Fiber bridge repair

The FRP must be used as a supplementary reinforcement.
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Repairing bridges effectively can be done with a technique developed in Japan. However, care must be taken, warn experts.

There are lots of great bridges over the Middle East. From the majesty of the new span to Saadiyat Island, to the length of the King Fahd Causeway, to the eccentricity of the Floating Bridge, it is clear that engineers will often take different and fascinating approaches to solve the same problem.

One area where bridges remain the same though, is the need for maintenance. You might have heard a seemingly never-ending and repetitive task is ‘like painting the Forth bridge’ due to the need for maintenance crews on a bridge over the Firth of Forth in Scotland, to start painting again as soon as one end has been finished.

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Conditions
While conditions in the Gulf are a little different to the North of Scotland, erosion of structures from winds effectively sandblasting the sides is common, as is decay of metal parts from working under load in an extremely hot and humid environment.

Factor in bridge strikes by vehicles, vandalism, fires, and in some cases indifferent quality concrete when the initial structure was poured, and you will understand why it is sometimes necessary for bridge repairs to take place.

The challenge for engineers is to work out a way to restore full structural integrity to a crossing, without having to start from scratch.

One method of doing this is FRP, otherwise known as Fibre Reinforced Polymer. Simply put, this involves applying panels, sheets or rods made from carbon fibre over the exterior of an existing structure.

The technique has been in use since the mid-1980s, when the Japanese government researched a way to put the strength back into an old bridge, as well as protecting is from seismic movements.

However, as Muneer Merchant, GM of Structural Preservation Middle East pointed out, this is not a matter of simply painting the goo on. First of all, one needs to be certain of the carbon fibre being the right grade for the job: “The most comprehensive code in carbon reinforcement design is ACI 440.2R-08” he explained.

“That is the most comprehensive guideline – it isn’t a code – but a guideline for external bonded reinforcement. In fact, the Canadian Highways Association has incorporated this guideline into its DOT codes and it extensively uses carbon fibre reinforcement to upgrade its bridges.”

Calculations
Careful calculations need to be made in terms of how much strength can be added to an existing structure. “One of the limitations is that this product can only be used as a supplementary reinforcement, not a primary reinforcement,” Merchant cautions.

“If the existing structure cannot withstand the loads involved, then carbon fibre cannot be used. In the past people have abused the technology and then there are failures because they haven’t used the right material.”

“If you use carbon fibre in the right application you will have benefits, but if you design it wrong or are not experienced enough to install it, you will get failures and you’ll blame the product – but there is nothing wrong with the product or technology” he said.

Merchant adds that he finds it crucial to have an experienced civil engineer involved in any area of bridge repair, who will consider such factors as vandalism or fire reinforcement.

Another interesting development is that of FRP rods, which although measure just 12mm in diameter have a strength of several times that of conventional rebar, though of course it will be applied externally, and corroded rebar left in situ.

“We can analyse the concrete to determine if, say, 25% of the strength of the rebar has rusted away. The strength can then be put back in with the carbon” Muneer explained.