Michigan DOT Uses New Sealing Techniques to Improve State Bridges
Michigan Department of Transportation (MDOT) Maintenance Supervisor Bob Helsel has sealed dozens of bridge decks using old methods. It had some drawbacks.
Workers would start by pouring 5-gallon portions of two-part epoxy into a larger pail, then mixing it by hand before dumping it on the bridge deck, squeegeeing it evenly, and applying an aggregate surface with a dusty blower.
"The rest would get on the wagon, the employee, and anything that got touched. It was very messy," Helsel said. "These 55-gallon barrels of epoxy were cumbersome to move and often times a second trip to the staging area was required to gather enough material to complete a large bridge."
New equipment incorporates 250-gallon reservoirs of the epoxy components, and a sprayer system that measures and mixes the materials precisely as they are applied, meaning less waste and a faster application rate. A pressurized system then applies a layer of specialized pea-sized chips that become embedded in the epoxy, but in half the time as the old method.
As with the old method, preparation is crucial. Contractors roughen and clear the surface in a process like sandblasting, removing loose material and pavement markings, and MDOT crews sweep the surface to make sure no debris remains. The deck must be completely dry for the process to work, and the materials are temperature-sensitive, meaning the work can only take place from late spring to early fall each year, usually May 1st through September 30th.
A Multitude of Advantages
The new way offers other advantages besides a cleaner process: more uniformity and better materials are expected to offer more years of protecting concrete bridge decks from the elements that attack them. A single bridge can be sealed with less than two weeks of work, and motorists usually encounter little or no delay when the work is under way.
"The main goal is to seal the bridge deck, to keep water out and extend the life of the bridges," said MDOT North Region Associate Engineer for Operations Bill Wahl. "It also provides a higher-friction surface, with a gritty texture that helps drivers maintain traction even when the surface is wet."
The treatment is targeted at newer bridges, those with concrete decks just a few years old that are just beginning to show small cracks due to shrinkage. The epoxy flows down into those cracks and seals the entire surface to prevent water - and salt water during winter - from reaching into the concrete.
"Water and salt water get into cracks in the concrete surface, then freeze and expand, damaging the bridge deck," Wahl said. "The chloride (road salt) also attacks the steel reinforcement, which corrodes and expands, breaking the concrete around it."
New MDOT bridges have a baseline design of 50 years, but with proper maintenance can last 75 to 100 years before requiring replacement. While a bridge substructure (the abutments and piers) and superstructure (usually steel or concrete beams) are somewhat shielded from many of the conditions that age bridges, the decks see it all: loading, fatigue, salt, ice, freezing and hot temperatures, and wear. As a result, they are the component that usually needs to be replaced first after the concrete breaks and steel reinforcing bars have rusted.
Tony Olson, MDOT North Region's Bridge Engineer, says he sees the benefits of the epoxy sealing every year when he inspects those bridges. Even in the few places where the old-method epoxy coating has worn away, the small cracks beneath are still sealed from the elements.
MDOT North Region highways have 335 bridges, and about 20 percent of them have decks sealed with epoxy. While the process has been focused on newer bridges with little deterioration, engineers and maintenance staff are looking to expand the program to include older bridges that have decks still sound enough to benefit from the sealant.
"It definitely slows the deterioration of the deck, and its money well-spent," Olson said. "Other MDOT regions have seen our program, and it's becoming part of their regular maintenance."
As the epoxy sealing process has evolved, so has the staffing to achieve it. In recent seasons, MDOT has contracted with county road commissions and city public works departments to provide additional maintenance employees to work alongside MDOT staff to seal bridge decks in what are being called "mixed crews," paying their wages and benefits.
"We have the equipment, materials and experience, but because our staffing levels have gone down, we're short of labor in the summer months," Wahl said. "The counties and cities typically have more labor available in the non-winter months, but don't have the experience or equipment."
Karl Hanson, Engineer with the Wexford County Road Commission, said a half-dozen of his employees have participated in the program. In addition to the experience they gain, the assignment gives them a chance to work alongside MDOT workers who they might usually only see from behind the windshield of a plow truck.
"I think it improves the relationships with the guys from MDOT crews that plow into our county," Hanson said. "It helps them put a face with a name."
Wahl said mixed crews have also worked on projects outside of the employees' home counties, going wherever in the North Region the work is needed. They're also performing other bridge deck and joint repairs, as well as catch basin and culvert cleaning and repair.
Tim O'Rourke, Manager of the Roscommon County Road Commission, said that when his employees have worked on these mixed crews, they've gained invaluable experience and knowledge that has been applied on their county roads. After working one season with MDOT staff, his employees were able to do their own deck joint replacement project on a county bridge.
"While we have not yet had the opportunity to staff on the epoxy work we look forward to that as well," O'Rourke said. "Job shadowing and cross training with MDOT staff would seem to be a win-win for both entities and the taxpayers."
"There's a spirit of cooperation at work and a focus on outcome," Wahl said. "We're matching the most-effective service provider with the work that needs to be done, and that benefits everyone."