CREAKING bridges could be made stronger by spraying on a coating of fibres
embedded in plastic. The coating is now being field-tested in Canada.
Engineers already use fibre-reinforced plastic sheets to help strengthen old
bridges. The wallpaper-like sheets, about a millimetre thick, contain long
fibres of glass or carbon lying side by side. When glued to a concrete bridge
deck, say, the plastic transfers stress to the fibres, which can be as strong as
steel.
But fibre sheets are awkward to use near bolts or support cables, and few
bridges offer clean, flat surfaces. So Nemkumar Banthia, an engineer from the
University of British Columbia in Vancouver working in conjunction with experts
in smart structures, wondered if a better solution would be to blast the
plastic-and-fibre mix onto a surface through a high-pressure hose.
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While a spray-on coating is easier to apply, Banthia and his colleagues
feared it wouldn’t be as strong as the preformed sheets because the fibres would
tend to be shorter. But they were surprised to find the opposite was true. In
lab tests on concrete girders from a 50-year-old bridge, the random orientation
of the shorter fibres made the spray better at absorbing stress applied from
several directions. Overall, it doubled the weight that the girder could carry,
while the improvement with the “wallpaper” version was less than 50 per
cent.
The coating’s success is thanks to the flexibility of the plastic, and a glue
that transfers stress more effectively, says Banthia. “Unidirectional wraps are
strong but very brittle,” he says. “Ours deforms before it fails.”
These properties could make his coating particularly good for protecting
structures against earthquakes, he adds. It could also be sprayed on
suspension-bridge decks, rock walls, or anything else in danger of collapse.
The coating is now being field-tested on the Safe Bridge, an old concrete
road bridge on Vancouver Island. It was sprayed on last month on top of a
network of fibre-optic sensors that will relay stress information back to the
lab via the Net. Banthia expects the coating to double the weight of traffic the
bridge can carry, and triple its resistance to earthquakes.