Study On The Evolution Mechanism Of Concrete Self-healing Performance Under The Condition Stimulated By Chemical Environment

As a kind of quasi-brittle material, concrete used in certain service environment was inevitable to produce cracks, which leads to catastrophic consequence. But if these microcracks could be timely repaired and healed, the serious consequence would be avoided. The crack of concrete material under certain conditions had the self-healing and self-repairment ability, which was of great significance to repair and eliminate the defects in concrete structure, improve the safety of the structure and prevent the further development of concrete cracks, and can also greatly reduce the maintenance cost of damaged concrete engineering.With the support of the national natural science fund project named self-healing mechanism and environmental stimulating factors of concrete(project number: 51372163), according to the difference of concrete material environment, research of concrete self-healing mechanism in chemical environment was carried out. First of all, hydraulic press preloading method of precasting concrete cracks was adopted in this study, then the influence discipline of different concentrations of sodium sulfate solution、sodium chloride solution、sodium carbonate solution and different alkalinitys of sodium hydroxide solution on concrete self-healing capacity were studied with concrete strength restoration ratio、changes of ultrasonic wave and crack width as the self-healing evaluation indicators, combining with crack width measuring instrument、ultrasonic testing、X-Ray Diffraction(XRD)、scanning electron microscope(SEM) and other testing methods, the evolution mechanism of concrete crack self-healing process under the conditions of different chemical environment were discussed, which provided a technical way to clear the best environment conditions so as to promote concrete self-healing ability. The main results were as follows:(1) Precracked concretes showed different degrees of self-healing ability under different concentrations of sodium sulfate solution, sodium chloride solution, sodium carbonate solution and different alkalinitys of sodium hydroxide solution. Within 60 days maintenance under different environmental incentive condition, along with the increase of solution concentration or alkalinity, self-healing effect of concrete was more and more obvious, and the age that concrete strength fully recovery needed was shorter and shorter.(2) Self-healing effect of concretes in sodium sulfate solution was the most prominent. The strength of precracked concretes in 6% sodium sulfate solution has fully recovered after about 15 days. It showed the best concrete self-healing performance in the curing age of 28 days, and the strength restoration ratio reduced from the maximum 137.8% to 75.8% at the curing age of 60 days. The crack repair and healing mechanism was that the sulfuric acid root ion and hydration products CH rapidly responsed and generated a large amount of hydration products such as ettringite in short time, which filled the concrete cracks and made it healed. But because the solution concentration was higher, along with the prolongation of curing age, hydration products such as ettringite, gypsum etc. in the crack gathered and inflated so as to produce crystallization stress, when the inflation was significant, it would cause the inhomogeneous expansion of concrete volume, leading to concrete cracking again after fracture healing.(3) Self-healing effect of concretes in sodium chloride solution was good, the strength of precracked concretes in 6% sodium chloride solution has fully recovered after about 35 days, and the strength restoration ratio reached the maximum 108.3% at the curing age of 60 days. The crack repair and healing mechanism was that under the incentive condition of sodium chloride solution, sedimentary Freide’s salt, hydration products of cement such as ettringite and continuous distribution of C-S-H gel on the fracture section, these hydration products made the crack healed gradully through filling action and cementation action, which effectively improved the structure of the concrete cracks.(4) Self-healing effect of concretes in sodium carbonate solution was good, the strength of precracked concretes in 6% sodium carbonate solution has fully recovered after about 49 days, and the strength restoration ratio reached the maximum 106.1% at the curing age of 60 days. The crack repair and healing mechanism was that under the incentive condition of sodium carbonate solution, carbonate ions reacted with calcium hydroxide and generated calcium carbonate deposited on the fracture section, filled the crack and made it healed step by step.(5) Self-healing effect of concretes in sodium hydroxide solution was relatively poor. The strength restoration ratio of concretes curing in the p H value of 13 sodium hydroxide solution increased the most obviously. But the strength restoration ratio has not yet reached 100% at the curing age of 60 days, only 97.6%. There were a large number of hydration products of cement such as ettringite and continuous distribution of C-S-H gel on the fracture section, which filled in the crack and made it healed gradully, and it derived from the further hydration of unhydrated particles around the crack.