The post-war baby boom sparked a mini building boom in Essex, and the council used Durox aircrete roof beams extensively in 60-odd new schools as well as some other buildings. Back then, the aircrete beams (or reinforced autoclaved aerated concrete planks to give them their grand title) seemed ideal. The air bubbles in the concrete would insulate the building, while the beams would provide a flat surface for the waterproofing membrane and also act as the ceiling to the classrooms. Up to 8.4m long and 600mm wide, the 100/150mm-thick beams also let the builders get the roof on fast.
Perfect. Until, that is, the beams began to sag. The problem wasn't the load but the span. The castellated steel beams supporting the roof were set 2.4m apart and that was simply too far for the aircrete. Even worse, while the aircrete sagged between the steel, its wire reinforcement didn't. Cracks started appearing.
Although Atkins surveyor Rob Moye says there was little danger of the roofs actually collapsing, the classrooms' light fittings, screwed directly into the aircrete, were coming adrift as the beams bent. The council decided its only option was to reroof, using the summer holidays to do the work.
Time is tight
But scheduling reroofing works during the long downtime of the summer holidays isn't as easy as it sounds. Builders too go on holidays in summer, and other jobs compete to make the most of good weather. Essex's costs went up and it became a race to finish before term started. It was also horribly slow, with the council managing to reroof only one school a year.
Hare suggested an alternative: nudging the roof back into place and then propping it up better. He partnered with steel specialist Metsec to create a lightweight lattice of steel beams to go between the original castellated steel beams, reducing the distance the aircrete planks had to span from 2.4m to 0.8m.
His big headache was how to push the aircrete beams back into place without cracking them. An odyssey of ingenuity saw Hare testing (and rejecting) one idea after another: hydraulic jacks (too complicated and expensive), expanding materials (don't work outside the lab and can't exert enough upward thrust), metal props with a screw in the middle (put pressure on a point rather than spread it across each beam), grout-filled pouches and water-filled hoses (in both cases leaks mess up the classroom).
We went through loads of processes. Air was a natural progression
John Hare
"We went through loads of processes," says Hare. "Air was just a natural progression." His roof cradle system involves laying tubes on top of the Metsec steel and slowly inflating them to lift the sagging aircrete back up by as much as 30mm. Inflating the tube exerts an even pressure over its length, removing p page 28 the risk of cracking the beams by putting great stress on specific points.
Hare imports the high-performance tubes, which can reach 100 pounds per square inch (psi) without leaking, and slowly inflates them to 40 psi. Having lifted the roof, Hare then puts an unshrinkable grout along the sides of the tube between the steel and the aircrete. The grout sets hard, transfers the roof load to the steel and extends the life of the roof by 20 years. Once the grout has set, the tube is redundant, and a false ceiling with new lighting is fitted.
Like Hare, Metsec has designed for simplicity. The supporting steel beams are light enough for a single worker to lift. There's no site welding or drilling: Metsec clamps the steel beams in place by putting bolts through the hexagonal holes of the castellated beams.
Because the roof cradle can go up piecemeal, schools don't have to close. Work takes place at night and at the weekend, with schools handed back to staff each weekday morning. The council typically fixes only the worst-affected third of a school's roof area (around 300-350m2, which takes two to three weeks), before moving on to the next school; most schools get three visits from Hare and Metsec before their entire roof is fixed. In local government, many customers can have the same problem, and a piecemeal approach satisfies more customers more quickly. Hare is halfway through the repair programme.
For Hare and Metsec, who patented the system last year, there's only one problem: the market for the roof cradle system may be running out. The air-filled tubes work best on a large smooth surface, but most UK roofs rest on close-set timber or steel purlins.
Source
Construction Manager
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