Self-healing concrete was voted the technology most likely to have the biggest global effect in a competition among presentations at the recent British Science Festival in Newcastle, UK.
‘I spoke about creating intelligent material for the construction industry that has the ability to sense and respond to damage and repair itself,’ says Diane Gardner, a civil engineer at Cardiff University. ‘We spend £40bn/ year repairing infrastructure in the UK. It’s not all concrete but a significant amount of it is.’
Three different strategies are used in the project to make the concrete. The first uses microcapsules, developed at Cambridge University, UK, which contains healing agents like resins or glues and which can be placed in concrete. They release these healing agents once damage occurs.
The second strategy, worked on at Bath University, UK, recruits a strain of Bacillus bacteria that can survive in the alkaline conditions in concrete. Bacterial spores are entombed along with a food supply. Once damage occurs and water ingresses, the spores rejuvenate and the bacteria produce calcite, natural limestone, which heals the cracks.
At Cardiff, meanwhile, Gardner uses polymers with a shape memory. ‘In their history they have been stretched and frozen in their stretched state. We use that form in the concrete and once they are heated – we activate by heat at the moment – they shrink and close the cracks in concrete and allow other natural healing mechanisms to occur,’ she says. With sufficient unhydrated cement in a mix, concrete can repair itself once water enters. The fibres, made from polyethylene terephthalate (PET), help the process along.
Henk Jonkers at Delft University of Technology in The Netherlands notes that a few groups are working on self-healing materials, but so far no construction material has been commercially developed. ‘The polymer fibres particularly increase the tensile strength of material, while the application of bacteria and encapsulated resin are, respectively, primarily meant for waterproofing and regain of compressive strength of construction materials. Of these the bacterial approach will probably be the most economical one,’ he predicts, ‘as bacterial production is relatively cheap.’ Delft University researchers have worked on bacteria-based self-healing concrete since 2006.
The main challenge is to produce the bacteria and their required feed, at relatively low cost, he notes. ‘This is easy to achieve for higher quality concrete constructions, particularly for those which are difficult to assess for manual repair,’ meaning high-way infrastructure, tunnels, liquid retaining structures and basement walls.
The plan, says Gardner, is for all three strategies to work cooperatively. She believes the polymer system might even be activated early enough to see off damage at the macro scale.