Macro-mesoscopic Study On Mechanical Behavior Of Grouted Semi-flexible Pavement Material

Semi-flexible pavement(SFP)refers to a new type of road construction material formed by pouring a special cement mortar into a porous asphalt(PA)mixture matrix,whose matrix porosity is generally between 20% and 30%.SFP has attracted increasing attention because of its excellent rutting resistance,but its promotion is also hindered by the disease of cracking.Focusing on the structural characteristics of its compact-skeletal interlocking,as well as the viscoelastic asphalt interface,the stiffness composition mechanism of SFP,the characteristics of asphalt interface and the failure modes at different temperatures were investigated at mesoscale.Specifically,the main work carried out in this paper and the corresponding findings were as follows:(1)Based on the framework of the meso-mechanical model,the two-phase and three-phase mesoscale predicting models on dynamic modulus,which consider the interlocking effect,were established by introducing elastic and viscoelastic interlocking factors into the 2S2P1 D constitutive model of the asphalt phase.The model quantified the stiffness composition of SFPs formed from different graded PA matrix.The contribution of the secondary skeleton of grouting cement on the overall stiffness was also evaluated.The results showed that the interlocking effect between the two skeletons accounts for 1/15 to 1/6 of the whole,which makes a significant contribution to the stiffness of SFP and should not be ignored in the numerical simulation.(2)Through electron microscopy scanning,energy spectrum analysis technology,and nanoindentation test,the asphalt phase,as well as its interface transition zone with the aggregate phase and the cement phase,were characterized from three aspects including microscopic morphology,chemical composition and mechanical modulus.Based on that,the mesomechanical model was then modified.At the same time,the viscoelastic parameters of the asphalt phase considering the interlocking and compaction effects were extracted from the model,which was verified valid using the results from the nanoindentation creep test.It is manifested that the extracted equivalent modulus of asphalt layer is a reliable and effective modeling parameter.(3)With the help of the CT scanning,the internal microstructure of SFP was obtained.The contact characteristics of SFP with different gradations were counted using image processing technology.After that,a method to distinguish between effective cement-coarse aggregate contact(cement acts as a skeleton)and ineffective cement-coarse aggregate contact(cement only acts as a binder)was proposed.As the statistical result,the number of effective contacts is about 1/6 of the whole cement-coarse aggregate contacts.Additionally,the morphological interlocking index was also defined and verified employing the interlocking factor from the meso-mechanical model.Furthoremore,the input parameters of the interlocking effect needed to establish the stiffness prediction model can be obtained from the description of meso-contact chatacteristics.(4)Using Acoustic Emission(AE)technology to monitor the uniaxial compression failure process of SFP and its PA matrix,the influence of the lignin fiber and that of the grouting cement on the overall mechanical properties,as well as on the AE signals,were studied.The analysis result of AE signals reflected that fiber can restrain accelerated shear failure.Also,porous asphalt matrix with 25% porosity has the most stable gradation.Based on the AE energy,the Weibull damage evolution equation was established,with the finding that the equation parameters are correlated with heterogeneity and skeletal interlocking,respectively.(5)The splitting processes of SFP at different temperatures were monitored using AE technology.The collected AE signals were classified through cluster analysis.The mesodamage mechanism was inferred based on the CT cross-sectional images after the destruction.Then,the damage source identification of the AE signals was realized.It was found that there is a temperature effect on the damage of SFP: the failure mode transfers from a main crack accrossing section to distributed meso-craks,as well as from adhesive damage,aggregate crack to cohesive damage in asphalt and cement.(6)On the foundation of the equivalent modulus of the asphalt layer,which considers the interlocking and compaction effects,the dynamic modulus test simulation of the mesorepresentative unit was carried out,the master curves of the dynamic modulus and phase angle were calculated,with verification with the result from the indoor tests.Besides,a mesoscale direct tensile model was established.The damage was characterized by inserting cohesive zone model units in batches.The validity of the model was also verified by comparing with the failure law summarized in(5).The sensitivity analysis of mechanical parameters of cement was carried out,and it was found that the tensile strength of cement should be adjusted to more than 3.5MPa and the Young modulus should be about 10000 MPa for material optimization under the condition of existing asphalt mixture matrix.