COMPARISON OF 2 AND 4 POINT FATIGUE TESTS AND HEALING IN 4 POINT DYNAMIC BENDING TEST BASED ON THE DISSIPATED ENERGY CONCEPT

Abstract

At the 4th Eurobitume Symposium a fatigue life definition was introduced, which was based on the dissipated energy concept (Hopman et al, 1989). This new definition is based on a change in a material related aspect (dissipated energy per cycle) instead of a specimen property. It was shown that, using the dissipated energy law and this new fatigue life definition, the results were exchangeable for strain and stress controlled fatigue tests even at different frequencies. These experiments were only carried out in a 4 point dynamic bending test. In this paper the results are presented of a comparison of fatigue lives measured in 2 and 4 point dynamic bending tests. In spite of the new fatigue life definition, the results are not comparable for both tests. This may be due to the composition of the mix at issue and the small dimensions of the specimen in the 2 point bending test. Also the healing phenomenon is studied in the 4 point bending test. Rest periods between loadings ought to enlarge the fatigue life considerably. After a resting period the stiffness modulus is largely restored. If instead of resting periods (with zero loading), load blocks are used with a smaller load amplitude, it is shown that during these pseudo rest periods the stiffness modulus will increase. Depending on the duration of the pseudo rest period and the amplitudes of the loadings, this increase will be followed by a decrease in stiffness modulus. These findings confirm the opinion that healing already takes place during the fatigue process and should be taken into account in the interpretation of the fatigue measurements and the characterization of the fatigue properties. Based on the dissipated energy concept a model is proposed taking into account the healing phenomenon. This model should enable a more fundamental description of the asphalt fatigue properties including healing.