In concrete structures, it is always a preferable idea to prevent the damage before it happens rather than to repair it afterwards, since it is usually less costly and in some cases the damage detection is impossible. Temperature and humidity fluctuations and/or external loading can trigger micro-cracking on a concrete structure, which in turn can open a pathway for harmful liquids and gasses. Those substances can degrade either the cement matrix or the embedded reinforcement and can cause an extended and irreversible damage. Prevention of damage or instant repair are not always achievable. Therefore, the idea to develop a cementitious material, which can sense the damage and repair it itself in order to mitigate the loss of durability, has gained ground in the last two decades.@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

Self-healing concrete can repair itself by closing micro-cracks and thus protect itself from ingress of deleterious gasses and liquids that can affect its durability. Many self-healing concepts have been developed in the recent years which target on the recovery of water tightness after cracking. Among those systems, the bio-based healing agents have shown promising results regarding the crack sealing performance. This paper studies the crack sealing efficiency of bio-based healing mortar with expanded clay particles. The investigation of sealing performance is conducted through experimental and computational approaches. Image processing and crack permeability test results are compared with results obtained by computer simulations. The study reveals that the experimental approaches might overestimate the crack closure percentage, while the computer simulation mostly underestimates the crack sealing. Finally, recommendations are given to improve the results obtained by both methodologies.

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Superabsorbent polymers (SAPs) have potential to be used as healing agent in self-healing concrete due to their property to attract moisture from the environment and their capacity to promote autogenous healing. A possible drawback, however, is their uptake of mixing water during concrete manufacturing, resulting in an increased volume of macro-pores in the hardened concrete. To limit this drawback, newly developed SAPs with a high swelling and pH-sensitiveness were developed and tested within the FP7 project HEALCON. Evaluation of their self-sealing performance occurred through a water permeability test via water flow, a test method also developed within HEALCON. Three different sizes of the newly developed SAP were compared with a commercial SAP. Swelling tests in cement filtrate solution indicated that the commercial and in-house synthesized SAPs performed quite similar, but the difference between the swelling capacity at pH 9 and pH 13 is more pronounced for the self-synthesized SAPs. Moreover, in comparison to the commercial SAPs, less macro-pores are formed in the cement matrix of mixes with self-synthesized SAPs and the effect on the mechanical properties is lower, but not negligible, when using high amounts of SAPs. Although the immediate sealing effect of cracks in mortar was the highest for the commercial SAPs, the in-house made SAPs with a particle size between 400 and 600 μm performed the best with regard to crack closure (mainly CaCO₃ precipitation) and self-sealing efficiency, after exposing the specimens to 28 wet-dry cycles. Some specimens could even withstand a water pressure of 2 bar.

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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

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

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

Within the European FP7 project HEALCON, Non-Destructive Testing (NDT) and monitoring techniques are developed and combined to characterize the effects of self-healing mechanisms in small and full-size specimens. In the first stage, healing mechanisms were evaluated at lab-scale. Specimens containing encapsulated polymer precursors were cracked and reloaded after the healing period. During loading, healing and reloading, NDT techniques (acoustic emission analysis, vibration analysis and ultrasonic measurement) were applied to help understanding the cracking behavior, capsule breakage and healing efficiency. Moreover, the effect of the flexibility of the polymeric healing agent on the crack re-opening during reloading was investigated on cracked and healed mortar specimens, using acoustic emission and digital image correlation techniques. The results show the applicability of NDT methods to evaluate the self-healing efficiency for small specimens. Comparing the NDT techniques, some of them (e.g. ultrasound) seem to be good candidates for in situ monitoring of the healing efficiency.

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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