Insulating concrete forms make the difference

Any contractor will confirm that there is a general reluctance in the industry to try anything different.

So when Freshfield Contracting insisted on a new structural concrete product for a development at Eastbourne Marina, the company was aware that the project was unlikely to be plain sailing.

The £5m project was won by Freshfield and MLRS Architects in a competitive tender that contractually required the winning bidder to construct the yacht club before any further development.

"Phase one was to develop the yacht club," explains Gary Pearson, project manager at Freshfield. "This was funded in a joint venture between us, Carillion [the landowner] and the yacht club. Basically, they were looking for a developer that would build a club subsidised by the opportunity to develop the land at the back."

The impressive Sovereign Harbour Yacht Club has now been constructed and work is underway on phase two - a development of 24 apartments - on the site of the club's temporary building. The club was constructed using a steel frame with pre-cast concrete floors and block external walls but, looking for a quick turnaround, Freshfield decided that an alternative construction method had to be found for phase two.

The contractor also realised that, being situated in a blustery coastal location, protecting the development from the elements as early as possible would be key to the project's success.

"We looked at steel frame, but considered it a little expensive for the project," continues Pearson. "With pre-cast concrete floors we would have had a problem with watertightness during construction. Although the frame is erected quickly and the floors go in fast we would still have to make the construction watertight before performing any internal finishing."

Another factor that influenced the specification was access to the site. Being on the coast, the whole of the building has to be built from one side. Loading and distribution must take place in a small area at the front of the development, ruling out the use of a static crane. This meant brick and block would have been problematic in terms of handling and, with space limited at the rear of the site, propping and struts for traditional formwork would have been difficult to erect.

"We looked around and found insulating concrete forms (ICF). So we asked our engineer, 'Have you considered ICF?' Having never designed with the product, he was unsure, but as a client we pushed for it. We knew it was financially viable," Pearson says.

Lightweight forms

It was decided that the shell of the building would be constructed using PolySteel ICF for all load-bearing interior and all exterior walls, with a Kingspan MD60 metal deck used for the flooring. Being effectively expanded polystyrene held together by integrated steel furring strips, the lightweight forms deal with the handling problems on the site, while providing a thermally and acoustically sound form in which to pour concrete.

Extensive groundworks have already been carried out on site to provide an underground, secure car park. Above this will sit the podium deck from which the main part of the building will rise. On this platform the walls are constructed using the ICF, slotted together with their tongue-and-groove profiles. An intermediate prop is erected for the deck, which Freshfield then fly in to use as an access to concrete pour the walls, before concreting the floor.

The process provides each floor with water tightness as soon as the concrete is set, leaving only window openings to be protected and allowing the contractor to start internal trades earlier.

"Usually we can't do this until the roof is on and the windows are in. We believe this gained us two to three months," Pearson adds.

"The polystyrene blocks can also be laid and concrete poured in any weather conditions, further contributing to the speed of the system."

However, being the first time that Freshfield has employed ICF, its execution was not without its problems.

The extensive use of reinforcement required for any construction above four storeys can make the product difficult to deal with, both in terms of placing the reinforcement and pouring the concrete.

"What our engineer designed meets all the requirements, but it proved difficult to build," says Pearson. "We could have saved a week on each floor on the programme if we had used a simpler design. I would say that, as the product is in its infancy here, a PolySteel engineer is crucial to its success. It is critical he advises the structural engineer on the design and placement of the reinforcement to ensure efficient construction. You really want to minimise the reinforcement as much as possible."

Pouring concrete

At Sovereign Harbour the contractor is pouring concrete into 3m-high troughs of ICF in a 200mm opening through protruding reinforcement. Two T16 bars are placed every 116 centres horizontally and two at 300 centres vertically. "To build the walls, stick them together and prop them is not difficult," explains Pearson. "But pouring the concrete is another skill altogether, the reinforcement makes the pour difficult and slows construction time. You must also pour precisely to prevent any air pockets forming, although we make doubly sure by lightly vibrating the concrete."

And it's not just pockets that are potential pitfalls, plenty of ancillary fixtures and fittings are required to hold the forms together and prevent any bursts. "You can't just turn up on site and expect that it will go up," Pearson says. Looking around the site it is obvious that this is the case, with random bands of plywood peppering the polystyrene. In fact, Pearson admits that he has mainly been employing carpenters due to the amount of timber in use. "It is critical to batten up with plywood otherwise it will take a gang of four an hour to clear up the mess," he continues. "I know this from bitter experience. It may look like belt and braces but you have to make sure."

When the concrete has set, work can begin on internal finishes regardless of the driving wind and rain on the south coast site.

Beyond providing weather protection and good thermal and acoustic properties, the permanent polystyrene shuttering also allows for a quick internal fit-out. All wiring can be placed by running a hot-knife through the polystyrene and pushing a cable into the cavity, while plasterboard can be screwed straight on top.

"Once we have mastered the pour, we can expect to gain real efficiency," Pearson says. "It's a complicated process, but once you get used to it you can go faster and faster, and being able to screw plasterboard straight on to the wall is a real advantage."

Pearson candidly admits that the use of ICF has been "a learning process - particularly on the engineering side", but he is confident its use will grow in the UK. "It's a new product to us and to the engineer, and there are plenty of hurdles to get over, but I like the product and I think it has a future," he says.

"Builders are reluctant to try anything new and until it is more established and contractors are specialising in ICF some contractors may be put off. It can be a little bit difficult to work with, but we have learned from this and would use it again." On leaving he issues a final warning: "But on a practical level its success comes from good design."