Factors affecting durability of concrete structures are generally associated with each other. Due to its brittle nature, concrete can crack under stress and these cracks are one of the main reasons for a decrease in service life in concrete structures. Therefore, it is crucial to detect and recover microcracks, then to repair them as they were developed to wider cracks. Recent research in the field of concrete materials suggested that it might be possible to develop a smart cement-based material that is capable of remediate cracks by triggering biogenic calcium carbonate (CaCO3) precipitaton. This paper summarizes a study undertaken to investigate the self-healing efficiency of Sporosarcina pasteurii (S. pasteurii) cells immobilized on both diatomaceous earth and pumice, to remediate flexural cracks on mortar in early ages (28 days after mixing). To obtain a two-phase bio additive, half of the minerals were saturated with a nutrient medium consisting of urea, corn-steep liqueur(CSL) and calcium acetate and the cells with immobilized to the other half without nutrients. Screening of the healing process was done with ultrasonic pulse velocity (UPV) testing and stereomicroscopy. With this approach, the cracks on mortar surface were sealed and the water absorption capacity of the so-called self-healed mortar decreased compared to its counterpart cracked mortar samples.

Recent research in the field of concrete materials showed that it might be possible to develop a smart cement-based material that is capable of remediating cracks by Microbial-induced calcium carbonate precipitation (MICP). The early remediation of microcracks enables the design of cement-based systems with an elongated service life with a sustainable approach. However, the main challenge of the application is to extend the viability of the cells against the restrictive environment of cement-paste. These cells have to tolerate the highly alkaline conditions of cement paste, survive the mixing process, and remain viable even when access to nutrients is limited. This paper summarizes a novel study undertaken to investigate the self-healing efficiency of Sporosarcina pasteurii (S. pasteurii) cells immobilized on zeolite and sepiolite minerals having the same particle size. This manuscript reports an extensive experimental study to understand the factors influencing the efficiency of immobilization barriers, such as composition and reactivity. To obtain the bio-additive, the bacterial cells were immobilized without nutrients and additional nutrients were only provided during the curing stage after crack initiation. Screening of the healing process was done with ultrasonic pulse velocity (UPV) testing and stereomicroscopy. Further evaluation on performance was done by evaluating the decrease in water absorption capacity. The healing precipitate was characterized through Environmental Scanning Electron Microscope (ESEM) and Fourier-Transform infrared spectroscopy (FTIR). With this approach, the cracks on mortar surface were sealed and the water absorption capacity of the so-called self-healed mortar decreased compared to its counterpart cracked mortar samples. Sepiolite was found to be a more suitable bedding for the microorganisms compared to zeolite, therefore samples containing sepiolite exhibited a higher performance in terms of crack healing. The results showed that while vegetative cell immobilization on locally available materials is a simple and economically feasible approach the healing capacity of bacterial cells can be hindered due to the reactivity of the mineral.

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Although steel reinforcements are used to withstand tensile forces in concrete, cracks are an unavoidable phenomenon. The presence of cracks, in fact, increases the risk for lowering the service life and durability of concrete structures. A critical issue occurs when due to splitting forces, cracks appear in concrete along the tensioned rebars which damage the bonding between the steel and concrete matrix. As a mitigation plan, the cracks should be healed at short notice and the bonding has to be recovered by the potential use of healing agents. This paper aims to investigate the bond behaviour of steel reinforcement in self-healing concrete. Two biomasses were employed as healing agents namely HTN (bacteria-based) and YEAST (fungi-based). The fresh and hardened properties of the normal and self-healing concretes were initially evaluated. The bond properties were investigated by performing pull-out tests on three different states of concrete: uncracked, cracked, and healed. Results revealed that the additions of biomasses did not induce negative effects on the compressive strength of hardened concrete. Moreover, the average bond strength of uncracked concretes containing HTN and YEAST improved by 20% and 8%, respectively, as compared with normal concrete. The introduction of a crack caused a significant reduction in bond strength regardless of the addition of healing agents. Nevertheless, it was found that the bond strength was slightly recovered after healing under water immersion.

As an alternative to the usage of axenic bacteria to achieve microbially induced calcium carbonate precipitation (MICP), this study evaluates the usage of two non-axenic biomasses as self-healing agents. A fungi-based consortium (Yeast) and heterotrophic nitrifiers (HTN) were harvested from the incubation of agricultural side streams. The characteristics of the two biomasses were identified through flow cytometry, total suspended solids and volatile suspended solids tests. The incorporation of the biomasses into concrete was evaluated in terms of compressive strength, flow and healing ability. Self-healing ability was analyzed through microscope imaging on prismatic (60x60x220 mm) samples. Cracks were induced with a three-point bending test where the widths were controlled with an LVDT sensor. A curing period of 56 days was applied to the samples and visual inspection was conducted at the start and end of the healing period with an optical microscope. Results compare and discuss the differing effects of active and autoclaved biomasses on the concrete properties and crack closure.