Stanton’s (1940) formative work on ASR indicated that expansive reaction is unlikely to occur when the alkali content of the cement is below 0.60% Na2Oe. This value has become the accepted maximum limit for cement to be used with reactive aggregates in the United States, and appears in ASTM C 150 Standard Specification for Portland cement as an optional limit when concrete contains deleteriously reactive aggregate. However, this criterion takes no account of the cement content of the concrete which, together with the cement alkali content, governs the total alkali content of concrete, and is considered to be a more accurate index of the risk of expansion when a reactive aggregate is used in concrete. Some national specifications take cognizance of this fact by specifying a maximum alkali level in the concrete; this limit is reported (Nixon and Sims, 1992) to range from 2.5 to 4.5 kg/m3 Na2Oe in the UK. In some countries (e.g., Canada), the limit may vary, depending on the reactivity of the aggregate.
There is currently no test method available that is suitable for determining the `safe level’ of alkali for a particular aggregate. Most test methods require an artificially high alkali content to accelerate the rate of reaction and conducting the test with low-alkali cement may fail to produce expansion in the laboratory even though the same combination of materials could result in long-term expansion under field conditions. In addition, small laboratory specimens are more prone to leaching of the alkalis during test and higher levels of alkali are required under laboratory conditions (e.g., concrete prism tests) to compensate for this phenomenon.
Aggregates that normally are not reactive when used in concrete with low- alkali cement may be deleteriously reactive in concrete of higher alkali content. This may occur through alkali concentration caused by drying gradients, alkali release from from aggregates, or the ingress of alkalis from external sources, such as deicing salts or seawater. Stark (1978) reported increases in soluble alkali from 1.1 to 3.6 kg/m3 Na2Oe close to the surface of some highway structures. Migra- tion of alkalis due to moisture, temperature, and electrical gradients has also been demonstrated by laboratory studies.
A number of workers have demonstrated that many aggregates contain alkalis that may be leached out into the concrete pore solution, thereby increasing the risk of alkali-aggregate reaction. ,
Stark and Bhatty (1986) reported that, in extreme circumstances, some aggregates release alkalis equivalent to 10% of the Portland cement content. Supplementary cementing materials (SCM), such as fly ash, silica fume, slag and natural pozzolans may also contain significant quantities of alkali, and this is discussed in the next section.
Alkalis may penetrate concrete from external sources such as brackish water, sulfate-bearing groundwater, seawater, or deicing salts. Nixon et al. (1987) showed that seawater (or NaCl solutions) present in the mixing water elevatesthe hydroxyl-ion concentration and increases the amount of expansion of concrete. Several researchers have also shown that exposure of concrete to saline environments, from which NaCl (and other alkali metal salts) penetrate into the material, can enhance expansion and cracking due to ASR (Chatterji et al., 1987; Oberholster, 1992; Kawamura et al., 1996; Sibbick and Page, 1998). Deicing salts, such as potassium acetate or sodium formate, typically used on airfield pavements as less corrosive alternatives to NaCl, may also be expected to exacerbate ASR, although there is little or no information about this in the literature.
