A new type of rapid hardening concrete promises to ease the pressure on contractors battling to complete time-sensitive jobs. Richard Johnstone reports.
When European contractors approached Cemex asking if it could supply a concrete that set fast and to correct compressive strengths, the building materials giant called in its research and development experts.
The men in white coats gathered at the company’s Swiss research centre and beavered away to develop the new solution, based on the use of two additional admixtures. The result is Promptis, which is claimed to be the first concrete with rapid hardening and standard useable properties for work on construction projects.
Limited access time
Cemex UK national director Steve Crompton says that the new concrete can be used where concrete framework buildings are constructed around formwork, and on transport projects where there is only a limited time to close roads and railways for repairs.
Traditionally, Crompton says, early strength concrete tends to be quite difficult to handle as it does not give those using it a lot of time to work with it. As a result it often goes into precast applications. The difference with Promptis is that it has all the benefits of high early strength without the concrete setting too quickly, Crompton says.
“Obviously high strength concrete wasn’t particularly new, and early strength concrete has been around for a while as well. But the key to this project was getting a product that can be delivered to site ready mixed, and still maintain its workability for normal time, from about 90 minutes to two hours,” he says.
“The product can also be used for repairs where there is only a short time available - on rail or road posessions”
Steve Crompton, Cemex
Within four hours of being placed the mixture would be set sufficiently to allow formwork to be stripped.
“The original driver was to get better utilisation of formwork, so getting two uses of the structure a day, rather than waiting for the traditional 24 hour cycle that you get with normal concrete,” adds Crompton.
Promptis will allow contractors to double the efficiency of their use of formwork, which he adds “more than offsets” the increased cost of the product, compared to ordinary concrete, is between 10% and 20%.
Depending on the project, the concrete mix can be altered, so that it can strengthen quicker, or provide more time for it to be worked with.
Crompton explains: “We’re controlling the set and we can control the amount of early strength you get. If you’re stripping formwork you only need strength of 4Nmm2 to 6Nmm2. If you wanted a road closure and you want the strength, then we can also set the mix to give you higher strength in 12
or 24 hours.
He says that UK customers have been reacting positively to the product, which will be available in the UK and worldwide next month. “We’ve had informal discussions with all our major customers on the principle of it, if it is something that would bring some benefits to them, and the response has been pretty positive. That has encouraged us to launch the product.”
In this country, the product’s use in concrete frames will be most prevalent. “I think the larger use is likely to be in frames and any application where you’re looking at reuse of formwork, because this enables you to get better utilisation of your formwork on site and better productivity,” Crompton says.
Other applications are also possible, particularly where there can be limited time for construction. “The product can also be used for repairs in situations where there’s only a short time available - any sort of rail possessions or road possessions,” Crompton says.
Anything that could improve efficiency will also be noted in Britain’s railway sector, where a value for money review led by Sir Roy McNulty is due to report next year. An interim report from McNulty’s team in June 2010 found that the “rail industry is perceived as being risk-averse and reluctant to introduce new technology”.
Here there is potential too. “I think there will be a specialist use for it where there are limited times for reinstatement, particularly with railway closure and the like where the client will have a weekend possession. They are often looking for an early strength solution. This product will lend itself to that application,” Crompton adds.
This has already been done in Ireland, where the product was used on the Dublin tram system as part of a bridge replacement scheme that required early strength concrete to ensure that the bridge - and the tram service - could be reinstated as quickly as possible.
“They didn’t have a lot of time to get the bridge back into use. Using Promptis meant the
team could pour it, strip the formwork very quickly and reuse the bridge within 24 hours.