Strength Mechanism And Degradation Of Porous Asphalt Mixtures Under Multi-physics

Porous asphalt mixtures?PAC?are an open-graded mixture with a maximum air void?AV?of 22% to 25%.PAC has the advantages of rainoff mitigation,safety improvement,noise reduction,and et al.Due to the special internal structure,the strength mechanism and performance of PAC is extremely different to the traditional dense graded asphalt mixtures,and PAC worked under complicated coupled conditions of vehicle loading,moisture,and temperature.Therefore,the objective of this thesis is to analyze the strength mechanism and degradation of PAC-13?PAC with a nominal maximum aggregate size of 13.2mm?under the coupled conditions.Firstly,the compaction characteristics of PAC were evaluated.A multisdirectional permeameter was designed to provide a more comprehensive evaluation of permeability which includes total permeability coefficient,vertical permeability,and horizontal permeability.Based on the balance design of permeability,skeleton structure,and durability,the design gyrations?Ndesign?of PAC are determined as in the range of 45 to 75 gyrations.Secondly,the parameter of cohesion?c?and friction angle???from Mohr-Coulomb failure theory was evaluated in the triaxial compression test.Further,the skeleton strength,adhesive strength,and cohesive strength of PAC were directly obtained from the developed tests of skeleton penetration test?SPT?,binder bond strength?BBS?test and modified binder bond strength?MBBS?test,to analyze the strength mechanism of PAC.Penetration strength and penetration energy were used to evaluate the influence aggregate size on the skeleton strength.The skeleton strength was decreased with the increment of percent passing of 2.36 mm,and the percent of aggregates passing 4.75 mm and retained on 2.36 mm acts a part of the skeleton bearing the loading.Aggregates size of 9.5mm has the most important influence on the skeleton strength.The design gradation of PAC-13 was optimized based on skeleton strength.The influence of binder type,temperature,and moisture damage on the adhesion and cohesion of asphalt,mastic?asphalt+filler?,and mortar?asphalt+filler+fine aggregates?was evaluated by the BBS test and MBBS test.Failure regime of asphalt-aggregate changes from adhesive failure to cohesive failure with the increasing temperature,and adhesive failure was occurred under moisture damage.The failure regime of matrix and motar changes from cohesive to adhesive failure,and cohesive failure was occurred under short-term moisture damage,while adhesive failure occurred under long-term moisture damage.Motar has the best correlations to strength and durability of PAC.Thirdly,a multi-physics repeated loading permanent deformation?MRLPD?test was developed to evaluate the rutting resistance of PAC under the coupled conditions of moisture,loading and temperature.X-ray CT scaning and staged test was used in the MRLPD test to analyze the degradation of rutting resistance.Four coulpled conditions of dry,raingall onset,ongoing raingall,and rainfall end were designed based on the rainfall process.Results indicate that the traditional moisture test is not appropriate for PAC to evaluate the influence of moisture on the performance.Under the rainfall onset and ongoing rainfall conditions,the rutting resistance of the PAC is lower than that under the dry condition.PAC shows the poorest rutting resistance under the ongoing rainfall condition.The presence of water in the pores creates pore pressure,which facilitates moisture damage and consequently decreases the rutting resistance.The rutting resistance of the PAC is more sensitive to temperature and overloading than to moisture damage.Crushing fracture was occurred under dry and rainfall onset conditions,and shear fracture was found in ongoing rainfall condition,and mixed crushing/shear fracture was found in rainfall end condition.Under the dry condition,the degradation of high-temperature performance of PAC undergoes the first stage of densification,the secondary stage of shear and densification,and the third stage of densification failure.In the first stage,AV is decreased and the horizontal deformation develops faster than vertical.In the secondary stage,AV and interconnected AV were increased under the shear and densification.In the third stage,the AV and interconnected AV decreased and the vertical deformation develops faster than horizontal deformation.Under saturated condition,the shear stress and the increment of AV are larger than dry condition in the firsr stage.In the second stage,AV and interconnected AV are increased under the functions of shear and densification,and the increament of AV and interconnected AV is larger than dry condition.In the third stage,PAC was shear failure under the saturated condition.The AV and interconnected AV continued increased,and the vertical deformation develops faster than horizontal deformation in the whole process.Lastly,the moisture sensibility and durability of PAC was evaluated.Also,the degradation of moisture sensibility and durability was also analyzed by the steged test under dry and saturated conditions.According to the residual strength after fatigue test,it was found that the degradation was mainly occurred in the early fatigue stage.Based on indirect tensile strength and indirect tensile resilience modulus,damage parameters of ?₁ and ?₂ were proposed to evaluate and predict the moisture damage.