| With continuous increase in the service life of asphalt pavements, durability has gradually become one of the very concerned issues for all road researchers. Due to the absence of systematic research of the long-term performance of pavements in China, it is insufficient to evaluate the durability of asphalt pavements only by the designed life and accumulative equivalent axles defined in the design specifications. Meanwhile, asphalt mixtures are typical viscoelastic material, the mechanical properties of which are notably influenced by loading time and temperature. This paper relies on the project "Research on the decay law of durability of each surface layer structure of semi-rigid base asphalt pavement" jointly undertaken by Jiangsu Ning-Hang Expressway company and Southeast University. A series of semi-circular bending test (strength test, fatigue test and residual strength test) were carried out to study the decay law of strength and the fatigue damage properties based on residual strength of asphalt mixtures under repeated loading. The research results of this paper has great significance for the understanding of durability of asphalt pavements.Firstly, several laboratory fatigue evaluation methods were summarized and semi-circular bending test was used to evaluate the fatigue performance of asphalt mixtures in this paper. SMA-13 mixture was chosen for this study, and mixture ratio design was conducted on the basis of tests of raw materials. To reduce test variability, a number of pre-tests were carried out with the goal of porosity control before the final formal test and specimens with unqualified porosity were ruled out after the final molding.Secondly, semi-circular bending strength test was conducted under different loading rate and temperature with the purpose to study the time-temperature characterization of asphalt mixtures. The test result showed that loading rate and temperature had significant influence on strength of asphalt mixture, which would increase as loading rate increases or the temperature decreases. The relationship between strength and loading rate could be fitted as a power function model. The axial displacement and cumulative energy consumption were analyzed with fatigue tests under five different loading level. Traditional fatigue equation and fatigue equation based on dynamic loading and real stress ration were proposed to describe the internal relations between strength fracture and repeated fatigue damage.Thirdly, the decay law of residual strength under different loading level was analyzed through residual strength tests after certain number of repeated loadings and a power function attenuation model was also established. The residual strength showed a nonlinear attenuation characteristic, it decayed slowly in an approximately linear mode in the earlier stage of the fatigue process, while in the later stage of fatigue process, it decayed sharply. Meanwhile, a nonlinear fatigue damage evolution equation was established on the basis of damage variable defined by residual strength. And the evolvement of fatigue damage under different loading level was analyzed. It was found that the fatigue damage increased faster under higher loading level, but the corresponding critical damage would be smaller.Finally, digital image correlation method was applied in the observation of transverse strain during semi-circular fatigue test. Strain field on the surface and the transverse distribution of tensile strain at the bottom were analyzed. A power function of strain evolution equation was established through the analysis of strain-cycle-index curve, which revealed the growing rule of tensile strain with cycle-index. At the same time, a unified equation between residual and tensile strain was built based on the same fatigue life ratio, through which we could get the corresponding residual strength with a given tensile strain.This paper is of significance for the further understanding of the decay law of durability of asphalt pavements. |
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