Bio-Engineered Concrete: A Critical Review on The Next Generation of Durable Concrete

Concrete is a prerequisite material for infrastructural development, which is required to be sufficiently strong and durable. It consists of fine, coarse, and aggregate particles bonded with a fluid cement that hardens over time. However, micro cracks development in concrete is a significant threat to its durability. To overcome this issue, several treatments and maintenance methods are adopted after construction, to ensure the durability of the structure. These include the use of bio-engineered concrete, which involved the biochemical reaction of non-reacted limestone and a calcium-based nutrient with the help of bacteria. These bio-cultures (bacteria) act as spores, which have the ability to survive up to 200 years, as they are also found to start the mineralization process and the filling of cracks or pores when in contact with moisture. Previous research proved that bio-engineered concrete is a self-healing technology, which developed the mechanical strength properties of the composite materials. The mechanism and healing process of the concrete is also natural and eco-friendly. Therefore, this study aims to critically analyze bio-engineered concrete and its future potentials in the Structural Engineering field, through the use of literature review. The data analysis was conducted in order to provide gradual and informative ideas on the historical background, present situation, and main mechanism process of the materials. According to the literature review, bioengineered concrete has a promising outcome in the case of strength increment and crack healing. However, the only disadvantage was its less application in the practical fields. The results concluded that bio-engineered concrete is a new method for ensuring sustainable infrastructural development. And also, it indicated that more practical outcome-based analysis with extensive application in various aspects should be conducted, in order to assess the overall durability.


INTRODUCTION
Concrete's weakness against tension force is always an issue of great concern as it generates cracks, which reduces the performance and durability of composite materials. Although, cracks generation is not a new topic, the remedies have always been significant issues of concern. Furthermore, they act as pathways for moisture, causing the corrosion of the embedded rebar inside the concrete. The generation of both micro and macro cracks also severely damage the properties of the material, which ultimately causes serious long-term problems (Zulfikar et al., 2021). This extremely leads to a decrease in the strength and durability of the concrete, as well as the decay of the structure (Thakur and Singh, 2017). After the construction, remedies to fill up the cracks are often carried out, using a costly and non-eco-friendly method.
Based on the construction industry, a durable and cost-efficient structure is always preferable, as various attempts are often conducted to create high-performance concretes. In this regard, the use of natural resources, e.g., Bio-engineered concrete, is becoming popular, due to having the ability to help eradicate the threat to the environment.
Furthermore, Sustainable Development Goals (SDGs) are being globally implemented, as the use of bio-resources is becoming a preferable topic for experts, based on the development of less energy-consuming bioconcrete. These materials mainly focus on the pre-construction measures, in order to make durable concretes. Therefore, the process of using bio-culture (bacteria) to heal cracks and improve mechanical properties is widely known as Bio-engineered concrete. This technique incorporates calcite precipitating bacteria within the material in certain concentrations. These bio-cultures precipitate calcium carbonate when in contact with water, and eventually solidifies the cracks (Wiktor and Jonkers, 2015). This biomineralization process is known as Microbiologically Induced Calcite Precipitation (MICP), which involves intracellular hydrolyzation of ammonia and Carbon dioxide. The ammonia present in this process causes an increase in the pH of the surroundings, which eventually leads to the mixture of the Calcium and carbonate cations that are deposited along the cell surface (Reinhardt and Joss, 2003).
Based on performance evaluation, several studies have been globally conducted on bioengineered concrete, as materials having different bio-culture provided good results than the conventional types (Raina et al., 2018). Furthermore, the properties of concrete were observed to effectively improve when bio-culture is used, based on the eco-friendly features of the material (De Belie, 2016). From Figure 1, the use of bacteria in concrete enriches the microstructural properties, which initiates a new and developed genus of durable material. This study aims to critically analyze the bioengineered concrete, as well as determine the key aspects and applications of its future potentials, through the use of the literature review method.

METHODOLOGY
This study was conducted by analyzing various published articles, based on the method adopted in using bio-cultures (bacteria) within concrete. The performance of this study was also based on significant results, obtained through key information analyses of the publications. Approximately 39 laboratory-based research articles were reviewed, such as scientific journals, dissertations, and conference proceedings on bio-agent utilization in concrete. Three. potential publications focusing on practical results related to the application of bioengineered materials in structural elements were also analyzed in section 8.2. These research articles were selected based on the titles, keywords and abstracts. In addition, Figure 2 represents the process flow being utilized in conducting this study. . dense microstructure of bio-engineered concrete (Priyom et al., 2020).

HISTORICAL BACKGROUND AND CURRENT PROSPECTS OF BIO-ENGINEERED CONCRETE
Although the term bio-engineered concrete was new in the field of Structural Engineering, the concept of using bio-cultures (bacteria) was very familiar in increasing microstructural properties.
In 1877, Ferdinand Cohn, a German biologist, claimed that the use of bacteria in the field of construction was an effective solution to the development of self-healing material genera. However, the study did not occur during this period, due to a lack of conception.
The use of bio-cultures to efficiently use the biomineralization process was first proposed by Me´tayer-Levrel et al. (1999). This study experimented on the use of bacterial carbonatogenesis, for the protection of architectural monuments. It also stated that the bio-mineralization process was a large field for future research, due to being a useful option to restore broken architectural monuments. , were prominent experts whose works in bio-agent (bacteria) concretes had generated a new possibility within the construction industry. Major studies based on this material had also been carried out in the last decade (2010)(2011)(2012)(2013)(2014)(2015)(2016)(2017)(2018)(2019)(2020). In addition, the first bioengineered concrete sample developed by Henk Jonkers through the encapsulation method, is shown in Figure 3.
Several laboratory and practical experiments had also been carried out on bio-engineered concrete, as various studies indicated that efficient use of bio-cultures developed and enriched self-healing and mechanical properties, respectively. The flow dynamics of publication in this study within the last decade is shown in Figure 4.
Based on Figure 4, a major increment of interest in the study of the bio-engineered concrete occurred between 2014 to 2020, as practical application related to the research became a major focus for future experts.

SELECTION OF BIO-AGENT (BACTERIA) IN CONCRETE
The precipitation of CaCO3 and other inorganic minerals by bacteria was strongly dependent on environmental conditions, which increased due to employing specific metabolic pathways. The designation of the 'Limestone producing bacteria' was always due to a tangible combination of the metabolic pathways, activities, and physicochemical environmental conditions. Therefore, a specific bacterium was 'limestone producing' in one environment, and different in another. The selection of bacteria was also highly dependent on their survivability within the concrete ambience. In addition, the pH value influenced the limestone precipitation, due to the urease enzyme being only activated for the specific acidic and basic level of Urea hydrolysis. This showed that the bacterial spores activated from the dormant stage had pH levels of 10-11.5, although they differed from the alkaliphile types and exposure conditions (Abo-El-Enein et al., 2013). Carbonate yield also played a vital role during the selection of bioagents, as Table 1 represents the different metabolic pathways of calcium carbonate precipitation .

METHODS OF APPLICATION OF BIO-AGENT (BACTERIA) IN CONCRETE
Based on various studies, the following methods are found more feasible, (1) Direct application method: Bio-cultures containing spores and calcium lactate are added directly into the concrete mix in this method. When cracks occur on the structure, the spores germinate and feed on the calcium lactate, to carry out healing.
(2) Encapsulation method: In this method, treated clay pellets containing bacterial spores and calcium lactate are used in concretes. These pellets are degraded when cracks occur on the structures, as spores are observed to start germinating.  (3) Vascular network method: This is a new method for using the self-healing mechanism, where temporary glass tubes are generally embedded in the concrete during casting. After this, these tubes are removed from the structures and tunnels produced within the structure. In addition, bio-cultures containing spores are then injected through the tunnel, when cracks are formed.
(4) Bio-agent as curing medium: One of the new and most promising techniques to produce bio-concrete is the curation of structural specimens within a medium containing bacterial spores. However, these bio-cultures are not directly applied to the concrete mix.
(5) Spraying bio-culture during curing: When the concrete specimens are cured for a desired limit, the spraying of bacteria-induced liquid is conducted, in order to produce a self-healing nature.
(6) Surface treatment agent: Bio-cultures are used as surface treatment cultures than conventional approaches, to protect the concrete from the ingression of water and other deleterious substances.
(7) Microcapsules enriched encapsulation method: In this method, microcapsule spores are applied into the concrete mix. These capsules are found to participate in self-healing processes, after the generation of cracks.

(8) Bacterial spores enriched powder method:
In this method, bacterial spores with calcite ingredients are dry-sprayed within the tank by a peristaltic pump, with the dry powder directly added to the concrete mix

MICROBIAL INDUCED CALCITE PRECIPITATION (MICP)
The autogenous healing process is a remarkable concrete capacity to naturally repair cracks. This mainly depends on the presence and absence of moisture and tensile stress, respectively. However, it is limited to crack widths below 0.2 mm (Beltran and Jonkers, 2015). Therefore, the development of a self-healing system filling the cracks of higher widths is becoming a popular topic for present studies. Furthermore, various studies had shown that microbial-induced calcite precipitation (MICP) was a promising technique for developing self-healing genera (Joshi et al., 2017;Silva, 2015). This method mainly focused on the mechanism of calcium carbonate (CaCO3) precipitation of bio-cultures, when cracks occurred on the concrete surface. In addition, the bacteria-induced CaCO3 sealed the cracks in the presence of moisture .
Bio-mineralization in the form of MICP is a process where CaCO3 minerals are formed from a supersaturated solution, in the presence of micro-organisms. Figure 5 represents the basic mechanism of this process. Furthermore, bioagent (bacteria) cells had the abilities to emit carbonate ions (CO3 2-), which reacted with calcium (Ca)-rich solution and precipitated insoluble CaCO3. Various species of bacteria also produced different phases of calcium carbonate (CaCO3). Therefore, calcite is the primary and most thermodynamically stable polymorph of calcium carbonate (CaCO3), mostly formed in the MICP reactions (Anbu et al., 2016).

MECHANISM OF BIO-ENGINEERED CONCRETE
As previously mentioned, cement mortar has high alkaline and dry ambience, which creates an inhospitable atmosphere for specified lifeforms. However, alkaliphilic bacteria have the viable capabilities to survive in severe and harsh conditions. Different activities such as the CaCO3 precipitation, is known as a typical metabolic pathway, leading to the increment of carbonate ion concentration and related saturation.
Based on several studies, the introduction of MICP in concrete was carried out through the enzymatic hydrolysis of urea, which was converted into NH3 and CO2 by bacteria. Therefore, the pH level increased from neutral to alkaline condition, causing the formation of carbonate. Although the process provided much effective self-healing capability in the concrete, a minor disadvantage was still observed, i.e., non-eco-friendly formation of ammonia. Furthermore, the long-term stability became limited in the alkaline environment within the material, due to the influence of urea. This indicated that the technique accurately performed as an externally applied repair mechanism than a self-healing agent.
Based on the process of MICP, a higher concentration of calcium carbonate was achievable within a short time. Urease also influenced the formation of minerals by four factors, namely concentration of Ca 2+ , dissolved inorganic carbon ratio, pH and presence of nucleation sites, and the latter of great importance for continuous and stable calcite crystals formation. However, it was generally carried out by the bacteria lying on the cell surface in Bio-mineralization. This charged with negative groups, as the divalent cations were anchored at a neutral pH level, which created the ideal nucleation sites for necessary calcite deposition. Meanwhile, magnesium ions made the bond more frequent than the calcium ions, due to having strong ionic selectivity. The bound cations (metal ions) also reacted with anions (carbonates) to form insoluble calcium carbonate. Bacterial cells further affected the type of minerals to be created, due to being nucleation sites. The enzymatic urea hydrolysis procedures are shown in the following chemical reaction (Vijay et al., 2017).
In this urease-mediated process, the reaction of urea (CO(NH2)2) and water yielded Carbon dioxide (CO2), and ammonia (NH3). Based on the high pK value (acid dissociation constant-a quantitative measure of acidic strength in a solution) of the NH3/NH4 + system (about 9.2), the reaction produced a pH increase and concomitantly shifted in the carbonate equilibrium (CO2 to HCO3 -and CO3 -2 ). This caused the precipitation of CaCO3, when a sufficient amount of calcium ions (Ca 2+ ) was present.
Based on Figure 6, the generation of calcium carbonate (CaCO3) in the mortar sample surface (inner and outer) was observed, showed due to the addition of bio-cultures. The generation of this compound did not only act as a crack healer, it also developed the mechanical properties of concretes. The use of these carbonate-producing bio-cultures filled the pores of the cement-sand matrix (Stooks-Fischer et al., 1999), causing the development of concretes with low permeability and higher strength.

Review of laboratory-based research study focusing on strength properties, microstructure and self-healing capability
This aspect focused on the laboratory-based articles that mainly investigated the healing capacity and ability of bio-genus as a strength increaser. Table 2 shows the analysis of the selected studies.  a. The viability of the bacterial spores within the concrete Ceramsite particles played an important role. b. Bacterial concrete showed a 20% increment in compressive strength than the control specimens. c. Bacteria concrete had 30% less water absorption ratio than the control specimens. d. Based on the mechanism of the self-healing process, the maximum width filled with precipitated calcite was 0.3 mm. e. Nutrients were frequently and easily accessed to the cells, when the bacteria and nutrients were incorporated within the Ceramsite particles.

Bio-agent used Methodology Major outcomes References
Bacillus cereus a. Direct application b. 35% of cement replaced by fly ash was also added a. At 28 days, 9.93% compressive strength was increased for the bacterial concrete, compared to the conventional specimens. b. Water absorption capacity for bacterial concrete was 1.98%, which was less than half of the conventional type.

Selvan and
Dharani, Paenibacillus muscilaginosus a. Direct application of bacteria powder in concrete mortar b. Bacterial powder of 1.2 kg/m 3 was added into the concrete mix After healing, a. Bio-concrete: 0 to 135 µA (corrosion current) and -150 to -550 mV (corrosion potential), Normal concrete: 50 to 150 µA (corrosion current) and -250 to -600 mV (corrosion potential) b. Bacterial concrete by healing cracks decreased the process of reinforced corrosion. c. The bio-concrete chloride contents at 5 and 10 mm depth were 0.02% lower than the conventional type.
Ling and Qian, (2017) S. pasteurii and Bacillus subtilis a. Direct application b. Two different bacterial concrete specimens were made and cured in both plain water and urea-CaCl2 solution a. Bacterial concrete cured in urea-CaCl2 was found to prolong cement hydration. b. Specimens cured in urea-CaCl2 showed less mass increment than those in water. c. Increment of bulk density and reduction of voids were found in bacterial concrete. d. Compressive strength of bacterial concrete approximately increased by 20%, which was more than the conventional type, as reduction of chloride penetration was also observed. e. S. pasteurii concrete showed better strength performance.

Nosouhian et al., (2015)
Bacillus pseudofirmus a. Encapsulation method b) Coated perlite with nutrient was applied to the mix a. A reinforced concrete wall was made with bacterial concrete for further investigations after trial. b. Healing of cracks was initiated in bio-concrete. c. Encapsulation method did not modify the basic properties of concrete.
Paine et al.,   (maximum), which was greater than the lower level structure, due to increased intensity of calcite precipitation. b. Bacterial concentration of 30x10 5 cfu/ml was optimal, due to obtaining positive characteristics from the concrete samples. c. For bacterial concentration of 30x10 5 cfu/ml, concrete samples had more calcite precipitation rate, due to its maximum compressive & flexural strength. d. In the construction industry, B. megaterium was part of green building material.
Andalib et al., Bacillus subtilis a. Direct application b. Depending on the cell concentration, 8 different bacterial agents were added to the concrete mix a. Bacterial specimens showed better strength performance than conventional concrete. b. The concentrations of 6.39 x 10 8 cells/ml were optimal for strength increment. c. 60% bacterial water provided better result. d. Bacterial concrete strength increment for 40MPa was 2-10% higher than the 20MPa specimens. e. The concentration of 6.39 x 10 8 cells/ml was highly optimal for strength increment.
Islam et al.,  were 48-80% and 18-50%. e. Maximum crack width healed by bacterial specimens was 970µm, which was 4 times lower in non-bacterial concretes at 250 µm. f. Bacterial specimens showed that water permeability was 10 times lower than the control concretes. g. At 95% RH, no self-healing was observed for all specimens, which strongly indicated the importance of the moisture presence.
Wang et al., Bacillus sphaericus a. Direct application b. 10 and 20 ml bio-cultures were directly added to the concrete mix a. For 10 and 20 ml addition, the compressive strength of bacterial specimens were 30.84 and 31.11%, respectively, which were higher than the conventional concrete. b. Increment in tensile strength was approximately 1.80 and 5.82% for 10 and 20 ml culture addition. c. Higher amount of culture addition showed better performance. Gandhimathi et al., (2015) Sporosarcina pasteurii a. Direct application b. Normal and light weight coarse aggregates were added into the concrete mix.
a. The coarse aggregates were left to soak in a bacterial inoculum with precursors for 6 days, and applied to the concrete mix. b. An average of 10% reduction in water absorption was observed for bacterial concrete. c. Bacterial concrete showed an average of 20% increment in compressive strength. d. The RCPT test showed a 20% reduction in chloride penetration than the controlled concrete. e. The SEM analysis showed denser and lower porosity of LWCA bacterial specimens.
Balam et al., Bacillus pumilus a. Direct application b. As a curing agent having 1.5X10 8 , b. 12X10 8 and 24X10 8 cells/ml a. In the case of direct application, 1.5X10 8 cells/ml showed better performance in compressive strength test. Also, a 6.3% increment in compressive strength was observed than the conventional specimen at 28 days. b. For bacterial specimens 24X10 8 cells/ml concentration showed better performance in compressive strength test.
Oriola et al., a. The bacterial strain as an isolated and super absorbent polymer, was employed for immobilization, and was also 30% volume for replacements of fine aggregates in coating mortars. b. At 28 days, compressive strength of the bacterial and conventional concretes were 40.7 and 38.9 MPa, respectively. c. Approximately 38% increment in compressive strength was observed in the bacterial concrete. d. The compressive strength coefficient after exposure of the samples in a 5% H2SO4 solution, was 1.02 and 0.97 for bacterial and conventional concretes, respectively. e. Due to H2SO4 solution reaction, gypsum production was 17% lower for bacterial concrete than the conventional specimens.
Yoon et al., Bacillus pseudofirmus Encapsulation process (1.4X10 9 , 3.2X10 9 , 5.5X10 9 , 8.6X10 9 and 13X10 9 cells/ml) a. Crack closure was observed for specimens containing 5.5X10 9 and 8.6X10 9 cells/ml, after 165 days. b. Sufficient healing compounds were not enough, as minimal bacterial spores were also required.   a. Cracks were formed in concrete specimen slabs with 10 cm di and 2.5 cm thickness, by the controlled application of compressive and tensile stresses. A crack width of 0.15 mm was also generated. b. Cracks with 0.15 mm width and 8 cm length were totally sealed. c. All specimens containing bacterial spores and controlled concretes showed 100 and 33% healing of cracks.

Jonkers (2011)
Bacillus sphaericus a. Encapsulation method b. Melaminebased capsule was added to the concrete mix a. Bacterial spores were able to grow and germinate in high pH range. b. Optimal pH range for growth was 7-9. Although the growth rate decreased at pH of 10-11, it did not stop. c. Crack width of 0.97 mm was healed by bacterial spores. d. Crack healing ratio for bio samples was 48 to 80%. Wang et al., (2017) Shewanella and E. coli a. Direct application b. Seven different cell concentration ranging from 10 to 10 7 per ml were directly mixed with water.
a. Cell concentration was determined by developing OD620 vs. bacterial cell numbers, standard curve. b. Mortar specimens containing Shewanella spores of 10 5 cells/ml showed 25% increment in compressive strength, than the conventional concrete. c. For E. coli, the increment in the compressive strength were less than 1%, compared to the conventional concrete.
Ghosh et al., a. Samples treated by calcium acetate showed better result in compressive strength increment, which was 2.45, 2.58, and 1.32 times higher than other samples. b. Samples of bacterial mortar with calcium acetate showed better tensile strength, which was 2.4 and 3.0 times than chloride and nitrate samples respectively. c. SEM and XRD analysis showed that the use of Ca(CH3COO)2 as calcium source for MICP, improved the mechanical properties and durability of the microbial mortar.
Zhang et al., Bacillus sphaericus a. Encapsulation method b. Bacterial spores were encapsulated into hydrogels, and incorporated into the specimens a. Prism specimens were subjected to multiple cracking by tensile load, with an average crack width of 150 mm. b. Maximum healing efficiency was observed in the specimens with bio-hydrogels, as 0.5 mm crack width was successfully healed. Approximately 40-90% healing ratio was also observed under wet-dry cycle. c. For non-bio hydrogel specimens, the healing width was 0-0.3 mm. d. Based on bacterial and control specimens, water permeability decreased by 68 and 15-55%% in average.
Wang et al., Bacillus a. Direct application b. Bacteria-based healing agent was directly incorporated into lightweight aggregates, and mixed with fresh mortar.
a. The liquid-tightness of mortar matrix with and without bacterial spores were evaluated through water permeability test, in both water immersion and wet-dry cycles. b. Water tightness of samples with and without bacterial spores were not different when immersed in water. c. During wet-dry cycles, samples with bacterial spores showed better performance than the control concrete. d. For bacterial samples, 96% of water tightness was achieved at day 56. Mucilaginous, were able to reduce water permeability coefficient from 7.9-8.3 x 10 -5 m/s to 0.8x10 -7 m/s, after 49 days of healing period.
Chen et al., Bacillus subtilis a. Direct application b. Bacterial spores were added to the mix, through the LWA and Graphite platelets a. Bacteria immobilized in graphite nano platelets provided better result in pre-cracked specimens, at 3 and 7 days old. b. Specimens immobilized in LWA were more effective in samples pre-cracked at 14 and 28 days old. c. Higher crack width was healed by bacterial samples with LWA, at 0.53 mm. d. Approximately 12% increment in compressive strength was observed for LWA immobilized specimens. Khaliq et al., (2016)

Application of bio-engineered mortar and concrete in structural elements
Although most of these studies were laboratorybased, a few conducted in the last five years were still related to the application of bio-cultures as healers and strength increasers in practical structural elements. Mors and Jonkers (2019), conducted several practical implementations on bio-engineered mortar and concrete. This was the largest practical study that mainly focused on the ability of bio-cultures, towards crack healing within a realistic environment. It also included two repair mortar and concrete construction demonstration projects, where a representative from the Bacillus genus was used as a healing agent.

Self-healing mortar:
This was applied into a damaged reinforced concrete column (Figure 7), and cracked with active leaking garage basement walls, which were located at 20m below ground ( Figure 8). To evaluate the performance of the self-healing mortar, visual determination of water tightness and hammer-knocking test was conducted with desired intervals. In addition, biennially monitoring for two subsequent years showed that the repaired patches were watertight, with the observation of sound bonding in both conditions.

Self-healing concrete:
The research team carried out two full-scale demonstrator projects, by using self-healing concrete. The first project was conducted on the construction of a wastewater purification tank of 7 X 2.5 X 0.15 m (Figure 9), while the second was carried out on a rectangular water reservoir of 47 m long X 5 m high dimension, where the south and east-facing walls were fully constructed by using selfhealing concrete ( Figure 10).
Based on the wastewater purification tank, a 10 kg healing agent per m 3 concrete mix was applied. The result showed that the tank completed three years of successful operation till September 2019, with no cracks or degradation on the surface. For the water reservoir, a 5 kg/m 3 self-healing agent was added to the concrete mix. Although the south-facing wall was more critical for cracking, the implementation of this method showed that no breaches were observed. However, minor cracks were found in the northfacing wall, where self-healing agent was not applied.    Zhang and Qian (2020) conducted the engineering application of self-healing concrete on ship lock walls, for practical outcomes. This study used Bacillus mucilaginous and calcium nitrate powder for the purpose of the process, as a dry-spray method was utilized for making microbial particle. Also, both laboratory-based and practical analysis were also conducted, respectively. In the laboratory-based analysis, there was no significant difference between the workability levels of the control and microbial structures, as the compressive strength of selfhealing concrete was slightly lower than the conventional type at the age of 28 days. During this period, 0.543 mm artificial crack was entirely healed by the self-healing material (Figure 11), as 2θ = 29.49° and 29.41° also indicated the production of minerals. Figure 11. Sealing of 0.543 mm crack of laboratory specimen after 28 days of curing (Zhang and Qian, 2020) Figure 12 shows the whole process of generating and applying self-healing concrete on the ship lock chamber. Based on temperature stress and shrinkage, cracks were generated on both the normal and self-healing concrete gates of the side wall.
At the age of 65 days, the cracks on the surface of the normal concrete were not healed. However, the generated breaches on the self-healing material were fully healed by calcite precipitation (Figure 13). In addition, the connectivity of the cracks were completely blocked on the bacterial enriched wall, as the leakages of water were not observed. Mullem et al. (2020), conducted a large scale application of self-healing concrete by directly mixing MUC + with the material (Figure 14). This compound (MUC + ) was obtained from the combination of ureolytic culture with anaerobic granular bacteria.
Furthermore, the roof slab of a drainage pipe inspection pit ( Figure 15) and laboratory prism specimens were casted by the same bacterial concrete mix ( Figure 16). In the self-healing concrete mix, two different water-reducing plasticizers and one mass (%) of bio-agent were added. From the laboratory analysis, a significant amount of strength was increased at 93 days, through the addition of the self-healing agent, as increment was also observed to be 9.7 MPa.
Based on the laboratory specimens, cracks were created through the application of the tensile stress, as widths varying between 0.52-0.58 mm were perfectly sealed during the healing analysis. For bottom cracks with 245 µm width, 86.3% closure was observed, as the formation of stalactites was also found during the wet-dry cycle analysis (Figure 17).
The roof slab was also installed in the inspection room after five weeks of casting. After this, an onsite inspection was conducted after one year of casting, as no cracking sign was observed at the bottom of the slab. Approximately 30% of the slab top was covered by large condensation drops, which indicated that it was is in a favourable condition to heal the cracks ( Figure  18). Figure 15. Roof slab made by bio-engineered concrete (Mullem et al., 2020). Figure 16. Laboratory prism specimens made by bioengineered concrete (Mullem et al., 2020). Figure 17. Formation of stalactites on a crack location of a prism specimen subjected to W/D cycles (Mullem et al., 2020). Figure 18. Condensation on the bottom side of the roof slab showing favorable condition for self-healing (Mullem et al., 2020).

HEALING CAPABILITY OF BIO-ENGINEERED CONCRETE AND TYPES OF CRACK HEALED
Based on the literature review, the maximum crack width completely sealed by the bioengineered concrete is 0.97 mm. The healing of crack widths ranging from 0.3-0.6 mm was also observed in several pieces of the study, which had been reviewed in Table 2. In a practical situation, the healing capacity depended on the surrounding environment (Mullem et al., 2020). When this environment supply sufficient moisture and oxygen, the healing capability of bio-engineered concrete increases. This material is capable of healing cracks to a width of 1 cm in a practical situation (Zhang and Qian, 2020). In this case, the selection of bio-cultures played a vital role, as the use of calcite reagent also increased healing capability. Most of the studies focused on the amount of crack width, healed with complete crack closure by the bioengineered concrete. However, Jonkers (2011), tried to focus on the length of crack that was efficiently sealed by the material. The study also showed that approximately 8 cm crack length was completely healed by the bio-engineered concrete, as the healing capability of bio-agent generally varied from 0.5 to 0.8 mm in practical situations .
Concrete structures are also susceptible to various cracks, with most of them occuring before and after hardening. From the literature analysis, bio-engineered concrete mainly focused on the cracks generated after the hardening of the structural specimens. Several pre-hardening cracks, such as construction movement and plastic breaches, were also healed by bio-engineered concrete.
The crack types healed by this method were often a topic of great interest among experts, as the implementation and further study in this field were more straightforward when summarized. Based on the literature reviewed, a diagram showing the crack type healed by bio-engineered concrete is shown in Figure 19.
According to the reviewed literatures, more than 80% of the studies were carried out by adopting a representative from the genus-Bacillus. This was due to its tremendous ability to precipitate carbonate during harsh environment. In addition, the usage of various bio-genus is shown in Figure 20, through the representation of a bar chart.

APPLICATION OF BIO-ENGINEERED CONCRETE
Sustainable urban amenities are very important for the better economy of a country, as most of the industrial works are infrastructurally related. This is because a small improvement on the durability of infrastructure has the ability to provide a larger result in the economy. Therefore, the use of bio-engineered concrete is becoming more popular in Structural Engineering. The roles and functions of this method are as follow, • Sealing of cracks in concrete structure: Bio-engineered concrete are an excellent option for making durable structures, due to their bio-mineralization abilities.
• Mechanical strength increaser by conducting reaction with cement-sand matrix.
• Preparation of less permeable concrete: Bacteria within the concrete reduce void volumes, leading to more compaction and less permeability.
• Enhancement of resistance towards freeze-thaw action.
• Construction of low-cost durable housing.
• Effect on sea-shore structures: It protects the structure from corrosion and prevents the deformation of rebar.

EFFECT OF BIO-AGENT ON THE PROPERTIES OF CONCRETE
• Compressive strength: The use of biocultures help in filling the pores between cement-sand microstructures, in order to acquire more compressive strength and become denser. According to Table 2, approximately 40% increase in this strength was observed through the use of bacteria.
• Reduction of permeability: Calcium carbonate helps in filling and reducing the pores and permeability of concrete specimens, respectively. Also, the permeability of bio-concrete is less than half, compared to the conventional structure.
• Setting time: Based on the different bioagent solutions, the set time of the concrete mix is accelerated or delayed. In addition, calcium lactate delayed this time, while Ca(HCOO)2 and Ca(NO3)2 (Calcium Formate and Calcium Nitrate) accelerated it (Jonkers et al., 2010).
• Development of the microstructure: The SEM and XRD analysis indicated that the application of bio-agent caused calcite precipitation on the concrete microstructure, which eventually enriched the property of the structural specimens (Jagadeesha et al., 2013;Jonkers and Schlangen, 2007).
• Self-healing: The use of bio agent enriches the ability of concrete, in order to heal cracks. From the literature analysis, the utilization of bacteria healed approximately 1 cm concrete crack width.

RECCOMENDATION FOR FURTHER STUDY
• Any new invention or improvement of the existing methodology in the construction industry needs a lot of practical research. Bio-concrete as a recent phenomenon is the most promising solution for durable structures. However, the long-term effect of this method is not well recognized. Previously, several studies were carried out on this topic, with most of them being laboratory-based.
Only a few demonstration project was conducted to assess the suitability of this new method. Therefore, large scale practical implementation and the regular inspection to sort out any problem should be the next step.
• The selection of bio-agent is also a matter of great concern, due to the difference in the climatic and environmental situation suitable for the microorganisms. Therefore, separation and categorization on various exposure conditions should be an important issue in the future.
• Based on practical implementation, bioconcrete should also be cost effective, with the budget mainly depending on the selection of bio-cultures. Therefore, the cost of the project should be focused on various exposure conditions.

CONCLUSION
Bio-engineered concrete is becoming a popular topic for research purposes. In this study, several publications focusing on the performance of bioengineered concrete were reviewed. Based on the literature reviewed, it was found out that such type of concrete are useful for harsh environment where density and compactness of the concrete microstructure is needed. Bioengineered concrete significantly improved the lifetime of construction, by reducing the repair and replacement costs. Therefore, the additional investment in the method was accurately worth the risk. In addition, future research on bioengineered concrete should understand the costeffectiveness and long-term effects.