Towards a sustainable bacterially -mediated self healing concrete H.M. Jonkers*, E. Schlangen Delft University of Technology, Faculty of Civil Engineering and GeoSciences / Microlab, Stevinweg 1, 2628 CN Delft, The Netherlands . *T: +31 15 278 2313 , [email protected]

Introduction Concrete is a strong and relatively cheap construction material and is therefore presently the most used construction material worldwide. However, the material also has a few drawbacks, e.g. its liability to cracking and that its mass ive production exerts some negative effects on the environment. Particularly cracking of the surface layer of concrete reduces material durability as ingress water and detrimental chemicals cause a range of matrix degradation processes as well as corrosion of the embedded steel reinforcement. One way to circumvent costly manual maintenance and repair is to incorporate an autonomous self -healing mechanism in concrete. One such an alternative repair mechanism is currently being investigated and devel oped in several laboratories, i.e. a technique based on the application of mineral -producing bacteria. The applicability of specifically mineral -producing bacteria for sand consolidation and limestone monument repair ( 1-2) and filling of pores and cracks i n concrete have been recently investigated ( 3-5). In several follow up studies (6-7) the possibility to use viable bacteria as a sustainable and concrete -embedded self healing agent was explored and results from ongoing studies will be discussed here. Viable bacteria as self healing agent The bacteria to be used as self healing agent in concrete should be fit for the job, i.e. they should be able to perform long -term effective crack sealing, preferably during the total constructions life time. The principl e mechanism of bacterial crack healing is that the bacteria themselves act largely as a catalyst, and transform a precursor compound to a suitable filler material. The newly produced compounds such as calcium carbonate based mineral precipitates should tha n act as a type of bio -cement what effectively seals newly formed cracks. Thus for effective self healing, both bacteria and a bio -cement precursor compound should be integrated in the material matrix. However, the presence of the matrix-embedded bacteria and precursor compounds should not negatively affect other wanted concrete characteristics. Bacteria that can resist concrete matrix incorporation exist in nature, and these appear related to a specialized group of alkali resistant spore -forming bacteria. Interesting feature of these bacteria is that they are able to form spores, which are specialized spherical thick -walled cells somewhat homologous to plant seeds. These spores are viable but dormant cells and can withstand mechanical and chemical stresses and remain viable for periods over 50 years. However, when bacterial spores were directly added to the concrete mixture, their life-time appeared to be limited to a few months only (8). The decrease in life -time of the bacterial spores from several decades to only a few months appeared to be due to continuing cement hydration resulting in matrix pore -diameter widths smaller than the 1 µm sized bacterial spores what causes cell collapse. However, pr otection of the bacterial spores by immobilization inside po rous expanded clay particles before addition to the concrete mixture (Figure 1) appeared to substantially prolong their life -time.

Currently running viability experiments show that still after 6 months concrete incorporation no loss of viability is observe d, suggesting that th eir original over 50 years viability is maintained.

Figure 1. Self healing admixture composed of expanded clay particles loaded with bacterial spores and organic bio -cement precursor compound. Autonomous crack repair of bacterial s elf healing concrete Concrete test specimens were prepared in which part of the aggregate material, i.e. the 2-4 mm size class, was replaced by similarly sized expanded clay particles loaded with the bio-chemical self -healing agent (bacterial sp ores plus caclium lactate) . Control specimen had a similar aggregate composition but in these expanded clay particles were not loaded with the bio -chemical agent. The self -healing capacity of pre -cracked concrete slabs sawed from 56 days cured concrete cylinders was determined by taking light microscopic images before and after permeability quantification. For the latter, pre cracked concrete slabs were glued in an aluminum ring and mounted in a custom made permeability setup. Subsequently, during a 24 hour period, pe rmeability and crack healing was quantified by automated recording of water percolation in time (Figure 2).

Figure 2. Pre -cracking of concrete slab and subsequent permeability testing.

Comparison between bacterial and control specimen revealed a signi ficant difference in self-healing capacity. In both type of specimens precipitation of calcium carbonate based mineral precipitates occurred. However, while in control specimen precipitation largely occurred near the crack rim leaving major parts of the cr ack unhealed, efficient and complete healing of cracks occurred in bacterial specimen as here mineral precipitation occurred predominantly within the crack itself (Figure 3).

Figure 3. Light microscopic images of pre -cracked control (A) and bacterial (B ) concrete specimen before and after healing. Mineral precipitation occurred predominantly near the crack rim in control but inside the crack in bacterial specimens. Efficient crack healing only occurred in bacterial specimens. Discussi on and concl usion The outcome of this study shows that crack healing in bacterial concrete is much more efficient than in concrete of the same composition but without added bio -chemical healing agent. The reason for this can be explained by the strictly chemical processes in the control and additional biological processes in the bacterial concrete. On the crack surface of control concrete 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 2O

The amount of calcium carbonate production in this case in only minor due to the limited amount of CO 2 present. As Portlandite is a rather soluble min eral in fa ct most of it present on the crack surface will dissolve and diffuse out of the crack into the overlying water mass. Subsequently, as more CO 2 is present in the overlying water, dissolved Portlandite will as yet precipitate in the form of calcium carbonate but somewhat away from the crack itself, as can be seen in Figure 4A. The self healing process in bacterial concrete is much more efficient due to the active metabolic conversion of calcium lactate by the present bacteria: Ca(C 3 H5O2)2 + 7O 2

→

CaCO 3 + 5CO 2 + 5H 2O

This process does not only produce calcium carbonate directly but also indirectly via the reaction of on site produced CO 2 with Portlandite present on the crack surface. In the latter case, Portlandite does not dissolve and diff use away from the crack surface, but instead reacts directly on the spot with local bacterially produced CO 2 to additional calcium carbonate. This process results in efficient crack sealing as can be seen in Figure 4B. The conclusion of this work is that the proposed two component bio -chemical healing agent composed of bacterial spores and a suitable organic bio -cement precursor compound , both immobilized in reservoir porous expanded clay particles, represents a promising bio -based and thus sustainable alt ernative to strictly chemical or cement based healing agents. References 1. Dick J, DeWindt W, DeGraef B, Saveyn H, VanderMeeren P, DeBelie N and Verstraete W (2006) Bio deposition of a calcium carbonate layer on degraded limestone by Bacillus species. Bi odegradation 17: 357 -367. 2. Rodriguez-Navarro C, Rodriguez -Gallego M, BenChekroun K and Gonzalez -Munoz MT (2003) Conservation of ornamental stone by Myxococcus xanthus -induced carbonate biomineralization. Appl Environ Microb 69: 2182 -2193. 3. Ramachandran SK, Ramakrishnan V and Bang SS (2001) Remediation of concrete using micro organisms. ACI Mater J 98: 3 -9. 4. De Muynck W, Debrouwer D, De Belie N and Verstraete W (2008) Bacterial carbonat e precipitation improves the durability of cementitious materials. Cement Concrete Res 38: 1005 -1014. 5. De Muynck W, Cox K, De Belie N a nd Verstraete W (2008) Bacterial car bonate precipitation as an alternative surface treatment for concrete. Constr Build Mater 22: 875 -885. 6. Jonkers HM (2007) Self healing concrete: a b iological approach. In Self healing materials - An alternative approach to 20 centuries of materials scie nce (ed. S. van der Zwaag), pp. 195 –204. Springer, The Netherlands. 7. Jonkers HM and Schlange n E (2008) Development of a bacteria -based self healing c oncrete. In Tailor made concrete structures - new solutions for our society. Proc. I nt. FIB symposium (ed. J. C. Walraven & D. Stoelhorst), pp. 425 -430. Amsterdam, The Netherlands. 8. Jonkers HM, Thijssen A, Muyzer G, Copuroglu O and Schlangen E (in press) Application of bacteria as self-healing agent for the development of sustainable concrete. Ecological Engineering