About 11.5% of the nation’s nearly 600,000 bridges are rated as structurally deficient. As the average age of America’s bridges pushes beyond 40 years, the status of our bridges looks only to deteriorate further, according to Transportation for America, a campaign that is calling attention to infrastructure. Funding for bridge repair and replacement lags substantially behind levels called for by the Federal Highway Administration to even maintain the status quo.
More than 25%, or about 16,500, of the structurally deficient bridges in the U.S. on record in 2011 can be found in just three states — Pennsylvania, Oklahoma, and Iowa. Pennsylvania leads the way with 26.5% of its bridges being structurally deficient.
The challenge for states is to either find additional funding to make up for the difference in federal dollars, which are becoming increasingly harder to plan for, or develop more economical ways of repairing aging bridges. One cost-effective alternative to complete deck replacement is overlay paving. “Virginia is aggressive with concrete overlays of bridge decks,” says Darrell Carman, construction manager for Infrastructure Corporation of America (ICA), Nashville, Tenn.
From the end of 2012 through the first two months of 2013, ICA has completed five concrete overlay projects in Virginia. The state’s aggressive approach to rehabilitation might be one reason why it is toward the bottom of the list for the number of structurally deficient bridges. Only about 9% of the state’s bridges are deemed structurally deficient.
Silica fume overlay
Offering 15 years of experience in highway and bridge construction and rehabilitation work throughout several southern states, ICA has extensively worked with the Virginia Department of Transportation to advance repair and reconstruction of structurally deficient bridges. ICA served as the design-build contractor for 23 bridge rehabilitation projects throughout DOT Region 3, from late 2009 through late 2012. This was part of the state’s push to repair or replace 119 structurally deficient bridges within a two-year period.
Most recently, ICA’s crews completed a bridge rehabilitation project on State Route 608 crossing the Roanoke River, straddling Pittsylvania and Bedford counties in southeast Virginia. “The bridge measured 513 feet long by 24 feet wide and required a silica fume concrete overlay,” explains Dail Stancill, ICA’s project manager.
For decks similar to the Route 608 Bridge that are candidates for an overlay, ICA starts the process by milling off the top one-half inch of the deck. “This removes from the concrete any remaining salt used by the road crews during winter,” says Carman.
The deck is cleaned and cracks are repaired with either a class B concrete for minor cracking or class C concrete for patches that go all the way through the deck. Crews then shotblast the deck and sandblast the ends, and the deck receives a second cleaning. Water is sprayed over the deck and a plastic covering is put in place. “The water soaks into the old concrete, which helps the overlay bond with the existing concrete,” says Carman.
For this project, and most of their other bridge deck overlay jobs, ICA uses a Terex Bid-Well 2450 automatic roller paver. With its 24-inch truss depth, this paver gives ICA’s crews the ability to pave a range of deck widths from 12 feet to 63 feet. For the Rt. 608 project, the silica fume concrete was placed ahead of the paver at an average depth of 1.5 inches, and the paver’s double flighted augers metered the concrete evenly in front of the paving carriage. Silica fume concrete can achieve very high strength and is extremely durable due to its very low permeability. This allows it to resist freeze-thaw damage and to protect the reinforcing steel from corrosion induced by deicing or marine salts, making it the perfect choice for bridge applications.
Just ahead of the finish rollers, the paver imparts vibration to the concrete. “The system delivers a more uniform concrete surface and desired densities, and it facilitates the sealing of difficult-to-finish concrete due to harsh mix designs, delays, low-slump specifications, and wind exposure,” says Larry Eben, district manager for Terex Bid-Well. The paver’s two 4-foot-long paving rollers finish the concrete to provide a smooth bridge deck. Following directly behind the rollers, the fully adjustable drag pan helps to seal the concrete, while the burlap drag delivers the specified deck texturing.
The narrow 24-foot-wide bridge was paved in two 12-foot passes so traffic could continue to cross the bridge during paving. To keep the paver’s end from sticking out into traffic, ICA equipped it with the available swing leg design. “The offset legs provide an additional 2 feet of length for variable-width and zero-clearance paving,” says Eben.
Full depth repair
When the bridge deck and substructure deteriorate to the point where concrete overlay is not feasible, ICA handles these full deck replacement jobs as well. This was the case for a design-build rehabilitation project that ICA completed last year for the 219-foot-long Rt. 257 Bridge crossing Interstate 81. The six-month project required repairing the substructure, removing the deck, installing overhangs, and full-deck paving of 7.5 inches. The 34-foot-wide structure was paved in two phases, requiring the paver to be set to a 14-foot paving width.
The 4000 psi concrete was pumped in front of the paver at approximately the required thickness. “We set the augers so they were halfway into the concrete,” says Carman. The crews once again used the swing leg to keep the paver’s end out of the traffic flow. “We also positioned one set of paver legs on the barrier rail and one on the concrete.” The crews paved the 14-foot-wide passes within four hours. The paver was heavy enough to pave through the harsh, 3-inch-slump concrete, yet it is light enough to be easily positioned on the deck. Its optional four-wheel travel dolly aids in on-site moving. “The paver is not so heavy that it can’t be moved with a lightweight vehicle rather than requiring a crane,” says Carman.
Crews used the paver’s fogging system to help keep the concrete moist and cool. This atomizes the water to produce a true, light fog, and the independently adjustable nozzles can be positioned to direct the fog to where it’s needed. A carriage fogger riser was used on the paver, which keeps the fogging system above and away from traffic, so crews can fog using a 12-foot section. The fogging system kept the concrete in front of the paver wet for bonding and behind the paver moist for curing.
With ample time to complete the project, ICA used a standard concrete mix design, which stayed wet for seven days and reached full cure in 28 days. Once one side was completed, traffic was moved over to the rehabilitated section and the other side of the deck was removed and paved. At the end of the project, vehicles driving along Rt. 257 in northern Virginia had one less structurally deficient bridge to cross.