Alkali Silica Reaction - Proactive Avoidance

1. Use a low alkali cement (<0.60% equivalent Na₂O). This may not remain a viable solution because changing technology used by the industry to conform to environmental regulations inherently produces cement with a higher alkali content. The alternative may be to use Type IP or IS cements.

• Implementing this recommendation does not imply that alkali-silica reaction will not occur. If a highly reactive aggregate has been used in the concrete, only a small amount of alkali is required to initiate the reaction. (Farny and Kosmatka, 1997)
• ASR may also occur in a low-alkali cement concrete if the structure experiences wetting and drying cycles. Even though only a small concentration of alkali is present, the alkali will become concentrated in the drying zone of the concrete and may be sufficient to initiate reaction. (Farny and Kosmatka, 1997)

2. Use fly ash, natural pozzolans, granulated blast-furnace slag, or silica fume as a cement replacement. They make the concrete mix less permeable therefore making it more difficult for water to reach the aggregate. However, before specifying them for a project mixture, test their effectiveness using ASTM C 311, C 595, C 989, C 1157, C 1260, or C 441 (Farny and Kosmatka, 1997).

• If Class C flyash is used, replace at least 25% of the cement by mass with it.
• Class F flyash is better than Class C flyash and is certain to solve the problem. Although any amount of class F fly ash will reduce alkali-silica reaction, 25% replacement of cement by mass is also recommended.
• If silica fume is used, it should replace 10% of the cement by mass.
• If blast-furnace slag is used, it should replace between 40% and 70% of the cement by mass.

3. Control access to moisture and alkali from external sources. Water imbibed into the gel causes expansion. Not allowing water access to the material and maintaining the concrete’s internal relative humidity below 80% will stop further gel growth and alkali-silica reaction. Accomplish this by applying a sealer such as paint or a moisture barrier to the concrete surface. Sealers must be reapplied periodically to remain effective. For example, a solvent-based silicon coating will provide protection for about 2.5 years (Hobbs, 1988).

4. Use proven non-reactive aggregates or "sweeten" (replace 30% of the aggregate with crushed limestone) the mixture with non-reactive aggregates. This may be impossible or very expensive. River gravel is often the most suspect because it contains only small amounts of reactive rock. The performance history of a given aggregate source is important in evaluating its usability with regard to ASR. Farny and Kosmatka (1997) suggest the following methods to identify reactive aggregate.

• Past field performance records are the best test.
• ASTM C 227 Mortar-Bar Method

• Aggregate is suspect if …
• expansion at 3 months > 0.05%
• expansion at 6 months > 0.10%
• This test has difficulty with aggregate of low reactivity.

• ASTM C 289 Chemical Method

• Determines aggregate to be either innocuous, deleterious, or potentially deleterious.
• This test has problems with slowly reacting aggregate.

• ASTM C 295 Petrographic Examination

• Determines mineral composition and form.
• Identifies potentially reactive minerals.

• ASTM C 1260 Rapid Mortar-Bar Test

• Tests aggregate only, not cement aggregate combinations.
• Innocuous aggregate ® < 0.10% expansion at 14 days.
• Potentially deleterious aggregate ® > 0.20% expansion at 14 days.
• Expansion falling between these two ranges may be defined as one or the other.

• ASTM C 1293 Concrete Prism Test

• Potentially deleterious aggregate ® > 0.04% expansion at 1 year.

5. Alter the alkali-silica gel.
6. Use concrete mixes with a low w/c ratio. This will make the hardened concrete less permeable thus allowing less of the water necessary for expansion of the gel to reach affected areas and limiting mobility of water and alkali around the concrete mass.
7. The use of lithium and barium salts as admixtures is known to reduce ASR.
8. Air-entrainment can reduce the effect of ASR expansion.
9. Use low cement concrete. Less cement provides less alkali to the system.
10. Use a coarse aggregate with a relatively porous microstructure.
11. If reactive aggregate must be used, either use only a small amount or a large amount. The worst situation is to incorporate an amount of reactive aggregate between large and small amounts.
12. Periodic cleaning of the structure may help prevent ASR by washing away salts before they dissolve and penetrate the concrete (Farny and Kosmatka, 1997).
13. Use beneficiation of the aggregate to remove the undesirable portions (Farny and Kosmatka, 1997).

PROACTIVE AVOIDANCE OF MECHANISM - REGIONAL SOLUTIONS

1. Some of the rivers in Kansas and Nebraska produce a sand-gravel that is highly reactive because of their grading and high silica content. They have caused map cracking in concrete structures. This problem is not avoidable with pozzolans and low-alkali cements. The solution is to replace 30% of the aggregate with crushed limestone.
2. In Nova Scotia and eastern Canada, another unique ASR problem has been identified. They are using sedimentary rocks composed of clay—graywackes, argillites, phyllites, siltstones, etc. The clay in these rocks is highly siliceous. The ASR apparently causes exfoliation of the clay minerals which causes them to expand from their normally compact orientation. Some normal ASR may also occur because of the microcrystalline quartz also present in these rocks.
3. Warm sea water in coastal areas can irritate ASR problems because of dissolved alkali. Use unreactive aggregate if it is available and design the mixture with pozzolans, low water-cement ratio, low cement content, and air-entrainment.
4. Alkali-reactive aggregate deposits are found to be wide-spread in the United States, eastern Canada, Australia, New Zealand, South Africa, Denmark, Germany, England, and Iceland.
5. Florida does not have ASR problems.
6. Kansas, Nebraska, and New Mexico have ASR problems.