The spread defense
The last time Wildcat Construction Co. repaired the I-135 bridge in Wichita, Kan., more than 20 years ago, it took nearly two years, and lanes were closed or diverted both day and night. This past summer, the repair project took about two months, and daytime traffic flow was never diverted.
The main difference between the two projects? Epoxy polymer overlay used in 2007 vs. a high-density concrete overlay used in 1986. The southbound lanes project, which encompassed nearly 7.2 lane-miles, is believed to be the largest epoxy-polymer bridge resurfacing project in the U.S.
The project is even more remarkable because Wildcat applied the overlay overnight, thanks to a quick-curing epoxy polymer from Unitex. One lane was always open for traffic at night, and all three lanes were open during the day.
Today’s strong, flexible epoxy-polymer overlays save money as well as time—and can offer the best protection available with lower life-cycle costs.
“We believe using an epoxy overlay can help save taxpayers’ dollars, because research shows that repairs made with this material are quicker, less costly, weigh less and are more waterproof than those using concrete,” said Dave Meggers, Kansas Department of Transportation research development engineer.
No milling around
The I-135 bridge—a reinforced concrete box girder bridge—was built in 1977. It is about 2½ miles long. Nearly 91,500 vehicles cross the bridge every day, heavy traffic for Kansas, according to Meggers.
The Kansas DOT has been using epoxy-polymer overlays for about 10 years, and the I-135 bridge fit the profile for an epoxy-overlay repair.
“It was cracked, but overall it was still in good condition,” said Meggers. It was time, though, to repair and protect the bridge to extend its life.
Meggers said the alternative to an epoxy overlay was a microsilica overlay. However, he said, this method has disadvantages. Surface milling would have been required to remove bridge weight in advance of the added weight from the microsilica overlay.
“Milling can leave the surface in worse shape,” said Meggers, who has concerns about the microcracks that can result. The milling process would have added about two months to the project, and lanes under repair would have been closed for much longer.
Just squeegee in
Wildcat, based in Wichita, Kan., began work last June, repairing bridge joints and patching the surface. This work also was done overnight. The overlay was executed in phases—two driving lanes and a shoulder in the first phase; the remaining lane and the acceleration or deceleration lane in the next phase; and the ramps on the final phase.
The epoxy-overlay portion of the repairs included four main steps:
- Shot blasting;
- Pouring and spreading; and
- Applying aggregate.
Throughout the project, the crew shut down a lane at 7 p.m. and then the second lane or other parts of the bridge at about 10 p.m. The crew ran shot-blasting machines over the lanes to roughen the bridge’s profile to match a No. 7 ICRI profile mat. Shot blasting created a clean, roughened profile to which the epoxy polymer could bond. A lighter brush blasting was done on the night when the first course of epoxy polymer was applied. Brush blasting cleaned the surface of debris and oil. The area also was blown or swept free of particles.
That same night, the crew mixed the Unitex Pro-Poxy Type III D.O.T. on-site in clean, plastic trash cans. The A side of the epoxy, the resin, was mixed with the B side, the hardener, in a 1:1 ratio. A drill with a “Jiffy mixer” mixed the epoxy at about 300 revolutions per minute for exactly three minutes. This mixing process is easier than with other epoxy products, such as those that require the epoxy to be heated.
The mixture was poured directly from the trash can onto the deck. The crew wore spiked shoes to avoid disturbing the epoxy. They spread the epoxy evenly using large squeegees with 3/8-in. notches. Aggregate, angular grain with a Mohs hardness of 7 or greater, was then blown onto the wet epoxy. The crew finished by about 3 a.m., allowing the epoxy time to cure before all lanes were re-opened at about 6 a.m.
The next night, the process was repeated: brush blasting and then epoxy application. More specifics regarding the application were:
- A gallon of epoxy covers about 40 sq ft for the first course and about 20 sq ft for the second course;
- About 10 lb of aggregate are applied per sq yd for the first course and about 14 lb per sq yd for the second course; and
- The curing time varies depending on the temperature. Temperatures must be at least 55°F. At 55°F, for example, the first course takes about four hours to cure. After the second course, the lanes would be open to traffic in about six hours. At 85°F, the first course takes about an hour to cure, with lanes open to traffic after the second course in about three hours.
Epoxy-polymer overlays can offer many advantages for the contractor, the DOT and the motorist, said John Grissinger, Unitex president.
Compared with using concrete overlays, the epoxy-overlay system reduces weight added to the structure by more than 50%, he said. Epoxy polymer also is more flexible and weather-resistant.
“[The] epoxy-polymer overlay prevents future salt damage to the concrete and rebar,” said Grissinger. “It seals the deck from deicing salts and allows the concrete underneath to dry.”
An epoxy overlay also provides more skid resistance and costs less than other options, said Grissinger. Compared with silica fume, for example, an epoxy overlay costs about 27% less.
Michael Sprinkel, associate director of the Virginia Transportation Research Council, said epoxy-polymer overlays can be more economical based on initial costs and life-cycle costs.
“Because epoxy polymers are thin, concrete removal and surface preparation costs may be less than for hydraulic cement concrete,” said Sprinkel.
“Cure time is typically less than for hydraulic cement concrete, which results in less inconvenience to the motorist and lower user costs,” he added.
The epoxy used for the I-135 bridge has undergone extensive lab and field testing by the U.S. Army Corps of Engineers, several state departments of transportation and many other state and federal agencies.
Hybrid technology engineered into the backbone of the epoxy retains more flexibility than nonreactive diluents, which may be initially flexible but lose more flexibility over time. This flexibility minimizes the chances of delamination and reflective cracking. The epoxy also has the durability necessary to withstand extreme road conditions without loss of aggregate.
Nearly 20 states have used the epoxy on multiple overlay projects. The product also has been used internationally.
Sprinkel said epoxy-polymer overlays are a good choice for decks in which the concrete has adequate strength; few cracks; reinforcement that is not corroding; is properly air entrained; and is not deteriorating.
“Overlays should not be used to cover concrete that should be removed,” Sprinkel advised.
Sprinkel cites several key factors to help achieve a long service life for overlays, such as choosing an epoxy polymer that achieves high bond strength and using good construction practices to also achieve high bond strength.
Stuart Johnson, president of Wildcat, said the success of any project also needs cooperation between the contractor, the manufacturer and the DOT.
Wildcat will be back on the job soon, continuing the epoxy-polymer overlay on the northbound bridge lanes.
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