Concrete Repair, Rehabilitation and Retrofitting II – Alexander et al (eds) © 2009 Taylor & Francis Group, London, ISBN 978-0-415-46850-3
Crack repair in concrete using biodeposition N. De Belie & W. De Muynck Magnel Laboratory for Concrete Research, Department of Structural Engineering, Ghent University, Ghent, Belgium
ABSTRACT: The ability of certain bacteria to promote the precipitation of calcium carbonate, has been used advantageously for consolidation of concrete and stone or for the densification of sandy soils. Previous research has illustrated a reduction of the capillary permeable porosity and an increased resistance to damage processes such as chloride ingress and carbonation by this biodeposition procedure. In order to explore the crack healing potential of a biodeposition treatment, standardised cracks of 0.3 mm were produced in concrete specimens by introducing thin copper plates in fresh concrete and removing them after 1 day, or by performing splitting tensile tests on concrete cores wrapped in fibre reinforced polymer sheets. The use of pure bacteria cultures as in the previously developed biodeposition procedure, did not result in sufficient calcium carbonate precipitation to completely bridge the cracks. Therefore, the bacteria were protected in a silica sol, resulting in the formation of a bioceramic material (sol-gel or biocer) which was able to bridge the cracks completely. The crack healing potential was illustrated by microscopic evaluation, ultrasound transmission measurements and low pressure water permeability tests. The treatment of cracks with the bacteria incorporated in the sol-gel resulted in a large reduction of the water permeability.
1 1.1
INTRODUCTION General
In concrete, cracking is a common phenomenon due to the relatively low tensile strength. High tensile stresses can result from external loads, imposed deformations (due to temperature gradients, confined shrinkage, differential settlement), plastic shrinkage, plastic settlement, expansive reactions (e.g. due to reinforcement corrosion, alkali silica reacion, sulphate attack). Without immediate and proper treatment, cracks tend to expand further and eventually require costly repair. Durability of concrete is also impaired by these cracks, since they provide an easy path for transportation of liquids and gasses that potentially contain harmful substances. If microcracks grow and reach the reinforcement, not only the concrete itself may be attacked (direct degradation). Also the reinforcement may corrode when it is exposed to water and oxygen, and possibly carbon dioxide and chlorides (indirect degradation). Microcracks are therefore precursors to structural failure. For crack repair, a variety of techniques is available. Injection with epoxy resin or cement grout are the most popular ones. Traditional repair systems such as those based on epoxy resin have a number of disadvantageous aspects, such as a different thermal expansion coefficient compared
to concrete and environmental and health hazards. Therefore bacterial induced calcium carbonate deposition has been proposed as an alternative and environment friendly crack repair technique.
1.2
Principle of microbiologically induced carbonate precipitation
Ureolytic micro-organisms can induce extracellular precipitation of CaCO3 by degradation of urea into ammonia and carbon dioxide. This increases the pH at the cell surface and promotes the microbial deposition of carbon dioxide as calcium carbonate. Through this process the bacterial cell is coated with a layer of calcium carbonate, resulting in death of the microorganism. However, in the meantime a loose carrier material such as sand may be bound together, or a porous material may be consolidated. This technology has also been successfully applied on limestone monuments (Castanier et al. 1999; Tiano et al. 1999). In a project carried out in collaboration between the Magnel Laboratory for Concrete Research and the Laboratory for Microbial Ecology and Technology of Ghent University, first the criteria for the selection of calcium precipitating Bacillus sphaericus strains were established: high urease activity, abundant EPS-production, a good biofilm production and
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a very negative ζ-potential (Hammes et al. 2003; Dick et al. 2006). Concrete and mortar samples of different porosity were treated with pure cultures and with ureolytic sludge in combination with different calcium sources and nutrient solutions (De Muynck et al. 2008; De Muynck et al., in press). CaCO3 deposition resulted in a decrease of the water absorption ranging from 65 to 90%. As a consequence, the carbonation rate and chloride migration respectively decreased by about 25–30% and 10–40%. Similarly, an increased resistance towards freezing and thawing was noticed. The use of calcium precipitating micro-organisms for crack remediation has not yet been investigated in depth, but the possibility was proven by Ramachandran et al. (2001). Cuts of 3 mm width could be sealed using B. pasteurii and up to about 12 MPa compressive strength was recovered. However, curing for 28 days in a urea-CaCl2 solution was necessary. In the currently presented research the possibility of crack repair through application of micro-organisms, is explored further.
2 2.1
MATERIALS AND METHODS Concrete samples
Concrete samples were prepared with ordinary Portland cement CEM I 52.5 N (300 kg/m³), aggregates 8/16 (790 kg/m³) and 2/8 (490 kg/m³), sand 0/4 (670 kg/m³) and water (150 kg/m³). They were cured for 28 days at a temperature of 20°C and a relative humidity (RH) of 90%. The compressive strength determined on cubes with 150 mm side amounted to 55 N/mm2. 2.2
Creation of cracks
Standardised cracks were made in concrete samples of 160 × 160 × 70 mm by introducing copper plates of 0.3 mm thickness up to a depth of 10 or 20 mm into the fresh concrete. These plates were removed after 24 h. More realistic cracks were obtained by splitting tests on FRP reinforced cylinders. From concrete cubes of 150 mm side, cylinders of 80 mm diameter were taken. These were protected with tape and a glass fibre reinforced polymer sheet was glued around the circumference with epoxy resin. Each wrapped cylinder of 150 mm height was cut in half, to obtain two cylinders of 75 mm height, which were subjected to a splitting test. From each 75 mm specimen, three samples of 20 mm height were taken afterwards. Only samples with a visible crack were selected for further use. Mostly the crack was running completely throughout the cylinder diameter.
The crack width was determined with an accuracy of 0.02 mm at 5 locations equally divided along the crack length, using a crack microscope. The crack width measured on 21 cylinders varied between 0.01 mm and 0.60 mm (mean value: 0.20 mm). 2.3
Crack repair
Cracks were repaired by traditional methods and by biodeposition treatments as listed below. 2.3.1
Traditional methods: epoxy and cement grout The two component epoxy resin was injected into the crack using an injection needle. The two component cement bound mortar (grout) was applied using a spatula. The area near the crack was protected with tape during the application. 2.3.2 Biodeposition treatments Bacillus sphaericus strains had been isolated earlier from calcareous sludge from a biocatalytic ureolytic calcification reactor and had been deposited at the BCCM culture collection in Ghent. The strain LMG 222 57 was used in this research because of its optimal CaCO3 precipitation capabilities (Dick et al. 2006). A lot of research on biodeposition has been conducted with CaCl2 as the calcium source (Bang et al. 2001; Adolphe et al. 1990; Ferris & Stehmeier 1992). As chloride ions are detrimental for the concrete reinforcement, the use of calcium nitrate Ca(NO3)2·4H2O as an alternative calcium source was investigated here. In order to protect the bacteria from the strong alkaline environment in concrete (at pH values above 11, the bacterial activity is stopped), the bacteria were immobilised in silica sol-gel in some treatments. This is an aqueous colloidal suspension of amorphous silica (SiO2). The sol-gel used was anionic with a specific surface area of 200 m²/g and a solids content of 30%. The different treatments were applied by placing the samples on plastic rods in the treatment solution, in such manner that the liquid level was 10 mm above the lower side of the specimens. Following treatments were carried out: – Medium The samples were immersed for 3 days in an equimolar solution of ureum (20 g/L) and CaCl2·2H2O (49 g/L) or Ca(NO3)2·4H2O (78 g/L). −BS + CaCl2 The samples were immersed for 24 h in a Bacillus sphaericus culture grown overnight (growth medium: 20 g/L yeast extract and 20 g/L ureum). After this inoculation, specimens were wiped with a paper towel to remove some bacteria from the surface. In this way ureolytic activity primarily resulted from bacteria
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inside the specimens. Then they were immersed for 3 days in an equimolar solution of ureum and CaCl2·2H2O.
pipet
−BS + sol-gel + CaCl2 or BS + sol-gel + Ca(NO3)2 Fifty ml of a culture grown overnight was centrifuged at 4°C and 7000 rpm. The resulting pellet was suspended in a solution of 10 ml demineralised water and 40 ml sol-gel. After addition of 1.2 g NaCl the solution was vortexed during 30 s. The suspension was applied in the cracks using a syringe. This was repeated at 5 min intervals until the entire crack was sealed. After gel formation, the samples were immersed for 3 days in an equimolar solution of ureum and CaCl2·2H2O or Ca(NO3)2·4H2O.
rubber seal
−Sol-gel The samples were immersed for 20 min in the silica sol and dried for 24 h at room temperature. This treatment was repeated three times. −Sol-gel + BS + CaCl2 The samples were treated with sol-gel as described above, followed by the BS + CaCl2 treatment. 2.4
Evaluation of crack repair
The effect of the crack repair was visualized with a crack microscope. Furthermore the effect of repair on the transmission time of ultrasound waves and on the water permeability coefficient was determined. 2.4.1 Ultrasound Ultrasound waves travel much easier in hardened concrete than in water or air: ultrasound velocities in those media amount to 4000–5000 m/s, 1480 m/s and 350 m/s (at 20°C and 100 kPa) respectively. Therefore they will travel around an open fissure, but when the crack is sealed they will be able to travel through the sealant and the travel time will be reduced. Ultrasound transmission experiments were carried out in triplicate on concrete prisms of 160 × 160 × 70 mm with a standardized crack of 10 or 20 mm deep. Measurements were taken in a direction orthogonal to the crack at 3 locations near the upper end of the specimens (where the crack is located) and compared to measurements in the middle and at the bottom end of the specimen (further away from the crack). 2.4.2 Water permeability The efficiency of crack repair was investigated by monitoring the water permeability of the specimens. The test method was a slightly modified version of the low pressure water permeability test described by Wang et al. (1997). Cylinders of 80 mm diameter and 20 mm height were protected with plastic tape at the upper and lower surface and then glued into a PVC ring using epoxy resin. The tape avoided that epoxy
specimen pvc ring epoxy resin rubber tube
Figure 1. Test device for water permeability.
would come into contact with the upper and lower side of the cylinder and it was removed after hardening of the epoxy. The samples were vacuum saturated in de-ionised water as described by NBN B05-202. Each specimen was then mounted between two plexiglas rings in the test device as shown in Fig.1. Rubber seals between the plexiglass and PVC rings were used to ensure a water-tight setup. At the outer end of the plexiglass rings, square cover plates with 2 holes each were applied and the cell was clamped together with four threaded bars. In one opening in the upper plate a glass pipet with inner diameter of 10 mm was positioned and it was covered to avoid evaporation. In the lower plate a rubber drain tube was attached and the free end of this tube was positioned level with the lower end of the concrete sample. Both upper and lower cell, including pipet and drain tube, were then filled with de-ionised water, creating a pressure head of 510 mm. The drop in water level in the pipet, due to water flow through the specimen, was measured at regular time intervals and water was restored to the original level. The water permeability will not immediately be constant but will decrease, mostly during several days. This is supposedly due to an incomplete saturation of the specimens and existence of air bubbles at the start of the test. A steady state flow was considered to be reached when similar results were obtained for the drop in water level during 5 subsequent days. In steady state conditions, Darcy’s law (equation 1) can be used to calculate the coefficient of permeability: k=
aT ⎛ h0 ⎞ ln ⎜ ⎟ At ⎝ h f ⎠
(1)
where a = cross sectional area of pipet (m²), A = cross sectional area of specimen (m²), T = specimen
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thickness (m), t = time (s), h0 and hf = initial and final water heads (m).
3 3.1
RESULTS AND DISCUSSION Visual evaluation of crack repair
Fig. 2 shows the level of crack bridging in a standard crack by different repair methods. Almost all tested biodeposition treatments, except the medium and BS + CaCl2 treatments, were able to bridge cracks. Treatment with medium only (results not shown), resulted in a limited deposition of calcium carbonate. For the BS + CaCl2 treatment CaCO3 deposition was visible at the crack edges and on the sample surface (Fig. 2a). The cross-sections indicate that the epoxy treatment and the treatments with BS + sol-gel + CaCl2 and BS + sol-gel + Ca(NO3)2 (not shown) resulted in a complete filling of the crack, while this was clearly not obtained in the case of the cement grout. 3.2
Ultrasound
With the ultrasound measurements the increase in transmission time due to the fissure could be determined: 0.22 ± 0.10 µs for the “10 mm” fissure and 0.43 ± 0.18 µs for the “20 mm” fissure (values
(a)
represent the average ± standard deviation on an individual measurement). By introducing the average wave velocities measured in the uncracked samples (around 5000 m/s in all specimens), the average depth of the fissure could be calculated: 9.0 ± 3.3 mm for the “10 mm” fissure and 12.8 ± 3.8 mm for the “20 mm” fissure. This means that the “20 mm” fissure was not 20 mm deep at all locations or that at least crack bridging occurred, allowing the ultrasound waves to take a shortcut. The effect of the epoxy, solgel and BS + CaCl2 treatments was also evaluated using ultrasound transmission. Decreases in transmission time through the crack, were corrected with decreases in transmission time at the reference locations to eliminate effects of continuing hydration or changes in humidity. For the “10 mm” fissure, the epoxy injection and the sol-gel treatment reduced the transmission time with 0.43 ± 0.20 µs and 0.27 ± 0.11 µs respectively. Although the variability on the measurements was quite large, considering the increase in transmission time due to the crack of 0.22 ± 0.10 µs, it can be concluded that the sol-gel treatment allowed to seal the fissure, whereas the epoxy treatment probably also had a consolidating effect on the concrete near the crack. For the “20 mm” fissure the epoxy, sol-gel and BS + CaCl2 treatments, decreased the transmission times with 0.13 ± 0.04 µs, 0.23 ± 0.02 µs and −0.006 ± 0.009 µs respectively. This would indicate that neither the epoxy, nor the sol-gel treatment allowed to seal the “20 mm” fissure completely, since before repair an increase in transmission time of 0.43 ± 0.18 µs was measured due to the fissure. As already noticed during visual inspection, the BS + CaCl2 treatment on its own was not at all able to bridge the crack.
(b) 1.8E-09 1.6E-09 1.4E-09
(c)
k (m/s)
1.2E-09
(d)
1.0E-09 8.0E-10 6.0E-10 4.0E-10 2.0E-10 0.0E+00 0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
Crack width (mm)
(e)
(f)
Figure 2. Visual evaluation of crack repair at the surface by BS + CaCl2 (a); BS + sol-gel + Ca(NO3)2 (b); Sol-gel + BS + CaCl2 (c); cross sections of BS + sol-gel + CaCl2 (d); epoxy (e) and grout treatments (f); the crack tip is encircled; arrows indicate the crack edges.
untreated epoxy BS+sol-gel+Ca(NO3)2 sol-gel
BS + CaCl2 grout BS+sol-gel+CaCl2
Figure 3. Water permeability coefficient k vs. crack width for the different crack repair systems.
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3.3
REFERENCES
Water permeability
Fig. 3 shows the coefficient of permeability k of the different samples versus the crack width. All treatments resulted in a decrease of permeability when compared to the untreated cracked samples. The grout and BS + CaCl2 treatments were least efficient in improving the watertightness, whereas the epoxy and BS + sol-gel + CaCl2 or BS + sol-gel + Ca(NO3)2 treatments were most efficient. This shows that with the alternative biodeposition treatments, in which the bacteria are incorporated in a sol-gel and provided with a calcium source, a similar effect can be obtained as with a traditional epoxy injection.
4
CONCLUSION
It was shown that crack repair can be obtained through biodeposition treatments in which a Bacillus sphaericus culture is incorporated in sol-gel. A calcium source such as CaCl2 or Ca(NO3)2 should be provided. Visual examination and ultrasound transmission testing proved that complete sealing of artificial cracks of 0.3 mm wide and 10 mm deep could be obtained. In contrast to traditional epoxy injection, a sol-gel treatment does not seem to affect the porosity of the material near the crack. In relation to watertightness, again treatments with Bacillus sphaericus, sol-gel and a calcium source appeared to be effective in healing real cracks of 0.01 to 0.6 mm. These repair materials were much more effective than application of sol-gel or bacteria only. They also reduced the concrete permeability much more than a cement grout repair technique and had a similar effect as epoxy injection.
ACKNOWLEDGEMENTS
Adolphe J.M., Loubière J.F., Paradas J. & Soleilhavoup F. (1990). Procédé de traitement biologique d’une surface artificielle. European patent 90400G97.0 (after French patent 8903517, 1989). Bang S.S., Galinat J.K. & Ramakrishnan V. (2001) Calcite precipitation induced by polyurethane-immobilized Bacillus pasteurii. Enzyme Microb Technol, 28(4–5): 404–409. Castanier S., Le Métayer-Levrel G. & Perthuisot J.P. (1999). Ca-carbonates precipitation and limestone genesis—the microbiogeologist point of view. Sedimentary Geology 126, pp. 9–23. De Muynck, W., Cox, K., Belie, N. & Verstraete, W. (2008). Bacterial carbonate precipitation as an alternative surface treatment for concrete. Construction and Building Materials, 22, 875–885. De Muynck, W., Debrouwer, D., Belie, N. & Verstraete, W. Bacterial carbonate precipitation improves the durability of cementitious material. Cement and concrete research, in press. Dick J., De Windt W., De Graef B., Saveyn H., Van der Meeren P., De Belie N. & Verstraete W. (2006). Biodeposition of a calcium carbonate layer on degraded limestone by Bacillus species. Biodegradation, 17(4), 357–367. Ferris F.G. & Stehmeier L.G.1992. Bacteriogenic mineral plugging. USA Patent US5143155 Hammes F., Boon N., de Villiers J., Verstraete W. & Siciliano S.D. (2003). Strain-specific ureolytic microbial carbonate precipitation. Applied and Environmental Microbiology, 69(8), 4901–4909. NBN B05-202. 1976. Experiments on building materials— frost resistance—porosimetry [in Dutch]. Ramachandran S.K., Ramakrishnan V. & Bang S.S. (2001). Remediation of concrete using micro-organisms. ACI Materials Journal, 98(1), 3–9. Tiano P., Biagiotti L. & Mastromei G. (1999). Bacterial biomediated calcite precipitation for monumental stones conservation: methods of evaluation. Journal of Microbiological Methods, 36, 139–145. Wang K., Jansen D.C. & Shah S.P. (1997). Permeability study of cracked concrete. Cement and concrete research, 27(3): 381–393.
This research was funded by a Ph.D. grant of Ghent University (BOF-fund) for Willem De Muynck. The authors wish to thank Tom Deloof for practical help.
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