Evaluation of Advanced Delivery Methods to Transport Bio-Functional Components in Cracked Concrete

The effective delivery of bio-functional agents into cracked concrete remains a critical challenge for achieving reliable microbially induced calcium carbonate precipitation (MICCP)-based self-healing. This study investigates a hydrogel-assisted delivery approach using sodium alginate (SA) to facilitate the transport and distribution of Lysinibacillus sphaericus within subsurface cracks. An artificial crack (~0.2 mm width) was created in mortar specimens to provide a defined and localized pathway for hydrogel delivery. SA hydrogels at different concentrations (2%, 4%, and 6 wt%) and temperatures (20 °C, 40 °C, and 60 °C) were injected inside the crack in both types of samples with and without channels. The transport behavior was quantified through ImageJ analysis, while biological performance was evaluated using optical microscopy, thermogravimetric analysis (TGA), and X-ray computed tomography (X-CT). The results showed that 2% and 4 wt% SA hydrogels achieved >95% infiltration of the crack volume and with a minor influence of the drilled channel, indicating effective transport through the crack and pore network. Optical microscopy confirmed bacterial distribution throughout the crack depth. TGA showed about 18.23 mg and 8.14 mg higher CaCO3 precipitation in bacteria-treated specimens than in controls at 7 and 28 days, respectively, demonstrating MICCP activity. X-CT visualization further showed substantial crack filling in the bacteria-treated specimens, with an estimated 86.4% of the crack volume occupied by bio-products and a hydrogel matrix. These results demonstrate that an appropriately selected SA concentration and temperature can facilitate targeted bacterial delivery and MICCP-based crack filling.