Characterisation of Asphalt Mixtures for Permanent Deformation
The study used triaxial compression test to characterize asphalt mixtures for permanent deformation. Various factors both structural and environmental that affect the permanent deformation of asphalt mixtures were investigated with an aim of understanding how they contribute to the permanent deformation of asphalt mixtures. The main aim of the study was to characterize asphalt mixtures for permanent deformation which was achieved by setting out six objectives namely; to investigate the effects of aggregates gradation, loading temperature, bitumen content, confining stress and bitumen consistency on permanent deformation of asphalt mixtures and finally to identify suitable models that best describe permanent deformation of asphalt mixtures. Asphalt mixtures made of well graded aggregate gradation were found to have high resistance to permanent deformation as compared to asphalt mixtures made of the gap-graded aggregate gradation. Permanent deformation of asphalt mixtures was found to increase with increase in loading temperature. Asphalt mixtures made of highly consistent bitumens (60/70 penetration grade bitumen) were found to have high resistance to permanent deformation as compared to the asphalt mixtures made of low consistent bitumens (180/200). High bitumen content was found to increase the permanent deformation of asphalt mixtures. High confining stresses were found to increase the resistance to permanent deformation of asphalt mixtures. Gap – graded asphalt mixtures made were found to be more susceptible to permanent deformation at high loading temperatures while their continuously graded counterparts were found to be less susceptible to permanent deformation at high temperatures. Continuously graded asphalt mixtures with low contents of highly consistent bitumen were found to be more suitable in resisting permanent deformation at high temperatures and high confining stresses. Logarithmic linear regression and power law equations were identified as the most suitable models to describe the test results for permanent deformation. Tangent stiffness (TS) was related with the loading temperature (T) using a natural logarithmic linear regression relationship of the form lnTS=C1+k1lnT where C1 and k1 are material constants. The maximum stress (fca) was related with the confining stress (σ3) by a power law of the form fca=aσ3b where a and b are model parameters. All the correlations were of high coefficients of determination (R2) ranging between 0.96 and 0.98.