Study Of The Macro And Microscopic Effects Of The Layer Interface And Its Shear Properties In Asphalt Pavement

The interlayer is a weak zone in asphalt pavement structure,which has an important impact on the service performance of pavement structure.In recent years,strengthening the interlayer bonding of pavement structure has attracted great attention among researchers.However,due to the lack of scientific and reasonable evaluation methods and standards of interlayer bonding,there is a certain blindness in interlayer treatment process and material selection design.The root cause is that the interlayer characteristics of pavement engineering are not well understood.In addition,the changing law of interlayer bonding condition and the formation mechanism of interlayer strength are not well understood.At the same time,the existing interlayer models based on macro domain analysis assumes that the interlayer surface is plane,which is far from the actual situation,and it is difficult to reveal the underlying cause of interlayer formation from the source.In view of this,this research started from studying the macro and micro domain"action effects"between the upper and lower interlayer surfaces of double-layered asphalt specimens.Nondestructive separation technology,three-dimensional laser scanning,and point cloud reverse reconstruction were utilized to reconstruct the three-dimensional morphology of the interface between the layers of asphalt mixture typical structures（AC,SMA,OGFC）and specimens with different gradations（9.5 mm,13.2 mm,16 mm）.On this basis,the"contact effect","interlocking effect"and the three-dimensional micro morphology index of the interface between different double-layered specimens were defined and calculated.Finally,combined with the interlayer shear strength（direct shear strength and inclined shear strength）of double-layered specimens,an interlayer shear strength model based on"macro effect"("effective contact area ratio A_r-r",three-dimensional"interlocking effect"coefficient IE_3D)and"meso effect"（key index of three-dimensional meso morphology）was established to reveal the strength formation mechanism of the interlayer bonding of double-layered asphalt specimens at multiple scales.The main work and research achievements are as follows:（1）The nondestructive separation of the interface of double-layered asphalt specimens was developed for the first time,and the complete upper and lower layer and their interfaces were obtained.On this basis,the three-dimensional point cloud data set of typical asphalt interlayer morphology was established by using the high-precision three-dimensional laser scanning technology.The point cloud reverse reconstruction technology was applied to realize the three-dimensional digital reconstruction of the original topography of the interlayer.The results provided a reliable data basis for the subsequent macro and micro morphological characterization of the interlayer and its effect analysis.（2）With the aid of solid rough surface contact theory,Delaunay triangulation method and Helen formula,the space curved surface area of the interface between double-layered asphalt specimens was calculated.The"contact effect"between layers of specimens was characterized from the macro scale by parameters such as"nominal contact area change ratio A_a-r","contour contact area ratio A_c-r"and"effective contact area ratio A_r-r".The results showed that with the increase of the nominal maximum particle size of the lower layer of the specimen,the spatial surface area of the interface between the upper and lower layers increased.When the upper layer was dense graded asphalt mixture（AC,SMA）,the"contact effect"between the layers of the specimen was greater than the condition that upper layer was open graded asphalt mixture（OGFC）.When dense graded asphalt mixture（AC,SMA）was used in both layers,the"contact effect"between the layers of the specimen mainly depended on the specific surface area between the aggregate particles in the upper and lower layers of asphalt mixture.The larger the specific surface area between the aggregate particles,the better the"contact effect"between the layers.When open graded asphalt mixture（OGFC）used as the upper layer,with the increase of the nominal maximum particle size of the substructure,its interlayer"contact effect"decreased first and then increased.（3）On the basis of the theory of"inlaying effect"of asphalt mixture spatial structure,and with the help of mortise and tenon and gear"inlaying"structure,the three-dimensional"concave-convex"structural space volume and its three-dimensional"inlaying effect"coefficient of the interface of different double-layer asphalt mixture specimens were defined and compared from the macroscopic scale.The results showed that when the mixture spatial structure is the same,with the increase of the nominal maximum particle size,the three-dimensional"convex"structure space volume of each interface gradually increased,while the"concave"structure space volume first decreased and then increased.In addition,the average value of the three-dimensional"interlocking effect"between layers of the specimen with the upper layer of dense grading（hereinafter referred to as"dense grading group"）was greater than the open grading（hereinafter referred to as"open grading group"）,that is,SMA-SMA>AC-AC>SMA-AC>OGFC-SMA>OGFC-AC>OGFC-OGFC.With the increase of the nominal maximum particle size of the aggregate in the lower layer,the three-dimensional"interlocking effect"between the layers of the dense gradation group decreased gradually.In the open grading group,the three-dimensional"interlocking effect"between layers of OGFC-AC and OGFC-SMA decreased first and then increased.However,the three-dimensional"interlocking effect"between layers of OGFC-OGFC gradually increased.It was determined that the three-dimensional overall"interlocking effect"between layers of double-layered specimens includes both the"mesoscopic interlocking effect"brought by aggregate particles（apparent texture）and the"macroscopic interlocking effect"formed by the upper and lower mixture layers spatial structure（"concave convex"structure）.（4）In terms of microscopic morphology characterization of layer interface,except for surface peak vertex density(S_pd),other interface microscopic peak characteristics(surface root mean square deviation（S_q）,surface height distribution skewness(S_sk),surface height distribution steepness(S_ku),and surface peak vertex arithmetic mean curvature(S_pc))showed great differences.The gradient S_sk of the height distribution of the interface between layers was less than 0,and the gradient S_ku of the height distribution was more than 3,indicating that the meso level height of the interface between asphalt layers is"sharp peak"relative to the reference surface.In addition,it was found that the damage rate of interlayer isolation membrane can effectively reflect the difference of"action effect"of interface.The damage rate of interlayer isolation membrane for different combinations of mixtures were ranked as follows:OGFC-AC<OGFC-SMA<AC-AC<OGFC-OGFC<SMA-AC<SMA-SMA.With the increase of the nominal maximum particle size of the asphalt mixture aggregate in the lower layer（9.5 mm→13.2 mm→16 mm while the nominal maximum particle size of the upper layer remained unchanged）,when the upper and lower layers of the specimen were dense graded asphalt mixtures（AC-AC,SMA-SMA,SMA-AC）,the damage rate of the interlayer isolation membrane gradually decreased.When the upper layer was open graded asphalt mixture（OGFC-AC,OGFC-SMA）,the damage of interlayer isolation membrane decreased first and then increased.Having the upper and lower layers as open graded asphalt mixture（OGFC-OGFC）,the damage rate of interlayer isolation membrane increased gradually.The meso peak characteristics of the interface of the specimen had a significant effect on the damage rate of the interlayer isolation film（"**"）,and the extent of the impact ranked as follows:root mean square deviation of the surface S_q>arithmetic mean curvature of the peak vertex S_pc>surface height distribution skewness S_sk>surface peak vertex density S_pd>surface height distribution steepness S_ku.（5）Based on the international standard ISO 25178-2 for 3D roughness evaluation and the 3D micro morphology analysis method,from the aspects of meso height(S_q,S_sk,S_ku,S_p,S_v,S_z,S_a),meso function(S_k,S_pk,S_vk,S_mr1,S_mr2,S_xp),meso space(S_al,S_tr,S_td),meso volume(V_mp,V_mc,V_vc,V_vv)and meso morphology(S_pd,S_pc,S_10z,S_5p,S_5v,S_da,S_ha,S_dv,S_hv),the differences of three-dimensional micro morphology indexes of interfaces of different specimens were calculated and compared systematically.By using the principal component analysis and correlation analysis,it was indicated that the comprehensive index I(S_10z,S_q,S_xp,S_5p,S_dv,S_pd),comprehensive index II(S_sk,S_mr1,S_mr2,S_pc),comprehensive index III(S_ku,S_da,S_ha)and comprehensive index IV(S_tr,S_al,S_td)can effectively represent the three-dimensional meso morphological characteristics of the layer interface.（6）Based on the interlayer bonding（direct shear strength and inclined shear strength）testing devices,the interlayer shear strength of different specimens was tested and compared.Then,the multivariate linear and nonlinear regression analysis were used for data driven modeling and the entropy weight method was utilized for combining the macro and micro"action effects"("effective contact area ratio A_r-r",three-dimensional"interlocking effect"coefficient IE_3D)of the interface.The internal relationship between"meso action effect"（key indicators of three-dimensional meso morphology）and interlayer shear strength has established an accurate"macro action effect"nonlinear interlayer shear strength model and a meso action effect"linear interlaminar shear strength mathematical model.As a result,the contribution of"contact"and"interlocking"effects between layers at the macro scale and key indicators of three-dimensional meso morphology at the meso scale to interlayer shear strength were quantified.The multi-scale analysis revealed the formation mechanism of shear strength under the coupling effect of multiple factors at the interlayer interface.Based on the special"double interface"working characteristics of asphalt mixture specimens,this thesis defined and calculated the"contact effect","interlocking effect"and key indicators of three-dimensional micro morphology of typical layer interface.Moreover,the impact of typical asphalt mixture spatial structure（AC,SMA,OGFC）and its grading differences（9.6 mm,13.2 mm,16 mm）on the above"action effect"were analyzed.The relationship between interlayer"action effect"and interlayer shear strength of specimens was explored,and the formation mechanism of interlayer strength of specimens was revealed by multi-scale analysis.The research results provide a theoretical basis and technical support for the design and optimization of asphalt pavement interlayer structure.