Study On Chloride Diffusion And Transmission Mechanism Of Self-Healing Concrete Based On Recycled Aggregate As Microbial Carrier

Recycled aggregate,is one of the products of waste concrete treated by a construction waste resource production line.Its secondary use can effectively reduce the large amount of natural resources needed for the development of emerging urbanization,as well as solve the environmental pollution and damage caused by the demolition of old buildings.However,the loose and porous nature of the mortar attached to the surface of the recycled aggregate makes the recycled concrete prepared thereby have inferior performance to the concrete prepared from natural aggregate,especially the durability of the recycled concrete is greatly impaired.In this paper,the recycled aggregate is used as a microbial carrier,and self-healing concrete is composed of original aggregate,old interface transition zone between original aggregate and adhesion mortar,adhesion mortar,new interface transition zone between new mortar and adhesion mortar,new mortar and repair products,as a six-phase composite material.The effects of recycled aggregate carrier on repair efficiency and the equivalent diffusion coefficient calculation model are used to analyze the chloride ion diffusion transmission mechanism of self-healing concrete that has not fully healed after cracking,and the calculation model is verified by experiments.The main research work and conclusions of this article include:(1)Recycled aggregate,expanded perlite,and diatom mud were used as microbial carriers,which were factors affecting the microbial repair efficiency.A control group was added to design 24 groups of concrete for preparation,and then experimental research on the self-healing effectiveness of the sample cracks was carried out.The test results show that the repair efficiency of the sample with the carrier is significantly better than that of the control group without the carrier.Even if the Bacillus pasteurii adsorbed by each carrier is in a state of inefficient response for repair,it still has a more significant acceleration effect than inefficient repair of cracks in the sample without carrier under the same conditions.Recycled aggregate as a carrier has a faster repair rate and a larger total repair volume than the expanded perlite and diatom mud carriers.The recycled aggregate carrier sample has the fastest repair response rate after cracks appear,and has the ability to repair cracks with a width of 0.27 mm when cracked for 28 days.(2)Using recycled aggregate as a carrier,the effects of carrier particle size,bacterial solution concentration,carrier ratio,and crack width on self-healing concrete repairing efficiency were investigated.The orthogonal design method was used to carry out the repairing efficiency experimental research.The experimental results show that the crack width has the most significant effect on self-healing efficiency than other factors,and the repair rate shows a significant downward trend with the increase of crack width.When the crack width reaches 1.0mm and above,the repair effect of cement-based self-healing concrete is very limited.Through the optimized mix ratio test,the effect of repairing the concrete with a crack width of 0.2 mm is most significant,when the recycled aggregate has a particle size of 5 mm or less,a bacterial solution concentration of 40%,and a carrier volume ratio of 30%.(3)The relationship between chloride ion concentration,diffusion depth,and repair efficiency in the samples determined by dry-wet cycle diffusion erosion experiment of self-healing concrete chloride ion was analyzed.Based on Fick’s second law,a time-varying repair model of self-healing concrete with recycled aggregate carrier and a six-phase model of chloride ion diffusion transport under crack repair state of self-healing concrete are proposed.At the same time,the chloride ion dry-wet cycle erosion experiment was designed,and the test data results were compared with the two models.The applicability and rationality of the six-phase numerical model for chloride ion diffusion of self-healing concrete and the equivalent diffusion coefficient calculation model for self-healing concrete cracks were verified.The physical significance of the self-healing concrete repairing efficiency parameters with Bacillus pasteurii as the repairing microorganism and the recycled aggregate carrier was also clarified.(4)The microbial self-healing concrete was prepared with the optimal solution mix ratio of the obtained repairing efficiency using the recycled aggregate as the carrier,and the dry and wet cycle diffusion experiment of chloride ion was designed with the crack repairing degree as an influencing factor.The test results show that the calcium carbonate generated at the cracks after the repair can effectively hinder the erosion rate of concrete by chloride ions.With the increase of the number of wet and dry cycles,it can be found that the concentration of chloride ions sampled at a depth of 5 mm at the crack of self-healing concrete with recycled aggregate carrier first increases and then decreases,and the initial measured erosion concentration was significantly lower than other groups without the addition of recycled aggregate carrier.This shows that the calcium carbonate generated by the recycled aggregate as a carrier effectively inhibits chloride ion erosion,making the concrete with recycled aggregate more resistant to erosion than the natural aggregate concrete.At the same time,through XRD ray diffraction characterization,it was found that with the increase of the degree of crack repair,the corrosion of the steel bars in the concrete showed a significant weakening phenomenon.After the dry and wet cycle experiment of the crack completely repaired samples,the steel bar corrosion did not appear.