# Bio-Influenced Self-Healing Mechanism in Concrete and Its Testing: A Review
## 3 Results and Discussion
### Type of Test: Destructive and Non-Destructive  
Destructive testing can be defined as the process in which the specimen is subjected to a physical destruction in order to evaluate its characteristics and behavior under different types of load. This includes different types of destructive testing methods, such as compression, tension tests, bend tests, Charpy impact tests, Pellini drop weight testing, peel tests, crush testing and pressure and fracture testing. It can be observed in Table 2 that a majority of the researchers conducted the destructive test, similar to the compressive test, in order to evaluate the additional strength improvement of concrete with bacteria. In this way, the percent increase in strength might justify the self-healing process of bacteria inside the concrete matrix. In order to quantify the reliability of the test results in the destructive method, microscopic tests can be done in order to support and validate the result of the self-healing efficiency. The data result that can be generated from the micro-test, such as Scanning electronic microscopy (SEM), can be in the form of an image which shows the formation of calcium carbonate that infiltrates through the cracks in concrete. Non-destructive testing is a non-invasive technique used by the industry to evaluate the properties of the material. This is typically used for quality testing in order to quantify the amount of damage present in the material. Typically, the local damage detection techniques for non-destructive methods include the following: ultrasonic; acoustic emission; eddy current; impact echo; x-ray method; magnetic particle; dye penetrant; rebound hammer. Acoustic emission has been used over the last decade since it is a very convenient and reliable method as a structural health monitoring tool, compared to other non-destructive methods. One of the advantages of this method is that it can detect early stage cracks based on the release of elastic waves that propagate inside the concrete material. The first scientific application of acoustic emission emerged in the year 1950 when [PERSON] conducted a series of tensile tests in metallic materials in order to find the behavior of acoustic emission with respect to the load [36]. The observation he found was named the Kaiser Effect after him, which describes that the material, under load, emits acoustic waves only after a primary load level is exceeded. After [PERSON]'s experiment, the first extensive research was conducted by [PERSON] in 1954 [37]. He carried out experiments in material engineering fields using the process of acoustic emission. He was able to conclude that acoustic emission is not a surface effect but rather a volume effect. [PERSON] [38] and [PERSON] et al. [39] observed that acoustic emission is different from other nondestructive testing methods in two primary aspects. First, acoustic emission can discover the energy released inside the test material and, secondly, this method is capable of finding the dynamic procedures associated with the deterioration of the structures. [PERSON] [40] mentioned that acoustic emission can be used in detecting the incremental damage of the specimen during the entire loading test without disturbing the property of the material. Recently, [PERSON] et al. [41] in 2018 conducted an extensive experiment on damage detection in reinforced concrete using acoustic emission with a convex hull algorithm. For this method, they used a convex hull volume algorithm to evaluate the behavior of sound propagating inside the concrete matrix. They found that the convex hull volume can be used to detect the damage until 60% of the ultimate load of the material. Multiscale modeling can also be applied in acoustic emission test. It was proven in the study made by [PERSON] et al. [42] that acoustic emission test can be used to detect the damage progression in a concrete specimen using computational geometry in a multiscale level.