At the Annual Meeting of the AAAS (American Association for the Advancement of Science) in Washington, D.C., Prof. Christian Grosse from the Technical University of Munich (TUM), Prof. Mo Li from the University of California (Irvine), and Prof. Erik Schlangen of Delft University of Technology talked about smart materials for sustainable infrastructure.
Grosse talked about a European Union research project on creating concrete that automatically repairs small cracks. Permanent loading and variations in temperature can cause small cracks to form in concrete. Grosse, chair of non-destructive testing (NDT) at TUM, said the cracks do not usually pose any direct threat to the stability of structures, “However, water and salts can penetrate the concrete (through such cracks) and damage the affected components.”
Traditional repairs to infrastructure are expensive and can result in long traffic jams. In the EU research project HealCON, an international team of researchers is working toward the development of concrete that can repair itself. The scientists are examining three different self-healing mechanisms.
Bacteria as mini construction workers
Certain bacteria produce calcium carbonate as a metabolic product. The scientists soak balls of clay with the spores of these bacteria and mix the balls into concrete. Once water penetrates the concrete, the microorganisms become active and release calcium carbonate, one of the main components of concrete. “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. They are used in diapers, among other things. Materials containing hydrogels can expand up to 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 further without expanding the cracks.
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. It forms a hard mass that seals the crack. It also has a positive side-effect: It increases structural stability.
Looking into concrete
Grosse and his colleagues specialize in testing how well these healing agents work in individual cases using non-destructive testing methods, such as acoustic emission technology.
When concrete cracks under pressure, it generates acoustic waves, which are measured using sensors. Using the measurement data, scientists can establish the precise location of the cracks. Following the healing process, the researchers carry out the experiment again. If the healing process was not successful, 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.
Testing structural components using ultrasound
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,” Grosse said.
Scientists measure the time required for ultrasound pulses to propagate through the concrete. Cracks prevent the transmission of the signal, which as a result needs more time to traverse the material. If the cracks have been filled, the pulses go through the material faster again. The strength of the signal also declines noticeably in the case of damaged material.
Promising results in creating self-healing concrete have been obtained from experiments carried out under laboratory conditions. The next stage will involve the use of the self-healing material in building actual components for bridges or tunnels. If the technologies succeed in real world application, they will have to be adapted for use in standard concrete production and construction methods.