Characterization Of Micromechanical Properties Of Aged Asphalt Mixtures By Nanoindentation

The aging of asphalt pavements severely deteriorates its performance and service life.Actually,aging effects and the following performance degradation inside asphalt pavements are not evenly distributed.This non-uniform aging behavior or aging gradient are difficult to characterize using conventional macro testing methods which brings a great impediment in understanding the actual aging behavior of asphalt mixtures.Hence,this paper utilizes nanoindentation,a new micromechanical testing technique,and typical macro experiments to investigate the non-uniform aging behavior and performance degradation in asphalt mixtures to reveal the internal effects of aging on mixture performance.This paper offers new ideas and tools for the aging research of asphalt materials.Firstly,on the basis of literature review and experimental research,this paper summarizes the nanoindentation testing methods of asphalt materials.Standardized sample preparation method and reasonable testing parameters are put forward.The effects of asphalt mixture gradations and polishing in sample preparation process on nanoindentation results were also discussed.Results show that asphalt mixtures of gap gradation are more suitable for nanoindentation test.Normative polishing operations will not affect the reliability of nanoindentation results and the law generalized.Secondly,the general aging behavior of asphalt mastic samples is explored with the help of nanoindentation and DSR frequency scanning test.It is suggested that the nanoindentation modulus and viscoelastic constitutive fitting parameters of mastic samples increase after aging treatment,indicating that aged mastic samples show more elastic characteristics and exhibit apparent agehardening behavior.DSR results suggest that the complex shear modulus of aged mastic samples grow while phase angles decrease and SBS modified asphalt show a better aging resistance.Moreover,the uniaxial creep compliance results calculated from DSR complex shear modulus are in good agreement with the nanoindentation creep compliance data,which verifies the feasibility and accuracy of nanoindentation in characterization of aging behavior of asphalt materials.The relationship between macro and micro test results was successfully establishedFinally,the micromechanical properties of asphalt mastic at different depths in four kinds of asphalt mixtures were measured by nanoindentation to describe the non-uniform aging behavior in aged asphalt mixtures with different air voids.The results suggest that nanoindentation can characterize the non-uniform aging behavior in asphalt mixtures by capturing the modulus gradient and changes of viscoelastic parameters.The modulus gradient in the aged mixtures is approximately in a logarithmic curve form.The range of aging effects in asphalt mixture samples depends largely on the air voids.In asphalt mixtures of 3% and 7% air void,the influence range of 8d laboratory oven aging is about 2-3 cm,while that for 12% air void mixtures is about 4-5cm.This study shows that nanoindentation can overcome the shortcomings of macro testing methods and achieve the intuitive characterization of non-uniform ageing behavior in asphalt mixtures from the micromechanical perspective by extracting the indentation curves,modulus and viscoelastic parameters.Nanoindentation is expected to become a powerful tool for research on aging of asphalt materials and other areas.