Air-entraining admixtures as a protection method for bacterial spores in self-healing cementitious composites: Healing evaluation of early and later-age cracks

Costs associated with the encapsulation process of bacterial spores continue to be a limiting factor for the commercialisation of self-healing cementitious materials. The feasibility of using air-entraining admixtures (AEAs) as an economical and straightforward encapsulation method for bacterial spores was evaluated to heal cracks (-0.50 mm) that were formed at an early (28 days) or later age (9 months). Three AEAs, commonly used in concrete industry, were compared to a successfully proven protection method (i.e., via aerated concrete granules (ACGs)). In this regard, only one of the three AEAs investigated improved the healing performance when compared to an equivalent mix using bacterial spores encapsulated in ACGs. Healing ratios obtained with this successful AEA were 59.6% and 46.2% higher than the results observed for the ACGs-containing mix when the cracking age was 28 days and 9 months, respectively. Moreover, water penetration resistance was increased by 18.1% or presented very similar values (-84%) after 56 days of healing for early or later-formed cracks, respectively. Moreover, a simple cost analysis was conducted to confirm the significant economic benefits of using AEAs to protect directly added bacterial spores. In this regard, the cost of using AEAs is about 13 times lower than for ACGs. Therefore, this study provides for the first time, evidence of the feasibility of using AEAs to protect bacterial spores, opening the doors to the development of bespoke AEAs to design cost-efficient selfhealing cementitious materials.