Recycled steel fibre concrete

By reusing steel wire from waste tyres, researchers at birmingham university are creating high- strength concrete with around one quarter less cost and carbon

Structural engineer Sakdirat Kaewunruen is a reader at the University of Birmingham and an expert in railway construction – so the reason for his interest in tyre recycling is not immediately obvious.

“It is to do with how to make concrete resist shear forces more efficiently,” he explains. “Shear forces happen when one part of a concrete structure tries to move relative to another, when sunlight heats a road pavement, for example, and the surface concrete expands relative to that underneath. Or when a train passes over a bridge, or an aircraft lands on a runway. In these circumstances you have to guard against significant shear forces causing delamination and cracking. The traditional solution is to use more reinforcement, and thicker, higher-strength concrete.”

This comes at a cost, however. Extra steel, cement and concrete increases expenditure on materials, and also the carbon footprint of the structure concerned. “But if you reinforce the concrete with steel fibres, it becomes much more resistant to shear forces,” says Kaewunruen. “Then the amounts of concrete and traditional reinforcement can be minimised.”

Kaewunruen and his former PhD researcher Xia Qin are working with the Concrete Society to update its TR63 guidance on steel fibres, to allow their use more widely and in higher-strength applications. This solution is not perfect, however, as industrial steel fibre (ISF) comes with its own financial and environmental costs. This is where the tyres come in: “Europe alone produces nearly four million tonnes of waste tyres every year. Many end up in landfill, though they can be burnt to produce electricity, or in cement production.” One problem with this is the steel wire used to reinforce the tyres: “There’s around 50m, or just over 1kg, in an average car tyre. When the tyre is burnt for electricity, this ends up as troublesome residue.”

Even if the wire is stripped out, it is usually recycled in power-hungry electric arc furnaces. “It would be better to reuse it. As it happens, the wire is usually around 1mm in diameter – a similar specification to the wire used to make steel fibre composite concrete.”

Kaewunruen and Qin have just completed new research into how well recycled steel fibre (RSF) would work in concrete. It is not entirely straightforward: even when the wire is removed from the tyre, it has to be cleaned, as rubber residue can affect hydration rates in concrete. “Fortunately the machinery to efficiently strip out the steel already exists, and the wire can then be vacuumed to remove rubber dust.”

Another issue is the shape of the individual wires or fibres. Those currently used in ISF composite concrete are specially manufactured with short bends, or ankles, at the end of each wire to increase the grip of the wires within the set concrete. “It’s not practical to add these features to recycled wire,” he explains. “So we had to check what effect using simple straight wires would have.”

Kaewunruen and his team tested a full range of mixes across all strength classes of concrete with different concentrations of RSF. “The good news is that our mixes proved highly effective at improving the shear resistance of the concrete. Our 0.8% mix, for example, is almost as effective as purpose-made steel fibre. It proved less effective at increasing tensile strength, probably due to the shape and surface characteristics of the fibre.”

The team also found that it was best to mix the fibres with the aggregate first before adding cement and water: “It improves their distribution and helps to maintain good flow by preventing them from balling and clogging.”

In fact, RSF improved fluidity: “You can have higher concentrations of straight fibres. Flow of ISF concrete becomes difficult much above 1%. With RSF, it is possible to go to 2% without significantly impacting flow.”
Clearly these particular characteristics of RSF composite concrete would have to be taken into account when specifying it – but the potential cost and carbon savings seem considerable. “RSF is cheap, and does not need to be manufactured in a blast furnace,” says Kaewunruen. “We calculated that a 0.8% mix delivers a 25% carbon saving and a 28% cost saving compared with ISF.”

A number of full-scale tests with composite concrete beams have been conducted at the university to study RSF’s capacity to improve structural performance. The team has also built an AI tool to help designers optimise RSF use for a particular concrete strength.

Kaewunruen hopes that RSF composite concrete can also be included in any update to TR63, which will boost many more engineering applications: “So we solve two problems with one solution, and hopefully make the world a better place.”

Interview by Tony Whitehead

^{Published in CQ Summer 2025}