Effect of calcium lactate on compressive strength and self-healing of cracks in microbial concrete

Abstract

This paper presents the effect on compressive strength and self-healing capability of bacterial concrete with the addition of calcium lactate. Compared to normal concrete, bacterial concrete possesses higher durability and engineering concrete properties. The production of calcium carbonate in bacterial concrete is limited to the calcium content in cement. Hence calcium lactate is externally added to be an additional source of calcium in the concrete. The influence of this addition on compressive strength, self-healing capability of cracks is highlighted in this study. The bacterium used in the study is bacillus subtilis and was added to both spore powder form and culture form to the concrete. Bacillus subtilis spore powder of 2 million cfu/g concentration with 0.5% cement was mixed to concrete. Calcium lactates with concentrations of 0.5%, 1.0%, 1.5%, 2.0%, and 2.5% of cement, was added to the concrete mixes to test the effect on properties of concrete. In other samples, cultured bacillus subtilis with a concentration of 1×10⁵ cells/mL was mixed with concrete, to study the effect of bacteria in the cultured form on the properties of concrete. Cubes of 100 mm×100 mm×100 mm were used for the study. These cubes were tested after a curing period of 7, 14, and 28 d. A maximum of 12% increase in compressive strength was observed with the addition of 0.5% of calcium lactate in concrete. Scanning electron microscope and energy dispersive X-ray spectroscopy examination showed the formation of ettringite in pores; calcium silicate hydrates and calcite which made the concrete denser. A statistical technique was applied to analyze the experimental data of the compressive strengths of cementations materials. Response surface methodology was adopted for optimizing the experimental data. The regression equation was yielded by the application of response surface methodology relating response variables to input parameters. This method aids in predicting the experimental results accurately with an acceptable range of error. Findings of this investigation indicated the influence of added calcium lactate in bio-concrete which is quite impressive for improving the compressive strength and self-healing properties of concrete.

Keywords

calcium lactate / bacillus subtilis / compressive strength / self-healing of cracks

Introduction

One of the most extensively used construction material is concrete. Due to the availability of raw materials, compressive strength, durability, and affordability. However, a lot of concrete structures certainly suffer deterioration and degradation in due course of time. This is due to penetration of water into the concrete which has an adverse effect on the efficiency of the concrete [¹]. One of such causes for deteriorations due to the formation of cracks at macro and micro levels which create the path for water ingress, dissolved particles in fluid sand unwanted acidic gasses. As a result, unwanted materials and other substances penetrate into the concrete, thus affecting the reinforcement and durability [²]. Few cracks formed will be at micro level, hence are invisible and difficult to access. The expansion, contraction, and permeation of materials cause an increase in both size and number of cracks. In this concern, the maintenance and inspection techniques for infrastructure are to be given utter importance. But due to economic constraints, continuous inspection and maintenance in case of large-scale infrastructures is difficult. Other factors such as damage location in the affected structure may make the repair challenging. To overcome the above-stated difficulties, a novel repairing methodology using bacteria is being developed [³]. These bacteria should be capable of influencing precipitation of calcium carbonate by producing urease enzyme. This precipitation occurs through heterogeneous nucleation of bacterial cell wall until supersaturation is achieved, the potential of bacteria when acting as a self-healing agent was studied in Ref. [⁴] and was proved to be effective. This was also supported and experimented by many other researchers [⁵–⁸]. It was found that all cracks were effectively healed by the application of isolated bacteria cultures and mixed cultures with fractured concrete, due to the metabolic activity of bacteria which precipitates calcium carbonate [⁹,¹⁰]. Figure 1 shows the image of calcium carbonates precipitation on the bacterial cell wall [¹¹].

In recent past, bacterial culture has been used with concrete for healing cracks using different approaches [¹²–¹⁴]. In Ref. [¹²], bacteria culture has been injected into the concrete surface to initialize self-healing. In an experiment carried out in Ref. [¹³], bacterial culture has been sprayed on the cracked surface of the concrete in a parking garage to study the self-healing capability. Instead of spraying or injecting bacteria on the cracked surface, it was added to the concrete at the time of mixing which showed better performance as reported in Ref. [¹⁴]. The cracks were healed due to the microbial precipitation caused by the urealytic activity of the bacteria. This microbial precipitation depends on pH, the concentration of dissolved inorganic carbon, amount of calcium ion present and nucleation site of bacteria used [¹⁵]. In self-healing concrete, the formation of any cracks leads to activation of bacteria from its stage of hibernation. By the metabolic activities of bacteria, during the process of self-healing, calcium carbonate precipitates into the cracks healing them. Once the cracks are completely filled with calcium carbonate, bacteria return to the stage of hibernation. In future, if any cracks form the bacteria gets activated and fills the cracks [¹⁶].

The calcium content in the cement decides the production of calcium carbonate. Hence calcium source is externally added to the concrete to study its influence on self-healing of cracks and compressive strength. Various types of bacteria are investigated for this purpose. Due to the better efficiency and ease of availability bacillus subtilis bacteria was used in this study. This also has the advantage to survive for a long duration and is also an effective crack healing agent. So far research has been carried out on studying effects of different bacteria into concrete for self-healing capability. The objective of the work mainly deals with studying the effect on concrete by adding calcium lactate along with bacteria. The effect of calcium lactates on compressive strength and cracks healing capability of bacterial concrete are studied, which are important parameters that influences the durability of concrete. In this study, bacteria mixed concrete with different proportions of calcium lactate were considered to analyze the effect of calcium lactate on the compressive strength and self-healing capability of concrete. The bacteria considered were in both spore forms as well in culture form.

Materials and methods

Materials

1) Cement

In this study, ordinary Portland cement is used. It is tested as per Indian standard specifications [¹⁷].

2) Fine and coarse aggregate

The natural sand having the specific gravity of 2.69 and maximum size of 4.75 mm is chosen as fine aggregate and has been tested as per Indian standards [¹⁸]. Crushed stone having 2.7 specific gravity and 20 mm maximum sizes are considered to be coarse aggregate.

3) Calcium lactate

Calcium lactate, which is also known by calcium salt pentahydrate (C₆H₁₀CaO₆) is a white powder with efflorescent odour. This powder is formed by the reaction of lactic acid with calcium hydroxide or calcium carbonate. Calcium lactate used in this research was purchased from Triveni Chemicals, Gujarat India, with the properties listed in Table 1.

4) Microbial sample

Bacillus subtilis spore powder samples are procured from the De Generic Bio-Tech Pvt Ltd, Hyderabad. The same was cultured in the following procedure:

The liquid medium is chosen for culturing the bacteria. This medium consists of 5 g of peptone added to 3 g of meat extract and 5 g of yeast extract per liter of distilled water to which 1.5% of agar was mixed to obtain solid medium per stock culture. Initially, this mixture was sterilized for 20 min at a temperature of 121 °C by autoclaving. This mixture is cooled to reach room temperature. Now, the bacillus subtilis spore powder samples were added to this mixture in a laminar flow chamber. These cultures are now incubated on a shaker incubator at 130 r/min maintaining a temperature of 30 °C for 72 h. Now, these bacterial cells are harvested by the process of separation from the 72-h grown cultures. These cultured samples were tested for identifying the isolated bacteria. The test data are given in Table 2.

Quantification of micro-organism

Optical density test was conducted using spectrophotometer for determining the quantity of culture solution required to mix. This test was conducted in bacteria growing medium which is considered to be blank. This solution is taken to be the reference, for experimentation of optical density of bacterial solution. A blank solution of 0.5 mL is placed in the spectrophotometer at a wavelength of 600 nm. After the machine reads this blank solution, it was replaced by the bacterial solution of 0.5 mL at the same wavelength. On this basis, the concentration of bacteria in the solution is measured using the expression Y = 8.59×10⁷X^1.3627 [¹⁹], where Y is the bacterial concentration/mL and X is the reading at OD600. The bacterial concentration was observed to be 2.8×10⁸ cells/mL using the spectrophotometer. From these results, culture concentrations in samples were maintained equal to 1×10⁵ cells/mL.

Fabrication and testing of concrete

Concrete mix is fabricated according to IS: 10262-2009 standards [²⁰]. Once the concrete is mixed, it is poured into steel moulds of volume 100 mm×100 mm×100 mm and then left to harden for 24 h. The concrete is displaced from the moulds after 24 h of water curing. The self-healing of cracks and compressive strength test was conducted for a maturity of 7, 14 and 28 d. The test was performed for three numbers of samples for obtaining the results. The averages of the test results are presented in Fig. 2. Nine altered mixes were prepared for this study. All the nine mix proportions contain 450 kg/m³ of ordinary Portland cement, 581.47 kg/m³ of fine aggregate, 1119.87 kg/m³ of coarse aggregates with 0.4 water to cement ratio. Mix 1 (M1) does not contain any specimens of bacteria. From Mixs 2 to 6 (M2 to M6) bacteria was added directly by mixing the bacterial spore powder with 2 million cfu/g concentration in concrete, with different proportionate of calcium lactate given in Table 3. In Mixs 7 and 8 (M7 and M8) only bacterial spore powder is added and in Mix 9 (M9) bacteria is added to water with 10⁵ cells/mL concentration and then mixed with concrete.

Results and discussion

Compressive strength

It is observed from Fig. 2 that there has been a considerable improvement in the compressive strength of specimens with calcium lactate and bacteria. There is a maximum of 12% increase in compressive strength with 0.5% calcium lactate and bacteria added to the concrete. It was observed that the increase in the percentage of calcium lactate in the mix results in the reduction of compressive strength of the concrete. The reduction in compressive strength is due to the materials added to the mix which do not take part in the process of hydration with the binder. The bacteria and calcium lactate added to concrete do not involve in the process of hydration directly, instead, the by-product, calcium carbonate helps in filling the pores in concrete and healing of cracks. It is observed from Fig. 2 that with the increase in the concentration of calcium lactate, there is a slight reduction of compressive strength. This shows that excess production of calcium carbonate affects the compressive strength of the concrete.

Scanning electron microscope analysis

Figure 3 shows the scanning electron microscope (SEM) analysis of bacterial and control concrete with 0.5%, 1%, 1.5%, and 2% concentration of calcium lactate. SEM images illustrate the formation of calcium silicate hydrate, calcite and needle type ettringite. Comparing the control and bacterial concrete specimens after 28 d of curing, bacterial concrete with 0.5% of calcium lactate has a compact and dense structure because of CaCO₃ production by bacteria and resulted in the increase in compressive strength than control concrete. Figures 3(d) and 3(e) also confirm that overproduction of calcium carbonate due higher concentration of calcium lactate decreases the compressive strength. Figure 3(f) shows the SEM analysis of bacterial cultured concrete with calcium lactate; this also shows the formation of calcium carbonate in the concrete. Figures 3(b)‒3(f) validates the effectiveness of bacteria added to concrete in both spore form and cultural form for calcium carbonate precipitation.

Energy dispersive X-ray spectroscopy

The quantity of calcium with respect to chemical analysis was examined using energy dispersive X-ray (EDX) instrument for the specimen with and without bacteria. The SEM and EDX analysis of structural concrete specimens without and with bacteria are illustrated from Figs. 3 to 7. The amount of Ca obtained for bacterial concrete with 0.5%, 1%, and 2%, of calcium lactate, was found to be 22%, 24% and 26% respectively using EDX instrument. Whereas calcium obtained for normal concrete is 15% which is less compared to bacterial concrete.

Self-healing of cracks

The 100 mm size cubes were pre-cracked at the age of 28 d by using compression testing machine and kept in water for curing. The most of the cracks were healed after curing of two weeks. From the experiment conducted it was observed that the process and efficiency of healing are quite impressive when the amount of calcium lactate is high. The results are shown in Fig. 8.

Electrical resistivity

The electrical resistance of concrete was measured using a Leader RCON^TM Concrete Electrical Resistivity Meter at one pre-determined location on each test specimens. For performing the test, two cubes for each mix in three conditions are considered. Firstly, at the age of 28 d cured sample, then cracks formed sample by applying constant load on all cubes and cracks healed sample. An average of two test cubes for each test age and type of the concrete was determined. The electrical resistivity with an average value of the electrical resistance is calculated by the following expression:

(1)\rho =RA/l,

where r is electrical resistivity (unit: Ω∙m), R is electrical resistance (unit: Ω), A is cross sectional area (unit: m²), and l is electrical path length (unit: m).

From Fig. 9, it can be concluded that the test cubes have the high electrical resistivity at the age of 28 d and resistivity decreases when cracks are formed. It was observed that resistivity increases with the proportionate increase in the calcium lactate. This increase is due to the presence of bacteria by filling the pores with the help of deposited CaCO₃. The added calcium source behaves as a catalyst to advance deposition of CaCO₃. The increase in precipitation of CaCO₃ by the calcium source in bacterial concrete fills pores within the concrete mix. This increases the electrical resistivity of concrete.

Response surface method

Response surface method (RSM) is a set of statistical methods used to improve, develop and optimize products. It is generally used where more than one factor has the influence on more than one performance characteristics or responses. It is used to optimize responses or to meet desired a set of specifications [²¹].

RSM in this study has been implemented for developing a regression equation to predict the strength of concrete. This allows obtaining the optimum values of the variables considered. In order to use RSM, the percentage of calcium lactate added to the mix, the age of specimen and the compressive strength of the concrete are considered as variables.

Multivariate analysis is carried out using MATLAB program with 95% confidence levels (α = 0.05). The parameter x₁, y₁ and z₁ where x₁ represents the percentage of calcium lactate added, y₁ is the age of specimen, and z₁ is the compressive strength respectively are considered. RSM applied to this study resulted in an empirical relation among compressive strength (z) and percentage of calcium lactate (x) and age of specimen (y) with the regression Eq. (2).

(2)z=a+bx+cy+dxy+e{y}^2,

where coefficients with 95% confidence bounds are a=19.25∈(16.51, 22), b=−3.25∈(−4.371, −2.129), c=2.021∈(1.742, 2.301), d=−0.04592∈(−0.1064, 0.01459), e=−0.03832∈(−0.04537, −0.03127).

The residual error obtained is tabulated as shown Table 4. It is clearly observed that percentage of error is less than 5% which concludes that curve obtained is within 95% confidence level. Figure 10 shows the response surface for regression model which express the compressive strength (z₁) versus the percentage of calcium lactate (x₁) and Age of the specimen (y₁). From Table 4 and Fig. 10, it is evident that the regression model obtained in Eq. (2) is a good fit.

Cross-validation

To decrease the computational complexity, the mechanical response Y is approximated by a polynomial (quadratic or linear) regression model with premise function P_X^T(X) = [X₁X₂… X₁²X₂²…] [²²,²³]. Distinctive techniques are accessible to know the goodness of fit in that coefficient of determination (COD) R² and the residual sum of squares are the most well-known measures for the goodness of fit.

a) Coefficient of determination

A valuable metric for show precision is the COD (R²) [²⁴,²⁵]. Let Ŷ_j be an approximation of the response of the k-parametric mechanical model Y_j. The coefficient of determination may ascertain by utilizing the relation.

(3)R^{\mathrm{2}}=\mathrm{1}-{\displaystyle \frac{{\displaystyle {\sum }_{j=\mathrm{1}}^N}{(Y_j-{\widehat{Y}}_j)}^{\mathrm{2}}}{{\displaystyle {\sum }_{j=\mathrm{1}}^N}{(Y_j-\overline{Y})}^{\mathrm{2}}}},

(4)\overline{Y}={\displaystyle \frac{\mathrm{1}}{N}}{\displaystyle {\sum }_{j=\mathrm{1}}^N}{Y}_j,

where Y_j is the experimental value and Ŷ_j is the predicted value from the regression model.

The value of the COD (R² = 0.9894) obtained confirms the appropriate fit of the model as the discrepancy of total variation is found to be 0.0106% which is an acceptable range of error. Table 5 shows the actual values and the predicted values obtained by regression expression for compressive strength. The large R² values prove that there are no notable discrepancies between the estimated and experimental values.

b) Residual sum of squares

The residual sum of squares is a standout amongst the most well-known measures for the “goodness of fit” of a regression curve to a plot [²⁶,²⁷]. It is also called the summed square of residuals and is usually labelled as RSS. Lower estimations of RSS show better fit which demonstrates that the model has a little arbitrary blunder part and that the fit will be more valuable for prediction.

(5)\mathrm{RSS}={\displaystyle {\sum }_{j=\mathrm{1}}^N}{(Y_j-{\widehat{Y}}_J)}^{\mathrm{2}}.

The value of the residual sum of squares (RSS=3.54) obtained confirms the appropriate fit of the model.

Conclusions

The effect of calcium lactate on bacterial concrete was tested in this study. The bacteria were added to the concrete both in spore form and culture form. The results verified that addition of bacteria in both the forms yields in the precipitation of CaCO₃. The results show that calcium lactate should be added in a lower concentration to the concrete so as to improve the compressive strength. Even though additions of higher concentration of calcium lactate increase in the precipitation of CaCO₃ decreases the compressive strength by a slight percentage. The compressive strength is increased due to plugging of the pores within the concrete cubes due to bacterial CaCO₃ precipitation this was confirmed by SEM and EDX analysis. Electrical resistivity test conforms the rehabilitation of concrete. The experimental data is validated through response surface method, and the error was found to be an acceptable range.

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