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By G. Terry Harris Sr., FACI
Before addressing alkali-silica reaction (ASR) in concrete, let’s start with what you already know. Concrete is essentially a mixture of two components: aggregates (stone, gravel and sand) and cement paste (water, cementitious materials, air and admixtures). The paste contains interconnected microscopic pores through which water can migrate. This pore water in concrete is a highly alkaline solution.
Alkali metal hydroxides in the pore solution chemically react with certain aggregates that contain silica. It is easier for these alkali metal hydroxides to combine with silica (quartz), which is in a more disordered, or reactive, form.
An expansive and destructive marriage
Since alkali hydroxides and unstable silica have a strong mutual attraction, these two compounds join hands, so to speak. Together they swell up as they draw in moisture from the surrounding paste, becoming a gelatinous chemical couple. And we should expect their union to grow progeny – or additional little chemical reactions. The problem is that the chemical result of their union is not so little.
Unfortunately, the gelatinous result of this chemical reaction, under certain conditions, may cause deleterious expansion. In fact, in extreme circumstances, the ASR gel expands so much that it cracks the concrete. As the gel absorbs more water and continues to expand, it induces internal pressures that may crack the aggregate, the cement paste or the surrounding concrete.
ASR is simply explained in three steps as follows:
10 ASR facts
How to avoid deleterious ASR
The surest way to prevent ASR is to avoid aggregates that are known to be reactive. Unfortunately, this is not always possible or practical. When proportioning a concrete mix for resistance to ASR, previous material history is extremely important. The use of a low-alkali cement, fly ash, slag, silica fume and metakaolin are all beneficial when proportioning for ASR resistance.
Lowering the total cement content has also proved to lower the total alkali load, providing that a cut in cement content does not compromise the strength spec. Chemical admixtures such as lithium nitrate or lithium hydroxidei are known to be effective in controlling ASR. As with any concrete mixture, testing with the selected materials for production is recommended (see the sidebar “ASR Test Methods”).
Knowledge is the best defense against ASR
As noted previously, most moderate ASR cases, although unattractive, do not adversely affect the load-bearing capacity of concrete structures. Of greater concern is that deleterious ASR expansion may “open up” the concrete to allow ingress for other deleterious processes. Adverse environmental elements such as chlorides, nitrates, sulfates, sea water and carbonation products can penetrate concrete through cracked surfaces and cause deterioration.
While ASR can be a devastating problem in concrete, especially when exposed to cycles of wetting and drying, information available today enables the concrete producer to design – either through historical knowledge or testing – the mix materials and proportions that will consistently produce ASR-free concrete.
Terry Harris is North American technical services manager, W. R. Grace & Co., Cambridge, Mass., and has 32 years of experience in the concrete industry, including ready mix, precast, block and admixtures. Terry is active in ACI, ASTM, NRMCA and NPCA and has a bachelor’s degree in concrete technology from Daytona State College. Contact him at Terry.Harris@grace.com.
American Concrete Institute, ACI 221.1R-98, “Report on Alkali-Aggregate Reactivity,” Farny, James A.
Kekhoff, Beatrix, “Diagnosis and Control of Alkali-Aggregate Reactions in Concrete”
(i) Lithium salts have been used to treat existing ASR-affected structures with limited success. It is very difficult for the lithium to penetrate the concrete sufficiently, even on heavily cracked surfaces.
(ii) Chert is a microcrystalline or cryptocrystalline sedimentary rock material composed of silicon dioxide (SiO2). It occurs as nodules, concretionary masses and as layered deposits. Chert breaks with a conchoidal fracture, often producing very sharp edges. Early people took advantage of how chert breaks and used it to fashion cutting tools and weapons. The name “flint” is also used for this material. Source: geology.com/rocks/chert.shtml