Optimization of bacteria-based self-healing cementitious composites: Effect of healing agents and the impact of carbonation and environment

Abstract

Microbially induced calcite precipitation (MICP) has been widely investigated for its application in self-healing concrete. Several studies in the past few years presented that MICP is a promising approach to seal and rehabilitate the shallow crack on concrete structure. Such studies have revealed that healing could occur in different conditions. In addition, bio-agents, including bacteria, calcium source, and nutrients, should be encapsulated in carrier to ensure the efficiency of healing.

Therefore, the main objective of this research was to optimize the efficiency of healing from the aspects of bio-agents’ content, carrier, calcium source, and evaluate the impact of carbonation and healing environment on the healing efficiency of bacteria-based self-healing mortar (BBSHM). The only bacterial strain used in this research was Bacillus Cohnii aiming to facilitate the comparison. To evaluate the healing degree, it requires an understanding of quantity and quality of healing, permeability and durability of concrete, and influence of the addition of bio-agents on the properties of concrete.

This research is divided into five steps from the aforementioned aspects. Step 1 compared the feasibility of two types of carriers, perlite and aerated concrete granules (ACG), for bio-agents in BBSHM. ACG showed generally greater feasibility in terms of density, water absorption, and healing performance of BBSHM. As BBSHM using perlite reached 97% healing ratio in water flow coefficient as the highest, but ACG-based BBSHM achieved about 100% healing. In addition, coating was used to better protect carriers from leaking, compared to double-layers (DL) coating consisting of sodium silicate and cement powder, PVA (polyvinyl acetate) coating was more feasible in BBSHM. Mortars using DL coating had maximum of 86% healing in crack width, however, crack of mortars using PVA coating achieved 100% healing in width.

To maximize the amount of precipitated calcite, step 2 evaluated the influence of bio-agents’ content, including bacteria, calcium acetate, and yeast extract, on the healing performance of BBSHM. It was found that the amount of yeast extract had relatively governing effect on the healing efficiency of BBSHM. Moreover, a hypothesis was raised for a proportional relationship between the content of each bio-agent. For certain amount of calcium acetate, a range of bacterial cells’ number may be presented in the process of ICP to achieve highest amount of precipitated calcite.

To facilitate the manufacturing process, step 3 determined the possibility of using calcium nitrate as the calcium source to non-ureolytic bacteria in BBSHM, as calcium nitrate can be directly added into cement matrix to compensate the retardation caused by yeast extract. Results suggested that non-ureolytic bacteria successfully precipitated calcite by using calcium nitrate and calcium nitrate-based BBSHM achieved high-level healing reaching 100% healing ratio of both crack size and water flow coefficient.

Step 4 investigated the effect of carbonation on the healing efficiency of BBSHM to simulate the aged concrete in-situ. It was found that carbonation had significantly negative impact on the healing. In addition, the encapsulation and higher amount of yeast extract contributed to the healing.

Finally, to evaluate the influence of the healing environment, step 5 investigated the healing performance of BBSHM in four healing regimes, including i) daily wet-dry cycle healing regime, ii) weekly wet-dry cycle healing regime, iii) deposition medium healing regime, and iv) semi-submerged healing regime. Water was generally shown to be more essential than oxygen in terms of MICP, and healing regime with simultaneously supplied oxygen and water worked greater for BBSHM containing directly added calcium and yeast extract. Moreover, extra supplied calcium source (10 g/l) and yeast extract (2 g/l) in the cycling water were shown to promote healing at early stage but damage the precipitated calcite afterwards. In addition, to better understand the requirement of oxygen, oxygen-releasing coating (ORC) containing calcium peroxide was used in BBSHM, results indicated great feasibility in BBSHM and promoted healing to some extent, where ORC-based BBSHM achieved 100% healing in crack width in daily wet-dry cycle healing regime, and more than 60% healing in water flow coefficient in weekly wet-dry cycle healing regime.

Date of Award	21 Jul 2021
Original language	English
Awarding Institution	University of Bath
Supervisor	Kevin Paine (Supervisor) & Veronica Ferrandiz-Mas (Supervisor)