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Use of bacteria for repairing cracks and improving properties of concrete containing limestone powder and natural zeolite
Construction and Building Materials 242 (2020) 118059
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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Use of bacteria for repairing cracks and improving properties of concrete containing limestone powder and natural zeolite Maedeh Sadat Jafarnia a, Mehdi Khodadad Saryazdi a,⇑, Seyed Mohammad Moshtaghioun b a b
Department of Civil Engineering, Yazd University, Yazd, Iran Department of Biology, Yazd University, Yazd, Iran
h i g h l i g h t s Cement substitution by natural zeolite increased compressive strength. Substitution of limestone powder decreased the compressive strength unless combined with zeolite. Properties of concrete improved by using bacteria cells in the mixtures. Utilizing bacteria in curing environment is very effective in crack healing.
a r t i c l e
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Article history: Received 27 February 2019 Received in revised form 25 November 2019 Accepted 2 January 2020
Keywords: Concrete Bacteria Limestone powder Natural zeolite Compressive strength Crack healing
a b s t r a c t Precipitation of calcium carbonate crystals caused by the metabolic activities of certain microorganisms is a relatively new method which can improve the properties of concrete and repair concrete cracks. The present study investigated the effects of Sporosarcina pasteurii bacteria on healing cracks, compressive strength, tensile strength, ultrasonic pulse velocity, electrical resistivity and microstructure of concrete containing various percentages of limestone powder and natural zeolite. Experimental results show that the microbial calcite precipitations enhanced compressive strength, tensile strength, ultrasonic pulse velocity and electrical resistivity of all specimens at all ages. The maximum values of these parameters are related to the bacterial specimen containing 10% zeolite without limestone powder. The SEM images of the specimens show that the amount of calcite crystals in the bacterial treated specimen containing limestone powder is more than the specimen without that. In addition, crack healing of the specimen containing limestone powder was also slightly better than the specimen without that. The results of Fourier-Transform Infrared spectroscopy show that the precipitation formed at crack surfaces of specimens is CaCO3. Ó 2020 Elsevier Ltd. All rights reserved.
1. Introduction Because of affordable prices, strength and durability properties, nowadays concrete is one of the most consumed construction materials in the world [1–3]. But the use of concrete also has its own issues. Cracks in concrete are caused by various factors including shrinkage, chemical reaction, tensile loading, differential settlement and thermal gradients. Without appropriate repairs, the cracks will grow and expanded cracks would jeopardize the durability of concrete. Furthermore, repairing expanded cracks costs more [2,4–6]. Nowadays, there are various techniques for repairing ⇑ Corresponding author at: Yazd University, Safaeeyah, Yazd, Iran. E-mail addresses: [email protected] (M.S. Jafarnia), mkhodadad@ yazd.ac.ir (M. Khodadad Saryazdi), [email protected] (S.M. Moshtaghioun). https://doi.org/10.1016/j.conbuildmat.2020.118059 0950-0618/Ó 2020 Elsevier Ltd. All rights reserved.
cracks of concrete. But traditional repair techniques are time consuming and have limitations. In recent years, utilizing microbial calcite precipitation in concrete to repair cracks and improve its properties has attracted attention of many researchers [7–13]. By filling the pores of the concrete matrix and the cracks with calcite precipitations, the possibility for corrosion of the embedded steel reduces. Bacteria are single-celled organisms that display various shapes and sizes. There are incredibly diverse bacterial species. Various bacterial species that contribute to carbonate deposition exist in different natural environments, including soils, oceans and lakes. Ureolytic bacteria are one of the most impressive species of microorganisms in producing large amounts of carbonate. The optimum pH range for ureolytic activity of soil urease bacteria is from about 6.5 to 9 and the high alkaline condition of
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So far, several studies have examined the effects of the use of microbial calcite precipitation in various types of concrete, including fly ash and silica fume concrete [31–33], rice husk ash concrete [34], and lightweight aggregate concrete [35]. But no such work has been reported on the effect of bacteria on repairing cracks and improving properties of concrete containing limestone powder and zeolite as partial replacement to cement. This paper investigates the effect of bacteria on mechanical properties and durability in terms of ultrasonic pulse velocity and electrical resistivity. Also, the crack healing of specimens was evaluated by handheld microscope and the microstructure of the concrete specimens and the cracks filler was investigated by SEM analysis and FourierTransform Infrared spectroscopy.
concrete is not the best pH for urease activity of the bacteria. Sporosarcina pasteurii is a non-pathogenic soil-inhabiting ureolytic bacterium that has been used throughout this study and is not harmful to human health. Sporosarcina pasteurii is a robust species in alkaline environments that can produce endospore in harsh environmental conditions [14–16]. Ureolytic bacteria such as Sporosarcina pasteurii need urea and calcium source to precipitate CaCO3. In this process, during the enzymatic hydrolysis of urea, urea is converted to ammonias and carbonate. If there is a source of calcium, bacteria act as a nucleation site to form calcite precipitation [5,17,18]. Cement industry has important environmental impact on carbon dioxide emission and global warming, since it is responsible for about 5–8% of total anthropogenic carbon dioxide emissions in the world [19–21]. Nowadays, researchers are seeking to reduce the pollution caused by cement productions by reducing cement consumption in concrete. In recent years, according to environmental benefits, Portland limestone cements (PLC) has been considered [22–24]. So that the European standard EN 197-1-2000 has different groups of PLC containing 6–20% and 21–35% limestone powder [25]. The Institute of Standards and Industrial Research of Iran ISIRI 4220-2005 also allows the use of 6–20% limestone powder as cement replacement in PLC [26]. Moreover, nowadays utilizing natural pozzolanic materials has become a common way to improve concrete properties, enhance its strength and decrease its permeability. One of the natural pozzolanic material is zeolite. A large number of studies have been devoted to investigate effects of natural zeolite on the mechanical and durability properties of concrete [27–30].
2. Materials and methods 2.1. Microorganism preparation In this study Sporosarcina pasteurii PTCC 1645 was used in concrete. Microscopic image of this bacterial strain is shown in Fig. 1. Bacteria were cultured in liquid medium containing 5.0 g of peptone and 3.0 g of meat extract per liter of distilled water (pH 7.0). Culture medium was autoclaved for 15 min at 121.5 °C. After inoculation under laminar flow hood the medium culture was incubated at 30 °C on a shaker for 72 h at 150 rpm. After 72 h of incubation the liquid medium was centrifuged at 4000 rpm for 15 min. After re-suspending bacterial cells in water, the optical density of solution was measured by spectrophotometer at 600 nm wavelength and the concentration of bacteria in the suspension was adjusted to about 107 cells/cm3. 2.2. Materials and mixture design Ordinary Portland cement (CEM I-42.5), natural zeolite, limestone powder, fine aggregate (nominal size of 4.75 mm) and coarse aggregate (nominal size of 12.5 mm) were used in this work. Physical properties of aggregates are shown in Table 1 and the chemical properties of cement, natural zeolite and limestone powder which were used in concrete mixes, are shown in Table 2. Two groups of mixtures were considered: one without bacteria (mix ID starts with letter C) and the other with bacteria in mixing water (mix ID starts with letter S). Composition of concrete mixture designs are presented in Table 3. As shown in Table 3, all mixtures were prepared with constant amount of water, binder, fine and coarse aggregate. Control mixtures (C-0-0 and S-0-0) containing only cement (450 kg) as the binder and in other mixtures, various percentages by mass of cement were replaced with limestone powder (first number followed by letter C or S in mix ID: 10, 20 and 30%) and/or natural zeolite (second number in mix ID: 10%). Therefore, water to binder ratio (W/B) of all mixtures is equal to 0.4, but the water to cement ratio (W/C) increases by reducing amount of cement. 2.3. Curing environment 24 h after casting, the specimens were demolded and cured in different solution for at least 27 days. As shown in Table 3, two group of specimens were used in this study and specimens were kept in two different curing environment. The first group which was cured in water, has no bacteria in mixture design. The second group has bacteria in mixing design water and cured in aqueous molar solution of urea and calcium chloride to investigate the effect of bacteria on properties of concrete. The concentrations of calcium chloride and urea used in curing environment were 49 g/L and 20 g/L, respectively.
Fig. 1. Light micrograph of gram-positive rods of Sporosarcina pasteurii PTCC 1645 (Grams stain, 1000X).
2.4. Testing procedure Table 1 Physical properties of aggregates. Material
Specific gravity (SSD)
Water absorption (% dry mass)
Fine aggregate Coarse aggregate
2.62 2.69
1.23 0.96
2.4.1. Compressive strength and tensile strength Compressive strength test was performed on cubic specimens with dimension of 100 mm 100 mm 100 mm in accordance with EN 12390 [33] at ages of 28 and 60 days. Split tensile test (Brazilian test) were conducted as per ASTM C496 [34] on cylindrical specimens with dimension of 100 mm in diameter and 200 mm in height at age of 28 days.
Table 2 The chemical composition of cement, natural zeolite and limestone powder (%).
Ordinary Portland Cement Natural Zeolite Limestone Powder
SiO2
Fe2O3
K2O
AL2O3
CaO
SO3
MgO
Na2O
TiO2
P2O5
Loss on ignition
21.86 68.95 0.32
3.7 0.97 0.1
0.6 0.95 –
4.8 11.14 0.02
64.4 4.83 55.3
2.4 0.07 –
1.2 0.79 0.23
0.55 0.95 –
– – 0.01
– – 0.02
0.55 10.64 43.43
Curing condition
Water Water Water Water Water Water Water Water
Urea-CaCl2 Urea-CaCl2 Urea-CaCl2 Urea-CaCl2 Urea-CaCl2 Urea-CaCl2 Urea-CaCl2 Urea-CaCl2
Bacteria content (cell/ml)
0 0 0 0 0 0 0 0
107 107 107 107 107 107 107 107
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2.4.2. Electrical resistivity Chloride ions can penetrate through the network of concrete pores and cause corrosion of reinforcement in concrete. Studies show that there is a strong correlation between electrical resistivity and chloride ions penetration in different concrete mixtures [37–39]. Electrical resistivity of concrete is a characteristic of concrete, indicating the resistance against the flow of the electrical current which is the result of ions movement in concrete. Electrical resistivity of concrete is the most important parameter to estimate the corrosion of the reinforcements in concrete. In general, these two parameters are inversely related. So that by increasing the electrical resistivity of concrete, the corrosion rate of the reinforcements in concrete decreases [36,38]. So far, several studies have been conducted on the relationship between electrical resistivity and other parameters of concretes [35–37]. In this study electrical resistivity was measured on cylindrical cores, taken from 100 mm cubic specimen, with diameters of 54 mm and heights of 100 mm. To measure the electrical resistivity, the specimens were placed between two copper plates. The test set-up is shown in Fig. 2. The electrical resistivity of the specimens was determined according to Eq. (1).
Coarse aggregate (kg/m3)
q¼
712.3 712.3 712.3 712.3 712.3 712.3 712.3 712.3 1040.6 1040.6 1040.6 1040.6 1040.6 1040.6 1040.6 1040.6 0 0 0 0 10 10 10 10
0 0 0 0 45 45 45 45
180 180 180 180 180 180 180 180
712.3 712.3 712.3 712.3 712.3 712.3 712.3 712.3 1040.6 1040.6 1040.6 1040.6 1040.6 1040.6 1040.6 1040.6 180 180 180 180 180 180 180 180 0 0 0 0 45 45 45 45 0 0 0 0 10 10 10 10
ð1Þ
2.4.3. Ultrasonic pulse velocity Ultrasonic pulse velocity measurement is a non-destructive test that are influenced by microstructure, mechanical properties and concrete quality. As the quality of concrete improves, the speed of ultrasonic waves through concrete specimen increases [27,40]. Cubic specimens with dimension of 100 mm 100 mm 100 mm were used for measuring ultrasonic pulse velocity as per ASTM C597 [41] at ages of 28 and 60 days. Two probes were positioned on two parallel and smooth surfaces of the test specimens. These two probes emit and receive the ultrasound waves and the device records the time required for the ultrasound pulse to pass through the width of specimen. The pulse velocity is computed by dividing the distance to the time measured.
0 10 20 30 0 10 20 30
0 45 90 135 0 45 90 135
2.4.4. Crack healing For measuring crack healing capacities, cylindrical specimens with heights of 200 mm and diameters of 100 mm were used. To induce crack in the specimen, it was loaded similar to splitting tensile test but only half of the height of the cylindrical specimen was placed under loading plates, in this way cracks with different widths were created without splitting the specimen in half. Points were marked along the induced cracks and the crack width at each point was measured by a hand-held microscope with a measurement accuracy of 0.02 mm. After measuring the initial crack width, the specimens were immersed in the curing environment, an aqueous molar solution of urea and calcium chloride. They were taken out of the curing media at different times and the crack width at the marked points were measured. Then they were immersed back into the curing environment. Crack widths of healing specimens were measured after 5, 15, and 30 days of curing. To evaluate the crack-healing capacity of different specimens, the percentage of crack healing was calculated as follows (Eq. (2)).
S-0-0 S-10-0 S-20-0 S-30-0 S-0-10 S-10-10 S-20-10 S-30-10 Second
450 405 360 315 405 360 315 270
0 10 20 30 0 10 20 30 450 405 360 315 405 360 315 270 C -0-0 C-10-0 C-20-0 C-30-0 C-0-10 C-10-10 C-20-10 C-30-10 First
(% by mass)
0 45 90 135 0 45 90 135
(kg/m3)
(% by mass)
(kg/m3)
Fine aggregate (kg/m3) Water (kg/m3) Natural Zeolite As cement substitution Limestone Powder As cement substitution Mixture I.D.
Cement (kg/m3)
VA IL
where q is electrical resistivity, V is potential difference, I is current, A is surface area of specimen and L is height of the concrete specimen.
Group
Table 3 Concrete mix designs.
3
Fig. 2. Electrical resistivity test set-up.
4
Healing percentageð%Þ ¼
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W0 Wt 100 W0
ð2Þ
where W 0 is the initial width of the cracks created and W 1 is the width of the cracks at the time of the measuring. Three mixtures were selected to be investigated. The first one is the bacterial specimen without limestone powder and zeolite (S-0-0). The second one is also a specimen with bacterial in its mixture but containing 20% limestone powder and 10% zeolite (S-20-10). This mixture was selected because it contains less cement but has good compressive strength. Third specimen (S-CE) was similar to first one (0% lime stone powder and 0% zeolite) but instead of using bacterial in the mixture, rather this specimen was cured in the suspension of bacteria and a reactive solution. The concentration of bacteria in curing environment was 107 cell/ml. 2.4.5. Microstructure of specimens To analyze the microstructure of the bacterial and nonbacterial specimens, Scanning electron microscopic (SEM; Vega3 Tescan) analysis at the accelerating voltage of 26 KV was performed on small broken pieces of concrete specimens. Hence, six different specimens (bacterial and nonbacterial specimens without limestone powder and zeolite, and mixtures containing 30% limestone powder and 10% zeolite) were examined with scanning electron microscope. Calcite precipitation of crack filler was also visualized by a field emission Scanning electron microscope (FESEM; Mira3 Tescan, Czech republic) analysis at the accelerating voltage of 15 KV. Moreover, these precipitates were examined by Fourier-transform infrared spectroscopy (FT-IR). Infrared spectroscopy (IR) is used to determine the molecular structure and identification of chemical species. FTIR measurement was performed using an Avatar spectrometer (Thermo, USA).
3. Results and discussion 3.1. Compressive strength The results of the compressive strength are provided in Fig. 3. The compressive strength of nonbacterial concrete containing varying percentages of limestone powder and zeolite was between 44.3 and 64.4 MPa at age of 28 days and between 50 and 65.8 MPa at 60 days. According to the results, compressive strength of concrete mixtures containing limestone powder decreased with increase in limestone powder content up to 30% as cement replacement and concrete mixtures containing 10% zeolite as cement replacement displayed higher compressive strength than specimens containing the same percentage of limestone powder without zeolite. The compressive strength of specimens containing zeolite with increase in limestone powder up to 20% is more than the compressive strength of the control specimen (C-0-0), but the specimen containing 30% limestone powder and 10% zeolite (C-30-10) displayed lower compressive strength than the control specimen (C-0-0). According to the results, the compressive strength of specimens containing varying percentages of limestone powder and zeolite with bacteria cells was between 46.9 MPa and 65.7 MPa at age of 28 days and between 51.5 MPa and 69.8 MPa at 60 days. Use of bacteria cells in the mixtures resulted in increase in
compressive strength of all the mixtures. So that the addition of bacteria to the control specimen enhances the compressive strength about 3.9% at 28 days. The compressive strengths of nonbacterial specimens containing 10%, 20% and 30% limestone powder (C-10-0, C-20-0, C-30-0) are 8.6, 11.8, 17%, lower than the compressive strength of the control specimen (C-0-0) at age of 28 days. While the compressive strengths of bacterial specimens S-10-0, S-20-0 and S-30-0 are 6.4%, 8.3%, 12.2% lower than the compressive strength of control specimen (C-0-0) at the age of 28 days. As stated, using 10% of zeolite as cement replacement enhances the compressive strength of the specimens. So that the compressive strengths of the specimens C-0-10, C-10-10 and C-20-10, are about 20.6% and 7.5% and 0.6% more than the compressive strength of the control specimen at 28 days. These enhancements in compressive strength for the bacterial specimens (S-0-10, S-10-10 and S-20-10) compared to the control specimen are respectively about 23%, 13.2% and 9%. Although the compressive strength of the specimen containing 30% limestone powder and 10% zeolite is about 8.2% less than the compressive strength of the control specimen at 28 days, the use of bacteria in this concrete mixture has reduced this reduction in compressive strength to 4%. The compressive strength of the bacterial specimen containing 20% limestone powder, without zeolite is about 4% more than the compressive strength of non-bacterial specimen containing the same percentage of limestone powder and zeolite at the age of 28 days; and use of bacteria in specimen containing 30% limestone powder enhances the compressive strength about 5.7% at 28 days. While these values are 3.9% for the control specimen.
Fig. 4. Tensile strength of specimens at 28 days.
Fig. 3. Compressive strength of 100 mm cubic specimens; (a) at 28 days, (b) at 60 days.
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Therefore, the effect of using bacteria on increasing the compressive strength of specimens containing 20% and 30% limestone powder is slightly higher than the control specimen. The variations in compressive strength of specimens at 60 days are similar to the observed change at the age of 28 days. 3.2. Split tensile test The test results are illustrated in Fig. 4. According to the results, the trend of tensile strength variations of the specimens is similar
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to the compressive strength changes. The results show that the presence of bacteria in mixture design has increased the tensile strength between 0.37 MPa and 0.77 MPa. The tensile strength of the bacterial specimen containing 10% limestone powder and 10% zeolite (S-10-10) is similar to the tensile strength of the control specimen (C-0-0). It means that by using bacteria in concrete mixture and reducing 20% cement weight, the tensile strength can be similar to that of the control specimen. The tensile strength of the bacterial specimen without limestone powder and zeolite is about 10% higher than the tensile strength of the control specimen.
Fig. 5. Electrical resistivity of specimens; (a) at 28 days, (b) at 60 days.
Fig. 6. Relationship between electrical resistivity and compressive strength of specimens; (a) at 28 days, (b) at 60 days.
Fig. 7. Ultrasonic pulse velocity of specimens; (a) at 28 days, (b) at 60 days.
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Fig. 8. Relationship between ultrasonic pulse velocity and compressive strength of specimens; (a) at 28 days, (b) at 60 days.
In fact, by filling the pores of the concrete matrix with calcite precipitations, the tensile strength of the specimens has increased to some extent compared to similar specimens without any bacteria.
3.3. Electrical resistivity Fig. 5 illustrates the electrical resistivity of all specimens at the ages of 28 and 60 days. According to the results electrical resistivity of specimens containing limestone powder decreased with increase in limestone powder as cement replacement. As the electrical resistivity of specimen containing 30% limestone powder is about 39.7% and 38.4% less than the electrical resistivity of the control specimen at ages of 28 and 60 days, respectively. While replacing 10% of cement weight with zeolite, has led to a significant increase in the electrical resistivity of the specimens. So that the electrical resistivity of the non-bacterial specimen containing 10% zeolite (C-0-10) is 53% higher than the electrical resistivity of the control specimen (C-0-0) at age of 28 days. According to the results, the electrical resistivity of the mixtures containing 20% and 30% limestone powder with 10% zeolite (C-20-10 and C-30-10) is approximately equal to the electrical resistivity of the control specimen (C-0-0). Thus the negative effect of cement substitution with lime stone powder (i.e. increasing W/C) can be compensated by adding 10% zeolite to the mix as well. According to the results, filling the pores by microbial calcite precipitation increases the electrical resistance of all the specimens. The bacterial specimen containing 10% zeolite (S-0-10), displayed highest electrical resistance (180.5 X.m and 208.7 X.m respectively at 28 and 60 days) among all specimens. Use of bacteria has increased the electrical resistivity of the control specimen by 6.7% at 28 days. Also, the electrical resistivity of bacterial specimens containing 10%, 20% and 30% are about 11.4%, 11.6% and 18%, respectively, more than nonbacterial specimens of similar mixtures at 28 days. This increment for the specimen containing 10% zeolite is about 7.5% and for the specimens containing 10% zeolite with 10%, 20% and 30% limestone powder are about 13%, 21% and 17.5% respectively at 28 days. This increment for both of the specimens without limestone powder and zeolite and without limestone powder, containing 10% zeolite is 6.3% at 60 days. While this increment is higher for the specimens containing limestone powder. So that the bacterial specimen containing 30% limestone powder has the highest increase and this amount is 14% for this specimen. This indicates that the use of bacteria in the specimens containing limestone powder has been more effective in increasing the electrical resistivity. Fig. 6 demonstrates the relationship between electrical resistivity and compressive strength of bacterial and non-bacterial
specimens made with different percentages of limestone powder and zeolite. The high value of coefficient R2 indicates a good relationship between these two parameters. 3.4. Ultrasonic pulse velocity Ultrasonic pulse velocity of specimens is shown in Fig. 7. According to the results, the ultrasonic pulse velocity of the bacterial specimen without zeolite and limestone powder (S-0-0) is about 0.98% and 0.93% more than the ultrasonic pulse velocity of the control specimen (C-0-0) respectively, at the ages of 28 and 60 days. The use of zeolite also increases the ultrasonic pulse velocity of the specimens. So that the ultrasonic pulse velocity of the nonbacterial specimen containing 10% zeolite (C-0-10) is about 2.1% and 0.37% more than the ultrasonic pulse velocity of the control specimen (C-0-0) respectively, at 28 and 60 days. The use of limestone powder as cement replacement has reduced the ultrasonic pulse velocity of specimens. The lowest value of ultrasonic pulse velocity is related to the specimen containing 30% limestone powder (C-30-0). So that the ultrasonic pulse velocity of this specimen is about 7% and 10.2% less than the ultrasonic pules velocity of the control specimen at 28 and 60 days. The highest rate of the ultrasonic pulse velocity is related to the bacterial specimen containing 10% zeolite (S-0-10). The ultrasonic pulse velocity of this specimen is about 3.7% and 2% more than the ultrasonic pules velocity of the control specimen respectively at 28 and 60 days.
Fig. 9. Relationship between ultrasonic pulse velocity and tensile strength of specimens at 28 days.
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Fig. 10. SEM images of (a) C-0-0; (b) S-0-0; (c) C-0-10; (d) S-0-10; (e) C-30-0, (f) S-30-0.
The ultrasonic pulse velocity of the nonbacterial specimen containing 10% zeolite and 10% limestone powder (C-10-10) is 0.98% less than the ultrasonic pulse velocity of the control specimen (C-0-0) at 28 days. While, adding the bacteria to this concrete mixture
has caused the ultrasonic pulse velocity to be 0.98% higher than the control specimen. The relationships between compressive strength and ultrasonic pulse velocity of bacterial specimens and non-bacterial specimens
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Fig. 11. Images of crack healing processes of specimens after different repair time, (a) bacterial specimen without limestone powder and zeolite (S-0-0); (b) bacterial specimen contains 20% limestone powder and 10% zeolite (S-20-10); (c) crack healing by using bacteria in curing environment (S-CE).
at the age of 28 and 60 days are presented in Fig. 8. The relationship between tensile strength and ultrasonic pulse velocity of specimens is illustrated Fig. 9. 3.5. Microstructure of concrete specimens Fig. 10a, c, e refer to the non-bacterial mixtures (without limestone powder and zeolite (C-0-0), containing 30% limestone powder (C-30-0) and containing 10% zeolite (C-0-10)) and the images of bacterial specimens (without limestone powder and zeolite (S-0-0), containing 30% limestone powder (S-30-0) and containing 10% zeolite (S-0-10)) are presented in Fig. 10b, d, f. The images of the specimens indicate the presence of calcite precipitation in bacterial specimens. It is obvious that the voids of the bacterial specimens are filled by calcite precipitations. Fig. 10a relates to the control specimen without limestone powder and zeolite (C-0-0). The existence of voids in the structure of this specimen is clearly evident. As shown in Fig. 10b, calcite precipitations created by bacteria have filled up the voids. Fig. 10c, d shows the bacterial and nonbacterial specimen containing 10% zeolite. The presence of a bacteria in the mixture design containing 10% zeolite
(S-0-10) provides dense structure than the nonbacterial specimen with the same mixture design (C-0-10). The SEM image of the specimen containing 30% limestone powder (Fig. 10e, f) revealed that the amount of calcite crystals in the bacterial treated specimen containing limestone powder (S-30-0) is more than the amount of crystals formed in the bacterial specimen without limestone powder (S-0-0). When the bacteria were exposed to limestone powder, they had better activities in order to produce calcite precipitations. Fig. 11 illustrates direct observation of cracks healing of specimens. As can be seen in this figure, compared with the specimens that bacteria were used in their mixture design, the amount of sediments in the cracks of the specimen cured in the suspension of bacteria and the reactive solution are much higher. Fig. 12 shows the microscopic images of crack healing processes of three different specimens. Fig. 12a refers to the crack healing of the bacterial specimen without limestone powder and zeolite (S-0-0) with a width of 0.28 mm that was nearly fully healed after 30 days and Fig. 12b shows the crack healing of the bacterial specimen containing zeolite and lime stone powder (S-20-10) with a width of 0.34 mm. According to the images, after 28 days of healing, this
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Fig. 12. Microscopic image of healing the cracks of different specimens with different widths during the time, (a) bacterial specimen without limestone powder and zeolite (S-0-0); (b) bacterial specimen contains 20% limestone powder and 10% zeolite (S-20-10); (c, d) crack healing by using bacteria in curing environment (S-CE).
crack was completely healed. In Fig. 12c and d a crack treated with using bacteria in curing environment is shown (S-CE). As it can be seen in Fig. 12c, completely healing of the crack with the width of 0.4 mm was at earlier healing time (5 days), while the crack with a width of 1.1 mm was only partially healed after 28 days (Fig. 12d). Fig. 13 illustrates the crack healing percentage of specimens at different healing times. As seen from this figure, the presence of bacteria in curing environment (S-CE) has the best healing results
among the other specimens. It can be seen that the trend line of the crack healing of the specimen containing limestone powder and zeolite is slightly higher than the specimen without limestone powder and zeolite. So, apparently the cracks of the specimen contains 20% limestone powder and 10% zeolite (S-20-10) seem to be healed better than the specimen without limestone and zeolite (S-0-0). As shown in Figs. 14, 15 all the cracks up to 0.58 mm were completely healed at 30 days, when the bacteria have been used in
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Fig. 13. Crack healing percentage of different specimens as a function of the initial crack width after curing time of (a) 5 days (b) 15 days and (c) 30 days.
Fig. 14. Maximum width of 100% healed crack.
curing environment and complete crack healing for the widths up to 1.1 mm were achieved for this specimen at 30 days. This values are respectively 0.16 and 0.42 for both of the specimens that bacteria have been used in their mixture design (S-0-0 and S-20-10). 3.6. Microstructure of cracks filler The carbonates have three to four intense bands in Infrared spectroscopy (IR) region. The 1420 cm1 and 876 cm1 region
Fig. 15. Maximum width of healed crack.
bands are characteristic for identification of carbonates and the 711 cm1 region band is characteristic of calcite among various carbonate rocks [36]. The results of Fourier-Transform Infrared spectroscopy of the precipitation at cracks surface show in Fig. 16. Existence of peaks at 1423 and 874 induced by the presence of carbonate and the 710 cm1 region band shows that the precipitation at cracks surface is calcite which matched with the mineralization by bacteria. The SEM image of the precipitation created on the surface of the cracks is presented in Fig. 17.
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Fig. 16. Details of the Fourier-Transform Infrared spectroscopy of the precipitation formed at cracks surface.
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53% and 31% higher than the electrical resistivity of the control specimen at 28 and 60 days, respectively. This values for the bacterial specimen containing 10% zeolite improves to values of 64% and 39%. According to the SEM images, the use of limestone powder has a positive effect on the formation of calcite crystals in the microstructure of the bacterial specimens. This effect is also reflected in the average percentage of crack healing. As the average percentage of crack healing of the specimen containing limestone powder is slightly higher than the specimen without limestone powder. The percentage of crack healing by using the bacteria in curing environment is significantly higher than the percentage of crack healing by using the bacteria in concrete mixture design. All the cracks up to 0.58 mm wide were completely healed at 30 days by using bacteria in curing environment, while if bacteria are used in concrete mixture only cracks with up to 0.16 mm width were completely healed. From the findings, considering a curing aqua containing bacteria for applying to the cracked surfaces of a floor slab is suggested. Using bacteria in the wet curing environment is very promising method for healing cracks. CRediT authorship contribution statement Maedeh Sadat Jafarnia: Investigation, Writing - original draft. Mehdi Khodadad Saryazdi: Project administration, Validation, Writing - review & editing. Seyed Mohammad Moshtaghioun: Writing - review & editing. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. References
Fig. 17. Details of the SEM image of the precipitation formed at cracks surface.
4. Conclusions Calcite precipitations in bacterial specimens increased compressive strength, tensile strength, ultrasonic pulse velocity and electrical resistivity of all specimens. Compressive strength, tensile strength, ultrasonic pulse velocity and electrical resistivity of all specimens decreased by increasing the percentage of limestone powder replacement of cement. But this decrease for specimens containing bacteria is lower than nonbacterial specimens. Partial replacement of cement with zeolite significantly increased compressive strength and electrical resistivity of the specimens. For example, when 10% zeolite is used, the 28days compressive strength of the non-bacterial specimen increased by 20% and that of bacterial specimen increased by 23%. These values for 60-days specimen without bacteria was 11% and with bacteria was 18%. Furthermore, the electrical resistivity of the specimen containing 10% zeolite is about by
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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Use of bacteria for repairing cracks and improving properties of concrete containing limestone powder and natural zeolite Maedeh Sadat Jafarnia a, Mehdi Khodadad Saryazdi a,⇑, Seyed Mohammad Moshtaghioun b a b
Department of Civil Engineering, Yazd University, Yazd, Iran Department of Biology, Yazd University, Yazd, Iran
h i g h l i g h t s Cement substitution by natural zeolite increased compressive strength. Substitution of limestone powder decreased the compressive strength unless combined with zeolite. Properties of concrete improved by using bacteria cells in the mixtures. Utilizing bacteria in curing environment is very effective in crack healing.
a r t i c l e
i n f o
Article history: Received 27 February 2019 Received in revised form 25 November 2019 Accepted 2 January 2020
Keywords: Concrete Bacteria Limestone powder Natural zeolite Compressive strength Crack healing
a b s t r a c t Precipitation of calcium carbonate crystals caused by the metabolic activities of certain microorganisms is a relatively new method which can improve the properties of concrete and repair concrete cracks. The present study investigated the effects of Sporosarcina pasteurii bacteria on healing cracks, compressive strength, tensile strength, ultrasonic pulse velocity, electrical resistivity and microstructure of concrete containing various percentages of limestone powder and natural zeolite. Experimental results show that the microbial calcite precipitations enhanced compressive strength, tensile strength, ultrasonic pulse velocity and electrical resistivity of all specimens at all ages. The maximum values of these parameters are related to the bacterial specimen containing 10% zeolite without limestone powder. The SEM images of the specimens show that the amount of calcite crystals in the bacterial treated specimen containing limestone powder is more than the specimen without that. In addition, crack healing of the specimen containing limestone powder was also slightly better than the specimen without that. The results of Fourier-Transform Infrared spectroscopy show that the precipitation formed at crack surfaces of specimens is CaCO3. Ó 2020 Elsevier Ltd. All rights reserved.
1. Introduction Because of affordable prices, strength and durability properties, nowadays concrete is one of the most consumed construction materials in the world [1–3]. But the use of concrete also has its own issues. Cracks in concrete are caused by various factors including shrinkage, chemical reaction, tensile loading, differential settlement and thermal gradients. Without appropriate repairs, the cracks will grow and expanded cracks would jeopardize the durability of concrete. Furthermore, repairing expanded cracks costs more [2,4–6]. Nowadays, there are various techniques for repairing ⇑ Corresponding author at: Yazd University, Safaeeyah, Yazd, Iran. E-mail addresses: [email protected] (M.S. Jafarnia), mkhodadad@ yazd.ac.ir (M. Khodadad Saryazdi), [email protected] (S.M. Moshtaghioun). https://doi.org/10.1016/j.conbuildmat.2020.118059 0950-0618/Ó 2020 Elsevier Ltd. All rights reserved.
cracks of concrete. But traditional repair techniques are time consuming and have limitations. In recent years, utilizing microbial calcite precipitation in concrete to repair cracks and improve its properties has attracted attention of many researchers [7–13]. By filling the pores of the concrete matrix and the cracks with calcite precipitations, the possibility for corrosion of the embedded steel reduces. Bacteria are single-celled organisms that display various shapes and sizes. There are incredibly diverse bacterial species. Various bacterial species that contribute to carbonate deposition exist in different natural environments, including soils, oceans and lakes. Ureolytic bacteria are one of the most impressive species of microorganisms in producing large amounts of carbonate. The optimum pH range for ureolytic activity of soil urease bacteria is from about 6.5 to 9 and the high alkaline condition of
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So far, several studies have examined the effects of the use of microbial calcite precipitation in various types of concrete, including fly ash and silica fume concrete [31–33], rice husk ash concrete [34], and lightweight aggregate concrete [35]. But no such work has been reported on the effect of bacteria on repairing cracks and improving properties of concrete containing limestone powder and zeolite as partial replacement to cement. This paper investigates the effect of bacteria on mechanical properties and durability in terms of ultrasonic pulse velocity and electrical resistivity. Also, the crack healing of specimens was evaluated by handheld microscope and the microstructure of the concrete specimens and the cracks filler was investigated by SEM analysis and FourierTransform Infrared spectroscopy.
concrete is not the best pH for urease activity of the bacteria. Sporosarcina pasteurii is a non-pathogenic soil-inhabiting ureolytic bacterium that has been used throughout this study and is not harmful to human health. Sporosarcina pasteurii is a robust species in alkaline environments that can produce endospore in harsh environmental conditions [14–16]. Ureolytic bacteria such as Sporosarcina pasteurii need urea and calcium source to precipitate CaCO3. In this process, during the enzymatic hydrolysis of urea, urea is converted to ammonias and carbonate. If there is a source of calcium, bacteria act as a nucleation site to form calcite precipitation [5,17,18]. Cement industry has important environmental impact on carbon dioxide emission and global warming, since it is responsible for about 5–8% of total anthropogenic carbon dioxide emissions in the world [19–21]. Nowadays, researchers are seeking to reduce the pollution caused by cement productions by reducing cement consumption in concrete. In recent years, according to environmental benefits, Portland limestone cements (PLC) has been considered [22–24]. So that the European standard EN 197-1-2000 has different groups of PLC containing 6–20% and 21–35% limestone powder [25]. The Institute of Standards and Industrial Research of Iran ISIRI 4220-2005 also allows the use of 6–20% limestone powder as cement replacement in PLC [26]. Moreover, nowadays utilizing natural pozzolanic materials has become a common way to improve concrete properties, enhance its strength and decrease its permeability. One of the natural pozzolanic material is zeolite. A large number of studies have been devoted to investigate effects of natural zeolite on the mechanical and durability properties of concrete [27–30].
2. Materials and methods 2.1. Microorganism preparation In this study Sporosarcina pasteurii PTCC 1645 was used in concrete. Microscopic image of this bacterial strain is shown in Fig. 1. Bacteria were cultured in liquid medium containing 5.0 g of peptone and 3.0 g of meat extract per liter of distilled water (pH 7.0). Culture medium was autoclaved for 15 min at 121.5 °C. After inoculation under laminar flow hood the medium culture was incubated at 30 °C on a shaker for 72 h at 150 rpm. After 72 h of incubation the liquid medium was centrifuged at 4000 rpm for 15 min. After re-suspending bacterial cells in water, the optical density of solution was measured by spectrophotometer at 600 nm wavelength and the concentration of bacteria in the suspension was adjusted to about 107 cells/cm3. 2.2. Materials and mixture design Ordinary Portland cement (CEM I-42.5), natural zeolite, limestone powder, fine aggregate (nominal size of 4.75 mm) and coarse aggregate (nominal size of 12.5 mm) were used in this work. Physical properties of aggregates are shown in Table 1 and the chemical properties of cement, natural zeolite and limestone powder which were used in concrete mixes, are shown in Table 2. Two groups of mixtures were considered: one without bacteria (mix ID starts with letter C) and the other with bacteria in mixing water (mix ID starts with letter S). Composition of concrete mixture designs are presented in Table 3. As shown in Table 3, all mixtures were prepared with constant amount of water, binder, fine and coarse aggregate. Control mixtures (C-0-0 and S-0-0) containing only cement (450 kg) as the binder and in other mixtures, various percentages by mass of cement were replaced with limestone powder (first number followed by letter C or S in mix ID: 10, 20 and 30%) and/or natural zeolite (second number in mix ID: 10%). Therefore, water to binder ratio (W/B) of all mixtures is equal to 0.4, but the water to cement ratio (W/C) increases by reducing amount of cement. 2.3. Curing environment 24 h after casting, the specimens were demolded and cured in different solution for at least 27 days. As shown in Table 3, two group of specimens were used in this study and specimens were kept in two different curing environment. The first group which was cured in water, has no bacteria in mixture design. The second group has bacteria in mixing design water and cured in aqueous molar solution of urea and calcium chloride to investigate the effect of bacteria on properties of concrete. The concentrations of calcium chloride and urea used in curing environment were 49 g/L and 20 g/L, respectively.
Fig. 1. Light micrograph of gram-positive rods of Sporosarcina pasteurii PTCC 1645 (Grams stain, 1000X).
2.4. Testing procedure Table 1 Physical properties of aggregates. Material
Specific gravity (SSD)
Water absorption (% dry mass)
Fine aggregate Coarse aggregate
2.62 2.69
1.23 0.96
2.4.1. Compressive strength and tensile strength Compressive strength test was performed on cubic specimens with dimension of 100 mm 100 mm 100 mm in accordance with EN 12390 [33] at ages of 28 and 60 days. Split tensile test (Brazilian test) were conducted as per ASTM C496 [34] on cylindrical specimens with dimension of 100 mm in diameter and 200 mm in height at age of 28 days.
Table 2 The chemical composition of cement, natural zeolite and limestone powder (%).
Ordinary Portland Cement Natural Zeolite Limestone Powder
SiO2
Fe2O3
K2O
AL2O3
CaO
SO3
MgO
Na2O
TiO2
P2O5
Loss on ignition
21.86 68.95 0.32
3.7 0.97 0.1
0.6 0.95 –
4.8 11.14 0.02
64.4 4.83 55.3
2.4 0.07 –
1.2 0.79 0.23
0.55 0.95 –
– – 0.01
– – 0.02
0.55 10.64 43.43
Curing condition
Water Water Water Water Water Water Water Water
Urea-CaCl2 Urea-CaCl2 Urea-CaCl2 Urea-CaCl2 Urea-CaCl2 Urea-CaCl2 Urea-CaCl2 Urea-CaCl2
Bacteria content (cell/ml)
0 0 0 0 0 0 0 0
107 107 107 107 107 107 107 107
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2.4.2. Electrical resistivity Chloride ions can penetrate through the network of concrete pores and cause corrosion of reinforcement in concrete. Studies show that there is a strong correlation between electrical resistivity and chloride ions penetration in different concrete mixtures [37–39]. Electrical resistivity of concrete is a characteristic of concrete, indicating the resistance against the flow of the electrical current which is the result of ions movement in concrete. Electrical resistivity of concrete is the most important parameter to estimate the corrosion of the reinforcements in concrete. In general, these two parameters are inversely related. So that by increasing the electrical resistivity of concrete, the corrosion rate of the reinforcements in concrete decreases [36,38]. So far, several studies have been conducted on the relationship between electrical resistivity and other parameters of concretes [35–37]. In this study electrical resistivity was measured on cylindrical cores, taken from 100 mm cubic specimen, with diameters of 54 mm and heights of 100 mm. To measure the electrical resistivity, the specimens were placed between two copper plates. The test set-up is shown in Fig. 2. The electrical resistivity of the specimens was determined according to Eq. (1).
Coarse aggregate (kg/m3)
q¼
712.3 712.3 712.3 712.3 712.3 712.3 712.3 712.3 1040.6 1040.6 1040.6 1040.6 1040.6 1040.6 1040.6 1040.6 0 0 0 0 10 10 10 10
0 0 0 0 45 45 45 45
180 180 180 180 180 180 180 180
712.3 712.3 712.3 712.3 712.3 712.3 712.3 712.3 1040.6 1040.6 1040.6 1040.6 1040.6 1040.6 1040.6 1040.6 180 180 180 180 180 180 180 180 0 0 0 0 45 45 45 45 0 0 0 0 10 10 10 10
ð1Þ
2.4.3. Ultrasonic pulse velocity Ultrasonic pulse velocity measurement is a non-destructive test that are influenced by microstructure, mechanical properties and concrete quality. As the quality of concrete improves, the speed of ultrasonic waves through concrete specimen increases [27,40]. Cubic specimens with dimension of 100 mm 100 mm 100 mm were used for measuring ultrasonic pulse velocity as per ASTM C597 [41] at ages of 28 and 60 days. Two probes were positioned on two parallel and smooth surfaces of the test specimens. These two probes emit and receive the ultrasound waves and the device records the time required for the ultrasound pulse to pass through the width of specimen. The pulse velocity is computed by dividing the distance to the time measured.
0 10 20 30 0 10 20 30
0 45 90 135 0 45 90 135
2.4.4. Crack healing For measuring crack healing capacities, cylindrical specimens with heights of 200 mm and diameters of 100 mm were used. To induce crack in the specimen, it was loaded similar to splitting tensile test but only half of the height of the cylindrical specimen was placed under loading plates, in this way cracks with different widths were created without splitting the specimen in half. Points were marked along the induced cracks and the crack width at each point was measured by a hand-held microscope with a measurement accuracy of 0.02 mm. After measuring the initial crack width, the specimens were immersed in the curing environment, an aqueous molar solution of urea and calcium chloride. They were taken out of the curing media at different times and the crack width at the marked points were measured. Then they were immersed back into the curing environment. Crack widths of healing specimens were measured after 5, 15, and 30 days of curing. To evaluate the crack-healing capacity of different specimens, the percentage of crack healing was calculated as follows (Eq. (2)).
S-0-0 S-10-0 S-20-0 S-30-0 S-0-10 S-10-10 S-20-10 S-30-10 Second
450 405 360 315 405 360 315 270
0 10 20 30 0 10 20 30 450 405 360 315 405 360 315 270 C -0-0 C-10-0 C-20-0 C-30-0 C-0-10 C-10-10 C-20-10 C-30-10 First
(% by mass)
0 45 90 135 0 45 90 135
(kg/m3)
(% by mass)
(kg/m3)
Fine aggregate (kg/m3) Water (kg/m3) Natural Zeolite As cement substitution Limestone Powder As cement substitution Mixture I.D.
Cement (kg/m3)
VA IL
where q is electrical resistivity, V is potential difference, I is current, A is surface area of specimen and L is height of the concrete specimen.
Group
Table 3 Concrete mix designs.
3
Fig. 2. Electrical resistivity test set-up.
4
Healing percentageð%Þ ¼
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W0 Wt 100 W0
ð2Þ
where W 0 is the initial width of the cracks created and W 1 is the width of the cracks at the time of the measuring. Three mixtures were selected to be investigated. The first one is the bacterial specimen without limestone powder and zeolite (S-0-0). The second one is also a specimen with bacterial in its mixture but containing 20% limestone powder and 10% zeolite (S-20-10). This mixture was selected because it contains less cement but has good compressive strength. Third specimen (S-CE) was similar to first one (0% lime stone powder and 0% zeolite) but instead of using bacterial in the mixture, rather this specimen was cured in the suspension of bacteria and a reactive solution. The concentration of bacteria in curing environment was 107 cell/ml. 2.4.5. Microstructure of specimens To analyze the microstructure of the bacterial and nonbacterial specimens, Scanning electron microscopic (SEM; Vega3 Tescan) analysis at the accelerating voltage of 26 KV was performed on small broken pieces of concrete specimens. Hence, six different specimens (bacterial and nonbacterial specimens without limestone powder and zeolite, and mixtures containing 30% limestone powder and 10% zeolite) were examined with scanning electron microscope. Calcite precipitation of crack filler was also visualized by a field emission Scanning electron microscope (FESEM; Mira3 Tescan, Czech republic) analysis at the accelerating voltage of 15 KV. Moreover, these precipitates were examined by Fourier-transform infrared spectroscopy (FT-IR). Infrared spectroscopy (IR) is used to determine the molecular structure and identification of chemical species. FTIR measurement was performed using an Avatar spectrometer (Thermo, USA).
3. Results and discussion 3.1. Compressive strength The results of the compressive strength are provided in Fig. 3. The compressive strength of nonbacterial concrete containing varying percentages of limestone powder and zeolite was between 44.3 and 64.4 MPa at age of 28 days and between 50 and 65.8 MPa at 60 days. According to the results, compressive strength of concrete mixtures containing limestone powder decreased with increase in limestone powder content up to 30% as cement replacement and concrete mixtures containing 10% zeolite as cement replacement displayed higher compressive strength than specimens containing the same percentage of limestone powder without zeolite. The compressive strength of specimens containing zeolite with increase in limestone powder up to 20% is more than the compressive strength of the control specimen (C-0-0), but the specimen containing 30% limestone powder and 10% zeolite (C-30-10) displayed lower compressive strength than the control specimen (C-0-0). According to the results, the compressive strength of specimens containing varying percentages of limestone powder and zeolite with bacteria cells was between 46.9 MPa and 65.7 MPa at age of 28 days and between 51.5 MPa and 69.8 MPa at 60 days. Use of bacteria cells in the mixtures resulted in increase in
compressive strength of all the mixtures. So that the addition of bacteria to the control specimen enhances the compressive strength about 3.9% at 28 days. The compressive strengths of nonbacterial specimens containing 10%, 20% and 30% limestone powder (C-10-0, C-20-0, C-30-0) are 8.6, 11.8, 17%, lower than the compressive strength of the control specimen (C-0-0) at age of 28 days. While the compressive strengths of bacterial specimens S-10-0, S-20-0 and S-30-0 are 6.4%, 8.3%, 12.2% lower than the compressive strength of control specimen (C-0-0) at the age of 28 days. As stated, using 10% of zeolite as cement replacement enhances the compressive strength of the specimens. So that the compressive strengths of the specimens C-0-10, C-10-10 and C-20-10, are about 20.6% and 7.5% and 0.6% more than the compressive strength of the control specimen at 28 days. These enhancements in compressive strength for the bacterial specimens (S-0-10, S-10-10 and S-20-10) compared to the control specimen are respectively about 23%, 13.2% and 9%. Although the compressive strength of the specimen containing 30% limestone powder and 10% zeolite is about 8.2% less than the compressive strength of the control specimen at 28 days, the use of bacteria in this concrete mixture has reduced this reduction in compressive strength to 4%. The compressive strength of the bacterial specimen containing 20% limestone powder, without zeolite is about 4% more than the compressive strength of non-bacterial specimen containing the same percentage of limestone powder and zeolite at the age of 28 days; and use of bacteria in specimen containing 30% limestone powder enhances the compressive strength about 5.7% at 28 days. While these values are 3.9% for the control specimen.
Fig. 4. Tensile strength of specimens at 28 days.
Fig. 3. Compressive strength of 100 mm cubic specimens; (a) at 28 days, (b) at 60 days.
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Therefore, the effect of using bacteria on increasing the compressive strength of specimens containing 20% and 30% limestone powder is slightly higher than the control specimen. The variations in compressive strength of specimens at 60 days are similar to the observed change at the age of 28 days. 3.2. Split tensile test The test results are illustrated in Fig. 4. According to the results, the trend of tensile strength variations of the specimens is similar
5
to the compressive strength changes. The results show that the presence of bacteria in mixture design has increased the tensile strength between 0.37 MPa and 0.77 MPa. The tensile strength of the bacterial specimen containing 10% limestone powder and 10% zeolite (S-10-10) is similar to the tensile strength of the control specimen (C-0-0). It means that by using bacteria in concrete mixture and reducing 20% cement weight, the tensile strength can be similar to that of the control specimen. The tensile strength of the bacterial specimen without limestone powder and zeolite is about 10% higher than the tensile strength of the control specimen.
Fig. 5. Electrical resistivity of specimens; (a) at 28 days, (b) at 60 days.
Fig. 6. Relationship between electrical resistivity and compressive strength of specimens; (a) at 28 days, (b) at 60 days.
Fig. 7. Ultrasonic pulse velocity of specimens; (a) at 28 days, (b) at 60 days.
6
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Fig. 8. Relationship between ultrasonic pulse velocity and compressive strength of specimens; (a) at 28 days, (b) at 60 days.
In fact, by filling the pores of the concrete matrix with calcite precipitations, the tensile strength of the specimens has increased to some extent compared to similar specimens without any bacteria.
3.3. Electrical resistivity Fig. 5 illustrates the electrical resistivity of all specimens at the ages of 28 and 60 days. According to the results electrical resistivity of specimens containing limestone powder decreased with increase in limestone powder as cement replacement. As the electrical resistivity of specimen containing 30% limestone powder is about 39.7% and 38.4% less than the electrical resistivity of the control specimen at ages of 28 and 60 days, respectively. While replacing 10% of cement weight with zeolite, has led to a significant increase in the electrical resistivity of the specimens. So that the electrical resistivity of the non-bacterial specimen containing 10% zeolite (C-0-10) is 53% higher than the electrical resistivity of the control specimen (C-0-0) at age of 28 days. According to the results, the electrical resistivity of the mixtures containing 20% and 30% limestone powder with 10% zeolite (C-20-10 and C-30-10) is approximately equal to the electrical resistivity of the control specimen (C-0-0). Thus the negative effect of cement substitution with lime stone powder (i.e. increasing W/C) can be compensated by adding 10% zeolite to the mix as well. According to the results, filling the pores by microbial calcite precipitation increases the electrical resistance of all the specimens. The bacterial specimen containing 10% zeolite (S-0-10), displayed highest electrical resistance (180.5 X.m and 208.7 X.m respectively at 28 and 60 days) among all specimens. Use of bacteria has increased the electrical resistivity of the control specimen by 6.7% at 28 days. Also, the electrical resistivity of bacterial specimens containing 10%, 20% and 30% are about 11.4%, 11.6% and 18%, respectively, more than nonbacterial specimens of similar mixtures at 28 days. This increment for the specimen containing 10% zeolite is about 7.5% and for the specimens containing 10% zeolite with 10%, 20% and 30% limestone powder are about 13%, 21% and 17.5% respectively at 28 days. This increment for both of the specimens without limestone powder and zeolite and without limestone powder, containing 10% zeolite is 6.3% at 60 days. While this increment is higher for the specimens containing limestone powder. So that the bacterial specimen containing 30% limestone powder has the highest increase and this amount is 14% for this specimen. This indicates that the use of bacteria in the specimens containing limestone powder has been more effective in increasing the electrical resistivity. Fig. 6 demonstrates the relationship between electrical resistivity and compressive strength of bacterial and non-bacterial
specimens made with different percentages of limestone powder and zeolite. The high value of coefficient R2 indicates a good relationship between these two parameters. 3.4. Ultrasonic pulse velocity Ultrasonic pulse velocity of specimens is shown in Fig. 7. According to the results, the ultrasonic pulse velocity of the bacterial specimen without zeolite and limestone powder (S-0-0) is about 0.98% and 0.93% more than the ultrasonic pulse velocity of the control specimen (C-0-0) respectively, at the ages of 28 and 60 days. The use of zeolite also increases the ultrasonic pulse velocity of the specimens. So that the ultrasonic pulse velocity of the nonbacterial specimen containing 10% zeolite (C-0-10) is about 2.1% and 0.37% more than the ultrasonic pulse velocity of the control specimen (C-0-0) respectively, at 28 and 60 days. The use of limestone powder as cement replacement has reduced the ultrasonic pulse velocity of specimens. The lowest value of ultrasonic pulse velocity is related to the specimen containing 30% limestone powder (C-30-0). So that the ultrasonic pulse velocity of this specimen is about 7% and 10.2% less than the ultrasonic pules velocity of the control specimen at 28 and 60 days. The highest rate of the ultrasonic pulse velocity is related to the bacterial specimen containing 10% zeolite (S-0-10). The ultrasonic pulse velocity of this specimen is about 3.7% and 2% more than the ultrasonic pules velocity of the control specimen respectively at 28 and 60 days.
Fig. 9. Relationship between ultrasonic pulse velocity and tensile strength of specimens at 28 days.
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Fig. 10. SEM images of (a) C-0-0; (b) S-0-0; (c) C-0-10; (d) S-0-10; (e) C-30-0, (f) S-30-0.
The ultrasonic pulse velocity of the nonbacterial specimen containing 10% zeolite and 10% limestone powder (C-10-10) is 0.98% less than the ultrasonic pulse velocity of the control specimen (C-0-0) at 28 days. While, adding the bacteria to this concrete mixture
has caused the ultrasonic pulse velocity to be 0.98% higher than the control specimen. The relationships between compressive strength and ultrasonic pulse velocity of bacterial specimens and non-bacterial specimens
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Fig. 11. Images of crack healing processes of specimens after different repair time, (a) bacterial specimen without limestone powder and zeolite (S-0-0); (b) bacterial specimen contains 20% limestone powder and 10% zeolite (S-20-10); (c) crack healing by using bacteria in curing environment (S-CE).
at the age of 28 and 60 days are presented in Fig. 8. The relationship between tensile strength and ultrasonic pulse velocity of specimens is illustrated Fig. 9. 3.5. Microstructure of concrete specimens Fig. 10a, c, e refer to the non-bacterial mixtures (without limestone powder and zeolite (C-0-0), containing 30% limestone powder (C-30-0) and containing 10% zeolite (C-0-10)) and the images of bacterial specimens (without limestone powder and zeolite (S-0-0), containing 30% limestone powder (S-30-0) and containing 10% zeolite (S-0-10)) are presented in Fig. 10b, d, f. The images of the specimens indicate the presence of calcite precipitation in bacterial specimens. It is obvious that the voids of the bacterial specimens are filled by calcite precipitations. Fig. 10a relates to the control specimen without limestone powder and zeolite (C-0-0). The existence of voids in the structure of this specimen is clearly evident. As shown in Fig. 10b, calcite precipitations created by bacteria have filled up the voids. Fig. 10c, d shows the bacterial and nonbacterial specimen containing 10% zeolite. The presence of a bacteria in the mixture design containing 10% zeolite
(S-0-10) provides dense structure than the nonbacterial specimen with the same mixture design (C-0-10). The SEM image of the specimen containing 30% limestone powder (Fig. 10e, f) revealed that the amount of calcite crystals in the bacterial treated specimen containing limestone powder (S-30-0) is more than the amount of crystals formed in the bacterial specimen without limestone powder (S-0-0). When the bacteria were exposed to limestone powder, they had better activities in order to produce calcite precipitations. Fig. 11 illustrates direct observation of cracks healing of specimens. As can be seen in this figure, compared with the specimens that bacteria were used in their mixture design, the amount of sediments in the cracks of the specimen cured in the suspension of bacteria and the reactive solution are much higher. Fig. 12 shows the microscopic images of crack healing processes of three different specimens. Fig. 12a refers to the crack healing of the bacterial specimen without limestone powder and zeolite (S-0-0) with a width of 0.28 mm that was nearly fully healed after 30 days and Fig. 12b shows the crack healing of the bacterial specimen containing zeolite and lime stone powder (S-20-10) with a width of 0.34 mm. According to the images, after 28 days of healing, this
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Fig. 12. Microscopic image of healing the cracks of different specimens with different widths during the time, (a) bacterial specimen without limestone powder and zeolite (S-0-0); (b) bacterial specimen contains 20% limestone powder and 10% zeolite (S-20-10); (c, d) crack healing by using bacteria in curing environment (S-CE).
crack was completely healed. In Fig. 12c and d a crack treated with using bacteria in curing environment is shown (S-CE). As it can be seen in Fig. 12c, completely healing of the crack with the width of 0.4 mm was at earlier healing time (5 days), while the crack with a width of 1.1 mm was only partially healed after 28 days (Fig. 12d). Fig. 13 illustrates the crack healing percentage of specimens at different healing times. As seen from this figure, the presence of bacteria in curing environment (S-CE) has the best healing results
among the other specimens. It can be seen that the trend line of the crack healing of the specimen containing limestone powder and zeolite is slightly higher than the specimen without limestone powder and zeolite. So, apparently the cracks of the specimen contains 20% limestone powder and 10% zeolite (S-20-10) seem to be healed better than the specimen without limestone and zeolite (S-0-0). As shown in Figs. 14, 15 all the cracks up to 0.58 mm were completely healed at 30 days, when the bacteria have been used in
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Fig. 13. Crack healing percentage of different specimens as a function of the initial crack width after curing time of (a) 5 days (b) 15 days and (c) 30 days.
Fig. 14. Maximum width of 100% healed crack.
curing environment and complete crack healing for the widths up to 1.1 mm were achieved for this specimen at 30 days. This values are respectively 0.16 and 0.42 for both of the specimens that bacteria have been used in their mixture design (S-0-0 and S-20-10). 3.6. Microstructure of cracks filler The carbonates have three to four intense bands in Infrared spectroscopy (IR) region. The 1420 cm1 and 876 cm1 region
Fig. 15. Maximum width of healed crack.
bands are characteristic for identification of carbonates and the 711 cm1 region band is characteristic of calcite among various carbonate rocks [36]. The results of Fourier-Transform Infrared spectroscopy of the precipitation at cracks surface show in Fig. 16. Existence of peaks at 1423 and 874 induced by the presence of carbonate and the 710 cm1 region band shows that the precipitation at cracks surface is calcite which matched with the mineralization by bacteria. The SEM image of the precipitation created on the surface of the cracks is presented in Fig. 17.
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Fig. 16. Details of the Fourier-Transform Infrared spectroscopy of the precipitation formed at cracks surface.
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53% and 31% higher than the electrical resistivity of the control specimen at 28 and 60 days, respectively. This values for the bacterial specimen containing 10% zeolite improves to values of 64% and 39%. According to the SEM images, the use of limestone powder has a positive effect on the formation of calcite crystals in the microstructure of the bacterial specimens. This effect is also reflected in the average percentage of crack healing. As the average percentage of crack healing of the specimen containing limestone powder is slightly higher than the specimen without limestone powder. The percentage of crack healing by using the bacteria in curing environment is significantly higher than the percentage of crack healing by using the bacteria in concrete mixture design. All the cracks up to 0.58 mm wide were completely healed at 30 days by using bacteria in curing environment, while if bacteria are used in concrete mixture only cracks with up to 0.16 mm width were completely healed. From the findings, considering a curing aqua containing bacteria for applying to the cracked surfaces of a floor slab is suggested. Using bacteria in the wet curing environment is very promising method for healing cracks. CRediT authorship contribution statement Maedeh Sadat Jafarnia: Investigation, Writing - original draft. Mehdi Khodadad Saryazdi: Project administration, Validation, Writing - review & editing. Seyed Mohammad Moshtaghioun: Writing - review & editing. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. References
Fig. 17. Details of the SEM image of the precipitation formed at cracks surface.
4. Conclusions Calcite precipitations in bacterial specimens increased compressive strength, tensile strength, ultrasonic pulse velocity and electrical resistivity of all specimens. Compressive strength, tensile strength, ultrasonic pulse velocity and electrical resistivity of all specimens decreased by increasing the percentage of limestone powder replacement of cement. But this decrease for specimens containing bacteria is lower than nonbacterial specimens. Partial replacement of cement with zeolite significantly increased compressive strength and electrical resistivity of the specimens. For example, when 10% zeolite is used, the 28days compressive strength of the non-bacterial specimen increased by 20% and that of bacterial specimen increased by 23%. These values for 60-days specimen without bacteria was 11% and with bacteria was 18%. Furthermore, the electrical resistivity of the specimen containing 10% zeolite is about by
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