Bacteria-based self-healing cementitious materials for cold and temperate climates

Student thesis: Doctoral ThesisPhD

Abstract

Various types of cracks occur inevitably in concrete, affecting the durability of the material and consequently the structure’s service life. Self-healing concrete offers an alternative strategy for confronting cracking. It shifts the focus from designing crack-free materials to designing materials that can manage their cracks when these occur. Bacteria-based self-healing concrete (BBSHC) utilises the ability of bacteria to induce the precipitation of calcium carbonate through their metabolic activities when certain conditions are met. The precipitated calcium carbonate serves for healing the cracks. Currently, there is a need to investigate the efficiency of BBSHC technologies at lower temperatures and develop techniques to encourage its application under realistic conditions. Thus, this PhD aimed to extend the application of BBSHC in cold and temperate climates by investigating key parameters that affect the healing at lower temperatures. First, bacteria-cement interactions were researched with regards to their effect on the mechanical properties of cementitious composites. Bacteria in live and dead state at different concentrations were introduced into cementitious composites, and their hydration and strength performance were evaluated. It was shown that the bacterial addition resulted in increased strength for most concentrations, with the optimum being 10^7 cells/ml. It was concluded that Microbially Induced Calcite Precipitation was not the driving force of the strength enhancement. Instead, it was suggested that strength improvement was related to interactions between the biological ionic components of the cell and the dissolved ions of the cement. Next, the effect of bacteria type and concentration on the self-healing performance of cement mortars at 20 °C and 7.5 °C was investigated. Two mesophilic and one psychrotrophic bacteria species, namely B. cohnii, A. pseudofirmus, and S. psychrophila were incorporated in mortars along with nutrients and precursors. The healing of the mortars at 20 °C and 7.5 °C was monitored using optical microscopy and water-flow tests for a healing period of up to 84 days and evaluated with Scanning Electron Microscopy, Energy Dispersive X-Ray Spectroscopy and X-ray computed tomography. The psychrotrophic strain outperformed the two mesophilic species as it was found that the addition of the S. psychrophila at a low concentration led to the highest healing at both temperatures with a decrease of the crack area by 62% and 74% at 20 °C and 7.5 °C respectively after 84 days of healing. Instead, similar healing was attained with the mesophilic A. pseudofirmus only when added at a higher concentration. The efficiency of BBSH in early and later age concrete was examined. Mortars cracked at three different ages, at 3, 28 and 270 days were investigated and their healing at 20 °C and 7.5 °C was assessed. The long-term healing efficiency of the method was demonstrated at both temperatures, as the crack area was decrease by 76% and 70% after 84 days of healing at 20 °C and 7.5 °C respectively. Samples cracked at 3-days, on the other hand, failed to present any healing. Finally, based on observations made in the previous investigations, modifications to the BBSHC system were examined. Thus, the adverse effect of yeast extract on the hydration and the strength of BBSHC was attempted to be minimised by replacing it with other organic compounds that could serve as nutrients for the bacteria. It was found that different bacterial species, namely A. pseudofirmus and S. psychrophila, responded differently to the modified nutrient source, consisting of 50% yeast extract and 50% sodium acetate. The partial replacement of yeast extract with sodium acetate enhanced the low temperature healing when using the mesophilic A. pseudofirmus and hindered it when using the psychrotrophic S. psychrophila strain, as the healing ratios were at 7.5 °C were 76% and 23% respectively. Preliminary modifications of the encapsulation process of the bacteria using electrospinning fibres were also in presented in this part of the work. Overall, this thesis provides insights on BBSHC at low temperature and identifies the key parameters that affect it. As the research on BBSHC progresses and moves from the laboratories to in-situ applications, this information becomes crucial for establishing the efficiency of the technology under different environmental conditions and is aligned with the vision for a new generation of self-sustaining resilient materials and structures.
Date of Award24 May 2023
Original languageEnglish
Awarding Institution
  • University of Bath
SupervisorKevin Paine (Supervisor) & Susanne Gebhard (Supervisor)

Keywords

  • concrete
  • cracking
  • self-healing
  • bacteria
  • calcite precipitation
  • mechanical performance

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