Cracking of concrete is a common phenomenon. Without immediate and proper treatment, cracks in concrete structures tend to expand further and eventually require costly repairs. Even though it is possible to reduce the extent of cracking by available modern technology, remediation of cracks in concrete has been the subject of research for many years.

When cracks appear in a concrete structure and water starts to seep in through the spores, the bacteria starts microbial activities on contact with the water and oxygen [1]. In the process of precipitating calcite crystals through nitrogen cycle, the soluble nutrients are converted to insoluble CaCO₃ . The CaCO₃ solidifies on the cracked surface, thereby sealing it up [2-3].

A technique is adopted in the formation of concrete by utilizing microbiologically induced calcite CaCO₃ precipitation. Microbiologically Induced Calcite Precipitation (MICP) is a technique that comes under a broader category of science called Biomineralization is the process by which living organisms produce minerals like CaCO₃ . Bacillus Subtilis, a common soil bacterium can induce the precipitation of calcite [4-5]. MICP Self-healing [12] is highly desirable because the calcite precipitation induced as a result of microbial activities is pollution free and natural. In this study investigation the above techniques are checked for the possibility of improving the performance of the bacterial concrete [7].

Growth of Bacteria–Bacillus Subtilis

The pure culture was isolated from the soil sample of JNTUH and is maintained constantly on nutrient agar slants. It forms dry white colonies on nutrient agar. Whenever required, a single colony of the culture is inoculated into nutrient broth of 25 ml in 100 ml conical flask and the ogrowth conditions are maintained at 38 ^oC temperatures and placed in 125 rpm orbital shaker.

The medium composition required for growth of culture is Peptone: 5 g/lt, NaCl: 5 g/lt, Yeast extract: 3 g/lt.

The present work aims at the preparation of bacterial concrete using fly ash and foundry sand [3]. Cement is partially replaced with fly ash, the percentages of fly ash [8- 12] used are 10% and 30%. Fine aggregate is replaced with foundry sand, the percentage of foundry sand used is 10% and 30% and Bactria used is Bacillus Subtilis 10⁵ cells/ml concretion. To achieve more sustainable and economical concrete, for every mix the different tests of, Compressive strength, Split tensile strength and Flexural strength will be done.

The objectives of the present work are as follows:

• To find the workability aspects of the bacterial concrete for M20 and M30 grade concretes using fly ash and foundry sand.
• To know the strength characteristics of bacterial concrete using fly ash and foundry sand [11] for the different percentages (0%, 10%, and 30%).
• To conduct Scanning Electronic Microscopy (SEM) investigations on both normal and bacterial concrete samples.

The experimental work involves conducting tests on Bacterial Concrete using partial replacement fly ash and foundry sand.

• Test on physical properties of the foundry sand and coarse aggregate are conducted.
• Test on chemical analysis of the fly ash were tested at IICT, Tarnaka Hyderabad.
• Cement is partially replaced with fly ash. The percentages of fly ash used in mix there are 10% and 30% and when fine aggregate is replaced with foundry sand the percentage of foundry sand used in the mix are 10% and 30%.
• Fly ash and foundry sand mix case added with 10⁵ cells/ml bacterial solutions.
• Role of bacteria on concrete for M-20 and M-30 grades using fly ash and foundry sand replacements have to be studied.

The present method deals with evolution of mechanical properties of concrete, compressive strength, split tensile strength, and flexural strength. Cement is partially replaced with fly ash, the percentages are 0%, 10%, and 30% and Fine aggregate is partially replaced with foundry sand, the percentages are 0%, 10%, and 30% of two standard Grades M-20 and M-30. The program involves casting and testing of specimens. The specimens were casted using standard cube (150 X 150 X 150 mm), standard cylinder (150 X 300 mm) and standard prism (100 X 100 X 500 mm).

3.1 Procurement of Materials

The materials used for the investigative study of bacterial concrete are below.

• Cement,
• Fine aggregate,
• Coarse aggregate,
• Fly ash,
• Foundry sand,
• Water, and
• Bacillus Subtilis 10⁵ cells/ml a model laboratory Bacteria is used.

3.2 Mixing of Concrete

Two mix cases with M20 and M30 grades of concrete have been considered for both normal concrete and bacterial concrete. Table 1 lists the mix proportions of concrete. Table 2 and 3 shows the workability test for M₂₀ and M₃₀ Normal concrete with the addition of fly ash, foundry sand without and with including bacteria, respectively. The mix design is adopted as per IS: 10262-2009 and mixes as follows.

• Normal mix of concrete in M-20 grade as per IS: 10262- 2009.
• For M-20 mix grade, Fly ash is partially replaced with cement and foundry sand is partially replaced with fine aggregate.
• Bacterial mix case of concrete using 10⁵ cells/ml of Bacillus Subtilis culture sample in M-20 grade as per IS:10262-2009.
• Normal mix of concrete in M-30 grade as per IS: 10262- 2009.
• For M-30 mix grade, Fly ash is partially replaced with cement and foundry sand is partially replaced with fine aggregate.
• Bacterial mix case of concrete using 10⁵ cells/mi of Bacillus Subtilis culture sample in M-20 grade as per IS:10262-2009.

Table 1. Mix Designation Proportions and Quantities

Table 2. Workability Test for M20 and M30 Normal Concrete with the Addition of Fly Ash, Foundry Sand

Table 3. Workability test for M₂₀ and M₃₀ Normal Concrete with the addition of Fly Ash, Foundry Sand including Bacteria

3.3 Mix Designation Proportions

• NAC - Natural aggregate concrete.
• NFAS1 - Normal concrete with the addition of 10% Fly ash, 10% Foundry sand.
• NFAS2 - Normal concrete with the addition of 30% Fly ash, 30% Foundry sand.
• NBC - Natural aggregate concrete with bacteria.
• NFASB1 - Normal concrete with the addition of 10% Fly ash, 10% Foundry sand including bacteria.
• NFASB2 - Normal concrete with the addition of 10% Fly ash, 10% Foundry sand including bacteria.
• B- Bacillus Subtilis Bacteria (10⁵ cell/ml).

Results obtained from experimental investigation are used. Mixes used in this investigation are M20 and M30 Standard grades and Cement, Fine aggregate are partially replaced with Fly ash and Foundry sand at various percentages. The study was conducted to find out the influence of Foundry sand and Fly ash on strength properties of bacterial concrete.

• Compressive strength for M20 grade concrete and M30 grade concrete, where cement is partially replaced with Fly ash and fine aggregate with Foundry sand at various percentages is (0%, 10%, and 30%) and bacteria concentration with 10⁵ cell/ml.
• Split tensile strength of M20 and M30 grade concretes, where cement is partially replaced with Fly ash and fine aggregate with Foundry sand at various percentages that is (0%, 10%, and 30%) and bacteria concentration with 10⁵ cell/ml.
• Flexural strength of M20 and M30 grade concretes, where cement is partially replaced with Fly ash and fine aggregate with Foundry sand at various percentages that is (0%, 10%, and 30%) and bacteria concentration with 10⁵ cell/ml.

4.1 Compressive Strength Test Results

Compressive strength is performed on standard cubes 150 X 150 X 150 mm and the following results are drawn based on Compressive strength test values.

4.2 Compressive Strength Result for M20 Grade Concrete

• Addition of bacteria increases the 7 days Compressive strength from 16.25 Mpa to 19.17 Mpa when compared with normal concrete (ie), percentage increase is 15.23%.
• When Cement, Fine aggregate are partially replaced with Fly ash and Foundry sand by 10%, the addition of bacteria increases 7 days Compressive strength from 16.9 Mpa to 20.15 Mpa (ie), percentage increase is 16.12%.
• When the percentage replacement of Cement and Fine aggregate with fly ash and Foundry sand by 30%, then the addition of bacteria increase 7 days Compressive strength from 18.12 Mpa to 20.75 Mpa (ie), percentage increase is 12.67%.
• Addition of bacteria increases the 28 days Compressive strength from 20.27 Mpa to 23.21 Mpa when compared with normal concrete (ie), percentage increase is 12.6%.
• When Cement, Fine aggregate are partially replaced with Fly ash and Foundry sand by 10%, the addition of bacteria increases 28 days Compressive strength from 20.77 Mpa to 21.42 Mpa (ie), percentage increase is 5.3%.
• When Cement, Fine aggregate are partially replaced with Fly ash and Foundry sand by 30%, the addition of bacteria increases 28 days Compressive strength from 20.23 Mpa to 23.24 Mpa, (ie), percentage increase is 12.95% as shown in Figure 1.
• The variation of the Compressive strength with the different percentages of Fly ash and Foundry sand is represented in a bar chart and given in Figures 1, 2, and 3.
• Addition of bacteria increases the 7 days Compressive strength from 25.63 Mpa to 27.17 Mpa when compared with normal concrete i.e., percentage increase is 5.66%.
• When Cement, Fine aggregate are partially replaced with Fly ash and Foundry sand by 10%, the addition of bacteria increases 7 days Compressive strength from 24.21 Mpa to 26.15 Mpa, (ie), percentage increase is 7.41%.
• When the percentage replacement of Cement and Fine aggregate with fly ash and Foundry sand by 30% then the addition of bacteria increases 7 days Compressive strength from 24.12 Mpa to 26.78 Mpa,(ie), percentage increase is 9.98%.
• Addition of bacteria increases the 28 days Compressive strength from 28.29 Mpa to 33.23 Mpa when compared with normal concrete i.e, percentage increase is 14.8%.
• When Cement, Fine aggregate are partially replaced with Fly ash and Foundry sand by 10%, the addition of bacteria increases 28 days Compressive strength from 30.19 Mpa to 33.66 Mpa, (ie), percentage increase is 10.93%.
• When Cement, Fine aggregate are partially replaced with Fly ash and Foundry sand by 30%, the addition of bacteria increases 28 days Compressive strength from 29.16 Mpa to 33.38 Mpa, (ie), percentage increase is 10.56%.
• The variation of the Compressive strength with the different percentages of Fly ash and Foundry sand is represented in a bar chart and given in Figures 4 and 5.

Figure 1. Variation of 7 days Compressive Strength with Different Percentages of Fly Ash and Foundry Sand for Normal and Bacterial Concrete

Figure 2. Variation for 28 days of Compressive Strength with Different Percentages of Fly Ash and Foundry Sand and Bacterial Concrete

Figure 3. Variation of Compressive Strength with Different Percentages of Fly Ash and Foundry Sand for Normal and Bacterial Concrete

Figure 4. Variation of Compressive Strength with Different Percentages of Fly Ash and Foundry Sand for Normal and Bacterial Concrete

Figure 5. Variation of Flexural Strength with Different Percentages of Fly Ash and Foundry Sand for Normal and Bacterial Concrete

4.3 Flexural Strength for M20 Grade

• Addition of bacteria increases Flexural Strength from 3.92 Mpa to 4.65 Mpa when compared with normal concrete i.e., percentage increase is 19.35%.
• When Cement Fine aggregate are partially replaced with Fly ash and Foundry sand by 10%, the addition of bacteria increases Flexural Strength from 3.62 Mpa to 4.89 Mpa, i.e., percentage increase is 33%.
• When Cement, Fine aggregate are partially replaced with Fly ash and Foundry sand by 30%, the addition of bacteria increases Flexural Strength from 3.82 Mpa to 5.1 Mpa, i.e., percentage increase is 33.14%, and it is shown in Table 4.

Table 4. Flexural Strength of Specimens for M20 28 Days

4.4 Flexural Strength Result for M30 Grade

• Addition of bacteria increases 28 days Flexural Strength from 4.85 Mpa to 5.5 Mpa when compared with normal concrete and the percentage increase is 28.50%.
• When Cement, Fine aggregate are partially replaced with Fly ash and Foundry sand by 10%, the addition of bacteria increases 28 days Flexural Strength from 4.78 Mpa to 5.71 Mpa, (ie), percentage increase is 24.67%.
• When Cement, Fine aggregate are partially replaced with Fly ash and Foundry sand by 30%, the addition of bacteria increases 28 days Flexural Strength from 5.1 Mpa to 5.7 Mpa, (ie), percentage increase is 34.75%. The results are shown in Table 5.

Table 5. Flexural Strength of Specimens for M30 28 Days

4.5 Scanning Electronic Microscopy Investigations

The Scanning Electronic Microscopy (SEM) investigation was conducted in University College of Technology (Osmania University) for both normal and bacterial concretes, and the photographs reveal the hydrated cement concrete structure of both concretes. The presence of bacteria and improved hydrated structure is found in bacterial concrete sample when compared with normal concrete. Figures 6-9 show different SEM investigations.

Figure 6. Plane Concrete M30 there is No Split in Structure

Figure 7. M-20 Grade with Partailly Replaced 10% Cement with Fly Ash in (Spiracle Shape) and Partially Replaced 10% Fine Aggregate with Foundry Sand (v shap)

Figure 8. M-20 Grade with Partailly Replaced 30% Cement with Fly Ash (Spiracle Shape) and Partially Replaced 30% Fine Aggregate with Foundry Sand Sand (v shap)

Figure 9. Bacillus Subtilis 10⁵ cell/ml, Calcite Layer is Formed in the form (Crystal Shape) for M-20 Grade

The following conclusions are obtained from the experimental results. Based on the present experimental investigation, the following conclusions are drawn that showed significant increase by 30%.

• 7 days addition of Bacillus Subtilis doesn't affect the workability aspects of concrete or it can be concluded that there is no change in the workability aspects of bacterial concrete when compared to normal concrete.
• The addition of Bacillus Subtilis increases the 7 days compressive strength for M20 grade concrete showed significant increase by 36%.
• The addition of Bacillus Subtilis increases the 28 days compressive strength for M20 grade concrete compressive strength of concrete for M30 Grade concrete with the addition of Bacillus Subtilis. The maximum percentage of increase is 25% when compared to normal concrete.
• By adding Bacillus Subtilis, the 28 days compressive strength of M30 Grade concrete increases by 29.74% (30% addition of both fly ash and foundry sand).
• The additional Bacillus Subtilis improves the flexural strength of concrete. The flexural strength of concrete showed significant increase for M20 & M30 Grade concretes by 30% & 34%, (For 30% replacement of both cements and fine aggregate by fly ash and foundry sand, respectively).
• The addition of Bacillus Subtilis improves the split tensile strength of concrete.
• The split tensile strength of concrete showed significant increase for M-20 and M-30 grades of concrete when cement and fine aggregates are replaced by 30% of fly ash and foundry sand, respectively. The percentage increase in strength for M20 and M30 grades are 33.14% and 34.14%, respectively.