**4.4 Bacteria-based self-healing in concrete (bioconcrete)**

Alkali-resistant endospore-forming bacteria that precipitate calcite through biological metabolism are used for self-healing in concrete. Examples of these bacteria are *B. cohnii*, *B. pseudofirmus* and *B. sphaericus*. The process involved in calcite production is termed as microbiologically induced calcite precipitation (MICP) [32]. There are two conventional MICP processes: firstly, the urease system, which

### *Self-Healing Concrete and Cementitious Materials DOI: http://dx.doi.org/10.5772/intechopen.92349*

is initiated by the hydrolysis of urea by the bacteria, secreted enzyme urease (urea aminohydrolase) as a catalyst [33] and secondly, calcium lactate-based MICP [65].

In the urea-based MICP process, hydrolysis of urea with urease results in ammonia and carbonate ions, which increase the pH value into the bacteria cell. Researchers have experimented with urea as a mineral precursor for biocementation using bacteria [33, 66]. In the presence of CaCl2 as a source of Ca2+, high pH content bacteria cause CaCO3 crystal precipitation from the solution. Typically, bacteria shell made with various ions are negatively charged to attract positive cautions Ca2+ ions surrounding the cell wall, which reacts with CO3 <sup>2</sup> and precipitate CaCO3 around the cell [66].

Calcium lactate (CaC6H10O6) is a crystalline salt, typically produced from the reaction of lactic acid with calcium carbonate or hydroxide. This was used as an alternative of urea-CaCl2, as a precursor for bacterial metabolism in concrete to avoid ammonia production in hydrolysis reactions. According to [65], metabolic absorption and breakdown of calcium lactate with bacteria lead to the precipitation of CaCO3.

Bacteria cannot survive long if they are mixed directly with fresh cement. The survivability of bacterial spores was optimized in [65], through the technique of packing bacterial spores and organic mineral precursor compounds in porous expanded clay particles before mixing in the concrete matrix. The pellets (2–4 mm) were principally made with the three components of a solid mixture, and they were used as a replacement of some of the similar size coarse aggregate. A high concentration of calcite precipitation has been found in concrete specimens with bacteria incorporated expended clay particles, which efficiently acted in crack-plugging and reduced permeability (**Figure 14**). About to micron sized (0.15 mm width), cracks were sealed. However, the main drawback in the bacterial pellet process is the

#### **Figure 14.**

*Microscopic images of bacteria based self-healing concrete, (a) Stereomicroscopic image of crack sealing, (b) Stereomicroscopic close-up image of massive columnar precipitate (c–e) ESEM images of top part of massive columnar precipitate indicated in image by dotted square (Reproduced from [65]).*

presented in **Figure 13**. The fundamental concept is that the surface of the coating may hydrate during initial production and mixing while the core healing mineral agent remains unhydrated; this may then dissolute and diffuse into the crack surface after crack propagation and form new products for self-healing.

*Concept of self-healing concrete with granules containing expansive mineral agents (Reproduced from [64]).*

*(a) Impregnation of LWAs to prepare pellet and self-healing concept: I-V (Reproduced from [49]), and (b) spraying PVA coating solution on the MgO pellets in the disc pelletizer and a microscopic image of a pellet*

Alkali-resistant endospore-forming bacteria that precipitate calcite through biological metabolism are used for self-healing in concrete. Examples of these bacteria are *B. cohnii*, *B. pseudofirmus* and *B. sphaericus*. The process involved in calcite production is termed as microbiologically induced calcite precipitation (MICP) [32]. There are two conventional MICP processes: firstly, the urease system, which

**4.4 Bacteria-based self-healing in concrete (bioconcrete)**

**Figure 12.**

**Figure 13.**

**206**

*covered by PVA (Adopted from [29]).*

*Advanced Functional Materials*

negative impact on the mechanical performance of concrete. About 50% of the total aggregate volume requires replacing with bacterial pellets for satisfactory self-healing performance, which negatively impacts the mechanical strength of concrete.

An encapsulation of bacterial spores inside microcapsules is a recent advancement in this field [26]. These microcapsules were reported flexible in humid/water conditions and becoming brittle in the dry environment. With their bacterial encapsulation systems, about 970-μm width cracks were healed successfully, which was four times greater than for non-bacterial mixes. Nevertheless, bacterial activity reduces dramatically with the increase in the pH (>12) value in concrete.
