‘Healing’ agents in microcapsules burst to fill cracks as they form
A self-healing concrete that mimics the human body’s natural ability to repair minor damage is being developed by researchers at the University of Cambridge.
The material is filled with millions of microcapsules that burst to fill cracks as they occur. With an average size of 0.1 to 0.5mm, the capsules contain ‘healing’ agents such as minerals, epoxy or bacteria.
Mixed into concrete, the tiny balls rupture when cracks start to form as a result of fatigue, releasing their payload to stabilise the material. In some cases the microcapsules could replace regular aggregate in concrete.
The research is being carried out by the Department of Engineering’s Geotechnical and Environmental Research Group as part of the £4.8 million ESPRC-funded project Resilient Materials 4 Life.
The science of the methodology is proven, but encapsulating the healing agents remains a hurdle. Livia Ribeiro de Souza, a postdoctoral researcher in the group, says: ‘That’s a big question we’re still trying to answer. Some materials are very reactive and don’t want to be encapsulated, some are not stable and do not remain as a healing agent for long. We have experimented with spray drying and also emulsification polymerisation, including the production of a double emulsion that forms a liquid core with a shell structure around it. We are still investigating the best and most cost-effective methodology to encapsulate the microcapsules.’
Particularly promising is a Dolomite Microfluidics system that binds strongly to the cement matrix, while also having thinner shell walls and higher core retention to improve the self-healing properties.
Self-healing technology has huge implications for the lifespan, maintenance and repair of concrete structures. In experiments with capsules containing sodium silicate it took an average of around four weeks for the healing process to complete. The chemical reacts with the internal face and opening of cracks to close them up.
‘Cracks are a preferential path for water and contaminants, so by closing this path you have effectively recovered the transport properties of the material [reduced the permeability of the damaged sample],' says de Souza.
Researchers are also devising ways to quantify the self-healing process and bring the technology to market by scaling it up from the laboratory. Industry partners include major contractor Costain, Cemex UK and Highways England.