The concept of self-healing concrete - a concrete which can autonomously repair itself after crack formation, with no or limited human intervention - has received a lot of attention over the past 10 years as it could help structures to last longer and at a lower maintenance cost. This paper gives an overview on the key aspects and recent advances in the development of the bacteria-based self-healing concrete developed at the University of Technology of Delft (The Netherlands). Research started with the screening and selection of concrete compatible bacteria and nutrients. Several types of encapsulated bacteria and nutrients have been developed and tested. The functionality of these healing agents was demonstrated by showing metabolic activity of activated bacterial spores by oxygen consumption measurements and by regain of material functionality in form of regain of water tightness. Besides development of bacteria-based self-healing concrete, a bacteria-based repair mortar and liquid system were developed for the treatment of aged concrete structures. Field trials have been carried out with either type of bacteria-based systems and the promising results have led to a spinoff company Basilisk Self-Healing Concrete with the aim to further develop these systems and bring them to the market.@en

Self-healing concrete has drawn a lot of attention in recent years. There are numerous projects worldwide that work on the development of self-healing agents. Among those, bacteria-based self-healing concrete is a very promising solution to prevent durability problems in concrete that are related with cracking. Bacteria-based self-healing concrete not only provides sealing of open micro-cracks that endanger the structure’s health, but it also has economical and environmental benefits, since it will extend the lifetime of the structure and reduce repair costs. In this study, the bacteria-based self-healing agent consists of alkaliphilic bacterial spores, organic mineral precursor compounds and LightWeight Aggregates (LWA). Although the success of the concept has been proven in previous research, in this study an optimized method for the incorporation of the organic compounds into the LWA has been developed. The method allows more of the organic compounds to be stored into the LWA. This paper focuses on comparing performance on mechanical properties and sealing efficiency of cracks of mortar samples from past and current research. The initial hypothesis of the study was that the lightweight mortar with the higher amount of healing agent will show faster and more efficient crack sealing capacity. The comparison revealed different results than expected. In fact, the sealing efficiency trend proved to be similar for the two studies which was speculated to be due to oxygen limitation rather than healing agent limitation in permanently water submersed specimens.@en

Presented is a modified test for generating crack permeability data for cementitious materials. Single-parallel cracks were generated in mortar specimens. The width of the cracks was analysed through stereomicroscope and computer tomography, and the water permeability of the cracks was determined. Reduction factors and crack flow models were generated, and the reliability of those predictions was assessed. Cracks analysed through stereomicroscope produced reliable crack permeability predictions (r 2 = 0.97–0.98), highlighting the importance of testing multiple (≥ 7) replicates. The modified test produced accurate cracks (i.e., cracks that were within 20 µm of their desired crack width) and was easy to use allowing rapid permeability data (i.e., 10 h for 21 specimens) to be generated. The modified test will be of great use for those wanting to generate rapid, accurate, and reliable crack permeability data for cementitious materials. @en

Biogenic self-healing cementitious materials target on the closure of micro-cracks with precipitated inorganic minerals originating from bacterial metabolic activity. Dormant bacterial spores and organic mineral compounds often constitute a biogenic healing agent. The current paper focuses on the investigation of the most appropriate organic carbon source to be used as component of a biogenic healing agent. It is of great importance to use an appropriate organic source, since it will first ensure an optimal bacterial performance in terms of metabolic activity, while it should, second, affect the least the properties of the cementitious matrix. The selection is made among three different organic compounds, namely calcium lactate (CaL), calcium acetate (CaA), and sodium gluconate (NaG). The methodology that was used for the research was based on continuous and non-continuous oxygen consumption measurements of washed bacterial cultures and on compressive strength tests on mortar cubes. The oxygen consumption investigation revealed a preference for CaL and CaA, but an indifferent behavior for NaG. The compressive strength on mortar cubes with different amounts of either CaL or CaA (up to 2.24% per cement weight) was not or it was positively affected when the compounds were dissolved in the mixing water. In fact, for CaL, the increase in compressive strength reached 8%, while for CaA, the maximum strength increase was 13.4%.@en

Bacteria induced calcium carbonate precipitation based on metabolic conversion of nutrients has been acknowledged for having potentials in self-healing cement-based materials. Recent studies have shown the development of bacteria-based repair solution (liquid) for concrete surface repair. This article demonstrates the feasible application of the solution as healing agent to be injected into porous network concrete (PNC). This type of concrete has a porous core which can be used as a media to transport healing agents into the fracture zone. The repair capacity of the solution have been assessed by monitoring the bio-mineral precipitation in the porous cylinder cores. The X-ray tomography and permeability tests at certain time interval were carried out before and after injection of the solution. Polished sections were prepared and examined under ESEM after healing period to investigate healing capacity. The healing potential was then tested by injecting the solution into PNC. The injection of tap water and bacteria based solution was performed through porous network until it reached and flew out through the crack which was formed by three-point bending loading. The healing efficiency was measured by water permeability test before and after injection at several time intervals. The specimens injected with bacteria based solution and cured in wet condition showed higher healing efficiency compared to dry cured specimens.@en

The innovative technology of self-healing concrete allows the material to repair the open micro-cracks that can endanger the durability of the structure, due to ingress of aggressive gasses and liquids. Various concepts of self-healing concrete have been developed, with target on the recovery of water tightness after cracking. Among those, bacteria-based self-healing concrete has shown promising results regarding the improvement of crack sealing performance. In this study, the bacteria-based healing agent is incorporated into lightweight aggregates and mixed with fresh mortar. By this means, autogenous healing of concrete is enhanced and upon cracking the material is capable to recover water tightness. The study focuses on the investigation of the effect of healing agent when incorporated into the mortar matrix and the evaluation of the recovery of liquid tightness after cracking and exposure to two different healing regimes (water immersion and wet-dry cycles) through water permeability tests. It was found that the compressive strength of the mortar containing lightweight aggregates is not affected by the presence of the healing agent. The study also reveals that the recovery of water tightness does not differ substantially either for specimens with or without healing agent when immersed continuously in water. Conversely, the recovery of water tightness increases significantly for specimens containing the healing agent compared to specimens without it, when subjected to wet-dry cycles. Oxygen concentration measurements and bacterial traces on calcite formations confirmed the bacterial activity on specimens containing the healing agent.@en

In this chapter an overview will be given of the biotechnological possibilities for repair of concrete with focus on application of limestone-producing bacteria and the different metabolic pathways involved, e.g., via hydrolysis of urea and heterotrophic CO2 production under alkaline conditions. The first paragraph comprises an overview of previously published reports on this subject. In the two succeeding paragraphs, two specific systems for biotechnological repair of concrete structures will be discussed. The first one covers liquid biobased repair systems for durable repair of cracked and porous concrete surfaces, and the second one addresses biobased mortar systems for repair of larger defects of concrete structures. The cases discussed here indicate that concrete repair applying biotech solutions results in improved material durability that can save money and at the same time lower the environmental impact of civil engineering activities.@en

This work presents a bacteria-based bead for potential self-healing concrete applications in low-temperature marine environments. The bead consisting of calcium alginate encapsulated bacterial spores and mineral precursor compounds was assessed for: oxygen consumption, swelling, and its ability to form a biocomposite in a simulative marine concrete crack solution (SMCCS) at 8 °C. After six days immersion in the SMCCS the bacteria-based beads formed a calcite crust on their surface and calcite inclusions in their network, resulting in a calcite-alginate biocomposite. Beads swelled by 300% to a maximum diameter of 3 mm, while theoretical calculations estimate that 0.112 g of the beads were able to produce ∼1 mm3 of calcite after 14 days immersion; providing the bead with considerable crack healing potential. The bacteria-based bead shows great potential for the development of self-healing concrete in low-temperature marine environments, while the formation of a biocomposite healing material represents an exciting avenue for self-healing concrete research.@en

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

The development of bacteria-based systems for the protection of concrete structures has gained lot of attention over the past few years as it could contribute to lower the maintenance cost and increase the durability of concrete structures. These systems are based on Microbial Induced Precipitation (MIP), a method by which calcium carbonate precipitation is induced by bacteria. This paper gives an overview of field applications with the bacteria-based systems developed at the Delft University of Technology (the Netherlands): self-healing concrete and repair systems. Field applications in collaboration with stakeholder parties involve casting of self-healing concrete as linings for irrigation canals in Ecuador (July 2014), patch repair for the waterproofing of leaking cracks with self-healing mortar (2013-2014) and improving the freeze-thaw resistance and sealing of cracks in parking decks with the application of a liquidbased repair system (2013-2016). These large scale applications proofed the functionality and market potential of these bacteria-based protective systems.@en

Self-healing of cracks in concrete can be achieved by application of bacteria which metabolically convert organic compounds under aerobic conditions yielding limestone. Added to the concrete mix as part of a healing agent, bacteria can, via metabolic activity resulting in limestone formation, seal cracks of up to 0.8mm width resulting in waterproofing and increased frost damage resistance of the concrete. Besides increasing the autogenous crack healing capacity of concrete, these bacteria can also be applied to improve bond strength of repair mortar and act as limestone producing agent in liquid concrete repair systems. This paper will review current state-of-the art bacteria-based self-healing concrete technologies investigated and applied by the Delft University research group specifically using bacteria which yield limestone after aerobic metabolic conversion of organic compounds under alkaline conditions.@en