Study On Mechanical Behavior And Performance Requirements Of Self-healing Microcapsules In Asphalt Pavement

The idea and inspiration for developing the self-healing microcapsule is derived from the natural ability of cells or tissues of the organisms to heal themselves.Therefore,some scientists always dream that if some materials have the self-healing ability,they will help extend the life and improve the reliability of the materials,and also reduce the cost of maintenance.Micro-cracks are inevitably produced in the interior of asphalt concrete under the normal usage.These micro-cracks will further propagate the macro-cracks affected by the load or temperature,which will reduce the service life of the asphalt pavement.With the development of microencapsulation technology,some researchers have applied the self-healing microcapsule to the asphalt pavement,which makes asphalt pavement with the ability to healing itself.in the current study,the research of self-healing microcapsule in asphalt pavement focuses on the repair efficiency and microcapsule preparation.However,there is still a gap on microcapsule applicability using in asphalt pavement which is tightly related to the idea of self-healing asphalt pavement.For using the microcapsule with the self-healing ability in asphalt pavement must meet the following requirements.Firstly,the microcapsule must have sufficient strength to bear the vehicle load and keep very good condition.Secondly,once the micro-cracks appear,the microcapsule can timely break and release the healing agent.Therefore,fully understanding the mechanical behavior in asphalt pavement,and fracture behavior under the action of micro-cracks are the most basic and key issues for developing the suitable microcapsule for self-healing asphalt pavement.1.The cracking resistance requirements of microcapsule under heacy traffic load in asphalt pavement.Firstly,the mathematical probability method and computer digital visualization technique were used to create the digital mixture specimen in meso-scale,and the contact and bond models in discrete elements was introduced in this thesis.Then,the contact and bond models were selected for the asphalt mixture,which can represent the viscoelastic behavior of asphalt mixture.More over,based on the relationship between the meso-scale parameters and macro performance of asphalt mixture,the lab tests were conducted to determine the input parameters of discrete elements in meso-scale.After that,the asphalt pavement model was established using the discrete element method.Combined with digital specimen,the macro-meso scale asphalt pavement was developed.Then,the mechanical response of pavement under heavy traffic load was analyzed.The results for mechanical response were compared with the Bisar program in order to validate the model and the parameters selected were correct.In addition,the mechanical response in asphalt mortar was studied under heavy traffic load.The results show that when the heavy load is 1.43 MPa,the average pressure load is 0.347 N,and the average tensile load is 0.093 N.Finally,the micro-scale self-healing model,which contains the microcapsule,was established to determine the actual load condition of microcapsule in asphalt pavement under heavy traffic load.The results show that the single microcapsule should keep good condition under 25 mN pressure load,which can meet the application requirements in the asphalt pavement.Finally,the ABAQUS was used to build a spherical microcapsule finite element model.The stress and displacement distributions of microcapsule wall and core were analyzed using finite element method,and the most easily damaged location was found according to the maximum tensile stress strength criterion.The result shows that the most easily damaged location is the outboard of compression zone,which is suffered the highest tensile stress value.In addition,the effects of size,wall thickness and the elastic modulus of microcapsule were also studied in terms of stress and displacement distributions.The results showed that under the compact loading the bigger the microcapsule size,the larger the stress,the more prone to rupture.And the small changes of microcapsule size can make a significant effect of stress distribution.The larger the wall thickness,the stronger of the ability to resist the load,the less likely to rupture.The larger the elastic modulus of microcapsule wall,the stronger of the ability to resist the deformation.But reduction of deformation led to the elevation of the maximum equivalent stress.Therefore,the quality of the wall's elastic modulus for resisting rupture still need further estimation.2.The cracking requirements of microcapsule in the presence of micro cracks in asphalt pavement.For the analysis of cracking requirements of microcapsule when micro-crack appear,analytical method is so difficult to solve fracture problems of complex materials.Due to the limitation of size effect,it is not suitable to study the cracking mechanism of microcapsule by laboratory test.As a result,the numerical method becomes a more powerful tool.There are lots of restrictions to figure out the coexistence of microcapsule particles and cracks using the traditional numerical methods,like finite element method and extended finite element method.Based on the advantages of the above two methods,an improved finite element method was used to analyze the cracking requirements under the action of micro-crack.Firstly,this paper introduces the improved implementation of the extended finite element method,typical problem solving the stress intensity factor,and compared with the conventional finite element method and the analytical solution,this paper proves that the precision of the present method.Finally,the influence of mechanical parameters,distribution and number of microcapsule are studied on the fracture parameters of asphalt matrix.And using the maximum circumferential stress criterion to simulate the crack propagation trajectories.The results show that the elastic modulus of microcapsule has much effect on the crack tip fracture parameters of asphalt matrix.When the elastic modulus of microcapsule was smaller than the asphalt matrix,the microcapsule would play important role to strengthen the crack tip fracture parameters.The micro-crack will move towards to the microcapsule,which will enable the microcapsule to serve as the self-healing function.When the elastic modulus of microcapsule was larger than the asphalt matrix,the microcapsule would weaken the crack tip fracture parameters.The crack would keep away from the microcapsule and propagate.In addition,the effect of microcapsule,which can heal the matrix material,on fatigue life and crack growth rate was also discussed.The results showed that the microcapsule had an effect on accelerating the crack growth rate,and reducing the fatigue life of the whole material,without considering the effect of self-healing of microcapsule.