Autonomous masonry repair with a bacteria-based self-healing mortar

Abstract

Cracking is an inherent form of damage in unreinforced masonry due to its limited tensile strength. Although conventional repair strategies such as repointing are effective, they often require repeated interventions throughout a structure’s service life. This thesis investigates whether engineered self-healing mortars can provide autonomous damage mitigation, with particular attention to historic masonry and lime-based mortars.
To this end, a dedicated framework was developed to assess self-healing mortars for masonry repair. It addresses recovery of performance (in terms of aesthetics, watertightness, and mechanical properties), identification of healing products, and impact on functional properties across three material levels: mortar, brick–mortar, and masonry component. The framework was applied using a commercially available bacteria-based polylactic acid (PLA) healing agent, originally developed for concrete repair, with solid clay bricks and a lime–cement mortar representative of historic Dutch masonry.
At the mortar level, tests on prisms with different lime-to-cement ratios (0, 0.33, 1, and 1.33) provided evidence of both autogenous and agent-enhanced healing, with their relative contributions depending on crack width. For fine cracks of approximately 0.07 mm, reference and agent-containing mortars showed comparable mechanical recovery, indicating a predominant contribution from autogenous healing. For cracks wider than approximately 0.3 mm, however, the agent-containing mortars showed greater aesthetic crack filling than the references, indicating an additional contribution from the healing agent. Infrared spectroscopy identified calcium carbonate as the main healing product in both mortar types, with the formation of this low-solubility compound pointing to possible watertightness recovery.
At the brick–mortar level, tests on couplets and cores constructed with solid clay bricks and the selected high-lime mortar confirmed both autogenous and agent-enhanced healing, with the presence of the agent extending recovery beyond the effective range of autogenous healing at larger crack widths. For fine cracks of approximately 0.065 mm, reference and agent-containing couplets achieved comparable flexural bond strength recoveries of 45% and 46%, respectively, again indicating a predominant contribution from autogenous healing. For larger cracks of approximately 0.16 mm, the reference specimens showed no recovery, whereas the agent-containing specimens achieved an average recovery of 27%, with values reaching 65% after 180 days under wet conditions. In addition, a permeability-based method, adapted from procedures used in self-healing concrete research, was developed in this thesis to assess watertightness recovery in brick–mortar cores. Partial watertightness and aesthetic recovery were observed. However, because the cores were cracked to failure, variability in crack geometry introduced considerable scatter and limited the interpretation of the results, highlighting the need for further research to refine the method.
At the masonry-component level, leaching of the PLA-based carrier from the mortar joints was observed during curing and subsequent wet healing, and no effective healing was achieved. This behaviour was attributed to scale-dependent moisture transport, highlighting the role of curing when healing agents are incorporated into masonry and the need for further research to establish suitable curing procedures that retain the agent and enable healing in larger components.
Overall, this thesis demonstrates the potential of incorporating engineered healing agents into masonry mortars to supplement autogenous healing and promote autonomous repair without external intervention, thereby opening a new research direction for masonry repair. The bacteria-based PLA agent investigated, originally developed for concrete and designed to promote calcium carbonate precipitation, showed potential for use in lime-based masonry mortars at the mortar and brick–mortar levels, supporting further investigation of its use in masonry repair. Beyond evaluating this specific agent, the thesis provides a structured experimental framework and identifies key parameters governing healing across material levels, establishing a basis for further systematic research in this emerging field.