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:
- Alkali + Reactive Silica = Alkali Silica Gel
- Alkali Silica Gel + Moisture = Expansion
- Expansion = Cracking
10 ASR facts
- ASR does not necessarily cause damage within a structure. If harmful ASR expansion does occur, these other nine facts could come into play:
- Map cracking – Map cracking (intersecting cracks) is not always indicative of ASR, but expansive ASR often reveals itself as map cracking. Map cracking is usually difficult to see when the concrete surface is dry, but it is more easily seen after the concrete is wetted and has begun to dry (see Figure 1).
- Surface deposits – ASR gel can sometimes be found along cracks and in voids within the concrete, and these deposits can range in color from white to dark gray.
- Efflorescence – Efflorescence (calcium hydroxide deposits) on concrete surfaces may be confused with ASR gel. Efflorescence can also develop from cracks in the concrete originally produced by ASR.
- Popouts – ASR does not cause all popouts. Pop-outs (the breaking away of small portions of a concrete surface) have a number of causes, such as porous aggregates that freeze and expand and aggregates contaminated with clay (clay – a champion of expansive force) or other deleterious compounds (pyrite). Popouts caused by ASR will almost always have a residual gel deposit at the location of the pop-out (see Figure 2).
- Petrographic tests – A petrographic examination is the only method to confirm the presence of deleterious ASR attack. Cracking, popouts and surface deposits can all be strong indicators of ASR attack, but taken separately, they are not definitive evidence.
- Structural integrity – Concrete’s load-bearing capacity and structural integrity is not significantly affected by moderate cases of ASR.
- Wetting and drying cycles – Deleterious ASR is amplified in structures exposed to continuous wet and dry cycles and external sources of alkalis such as deicing salts.
- Available technology – ASR can be avoided through the use of available concrete-mix material histories and an informed mix design for production.
- Test your mix materials – The best way to control ASR is to test your mix materials slated for production.
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 [email protected].
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