# Bio-Influenced Self-Healing Mechanism in Concrete and Its Testing: A Review
The bacteria can be incorporated into the cementitious composites during the mixing process in order to control the crack through bacterial precipitation [14]. In comparison with other conventional healing materials, such as silicone-based polymer and acrylic resin, bacteria are more preferable, since the substances produced, such as calcium carbonate, have good thermal compatibility and outstanding mechanical property with cementitious composites [15; 16]. Bacterial precipitation can be carried out by applying the bacteria into cementitious matrix using various methods. The three widely used methods are the following: (1) Direct Application; (2) Immobilization; (3) Encapsulation. The first method is the direct application. For this method, the bacteria can be directly applied to the solution with nutrients or the direct incorporation of bacteria during the mixing process into cementitious matrix. [PERSON] et al. [17] used a solution with urea-CaCl\({}_{2}\) and Bacillus Pasteurii for the pre-cracked specimen. He found that the permeability of all the specimens with cracks of decreasing width accelerated the precipitation of bacteria after treating the cementitious matrix with urea-CaCl\({}_{2}\). [PERSON] et al. [18] applied the alkaliophilic bacteria with nutrient solution to the cementitious matrix and found that precipitation can fill the cracks in concrete composites, and it would be more efficient to use multiple applications of bacterial treatment in order to obtain a substantial bacterial precipitation. [PERSON] et al. [19] mentioned that the bacterial precipitation could occupy the pore spaces concrete, which can reduce the total volume of the pores in the matrix, consequently improving the compressive strength. One of the drawbacks of direct application of the bacteria in a cementitious matrix is the harsh alkaline environment during the concrete mix. It also seems that unprotected bacteria in concrete for a long time period is not a feasible option. The bacteria may not be able to fill and repair the cracks completely if there is no protective cover for the bacteria inside the concrete matrix [20]. In order to ensure the effectiveness and long term survivability of the bacteria, an immobilization method was proposed to apply the bacteria with a protective material in cementitious composites. The efficiency of the self-healing process of immobilized bacteria was more prominent than those with the direct incorporation [21]. [PERSON] et al. [22] noticed that the bacteria immobilized in graphite nano-platelets were more substantial in healing the specimens at 3 and 7 days, while bacteria immobilized in light weight aggregates yielded a better result at 14 and 28 days. The deterioration of the bacterial cells might happen during the process of mixing, hardening and setting in the cementitious matrix [23]. In order to ensure the effectiveness of the self-healing process in bacterial concrete, the process of encapsulation is more preferable than the other previous methods. To protect the bacteria from the harsh environment, suitable carriers can be used. Typically, the function of encapsulation is to protect the bacteria, and ensure that the suitable carriers have no negative effect on the precipitation of bacteria or on the other components of cementitious composites [23]. The healing agent is released in the encapsulation once microcracks occur in the concrete matrix [24]. Autogenous self-healing is one approach that can be used to mitigate the extent of the macrorack damage. It is imperative to conduct a comprehensive review on recent studies on the behavior of the bacteria and its effect on the macro, micro and nanostructure of the concrete matrix. This research undertaking can fill the potential research gap, emerging research topics and existing problems or challenges about the bio-influenced self-healing mechanism in concrete.