Self Healing Concrete: A Biological Approach Henk M Jonkers Delft University of Technology, Faculty of Civil Engineering and GeoSciences/Microlab, Stevinweg 1, The Netherlands E-mail: h.m.jonkers@tudelft.nl 1 An Introduction to Concrete Concrete can be considered as a kind of artificial rock with properties more or less similar to certain natural rocks. As it is strong, durable, and relatively cheap, concrete is, since almost two centuries, the most used construction material worldwide, which can easily be recognized as it has changed the physiognomy of rural areas. However, due to the heterogeneity of the composition of its principle components, cement, water, and a variety of aggregates, the properties of the final product can widely vary. The structural designer therefore must previously establish which properties are important for a specific application and must choose the correct composition of the concrete ingredients in order to ensure that the final product applies to the previously set standards. Concrete is typically characterized by a high-compressive strength, but unfortunately also by a rather low-tensile strength. However, through the application of steel or other material reinforcements, the latter can be compensated for as such reinforcements can take over tensile forces. Modern concrete is based on Portland cement, a hydraulic cement patented by Joseph Aspdin in the early 19th century. Already in Roman times hydraulic cements, made from burned limestone and volcanic earth, slowly replaced the widely used non-hydraulic cements, which were based on burned limestone as main ingredient. When limestone is burned (or “calcined”) at a temperature between 800 and 900◦ C, a process that drives off bound carbon dioxide (CO2 ), lime (calcium oxide; CaO) is produced. Lime, when brought into contact with water, reacts to form portlandite (Ca(OH)2 ) which can further react with CO2 , which in turn forms back into calcite (CaCO3 ), or limestone, the pre-burning starting material. However, a major drawback of this non-hydraulic cement is that it will not set under water and, moreover, its reaction products portlandite and limestone are relatively soluble, and thus will deteriorate rapidly in wet and/or acidic environments. In contrast, portland cement produces, upon reaction with water, a much harder and insoluble material that will also set under water. For portland cement production a source of calcium, silicon, aluminum, and iron is needed and therefore usually limestone, clay, some bauxite, and iron ore are burned in a kiln at temperatures up to 1, 500◦ C. The cement clinker produced is mainly composed of the minerals alite (3CaO.SiO2 ), belite (2CaO.SiO2 ), aluminate (3CaO.Al2 O3 ), and ferrite (4CaO.Al2 O3 .Fe2 O3 ), which all yield specific hydration products with different characteristics upon reaction with water. S. van der Zwaag (ed.), Self Healing Materials. An Alternative Approach to 20 Centuries of Materials Science, 195–204. c 2007 Springer. 195 196 H.M. Jonkers The contribution of these clinker minerals to the composition of general-purpose portland cement in weight percentage is typically 50%, 24%, 11%, and 8% respectively. Important characteristics of clinker minerals are reaction rate and contribution to final strength of the product. For example, of the two calcium silicates, alite is the most reactive and contributes to early strength, while the slower-reacting belite contributes more to longer-term strength. Aluminate contributes to early strength as its hydration reaction is fast but it also generates much heat. The final properties of cement-based materials can thus vary widely as they strongly depend on the mineral composition of the cement used and therefore, different types of cement, each suitable for specific applications, are produced. Quantitatively most important hydration product of general-purpose portland cement is calcium silicate hydrate (C–S–H), an amorphous mineral somewhat resembling the natural mineral tobermorite. A secondary reaction product is calcium hydroxide (portlandite), which together with the very soluble sodium and potasium oxides (Na2 O and K2 O) also present in portland cement, contribute to the high alkalinity of the concrete’s pore fluid (pH ≈ 13). The high matrix pH is important in structural concrete as it protects the embedded steel reinforcement from corrosion. The protective oxidized thin layer of Fe3+ oxides and oxyhydroxides on the reinforcement steel (the passivation film) rapidly degrade when the matrix pH drops below 9, leading to further oxidation and deterioration of the concrete structure due to expansion reactions and loss of strength. Corrosion of the steel reinforcement is in fact one of the major causes limiting the durability, or lifetime, of concrete structures. For further and more detailed information on general concrete properties the reader is referred to Reinhardt (1985) and Neville (1996). 2 Concrete Durability, Deterioration, and Self Healing Properties A variety of additives or replacements of cement can be applied in order to improve the durability of the final concrete product. Also certain industrial waste or recycled materials can be used to improve the sustainability, or environmental friendliness, of concrete and some even improve certain properties. The production of cement is high-energy consuming as raw materials are burned at 1, 500◦ C, a process that contributes to a significant amount of atmospheric CO2 release worldwide. Thus, for both economical and environmental reasons, cement production and use should be minimized. Examples of industrial waste products, which can partly replace and even improve cement properties, are fly ash, blast furnace slag, and silica fume. Fly ash, a waste product from coal-burning power plants, is a source of reactive silica and can substitute 35–75% of cement in the concrete mix. Application of fly ash increases concrete strength as it reduces the required water/cement ratio and also improves resistance against chemical attack as it decreases the matrix permeability. Similarly, silica fume from the silicon industry and blast furnace slag from steel industries can partially replace cement in the concrete mix, as these are sources of reactive silica and Self Healing Concrete: A Biological Approach 197 both reactive silica and calcium respectively. Other commonly applied additives that improve or change certain concrete characteristics needed for specific applications are air-entraining agents to improve freeze/thaw resistance, setting or retarding agents and plasticizers to enable a lower water/cement ratio to increase concrete strength. A number of processes negatively affect the durability and result in the unwanted early deterioration of concrete structures. One major cause that initiates various mechanisms of concrete deterioration is the process of cracking what dramatically increases the permeability of concrete. The microstructure of hardened cement paste is porous as it contains isolated as well as interconnected pores. Specifically the connected pores determine permeability, as these allow water and chemicals to enter the concrete matrix. As cracking links both isolated and connected pore systems, this results in a substantially increased permeability. In most concrete-deterioration mechanisms permeability plays a major role. Intrusion of sulfate ions into the matrix may result in ettringite formation, a conversion reaction in which a high-density phase is transformed into a low-density phase, causing expansion and further cracking of the material. Chloride ions penetrating the matrix through the connected pore system will destabilize the passivation film of the steel reinforcement and by doing so accelerate further corrosion. Similarly, in a process called carbonation, CO2 diffusing through the pore system will react with alkaline pore fluid components such as Ca(OH)2 which will result in a lowering of matrix pH and again depassivation of the protective film on the steel reinforcement. These examples make clear that cracking of concrete should be minimized and that a potential healing mechanism should ideally result in the sealing or plugging of newly formed cracks in order to minimize increases in matrix permeability. An active self healing mechanism in concrete should be ideal as it does not need labor-intensive manual checking and repair what would save an enormous amount of money. A self healing mechanism or self healing agent in concrete should comply ideally with all, or at least with some, of the following characteristics: 1. Should be able to seal or plug freshly formed cracks to reduce matrix permeability 2. Must be incorporated in the concrete matrix and able to act autonomously to be truly “self-healing” 3. Must be compatible with concrete, i.e. its presence should not negatively affect material characteristics 4. Should have a long-term potential activity, as concrete structures are build to last typically for at least 50 years 5. Should preferably act as a catalyst and not be consumed in the process to enable multiple healing events 6. Must not be too expensive to keep the material economically competitive Different types of potential self healing mechanisms or agents for autonomous concrete repair can be thought of. One series of mechanisms could involve the secondary formation of minerals which are compatible with the material matrix, i.e. will not negatively affect but rather increase concrete durability by sealing freshly formed cracks and so decrease matrix permeability. A chemical agent such as the inclusion 198 H.M. Jonkers of still nonreacted cement particles in the concrete matrix is feasible as it complies with at least some of the listed self healing properties. Besides this, other agents could work equally well or can contribute to the self healing property of concrete in concert with the previous one. Next to chemicals one could think of an agent of biological origin, and in the next part the possible application of bacteria as healing agent will be considered. 3 The Self Healing Mechanism of Bacterial Concrete Do bacteria exist which could potentially act as a self healing agent in concrete, and if so, what would be the healing mechanism? From a microbiological viewpoint the application of bacteria in concrete, or concrete as a habitat for specialized bacteria, is not odd at all. Although the concrete matrix may seem at first inhospitable for life, as it is a very dry and extremely alkaline environment, comparable natural systems occur in which bacteria thrive. Inside rocks, even at a depth of more than 1 km within the earth crust, in deserts as well as in ultra-basic environments, active bacteria are found (Jorgensen and D’Hondt 2006; Fajardo-Cavazos and Nicholson 2006; Dorn and Oberlander 1981; DelaTorre et al. 2003; Pedersen et al. 2004; Sleep et al. 2004). These desiccation- and/or alkali-resistant bacteria typically form spores, which are specialized cells able to resist high mechanically and chemically induced stresses (Sagripanti and Bonifacino 1996). A low-metabolic activity and extremely long lifetimes also characterize spores, and some species are known to produce spores which are viable for up to 200 years (Schlegel 1993). In a number of recent studies the potential for application of bacteria in concrete technology was recognized and reported on, e.g. for cleaning of concrete surfaces (DeGraef et al. 2005) as well as for the improvement of mortar compressive strength (Ghosh et al. [53]). Moreover, bacterial treatment of degraded limestone, ornamental stone, and concrete structures for durability improvement has been the specific topic of a number of recent studies (Bang et al. 2001; Ramachandran et al. 2001; Rodriguez-Navarro et al. 2003; De Muynck et al. 2005; Dick et al. 2006). Due to bacterially controlled precipitation of dense calcium carbonate layers, crack-sealing, as well as significant decreases in permeability of concrete surfaces were observed in these studies. In these remediation and repair studies the bacteria and compounds needed for mineral precipitation were brought into contact with the structures’ surface after setting or crack formation had occurred, and were not initially integrated as healing agents in the material’s matrix. The mechanism of bacterially mediated calcite precipitation in those studies was primarily based on the enzymatic hydrolysis of urea. In this urease-mediated process the reaction of urea (CO(NH2 )2 ) and water yields CO2 and ammonia (NH3 ). Due to the high pK value of the NH3 /NH+ 4 system (about 9.2) the reaction results in a pH increase and concomitant shift in the − carbonate equilibrium (CO2 to HCO− 3 and CO32 ) which results in the precipitation of calcium carbonate (CaCO3 ) when sufficient calcium ions (Ca2+ ) are present. Self Healing Concrete: A Biological Approach 199 Fig. 1 Scenario of crack-healing by concrete-immobilized bacteria. Bacteria on fresh crack surfaces become activated due to water ingression, start to multiply and precipitate minerals such as calcite (CaCO3 ), which eventually seal the crack and protect the steel reinforcement from further external chemical attack Whether this system can be integrated in the concrete matrix in order to act as truly self healing agent or that its application is tailored for remediation purposes remains to be investigated as both enzyme (urease) activity and substrate (urea) may not have a long-term (years) stability when immobilized in the concrete matrix. One drawback of the urease-based system from an environmental viewpoint is the excessive (equivalent) production of ammonium to carbonate ions. Thus ideally, for bacterially based self healing concrete, both the bacteria and environmental friendly biomineral precursor compounds should be embedded in the material. The immobilized bacteria should immediately start to precipitate minerals and seal cracks upon concrete cracking and water entrance. Such a scenario is schematically depicted in Figure 1. 4 Experimental Evidence for Bacterially Controlled Self Healing in Concrete In a number of published studies the potential of calcite precipitating bacteria for concrete or limestone surface remediation or durability improvement was investigated (Bang et al. 2001; Ramachandran et al. 2001; Rodriguez-Navarro et al. 2003; De Muynck et al. 2005; Dick et al. 2006). However, as the bacteria and mineral 200 H.M. Jonkers precursor compounds were not initially part of the material matrix but rather externally applied, the remediation mechanism in those studies cannot be truly defined as self healing. Therefore, in order to investigate the potential of autonomous bacterially mediated self healing in concrete, a series of experiments were performed. Firstly, a number of potentially suitable bacterial species were selected. Four species of alkalitolerant (alkaliphilic) spore-forming bacteria of the genus Bacillus were obtained from the German Collection of Microorganisms and Cell Cultures (DSMZ), Braunschweig, Germany. These bacteria were cultivated and subsequently immobilized in concrete and cement stone (cement plus water in a weight ratio of 2:1 without aggregate addition) to test compatibility with concrete and bacterial mineral production potential respectively. As was listed above (paragraph 2), the ideal self healing agent should comply with certain characteristics, and one of them is that its presence should not negatively affect the material characteristics. To test this, a dense culture of Sporosarcina pasteurii was washed twice in tap water and the number of bacteria in the resulting cell suspension quantified by microscopic counting before addition to the concrete mix make up water. Two parallel series of nine concrete bars (with and without bacteria) of dimensions 16 × 4 × 4 cm were prepared and triplicate bars of both series were subsequently tested for flexural tensile and compressive strength after 3, 7, and 28 days curing. Table 1 shows the composition of the concrete mix and Figure 2 depicts the strength development of both types of concrete in time. The results of the concrete compatibility test show that the addition of bacteria to a final concentration of 109 cm−3 does not affect strength characteristics. Moreover, incubation of cement stone pieces in a medium to which yeast extract and peptone (3 and 5 g L−1 respectively) was added as a bacterial food source revealed that on the surface of bacteria-embedded specimen, but not on control specimen, copious amounts of calcite-like crystals were formed (Figure 3). From the latter experiment it can therefore be concluded that suitable bacteria, in this case alkali-resistant sporeforming bacteria, embedded in the concretes’ cement paste are able to produce minerals when an appropriate food source is available. Table 1 Cement, water, and aggregate composition needed for the production of nine concrete bars of dimensions 16 × 4 × 4 cm. The washed cell suspension used for bacterial concrete was part of totally needed makeup water Compound Cement (ENCI CEMI 32.5) Water Aggregate Size Fraction (mm): 4–8 2–4 1–2 0.5–1 0.25–0.5 0.125–0.25 Weight (g) 1, 170 585 1, 685 1, 133 848 848 730 396 Self Healing Concrete: A Biological Approach 201 Fig. 2 Flexural tensile (a) and compressive (b) strength testing after 3, 7, and 28 days curing revealed no significant difference between control and bacterial concrete. The latter contained 1.14 × 109 S. pasteurii cells per cubic centimeter of concrete The mechanism of bacterial mineral production here is likely metabolically mediated. As the bacteria metabolize the available organic carbon sources (yeast extract and peptone) under alkaline conditions, carbonate ions are produced which precipitate with access toward calcium ions present in the concrete matrix. The produced carbonate ions can locally reach high concentrations at the bacterially active “hot 202 H.M. Jonkers Fig. 3 Cement stone samples, which were cured for 10 days and subsequently further incubated in yeast extract- and peptone-containing medium. (A) Control (cement stone without added bacteria) and (B) cement stone containing 109 cm−3 spores of B.pseudofirmus. The inset in Figure 2B (×5000 magnification) shows a close up of the massive calcite-like crystals formed on the specimen surface Table 2 Flexural tensile- and compressive-strength characteristics of control and organic carbonamended concrete bars after a 28 days curing period Type of Concrete Control Na-aspartate Na-glutamate Na-polyacrylate Na-citrate Na-gluconate Na-ascorbate Tensile Strength (MPa) 7.78 ± 0.38 7.33 ± 0.37 7.16 ± 0.19 6.42 ± 0.47 3.48 ± 1.72 0 0 Compressive Strength (MPa) 31.92 ± 1.98 33.69 ± 1.89 28.52 ± 3.56 20.53 ± 4.50 12.68 ± 1.82 0 0 spots” and here calcium carbonate (calcite) crystals form. For autonomous self healing, however, all compounds needed for the healing reaction should ideally be incorporated in the material matrix. As in the previous experiment bacterial food sources were part of the medium but not of the concrete matrix, an additional experiment was done to investigate the compatibility of concrete and various organic compounds. The compounds chosen for this test, all suitable food sources for the applied bacteria, were added to concrete in a concentration of 0.5% of cement weight (see Table 1 for composition of the concrete mix). After 28 days curing, some compounds appeared to be better compatible with concrete then others (Table 2). No significant difference was found in flexural tensile and compressive strength between control and amino acid (aspartic acid and glutamic acid)-containing concrete bars. Concrete to which polyacrylic acid and citric acid was added suffered significant strength loss, while gluconate- and ascorbic acid-amended concrete did not develop any strength at all. Specific organic compounds such as amino acids appear thus suitable candidates to act as self healing agent in concert with suitable bacteria. Self Healing Concrete: A Biological Approach 5 203 Conclusions and Future Perspectives Some previous studies reported on the successful application of bacteria for cleaning of concrete surfaces as well as concrete-, limestone-, and ornamental-stone crack repair (DeGraef et al. 2005; Dick et al. 2006; Rodriguez-Navarro et al. 2003; Bang et al. 2001; Ramachandran et al. 2001). As the bacteria in these studies were brought into contact with the material only after damage had occurred, these examples cannot be considered as truly, autonomous, self healing mechanisms. The experiments presented in this study, focused on the healing potential of concrete-immobilized bacteria, i.e. bacteria that are part of the concrete matrix. The results of the experiments show that immobilized bacteria mediate the precipitation of minerals and, moreover, the bacteria and certain classes of needed food sources do not negatively affect concrete strength characteristics. It can therefore be concluded that bacterially controlled crack-healing in concrete by mineral precipitation is potentially feasible. The concept, however, needs further developments on some areas. It should still be clarified whether bacterial mineral precipitation effectively seals cracks, i.e. significantly reduces the permeability of cracked concrete in order to protect the embedded reinforcement from corrosion and thus increases the durability of the material. Furthermore, bacterial species must be selected which, when part of the concrete matrix, remain viable for at least the expected lifetime of the construction. If so, the bacterial approach can successfully compete with other (abiotic) self healing mechanisms as such bacteria comply with all the listed characteristics of the most ideal self healing agent. Acknowledgements Arjan Thijssen is acknowledged for help with ESEM analysis and for providing ESEM photographs. Financial support from the Delft Centre for Materials (DCMat: www.dcmat.tudelft.nl) for this work is also gratefully acknowledged References Bang S.S., Galinat J.K., Ramakrishnan V. (2001) Calcite precipitation induced by polyurethaneimmobilized Bacillus pasteurii. Enzyme Microb Tech 28:404–409 DeGraef B., DeWindt W., Dick J., Verstraete W., DeBelie N. (2005) Cleaning of concrete fouled by lichens with the aid of Thiobacilli. Mater Struct 38(284):875–882 DelaTorre J.R., Goebel B.M., Friedmann E.I., Pace N.R. 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