Study of permanent deformation mechanism in asphalt mixes in relation to microcrack growth

Abstract

Recent developments by Lytton et al. (1998) indicate phics. that the growth of micrographs in asphalt layers under repeated loading is also a cause for the accumulation of permanent deformation or rutting in pavements besides the plastic strain accumulation. Lytton developed a model to account for the permanent strain accumulation due to the growth of micrographs or some other form of microdamage. He also hypothesized that pavements accumulate additional amount of permanent strain in the process of repeated loadings due to microcrack growth up to 25 OC and due to the combined effect of microgramming and microdamage after 25 []. This thesis uses Lytton's model to analyze repetitive axial load permanent deformation test results and presents the experimental evidence of factors other than plastic strain accumulation in the process of damaging the sample. This thesis presents the graphical representation of evidence of other factors that are accelerating the permanent deformation accumulation and recommends for further research to specifically confirm the presence of micrographs. Threshold values for the parameters of equations for dissipated pseudo strain energy are defined for asphalts used in this study in order to indicate that once those threshold values are exceeded one may expect additional damage to the pavements due to the growth of micrographs or some other form of microdamage. This thesis uses a new technic, developed by Leseur and Little (1997), to calculate linear viscoelastic stress with the help of Fourier series. This process greatly simplifies the conventional laborious process of calculating linear viscoelastic stress by Schapery's process. The theory behind Lytton's model and the new method of calculating linear viscoelastic stress are extensively discussed, and step by step process of analysis is also presented in this thesis.