Experimental Research On Void Characteristics And Permeability Of Asphalt Mixture Under Cyclic Loading

With the rapid growth of expressway construction and traffic in China,the early water damages of asphalt pavement are becoming increasingly common and severe.When the free water on road surface infiltrates in road structure along void and crack and is stuck,excess pore water pressure will be produced under the repeated traffic loads,which accelerates the damage of asphalt mixture,and the constantly damage of material further intensify the infiltrating of water.In this way,asphalt pavement is in the multi-field coupling of Load-flow-damage and the permeability changes dynamically with injury,load-stress and water pressure.In this context,the dynamic changes of permeability of SMA-13 asphalt mixture under cyclic loading is discussed in the paper.The main achievement are as follows:(1)A penetration testing system for pavement materials under multi-field coupling condition is developed by the team.Combined with MTS test systems and industrial CT scanner,it makes it possible to implement fatigue test of asphalt mixture under the multi-field coupling condition,to test the permeability of pavement materials under different damage stage with different axial load and hydrodynamic pressure,and to get CT images of materials during fatigue-loading.(2)First,four different porosities of SMA-13 asphalt mixture smaples with two forming methords are prepared.Then triaxial compression tests and permeability tests on different stages of fatigue damage are conducted on the permeability measurement system and MTS test platform.It reveals that the main failure form of SMA-13 under Triaxial compression is flow failure,without obvious macroscopic crack;Permeability enhances with the increase of porosity and SMA-13 with more than 8%porosity has greater coefficient of permeability,but with the loading of triaxial compression increases,the permeability decreases.SMA-13 with 6%and 4%porosity is impervious during the initial phase,but the permeability is increase at first and then decrease during the fatigue loading.In addition,the permeability has a significantly enhanced feature with the increasing water pressure while axial pressure has a relative weak effect to the permeability of SMA-13.(3)Parameters such as porosity,void number,void equivalent diameter,void roundness,void profile fractal dimension and so forth are used to characterize the internal void mesoscopic structure of asphalt mixture.Industrial computed tomography(CT)technology and digital picture processing technology are utilized to obtain the internal mesoscopic structure feature of SMA-13 on different damage stages.The internal voidage of SMA-13 compacted by Marshall method presents a approximate symmetrical distribution along the height direction of specimen;the internal voidage of SMA-13 compacted by Gyratory method exhibit a "bathtub" shape,whereby more voids are present at the top and bottom parts of a specimen,with larger variability along the height direction of specimen.In the process of triaxial compression fatigue,the SMA-13 chronologically experienced the process of compaction,destruction and recompaction.