| The problem of preventing and controlling reflective cracks in the bituminous retrograde layer of old cement concrete pavements is a difficult problem in road reconstruction projects.There are joints and damaged cracks in old cement concrete pavements,and reflection cracks are highly likely to occur at locations where the asphalt overlay corresponds to cracks and joints in the underlying old pavement panels.Research based on the microscopic scale has important theoretical implications for the mechanism of reflective cracking.Therefore,it is necessary to study the reflection cracking mechanism of asphalt paving layers at microscopic scale.In this thesis,a three-point bending test of composite specimens with jointed cement concrete slabs and asphalt pavement is used to investigate the mechanism and development of reflection cracking at the microscopic scale,in combination with the discrete element method.The main research contents and conclusions of this paper are as follows:(1)The laws of changes in microscopic morphological characteristics of asphalt mixtures at different stages of reflection cracking were revealed.Firstly,the three-point bending test of acoustic composite specimens,combined with acoustic emission technology,divided the reflection cracking stages into three stages under microscopic scale.Secondly,the microscopic morphological characteristics and changes of reflection cracking of the specimens in each stage were observed and analysed.The results show that the acoustic emission amplitudes of 37 d B,48 d B and 55 d B appear when the deflection is close to 0.2mm,4mm and 11 mm,thus dividing the reflection cracking into initial,failure and residual phases at the microscopic level;the cracks in the initial phase mostly occur in the asphalt part,the cracks in the failure phase mostly occur at the asphaltaggregate interface,and the cracks in the failure phase are found all over the surface.(2)The changes in the microscopic characteristics of the asphalt-aggregate interface during the reflection cracking process were revealed.Firstly,a three-point bending test was carried out on mortar specimens by means of a home-made loading device to investigate the morphological characteristics of mortar specimens at each stage of the reflection cracking process.Secondly,the microscopic features were quantified and analysed using the software Image-Pro Plus to achieve the microscopic cracking stages.The results of the study show that microscopic cracking occurs at the asphalt-aggregate interface,expands to the asphalt mastic region and eventually connects in the asphalt mastic region;the microscopic crack development is divided into two periods: the crack emergence period and the structural failure period,during which the microcrack area increases by 27,111 μm2 and the number of microcracks decreases by 113.(3)The mechanism of micro-crack generation during reflection cracking of composite specimens was elucidated.Firstly,the correctness of the model was verified by comparison with the indoor tests.Secondly,the effects of different loading positions and joint widths on the evolution of force chains,microcrack development patterns and internal displacement clouds of the composite specimens were investigated.The results of the study show that the difference in peak load between the actual and simulated specimens at damage is 5.51% and the model is correct.The 4cm offset load case required the application of 2.92 times the central load to cause upward expansion of the reflective cracking;the lower cement slab pre-joint joint width had little effect on the reflective cracking process mechanism at the 15 mm joint width condition. |
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