The importance of concrete had been proven without a doubt. Used as a building material since ancient times, today it is said to be the second most consumed substance on earth after water[1]. But concrete cracks. This consequence is systematic, and therefore expected. However, this does not prevent repairs carried out after commissioning from being costly and technically difficult. Gels, fungi or bacteria… several research centres around the world are working on proactive self-healing concrete processes, anticipating cracks as early as the design or the placing of the concrete.

Concrete cracking, a common and ancient phenomenon

Concrete cracking is a common phenomenon that occurs for a variety of reasons: the nature of its composition, its drying time, the tensile stresses applied or vibrations, to name a few. Cracks are acceptable as long as they do not jeopardise the stability and durability of the infrastructure. The use of polymers makes it possible to fill cracks and restore the material’s resistance, especially if an anti-corrosive product is added to limit corrosion of the reinforcements. It can be seen that Byzantine constructions (4th century AD) such as the Hagia Sophia in Constantinople or Roman constructions, contain crystals resulting from chemical reactions caused by calcium silicates and water, thus preventing the enlargement of cracks some time after the commissioning of the building. This phenomenon of self-healing of concrete is therefore not new and comes either from the hydration of residual cement or from carbonation (capture of ambient CO2). However, this only applies to relatively small cracks of a few tenths of a millimetre. Several research centres have been working in recent years on self-healing concrete processes, also called self-repairing or self-regenerating concrete. Solutions are based on the use of bacteria, fungi or gels.

Solutions

The use of bacteria is a solution to promote this self-healing. The idea is to produce calcium carbonate through bacteria or micro-organisms and a nutrient broth. The idea is not new, because this bio-mineralisation has been used for about twenty years for treating stone by the Amonit company in the form of a patch: the Café de la Paix and the Chamber of Commerce in Toulon were already treated more than 10 years ago. The idea here is to integrate it into the concrete during its manufacture so that it is only activated when a crack appears. For the past ten years or so, research on this subject has been flourishing. Recent articles report on a team of researchers at the University of Colorado working on the integration of photosynthesis in concrete. The principle involves introducing bacteria into the concrete to capture light and carbon dioxide present in the air to produce calcium carbonate. This “living concrete” would be a way to seal cracks in the concrete but this method requires many adjustments such as the life span of bacteria and the physical capacities of the concrete for example. Other teams of researchers are exploiting the potential of bacteria mentioned above. At the University of Delft, research has been carried out on the incorporation of bacteria and calcium lactate into clay capsules, which are used as aggregates in the manufacture of concrete. The capsules crack along with the concrete and cause bacterial activity, leading to the production of calcium carbonate which seals the cracks. In the same vein, researchers in the US states of New York and New Jersey are working on the implementation of fungi in concrete. Like bacteria, they would react with the water and oxygen brought in during the cracking of the concrete and produce calcium carbonate that would plug the cracks.

Practical implementation and market realisation

The principle developed by researchers at the University of Delft was tested from 2011 and resulted in a product for self-healing concrete in collaboration with the University. The solution marketed by the Dutch startup Basilisk can be used on existing or new structures. It can repair cracks up to 0.8mm wide in existing infrastructures and has been tested in a car park in Apeldoorn in the Netherlands. On the other hand, the use on new infrastructures can prevent cracks up to 1mm wide, once the solution is mixed with the rest of the concrete ingredients during the preparation of the concrete. This process was used for the concrete water basin construction project in the port of Rotterdam in 2017. In Belgium, a recent full-scale test was carried out on a similar principle in the cover slab of a concrete manhole cover. The bacteria-based self-healing solution was added to the concrete mixer before pouring. While the results obtained in the laboratory are encouraging, it is too early for Belgian researchers to draw conclusions on the application of this method. Indeed, although the conditions are met for good self-healing, the concrete had not yet cracked at the time of an inspection one year later. However, it is an important step in convincing the construction industry of the value of the self-healing method for concrete, especially as the number of solutions is increasing. If feedback from real size application experiences is scarce, it is also because the cost of this method is substantial. According to the estimates of the Dutch startup, the price per cubic metre would have to be doubled[2] compared to conventional concrete. It is also difficult to know at what crack size the process is no longer operative, and what mechanical continuity this self-repair offers. Otherwise, it would play a protective role with regard to sustainability. A delicate subject is also the resistance of the capsules, especially during the concrete mixing phases, which can break them and therefore alter or cancel the effectiveness of the solution. On the same subject, researchers are interested in polymers in the form of gels that, once incorporated into materials, would absorb CO2 from the ambient air to strengthen and solidify. Today they are not strong enough to be considered building materials. The search continues…   [1] “Concrete: the most destructive material on Earth”, The Guardian, 25/02/2019 https://www.theguardian.com/cities/2019/feb/25/concrete-the-most-destructive-material-on-earth [2] “Self-healing concrete”, Ingenia magazine, March 2011