A new approach to characterizing asphalt concrete in the linear/nonlinear small strain domain
Asphalt concrete (AC) material characterization is usually performed on laboratory prepared specimens, a procedure that does not ensure that the material tested reproduces the aggregates arrangement of the field material. Currently, field specimens are tested on cores in the Indirect Tensile Test (IDT) and on cylindrical specimens cored horizontally from relatively thick AC layers. As AC materials may exhibit cross anisotropy, testing cylindrical specimens requires two specimens, one in the vertical direction and the other in the horizontal direction. The above limitations can be overcome by testing prismatic field specimens in a triaxial cell with controllable vertical stress and confining pressure. The paper presents a study of the cross-anisotropic relations in two different specimen shapes, cylindrical and prismatic, and shows that prismatic specimens are preferred in the case of cross anisotropy.
The use of the triaxial cell offers the possibility of applying different 2D stress paths and measuring the resulting 3D deformations. Creep and recovery tests on a prismatic specimen cut from a test section are conducted to illustrate the feasibility of the procedure. Before embarking in an extensive research project, six creep and recovery tests are conducted, under different stress paths that represent the closest stress/strain condition under a wheel load. The stress/strain conditions were kept within the small strain range, within the linear nonlinear stages. The linear viscoelasticity for cross-anisotropic materials is formulated, implying the derivation of four independent creep compliance functions. The test results are analyzed using numerical integration. The analyses show that the creep compliance functions vary with the stress/strain level, even in the so-called small strain range. Recommendations for further research are suggested.
KeywordsCross anisotropy Prismatic specimen Creep and recovery Linear viscoelasticity Triaxial cell
Special thanks are due to Mr. Meir Ovadia who put together the system and conducted the tests with great care.
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