Los Alamos National Laboratory James E. Rickman, (505)665-9203/[email protected]
"CRACK" RESEARCH TEAM DEVELOPS SIMPLE TEST FOR EARLY DETECTION OF FAILING CONCRETE
LOS ALAMOS, N.M., Feb. 18, 1997 -- Researchers at Los Alamos National Laboratory have developed a simple, environmentally friendly test to detect concrete flaws and are one step closer to cracking a 50-year-old problem of how to stop a significant cause of concrete fractures.
The test identifies one of the most common causes of concrete degradation, even before cracks and other signs of failure become visible. It gives better results than currently used tests and does not involve hazardous materials.
George Guthrie and Bill Carey of Los Alamos' Geology and Geochemistry Group are using their expertise in mineralogy to understand how concrete ages. Guthrie and Carey are studying chemical reactions that occur within concrete and cause the building material -- of which an estimated 260 million cubic yards were used in the United States last year -- to weather and decay.
Of particular interest to Guthrie and Carey is the so-called alkali-silica reaction. The reaction is responsible for premature degradation of concrete. Its effects can easily be seen on sidewalks as the spider-web network of cracks celebrated in childhood rhymes as things that might break a mother's back if stepped on.
Concrete is a mixture of cement and aggregate, typically rock and sand. The alkali-silica reaction can occur when cement is mixed with aggregate rich in certain types of silica materials. The silica reacts with alkalis in cement and forms a gel inside the concrete. The gel expands dramatically in the presence of moisture and creates a network of microscopic cracks. Freezing and thawing make the cracks worsen over time, and eventually the concrete becomes structurally unsound.
"People have been studying the alkali-silica reaction for about 50 years," said Carey, "but there are still a lot of unsolved mysteries. This work is giving us additional insight into ASR."
The test that Guthrie and Carey have developed not only shows when the reaction is occurring, but has given them insight into how it occurs.
Many areas in the United States -- including New Mexico with its abundance of volcanic rock -- have aggregates that promote alkali-silica reaction in concrete. Because so many highways and highway bridges are constructed with concrete, the alkali-silica reaction is of great concern to transportation officials.
Guthrie and Carey are working with personnel in the New Mexico State Highway and Transportation Department to identify structures that may fall prey to alkali-silica reaction -- long before the cracks formed by expanding gel are apparent.
"In some ways, a diagnostic test that allows you to determine whether ASR is occurring in concrete before there are any symptoms is a lot like catching a disease in the very early stages -- the chances of successful treatment are much better," said Guthrie. "Right now there are no proven methods for reversing ASR, but Bill (Carey) and I are learning a great deal and we hope to come up with a cure."
To aid in early detection of alkali-silica reaction, Guthrie and Carey have developed a simple, environmentally benign test that will identify ASR in concrete: a set of chemicals that can be poured on concrete core samples in the field. If the degrading gel is there, the chemicals stick to it and color it. Results can be obtained in minutes.
Right now, one of the more common tests to detect ASR -- and the primary test recommended to state highway departments by the Federal Highway Administration -- involves the use of uranyl acetate. Concrete core samples treated with the radioactive uranium compound must be viewed under a black light in total darkness; areas where gel may have formed fluoresce under the light.
"We believe our test is a much better diagnostic than the uranyl acetate test and is much easier to use in the field," said Guthrie. "Our test is environmentally friendly, doesn't require any special viewing apparatus and is much cheaper, especially in terms of costs associated with disposal of uranium-bearing materials. Not only that, but the uranyl acetate test can lead to false positives."
As a consequence of their research, Guthrie and Carey have identified two distinct types of gels that form during the alkali- silica reaction and have developed their set of chemicals to indicate the presence of the second gel type. The pair also believe there is a third type of gel and are developing a color test to detect its presence as well.
"These tests are chemically specific and we have greatly reduced the chance of false positives," Carey said.
To test the effectiveness of the chemicals, Guthrie and Carey squared off against four concrete core samples taken from a major urban street in Albuquerque, N.M. Other concrete testing labs looked at the samples as well. Guthrie and Carey were able to successfully determine which samples were inundated with ASR gels. The test also was able to find tiny amounts of ASR that weren't readily apparent.
The new test has the potential to save millions of dollars each year. With early detection of ASR, contractors can identify whether concrete is going to undergo quick degradation. If so, they can use a different source of cement or aggregate. In addition, the test may allow concrete-industry personnel to identify whether an aggregate quarry is going to produce, or is producing, material that leads to ASR.
"But more important," said Guthrie, "we believe that with a greater understanding of these gels as a result of our research, we can now possibly determine exactly why these gels are forming and how to stop them. If you could stop these destructive gels from forming, then we have the potential to save lots and lots of money for the construction industry. That's why this problem is important. Not to mention all the mothers' backs we'll save."
Guthrie and Carey have applied for a patent for the indicator chemicals.
Funding for the project comes from Los Alamos' Laboratory- Directed Research and Development program. The LDRD program uses about six percent of the Laboratory's total budget to support outstanding science and engineering efforts. The program in past years has accounted for nearly 40 percent of the Laboratory's patents and nearly 60 percent of its R&D 100 Awards, and helps Los Alamos retain its intellectual vitality.
In addition, Los Alamos has been working with the New Mexico Alliance for Transportation Research to develop a broader effort in improving the durability of concrete.
Los Alamos National Laboratory is operated by the University of California for the U.S. Department of Energy. -30-
For more Los Alamos news releases, visit World Wide Web site http://www.lanl.gov/Internal/News/pressreleases
RSS