Test Gauges Degradation in Concrete
Two geologists are down on their knees, bathing chunks of concrete in chemicals and watching the grayish material change to yellow and pink.
George Guthrie and Bill Carey then offer their diagnosis: The 12-year-old bridge over Interstate 25 is old before its time.
The stain test developed by the geologists from Los Alamos National Laboratory’s geology and geochemical group detects flaws in concrete more quickly and cheaply than tests now in use.
Finding flaws before they become all too apparent is important because “we can take some action and do it early enough to extend the life of the structure,” said Joe Barela, materials testing engineer for the New Mexico Highway and Transportation Department.
And that’s a big deal because concrete is everywhere--in bridges, roads, dams, runways, culverts, sidewalks and buildings. Millions of cubic yards of it are used in the United States each year.
Problems such as cracks, potholes and the like develop because of chemical reactions within concrete. They can become severe enough to force repair or replacement before the end of the structure’s expected life.
Concrete is a mixture of cement and aggregate, typically sand and gravel. But sometimes the aggregate and cement react with each other in a process known as alkali-silica reaction (ASR), or alkali aggregate reaction (AAR).
The reaction forms a gel inside the concrete, Guthrie said.
“The gel expands dramatically in the presence of moisture and creates a network of microscopic cracks. Freezing and thawing make the cracks worse over time and the concrete becomes structurally unsound,” he said.
The test developed by Guthrie and Carey shows how far ASR has progressed, as evidenced by the differences between the yellow and pink stains. Guthrie estimates it will be six months before the test is validated well enough to give a highway department confidence in it.
“I’m very excited about it,” said Roger Surdahl of the Federal Highway Administration in Washington. “I think it tells us much more” than the test now most commonly used and has the advantage of being nontoxic and easily viewed on site.
A 1988 survey by the Federal Highway Administration found at least 26 states had a problem with ASR, Guthrie said.
There are roughly a million miles of concrete highways in the United States out of a total of 3.9 million miles, according to administration statistics. However, Paul Svercl, a highway engineer for the agency, said the figure was based on a sample and does not include concrete highways that may have been overlaid with asphalt.
About 26 U.S. dams are listed as having a concrete reaction problem, according to a worldwide Internet listing set up in 1995 after the Second International Conference on Alkali Aggregate Reactions in Hydroelectric Plants and Dams.
Larry Nuss of the U.S. Bureau of Reclamation in Denver said ASR can present a maintenance problem for dams, largely those built before the 1940s, because swelling of the concrete can bind spillway gates, for example.
“If the reaction continues, it can degrade the concrete to the point it’s no good, it can crack and fall apart,” he said.
But Nuss and Robert Grabini, executive vice president of the National Ready Mix Concrete Assn. in Silver Spring, Md., caution that the presence of ASR does not mean a structure is ready to collapse.
Rather, Grabini believes the easy diagnosis that may be afforded by the Los Alamos test would help in maintenance.
“The more we know about the performance of our structures, the better off we are,” he said. “If we can forecast, we can do our maintenance program and forestall maintenance that is not necessary.”
The test also is another step in trying to understand ASR, Grabini said.
“It’s going to be able to help forensic engineers looking into potential problems say, ‘We don’t have a problem or we do have a problem, and we’re going to take some remedial steps,’ ” he said.
In the case of the Interstate 25 bridge, even the distress indicated by the pink stain does not mean the bridge is going to fall, Barela said.
“It’s a materials problem instead of a structural problem,” he added.
But the flaws do mean potholes and other surface problems will develop faster than expected and cost taxpayers more, Barela said. And he said it probably will force the bridge to be rebuilt at 20 to 25 years, rather than at its design life of 30 years.
Researchers are not sure precisely how ASR occurs but say the Los Alamos test is offering new insights.
Eventually, researchers hope it will lead to a new concrete recipe to stop the reaction or prevent concrete from swelling.
