Anisotropy in compressive strength and elastic stiffness of normal and polymer-modified asphalts

Anisotropy in the compressive strength and the elastic stiffness of normal hot-mix asphalt (HMA) and polymer-modified asphalt (PMA) were investigated experimentally. Two types of prismatic specimen with planes of compaction either normal to or parallel to the applied axial compression were used. Both axial and lateral principal strains were measured locally. Continuous monotonic loadings at a constant strain rate were axially applied to investigate the anisotropy in the compressive strength. Very small strain–amplitude cyclic stresses were also applied by means of another load-controlled apparatus to investigate the anisotropy in the elastic stiffness. The followings were found: (i) compressive strengths of both HMA and PMA are anisotropic in that the values for the vertical direction were significantly larger than those of horizontal direction; (ii) the small strain stiffness of both HMA and PMA are anisotropic with the vertical elastic Young׳s modulus being greater than the horizontal elastic Young׳s modulus; (iii) the elastic Young׳s modulus is stress level dependent, exhibiting a hypo-elastic behaviour; and (iv) the elastic Young׳s modulus becomes greater with the applications of the normal and the polymer-modified asphalt cement binders.