Modeling of complex modulus of bituminous mixtures measured in tension/compression to estimate secant modulus in indirect tensile test

Abstract

For mechanistic based pavement design, it is necessary to know the stiffness of materials used for road construction. In the European standard EN 12697-26, several experimental tests are proposed to measure the modulus of bituminous mixtures. However, bituminous materials exhibit strong viscoelastic behavior. Hence, the stiffness of this specific material depends not only on the sample geometry but also on the loading law (strain or stress curve versus time). As a consequence, comparison between these different tests, performed in time or frequency domain, is not straight forward. The present paper focuses on measurement of secant modulus using the Indirect tensile (IT) test and comparison with complex modulus. In the IT test, the loading law is not systematically controlled. So, it is important to investigate the influence of the loading waveform on the test result. For this study, the case of a High Modulus Bituminous Mixture for base course (French EME) has been considered. Its viscoelastic behavior has been firstly determined using complex modulus measurements and using Prony series model. An intermediate step, based on an original master curve construction method is used. Doing so, the viscoelastic model is validated on the time domain by simulating direct tensile tests. The case of IT test is then considered. The Prony series model is conformed against IT test results for which the force waveform is known. For this last test, the dependance of the Poisson ratio with temperature and loading time is highlighted. Assuming a purely elastic behavior in isotropic compression, a formula, able to derive the viscoelastic Poisson ratio from the complex modulus is presented. Finally, a theoretical parametric study considering the IT loading waveform is undertaken. It appears that the correction factor given in the standard EN 12697-26 cannot be applied at all temperatures. Actually, this correction factor should be material dependent. This study has been carried out in the framework of collaboration between LCPC and USIRF (Union des Syndicats de l’Industrie Routiere francaise).