Protective carrier is essential for the self-healing of concrete cracks by microbially induced CaCO3 precipitation, owing to the harsh conditions in concrete. In this paper, porous ceramsite particles are used as microbial carrier. Heat treatment and NaOH soaking are first employed to improve the loading content of the ceramsite. The viability of bacterial spores is assessed by urea decomposition measurements. Then, the self-healing efficiency of concrete cracks by spores is evaluated by a series of tests including compressive strength regain, water uptake, and visual inspection of cracks. Results indicate that heat treatment can improve the loading content of ceramsite while not leading to a reduction of concrete strength by the ceramsite addition. The optimal heating temperature is 750°C. Ceramsite particles act as a shelter and protect spores from high-pH environment in concrete. When nutrients and spores are incorporated in ceramsite particles at the same time, nutrients are well accessible to the cells. The regain ratio of the compressive strength increases over 20%, and the water absorption ratio decreases about 30% compared with the control. The healing ratio of cracks reaches 86%, and the maximum crack width healed is near 0.3 mm.
Since the invention of modern concrete, it has become the most widely used building material all around the world. It stands out for its high compressive strength, low cost, and flexibility in casting. However, durability is the primary concern for concrete engineers. Concrete has a high tendency to cracking, which allows aggressive species to penetrate into the matrix. External loads, temperature gradients, and restrained deformation are the main factors contributing to the crack [
Bacterial spores, together with nutrients and mineralization precursors, are mixed in concrete mixtures during casting. After hardening, spores will stay as dormant. When the cracks form, bacterial spores are exposed to moisture and air. Then, the spores will rejuvenate and produce minerals, which mostly appear as calcium carbonate, to seal the cracks [
In order to allow the bacterially induced concrete self-healing system work, bacterial spores should be incorporated in fresh-state concrete. Basically, there are two ways to add bacterial cells into concrete matrix: directly mixing and carrier immobilization. Directly mixing is much straightforward but will expose bacteria to the harsh environment of concrete, which is harmful to the bacterial activity. Moreover, the process of mixing and continuous hydration could apply physical stress on bacterial cells [
For the evaluation of self-healing effects, most of the prior works concentrated on the assessment of crack healing and permeability. The quantitative analysis of the crack width and the ratio of cracks that could be healed was studied. Permeability of concrete, which is a direct reflection of the durability, was evaluated after self-healing [
In this work, ureolysis-based bacterial spores were applied in mortar to serve as self-healing agents. Compressive tests before and after healing were performed to evaluate the healing effects from the point of mechanics. The efficiency of healing was also studied by crack imaging analysis and water uptake measurements. In order to provide a protection for spores, porous ceramsite particles, which are a type of expanded clay, were employed. Pretreatments on the ceramsite particles were carried out beforehand for the purpose of improving the loading content of the protective carriers.
An ureolytic bacterium
The porous ceramsite particles were used as protective carriers for bacterial spores and healing agents. The mean particle size was 2–5 mm, and the bulk density was 1036.5 kg/m3. For the purpose of improving the loading content of ceramsite particles, different pretreatment procedures, such as alkali erosion and sintering treatments, were carried out.
For the alkali erosion treatment, ceramsite particles were first treated by NaOH solution with concentrations of 0.5 mol/L, 1.0 mol/L, and 1.5 mol/L. After immersion in solution for 24 h, ceramsite particles were rinsed repeatedly by distilled water and then oven-dried at 105°C. For the sintering treatment, the selected sintering temperature varied from 400°C to 1000°C. The heating rate was 5°C/min and then kept for 2 h at the maximum temperature, followed by a cooling process in a furnace. After pretreatment, the particles were oven-dried at 105°C and weighed. Then, the loading content was evaluated by immersing the particles in distilled water. After 24 h, particles were taken out, wiped by wet towel to remove water on the surfaces, and weighed. The mass difference before and after immersion was considered as an indicator of loading content.
Considering the possible negative effect of the heating process on the mechanical properties of concrete matrix, compressive strength and flexural strength of concrete matrix with ceramsite treated at different temperatures were determined. For the mechanical tests, 40 × 40 × 160 mm mortar specimens containing 250 g ASTM Type I Ordinary Portland cement, 338 g local natural sand with a specific density of 2.65 g/cm3, 125 g water, and 196 g ceramsite particles were fabricated. Specimens were cast and cured in a standard curing room at a temperature of 20°C and relative humidity (RH) of 90%. After 24 h, all samples were demolded and then stored in the same room until tests. Triplicate sets for each group were fabricated. The mechanical tests were performed according to standard GB/T 17671-1999.
Spores were loaded in ceramsite particles by simply immersing 10 g ceramsite in 7.5 mL spore suspension (109 cells/mL) for 2 h. Then the particles were dried in an oven at 40°C until the weight remains constant. The viability of spores with or without the protection from ceramsite was evaluated by a treatment in a simulated concrete pore solution. The solution was prepared by mixing cement (the same as described in Section
For the treatment of spores without protection, 1 mL spore suspension (109 cells/mL) was mixed with 10 mL simulated solution and left to rest for 24 h. After this, spores were collected by repeated centrifugation and washing by sterile distilled water. For the treatment on loaded spores, 10 g dry ceramsite particles were put into a filtering tea bag, which was then submerged in 10 mL simulated solution for 24 h. After this, the tea bag was taken out and rinsed repeatedly by sterile distilled water. Then, the tea bag was oven-dried at 40°C until the mass remains constant. Ceramsite particles were collected from the opened tea bag.
The amount of urea decomposed by spores was used as an index to evaluate the viability of the spores with or without protection. Urea concentration measurement was based on a colorimetric method as described by Douglas and Bremner [
The cement and sand used were the same as described in Section
The mixing proportion.
Group | Cement (g) | Sand (g) | Water (g) | Ceramsite (g) | Beef extract (g) | Peptone (g) | Urea (g) | Calcium nitrate (g) | Basalt fiber (g) | Water-reducing agent (g) |
---|---|---|---|---|---|---|---|---|---|---|
C | 250 | 338 | 125 | 196a | — | — | — | — | 8 | 1.0 |
N | 250 | 338 | 125 | 197.6b | — | — | 2.5 | 1.5 | 8 | 1.2 |
S | 250 | 338 | 125 | 197c | 0.3 | 0.5 | 2.5 | 1.5 | 8 | 1.2 |
SN | 250 | 338 | 125 | 197.5d | — | — | 2.5 | 1.5 | 8 | 1.2 |
Note: aceramsite without loading; bceramsite loaded with 0.6 g beef extract and 1 g peptone; cceramsite loaded with 1 g spores; dceramsite loaded with 0.3 g beef extract, 0.5 g peptone, and 0.7 g spores.
Mortar cubes of size 50 × 50 × 50 mm were cast and cured in a standard curing room at a temperature of 20°C and RH of 90%. After 24 h, all samples were demolded and then stored in the same room until tests. Triplicate sets for each group were fabricated.
Cracks were created by a compressive loading program. The compressive tests were performed by using a mechanical testing system (TSY-2000). The displacement control mode was used with a loading rate of 0.1 mm/min. After peak load, the final displacement was controlled in the same way. All specimens were subjected to wet-dry cycles for 4 weeks at 20°C. For one cycle, specimens were submerged in water for 1 h and then exposed to ambient condition (20°C, RH 60%) for 11 h.
The self-healing effectiveness was evaluated as follows.
The compressive tests were performed using the same mechanical testing system, and the compressive strength of each specimen was recorded. At the same time, cracks were produced. After 28 days healing, compressive strength was tested again under the same circumstance. The strength regain ratio
Since water uptake is one of the indicators of durability, the capillary water absorption test was performed to evaluate the durability of specimens after self-healing. The mortar cubes were put into the oven at 70°C and dried until their mass loss was less than 0.1% between two measurements at 24 h intervals. After drying, the specimens were then submerged in water, 80 ± 2 mm deep. This was done in an atmosphere of 20°C and an RH of 60%. While the water level was maintained, all specimens were removed from the water every three minutes, dried on the surface with a towel, and weighed. Immediately after this measurement, the specimens were submerged again. The procedure was repeated until weight remains constant.
Pictures of the sample surfaces were taken before and after self-healing. In each image, crack widths and lengths that can be healed were analyzed by the image processing software “Image J.” By setting a threshold value, the cracks can be distinguished from the uncracked area. Thus, the length and width of each crack can be obtained.
Porosity is positively correlated with the loading content of ceramsite. The main composition of ceramsite is silica. Therefore, alkali erosion by NaOH solution was first considered to increase the porosity of ceramsite particles. Figure
Loading content by alkali erosion at different concentrations of NaOH solution.
Figure
Loading content by heat treatment with different sintering temperatures.
Figure
SEM images of ceramsite heated to different temperatures: (a) room temperature; (b) 400°C; (c) 800°C; (d) 1000°C.
Table
Strength of mortar specimens containing ceramsite treated at different temperatures.
Mechanical tests | Room temperature | 600°C | 750°C | 900°C |
---|---|---|---|---|
Compressive strength (MPa) | 36.39 | 33.42 | 35.58 | 35.68 |
Flexural strength (MPa) | 8.10 | 8.32 | 7.77 | 8.48 |
The amount of urea decomposed along 6 days, which is an index of bacterial viability, is shown in Figure
Urea decomposition by free spores and loaded spores with or without treatment by high-pH-simulated solution.
SEM images of spores loaded in ceramsite.
In order to simulate the process of self-healing, cracks were first introduced by compressive loading. After 28 days of incubation, the self-healing effects were evaluated by compressive strength, water uptake, and image analysis tests. Figure
The regain ratio of the compressive strength by self-healing.
Figure
The water uptake after self-healing.
Figure
Images of cracks before and after healing: (a) group C; (b) group SN.
Table
Summary of the crack healing.
Group | Average healed crack width ( |
Maximum healed crack width ( |
Percentage of crack healed |
---|---|---|---|
C | 36 ± 4 | 56 ± 4 | 6 ± 4 |
N | 70 ± 34 | 124 ± 20 | 16 ± 11 |
S | 46 ± 15 | 111 ± 13 | 7 ± 3 |
SN | 153 ± 58 | 273 ± 42 | 86 ± 5 |
For both S and SN groups, bacteria were incorporated in ceramsite and enabled the production of CaCO3, while for C and N groups autogenous healing could occur. We found that the healing effectiveness is highly related to the way of loading. When bacteria were loaded in carriers only, a low healing efficiency, even lower than the control, was observed. This could be owing to the fact that spores instead of live cells were used in the self-healing system. Spores might not be able to germinate without the presence of some organic substances nearby. Besides, some cells could further disintegrate and impair the interfacial region between ceramsite particles and paste matrix. When the organics were loaded in company with bacteria, although the cracks cannot be healed completely, the maximum width of cracks that can be healed was near 0.3 mm. Another benefit was that the negative impact of organic nutrients on the concrete matrix could be avoided if nutrients are loaded in carriers.
Heat treatment, instead of NaOH soaking, could increase porosity of ceramsite, which thus improves the immobilization capacity. The optimal heating temperature was 750°C, which results in the highest loading content and a negligible decrease in mechanical strength. Ceramsite particles provide a preferable microenvironment for bacterial spores that the viability of spores can be preserved during the urea decomposition process.
When nutrients and bacterial spores are incorporated into ceramsite particles, nutrients are well accessible to the cells and significant healing effects can be observed. The regain ratio of the compressive strength increased over 20%, and the water absorption ratio decreased about 30% compared with the control. The healing ratio of cracks reaches 86%, and the maximum crack width healed was near 0.3 mm.
The authors declare that there are no conflicts of interest regarding the publication of this paper.
The authors would like to acknowledge the financial support for this study from the National Natural Science Foundation of China (51378011), the Natural Science Foundation of Shanghai (17ZR1441900), and the National Key Research and Development Program of China (2016YFC0700802).