By now, just about everybody has heard the sensational reports that many of America’s bridges are “structurally deficient or functionally obsolete.” Senators get alarmed about this statistic and headlines scare us with it, although few people know what those terms actually mean in bridge inspector’s terminology. Since the big headline is drawn from bridge inspection data, that little detail is of significance.
“Structurally deficient” means that the condition of one or more parts of the bridge (deck, superstructure, substructure, or culvert) have deteriorated and need to be monitored or possibly repaired. “Functionally obsolete” means that the bridge was not designed for the kind of traffic or safety standards of its current use.
These ratings do not mean the bridge is unsafe or about to collapse. If it’s unsafe, they close it. Period. End of discussion. (And by the way, as of this year, the percentage of “structurally deficient or functionally obsolete” bridges is down, not up, at about 25%.)
In many structurally deficient cases, is it not the main support structure that is lacking, but the bridge deck. The roadway surface gets all the direct wear, it gets the rain and the snow accumulating on it, petro-chemicals that leak out of cars and trucks, and de-icing salts that the DOT spreads in the winter. Some of these old decks are wood, sometimes with a thin concrete or asphalt layer over it.
Replacing the deck with a lighter weight material can extend the life of a bridge in two ways. First, the direct way, the deck is new and ready for a long service life. But also, the weight reduction can help make the bridge better-suited to today’s traffic.
Many people don’t realize that the most significant load a bridge has to support is not the weight of the traffic crossing it (the live load), but the weight of the bridge itself (the dead load). Reducing the weight of the deck makes more load-bearing capacity available for traffic, for today’s bigger trucks and higher speeds. Simply by replacing the deck and lightening the dead load, the bridge was made more modern.
Composite Advantage of Dayton, OH has been making glass-fiber composite bridges and bridge decks for several years now. One recent project, the Rocks Village Bridge in Haverhill, MA, was an historic steel truss bridge. Built in 1883, it is the state’s oldest moveable bridge, and its six spans cover a length of 809 feet. It had a 7” thick wooden deck with a chipseal (concrete and gravel) topping.
It was replaced with a glass fiber composite deck, also 7 inches thick, that weighed about half as much, just 19 lbs per square foot. (For comparison, 7 inches of concrete weighs upwards of 80 psf.) The new deck panels are 9.25 ft X 21.25 feet or 25.3 feet. It has a polymer concrete wear surface that was applied after the composite panels were in place. At 18,776 square feet, it is the largest composite vehicle bridge deck in the world.
One of the advantages of a composite bridge deck is that it’s quick to install. It is made is large sections that are light and easy to handle. Scott Reeves, president of Composite Advantage, estimates that it can take about four weeks to take out an old wooden deck, but only about 3 days to install the new composite deck. This can be done even in cold weather when concrete could not be poured. Speed is a major concern in bridge repairs. One of the significant costs of bridge repair is the local economic impact of delayed traffic, and rapid bridge replacement is a major area of study in the highway industry.
The composite deck has another advantage over concrete: it is virtually impervious to the elements. Concrete is really badly-suited to bridge decking in cold-weather climates. In freezing temperatures, the roads are often de-iced with salts or chemicals that become dissolved in the melt water. These dissolved salts penetrate microcracks in the concrete surface , and can initiate corrosion of the steel reinforcement bars. The corrosion products occupy greater volume than the steel, so they crack the concrete further, ramping up the deterioration. None of that happens to FRP. It doesn’t corrode, it doesn’t absorb water so it is impervious to freeze-thaw damage, and it is chemical resistant.
The main vulnerability of FRP is ultra violet (UV) light, which is an inevitable part of sun exposure. UV resistant coatings are required to protect the composite bridge decks.
All images courtesy of Composite Advantage.