Colorado Springs – Airport maintenance manager Troy Stover knelt on the massive expanse of a 13,500-foot runway at the Colorado Springs airport to examine a web of cracks in the concrete.
Nearby, an impact roller broke up the 14-inch thick pavement, and bucket loaders filled huge dump trucks with the rubble. All of Runway 35R/17L – 150 feet wide – must be replaced.
Engineers say a chemical reaction made worse by pavement de-icing fluids ruined major sections of the Springs runway and may also be afflicting portions of runways at Denver International Airport.
“We can continue to maintain it, but there’s so much of it – we’re putting ourselves and our passengers at too much risk,” Stover said of the patching program the airport used before deciding to replace the runway.
The cracked concrete in the Springs and at DIA shows premature aging of runways that are among the nation’s newest.
The runway in Colorado Springs that is being replaced opened in 1992. Runways are expected to last up to 30 years. The Federal Aviation Administration is paying 94 percent of the $32.4 million cost in the Springs.
Contractors will build the new runway using the latest concrete chemistry, and officials are confident that the pavement will get its full 30 years of life, said Colorado Springs airport director Mark Earle.
The demolition of 35R/17L will take about three months, and its reconstruction with new concrete 16 inches deep will take about seven months.
DIA’s runways opened in 1995, and engineers have found surface cracking on the pavement that could be the result of the same chemical reaction found in Colorado Springs.
The airport has launched a study to examine “every inch” of the airport pavement, said Brooks Allshouse, project manager for DMJM Aviation, DIA’s paving consultant.
There are a lot of runway inches at DIA. The airport’s first five runways – each 12,000 feet long – began hand ling air traffic in February 1995. The sixth is 16,000 feet long and opened in 2003.
Engineers will take concrete core samples and do other tests to determine whether the pavement cracking at DIA is due to freeze-thaw cycles, shrinkage because the concrete was put down during low humidity, or a chemical reaction that can cause runways to heave and buckle over time.
The degradation of the concrete appears to be an unintended consequence of airports using new, environmentally friendly de-icing and anti-icing fluids.
A study by Clemson University civil engineering professor Prasad Rangaraju and two colleagues shows that potassium acetate, one of the few de-icers approved for airport runways, may catalyze a reaction between elements in the cement and aggregate, or rock, that are key ingredients in concrete. The local aggregate used to make the concrete may be especially susceptible.
It’s called “alkali-silica reaction,” or ASR, a condition in which alkalis in cement and silicas in the rock react in a way that can cause a gel to form.
Swelling of the gel can lead to expansion and cracking of the pavement, according to an FAA document.
Michael McNerney, a DMJM Aviation civil engineer and paving expert, said his hunch is that the cracking on DIA’s runways has not been caused by an alkali-silica reaction. But if further investigation shows it is the root of the problem, it would be more serious than other causes because ASR-damaged runways cannot be fixed with surface patches over the long term.
There is a strong suspicion that the de-icer has exacerbated alkali-silica reactions in pavements, and the Springs airport was the first in the country to show the seriousness of the problem, said Jack Scott, an engineer and pavement specialist with the airports division of the FAA’s regional office in Renton, Wash.
“When potassium acetate came along, the ballgame changed,” he said. “It provided the material that caused the reaction to go.”
Staff writer Jeffrey Leib can be reached at 303-820-1645 or jleib@denverpost.com.
