Effect of carbonation on bacteria-based self-healing of cementitious composites

Self-healing cementitious composites are being developed to respond to the high cost of repair and maintenance of infrastructure. A promising solution is the use of bacteria to induce calcium carbonate precipitation within cracks when they occur and prevent further deterioration. Previous work has shown successful bacteria-mediated self-healing of cementitious composites at early-ages, in conditions where the material was uncarbonated prior to cracking. However, as cementitious composites often crack when they have reached a more aged state and are likely carbonated at the time of crack formation, these previous experiments did not fully reflect the real-world situation. In the present study, we show that for cementitious composites that do not carbonate prior to cracking the calcium hydroxide created as a hydration product is a sufficient source of Ca2+ ions to provide effective bacteria-induced healing. We note that supplying an extra source of Ca2+ ions at the moment of cracking, delivered via encapsulation, further enhances the degree of healing. Importantly however, in carbonated mortars calcium hydroxide is not available as a source of Ca2+ ions. Consequently, we show for the first time that 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. Our study therefore provides important insights for the rational design of self-healing concrete, where the conditions of the concrete during service life need to be taken into consideration when choosing between direct addition or encapsulation of calcium sources to ensure optimal performance.<br/

Effect of carbonation on bacteria-based self-healing of cementitious composites

Self-healing cementitious composites are being developed to respond to the high cost of repair and maintenance of infrastructure. A promising solution is the use of bacteria to induce calcium carbonate precipitation within cracks when they occur and prevent further deterioration. Previous work has shown successful bacteria-mediated self-healing of cementitious composites at early-ages, in conditions where the material was uncarbonated prior to cracking. However, as cementitious composites often crack when they have reached a more aged state and are likely carbonated at the time of crack formation, these previous experiments did not fully reflect the real-world situation. In the present study, we show that for cementitious composites that do not carbonate prior to cracking the calcium hydroxide created as a hydration product is a sufficient source of Ca2+ ions to provide effective bacteria-induced healing. We note that supplying an extra source of Ca2+ ions at the moment of cracking, delivered via encapsulation, further enhances the degree of healing. Importantly however, in carbonated mortars calcium hydroxide is not available as a source of Ca2+ ions. Consequently, we show for the first time that 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. Our study therefore provides important insights for the rational design of self-healing concrete, where the conditions of the concrete during service life need to be taken into consideration when choosing between direct addition or encapsulation of calcium sources to ensure optimal performance.<br/

The effects of biomineralization on the localised phase and microstructure evolutions of bacteria-based self-healing cementitious composites

Microbially induced calcite precipitation (MICP) is one of the most effectivemechanisms to achieving self-healing abilities in cementitious composites. However, there has only been limited understanding of the effect of the MICP process on the mineralogy and microstructure of the cementitious matrix closely mixed with the healing products. This study systematically assessed the effect of biomineralization on the localised cementitious binders at micro and atomic level combining different characterisation techniques (i.e. XRD, FTIR and μCT). The results show that, in addition to the formation of CaCO3 polymorphs that close the crack space, the MICP process will also modify the phase assemblages near the healed cracks. For the first time we observed that when the most common source of calcium for the MICP process (calcium hydroxide) is limited, ettringite and C-S-H can also act as the providers of the calcium for the biomineralization process to take place. The detailed microstructure characterisations support that, apart from the dense thin layer (around 0.5 mm) of healing products formed on the surface of the cracks, loose particle-like calcium carbonate crystals can also form in pores and voids, suggesting that healing can also be generated in deeper sections of the crack. The outcomes of this study advance the fundamental understanding of the MICP process in Portland cement binders, and will also assist the further evaluation of the durability performances of these self-healed cementitious composites

Piezoresistivity and piezopermittivity of cement-based sensors under quasi-static stress and changing moisture

Integrated cement-based sensors offer an economic alternative to extrinsic sensors for health monitoring applications in concrete structures due to their high strength to cost ratio, geometrical versatility, low shrinkage, and natural compatibility. Nonetheless, their performance under in-service conditions were in lack of investigations. While the piezoresistivity (change in resistance with stress) has been commonly used for mechanical sensing, the piezopermittivity (change in capacitive reactance with stress) is rarely characterized. Exploiting the high relative permittivity and electrical conductivity of carbon fibre reinforced cement-based sensors, this study investigates the piezoresistivity and piezopermittivity under changing stress and moisture using electrochemical impedance spectroscopy (EIS). Two types of sensors were evaluated: one containing 0.5 vol% of carbon fibres whose electrical conductivity was ionically dominant, and another with electronically dominant (1.2 vol% of carbon fibres) conductivity. Results highlighted that the piezopermittivity is “moisture content-dominant” whilst the piezoresistivity is “fibre content-dominant”. As the moisture content decreased, the sensitivity of piezopermittivity for both sensor types decreased, while the sensitivity of piezoresistivity decreased for the ionically dominant sensor but increased for the electronically dominant sensor. The piezoresistivity of the electronically dominant sensor was less sensitive than piezopermittivity at a water saturation of 80%. Conversely, the piezoresistivity of the ionically dominant sensor was more sensitive than piezopermittivity at the tested water saturations ≤ 80%. For the first time, this study presents the combined effects of moisture and fibre content on the pressure sensitive response of cement-based sensors through a dual-phase (i.e., piezoresistivity and piezopermittivity) EIS interpretation technique, providing valuable information to benefit further behaviour prediction and single-effect recognition in the field scenario where the sensors are subject to simultaneous environmental effects causing moisture variations such as temperature and humidity variations, freeze-thawing, and so on

Robust estimation of bacterial cell count from optical density

Optical density (OD) is widely used to estimate the density of cells in liquid culture, but cannot be compared between instruments without a standardized calibration protocol and is challenging to relate to actual cell count. We address this with an interlaboratory study comparing three simple, low-cost, and highly accessible OD calibration protocols across 244 laboratories, applied to eight strains of constitutive GFP-expressing E. coli. Based on our results, we recommend calibrating OD to estimated cell count using serial dilution of silica microspheres, which produces highly precise calibration (95.5% of residuals &lt;1.2-fold), is easily assessed for quality control, also assesses instrument effective linear range, and can be combined with fluorescence calibration to obtain units of Molecules of Equivalent Fluorescein (MEFL) per cell, allowing direct comparison and data fusion with flow cytometry measurements: in our study, fluorescence per cell measurements showed only a 1.07-fold mean difference between plate reader and flow cytometry data

The effects of biomineralization on the localised phase and microstructure evolutions of bacteria-based self-healing cementitious composites

Microbially induced calcite precipitation (MICP) is one of the most effectivemechanisms to achieving self-healing abilities in cementitious composites. However, there has only been limited understanding of the effect of the MICP process on the mineralogy and microstructure of the cementitious matrix closely mixed with the healing products. This study systematically assessed the effect of biomineralization on the localised cementitious binders at micro and atomic level combining different characterisation techniques (i.e. XRD, FTIR and μCT). The results show that, in addition to the formation of CaCO3 polymorphs that close the crack space, the MICP process will also modify the phase assemblages near the healed cracks. For the first time we observed that when the most common source of calcium for the MICP process (calcium hydroxide) is limited, ettringite and C-S-H can also act as the providers of the calcium for the biomineralization process to take place. The detailed microstructure characterisations support that, apart from the dense thin layer (around 0.5 mm) of healing products formed on the surface of the cracks, loose particle-like calcium carbonate crystals can also form in pores and voids, suggesting that healing can also be generated in deeper sections of the crack. The outcomes of this study advance the fundamental understanding of the MICP process in Portland cement binders, and will also assist the further evaluation of the durability performances of these self-healed cementitious composites