Study On Crack Propagation And Crack Resistance Mechanism Of Large-size Asphalt Mixture

Large-size asphalt mixture is widely used in subsurface pavement layer of semi-rigid base asphalt pavement,flexible base and cement concrete overlay asphalt pavement.As the subsurface pavement layer of asphalt pavement,large-size asphalt mixture is not only subjected to repeated action of traffic loading,but also to temperature variation.As the connecting layer of pavement structure,large-size asphalt mixture subjected to the load of pavement base,and controls the reflection cracks from subsurface pavement layer of semi-rigid base or cement concrete overlay to expand upwards.The problems of reflective cracks and fatigue cracks of asphalt pavement have become more and more prominent under the conditions of low temperature,long-term aging and fatigue.At present,there is no research on crack propagation behavior and crack resistance mechanism of large-size asphalt mixture.The research on crack propagation behavior and crack resistance mechanism of large-size asphalt mixture not only deepens the understanding of crack resistance mechanism of large-size asphalt mixture and develops new structural types of asphalt mixture,but also has important theoretical and applied value in controlling reflection crack propagation of semi-rigid base and rigid base to asphalt pavement and preventing fatigue cracking of Asphalt Pavement underlying layer.In this paper,overlay tester(OT)test is used to simulate the pavement structure and study the crack propagation of large size asphalt mixture.Through the finite element analysis,the strain range,the reasonable thickness of the specimen and the strain distribution characteristics are determined.Based on the concept of disturbance state,the constitutive model,disturbance evolution law and damage evolution law of large size asphalt mixture are established.The specimens were formed by wheel rolling method,and the effect of nominal maximum particle size of coarse aggregate was highly considered.The specimens were carried out on the specimens with a thickness of 60 mm.All the specimens were cut out from the rutting plate to ensure that the forming conditions and thickness of the specimens can simulate the actual road conditions more truly.1.Based on OT test method,the effects of loading rate,tensile displacement,asphalt content and aging degree on fracture properties of large size asphalt mixture under monotonic eccentric loading were studied.The results show that the crack propagation of large size asphalt mixture has obvious fractal characteristics,the fracture energy is insensitive to the variation of asphalt-aggregate ratio,and the failure load has no obvious variation.Fractal dimension and fracture energy decrease with increasing temperature,but increase with aging degree.With the increase of loading rate,the failure load increases and the fracture strain does not decrease significantly.These characteristics indicate that reducing the asphalt-aggregate ratio,reducing temperature,increasing loading rate and aging degree have no obvious adverse effects on crack resistance of large-size asphalt mixtures.Based on the disturbed state concept theory(DSC),the constitutive relationship of large-size asphalt mixture under eccentric tension load is established.The selected disturbed function fits the stress-strain curve,which is in good agreement with the test results.It provides a basis for structural analysis and checking calculation of large-size asphalt mixture.2.Crack propagation in large size asphalt mixture under repeated loading has attracted much attention.Based on OT test and the concept of pseudo displacement,the crack propagation law of large size asphalt mixture is studied by Paris' Law,and the stress intensity factor in Paris' Law was replaced by J-integral.A fracture propagation model was proposed and parameters A and n in Paris' Law were given,by studying the effects of different temperatures,different tensile displacements,different oilstone ratios and different aging conditions on crack propagation under repeated loading.The smaller n value in Paris' Law of large-size asphalt mixture indicates that fatigue crack propagation is slow and is insensitive to repeated strain.Large particle size asphalt mixture has obvious stress relaxation characteristics,which is conducive to controlling fatigue crack propagation.3.In order to avoid the dispersion,randomness and heterogeneity of damage in asphalt mixture fatigue process,the OT test method is improved in this paper.Materials in representative regions are selected to study the effects of different temperatures,different tensile displacements,different oilstone ratios and different aging conditions on damage evolution under repeated loading.The pseudo stiffness evolution law is established based on the viscoelastic continuum damage model(VECD).The parameter Z reflects that material damage decreases with the increase of frequency,load and aging degree.The concept of disturbance parameter is proposed,which can represent the damage evolution in fatigue process.The results show that the disturbance increases with frequency,load and aging degree,and the life decreases gradually.The fatigue constitutive model based on disturbance state is in good agreement with the experimental results.In summary,the large-diameter asphalt mixture has good crack resistance,low temperature resistance,fatigue resistance,and is insensitive to stress and repeated strain.It is an excellent structural material for the underlying layer of the asphalt pavement and for preventing reflection cracks.