What’s A ‘Bridge In A Backpack’?

Before we discuss the specifics of this bit of engineering ingenuity, can we all just appreciate for a moment the marketing savvy of “bridge in a backpack”? Based on name alone, we want lots of them built in our town. It tickles our brains in just the right way. Our only question: the people capable of thinking up brand names that snappy tend to be retained by companies that make “age defying” wrinkle creams or “scrubbing bubble” bathroom cleaners–not a civil engineering solutions labs at the University of Maine. But perhaps the secrets are slipping out?

The structure itself is the brainchild of an engineer named Habib Dhagger, who devised a process for building a bridge out of things that look like giant carbon fiber socks. When it’s time for construction to start, one simply inflates the socks, treats the fabric with a hardening epoxy resin, and them fills the resulting tubes with concrete. Voila!, you’ve got the skeleton of your bridge. You essentially just throw on a plastic deck, pave it, and you’re ready to go. With Dagher’s technology, it’s suddenly possible to construct highway bridges in a few days, instead of a few weeks or months as those tedious old non-backpack bridges require.

(More pics after the jump)

One of these backpack bridges–so named because a carbon sock can fit in a backpack–was put up in rural Maine earlier this year, and was much celebrated there as something that might bring lots of jobs and other peoples’ money to the state. Now it is in the news again because on Monday our Secretary of Transportation, Ray LaHood, went up to Maine pay homage to it. (Though, sadly, he didn’t blog about it.)

Not unimportant in these days of government red ink: Dagher expects the backpack bridge will eventually cost about 20 percent less to build. Admittedly, the one that’s already been built–a 44-foot span near Pittsfield–came in $581,000, which was merely “comparable” to a regular bridge. (In the lexicon of homo economus, that tends to mean “somewhat more expensive.”)

On a lifetime basis, though, it would seem to be a great bargain even at that price. Dagher estimates that his bridge will last two or three times as long as regular bridges. Primarily this is because the concrete isn’t exposed in elements. Also, there’s no steel rebar or structural steel–which are expensive and also degrade–required because the carbon socks are very strong.

As Dagher told a local paper: “You know what happens to concrete and rebar with the environment in Maine. Water gets in there and it cracks, it freezes, and breaks up the concrete. Now water can’t get in there. The concrete is completely protected from the environment so bridges can last quite a bit longer.”

We tried to reach Dr. Dagher in Maine to get more juicy quotes like that, but apparently he’s out of the country and can’t talk to us.

From other media organizations though we know that Maine is considering building a 500 foot bridge using a version of this technology as well as several other smaller ones around the state. Massachusetts is also sniffing around and might be a buyer.

For those intrigued by the process, here are more pictures of the construction process for these newfangled bridges. We will note sadly, though, that there aren’t any pictures that actually involve putting one of the unhardened carbon molds into a backpack.

The concrete-filled carbon fiber forms are put in place:

… and so on…

Now it just needs to be decked (using a corrugated plastic) and paved:

Luminaries at the grand unveiling:

Pics: UMaine
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The molds, when inflated and coated in epoxy are so light even this 98-pound weakling engineering student can carry them around:

Pic: Bangor Daily News
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Hey, come drive on me!