Research On Opening-mode Reflective Cracking Behavior Of Asphalt Overlay On Old Cement Pavement

Adding asphalt concrete on old roads is a commonly used method for repairing old roads in actual projects,and reflective cracks are the main disease type of this type of composite structure pavement.The development process of reflective cracks can be divided into two stages:crack initiation and crack propagation.The initiation stage of cracks is dominated by opening-mode reflective cracking.At present,there are few studies on the mechanism and influencing factors of opening-mode reflective cracking.In view of the fact that opening-mode reflective cracking is the dominant position in the initial development of reflective cracks,carrying out relevant research is useful for understanding the formation and damage evolution process of reflective cracks in asphalt overlays.Significance.This paper will carry out the three-point bending test of the asphalt layer structure of the cement concrete slab with joints to explore the expansion mechanism,influencing factors and evaluation indicators of the expansion type reflection cracking behavior.The main research contents and results are as follows:In the exploration of the mechanism of opening-mode reflection cracking behavior,acoustic emission technology is used to dynamically monitor the damage process of composite beams,and the horizontal strain field of the damage process is obtained with the help of digital imaging software Ncorr,based on the expansion of acoustic emission technology and digital image technology.The investigation of the mechanism of reflective cracking shows that the acoustic emission technology has a good recognition effect on the development of microcracks and macro cracks in the asphalt overlay;based on the AE parameters,the opening-mode reflective cracking process can be divided into silent period,rising period,There are 4 stages of development and residual period.There is basically no AE signal in the silent period;the damage increases rapidly in the rising and developing phases,which is manifested by the dense and active AE signal,which is in good agreement with the load curve,showing a unimodal characteristic;the composite beam is approaching damage in the residual period,and the damage increases slow.The expansion process of the open reflection cracks first develops along the two sides of the joint in the form of multiple cracks,and then one of the cracks develops into the main crack and continues to expand vertically upwards,and the secondary crack enters a stagnant state;and the amount of damage based on ringing is proposed.The damage evolution characterization method of the composite beam’s expansion reflection cracking behavior.Through the comparison of acoustic emission characteristic parameters,the bending fracture process of the two types of composite beams with or without the glass fiber grid sandwich is analyzed,and the evolution characteristics of the AE parameters during the damage process of the specimen are explored.The fracture mode of the composite beam is analyzed through the RA value and the AF value,and combined Fractal theory studies the effect of glass fiber grid on the bending and fracture characteristics of composite beams.The study shows that the acoustic emission characteristic parameters can effectively identify the period of action of the glass fiber grid;the glass fiber grid gradually begins to bear the bending and tensile load after the load peak,delaying The expansion of the macroscopic cracks has a significant effect on the post-peak load-holding capacity of the composite beam,but the bearing period lags behind the asphalt overlay,which has no effect on the bending failure load;the existence of the glass fiber grid interlayer reduces the composite beam The tensile failure trend improves the orderliness in the bending failure process of the composite beam,and makes the composite beam a more orderly and gradual process from the generation,development,and integration of micro-cracks to the final bending failure of the composite beam.Fractal features.In terms of the influence of open reflection cracking factors,based on two types of evaluation indicators,peak load and fracture energy,the effects of joint width,geotechnical interlayer type,maximum nominal particle size and overlay thickness on crack resistance are analyzed separately.The core of the anti-sandwich composite beam resisting Type I reflective cracking lies in the cohesive force between the overlay asphalt and asphalt mortar and the aggregate;different joint widths and different maximum nominal particle diameters have little effect on the cohesive force;asphalt The amount directly affects this performance;the geotechnical interlayer provides tensile performance equivalent to cohesion（the tensile performance of the bottom layer）.Among them,the peak load and fracture energy of composite beams with geotechnical interlayer have been significantly improved.The evaluation method based on fracture energy has good consistency with the existing indoor anti-reverse interlayer fatigue test results.In terms of evaluation index research,three types of anti-cracking evaluation indexes CT_index,FI and CRI,which are based on fracture energy and consider the characteristics of macroscopic crack propagation,are introduced to conduct in-depth analysis of various composite beams.Firstly,with the help of image measurement software,it is verified that the macroscopic crack propagation rate has a good correlation with the slope at the inflection point of the curve after the peak.Secondly,the analysis of composite beams with different overlay thicknesses found that all three types of evaluation indicators have obvious size effects.Finally,analysis of composite beams of the same size with a thickness of 50mm overlaid on the basis of three types of indicators shows that the composite beams show good crack resistance under the"strong"influencing factor of the geotechnical interlayer,which is consistent with the evaluation method based on fracture energy.It has good consistency with the existing indoor anti-interlayer fatigue test results;for the weaker influencing factors such as different maximum nominal particle diameters and increasing asphalt content,compared to fracture energy,FI and CRI consider macroscopic cracks Expansion characteristics can better characterize the crack resistance of composite beams.At the same time,FI and CRI show a good correlation,the coefficient of determination is as high as 0.9951.Considering that the calculation of CRI is simple,it is more suitable for the evaluation of the crack resistance of composite beams.