The paper reports on work being carried out on bacteria-based self-healing of cementitious composites as part of the UK’s Resilient Materials for Life (RM4L) programme grant. RM4L is a follow-up to a previous project that developed bacteria- based self-healing using non-ureolytic bacteria encapsulated in lightweight aggregates and microcapsules. This led to the UK’s first full-scale trial of bacteria-based self- healing concrete. In RM4L, research has been undertaken to overcome a number of application issues with use of bacteria-based self-healing in practice. In particular, a large number of environmental bacteria have been screened that are potentially capable of working in cold and saline conditions. It has been shown that different microbial metabolisms can result in different mechanisms of precipitation, possibly impacting on performance in application. Further work has shown that it is possible to get healing to occur in aged concretes provided the healing agents are included appropriately. Work has also demonstrated to what extent wet/dry conditions are necessary for healing. Finally, the paper reports on work to genetically engineer bacteria to obtain more effective healing

Gebhard, Susanne

Paine, Kevin

Reeksting, Bianca

English

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        Citation for published version:Paine, K, Reeksting, B & Gebhard, S 2020, 'Developments in bacteria-based self-healing of cementitiouscomposites', Paper presented at 1st International Conference (Online) on Microbial Biotechnology inConstruction Materials and Geotechnical Engineering, Nanjing, China, 6/11/20 - 7/11/20 pp. 50.Publication date:2020Link to publicationUniversity of BathAlternative formatsIf you require this document in an alternative format, please contact:openaccess@bath.ac.ukGeneral rightsCopyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright ownersand it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights.Take down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.Download date: 17. Nov. 20201st International Conference on Microbial Biotechnology in Construction Materials and Geotechnical Engineering Nanjing, China. 16-18 August 2020 DEVELOPMENTS IN BACTERIA-BASED  SELF-HEALING OF CEMENTITIOUS COMPOSITES  Kevin Paine1, Bianca Reeksting2 and Susanne Gebhard2  1BRE Centre for Innovative Construction Materials, University of Bath, UK  Email: k.paine@bath.ac.uk 2Department of Biology and Biochemistry, University of Bath, UK  Email: b.j.reeksting@bath.ac.uk; s.gebhard@bath.ac.uk   Keywords: Bacteria, Self-healing, MICP, Carbonation, Moisture   Introduction Concrete remains the principal construction material used in buildings and infrastructure worldwide. However, it is susceptible to degradation which, given current design guidance, is nearly always inevitable over its lifetime. This means that intervention and mitigation, in the form of maintenance and repair, is necessary to retain and recover high levels of performance and durability. However, in recent years a range of ‘smart’ concretes has been developed, including those with autogenous and autonomic self-healing and self-repairing capabilities. Consequently, it is now possible to embed self-immunity and resilience in our concrete buildings and structures such that they evolve over their lifespan. This has significant implications for enhancing durability and serviceability, improving safety and reducing maintenance costs.  Resilient Materials for Life Research to develop a biomimetic approach to the repair of concrete is being undertaken in the UK as part of the £5M multi-disciplinary research project, Resilient Materials for Life (RM4L), a collaboration between Cardiff University, and the Universities of Bath, Bradford and Cambridge [1]. RM4L has a broad remit to focus on tailoring self-healing cementitious systems for use in specific applications and to address different damage scenarios and conditions. Furthermore, important damage scenarios have been identified, including time-related and cyclic damage as well as chemical damage, including corrosion. In addition to self-healing attributes, RM4L is initiating novel research aimed at embedding self-sensing, self-diagnosing, self-immunisation and self-reporting capabilities into cementitious systems in order to develop truly biomimetic responses in our infrastructure materials and structures.  Within RM4L, research on bacteria-based self-healing of cementitious composites is being undertaken at the University of Bath. In particular, research has been undertaken to overcome a number of application issues with use of bacteria-based self-healing in practice. This paper reports on work that has: (i) determined that different microbial metabolisms result in distinctive mechanisms of precipitation, impacting on performance in application, and (ii) investigated the potential for healing to occur in aged concretes. The extent to which wet/dry conditions are necessary for healing and the possibility for genetically engineering bacteria to obtain more effective healing is also commented on.  Mechanism of Calcium Carbonate Precipitation Bacteria-based self-healing is achieved by embedding bacterial spores within the concrete and providing them with the nutrients that they need to grow. Whilst the bacteria are in their spore form, they are inactive. However, when conditions become favourable, they germinate into active cells and these multiply. These more favourable conditions occur when a crack forms in the concrete and water and oxygen ingress.  The presence of bacteria near a crack will aid the formation of calcium carbonate should the local environment be rich in dissolved inorganic carbon (DIC) and Ca2+ ions. For this reason, growth media containing both carbon and calcium sources are normally used simultaneously with the spores as healing agents. In general, bacteria-based self-healing of concrete can occur because of either ureolytic or non-ureolytic metabolism.   Kevin Paine, Bianca Reeksting and Susanne Gebhard  Research at Bath has performed a comparison between how ureolytic and non-ureolytic environmental bacteria precipitate calcium carbonate in order to understand how these mechanisms differ and how we can apply this knowledge to the improvement of self-healing concrete [2]. In this study, environmental bacteria were surveyed for their ability to precipitate calcium carbonate and both the mechanism and resulting precipitates were examined. It was shown that while ureolytic bacteria cause the most rapid precipitation, non-ureolytic bacteria could produce similar amounts of calcium carbonate. In addition, the crystals resulting from non-ureolytic carbonate precipitation appeared to have more organic components and subsequently occupy larger volumes (Figure 1, left), this is likely beneficial for crack sealing application as for the same amount of calcium a larger precipitate can be formed. Indeed, non-ureolytic bacteria caused a more consistent crack healing and subsequent recovery of water tightness (Figure 1, right).   Figure 1: Crystal morphology and crack sealing in mortars containing either non-ureolytic or ureolytic bacteria. Left, electron micrographs of representative precipitate produced by non-ureolytic (top) and ureolytic (bottom) bacteria. Right, crack healing down the side of cement mortars at 8 weeks healing. Non-ureolytic bacteria (top) seal side cracks in a more consistent manner than ureolytic bacteria (bottom). White arrows, complete crack closure. Black arrows, incomplete crack closure.   Engineered Bacteria It has been considered unlikely that a singular bacterium will be able to meet all requirements for self-healing in concrete and that a mixture of bacteria will be needed in practice. However, speculatively, it has been investigated whether bacteria can be engineered through the introduction of novel “calcite precipitation modules” into weak calcite precipitators to obtain better bacteria for self-healing of concrete. Initial research has shown that this is the case and that Bacillus subtilis W168, a bacterium that does not precipitate calcite, can be made to do so by introduction of the correct modules.  These results are proving positive in terms of understanding how and why calcite precipitation occurs. Non-ureolytic2 mm 2 mm 10 µm             .Precipitate morphology Crack closure (side view)Ureolytic2 mm 2 mm 10 µm                                         .1st International Conference on Microbial Biotechnology in Construction Materials and Geotechnical Engineering Nanjing, China. 16-18 August 2020 Healing in Aged Concrete The ability of bacteria to autonomously heal cracks in cementitious composites by precipitating calcium carbonate has been verified in studies using several technologies. Since both Ca2+ and CO3-2 ions are required, technologies have been developed that provide these as separate additions (e.g. urea plus calcium nitrate) [3], or by the use of organic calcium salts (e.g. calcium acetate) which supplies both as a single addition [4]. Although the precise source of Ca2+ ions used by the bacteria during self-healing is unclear, it has been proposed that calcium hydroxide is likely to be an important source. Therefore, it is interesting to note that in nearly all previous studies self-healing of the cracks has been tested on water-cured mortars or concretes at a relatively young age (~28 days) where calcium hydroxide is present as a hydration product and provides a plentiful supply of soluble Ca2+ ions.  However, in practice concrete is often subject to alternate wetting and drying, and a process of carbonation can occur before the concrete cracks. The outcome is that Ca2+ ions may become trapped in an unusable form. For this reason, research was carried out at Bath to compare mortars that were cast with calcium sources added directly to the mortar and with the calcium source encapsulated in lightweight aggregates.  It was verified that calcium hydroxide is an important source of Ca2+ ions for bacteria-based self-healing of mortars. For mortars that do not carbonate prior to cracking this calcium hydroxide is sufficient to provide an efficient level of Ca2+ ions for healing. However, carbonation of this calcium hydroxide eliminates this source of Ca2+ ions.  Consequently, bacteria-based self-healing in mortars that have carbonated prior to cracking is almost totally dependent on the availability of Ca2+ ions released from an encapsulated source. Therefore, whilst the direct addition and encapsulation of calcium nitrate are both suitable for providing self-healing of concrete, the conditions of the concrete during service life need to be considered when choosing the most appropriate option.  Wet/Dry Conditions Water has an important role to play in bacteria-based self-healing.  It is essential for germination of bacterial spores and furthermore the mineral precursors need to be dissolved in water in order to be available to the bacteria [5] . Consequently, bacterial precipitation of calcium carbonate cannot occur if no water is present in the crack zone. A number of researchers have looked to determine to what extent self-healing can occur in different environments, and work has been carried out at Bath to compare self-healing in different conditions (Figure 2).      Figure 2: Healing conditions for bacteria-based self-healing concrete: (a) permanently wet, (b) semi-submerged and (c) cyclic wet-drying; 16 hours wet, 8 hours dry  The results show that self-healing is possible in all conditions. However, healing in semi-submerged conditions appears to occur more rapidly and may be related to the continual supply of oxygen. Healing in cyclic wet-dry conditions was better than that in permanently wet conditions.  Conclusions Bacteria-based self-healing of concrete is a reliable and useful technology for maintaining the durability and resilience of our infrastructure. However, several fundamental and application-related challenges remain to be solved before it will become a readily available and widely used construction material. This paper has reported on work that has further developed bacteria-based self-healing concrete in relation to these challenges.    Kevin Paine, Bianca Reeksting and Susanne Gebhard  Acknowledgements The authors wish to thank the EPSRC for their funding of the Resilient Materials for Life (RM4L) programme grant (EP/P02081X/1). We also thank all current members of the Bath team working on bacteria-based self-healing of concrete for their contributions: Veronica Ferrandiz-Mas, Charlotte Hamley-Bennett, Andrew Heath, Timothy Hoffmann, Ismael Justo-Reinoso, Xinyuan Ke, Lorena Skevi and Linzhen Tan.  References  [1] A. Al-Tabbaa, B. Lark, K. Paine, T. Jefferson, C. Litina, D. Gardner, T. Embley, Biomimetic cementitious construction materials for next-generation infrastructure, Proc. Inst. Civ. Eng. - Smart Infrastruct. Constr. (2018) 1–35. doi:https://doi.org/10.1680/jsmic.18.00005. [2] B.J. Reeksting, T.D. Hoffmann, L. Tan, K. Paine, S. Gebhard, In-depth profiling of calcite precipitation by environmental bacteria reveals fundamental mechanistic differences with relevance to application, Appl. Environ. Microbiol. (2020). doi:10.1128/AEM.02739-19. [3] J.Y. Wang, H. Soens, W. Verstraete, N. De Belie, Self-healing concrete by use of microencapsulated bacterial spores, Cem. Concr. Res. 56 (2014) 139–152. doi:10.1016/j.cemconres.2013.11.009. [4] M. Alazhari, T. Sharma, A. Heath, R. Cooper, K. Paine, Application of expanded perlite encapsulated bacteria and growth media for self-healing concrete, Constr. Build. Mater. 160 (2018) 610–619. doi:10.1016/j.conbuildmat.2017.11.086. [5] K. Paine, Bacteria-Based Self-Healing of Concrete: Effects of Environment, Exposure and Crack Size, Proc. RILEM Conf. Microorg. Mater. Interact. (2016) 1–15.   

Developments in bacteria-based self-healing of cementitious composites

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Developments in bacteria-based self-healing of cementitious composites

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