Self-healing materials receive a boost
Four UK universities are collaborating on a mission to develop self-healing materials. Simon Frost reports.
Civil engineers strive to guarantee that their structures will endure the elements and remain in safe use for generations, but an estimated £40 billion is spent maintaining, fixing and restoring civil structures in the UK every year – so what could be more desirable than a structural material that repairs itself? That is what the Materials 4 Life project has been pursuing for the past four years, and now a five-year successor project, Resilient Materials 4 Life (RM4L), has been granted £4.7m funding from the EPSRC.
Led by Professor Bob Lark at Cardiff University’s School of Engineering, UK, RM4L is a collaboration between Cardiff and the UK universities of Cambridge, Bath and Bradford, along with industrial partners. Lark brought Materials World up to date with where the forerunner, M4L, left off.
‘The key achievements of M4L were the development of several complementary self-healing technologies for healing damage in conglomerate materials at different time and dimensional scales and showing how they could be combined in real applications,’ he said.
Those technologies include microcapsules dispersed into the matrix of cement that rupture when microcracks appear, releasing healing agents to repair them. The capsules, formed of thin-walled soda glass or polyurethane, variously contain liquid and powdered agents including sodium silicate, magnesium oxide, bentonite clay, quicklime, colloidal silica and tetraethyl orthosilicate.
M4L also examined shape-memory polymers, which are produced by heating polymers and drawing them through a circular die, then cooled in a controlled manner to ‘lock in’ an extension, which can later be released by heating. ‘When embedded in concrete, this “shrinkage” is restrained by the surrounding concrete, thereby generating small pressures in the concrete that can both close cracks that have formed and prevent further cracks from forming,’ Lark explained. ‘Currently, we can heal and prevent mechanical damage, but we’re still working on how it will be diagnosed.’
The project culminated with a set of site trials in which five wall panels were constructed on a live site using the healing systems developed, and then subjected to a range of short- and long-term tests. ‘These trials showed that the technologies can be scaled up and deployed together in real structural elements,’ Lark said.
As with M4L, cementitious construction materials will be the focus of RM4L, but Lark notes, ‘The techniques that we are exploring will potentially be applicable to a wide range of composites which we know other research groups are working on and with whom we regularly share thoughts and ideas.’ All of the universities in the project will contribute to all of the research themes, but Cardiff will lead on the healing of mechanical damage, Bath on bacterial healing and diagnostic techniques, Cambridge on chemical damage and Bradford will focus chiefly on shape-memory polymers.
Dr Kevin Paine, Reader in Civil Engineering at the University of Bath, told Materials World, ‘We are carrying out research in RM4L to further develop the bacteria-based self-healing approach developed during M4L. Here, we developed a method for encapsulating both spore-forming bacteria and nutrient sources in concrete. Once triggered, by a crack, calcium carbonate was precipitated in cracks as a healing compound. We now want to extend this work to a much wider range of environments. For this we will need to isolate alternative strains of bacteria.’
Researchers at Bath aim to characterise isolates for the quantity of calcite produced and their growth under the range of conditions that structures are required to endure in different applications, such as cold, alkalinity and salinity. ‘As it is unlikely a singular species of bacteria will be able to meet all requirements, we may look to create a mixture of bacteria that will work in a wide range of concrete types and conditions,’ Paine said.
Bacterial healing could offer advantages over other forms of healing because it has the potential to be repeatable. ‘It is assumed that once bacterial healing has been completed, the bacteria will return to the spore form and hence will be available for future healing. However, to date it has never been proven that the bacterial cells used in self-healing concrete will return to spores on conclusion of their healing role, nor whether they would remain in a dormant state until their activity is needed once again,’ Paine explained. The Bath researchers aim to demonstrate that live cells can sporulate in concrete, forming the basis of a repeatable healing system.
Linked to industry
More than 20 businesses throughout the supply chain are also involved in the project, from producers and suppliers of components that will be used to the end users in the construction industry. The main industrial sponsor is engineering consultancy Costain, UK, which will sponsor at least one PhD student, provide support for planned site trials on its own projects and lead an industrial advisory panel to keep the project’s outcomes aligned with industrial challenges.
Oliver Teall, Innovation and Research Manager at Costain, described the company’s rounded view of RM4L’s potential. ‘We aim to develop solutions rather than products. This could be the construction of new assets with improved durability and resilience, or intelligent repair systems with self-sensing/healing capabilities. We see the RM4L research as a collaborative effort to improve infrastructure resilience, and would look to work with supply chain partners to deliver these outcomes.’
As an EPSRC-funded project, RM4L is not a commercialisation venture itself, but Teall notes, ‘We would like to support spin-offs to accelerate commercialisation of the most promising techniques being investigated throughout the five-year programme, particularly in the last two years of funding, from 2020 onwards.’ Teall says that the most obviously promising applications would be those in aggressive environments where durability is key, and where it is expensive and potentially unsafe to revisit for regular repairs and maintenance.
There is plenty for the project partners to achieve in the next five years, but Lark is confident that they can achieve it. ‘What we have to do now is improve the reliability and reduce the cost of the techniques that we have developed so far, but we also need to find other, more efficient and perhaps more tailored approaches that can ensure we address the full range of damage scenarios that structures can experience,’ he said.
‘Self-diagnosis and self-immunisation are also topics that are yet to be addressed, as is how we achieve multiple cycles of healing. Finally, we also want to prove the performance of the techniques that we come up with in real-life applications to assess their resilience and really demonstrate their value.’
If their research, as hoped, can shrink the maintenance costs for public buildings, bridges, tunnels, sea defences and roads, then its value will be considerable.