Comparison of improved autogenous and bio-based self-healing techniques in fiber-reinforced mortar: Effect of bacteria incorporation strategy and fiber hybridization

Abstract

Efficacy of autogenous and bio-based self-healing in cement-based materials varies depending on the mix design strategy and the imposed damage level. This research aims to compare the crack sealing and recovery of strength and permeability in damaged mortars using three techniques – (1) improved autogenous with superabsorbent polymer (SAP) and fibers, (2) bio-based strategy using directly added bacteria with SAP and fibers and (3) bio-based strategy using biochar-immobilized bacteria and fibers. Bio-based approach involves self-healing through calcium carbonate precipitation generated by cell metabolism of the added bacteria (Bacillus Sphaericus). Two fiber combinations have been employed – only PVA fiber and hybridization of PVA and steel fibers. Experimental results show that the bio-based approach is more efficient in sealing cracks wider than 150 μm compared to autogenous healing, while type of fiber does not significantly affect sealing of surface cracks. Hybridization of fibers offers 8–12% higher recovery of water penetration and initial sorptivity compared to PVA fiber alone. Irrespective of the mix design, the hybridization strategy prevents drastic reduction in recovery ratio of sorptivity compared to mortar with only PVA fiber as the damage level is increased from 60% to 80% of the peak compressive stress. Autogenous healing and bio-based healing using directly added bacteria spores show similar depth of water penetration after healing, while the biochar-immobilized spores reduces depth of penetration and increases its recovery ratio by 7–10%, attributed to crack pinning effect of biochar. Bio-based self-healing strategy reduces rate of water sorptivity through capillary suction in healed mortars by 40–80% compared to autogenous strategy due to more complete filling of cracks and interfacial zones by bio-mediated calcium carbonate than hydration products. Finally, a regression model to predict the strength after healing as a function of water sorptivity and water penetration is developed.