BACTERIA MEDIATED REMEDIATION OF CONCRETE STRUCTURES

Transcription

1 BACTERIA MEDIATED REMEDIATION OF CONCRETE STRUCTURES Henk M. Jonkers and Arjan Thijssen Delft University of Technology, CITG, Microlab, P.O. Box 5048, 2600 GA Delft, The Netherlands. Abstract One aspect that could particularly contribute to increased service life of concrete structures is autonomous crack-healing potential. Substantial self-healing of cracks in concrete will decrease material permeability and reduce risk of premature reinforcement corrosion and concrete matrix degradation. Since several years a novel type of self-healing concrete in which bacteria mediate the production of crack-filling material is under development in our laboratory. The specific mechanism of crack-healing in this case is based on concrete matrix-incorporated dormant but viable spores of specific alkali-resistant bacteria which, after activation by crackingress water, produce inorganic mineral precipitates by conversion of organic precursor compounds. Experimental results indicate that the quality of mineral precipitates produced depend on species of bacteria and type of precursor compound involved. Environmental scanning electron microscopic (ESEM) analysis revealed that a bacterial isolate originating from soil samples produced robust elongated sphere-shaped µm-sized precipitates from the conversion of calcium lactate. In contrast, another isolate originating from an alkaline soda lake produced larger mm-sized plate-like precipitates from sodium glutamate. Energy dispersive X-ray analysis (EDAX) indicated that the plate-like mineral precipitates are probably calcium carbonate-based. Light microscopic analysis of cracked and subsequently water incubated concrete specimens revealed perfect crack-healing of bacteria-based but not of control specimen. We therefore conclude that bacteria-based self-healing concrete represents a durable and particularly sustainable alternative to classical, but also to strictly chemicallybased self-healing concretes. 1. INTRODUCTION Since the last decade efforts are undertaken by several research groups to develop a new 833

2 type of concrete which features an inbuilt active crack-healing mechanism. The challenge is to apply a mechanism with a potential life time similar to that of the concrete construction and that only becomes active when needed. The rationale behind the idea is that due to crack formation concrete constructions become more and more permeable over time enabling aggressive ingress chemicals to deteriorate the concrete matrix and embedded reinforcement [1-2]. Connected cracks may even result in leakage of barrier constructions. To delay these deteriorating processes the self-healing mechanism should become activated upon crack formation resulting in the sealing or effective plugging of the cracks [3]. Such a self-healing mechanism could substantially decrease maintenance and repair costs and furthermore extend the service life of concrete constructions. Most types of commonly used concretes do in fact already feature a kind of self-healing mechanism [1-3]. However, these are a passive and have a rather limited crack-healing capacity. The passive healing activities are related to the amount and type of still non-reacted binder present in the concrete matrix. Particularly high performance or high strength concretes based on mixtures with a low to very low water-tobinder ratio feature a substantial crack-healing capacity. This phenomenon is due to the presence of a high fraction of non- to only partially hydrated binder particles present in the concrete matrix which can undergo delayed or secondary hydration upon reaction with crack ingress water [4]. However, as current policies advocate the reduction of particularly the amount of cement in concrete mixtures for environmental reasons [5], an alternative more sustainable crack-healing mechanism is needed. One such a mechanism could be provided by life bacteria, e.g. by those who mediate sand stone formation in nature. It has been demonstrated that various taxonomic groups of bacteria are able to produce mineral precipitates, a process that can result in the consolidation of sands and soils [6-7], but also to a substantial decrease in concrete surface permeability [8-9] and a certain amount of crackhealing in concrete [10-12]. In previous studies it has been shown that a specific group of alkali-resistant spore-forming bacteria of the genus Bacillus is not only able to survive concrete incorporation but also to produce crack-sealing mineral precipitates upon activation by crackingress water [13-15]. Previous microbiological studies with pure cultures of bacteria in liquid media have shown that, largely depending on growth and incubation conditions, different species produce different types of mineral precipitates [16-17]. Whether this also holds for concrete-incorporated bacteria is not known. In this study we therefore investigated mineral production by two different types of concrete incorporated bacteria. Morphological differences of bacterially produced mineral precipitates deposited on crack surfaces were investigated by environmental scanning electron microscopy and further characterized by EDAX analysis. 2. METHODS 2.1 The bacteria Spore-forming alkali-resistant bacteria were isolated from mud slurries originating from a natural soda lake (strain C2-C2-1A) or purchased from the German Collection of Microorganisms and Cell Cultures (DSMZ), Braunschweig, Germany (strain Bacillus cohnii DSM 6307). To induce copious spore formation both strains were cultivated in an alkaline 834

3 medium containing 20 mm sodium citrate as energy and carbon source for growth. The medium was further composed of 0.2g NH 4 Cl, 0.02g KH 2 PO 4, 0.225g CaCl 2, 0.2g KCl, 0.2g MgCl 2.6H 2 O, 0.375g KNO 3, 50 mm NaHCO 3, 50 mm Na 2 CO 3, 1 ml trace elements solution SL12B and 0.1g yeast extract per liter Milli-Q ultra pure water. Before addition to the cement mixture for test specimens preparation, bacteria were cleaned from culture residues by repeated centrifugation and resuspension of obtained cell pellet in clean tap water [15]. 2.2 Preparation of cement stone specimens and incubation conditions Cement stone specimens with and without (controls) incorporated bacterial spores were prepared to investigate mineral precipitate deposition on crack surfaces. Ordinary Portland cement (CEM I 42.5N, ENCI, The Netherlands) was mixed with aliquots of tap water in a water-to-cement weight ratio of 0.5. Bacteria-containing specimens were prepared by addition of washed spore suspensions replacing part of the makeup water. Liquid paste was poured in moulds (with dimensions of 4 x 4 x 4 cm), gas-tight sealed and cured at room temperature. Bacterial specimens contained spores cm -3 cement stone. Specimens were cured for 7 or 28 days after which they were broken and further suspended in tap water at room temperature to allow development of mineral precipitates on crack surfaces. Two different series of specimens were prepared. One contained B.cohnii spores ( spores cm -3 ) plus additional calcium lactate (0.5% of cement weight) representing the bacterial 'food' or mineral precursor compound. The other series included the bacterial species C2C21A which was isolated from an alkaline soda lake ( spores cm -3 ). To the latter series no additional bacterial food was added but applied instead to the water in which cracked specimens were afterwards incubated (20 mm sodium glutamate plus 6 g/l yeast extract) [15]. 2.3 ESEM and EDAX analysis Cracked specimens were rinsed with dematerialized water after the incubation period and without any further preparation directly studied for mineral formation at crack surfaces by environmental scanning electron microscopy (Philips ESEM XL30 Series) equipped with an X-ray microanalysis (EDAX) system. 3. RESULTS 3.1 ESEM analysis of crack surfaces Control specimens (no bacteria included) cured for 7 or 28 days produced small, typically 1-5 µm-sized particles on crack surfaces (Figure 1). A prominent feature on crack surfaces of young specimens (7 days cured) was massive production of fibrous material not visible on crack surfaces of older (28 days cured) specimens. Crack surfaces of bacteria-based specimens appeared very different. One those of the first series, (B.cohnii plus calcium lactate) copious amounts of larger µm-sized robust sphereshaped particles were formed at the crack surface of 7 days but not on those of 28 days cured specimens (Figure 2). Crack surfaces of all 28 days cured specimens (bacterial and controls) did in fact not appear very different (Compare Figures 1C-D and 2C-D). 835

4 Figure 1: ESEM photographs of crack surfaces of control (no bacteria-containing) specimens cured for 7 (A and B) or 28 days (C and D). Scale bars: 100 µm (A), 20 µm (B), 50 µm (C) and 10 µm (D). On crack surfaces of the second series of bacteria-based specimens (species C2C21A with sodium glutamate and yeast extract provided in the incubation water) also copious amounts, but in this case plate-like precipitates, were formed (Figure 3). 3.2 EDAX analysis of mineral precipitates X-ray analysis indicated that the plate-like precipitates observed in the second series were calcium carbonate based as the Ca/C ratio of these were closer to 1 than that of the crack surface of a control specimen. Moreover, the plate-like precipitates featured a very low silica content compared to the control cement stone surface (Table 1). Table 1: X-ray micro analysis of specimen surface. Elements expressed as percentage of atom number in sample. Element: Control C2C21A Calcium Silicon Carbon Ca/C Ca/Si

5 2nd International Symposium on Service Life Design for Infrastructure Figure 2: ESEM photographs of crack surfaces of the first series of bacteria based specimens (B.cohnii plus calcium lactate) cured for 7 (A and B) or 28 days (C and D). Scale bars: 100 µm (A), 20 µm (B), 50 µm (C) and 10 µm (D). Figure 3: ESEM photograph of crack surfaces of the second series of bacteria based specimens (species C2C21A) cured for 7 days in sodium glutamate and yeast extract amended water. Scale bar: 200 µm. 837

6 4. DISCUSSION Main goal of this study was to investigate whether viable bacterial spores immobilized in the cement stone matrix can act as self-healing agent to catalyze the process of autonomous repair of freshly formed cracks. One major problem associated with crack formation is that the process results in a drastic increase in material permeability increasing the risk of matrix and embedded reinforcement degradation by ingress water and other aggressive chemicals [1-3]. Active bacterially-mediated mineral precipitation could result in crack-plugging and concomitant decrease in material permeability [12]. As bacteria function as catalyst, a suitable mineral precursor compound needs additionally to be incorporated in the material matrix to provide a truly autonomous repair mechanism. In this study we found that bacteria-based cement stone specimens produced various forms of mineral precipitates on crack surfaces of particularly young (7 days cured) samples. This indicates that crack-healing in bacterial concrete is potentially much higher than in concrete without added bio-chemical healing agent (bacterial spores plus food). The increased mineral precipitate production potential in bacteriabased specimens can be explained by metabolic activity of bacteria. In contrast, crack healing in control specimens is strictly due to chemical processes. Here, unhydrated cement particles exposed on the crack surface will undergo secondary hydration producing CSH like filamentous structures as can be seen in Figure 1A and B. In addition some calcium carbonate will be formed due to the reaction of CO 2 present in the crack ingress water with Portlandite (calcium hydroxide) present in the concrete matrix according to the following reaction: CO 2 + Ca(OH) 2 CaCO 3 + H 2 O (1) The amount of calcium carbonate production in this case is only minor due to the limited amount of CO 2 present in the crack ingress water, and the fact that Portlandite is rather soluble in water and will therefore dissolve and diffuse away from the crack surface. In bacterial specimens, however, the crack healing process is much more efficient. This partly due to the direct metabolic production of calcium carbonate by conversion of e.g. calcium lactate by the present bacteria: Ca(C 3 H 5 O 2 ) 2 + 7O 2 CaCO 3 + 5CO 2 + 5H 2 O (2) Furthermore, metabolic conversion of organic compounds such as lactate, glutamate or yeast extract produces substantial amounts of CO 2 what, produced on the spot, can directly react with present Portlandite crystals to form additional calcium carbonate precipitates. During our ESEM investigations we found several examples of the apparent conversion of Portlandite hexagonal crystals to calcium carbonate particles (see Figure 4). 838

7 Figure 4: Apparent conversion of hexagonal Portlandite crystals to calcium carbonate particles Older bacterial specimens as well as control specimens who did not contain bacteria did not produce copious or larger precipitates. The reason why older bacteria based specimens produced insignificant amounts of mineral precipitates is likely due to loss of viability of concrete matrix-embedded bacterial spores. Ongoing cement hydration results in continuing decrease in matrix pore diameter sizes what may in combination with the high matrix ph values hamper bacterial spore viability. A possible solution to the problem of loss of viability and related functionality of incorporated bacterial spores could be provided by encapsulation or immobilization of spores in a protective matrix prior to addition to the concrete mixture. A possibility could be embedding of bacterial spores prior to concrete mixture addition in sol-gellike materials or porous aggregate material. Another possible strategy to avoid crushing of bacterial spores in aging concrete could be the addition of air-entraining agents to the concrete mixture to create isolated micro pores in the concrete matrix in which spores can survive. 5. CONCLUSION The conclusion of this work is that the two component bio-chemical healing agent, i.e. viable bacterial spores plus a suitable organic bio-mineral precursor compound represents a promising bio-based and thus sustainable alternative to strictly chemical or cement-based healing agents. Current investigations focus on further clarification of both composition of mineral precipitates formed and efficiency of crack-healing of this novel type of bacteria-based concrete. REFERENCES [1] Edvardsen, C. (1999) Water permeability and autogenous healing of cracks in concrete. ACI Materials Journal 96(4): [2] Reinhardt, H.W., and Jooss, M. (2003) Permeabilitiy and self-healing of cracked concrete as a function of temperature and crack width. Cement and Concrete Res 33: [3] Neville, A.M. (2002) Autogenous healing - A concrete miracle? Concrete Int 24(11):

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