Montana State University

Saving America's highways, one concrete block at a time

November 8, 2006 -- By Tracy Ellig, MSU News Service


Tongyan Pan (left) and Xianming Shi, researchers at the Western Transportation Institute at Montana State University, stand on some of the nine tons of concrete they will test at MSU to better understand how corrosion affects the nation's roads and bridges. (MSU photo by Jay Thane.)   High-Res Available

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Tel: (406) 994-4571
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A never-tried experiment on nine tons of concrete may shed some light on how to save billions of dollars in maintenance, repair and replacement of the nation's highway bridges and roads.

The experiment by Xianming Shi and Tongyan Pan, researchers at the Western Transportation Institute at Montana State University, addresses the most destructive force on the nation's highways: corrosion.

"Corrosion isn't as interesting-sounding as hurricanes, but this silent killer costs billions annually. There is no way to stop it, but you can slow it down," Shi said.

The roughly nine tons of concrete Shi and Pan are experimenting with is in the form of 150 blocks, custom poured at the direction of the Washington Department of Transportation, which is funding the research.

The blocks are made from the same concrete typically found in Washington state highways and bridges and are embedded with rebars or dowel bar. The blocks will be exposed to four different highway de-icers, from straight salt to de-icers containing corrosion inhibitors.

"When salt is spread on a road, it eventually migrates through the concrete onto the metal rebar surfaces and causes them to rust," Shi said. "When the rebar rusts, it expands and puts stress on the concrete that can cause it to crack, chip and flake."

Though the blocks have been poured to Washington specifications, Shi and Pan expect the results will be applicable to any snow-belt state, including Montana. Salt and chemical de-icers have replaced sand along many highways to prevent water pollution. While salt and chemical de-icers can be safely diluted by runoff, sand can damage streams and rivers.

In the Northwest, salt has been replaced by far more expensive chemical de-icers, such as magnesium chloride, that contain corrosion inhibitors, mainly to protect vehicles. However, tests to determine whether such expensive de-icers actually protect vehicles outside of the laboratory
have proven inconclusive, Shi said.

"There are too many variables, such as large changes in temperature and moisture to get good results," Shi said.

The concrete block research is an attempt to get rid of those variables and focus on a different area.

"Could the corrosion inhibitors seep into the concrete and protect the rebar or dowel bar in roads and bridges? This is pretty much a mystery," Shi said. "We think we've designed an experiment that can give us usable data."

The blocks were molded to hold a small pool of de-icing liquid on their surface. A plastic window will be fitted over the top of the block and then the pool of liquid will be pressurized. This will be done to speed the de-icer into the concrete. Sensors embedded in the blocks will detect when a de-icer and its corrosion inhibitor have reached the rebar.

"Normally, it would take years, maybe decades to seep enough de-icers through the concrete to cause the steel to corrode," Pan said. "We don't have that long."

Shi and Pan hope their accelerated experiment will give them results in six to 12 months.

The blocks will also be kept in an environmental chamber to simulate the freeze-thaw and wet-dry cycles that normally occur on winter roads.

This kind of research is the first of its kind and if successful could lead to a national, standardized method for testing how chemicals seep into concrete, Pan said.

Any knowledge that might extend the life of the nation's roads and bridges would be useful, Shi said.

"Cement is a valuable resource: To produce one ton of cement, one ton of carbon dioxide is produced," Shi said. "Also, the cost of repairing a road or bridge can be five to six times the cost of doing good design work up front to lengthen its life."

Considerable know-how has gone into designing the experiment, but eventually some raw muscle may be needed as well. At some point after the sensors indicate the steel bars are starting to corrode, Shi and Pan want to break the blocks open and look at the metal.

"Well ...," Shi said, looking thoughtfully at one of the stout blocks, "that may be a challenge."

Xianming Shi (406) 994-6486 or xianming_s@coe.montana.edu.