Simplified Skeleton Construction And Identification Of Internal Stress Paths For Coarse Aggregates In Asphalt Mixtures

Asphalt mixes are non-homogeneous materials consisting of aggregates,asphalt mastic and voids,whose macro-mechanical properties are closely related to the fine structure.There are limitations in the current empirical approach to mix design.A deeper understanding of the fine structure characteristics and mechanical behaviour of asphalt mixtures is needed to elucidate the relationship between the internal stress transfer paths and the macroscopic properties of the material,and to optimise the current asphalt mixture design methods based on a clearer mechanical basis.This study attempts to use a CCD digital camera to obtain the internal structure of the asphalt mixture,combine it with digital image processing techniques to analyse the fine-scale characteristics of the mixture,and construct a simplified skeleton based on the coarse skeleton contact parameters.Based on the finite element model and skeleton parameters,the internal stress paths of the mix are identified and the intrinsic correlation between the stress path characteristics and the macroscopic mechanical properties is explored based on the generated force chain network.The main work and findings of this study are as follows.(1)Image processing techniques were applied to pre-process the mix images to remove image noise and fine aggregates below 4.75 mm,and an aggregate search and contact point calculation program was developed based on MATLAB to calculate the aggregate quantity,aggregate centre-of-mass coordinates,contact point quantity and contact point coordinates.The results show that the self-researched contact point calculation program can effectively locate aggregate particles and calculate skeleton parameters such as aggregate centre-of-mass coordinates and contact point coordinates.(2)The definition of simplified skeleton is proposed,the gradation curves of the original section skeleton and simplified skeleton,the fractional dimension of the original section and simplified skeleton are calculated,and the relationship between the fractional dimension of the simplified skeleton and the macro-mechanical test is obtained.The results show that the simplified skeleton has gradation properties and can express the internal structure of the mix and the macro-mechanical properties more effectively than the design gradation.(3)The Abaqus contact force extraction program based on MATLAB can accurately extract the stress state in the target region within the mix.The force chain analysis shows that the number of strong and weak force chains is influenced by the type of load and the location of action,indicating that the asphalt mix is a stress-dependent material.Isolated force chains are undesirable and the mastic properties should be ensured in the mix design to resist the effects of isolated force chains.(4)The analysis of stress dissipation efficiency and contact force distribution along each direction shows that the contribution of stresses transmitted by both the skeleton and the asphalt mastic is related to the gradation.The force chain network analysis based on the simplified skeleton can effectively quantify the contribution of both,which suggests the possibility of digital and refined design of asphalt mixtures.