# Bio-Influenced Self-Healing Mechanism in Concrete and Its Testing: A Review
## 3 Results and Discussion
### Type of Test: Destructive and Non-Destructive
Thus, acoustic emission is a promising method among the non-destructive methods and can be used in order to monitor the propagation of cracks and the performance of bacteria in the concrete. Aside from acoustic emission, ultrasonic waves can also be used to detect the defects in concrete. [PERSON] et al. [43] used a feed-forward back-propagation artificial neural network (ANN) to determine the significant effect of various ultrasonic parameters on the mechanical load applied. This study can be used as a reference for selecting the most appropriate ultrasonic parameters in assessing the damages in concrete with varying water/cement ratios and reinforcement. For reinforced concrete, [PERSON] et al. [44] showed that the sensitivity of the noncontact ultrasonic test is higher for a concrete structure made by a higher water/cement ratio, where, for the contact ultrasonic test, a lower water/cement ratio concrete structure provides good sensitivity. The loading pattern greatly influenced the generation of higher harmonics in acoustic emission, as reported by [PERSON] et al. [45]. They noticed that the second harmonic generation demonstrates significant changes with incremental load in concrete cubes. [PERSON] [46] carried out an ultrasonic technique in order to monitor the progression of the healing of fine cracks in concrete. He found that the periodic monitoring of the transmitted pulse throughout the healing period revealed that signal amplitude can be correlated with the extent of healing. [PERSON] [47] believed that the most advanced non-destructive test methods for self-healing concrete are mercury intrusion porosimetry and X-ray tomography. Ultrasound can be defined as an acoustic energy or sound waves with frequencies above 20 kHz. For less dense material, such as concrete, an ultrasound frequency range of 50 to 500 kHz is typically used. The process of ultrasonication is commonly perceived as harmful to the growth of bacteria. However, the cells of the bacteria can actually grow in low intensity insonation, due to the ability of the ultrasound to transport the small molecules in solution and its inability to completely remove cells from surfaces [48]. The study made by [PERSON] et al. In 2001 found that ultrasonication with a frequency of 35 KHz and intensity of 0.66 W/cm\({}^{2}\) for 60 min alone did not cause any significant damage to the bacteria [49]. Thus, it can be concluded that ultrasonic frequency signal alone does not have any detrimental effect on the cells of bacteria inside the concrete. Table 2 shows the application methods of bacteria and the corresponding property tests for the twenty-eight journals considered in this study. For the first paper, [PERSON] et al. [50] placed the bacteria directly in the concrete mix with geopolymer and found that the crack width of \(89\pm 3\) um was sealed completely and the cementitious composites did not cause any significant damage to the bacteria cells. The presence of large pores that are interconnected in the geopolymer was the main reason for the efficient healing capacity and provided the necessary space for the development and reactivation of the bacteria in cementitious composites. Meanwhile, [PERSON] et al. [22] concluded that specimens incorporated directly with bacteria did not show any effects in the crack healing of concrete. [PERSON] et al. [51] observed that bacterial protein-impregnated mortar samples form nanotube-like structures of silica which fill the micropores of the mortar samples, which enhance the compressive strength and durability of the samples. [PERSON] and [PERSON] [52] assessed the self-healing efficiency of concrete using the nanostructure test. They found that, in the interfacial transition zone, the nano mechanical values were 20% higher than the precipitates in the outer cell wall, which represented a solid bond between the deposited layer and the concrete matrix.