Research On Internal Stress Distribution Of Asphalt Mixture And Its Impact On Viscoelastic Performance

Asphalt pavement has been widely used in the modern high grade road engineeringbecause of its smooth surface, without seams, facilitate comfortable driving and theadvantages of low noise. However, in order to investigate the mechanical characteristics andfailure mechanism of the asphalt mixture, the discussion on mechanical properties of asphaltmixture has never been terminated. In previous studying, asphalt pavement and asphaltmixture were assumed as isotropic homogeneous material in general engineering analysis.The elastic or viscoelastic mechanical parameters of asphalt mixture for these analysis werenecessarily tested by experiments. In actuality, asphalt mixture is a multiphase compositematerial that consist of asphalt mortar, aggregate, filler and voids. Therefore, the assumptionof isotropic materials has a big gap with real mixture as a result of the complexity ofirregular aggregates arrangement. The internal mechanical characteristics and deformationproperties of mixture were complex and different form the isotropic material. Wherefore, Itis needed find a new method that can reflect the real mixture structure for internalmechanical characteristics evaluation.Combining with Project supported by the National Natural Science Foundation ofChina (Grant No.51278222)"Investigation on anisotropic properties of asphalt mixturebased on digital image processing technology" and Project20121079Supported by GraduateInnovation Fund of Jilin University, the internal stress distribution and the viscoelasticdeformation characteristics of asphalt mixture were investigated. The following results wereobtained with research of this paper:1. The asphalt mixture digital images were obtained with CCD digital camera.Aggregates has a serious adhesion phenomenon because of the influence of noise and pixelaccuracy. The watershed segmentation method was proposed for break up aggregateadhesions in digital images. It was proved that this method was suit for aggregate adhesionsegmentation. And then the method of equivalent diameter was adopted to distinguish the area gradation of mixture. According to the particle size of aggregate has a large gap,equidistant grouping can not considered the defect of coarse and fine aggregatessimultaneously. The logarithmic spacing block method was proposed to transform areagradation to real gradation, and speculated the volume gradation of the asphalt mixture. Thecorrected and designed gradation are consistently well. This method can simultaneouslyidentify the coarse and fine aggregates, the problem of identify aggregates gradationsuccessful solved.2. Vectorized method of asphalt mixture image was investigated, the split strength andmerge strength were adopted as judgment indexes which has nothing to do with the diffrentscale of aggregates. Feature points of aggregates can be reserved effectively under thecondition of uneven distribution. The aggregates edge contour and voids edge contour werevectorized with polygon approximation iteration. Root Mean Square Error was used asiterative convergence control index based on the approximation error of the reliabilityprinciple as is presented in this paper. The iterative program was made for contourvectorization. Edge contour of aggregates and voids in asphalt mixture digital image werevectorized. The analysis of iteration convergence efficiency and stability show that thepresented polygonal approximation method can effectively deal with the problem ofmulti-scale contour vector problem. It has a high convergent efficiency. The comparativeanalysis of the numerical model for different Root Mean Square Error shows that thepresented polygonal approximation method can effectively reduce the scale of the numericalmodel, reduces the model dependence on computer hardware, and improves the computationefficiency for numerical analysis.3. The internal stress state of asphalt mortar in mixture was investigated. Stress state ofasphalt mortar was complex and can’t be characterization with a index. And then Gaussfunction was used to describes the state of asphalt mortar under the uniaxial stress.Numerical model of different gradation asphalt mixture were established. Effect of materialparameters, grading form, void content and manufacture method on stress state of mortarwere investigated. The results showed that statistical average value of its stress and stressdeviation of the skeleton dense structure are better than other types of asphalt mixture. The existence of voids can reduce the statistical average stress of asphalt mortar, and stressdistribution of asphalt mortar is more uniform. The frequency of the high stress area appearsincrease obvious. In addition, the multi-scale model of asphalt pavement was established.through comparative analysis of asphalt pavement was carried out for multi-scale model andisotropic model. It suggests that stress value comes from traditional theory is lower than thecalculation results of multi-scale model.4. A optimal segmentation method for Burgers viscoelastic parameters identificationwas proposed. A optimal segmentation scheme of time that has a min fitting residual errorwas obtained. The best parameters of Burgers model was obtained with time-domainsearching method and nonlinear fitting method. This method was compared with existingmethods. It shows that the residual quadratic sum produced by identify is the smallest of allpossible segmentation programme if parameter fitting in accordance with this method. Sumof residual error squares tends to be stable as the creep time increase. And residual errorsquares sum of improved methods reduced significantly compared with the original method.5. Asphalt mortar specimens were prepared in the room. The uniaxial compressivecreep tests were conducted in order to investigate the nonlinear viscoelastic behavior ofasphalt mortar. A practical nonlinear viscoelastic model was used to describe the nonlinearviscoelastic properties of asphalt mortar. And then the parameters of nonlinear viscoelasticmodel were obtained by nonlinear fitting. The parameters function of stress level wereestablished finaly. It shows that the elastic parameters of the model were power function ofstress. The relaxation time changes render non-linear trend as the level of stress. In addition,the nonlinear viscoelastic model of asphalt mastic was applied in the viscoelasticitydeformation prediction by the secondary development. The results show that the predictedviscoelastic deformation of plane multi-scale numerical model is lower than test resultswithout considering the influence of aggregate in lateral strength. The predicted viscoelasticdeformation with nonlinear viscoelastic model is higher than that of linear viscoelasticpredictive value, and it is more close to experimental results. Therefore the nonlinearviscoelasticity effect of asphalt mortar will be reflect the real viscoelasticity deformationcharacteristics of the asphalt mixture better. Asphalt mixture specimens were prepared by rolling. The simulation and test results of specimens in different directions show that asphaltmixture appears weak anisotropic characteristics. This is different with the assumption ofisotropic materials.