The comparative study of epoxy and bacterial injections for cracked concrete beam flexural strength

. Cracks in concrete beams can significantly affect the performance of reinforced structures. To repair cracks in concrete, Traditional crack repair methods typically involve epoxy injection, but studies have suggested that epoxy may not fully restore the original strength. Therefore, concrete structure repair requires the development of new and more efficient materials. This study focuses on investigating the potential of Bacillus megaterium, a bacterium known for its self-healing properties in concrete. This bacterium has the capability to precipitate minerals like calcium carbonate, which can fill the cracks and increase the strength of concrete. Testing is carried out on reinforced concrete beams with existing cracks, sized at 100 mm x 100 mm x 500 mm. The specimens are divided into two groups, one repaired with epoxy injection and the other with bacterial injection, with varying additions of 10% and 15%. The research results show that the injection process with the addition of 15% bacteria is able to restore approximately 99.72% of the initial strength of the concrete beams. The results of this research contribute to the development of improved repair techniques that can restore the flexural strength of cracked concrete beams and extend the service life of concrete structures.


Introduction
Cracks in concrete are inevitable and cannot be prevented.Cracks that occur in reinforced concrete greatly affect the performance of the reinforcement.If it cracks to the reinforcement it will cause delamination (removal of the concrete cover from the concrete slab structure due to oxidation of the reinforcement) and spalling (peeling of the concrete surface due to corrosion of the reinforcement or collision) [1].The crack was caused by several things, such as environmental influences, less than optimal initial design, poor maintenance, and natural events such as earthquakes.Cracks in concrete can usually be repaired by injection of the crack using an epoxy material [2].
But several studies that have been carried out to repair concrete cracks using the injection method using epoxy materials show that epoxy injection is not able to restore the flexural strength of the beam like the initial strength and there is a decrease of 16% [3].Likewise, the epoxy injection was able to fill the cracks that occurred in the concrete beam test object, but the injection could not completely fill the entire crack area, which was indicated by the wave velocity value which was smaller than when the samples had not experienced cracks [4].This proves that injection using epoxy is able to repair cracks but is less able to restore strength to the initial strength of the concrete.
Therefore, to overcome this problem, a new innovation has been found to repair cracks in concrete, especially micro-cracks, which are called bio-concrete using the bacterium Bacillus Megaterium [5].Bacterium Bacillus Megaterium is a product that can repair independently after cracked concrete (self-healing concrete).Previous research used the bacterium Bacillus Megaterium by mixing it into a sample in the form of a 100 mm x 100 mm x 500 mm beam and testing its flexural strength.The results of this study concluded that concrete mixed with bacteria can increase the flexural strength of concrete by up to 14.64% compared to normal concrete.[6] and compressive strength test with specimens printed into cylindrical with 150 mm x 300 mm with different percentages of fly ash and bacteria.It was concluded that the compressive strength of bacteria-based concrete increased by up to 55.178% compared to normal concrete [7].
With the addition of this material through injection into cracked concrete, it is hoped that Bacillus Megaterium bacteria can assist in repairing the concrete, enhancing its load-bearing capacity, and achieving better crack closure compared to epoxy injection material.Research is conducted to determine the effectiveness of the injection process using Bacillus Megaterium bacteria in restoring concrete strength compared to the use of epoxy material by conducting a comparison between the flexural strength values of beams before cracking occurs and after the injection process is carried out.

Material
In this study, two types of materials are being injected into the cracked concrete beams, namely epoxy and Bacillus megaterium bacteria.

Epoxy
Epoxy is a material used in the reinforcement of reinforced concrete structures.Epoxy material for injection is in liquid form and has the property of hardening quickly and is able to penetrate into cracks that cannot be reached by cement.This material is very helpful for bonding cracked concrete and preventing the concrete reinforcement from the threat of corrosion.The reliability of this injection method is highly dependent on the viscosity level of the epoxy used [8].Cracks 0.05 mm to 10 mm wide can be filled by epoxy injection [9].Before concrete repairs are carried out, the cause of the damage must be assessed.Injection into cracks is needed to re-stick concrete and produce a stronger concrete structural state [10].Epoxy injection is a widely used and recommended procedure for structural reinforcement.

Epoxy injection consists of two components :
Epoxy material for crack sealing (hardener) should have low viscosity with sufficient compressive strength, ideally surpassing that of concrete.The material must exhibit strong adhesion properties to ensure that the repairs on the cracks do not reoccur.
High-strength epoxy is used to seal cracks on the concrete surface (base) and is also employed for injection through the nozzle (valve for introducing injection material), allowing it to withstand the pressure from the epoxy injection pump.
Epoxy injection is a highly technical work, requiring great skill to obtain good results, and its use technique is limited by ambient temperature, and requires the highest degree of care in its execution, even though it is based on a simple procedure.All cracks visible to the eye should be properly recorded and carefully marked in length, width and location.

Bacteria bacillus megaterium
Bacterial Based-Concrete is intelligent concrete that exhibits human-like self-healing characteristics capable of increasing structural strength, especially under stress.Other advantages such as increased service life of the structure as a whole, effective against corrosion due to the presence of water vapor produced is used as a catalyst to maintain the quality of concrete continuously.Self-healing concrete is better than ordinary concrete due to its environmental friendliness.The presence of bacteria in concrete is an alternative to many other conventional technologies because it is environmentally friendly, and has the ability to act as self-healing agents [11].In self-healing concrete applications, calcium lactate can be useful as a binder, hardener, and compactor for the performance of Bacillus megaterium bacteria.After a curing period of 28 days, compression and flexure tests can be carried out until it cracks [12].
One type of bacillus bacteria is Bacillus Megaterium which can increase the compressive strength of concrete by 48% [5].Bacteria Bacillus Megaterium has the highest compressive strength at 28 days of age of 37 MPa as shown in Table 1 and after the SEM test the bacteria Bacillus megaterium can block the entry of other harmful substances to increase its impermeability and this causes the concrete to be well hydrated and the curing process in the concrete.may continue due to residual moisture [13].
Table 1.Effect of several types of bacteria on increasing the compressive strength of concrete [5].Autogenous healing techniques are a good approach in the repair of micro cracks on the concrete surface, the addition of bacteria causes the deposition of calcium carbonate which forms a layer to cover the cracks in the concrete [15].The added bacteria must be able to survive in a highly alkaline or alkaline environment because concrete is wet [16].Bacillus sphaericus can precipitate CaCO3 in a highly alkaline environment by converting urea to ammonium and carbonate [17].Calcium carbonate helps fill in micro-cracks and bonds other materials such as sand and gravel in concrete [18].Any form of cracks in the concrete will cause the bacteria to become active from its hibernation stage.Bacterial metabolism produces deposits of calcium carbonate which fill in the cracks.After the crack is completely filled with calcium carbonate, the bacteria return to the hibernation stage and when a crack occurs again, the bacteria are active again and fill in the cracks so that the bacteria can be said to be a long-lasting self-healing agent.This mechanism is referred to as Microbiologically Induced Calcium Carbonate Precipitation (MICP) [19].

Bacteria
The concept of self-healing bacteria-based concrete suggests that in the presence of oxygen and water in the cracks, dormant bacterial spores are activated.Then, active bacterial cells convert calcium lactate (CaC6H10O6) to CaCO3 plus oxygen into calcium carbonate.

CaC6H10O6 + 6O2 → CaCO3 + 5CO2 + 5H2O.
(1) [20] 3 Methods In conducting research, thorough planning is essential to ensure that its implementation proceeds effectively and efficiently.The type of this reasearch is experimental.The following are the stages of research execution.

Beam test specimen preparation
The initial step in this research involves preparing the materials to be used in the concrete mix preparation.The test specimen used is a reinforced beam with dimensions of 100 mm x 100 mm x 500 mm and a planned concrete strength of f'c 30 MPa, as shown in Fig. 2. The concrete beams will be produced using a concrete mixture with a water-to-cement ratio (FAS) of 0.5.The materials used will be consistent, and the casting process will be carried out simultaneously.Through this consistent treatment, it is expected that homogeneous test specimens will be created, reducing doubts about their quality uniformity.The sample production will be divided into several injection variations, as detailed in Variations are categorized into two sample types, one filled with epoxy as the injection filler, and the other with a mixture of epoxy along with the addition of Bacillus megaterium bacteria.To determine the optimal bacteria addition, several concentration variations are established, including additions of 10%, 15%, 20%, and 25%.The most optimal value is selected from the highest bacteria addition concentration that restores the initial strength, and it is compared to epoxy without bacteria addition.To determine if bacteria have crack-sealing capabilities equivalent to epoxy, testing will be distinguished by the testing time.Some epoxy specimens will be tested after 3 days, following the typical epoxy curing time, and some specimens will be tested after 28 days, which represents the optimal concrete strength period.

Cultivation of bacillus megaterium bacteria
The cultivation of Bacillus Megaterium bacteria involves two types of media: agar and liquid.Nutrient Agar serves as the agar medium, while Nutrient Broth serves as the liquid medium.For cultivation on solid media, the first step is to sterilize the equipment and agar medium in an autoclave for 15 minutes at 1 atm pressure.After reaching normal temperature, the bacteria are inoculated by taking 1 loopful and streaking it onto Nutrient Agar using a zig-zag motion.Subsequently, the specimens are placed in an incubator at a temperature of 30-35 degrees Celsius.
Regarding the process with liquid media, Nutrient Broth is mixed with 500 mL of distilled water, then heated and stirred on a hot plate until it becomes homogeneous.This liquid medium is then sterilized in an autoclave at 1 atm pressure for 15 minutes.After sterilization, 1 tube of bacteria is transferred into an Erlenmeyer flask.The Erlenmeyer flask is then placed in a shaker incubator at a temperature of 37 degrees Celsius for 24 hours.After 24 hours have passed, the samples are removed and stored in a refrigerator at 0 degrees Celsius.The bacterial solution can be seen in Fig. 3 , 04003 (2024) E3S Web of Conferences https://doi.org/10.1051/e3sconf/202447904003479 ISSAT 2023 Fig. 3. Bacillus megaterium bacterial solution.

Initial loading (P0)
After creating specimens for conducting a Flexural Strength Test, the standard employed for this test is ASTM C.293.The flexural strength test is conducted on beams measuring 100 mm x 100 mm x 500 mm using the three-point bending method.Testing is performed after 28 days.Subsequently, the testing is carried out using a three-point bending machine as depicted in Fig. 4.

Specimen repair with injection
After the specimen is subjected to a load, it will result in deflection, causing the beam to crack.The beam is designed to withstand a permissible crack width of 5 -10 mm.Subsequently, when the beam experiences cracking, repairs will be carried out on these cracks using a mixture of injection filler material, consisting of epoxy , and the addition of Bacillus Megaterium bacteria.The injection filler epoxy uses estorex EP10 product from PT. Hissan Indonesia.The composition of the filler material, as indicated in Table 3.

Load testing after injection (P1)
Because epoxy and Bacillus Megaterium bacteria require time to harden, testing is conducted after 28 days from the injection process.After 28 days since the injection, the beams are subjected to additional loads to determine the load after epoxy injection (P1).Subsequently, load testing after injection will be followed by an analysis comparing the flexural strength before and after injection.This analysis will include a graph illustrating the relationship between deflection and the capacity of epoxy and Bacillus Megaterium as factors for concrete flexural strength improvement.

Flexural strength results before injection
After the flexural testing is conducted, the experimental results are summarized in Table 4.The following are the flexural strength values for beams that have undergone 28 days of curing, and these results will serve as a comparison to the beams injected with epoxy and epoxy with the addition 10% , 15%, 20% and 25% Bacillus Megaterium bacteria.

Comparison of flexural strength recovery after injection
Beams that have cracked are then injected with epoxy resin and Bacillus Megaterium bacteria according to the variations listed in Table 2.After curing have passed since the injection, the beams are subjected to additional loads to determine the load after epoxy injection (P1) then the results will be compared with the condition before the injection (P0).

Epoxy injection beams
After being tested for 28 days on the specimens, a decrease in strength is observed, as seen in the flexural strength after epoxy resin injection, which is lower compared to the flexural strength before injection.This decline occurred in three different specimens, with the largest decrease in beam BL1. up to 87.31% of its initial strength.Meanwhile, the smallest percentage decrease occurred in beam BLI.1, from an initial load of 13.23 MPa to 12.79 MPa, and this beam was able to restore flexural strength up to 96.71%.The average decrease in strength across these three specimens is 7.56%.This indicates that beams with epoxy injection can only restore their strength up to 92.44% of their initial strength.The magnitude of the flexural strength comparison using epoxy injection can be seen in the diagram in Fig. 6.Fig. 6.The graph that shows the comparison of flexural strength before injection and after injection using epoxy.

Epoxy injection with the addition of bacillus megaterium bacteria 4.2.2.1 The addition of 10% bacillus megaterium bacteria
A decrease in strength occurred in beams injected with epoxy and supplemented with 10% Bacillus megaterium bacteria.This decline was observed in three different specimens, with the most significant decrease occurring in beam BLII.3, amounting to 18.61%, which could only restore the strength to 81.39% of its initial level.Meanwhile, the smallest percentage decrease was observed in beam BLII.1, which was able to restore flexural strength up to 96.11%.The average decrease in these three specimens was 12.58%.This indicates that beams injected with epoxy could only restore their strength to 87.42% of the initial level.In comparison, the addition of 10% bacteria proved less effective compared to the epoxy resin injection's ability to restore up to 92.44%.However, this is an improvement compared to a previous study that only achieved an 84% restoration [3] of the initial strength.The magnitude of the flexural strength comparison using epoxy injection can be seen in the diagram in

The addition of 15% bacillus megaterium bacteria
Different results were observed in the injection of beams using epoxy and the addition of 15% Bacillus megaterium bacteria.It turns out that in the three tested beam specimens, one of them, namely beam BLIII.3, was able to restore the initial strength and increase the flexural strength of the beam by 3.69%.The largest percentage decrease occurred in beam BLIII.1, at 0.83%, which could only restore strength to 99.17% of its initial level.The average percentage of strength recovery in the three specimens was 99.72%.In terms of flexural strength, the addition of 15% bacteria is significantly more effective compared to epoxy resin injection, which could only restore the initial strength to 92.44%.The magnitude of the flexural strength comparison using epoxy injection can be seen in the graph in Fig. 8. Fig. 8.The graph that shows the comparison of flexural strength before injection and after injection using epoxy and the addition of 15% Bacillus megaterium bacteria.

The addition of 20% bacillus megaterium bacteria
After conducting flexural strength testing, the addition of 20% bacteria after a 3-day curing process also yielded quite significant results.Improvement was observed in two out of three specimens, namely beams BLIV.1 and BLIV.2, which were able to increase their initial strength from 14.58 MPa and 14.82 MPa to 15.18 MPa and 15.37 MPa, respectively.The increase in flexural strength was 4.14% and 3.69%.This indicates that adding 20% bacteria to the epoxy injection mixture is effective, although a significant decrease occurred in beam BLIV.3, from the initial strength of 15.53 MPa to 13.39 MPa, indicating a 13.75% decrease and able to restore strength to 86.25%.This could be due to technical errors in the injection process or the flexural strength testing.Therefore, the average percentage of strength recovery in the three specimens is 98.03%.However, this addition is still considered effective compared to epoxy resin injection, which can only restore the initial strength to 92.44%.This also demonstrates that a 3-day curing period with the addition of bacteria already has a significant impact.The magnitude of the flexural strength comparison using epoxy injection can be seen in the diagram in Fig. 9. Fig. 9.The graph that shows the comparison of flexural strength before injection and after injection using epoxy and the addition of 20% Bacillus megaterium bacteria.

The addition of 25% Bacillus Megaterium bacteria
To achieve a margin above the bacterial addition limit, an addition of 25% Bacillus Megaterium bacteria was performed.The results indicate a decrease in strength in all three specimens.The largest decrease occurred in beam BLV.1, which initially had a strength of 14.72 MPa but decreased to 12.72 MPa, resulting in a decrease of 13.59%.Consequently, this beam could only restore its strength to 86.41% of the initial level.The average strength decrease across the three specimens is 10.21%.This reveals that the beams could only recover their strength to 89.79% of the initial level.In comparison, adding 25% bacteria is no longer effective when compared to the ability of epoxy resin injection, which can restore strength up to 92.44%.The magnitude of the flexural strength comparison using epoxy injection can be seen in the diagram in Fig. 10.Here is a summary result of the flexural strength comparison before and after injection in all tested specimens, as shown in Table 5.In the epoxy injection process, it can restore an average of about 92.44% of the initial strength.This result is considered good, especially when compared to previous research that could only restore about 84%.Several factors affect the strength reduction in crack injection processes, including inadequate bonding if epoxy injection is not performed properly and cannot strongly bind to the cracked concrete walls.Additionally, it's essential to adhere to established guidelines.If the injection technique is not executed correctly, such as incorrect injection pressure or an improper injection material amount, the strength of the repair can be affected.Meanwhile, in the injection process with the addition of 15% bacteria, it can restore around 99.72% of the initial strength.Therefore, it can be concluded that the most effective method in restoring initial strength after cracks occur is by using bacterial injection with an addition of 15% of the total injection solution weight.

Conclusion
Based on mechanical testing of beam flexural strength, the most effective method for restoring the initial strength after cracking is by using bacterial injection with an addition of 15% of the total weight of the injection solution.A decrease or increase in the bacterial content in the epoxy mixture will reduce the strength after injection.This also proves that adding bacteria to the epoxy injection filler material has a strengthening effect compared to pure epoxy.The test results also demonstrate that a 3-day curing period with the addition of bacteria already has a significant impact.This research is still in the development stage, therefore further testing is needed to assess the potential of bacteria in the self-healing process when cracks occur for the second time.
This research is supported by Postgraduate Research (PPs), Politeknik Negeri Bandung, Indonesia

Fig. 7 .
Fig. 7.The graph that shows the comparison of flexural strength before injection and after injection using epoxy and the addition of 10% Bacillus megaterium bacteria.

Fig. 10 .
Fig.10.The graph that shows the comparison of flexural strength before injection and after injection using epoxy and the addition of 25% Bacillus megaterium bacteria.

Table 3 .
Mixture of injection filler material.

Table 4 .
Flexural strength results of concrete beams.

Table 5 .
Comparison of Flexural Strength before and after injection.