# Bio-Influenced Self-Healing Mechanism in Concrete and Its Testing: A Review
## 3 Results and Discussion
### Type of Test: Destructive and Non-Destructive
[PERSON] [53] found that B. spahericus improved  
the compressive strength of concrete by 30.76% in 3 days, 46.15% in 7 days and 32.21% in 28 days, as compared to conventional concrete. [PERSON] [54] revealed that an increase in one optical density (OD) of the bacterial cells mixed with water produced a 33% strength improvement in the mortar compressive strength. [PERSON] [55] found that bio-concrete reached a strength improvement of 22% in compressive strength, 11% in flexural strength and 16% in split tensile strength relative to the conventional concrete. [PERSON] [56] found that the compressive strength and split tensile strength of the species Bacillus subtilis, Bacillus megaterium and consortia of both were observed to be higher than 15% and more, when compared with conventional concrete with M30 concrete in 28 days. [PERSON] [57] revealed that a maximum 32% increase in compressive strength, a 14% increase in split tensile strength and a 29% increase in flexural strength were observed in the specimen, having a bacterial concentration of 10\({}^{5}\) cells/mL of water. [PERSON] [58] found that the inclusion of S. pasteurii in fly ash concrete enhanced the compressive strength and reduced the porosity and permeability of fly ash concrete. From these studies, it can be concluded that the direct incorporation of bacteria has a significant effect on the strength of concrete. However, it can be noted that their results are valid only for a limited period of time. There must be a validation for the survivability and effectiveness of the bacteria over a long period of time. It can be observed that, under the direct incorporation method, almost all of the researchers carried out both destructive and non-destructive tests. The authors usually conducted a non-destructive test, such as the ultrasonic pulse velocity test, in order to determine the dense nature of concrete, and found that certain species of bacteria have a positive effect to the mechanical properties of concrete. It is also interesting to note that the frequency signal of ultrasonic alone does not have any detrimental effect on the viability of bacteria in concrete, since there is an improvement in the mechanical properties. Multiple protective materials can be used for the immobilization of bacteria in concrete. Limestone powder (LSP), as a protective material was studied by [PERSON] et al. [59], iron oxide nano-sized particles (IONNPs) were studied by [PERSON] et al. [60] and iron oxide nanoparticles were studied by [PERSON] et al. [61]. [PERSON] et al. [60] showed that Bacillus subtilis can improve the compressive strength of concrete and the iron oxide nano-sized particles (IONPs) were considered to be the most effective immobilizers for preserving the bacteria until the generation of cracks followed by the siliceous sand and limestone particles. In addition, the study made by [PERSON] [61] revealed that the addition of immobilized Bacillus species with iron oxide nanoparticles would increase the compressive strength of concrete. Meanwhile, some researchers have utilized the waste and recycled construction materials as a protective agent, and [PERSON] et al. [62] are among those who used a crushed brick aggregate. [PERSON] et al. [63] found that thermogravimetric analysis revealed that bacteria with silica gel were able to precipitate CaCO\({}_{3}\) crystals inside the cracks and these precipitated crystals filled the cracks. [PERSON] et al. [64] used an internal nutrient reservoir, which resulted in the growth of vegetative cells without incurring a significant loss in strength. [PERSON] et al. [22] revealed that bacteria immobilized in graphite nano platelets gave better results than the concrete specimens pre-cracked at 3 and 7 days, while bacteria immobilized in light-weight aggregates were more effective in samples pre-cracked at 14 and 28 days. [PERSON] et al.