Concrete plan

Ultra-High Performance Fibre-Reinforced Concrete (UHPFRC) could help protect UK landmarks and public areas against bomb blasts. Siobhan Wagner reports.

An advanced composition of concrete that incorporates needle-thin steel fibres has compression strength 500 per cent greater than conventional concrete.

Engineers at the University of Liverpool and the University of Sheffield believe this concrete, called Ultra-High Performance Fibre-Reinforced Concrete (UHPFRC) could be used to reduce the impact of bomb blasts in public areas across the UK. The concrete has already been used in the roof of the Australian Embassy in Baghdad to protect it from mortar attack.

The research team began testing the concrete three years ago under a government-funded programme entitled Think Crime-4 Initiative. The leader of the Liverpool research team, Steve Millard, a professor in the university’s engineering department, noted that at the time, concrete was not being used in a smart way.

For example, he said, concrete blocks were initially installed outside the Houses of Parliament and other key London landmarks a few years ago to ‘protect’ those areas from terrorism, but security officials soon realised that those concrete barriers could be dangerous if subjected to intense explosive loading.

‘If a bomb went off, it would send a pressure wave through a conventional concrete wall,’ Millard said. ‘The wave would then rebound on the far side and blow the concrete cover off. If the concrete between the steel bars and the surface gets blown away, shrapnel can kill people on the other side. The wall is in good shape, but the people behind it who are meant to be protected are not.’

Potential instances such as this, he added, highlight the need for explosion-resistant infrastructure that can be quickly and easily positioned or retro-fitted in such high-security areas.

The Liverpool and Sheffield engineers worked with the Centre for the Protection of National Infrastructure to explore the UHPFRC’s tensile strength and ductility and conducted a series of high-explosion blast tests at RAF Spadeadam, in Cumbria. Each explosion, which mimicked a typical car bomb, was moved closer to the concrete sample to examine its bending strength and capacity to absorb energy.

After these tests, the researchers were able to demonstrate that the UHPFRC could resist a high-explosion blast without any disintegration from the back of the panels causing shrapnel.

There is a commercially patent form of UHPFRC called Ductal and the researchers mostly stuck to the material’s original recipe. However, Millard said they did vary the dosage of steel-fibre reinforcement. The team also looked at cement replacements, such as pulverised fuel ash (PFA) and slag, which are much less-expensive materials.

Millard describes UHPFRC as having ‘three times as much cement than is normal’. UHPFRC has 1,100kg/m3 while most other types have 250-350kg/m3. It also has very little water, containing 0.02 per cent, whereas most construction cement contains 0.35 to 0.5 per cent.

‘The more water you put into concrete the more mixable it gets,’ he said, ‘but it also gets weaker.’

In place of the water, the cement contains high dosages of a superplasticising additive, which is needed to maintain the cement’s workability.

All cement materials contain aggregates, which are inert granular materials such as sand, gravel or crushed stone, that are used to resist compressive stress. The only aggregate present in UHPFRC is fine silica sand. The short, straight steel fibre provides the concrete’s ductility.

UHPFRC can be mixed and cast like normal concrete with no special facilities or handling, but Millard said it does require post-set heat treatment at 90°C in the first week for 12-24 hours to improve microstructure and mechanical properties. The researchers’ tests concluded that UHPFRC has compressive and tensile strengths of 200MPa and 30MPa respectively and fracture energies up to 30kJ/m2.

He said this concrete could have a range of security applications, including protection barriers. It could even be used for civilian purposes.

Millard suggested a slightly varied version of the UHPFRC could be used to make paving flags that never crack.

‘We are currently looking for further funding to figure out where else we can go with this material,’ he added.