Microbial induced calcium carbonate precipitation (MICP) technology has been successfully used to enhance the properties of recycled concrete aggregates. However, the complex source and varied physical and chemical properties of recycled aggregates may have influence on the modification efficiency since microbes are often sensitive to the surroundings. In this study, two representative types of recycled aggregates, recycled concrete aggregates (RCA) and recycled brick aggregates (RBA), were subjected to two kinds of MICP treatments, basic MICP treatment and sodium alginate (SA) aided MICP treatment. The absorption and desorption of bacteria in/on aggregates during MICP treatments were quantified. The physical and chemical properties of aggregates after the bio-treatments were tested, and the influence of alkalinity and pore structure of aggregates under various MICP treatment methods on treatment efficiency were detailed investigated. Results indicated that, when the aggregates were subjected to the basic MICP treatment, the treatment efficiency was more remarkable in RBA, because of its high porosity and moderate pH (around 8–9), which facilitated the absorption of bacteria in/on aggregates and urease activity respectively. While under SA-aided MICP treatment, the influence of pore structure and alkalinity of aggregates on the treatment efficiency was not significant compared with that under the basic MICP treatment, especially the mass of CaCO₃ on the aggregates. The biogenic CaCO₃ generated by SA-aided MICP treatment not only plugged micropores, but also distributed all over the entire surface of the aggregates, resulting in a sufficient repair of microcracks. Meanwhile, the surface repair may reduce the influence of pore structure and pH of aggregates on the precipitation process, thereby reducing the impact of varied physical and chemical properties of aggregates on the treatment efficiency, which was conducive to the widely unified application of SA-aided MICP treatment in the modification of recycled aggregate based on construction solid waste.

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The suitability of using a spore-forming ureolytic strain, Bacillus sphaericus, was evaluated for self-healing of concrete cracks. The main focus was on alkaline tolerance, calcium tolerance, oxygen dependence, and low-temperature adaptability. Experimental results show that B. sphaericus had a good tolerance. It can grow and germinate in a broad range of alkaline pH. The optimal pH range is 7 ∼ 9. High alkaline conditions (pH 10 ∼ 11) slow down but not stop the growth and germination. Oxygen was strictly needed during bacterial growth and germination, but not an essential factor during bacterial urea decomposition. B. sphaericus also had a good Ca tolerance, especially at a high bacterial concentration of 10⁸ cells/mL; no significant influence was observed on bacterial ureolytic activity of the presence of 0.9M Ca²⁺. Furthermore, at a low temperature (10 °C), bacterial spores germinated and revived ureolytic activity with some retardation. However, this retardation can be counteracted by using a higher bacterial concentration and by supplementing yeast extract. It can be concluded that B. sphaericus is a suitable bacterium for application in bacteria-based self-healing concrete.

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Self-healing concrete has created a lot of public interest in recent years. Several research groups worldwide are currently working on creating durable and sustainable self-healing concrete structures. HEALCON (the concrete which repairs itself) is a European Union funded project, which focuses on developing cementitious materials with different selfhealing mechanisms. The self-healing mechanisms can either repair the cracks and regain liquid-tightness, bridge the cracks and recover structural performance, or do both. One of the promising materials that have been studied within the project is the bacteria-based selfhealing mortar, which is able to regain liquid tightness after cracking and healing. Within HEALCON an experimental methodology, which comprises of tests for evaluating the ability of the cementitious material to regain liquid-tightness and mechanical properties, has been developed. This study focuses on evaluating the suggested experimental methodology through a round robin test (RRT) among five laboratories within the framework of RILEM/TC 253 MCI (Micro-organisms-Cementitious Materials Interactions), WG4 (Engineered bacteria-based protective systems for cementitious materials) and it concerns only the part that examines the sealing efficiency. The testing sequence includes: - tests for material characterization, - crack introduction on mortar prisms, - healing treatment and - water tightness examination. Specimens with and without bacteria-based self-healing agent were tested. After the completion of the tests the results of the different laboratories were gathered for purposes of comparison. The comparison revealed high scatter in the results of the suggested methodology. Therefore, the current paper gives some recommendations, for improving the tests procedures, which will later be adapted to the second RRT that will follow.@en