## 1 Introduction  
Concrete, as an engineering material, has been widely used worldwide. In the application, one of the prominent problems is the high probability of cracking in concrete due to its low tensile strength. These cracks can be caused by both non-load factors, such as shrinkage, thermal effects, and chemical reactions, as well as external loads from events like earthquakes and explosions [1, 2]. Cracking increases the likelihood of corrosive substances penetrating the concrete, thereby compromising its durability [3, 4, 5]. Concrete cracks must be repaired to meet the use strength and service life. Among them, manual repair is the most common solution, but the economic cost is very high [6, 7]. The Federal Highway Administration of the United States reported that annual investments in maintaining concrete highway bridges amount to nearly USD 4 billion [8]. In Europe, nearly 50% of the annual construction budget is used to repair existing concrete facilities [9]. Inaddition, although the quality of manual repair can be guaranteed, durability is difficult to achieve, and manual repair methods must be carried out only after cracks are detected manually. Passive methods can no longer intervene in the generation and expansion of cracks at an early stage. This is also one of the problems that need to be solved in concrete crack repair. Early repair methods for cracks mainly include grouting, surface treatment, filling, etc. These physical methods cannot effectively repair internal damage to concrete. The use of chemical reagents will inevitably have adverse effects on the environment. With the self-healing materials proposed later, the repair methods for cracks change from passive to active, such as microencapsulation technology and adding a crystalline admixture. The effect of microcapsule distribution on the self-healing properties of concrete is important. The self-healing width of cracks in concrete with the crystalline admixture is very small. Now, biotechnology is developing rapidly, and microbial self-healing technology for concrete has quickly become the focus of research and engineering applications. Microbial self-healing technology for concrete introduces microorganisms into the concrete. It relies on the metabolic reactions of microorganisms to generate mineral compounds (calcite with calcium carbonate as the main component) to repair the surface and internal microcracks of concrete [9]. The metabolic process of microorganisms not only does not cause any adverse effects on the environment but also repairs cracks in concrete actively, and the self-healing width of cracks is large. To some extent, self-healing can even enhance the physical and mechanical properties of concrete [10; 11]. In 2007, [PERSON] [12] formally proposed the concept of using microbial technology to repair concrete and verified through experiments that, compared with other concrete self-healing technologies, microbial self-healing technology has certain advantages. The study of the mechanisms and effects of self-healing in concrete using various microorganisms reveals that factors such as microorganism concentration, type, and state, as well as the type and concentration of calcium sources and the type and amount of carriers, significantly influence the healing efficacy of concrete [13; 14]. [PERSON] et al. [15] mixed different concentrations of microorganisms into cement mortar, and discussed the influence of microbial concentration on the self-healing effect of cement mortar by using self-healing percentage and permeability. The results showed that the fracture healing effect increased with the increase in microbial concentration. [PERSON] et al. [16] discussed the influence of microbial concentration on the mechanical properties of cement mortar.