Self-healing concrete: a potential smart material to apply for underground construction

The crack of the concrete in underground constructions is generally difficult to detect and repair and consequently causes structural deterioration. Smart concrete with self-healing ability to autonomously repair micro cracks is a potential smart material to apply for underground constructions with sustainable development proposes. This paper reviews healing materials, the self-healing process, and the mixing method of self-healing concretes using bacteria, mineral admixtures, and fibers. The bacteria such as Bacillus sphaericus, Bacillus megaterium, Bacillus subtilis, Bacillus pasteurii, and Bacillus subtilis can produce a calcareous product for healing exterior cracks through microbial metabolic processes. As micro cracks are formed in the smart concrete, the bacteria spores contact nutrients and water and generate Calcium Carbonate (CaCO3). The addition of mineral admixtures based on silica including fly ash and granulated blast furnace slag (GGBS) heals the inner cracks through Calcium silicate hydrate (CSH) gels ( hydration reaction products. Other minerals such as expansive materials, geo-materials, crystals, and chemical additives change their forms or volumes to close cracks. Fibers including steel, carbon, PVA, PE, and carbon fibers are utilized to develop self-healing concretes based on controlling the crack width. The addition of fibers generates multiple micro-cracks, decreases the crack width, and enhances autogenous crack healing. Besides, healing agents can be easily dispersed into self-healing concretes by using the dry mixing, the wet mixing, or the latter mixing. Hence, all smart concretes with self-healing ability demonstrate potential and suitable characteristics for underground constructions. A combination method of fibers and bacteria or mineral admixtures can be applied for better sealing crack and durability enhancement of underground concrete structures.