Meso-scale Investigation Of Anisotropy Of Asphalt Mixture

The anisotropy of asphalt mixture has always been a concern for road researchers.For a long time,due to the limitation of testing methods and mechanical analysis methods,this problem has not been fully discussed.At present,in the structural design of asphalt pavement in China,the pavement materials are assumed to be isotropic.This assumption simplifies the calculation process and facilitates the solution of mechanical response,but whether asphalt mixture composed of aggregate particles and gradation-aggregate can be regarded as isotropic materials has always been doubted.In this paper,the internal and external factors that cause the anisotropy of asphalt mixture are closely followed.Firstly,the scanning of asphalt mixture is carried out through X-ray CT,Mimics and Matlab,and the relationship between the anisotropy microscopic index of asphalt mixture and the number,flattening rate and long axis orientation of coarse aggregate and void in the mixture is analyzed by image processing technology and fabric tensor method.In order to further expands the application of anisotropy in practical engineering,the principal curve and Prony series model of asphalt mastic are obtained by using linear programming and improved configuration theory,and the viscoelastic parameters of asphalt materials is applied to the numerical model for the numerical simulation test of asphalt mixture.The two-dimensional and three-dimensional numerical models were reconstructed based on the scanning images of asphalt mixture,and the two dimensional numerical models were used to conduct indirect tensile tests in different loading directions to reveal the relationship between anisotropic microscopic evaluation index and transverse mechanical index.And dynamic modulus test,uniaxial tensile tests and indirect tension test were conducted after cutting models of the same size along the horizontal and vertical directions with three-dimensional numerical models.At the same time,the anisotropy of asphalt mixture is comprehensively characterized by the different direction dynamic modulus,stress and train distribution.It is found that the shape,size,long axis orientation and flatness of aggregate and void are important factors affecting the anisotropy of asphalt mixture.Asphalt mixture presents obvious anisotropy in vertical direction but little in horizontal direction,and the anisotropy is more significant with the increase of stress.The root cause of the difference is the difference of the morphology and distribution of aggregate and void in vertical and horizontal direction.The research method and content of this paper not only take the advantage of mechanical calculation efficiency of continuous medium,but also accurately describe the influence of microstructure distribution on the macroscopic response of materials,providing an important tool for revealing the anisotropy of asphalt mixture.In addition the typical diseases such as ruts,cracks and fatigue can also be explained more reasonably when anisotropy is considered.