Concrete experiment yields baffling results

A leading Iowa State University expert with the school’s National Concrete Pavement Technology Center (NCPTC) admits he and his colleagues are baffled by developments that have affected an experimental section of the new southbound Route 141 roadway south of Olive in West St. Louis County.

A 1,500-foot stretch of the road that was opened last summer is topped with a 2-inch layer of concrete mixed with titanium dioxide. The chemical was added to serve as a photo-catalytic agent that absorbs smog, using sunlight to break down harmful nitrogen oxides and releasing nitrogen and carbon dioxide as end products.

“But when you’re doing research on experimental methods such as this, sometimes you learn things you don’t expect,” said Dr. James Alleman of Iowa State’s Civil, Construction and Environmental Engineering Department and a researcher for the NCPTC.

The experimental section of roadway was paved in the fall of 2011 and air quality monitoring has been conducted since then to measure the effectiveness of the concrete-titanium dioxide mix in reducing nitrogen oxides.

“Early results showed 40 percent removal of the nitrogen oxides in the air, a very significant amount,” Alleman said.

But subsequent testing showed a dramatically different outcome, with removal plummeting to 4 percent.

In an effort to find an answer, researchers responded with an analysis of some core samples taken from pavement in the experimental area and a similar number from a section of the roadway where conventional paving was applied.

“What we found was a thin layer of calcium carbonate had formed in the test area,” Alleman said. “It seemed the experimental pavement had literally sucked carbon dioxide out of the air and combined it with calcium in the concrete to create a layer that prevented sunlight from reaching the titanium dioxide.”

The result definitely wasn’t what the research team had anticipated. That’s because the calcium carbonate problem has not been reported in Europe where titanium dioxide has been used for a number of years, not only for roadways but as a coating for architectural structures to limit nitrogen oxide exposure that causes discoloration.

However, research literature from the European applications appears to be limited to short-term results, Alleman said.

The Route 141 paving represents what officials believe is the first U.S. location for testing the smog-alleviating concept. And the technology isn’t cheap, adding some $130,000 to the price tag for paving.

Alleman noted that a conference call will be held to review the situation with officials from the Missouri Department of Transportation, the Federal Highway Administration, contractor groups and others with an interest in the technology.

“What we now plan to do is physically scrub the pavement in the test area to remove the calcium carbonate,” Alleman said. “We’ve made arrangements with a contractor to handle the job when he has equipment nearby in connection with another project. The work is being done on a not-for-profit basis, so we need to be flexible as to when it’s done. Weather, as well as the timing of the other project, will affect when the scrubbing takes place, but we anticipate it will be later this spring.”

Alleman said the scrubbing cost will be paid from discretionary funds the NCPTC maintains for such situations. MoDOT will provide traffic control during the operation.

Will scrubbing the pavement ultimately lead to different results?  Or will another layer of sunlight-blocking calcium carbonate build up?

“We think the problem may not happen again,” Alleman said, “but at this point we don’t know for sure.”

One thing is certain though: Results from studies on the experimental segment of Route 141 will become a new chapter to be written about a technology that may, or may not, have a beneficial long-term impact on improving air quality.

Source: News Magazine Network