Researchers develop a dye test for alkali-silica reaction in concreteResearchers at Los Alamos National Laboratory, Los Alamos, N.M., have begun a second round of field testing of a simple, environmentally friendly test to detect alkali-silica reaction (ASR). The Los Alamos approach, developed by George Guthrie and Bill Carey, does not require any special viewing apparatus or a laboratory setting. In addition, their research may be provide new insight into how ASR occurs, or doesn't occur. Their previous research on the basic energetic and kinetic factors governing the crystallization and dissolution of minerals led the team to study concrete. Guthrie and Carey had been investigating geological and hydrological conditions at Yucca Mountain, a proposed radioactive waste-disposal facility in Nevada. "Since many of the problems in concrete degradation are analogous to geological processes, we began to investigate the geochemical and mineralogical issues related to ASR," says Carey. The test method recently received a R&D 100 award from R&D Magazine. The program honors the most significant products, materials or processes with commercial promise developed by research groups from across the country.
Watching the gels grow ASR can occur in hardened concrete made with aggregates containing some reactive types of silica. Highly alkaline water (pH>12.5) in the pores of hardened concrete reacts with the silica minerals to produce an alkali-rich silica gel. This silica gel swells dramatically in the presence of moisture and creates a network of microscopic cracks. Freezing and thawing cycles widen the cracks over time, and eventually the concrete may become structurally unsound.One area of the Guthrie and Carey research gave insight into the swelling processes of gels in concrete based on the thermodynamics of water sorption. They identified two distinct types of swelling gels formed during ASR. One type is the alkali-rich gel, normally associated with ASR. The other type is a calcium-rich, low-alkali gel.
The researchers suspect other reaction products also form, and are working on a method to detect these products. "We believe that, with a greater understanding of these gels as a result of our research, it may be possible for us determine exactly why these gels are forming and how to stop them," says Guthrie. "If you could stop these destructive gels from forming, you would have the potential to save a lot of money for the construction industry."Effective test and cost savingsThe Los Alamos test utilizes special, environmentally benign dyes that are poured on core samples in the field. If either of the gels is present, the dyes stick to them and are visible in natural light. The alkali-rich gel is stained yellow, while the calcium-rich gel is stained red. If field tests prove accurate, the gel dye tests will reduce the time needed to determine the presence of ASR. It would no longer be necessary to send all samples out for petrographic analysis. Nor would there be a need for labs to use ASTM C-856, a test procedure for ASR that uses uranyl acetate on core samples. Samples treated with this radioactive solution tag ASR, which then fluoresces under a black light. Drawbacks to this method include the process time and cost plus the use of a radioactive chemical. In addition, Guthrie also says because the uranyl acetate test is not chemically specific, it can lead to false positives.The Los Alamos researchers have tested the dyes on concrete cores from more than 20 structures. In addition, they matched the results of their dye test to the results of petrographic and uranyl acetate testing procedures in a blind test on four core samples. The staining method was able to detect small amounts of ASR that weren't readily apparent with either of the other test methods.The researchers hope to expand dye-method applications to testing of concrete that is only 28 days old. Eventually they expect the test to also predict the probable reactivity of aggregates from a given source such as a quarry or pit.
KEYWORDS: alkali-silica reaction, ASR, testing