Long-term viable SF immobilized bacterial cells as sustainable solution for crack healing in concrete

Utilization of microbially induced calcium carbonate precipitation (MICCP) via biomineralization process has been considered a novel method in self healing of concrete structures. To develop MICCP as ready product for field scale construction, mineral based inoculum of higher shelf life is required. The present study focuses on this aspect of field based application of MICCP. In this study, mineral admixture (silica fume (SF), cement kiln dust (CKD) and rice husk ash (RHA)) based bacterial inoculum were developed to immobilize bacteria for self healing capabilities in concrete. The prepared inoculum was stored at varying temperature (4 °C and 25 °C) and the survival of bacterial cells in carrier based materials were tested on weekly basis until 180 days of storage at both temperature. After 180 days of storage, the most efficient bacterial inoculum based upon viability studies was incorporated in concrete and tested for crack healing performance. For this, crack of approximately 0.5 mm width was generated during the casting and healed using a bacteria-based healing agent (nutrient broth, urea and CaCl2). The healed cracked surface was examined through optical imaging to monitor the crack width reduction. Along with this, Electromechanical Impedance (EMI) technique was used to monitor the crack healing potential until the full healing of cracked surface was achieved. Statistical EMI indicators such as root mean square deviation (RMSD), mean absolute percentage deviation (MAPD) and correlation coefficient deviation (CCD) were employed to quantify crack healing process. RMSD was found to be promising parameter to monitor the crack healing process and reflected an increasing trend from 1 % to 14 % in healed bacterial specimen. At the end of test, the healed specimens were subjected to bending failure to assess the strength regain. Significant regain was noticed in healed bacterial specimen (approximately 33 %) in comparison to the control. The healing mineral precipitated inside the cracks was examined through field emission scanning electron microscopy (FESEM), energy dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD) and thermogravimetric analysis (TGA) to evaluate its physicochemical attributes. The results give clear proof that the SF-based carrier material can be effectively used in healing the cracks and the crack healing process can be efficiently monitored by the EMI technique. Results further indicated that RMSD values were very effective in quantifying the progressive crack healing achieved due to MICCP precipitation.