Study Of Cracking Behavior Of Self-healing Microcapsules Used In Cement Concrete Pavement

With the development of modern science and technology,how to realize self-repairing of materials has become a hot spot in the field of material research.At present,there are lots of investigations focusing on self-repairing in cementitious materials,as one of the commonly used passive repairing methods,self-repairing through the utilization of microcapsules has attracted more and more attention.When using microcapsules for self-repairing of cement-based materials,researchers often implant microcapsules which contain liquid adhesive encapsulated into cement-based materials in order to analyze the mechanical and durable properties of cement-based materials before and after repairing and finally evaluate the repairing efficiency of microcapsules.However,to apply microcapsule repairing method in concrete pavement and investigate the mechanical behavior of microcapsules under traffic and environmental loads have not been reported.The primary task for self-repairing of concrete pavement through microcapsule method is to ensure that microcapsules can withstand the traffic and environmental loads without premature rupture,in other words,the microcapsule wall should have sufficient strength to resist cracking.And the ultimate objective of the microcapsule method is that the microcapsule wall could be ruptured when a crack occurs in the concrete,in other words,the wall of the microcapsule should have cleavability.Based on this purpose,simulations were carried out on the study of cracking resistance and cleavability of microcapsules embedded in concrete pavement.In comparison with the concrete pavement,the size of microcapsules is relatively small(usually less than 1000? m).Therefore,it is impossible to study mechanical behaviors of both the two at the same scale,and multi-scale method must be adopted to solve this problem.In this study,a two-scale macro-micro analysis model was established to analyze the mechanical behavior of microcapsules.At the macro scale,the concrete pavement containing microcapsules was regarded as a kind of isotropic homogeneous material,while at the micro scale,it was regarded as a two-phase composite material composed of concrete and microcapsules.On the basis of scale division,the volume fractions of materials on each scale were calculated according to the content of the microcapsule and the mix proportion of the concrete,which were served as the basis for the estimation of equivalent thermodynamic constants of the microcapsule and the concrete.At the same time,the establishing method of the geometric model used for finite element analysis was introduced.The coupling multi-scale method was introduced to analyze the stress of the microcapsule in self-repairing concrete pavement under the combined effect of traffic load and temperature load.According to the principle of the coupling multi-scale method,periodic boundary condition was implemented in the mesoscopic model,and based on this technique,the components of strain of the stress sensitive unit in the macroscopic model were extracted,which were considered as the average strain components to apply to the mesoscopic model.Accordingly,the stress of the microcapsule wall could be obtained.In addition,parameter sensitivity analysis was carried out on the factors that may affect the stress level of the capsule wall,such as the elastic modulus of the capsule wall,the wall thickness/core radius ratio,the size of the microcapsule and the overload.Finally,the cracking resistance of the capsule wall was evaluated.When a crack appears in the concrete,the propagation of the crack could break the microcapsule wall to realize the self-repairing function is the ultimate goal of the microcapsule repairing method.When microcapsules were added to the concrete,the interfacial strength between the two materials may affect the propagation path of the crack.In this paper,the cleavability of the microcapsule wall in concrete was analyzed through the combined use of extended finite element method and cohesive zone model.The crack propagation in concrete was simulated by extended finite element method,and in the interface between microcapsule wall and concrete the crack was simulated by cohesive surface model.The influences of the elastic modulus of the microcapsule wall,the wall thickness/core radius ratio,the size of the microcapsule and the bonding strength on the cleavability of the microcapsule wall were analyzed in order to evaluate the cleavability of the microcapsule.The research methods established in this paper to investigate the cracking resistance and cleavability of microcapsules could be served as a reference for the application of microcapsules in concrete pavement,and promote the optimization of design parameters of microcapsules.In addition,this research put forward some rationalization proposals on the modulus of the shell of the microcapsule,the size of the microcapsule,the ratio of the thickness of microcapsule wall to the radius of microcapsule core,and the bonding strength of concrete and the microcapsule.