Circle Permeable Concrete

Crushed waste seashells can be used as aggregate to create permeable concrete surfaces – an ingenious, local solution for flood-prone coastal areas

The seafood industry produces surprisingly large quantities of waste shells – some 30,000 tonnes of scallop, cockle and whelk shells in the UK alone every year. While the majority end up in landfill, some simply accumulate outside processing plants where they form mountainous shell “slag heaps”. It is an under-appreciated problem, but one for which there may soon be a solution thanks to new concrete technology.

“We can use shells to replace up to 20% of the aggregate in concrete,” explains Professor Karl Williams, director of the Centre for Waste and Resource Management at the University of Central Lancashire (UCLan). “Shells are basically calcium carbonate and, when crushed and put into a concrete mix, they behave much like limestone. They are inert, but bind well with cement.”

Better still, says Williams, the curved shape of shell fragments can create the small voids that make permeable concrete. UCLan has now laid a 50m2 trial area of permeable shell-concrete in a community garden in Blackpool, working with local arts organisation LeftCoast.

“The technology was first investigated by French researchers at the BUILDERS École d’ingénieurs, and they originally looked at making permeable concrete paving slabs,” says Williams. “The CIRCLE project developed a ready-mix concrete material based on formulations developed at UCLan, and we continue to work closely with them. Our Blackpool trial has made use of local shells from Fleetwood and has been laid in-situ.”

The mix has been designed to produce a low-load surface, suitable for footpaths, cycle paths and car parks – applications where its permeable nature can reduce the risk of puddles and flooding without expensive drainage systems.

To help form the voids that allow water to drain through, there is no sand in the mix, and 20% of the aggregate has been replaced by whelk and scallop shells that have been crushed and sieved to produce 2-4mm fragments. “In laboratory tests it has performed well above the current drainage levels required for permeable concrete and better than many similar existing products,” says Williams. “At Blackpool, we have laid a 150mm layer above a 150mm sub base.

It will be exciting to see how it works in practice and how, for example, it copes with seedlings and encroaching vegetation.” More work needs to be done, however: “We want to optimise the crushing to reduce waste, and also to continue looking at how different shell combinations perform. For example, we cannot use mussel shells because they have a laminate structure that tends to splinter and form shapes that don’t work well in a mix.

Seasonal supply is also an issue. Some shells are only available when there isn’t an ‘R’ in the month!”

Replacing aggregate with shells brings a number of benefits. “Most obviously it finds a permanent place for this material, reduces waste to landfill and the need for virgin aggregate,” says Williams. “But there are carbon savings too, resulting from reduced aggregate extraction and potentially also reduced transport to landfill.”

There is further scope to cut carbon, adds Dr Emmanuel Anike, senior product developer and concrete materials specialist at UCLan. “For example, we are investigating lower-intensity cements to reduce the footprint of the concrete even more,” he says.

If the shell-aggregate is transported more than 80km from where it arises, much of the carbon benefit is lost. But, says Williams, waste shells tend to arise near the coast. “These are usually lowland areas, often prone to flooding. Permeable concrete can help with that – so we see shells in concrete very much as a local solution to a local problem. In any case, as we reduce waste and optimise our shell mixes, that 80km figure should extend quite a bit.”

A total of six organisations have come together to help develop the shell-based concrete. Known as the Circle Project, it comprises BUILDERS École d’ingénieurs, EQIOM Bétons, the Communauté d’Agglomération des 2 Baies en Montreuillois (CA2BM), UCLan, the University of East Anglia, and the Golfe du Morbihan – Vannes agglomération (GMVA). It is funded through the EU’s Interreg VA France (Channel) England programme.

Interview by Tony Whitehead

Photos Paul Burroughs