Study On Meso-structure And Damage Effect Of Cement Stabilized Macadam

Cracking has been recognized as the typical failure mode of semi-rigid base materials.Load is the most direct cause of the structural damage.Cement stabilized macadam is one of the typical semi-rigid base materials.Its damage under load is micro-defect inside the material with crack initiation,expansion and convergence step by step.The base layer of the pavement is below the asphalt surface layer and belongs to the concealed structural layer.It is relatively difficult to observe the structural damage process.Therefore,there has been a long-standing problem that the material and structural damage state and their parameters deviate greatly from the actual macroscopic expression indicators.So,it is necessary to improve the understanding of material damage under test support.On the basis of summarizing the existing research results at home and abroad,this paper focuses on the problem of semi-rigid base material damage as a small-scale,complex and procedural variable.It used the image technology to analyse the damage evolution process of gravel under load based on the meso-analysis perspective.The mesostructure index system of cement stabilized macadam mixture was established with the combination of image processing technology and mechanical testing technology,The paper Conducted the effects of the mesostructure damage by analyzing the internal aggregate and void distribution of the mixture with the research object of mesostructure of the mixture core sample.specifically:Firstly,The mesostructure index system of cement stabilized macadam mixture was established.The paper analyzed the image of the cement stabilized macadam mixture core sample,by using the self-developed image processing program with the needs of meso-structure analysis.And it extracted the image of the aggregate shape,composition,structure indicators and the structural indicator system of pore distribution in the mixture.It verified the sensitivity and effectiveness of the indicators.Secondly,the paper designed the CT scanning and loading test of cement stabilized macadam material.It Analyzed the CT composition and imaging principle,and calibrated the equipment used in four aspects: industrial CT ray energy,density resolution,detector receiving mode,and reconstruction algorithm.It objectively evaluated the performance of CT equipment and the scanning image effect of cement stabilized macadam material.It was determined that the image of cement stabilized macadam material meets the problem analysis requirements.The screening rates are 30%,35%,40%,45% and 50% based on the pass rate of 4.75 mm.respectively.It designed the loading scheme,load level and sampling position.It Loaded and scan the water stable aggregates B1-N2 and B2-N9 with differential grading(4.75 mm pass rate of 30% and 45%),and obtained the tomographic images.And it Analyzed the applicability to existing image threshold segmentation methods.Based on these,it improved the quality of the image recognition and segmentation of tomography by use the Ring improvement Otsuka.Thirdly,it Statistically Analyzed the pore mesostructure distribution characteristics of the image with the two-dimensional CT tomographic image analysis of cement stabilized macadam based on the microscopic damage evolution of CT images.IT calculated the CT scan fault of cement stabilized macadam in different loading and scanning stages with the combined theory of differential box dimension method and fractal method.It analyzed the correlation between the fractal dimension of the image and the load,and it revealed the mesoscopic damage evolution process of the material.It analyzed the influence of the skeleton structure on the material properties.It analyzed the correlation between the composition ratio of the coarse aggregates,the aggregate spacing,the aggregate contact area and the macroscopic mechanical indexes at the image scale.It is preliminarily verified that there is a certain negative correlation between the particle number and the compressive strength under the unconfined compressive strength;the number of contact units of the particles is also weakly linearly related to the compressive strength.Finally,it analyzed the damage and fracture process of brittle materials.And it proposed the incremental recursive exponential model of quasi-brittle material damage,which was verified by B1-N2 and B2-N9 grading materials under step-by-step loading.The type of B2-N9 specimen damage meets the damage characteristics of the quasi-brittle material,that is,the threshold value of the damage exists.Before the threshold value,the material damage is not obvious.After the damage threshold value,the material has an accelerated damage process.However,the skeleton type mixture cannot be simply described by an exponential model due to internal mesoscopic inclusions and the like.The research results in this paper can help to understand the meso-damage performance of the semi-rigid base layer.It is also rich and perfect for the current design index system,which has very important engineering application value.