Evolution Analysis Of Mechanical Properties Of Ultra-high Performance Concrete Considering Self-healing Process

Manual inspection and damage repair of in-service structures are costly and difficult.The development of new materials with self-healing or self-healing ability to prolong the service life of structures is one of the important directions to be broken through in concrete technology in recent years.Ultra-high performance concrete(UHPC)has a good crack width control ability and a low water-cement ratio,and has a better potential self-healing ability than ordinary concrete.It has been widely used in super high-rise buildings,tunnels,large-span structures,military protection projects and other fields,and has a very broad prospect.At present,the researches on the self-healing behavior of UHPC,especially the researches on the self-healing behavior of damage under long-term load,are mainly experiments,and there are few quantitative numerical simulation studies.Therefore,in order to explore the internal self-healing mechanism of UHPC and its damage self-healing mechanism,this paper developed a quantitative characterization method of UHPC damage self-healing behavior based on the self-healing mechanism and damage theory,and established an analysis model of fatigue damage evolution of UHPC components considering the self-healing process,providing calculation method and analysis theory for the study of UHPC damage self-healing behavior.The main work and research results are as follows:1.Based on the self-healing mechanism of unhydrated cement rehydration and strain hardening damage constitutive relationship of UHPC,the internal variables used to characterize the damage self-healing behavior were defined.Combined with COMSOL Multiphysics and ABAQUS platform for secondary development,a numerical analysis model of the damage self-healing behavior of UHPC was established.Taking uniaxial tensile specimen as an example,the influence of damage degree and self-healing time on damage self-healing degree was simulated and summarized.The results showed that the healing effect of self-healing on UHPC specimens decreases with the increase of damage degree,and the recovery degree of elastic modulus increases with the increase of self-healing time.As for the rehydration mechanism,due to the limited amount of unhydrated cement remaining in the material,the self-healing ability of UHPC will be limited after the self-healing time exceeds 7 days.Prolonging the self-healing time has no obvious effect on improving the self-healing effect.2.Based on the carbonization self-healing mechanism and high-cycle fatigue damage accumulation theory,a multi-field coupling analysis method for fatigue damage and self-healing behavior of UHPC in long-term service was summarized,and the algorithm was implemented through COMSOL Multiphysics and ABAQUS secondary development interface.The effects of self-healing constitutive parameters on fatigue damage evolution of four-point bending beams were analyzed,and the effects of self-healing on fatigue damage evolution of UHPC beams were summarized.The results show that the self-healing efficiency of fatigue damage is limited,and the lower damage level is more favorable to the damage repair.In the initial stage of injury accumulation,the self-healing process may lead to the reversal of injury.But over time,the damage accumulates irreversibly.The increase of reaction rate constant is beneficial to the self-healing of surface damage and internal damage,while the increase of diffusion coefficient is only beneficial to the self-healing of internal damage.Due to the limited amount of substances available for reaction in UHPC,the increase of the two kinds of parameters gradually decreases the promoting effect on the self-healing of damage.For typical load conditions,the fatigue life of UHPC members can be increased by about 8% under self-healing.The self-healing effect is important to prolong the service life of structures.3.Furthermore,the relationship between UHPC damage internal variables and water transport diffusion coefficient was introduced to establish a bidirectional coupling model of UHPC fatigue damage field and water transport field considering the self-healing process.Comparing the difference between the results of bidirectional and unidirectional coupling analysis in the mechanical property evolution analysis of UHPC,the applicable conditions of the two analysis methods were summarized.The results show that in the framework of bidirectional coupling analysis method,the damage evolution leads to the increase of diffusion coefficient,which can promote the self-healing effect.The damage evolution rate calculated by bidirectional coupling analysis method is obviously lower than that calculated by unidirectional coupling analysis method.The bidirectional coupling analysis method is suitable for the damage analysis inside the material,while the unidirectional coupling analysis method is more suitable for the damage analysis near the material surface.