# Bio-Influenced Self-Healing Mechanism in Concrete and Its Testing: A Review  
[PERSON]. 1 Department of Civil Engineering, De La Salle University, 2401 Taft Avenue, Manila 1004, Philippines;  
[EMAIL_ADDRESS] (M.K.M.D. ); [EMAIL_ADDRESS] (J.M.C.O. )1  
[PERSON] [PERSON] and [PERSON]  
1 Department of Civil Engineering, De La Salle University, 2401 Taft Avenue, Manila 1004, Philippines;  
[EMAIL_ADDRESS] (M.K.M.D. ); [EMAIL_ADDRESS] (J.M.C.O. )1  
Footnote 27: email: {althe integrity of the material. Thus, the presence of cracks in the concrete material might compromise the strength and durability of the concrete structure. Traditionally, concrete material can be repaired through retrofitting in order to prolong the serviceability of a concrete structure. However, the process of retrofitting the concrete is very costly and time consuming. In addition, it is very difficult to repair all the cracks, considering the exact crack location, depth and extent of the damage inside the concrete material. Therefore, a self-healing concrete is needed to repair the cracks in a concrete structure. Previous researchers have shown that only macro-cracks of up to 0.3 mm can be healed completely through the carbonation of dissolved calcium hydroxide or the hydration of unused cement particles, which can be considered as an autogenous self-healing process [3]. The chemical reaction and mechanism of the carbonation of calcium hydroxide is described in Equation (1). \[\mathrm{Ca(OH)}_{2}+\mathrm{CO}_{2}\rightarrow\mathrm{CaCO}_{3}+\mathrm{H}_{2}\mathrm{O} \tag{1}\]  
Bio-concrete is a self-healing technique using bacteria that can fill the cracks in concrete by inducing a calcium carbonate precipitation through biomineralization. The basic mechanism of bacteria-based crack healing happens when the concrete crack occurs. The water will then leak via the formed cracks and the capillary pores of concrete [4]. With the presence of the water and nutrients inside the concrete matrix, the process of the germination of bacteria will take place and the limestone produced can fill the cracks. According to the intensive literature review by [PERSON] and [PERSON] (1956), they stated that the healing process was discovered first by the French Academy of Science in 1836, which concluded that the self-healing process is the conversion of calcium hydroxide from hydrated cement to calcium carbonate by exposing it to the atmosphere [5]. [PERSON] [6] was the first to examine the development of crack-healing via polymeric materials in 1973. In his experiment, crack healing was observed by considering the following parameters: temperature dependence; delamination; the relaxation time of the polyvinyl acetate (polymer). Since then, the process of healing the crack in cement-based products has been intensively used by the researchers to develop various healing agents [7]. Polymers can be seen as one of the promising materials that can be used in the self-healing of concrete. The process of healing in polymers depends on the force interaction between the crack surface and polymeric material, which can result in crack filling and restoration [8]. The substance that is produced and fills the cracks is not enough to fully recover its mechanical properties, due to the lack of adhesion between the crack wall and the polymer [9]. Alternately, Super Absorbent Polymers (SAPs) can be used to carry the water into the cementitious matrix, which might lead to an autogenous self-healing process [10]. In 1974, [PERSON] and [PERSON] [11] noticed the self-healing process and performance of hydraulic concrete. In 1984, [PERSON] [12] found that, under the condition of water curing, the autogenous healing degree of the interfacial zone between steel fibers and the cement mortar matrix was higher than that of fractured plain mortar or concrete. In 1995, silica fume was added to non-air entrained concrete by [PERSON] et al. [13] in order to produce a self-healing concrete.