Shrink-wrap repairs fast (Image: Teh Eng Koon/AFP/Getty Images)

WHEN an earthquake strikes, modern bridges flex rather than break, but older bridges can sustain serious damage. Now a repair technique that wraps up damaged columns using shape memory alloys could make the bridges stronger than ever.

Bridges are usually repaired using either sheets of fibre-reinforced polymer or concrete jackets. Although FRP sheeting is quick to apply, it can take at least a week for the resinous material to cure to the required strength. Concrete jackets, on the other hand, take days of construction work to install.

A team led by Bassem Andrawes, at the University of Illinois at Urbana-Champaign, is developing a repair method based on shape memory alloy (SMA) wire. This “remembers” its previous shape and returns to it on heating. The first stage of the repair is to remove loose concrete from the damaged column and replace it with quick-setting mortar.


Then a spiral of wire made of a nickel-titanium-niobium alloy is wrapped around the column. The wire has been stretched, but it contracts again when heated with a blowtorch, effectively shrink-wrapping the damaged column. This applies pressure to the column and strengthens it.

The wrapping method was tested on columns one-third the usual size and the repair took less than 15 hours to complete. Tests showed that the columns were restored to their original strength. The ductility ratio, which describes the column’s ability to withstand further damage, was higher after repair than when the columns had first been built.

In the next few months spirals will be installed in one or more road bridges in association with the Illinois Department of Transportation to test the SMA’s durability.

“Other methods need skilful labour and time, days or weeks, while this new technique needs no skills and the SMA spirals could be wrapped and heated in a few hours,” says Andrawes.

The shape memory alloy needs no skills to apply and the spirals could be wrapped in a few hours

David Grant, a materials scientist at the University of Nottingham, UK, says that extreme temperatures may have significant effects on the SMAs in this application. “It would be interesting to see if the long-term recovery stress holds for long periods of time, especially in cold climates where the temperature is well below freezing,” he says.