Who Needs Concrete?

Who Needs Concrete?

The high strength and minimal weight of FRP have been used simplify bridge construction by making FRP formwork to cast the concrete bridge deck (See A New Form of Formwork).

Today we examine a bridge that says, “Why bother with concrete decking?”

The particular bridge in question is a pedestrian bridge, although the same technology has been used for vehicular bridges, as well. It is situated just outside Washington, DC, in Wolf Trap National Park, the national park for the performing arts. The bridge connects the park’s two main performance venues, the Filene Center and The Barns. The river it crosses is a river of cars, the 10-lane main access road to Dulles Airport, the larger of the two commercial airports serving the capital area.

One of the main concerns with building a bridge over a main highway is the disruption of traffic. The highway would have to be closed while the spans are erected over the road, and closed again when the concrete deck is being poured. The larger and more important the highway, the greater the social and economic disruption that’s caused by closing it.  Rapid  erection has become one of the main goals in planning to span an active traffic artery.

Composite Advantage, of Dayton, Ohio, has a solution to that dilemma: Build the bridge off to the side of the road, and make it lightweight enough that you can pick it up and move it into place with a crane. They achieve the necessary weight reduction by eliminating the heavy concrete deck and replacing it with FRP.

The bridge at wolf Trap is made up of three spans, each supported by a steel superstructure. The steel trusses are assembled and fitted with FRP decking at a location off the roadway near the installation site. Then they are hoisted and placed, with only a 15-minute traffic closure for each span. The largest span weighs 132,000 lbs, with FRP accounting for only 10% of that weight. With a concrete deck, it would have been over 200,000 lbs.

The deck construction is a technology that its maker calls FiberSPAN™.  Decks are usually 3-5 inches thick.  The Wolf Trap bridge is made up of panels 8.3 ft x 15.5 ft, and the deck is about 4 inches thick, plus a molded curb at each outer edge.   Each panel has top and bottom skins of fiberglass fabric. In between is a structural core made of bars of foam. Each bar is 3 inches wide and almost 4 inches tall. Each is individually wrapped in a layer of fiberglass fabric, and a solid layer of bars are stacked between the skins. When vinyl ester resin is vacuum-infused into the assembly, it saturates all the fiberglass, including the vertical fabric between the foam bars. Upon resin curing, each of the bars becomes something like a small box-beam that derives its strength from the “webs” of vertical fiberglass. Like a box-beam or an I-beam, the deeper the web – the greater the height of the foam bars – the more load it can bear.

According to Composite Advantage’s Scott Reeve, the cost of using FRP instead of concrete increased the price of the Wolf Trap bridge by about $80,000-90,000.  It’s easy to see that the economic impact of a long road closure would be far greater.

Composite Advantage has made five vehicular bridges using this technology, as well as 25-30 pedestrian bridges. In addition to the light weight and rapid construction enabled by it, these decks have a significant durability advantage over concrete or wooden decks. In environments subject to freezing, concrete decks are  vulnerable to corrosion of their reinforcing steel due to the de-icing salt and chemicals used in road maintenance. When the steel corrodes, it expands, and destroys the concrete.  Wood is likewise prone to deterioration because of exposure to the elements.  These conventional bridge decks have notoriously short service lives.   FRP decks suffer no corrosion. They are durable in exterior environments, due in part to chemical UV inhibitors in the resin, and a polyurethane coating that offers additional protection against UV damage as well as providing a non-slip surface. The decks have an estimated service life of 75 years (not including surface re-coating.)

Photos courtesy of Composite Advantage LLC.