Doctoral candidate Fabian Malm at the Technical University of Munich calibrates the sensors used to monitor the formation of cracks in concrete. Photo: Werner Bachmeier/TUM.
Doctoral candidate Fabian Malm at the Technical University of Munich calibrates the sensors used to monitor the formation of cracks in concrete. Photo: Werner Bachmeier/TUM.

At the recent Annual Meeting of the American Association for the Advancement of Science (AAAS) in Washington, D.C., Christian Grosse from the Technical University of Munich (TUM) in Germany revealed details about a European Union (EU) research project developing concrete that can repair itself.

Small cracks can form in concrete due to permanent heavy loads or variations in temperature. As Grosse explains, these cracks do not usually pose any direct threat to the stability of structures: "However, water and salts can penetrate the concrete and damage the affected components." For infrastructure such a bridges and tunnels, repairing this damage is expensive and can lead to long traffic jams.

Through the EU research project HealCON, an international team of scientists is now examining three different approaches to creating concrete that can repair the damage itself.

Bacteria as mini construction workers

The scientists have tried soaking balls of clay with the spores of bacteria that can produce calcium carbonate, one of the main components of concrete, as a metabolic product and mixing these balls into concrete. Once water penetrates the concrete, the microorganisms become active and release calcium carbonate, which fills up the cracks. "The bacteria can close cracks of up to a few millimeters in width in a matter of a few days," says Grosse.

Hydrogels as gap fillers

Hydrogels are polymers that absorb moisture, able to expand to 10 or even 100 times their original size. Cracks that form in concrete can be healed by a hydrogel that expands when it comes into contact with moisture, thus preventing the water from penetrating any further.

Greater strength thanks to epoxy resin

Epoxy resins or polyurethane can be encapsulated and mixed into the concrete. When the concrete cracks, the capsules break open and the polymer is released, forming a hard mass that seals the crack. This process also has another positive effect, as it increases structural stability.

Grosse and his colleagues specialize in testing how well these healing agents work in individual cases. To do this, they employ non-destructive testing methods such as acoustic emission technology, which involves exerting pressure on a concrete block containing one of the healing agents until it cracks. This cracking generates acoustic waves, which are measured with sensors. Using this measurement data, the scientists can not only establish that cracks have formed but can also determine precisely where.

Following the healing process, the researchers carry out the experiment again. If the healing process was unsuccessful, there are few new acoustic waves, as the cracks are still there. If the cracks have been filled, new ones arise, but in different places. "The localization of the crack sounds clearly indicates whether a remedy works or not," explains Grosse.

While acoustic emission analysis is suitable for laboratory applications, a different technology must be used for real-world on-site testing of large concrete components. "In this case, we use continuous ultrasound pulses," explains Grosse.

The scientists measure the time required for ultrasound pulses to propagate through the concrete. Cracks prevent the transmission of the signal, increasing the time taken by the pulses to traverse the material; if the cracks have been filled, however, the pulses take less time to travel through the material. The strength of the signal also declines noticeably for damaged materials.

Promising results have already been obtained from experiments carried out under laboratory conditions. The next stage will involve using the self-healing material in actual building components (sections of bridges or tunnels). After this, the technologies will have to be adapted for use in standard concrete production and construction methods.

The HealCON project is being funded as part of the European Union's 7th Framework Programme for Research and Technological Development. The project is coordinated by Ghent University in Belgium.

This story is adapted from material from the Technical University of Munich, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.