the specimen compaction and the direction of the axial load direction in the triaxial test. The mechanical properties investigated in the laboratory test program include the compressive strength (defined as the peak deviator stress), the modulus of elasticity (defined as initial portion of the stress-strain curve), the strain at the peak strength (used as an indication of the ductility of the mix), and the toughness of the mix (determined as the area under the stress-strain curves). Also, a comprehensive testing program using the repeated triaxial testing procedure is carried out on the specimens with three orientation directions to investigate the accumulation of permanent deformation under repeated loadings. This paper presents pertinent test results and exams the significance of anisotropy. Based on the experimental results of the laboratory test program, the anisotropic behaviour of HMA is found to be significant. Therefore, in order to accurately predict the stress and strain regimes in the pavement layers under traffic load, the HMA should be modeled as an anisotropic media. Predictions of permanent deformation (rutting) require proper consideration of material anisotropy as well. INTRODUCTION The characterization and modeling of the anisotropic properties of soils have been widely explored in geomechanics and geotechnical engineering. However, very few research studies have focused on the characterization and modeling of the anisotropic properties of the asphalt concrete mixtures. It has been realized that the anisotropy of HMA materials greatly depends on specimen preparation and testing conditions (1). The behavior is further influenced by the magnitude of stresses and strains induced within the HMA material and the rate of load application. The mechanical behavior of many bound materials such as asphalt concrete mixtures is anisotropic in nature (2). However, the majority of the current mechanical tests and analytical models for asphalt concrete mixtures are based on the assumption of isotropic material properties. No known pavement structural response model based on layered theory considers the anisotropy of the asphalt concrete materials (3, 4). Most known models are based on the linear elastic or linear viscoelastic theory. In the linear elastic theory, the asphalt concrete material is assumed to be homogenous, isotropic and linear elastic. The viscoelastic theory considers the time rate of stresses and strains in the asphalt concrete layer, with the similar assumptions of homogeneity and isotropy. This paper presents a laboratory program to investigate anisotropic behaviour of HMA under both static and repeated loading conditions. The mechanical properties investigated in the laboratory test program include the compressive strength (defined as the peak deviator stress), the modulus of elasticity (defined as initial portion of the stress-strain curve), the strain at the