Cyclic Healing of Later-Aged Cracks in Bacteria-Based Self-Healing Cementitious Composites.

Concrete is extensively used in the construction industry worldwide because of its availability and affordability, but it is vulnerable to crack formation. Cracking of concrete is a natural phenomenon that severely reduces its durability due to the ingress of aggressive gases and liquids, which may result in corrosion of internal reinforcement bars. To extend the service life of concrete structures it is essential to repair cracks when they appear. However, frequent inspection to detect and repair these cracks becomes a complicated process with high associated direct and indirect costs. To counteract the negative effects that cracks have on the durability of concrete structures, self-healing technologies have emerged in the last decade as a viable and sustainable option to produce concretes with the potential to self-heal their cracks. Therefore, self-healing technologies are known strategies to contain cracking and recover the original permeability properties of concrete as well as improve durability. Many laboratory studies have shown that bacteria-based self-healing (BBSH) cementitious materials work when the element is cracked and allowed to heal at a relatively early age. However, little evidence exists on the capability of these BBSH cementitious materials to successfully heal later-aged cracks or re-heal previously healed cracks. This paper reports on work done at the University of Bath (UK), where the ability of BBSH cementitious composites to heal later-aged cracks (i.e., after 20 months) and re-heal reformed cracks that were previously healed at an early age (i.e., 28-days) were investigated. The results showed that BBSH cement mortar cracks that were successfully healed at an early age were not able to successfully re-heal when cracks were reformed in exactly the same location 20-months later; even when exposed to extremely favourable conditions (i.e., high humidity, temperature, calcium source and nutrients) to promote their re-healing. It was observed that after 20 months, no bacterial spores were likely available within the initially healed crack to successfully start a new self-healing cycle. However, it was observed that if entirely new cracks were intentionally generated at a different position in 22-months old mortars, the encapsulated bacterial spores remained viable inside the cementitious matrix and these cracks were able to self-heal. The results reported in this paper provide important insights into the appropriate design of practical self-healing concrete.