Engineers at Florida International University (FIU) have created a system that sprays ultra-high-performance concrete (UHPC) onto structures. Such concrete not only has the potential to patch walls, fix pipes and restore drainage culverts, it can make these structures stronger than they were in the first place.
More than 40,000 bridges in the USA are deemed structurally deficient, and as many as 221,000 deemed in need of repair, according to the American Road and Transportation Builders Association. The new system developed by the FIU researchers could play a key role in restoring them.
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UHPC is many times stronger and more water-resistant than regular concrete. Despite its inception some 30 years ago, its use in the USA has been limited due mainly to high cost, but that might be about to change thanks to the new system.
‘UHPC has historically been very expensive to use, mainly because most of those available [mixes] on the market have been commercial versions. Even a very thin layer of UHPC can drastically improve the strength of a structure and protect it from water,’ said Atorod Azizinamini, a professor of civil engineering and director of a US Department of Transportation-funded research centre at FIU. ‘We have developed an affordable type of UHPC, and a system that can spray it. This could become a major tool in repairing or upgrading structurally deficient bridges quickly and efficiently.’
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At around US$500 per cubic metre, the FIU-made UHPC mix comes at a fraction of the cost of many comparable commercial versions of UHPC, Azizinamini said. And since only a thin layer of it is needed, the overall costs are manageable for many projects.
The system deposits thin layers of concrete onto surfaces through a nozzle, similar to the way spray paint is applied. These layers can be added, one upon another, in coats. Their light weight doesn’t significantly alter the mass of a structure.
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Azizinamini’s team applied UPHC to a bridge in Virginia last year. ‘The reason that we invited the FIU team here to Virginia is that we had a bridge abutment wall that had corrosion issues due to the de-icing salt that is used here,’ said Sam Fallaha, an engineer with the Virginia Department of Transportation. ‘We liked it because it is easy to apply a thin layer to vertical surface, creating a durable solution for problems like these that occur in cold climates.’
The Virginia job was the first-ever application of its kind in the USA. The Risk and Resilience Tech Hub is investing roughly US$1million into the FIU researchers’ system to commercialise the technology and help the university train more engineers and builders to use it.
‘Anyone who has used a glue gun for an arts and crafts project knows how difficult it can be to keep the nozzle from getting clogged,’ said Morgan Dickinson, a PhD student in Azizinamini’s lab. ‘The same goes with our UHPC system. If you don’t know how to clean it, the concrete hardens, and you might have to throw parts away. Fortunately, we are seeing widespread interest in people who want to learn about our system and use it properly.’
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Kia Hajifathalian is a vice president and market general manager at Baker Construction, the leading commercial concrete contractor in the USA. He sees the FIU system as a practical way of bringing the benefits of UHPC to more buildings and bridges around the nation. ‘This is not just research in a lab that will only have a couple of specific uses,’ he said. ‘I think that FIU has figured out how to utilise UHPC in a way that can be used to repair buildings and bridges at scale.’
Hajifathalian added that for the system to continue to gain popularity, it will need to be further tested for compatibility with other aspects of construction, such as painting and architecture aesthetics. But right now, it holds a lot of promise. ‘The biggest nightmare of a concrete contractor is pouring concrete, testing it later and finding out that it is not as strong as you thought it would be because something unexpected happened,’ he said. ‘Having that concrete in a pre-bagged system like FIU is doing, where you have quality control, is so important.’