Self healing anti corrosion concrete

The university of East London’s concrete lab is formulating new self-healing mixes, and pushing them to their limit.

At the University of East London, Dr Ali Abbas is inflicting highly controlled damage on cubes and cylinders of concrete. The cracks his machines create are only just visible, less than 1mm wide, similar to those that first appear in old or distressed concrete. “We then monitor the cracks to see how our new concrete formulation is performing,” says Abbas. “All being well, the cracks repair themselves.”

Self-healing concrete relies on a bio-chemical phenomenon known as microbially induced calcite precipitation (MICP). Originally developed in Holland, it has since been progressed by a number of British universities. “Bacterial spores are added to the mix and then remain dormant until cracks appear,” explains Abbas. “When they do, the bacteria are exposed to water and oxygen, and begin to precipitate calcite which seals the crack.”

If you can seal cracks at an early stage, he says, this stops them getting wider, protects rebar from moisture ingress, reduces corrosion, and results in longer-lasting structures. “This not only saves the cost of maintenance or replacement – it saves the carbon associated with having to produce new concrete.”

But getting innovation out of the laboratory and on to real world building sites is not straightforward, he adds. “It requires the sort of research we are doing here. If we are to decarbonise concrete and make an impact in the outside world, this kind of work is absolutely vital.”

Abbas speaks from experience. A structural engineer by training, he completed his PhD (modelling cracks in concrete) at Imperial College London. He then spent four years working in Atkins’ R&D department: “We were concerned with heavy-duty infrastructure: the nuclear industry and also London Underground. I realised that we have to find ways to make this stuff last longer as replacing it is so costly, economically and environmentally.”

The close study of the corrosion resistance of healed cracks is a good example of UEL’s real-world focus: “Most testing of MICP to date has been carried out on unreinforced concrete” says Abbas. “Sealing cracks with MICP should protect rebar – but does it? To find out if it really works we have cylinders with a length of rebar down the middle, and we pass a current through that to accelerate any corrosion that might occur.”

In fact, the mix Abbas is currently testing involves a little more than just MICP: “We’ve also added calcium nitrate which is known to have an anti-corrosion effect, helping rebar last longer. So if we combine this with MICP, we should get an extremely resilient and long- lasting concrete. But, again, we have to check. Do the mixes work well together? In addition, the mix includes GGBS to lower its carbon content, so once again we have to check these ingredients perform as they should when all are present.”

Results so far are encouraging. “The good news is that the calcium nitrate admixture (sold as NitCal) does not kill the bacteria – in fact it seems to help it perform better,” says Abbas. “Setting times are, if anything, reduced and there is no reduction in either compressive or tensile strength. It may even be that, over time, strength is improved.”

And the effect on corrosion? “Depending on the dosage, concrete containing NitCal lasted 6-8 times longer before becoming corroded – and the severity of corrosion was reduced by 30-50%. Concrete that also contained the crack-healing admixture showed a crack-healing efficiency of 90% with a further significant reduction in corrosion.”
Abbas stresses that research like this must keep a steady eye on real-world applications: “This work has been funded by Innovate UK and carried out with industry partners including JP Concrete and Sensicon, where we are working with innovation director Vighnesh Daas.

They are incorporating our findings into their product ranges, so hopefully we will soon be able to test how these formulations perform in actual structures.”

Interview by Tony Whitehead

Photos Paul Burroughs

^{Published in CQ Spring 2024}