Use of Bacteria Externally for Repairing Cracks and Improving Properties of Concrete Exposed to High Temperatures

Abstract

The current paper presents the results of an experimental study on the application of calcium carbonate precipitation bacteria as a new approach to repairing damaged concrete when exposed to high temperatures. To do so, cylindrical and cubic concrete specimens were initially exposed to heat in a furnace for 1 h, after reaching two different temperatures of 600 and 800 °C. A heat rate of 5.5 °C per minute was used to achieve the target temperatures. Then, two types of bacteria, namely Sporosarcina pasteurii and Bacillus sphaericus, with cell concentration of 10⁷ cells/mL, were utilized externally, to repair the thermal cracks, enhancing the mechanical properties and durability of the damaged concrete. The efficiency of the bacterial remediation technique was then evaluated through compressive strength, ultrasonic pulse velocity (UPV), and electrical conductivity tests on the control specimens (unexposed to heat), and those exposed to high temperature with or without bacterial healing. The experimental results demonstrate that the compressive strength of the test specimens exposed to temperatures of 600 and 800 °C decreased by about 31–44% compared with the control ones. However, compared to those damaged at 600 and 800 °C, the compressive strength of specimens repaired by the S. pasteurii and the B. sphaericus showed increases of 31–93%. This increase is associated with the precipitation of calcium carbonate in the deep and superficial cracks and pores of the damaged specimens. Furthermore, the ultrasonic pulse velocity of the specimens subjected to bacterial remediation had a significant increase of about 1.65–3.47 times compared with the damaged ones. In addition, the electrical conductivity of repaired specimens decreased by 22–36% compared with the damaged specimens.

1. Introduction

Mainly composed of a mixture of Portland cement, water, and aggregates, concrete is one of the most commonly used construction materials, whose consumption increases all over the world. The affordability and availability of its ingredients have made concrete one of the most crucial building materials. In contrast to concrete’s advantages, such as its high compressive strength, one can point to its disadvantages, such as its low tensile strength, which triggers cracking and ultimately failure of reinforced concrete’s constituent elements. The composite is reinforced with internal steel rebars to compensate for the disadvantages. Factors such as low tensile strength of concrete, concrete structure exposure to high thermal stresses, structural design errors, chemical attacks, and exposure of the structure to overloading can form cracks in reinforced concrete members [1,2,3,4]. Although the cracks may not pose a severe danger to the structure in the early days, with the development and increase of these cracks in size, in the long run, the possibility of penetration of chlorine ions, sulfate, and acid, etc., exists. As a result, corrosion of internal steel rebars is likely, leading to the durability and life of the structure being reduced [5,6,7,8,9]. Many costs are incurred annually for healing cracks in reinforced concrete structures. In Europe, for example, about 50% of construction costs are spent on healing and improving existing structures to increase their lifespan [10]. As this cost rises every year, it is essential to employ new methods to prevent and heal the cracks. The occurrence of fire, and consequently, exposure to heat, causes cracking in the reinforced concrete structural members. In addition to causing severe damage to reinforced concrete structures, in some cases, overheating may destroy a concrete structure [11]. Previous studies illustrate that fire can reduce the strength and shell-shaped explosive spalling in reinforced concrete structures [12,13,14].

The behavior of concrete and reinforced concrete structures exposed to high temperatures during fire is one of the subjects that structural researchers have investigated. Most studies are dedicated to the strength and durability of concrete. The mechanical properties of concrete materials are the most influential factors in the efficiency and durability of heat-exposed concrete structures that often do not behave similarly. The concrete properties are unpredictable and complicated. Reviewing the literature shows that the first study investigating concrete behavior against high temperatures was conducted in 1922 [13]. Exposure of concrete to fire or high heat causes physical and chemical changes in the structure of concrete. Overheating can change the mechanical properties of concrete structures and reduce their durability [13,14]. Cement paste and the aggregates used are the primary sources of changes in the mechanical properties of concrete. Exposure of cement mortar and concrete aggregates to high heat causes physical and chemical changes inside them and thermal incompatibility between aggregates and cement mortar, thus adversely affecting their mechanical properties (Figure 1) [13]. Furthermore, the heat increase rate is among the environmental factors that affect the loss of concrete properties. Therefore, the same behavior is not expected from different concretes exposed to heat, since the type of aggregate and cement and the amount and rate of heat flow is often not the same. Only if all these factors are the same, can one expect the same reactions. In a study conducted on changes in heat-exposed concrete’s mechanical properties, Khoury showed that overheating may form cracks in a concrete structure due to significant reduction in its tensile and compressive strength. Changes in hydrated cement cause the loss of strength in ordinary concrete due to physical and chemical changes in concrete’s microstructure; in addition, the strength of concrete at 100–300 °C increases by 20–30% [13].

However, most concrete structures face a decrease in their compressive strength at temperatures above 300 °C [13]. Moreover, the types of aggregate and cement used are influential in reducing their compressive strength. Calcium hydroxide (Ca(OH)₂), a significant hydration material component, decomposes at temperatures above 400 °C. Silicate gel, known as the main component of cement paste and the main factor for concrete strength, deteriorates at 550–600 °C, causing a significant increase in basic creep in Portland cement paste. Therefore, concrete loses its major strength in extreme heat and is not structurally reliable [15]. When concrete is exposed to a temperature of 500 °C or higher, small and large cracks appear on its surface. Moreover, these cracks expand continuously with the increase in the temperature, spreading over the whole concrete; their width also increases, and finally, it reduces the concrete strength and durability and even affects the rebars inside the concrete. In recent years, various methods for repairing concrete cracks have been investigated. The treatment methods can be divided into active and passive. Applying epoxy resins is one of the most common passive methods for healing cracks. Environmental impacts on the method pose several limitations, such as inapplicability in harsh weather conditions, moisture sensitivities, differences in thermal expansion coefficients, and low thermal resistance [7,16]. Due to the limitations and obstacles of the previous method, the use of alternative methods for healing cracks using mineral deposits caused by bacteria has been widely considered by researchers [17,18,19]. Proposed as a powerful and environmentally friendly method, bacterial calcification for filling concrete cracks and microcracks has been investigated in various studies [20,21,22]. In the sediment formation process, the first hydration of calcium oxide (CaO) causes the production of Ca(OH)₂. From the reaction of this substance with carbon dioxide (CO₂) in the air, calcium carbonate (CaCO₃) is produced. It is one of the best methods for filling cracks and joints due to its compatibility with cement hydration products (see chemical Equation (1)) [23].

$$\mathrm{CaO}+{\mathrm{H}}_2\mathrm{O}\to \mathrm{Ca}{\left(\mathrm{OH}\right)}_2$$

(1)

$$\mathrm{Ca}{\left(\mathrm{OH}\right)}_2+{\mathrm{CO}}_2\to {\mathrm{CaCO}}_3+{\mathrm{H}}_2\mathrm{O}$$

Extensive research has been conducted on self-healing concrete to date. Lu et al. used bacteria that produce CaCO₃ for concrete crack healing. Considering the crack width, crack age, and processing conditions of microorganisms, the results showed that the ability of bacteria to repair cracks depends on various factors, such as crack width, crack age, and microorganism processing methods. When the cracks are older than two months, the possibility of healing decreases significantly, since, after this time, the bacteria either die or become inactive. Moreover, the study found that repairing concrete cracks with a width of more than 0.8 mm with a microbial repair agent was difficult [20]. Muynck et al. studied the effect of temperature on the activity of microorganisms. The study compared bacteria’s ability to precipitate calcium carbonate (CaCO₃) at 10, 20, 28, and 37 °C for 7 and 21 days in a container containing agar. The study also showed that B. sphaericus bacteria had better efficiency and a higher activity rate than other microorganisms. The optimum temperature for the growth and activity of B. sphaericus and S. pasteurii was determined to be 37 °C [24].

Chahal et al. evaluated the effect of S. pasteurii with different concentrations (0, 10³, 10⁵, and 10⁷ cells/mL) on compressive strength, rapid chloride permeability, and water absorption of concrete without and with fly ash at 28 days of age. The study indicated that specimens containing S. pasteurii with a concentration of 10⁵ cells/mL had the highest compressive strength (about 22%, compared with the control specimens) and the lowest chloride permeability [6]. Bacterial activity significantly reduces in highly alkaline environments (pH greater than 12). Wang et al. compared polyurethane and silica gel’s efficiency as a carrier for protecting bacteria in mortars. The results showed that a polyurethane coating is a better option than silica gel as a self-healing agent for cracks created in specimens. The specimens containing polyurethane have less water absorption and higher self-healing ability than specimens containing silica gel, which had a higher strength regain [25].

Wang et al. encapsulated the bacteria into hydrogels, and then added them to the mortar to examine the healing efficiency using thermogravimetric analysis. The test results displayed that the maximum width of healed cracks in mortar specimens containing hydrogel-encapsulated spores is about 0.5 mm, and water permeability is reduced by about 68% [22]. Wang et al. experimented with encapsulate bacterial spores, determining the efficiency and viability of the spores through scanning electron microscopy (SEM) on mortar specimens during the breakage of the microcapsules. The results showed that the microcapsules opened due to cracks in the mortar, and the self-healing agents started to work. The results indicated that the healing ratio in the specimens with bio-microcapsules was higher (48–80%). The maximum crack width repaired in the bacteria series specimens was 970 μm, nearly 4 times that of the non-bacteria series. Moreover, the permeability of specimens containing bacteria is about ten times less than specimens without bacteria [26]. By adding two microbial organisms, S. pasteurii and B. subtilis, to the specimens at 28 and 91 days of age, Tayebani et al. examined their efficiency. The results showed that the water permeability and chloride ion penetrability of the specimens containing bacteria decreased. Moreover, the compressive strength and electrical resistivity increased compared to control specimens [27].

Parastegari et al. investigated the effect of S. pasteurii on improving electrical resistivity and chlorine ion penetration in autoclaved aerated concrete (AAC) [28]. In addition, Jafarnia et al. conducted a study using S. pasteurii in concrete containing limestone powder and zeolite, showing the significant effect of these microorganisms on concrete durability and mechanical properties of specimens [29]. Application of B.subtilis and S. pasteurii in fibrous and lightweight concrete also increased the compressive strength and reduced water permeability and absorption [30,31,32]. Using these microbial repair agents in sulfate medium, Mostofinejad and Nosouhian indicated that specimens surface repairing reduced water absorption by 7% and chlorine ion penetration by 12% compared with control specimens, improving concrete durability in a sulfate medium [33]. Muynck et al. investigated the ability of bacteria to precipitate calcium carbonate (CaCO₃) using chlorine ion migration test and water permeability in mortars with different porosity. The results of the study showed that the water permeability of specimens containing bacteria was reduced by 65–90%, and the migration of chlorine ions was reduced by 10–40% compared with control specimens [34].

The results of the conducted research show that exposing concrete to high temperatures reduces the mechanical properties and durability and triggers surface cracks, which could be developed in different directions [35,36,37,38]. As a result, repairing thermal cracks in concrete is essential, especially for fire exposed concrete structures. Therefore, this type of healing can be employed in these structures to repair deep and superficial thermal cracks. The positive effect of bacteria for improving mechanical properties of concrete and repairing cracks has been proven in various research papers [39,40]. However, bacterial effectiveness in repairing thermal cracks has not been fully evaluated. In this study, the efficiency of two bacteria including S. pasteurii and B. sphaericus in repairing thermal cracks and mechanical properties are examined.

2. Materials and Methods

2.1. Bacterial Strain and Their Growth Condition

In the present study, two types of bacteria were employed as repair agents with 10⁷ cell/mL concentration. The first type is Sporosarcina pasteurii (S. pasteurii) with PTCC 1645, a gram-positive bacterium, formerly known as bacillus pasteurii from older taxonomies. Through the process of microbiologically induced calcite precipitation (MICP), S. pasteurii has the ability to solidify sand and produce CaCO₃ precipitation [41,42,43]. The second type of bacteria is Bacillus Sphaericus (B. sphaericus) with PTCC 1490. Type II bacteria is a gram-positive, strict aerobic bacterium, which produces round spores in a swollen “club-like” terminal or subterminal sporangium at the end of its vegetative life cycle. It also has the ability to produce CaCO₃ precipitation with high urea activity [44,45]. In addition, applying bacteria to produce CaCO₃ precipitation has many advantages, such as simple technology, low cost, and no contamination, which have become a research hotspot [34,46,47].

Nutrient agar medium was employed to make S. pasteurii culture medium according to the protocol of manufacturer. First, 0.8 g/L of nutrient agar medium was weighed with distilled water to a volume of 100 mL. The prepared culture medium was sterilized in an autoclave for 20 min at 120 °C. A measure of 20 g of urea with distilled water was made up to 100 mL. Then the solution was sterilized by passing through a micropole filter. A measure of 10 mL of 20% filtered urea solution was added to 100 mL of autoclave nutrient agar medium (under laminar hood and sterile conditions). The culture medium was then immediately dispersed in 10 cm sterile pterygiums. It was incubated for 24 h at 37 °C to ensure no bacterial contamination. The strain, prepared from the collection center of the industrial microorganisms of Iran, was cultured linearly under a laminar hood and sterile conditions on a nutrient agar medium, containing urea.

The culture medium for incubation of bacterial colonies was incubated for 24 h at 30 °C. After full growth of bacteria and its combination with culture medium and distilled water, half of the damaged specimens were placed in this solution for four days at 30 °C.

The culture medium, rich in B. sphaericus, contains Mueller–Hinton (MH) agar at a ratio of 23 g/L. A measure of 2.3 g of the culture medium was poured into an Erlenmeyer flask and made up to 100 mL with distilled water to prepare it. The prepared culture medium was sterilized in an autoclave for 20 min at 120 °C. The strain, designed from the Persian Type Culture Collection (PTCC), was placed under a laminar hood and cultured linearly under sterile conditions on MH agar medium. The culture medium was incubated for 24 h at 37 °C to allow bacterial colonies to grow. The other damaged specimens were then placed in a solution containing bacteria, its culture medium, and distilled water at 30 °C for 4 days.

2.2. Concrete Mix Design

The concrete mixing design employed in this experimental study was developed to achieve a 28 day compressive strength of 40 MPa, based on ACI-211. The cement, gravel (coarse aggregate), and sand (fine aggregate) were 450, 848, and 751 kg, respectively, and the water–cement ratio was 0.52. The maximum coarse aggregate size was 12.5 mm—Portland cement type I was used in making the specimens. After making the concrete mix design, the mixture was poured into cubic molds with dimensions of 150 mm or cylinders with a diameter of 100 mm and a height of 200 mm. After 24 h, the specimens were transferred to the treatment pond and treated for 28 days at 25 °C.

Table 1. Mix design for 1 m³ of concrete.

Portland Cement	Water	Coarse Aggregate	Fine Aggregate	W/C
450 kg/m3	235.5 kg/m3	848 kg/m3	751 kg/m3	0.52

shows the mix ratio for concrete.

2.3. Test Procedure

In the present study, the efficiency of bacteria in repairing specimens exposed to heat was evaluated by performing compressive strength, UPV, and electrical resistivity tests. Accordingly, 72 cubic specimens with dimensions of 150 × 150 × 150 mm and 24 cylindrical specimens with a diameter of 100 mm and a height of 200 mm were made. Each of the compressive strength and UPV tests were performed on 36 cubic specimens (4 sets of 9 specimens), and the average of the experimental results was reported on three specimens.

In addition, the bulk electrical resistivity test was performed on 24 cylindrical specimens (4 sets of 6 specimens), and the average results of both specimens were presented. 24 cubic specimens were placed in a furnace with a heat capacity of 1200 °C for 1 h at 600 °C using a heat rate of 5.5 °C per minute. Moreover, 24 other specimens were exposed to the same heating regime for 1 h, at 800 °C. Eight cylindrical specimens were also exposed to a temperature of 600 °C for 1 h using a heat rate of 5.5 °C per minute. Eight were exposed to a temperature of 800 °C for 1 h using the same heating regime.

Figure 2a and Figure 3a illustrate specimens of heat damaged cubes and cylinders at 600 and 800 °C. In order to repair damaged concrete samples, a number of the specimens (including 12 cubic specimens and 4 cylindrical specimens) were placed for 4 days in a plastic tank containing S. pasteurii with a concentration of 10⁷ cells/mL in a culture medium, required for bacterial urea activity (Figure 2b). Likewise, the same number of damaged samples were placed for four days in another plastic tank containing B. sphaericus. For the full growth of bacteria, the temperature of bacterial culture medium was kept constant at about 30 °C for this period (Figure 3b).

Table 2. Specification and labels of the specimens.

Test	Shape	Series	Temperature	Bacteria	Specimen’s Label
Compressive strength, Pulse velocity through concrete	Cubic (150 mm)	1 2 3 4	- 600 800 600 800 600 800	- - - Sporosarcina pasteurii Sporosarcina pasteurii Bacillus sphaericus Bacillus sphaericus	RCα DCα-600 DCα-800 BCα-600-SP BCα-800-SP BCα-600-BS BCα-800-BS
Bulk electrical resistivity	Cylindrical (200 × 100 mm)	1 2 3 4	- 600 800 600 800 600 800	- - - Sporosarcina pasteurii Sporosarcina pasteurii Bacillus sphaericus Bacillus sphaericus	RCα DCα-600 DCα-800 BCα-600-SP BCα-800-SP BCα-600-BS BCα-800-BS

Note: α shows the number of the series of the specimens.

summarizes the referencing of the specimens, labelled as follows: the control specimens—“RC” (Reference Concrete); the heat-damaged specimens—“DC” (Damaged Concrete); and the specimens repaired with bacteria—“BC” (Bacterial Concrete). The numbers after the letters “RC”, “DC”, and “BC” indicate the series of specimens. Further, the numbers 600 and 800 °C indicate the furnace temperatures. In bacterial specimens, the last letters indicate bacterial type (S. pasteurii and B. sphaericus).

2.4. Compressive Strength

Compressive strength is a concrete quality indicator for determining overall strength. For this experiment, standard cubic specimens with dimensions of 150 mm were used, and loading was performed according to ASTM C39-18, at a rate of 0.25 MPa per second until failure.

2.5. The Ultrasonic Pulse Velocity (UPV) Test

The (UPV) test is a non-destructive test of concrete, used to control the quality of concrete materials and identify structural damage. Moreover, it is possible to estimate compressive strength of concrete by determining the ultrasonic pulse velocity. The passage time of sound waves is related to the tensile properties and density of materials [48]. Ultrasonic transmission time indicates the internal conditions of the test area. In general, longer passage time for a given path indicates lower quality concrete with defects, while shorter passage time indicates higher quality concrete. As the ultrasonic wave propagates within the test area, the wave is reflected at the edge of cracks, increasing the passage time. Therefore, the wave propagation time is lower in high-quality concrete, and the wave propagation time (lower wave velocity) is lower in low-quality concrete. In the present study, this experiment was performed according to the ASTM C597 standard [49]. For this experiment, 28 day standard cubic specimens of 150 × 150 × 150 mm were employed. After determining the wave transmission time and specimen length, the ultrasonic pulse velocity of each specimen can be calculated based on Equation (2). Three identical specimens were tested to determine the ultrasonic pulse velocity of specimens, and the average results were reported.

V = L/T

(2)

where:

V = ultrasonic pulse velocity (m/s);

L = distance between transducers faces (m);

T = transmission time (s).

2.6. Bulk Electrical Resistivity Test

The Bulk electrical resistivity test is one of the non-destructive tests for determining the durability of concrete and is performed using electrical resistivity apparatus. In the present study, the test was performed according to ASTM C1760 [50]. In this experiment, the proportion of pores directly affects the amount of electrical resistivity [51]. Twenty-four specimens of standard 28 day cylinders, with a diameter of 100 mm and a height of 200 mm, were placed in water for 48 h to reach saturation in order to perform this experiment. The electric current measuring plates were then connected to both sides of the specimens. Then, the electric potential difference of 60 V was passed through the specimens for 1 min. The converse of electrical resistivity was calculated via Equation (3), as follows:

$$\mathsf{\sigma }=\mathrm{K}\frac{{\mathrm{I}}_1}{\mathrm{V}}\frac{\mathrm{L}}{{\mathrm{D}}^2}$$

(3)

where σ is the electrical conductivity in $\frac{\mathrm{m}}{\mathsf{\Omega }.\mathrm{m}}$; I₁ is the electrical current measured after 1 min in mA; V is the applied voltage in volts taken as 60; D represents the average specimen diameter in mm and equal to 100; L is the average length of the specimen, which was 200 mm; and k is a constant coefficient of 1273.2. The electrical resistivity was determined by Equation (4), as follows:

$$\mathrm{k}=\frac{{10}^3}{\mathsf{\sigma }}$$

(4)

where R is the electrical resistivity in Ω.m.

2.7. SEM/EDS Images

Employing a focused beam of electrons, the SEM produces signals at the surface of specimens, revealing information about the chemical composition, crystalline structures, and orientation of materials making up the specimens. In the present study, the specimens were cut into areas ranging from approximately 5 mm to 10 mm in width and were coated with a thin gold layer as a conducting material. The SEM analyses were conducted under low vacuum mode with low nitrogen pressure at 10–15 kV of acceleration voltage. The SEM photos were used to investigate the orientation of CaCO₃ crystals and bacterial efficiency for healing the high temperature-produced cracks. Energy dispersive X-ray spectroscopy (EDS or EDX) is a chemical microanalysis technique used in conjunction with the SEM. The EDS technique detects X-rays emitted from the specimen during bombardment by an electron beam to characterize the analyzed elemental composition of volumes. Features or phases as small as 1 µm or less can be investigated [52].

3. Experimental Results and Discussion

3.1. Compressive Strength

Table 3. Results of compressive strength.

Specimens	Compressive Strength (MPa)
RC1	Avg STD CV	41.58 0.57 0.01
DC1-600	Avg STD CV	28.8 1.42 0.04
BC1-600-SP	Avg STD CV	55.6 0.82 0.01
RC2	Avg STD CV	41.71 1.31 0.03
DC2-600	Avg STD CV	29.11 1.25 0.04
BC2-600-BS	Avg STD CV	44.38 0.66 0.01
RC3	Avg STD CV	44.75 0.55 0.01
DC3-800	Avg STD CV	25.26 1.78 0.07
BC3-800-SP	Avg STD CV	48.4 0.28 0.005
RC4	Avg STD CV	51.35 0.89 0.01
DC4-800	Avg STD CV	33.95 1.52 0.04
BC4-800-BS	Avg STD CV	44.66 0.58 0.01

represents the results of the compressive strength test for standard 28 day cubic specimens measuring 150 mm. The effect of heat is visible on the compressive strength reduction of the samples. In the first series, the compressive strength of the samples decreased by 31% after exposure to 600 °C, compared with control specimens. After adding type I bacteria and filling high heat-induced surface and deep cracks, the compressive strength of the repaired samples increased by 93% and 33% compared with the damaged and control samples of the same series, respectively. In the second series, the compressive strength of the damaged specimens at 600 °C was reduced by 31% compared with the control specimens. By adding type II bacteria and repairing damaged samples with calcium carbonate deposit, the compressive strength of the repaired samples increased by 52% and 6% compared with the damaged and control samples of the same series, respectively (Figure 4).

In the third series, the specimens damaged at 800 °C experienced a 44% reduction in compressive strength compared with the control specimens of the same series. When the samples were placed in a tank containing type I bacteria, the compressive strength of repaired samples increased by 91% and 8%, relative to damaged and control specimens of the same series, respectively. In the fourth series, the compressive strength of samples damaged at 800 °C decreased by 34% in comparison with reference samples of the same series. By adding type II bacteria and repairing heat-induced surface and deep cracks, the compressive strength of repaired samples increased by 31% relative to damaged samples and reached 0.87 of the compressive strength of control samples of the same series (Figure 5).

The results of tests show a better performance of type I bacteria than type II bacteria to repair the samples leading to the increased compressive strengths of about 93% and 91% in the samples damaged at 600 and 800 °C, respectively. For type II bacteria, on the other hand, increased compressive strengths of about 52% and 31% were observed in specimens exposed to 600 and 800 °C, respectively. The increased compressive strength of containing bacteria samples resulted from the formation of calcium carbonate deposits inside the samples and the filling of surface and deep cracks and pores, thereby increasing the compressive strength of the samples. It is noteworthy that microcracks may be filled with calcium carbonate deposits in bacteria-repaired specimens, whereas microcracks are usually present in concrete specimens. Therefore, an increase in strength is observed in bacteria-repaired samples in comparison to the control (RC).

3.2. Ultrasonic Pulse Velocity for Concrete

The UPV test results of standard 28 day cubic specimens measuring 150 mm are illustrated in

Table 4. Results of ultrasonic pulse velocity.

Specimens	Ultrasonic Pulse Velocity (Km/s)
RC1	Avg STD CV	4 0.02 0.005
DC1-600	Avg STD CV	1.058 0.10 0.01
BC1-600-SP	Avg STD CV	3.671 0.26 0.070
RC2	Avg STD CV	3.894 0.37 0.09
DC2-600	Avg STD CV	1.974 0.65 0.33
BC2-600-BS	Avg STD CV	3.268 0.39 0.12
RC3	Avg STD CV	3.966 0.27 0.06
DC3-800	Avg STD CV	1.832 0.79 0.43
BC3-800-SP	Avg STD CV	3.583 0.08 0.02
RC4	Avg STD CV	4.183 0.24 0.05
DC4-800	Avg STD CV	1.501 0.23 0.15
BC4-800-BS	Avg STD CV	3.548 0.24 0.06

. Figure 6 and Figure 7 show that the damaged specimens experienced a sharp drop in UPV after being placed in the furnace. The drop rate in the damaged specimens at about 600 °C was about 50 and 74%, and at 800 °C was 54 and 65%, respectively. However, with the addition of bacteria to the specimens and the formation of CaCO₃ precipitation in surface and deep heat-induced cracks, the ultrasonic pulse velocity for the damaged specimens (at 600 °C), repaired by type I and II bacteria, increased by about 3.46 and 1.65 times, respectively. Moreover, it rose nearly by 95 and 136% in damaged specimens at 800 °C after repairing. By comparing the efficiency between the two microorganisms, it can be seen that type I and II bacteria had a better efficiency in increasing pulse velocity of the damaged specimens at 600 and 800 °C, respectively.

3.3. Bulk Electrical Resistivity

The bulk electrical resistivity test results, obtained from the average of two replications on a cylindrical specimen, are given in

Table 5. Electrical conductivity results.

Specimens	Electrical Conductivity (Ω-m)
RC1	353
DC1-600	591.5
BC1-600-SP	462
RC2	305.5
DC2-600	674.5
BC2-600-BS	466.5
RC3	358
DC3-800	484
BC3-800-SP	364.5
RC4	335
DC4-800	574
BC4-800-BS	370

. As shown in Figure 8, the electrical resistance of damaged specimens at 600 °C increased by about 67–121%. After the addition of bacteria and the filling of cracks and pores formed by heat, the electrical resistance for types, containing microbial organisms, decreased. The electrical resistance for the damaged specimens (at 600 °C), repaired by type I and II bacteria, reduced by about 22 and 31%, respectively. Moreover, the electrical resistivity of the damaged specimens at 800 °C increased by about 35–71%. After adding the microbial organisms, types I and II, the electrical resistance decreased by 25 and 36%, respectively, compared with the damaged specimens (Figure 9). By comparing the efficiency between the two microorganisms, it can be seen that type II bacteria had a better efficiency than type I bacteria in reducing the pores of the specimens and repairing the heat-induced cracks.

3.4. SEM/EDS Images

Microstructural analysis was performed to evaluate the efficiency of microorganisms and determine the heat-induced pores and cracks and the CaCO₃ precipitation produced by the microorganisms. The SEM images of the damaged specimen at 600 and 800 °C are shown in Figure 10, and the SEM images of the specimens repaired by type I and II bacteria are shown in Figure 11 and Figure 12, respectively. The results of comparing the images are described below. Figure 10 shows that continuous pores and small cracks formed at 600 °C. Solid CaCO₃ precipitation filling pores and cracks can be seen in Figure 11. In Figure 12, CaCO₃ precipitation deposits produced by type II bacteria fill the heat-induced cracks and pores. In addition, SEM images of the repaired specimens by bacteria type I and II and their comparison with SEM images of calcite, vaterite, and aragonite crystals prove that crystals created in the specimens are aragonite [53].

Figure 13 shows its EDS diagram, and

Table 6. The weight percentage of elements in concrete containing type I bacteria.

Elements	O	Na	Mg	Al	Si	S	K	Ca	Fe
Weight percentage	27.6	3.55	1.43	1.68	7.24	16.87	0.14	40.38	1.06

illustrates the weight percentage of the ingredients in the composition of this specimen. As observed, there are elements such as silicon (Si), oxygen (O), sodium (Na), magnesium (Mg), aluminum (Al), calcium (Ca), sulfur (S), potassium (K), and iron (Fe) in the concrete composition. Moreover, large amounts of calcium in the concrete structure indicate an increase in this type of bacterial activity.

Figure 14 shows the EDS diagram of concrete containing B. sphaericus bacteria. This specimen includes elements such as silicon (Si), oxygen (O), sodium (Na), magnesium (Mg), aluminum (Al), calcium (Ca), potassium (K), sulfur (S), and iron (Fe). Increasing the amount of calcium in this specimen indicates an increase in bacterial activity in the concrete.

Table 7. The weight percentage of elements in concrete containing type II bacteria.

Elements	O	Na	Mg	Al	Si	S	K	Ca	Fe
Weight percentage	16.36	0.43	0.08	0.09	4.25	8.29	0.56	61.71	8.16

displays the weight percentage of ingredients in concrete containing type II bacteria.

4. Conclusions

In the present study, thermal cracks on damaged specimens were repaired by applying two different types of microorganisms externally. For this purpose, the specimens were exposed to bacterial solution and culture medium for growth and activity for four days at 30 °C. Then, the efficiency of bacteria was evaluated by the compressive strength, UPV, bulk electrical resistivity, and SEM tests. The results were compared with each other, as summarized below.

1. The results showed that the compressive strength of the repaired specimens was 93% higher than the damaged specimens and 33% higher than the control specimens. Whereas, in previous studies, the compressive strength of specimens containing bacteria increased by a maximum of 24% compared with control samples [54].

2. The results of the SEM/EDS images of the repaired specimens showed CaCO₃ precipitation produced by microorganisms inside the pores.

3. The results showed that, by adding bacteria to the damaged specimens, the compressive strength of the repaired specimens increased by approximately 1.31–1.93 times compared with the damaged ones. These values vary depending on the number of cracks, the applied temperature to the specimen, and the repairing bacterial type.

4. The optimal efficiency of bacteria in the UPV test was also shown in the study. Once the damaged specimens were placed in the bacteria, their deep and surface heat-induced cracks were filled and repaired by CaCO₃ precipitation, resulting in a significant improvement in the pulse velocity passing through the repaired specimens.

5. Another finding of this study was the direct effect of pores on the bulk electrical resistivity test results. The damaged specimens have higher electrical resistivity than the control specimens. However, these values were significantly reduced after adding bacteria and repairing the cracks and pores by CaCO₃ precipitation.

6. The difference of the efficiency of both types of bacteria at different temperatures was also found in the present study. The results showed that the efficiency of microorganisms in repairing the cracks created at 600 °C was higher than the cracks created at 800 °C. These results indicated a difference between the cracks created at 600 and 800 °C, in that the thermal cracks at 600 °C were finer and evenly spread on the concrete surface. Still, the heat-induced cracks at 800 °C were larger. They were also wider, which probably directly affected bacteria’s efficiency compared with the conditions at 600 °C.