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Federal Highway Administration Research and Technology
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Publication Number: FHWA-HRT-06-073
Date: July 2006
Chapter 7. Conclusions and Recommendations for Future Work
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This report has reviewed the basics of ASR, summarized past research and field applications of lithium compounds in concrete construction, and presented revised recommendations for using lithium in new concrete and as a post-treatment for existing structures. Some of the main findings and conclusions from this report are:
- ASR is a significant problem in the United States and elsewhere, but there are several methods available for preventing ASR-induced expansion, including the use of nonreactive aggregates, low-alkali concrete, SCMs, and lithium compounds.
- The mechanisms by which lithium compounds suppress ASR expansion are not understood fully, but it is evident that lithium is incorporated into ASR gel, rendering the gel essentially nonexpansive. ASR gels still form in the presence of lithium; however, their altered structure (perhaps lithium substituting for calcium) inhibits water absorption and expansion.
- A variety of lithium compounds, including LiCl, Li2CO3, LiF, Li2SiO3, LiNO3, LiOH·H2O, LiNO2, and Li2SO4 have been shown in laboratory studies to inhibit ASR-induced expansion effectively, provided that they are used at a sufficiently high dosage. LiNO3 has been found to be the most efficient of the above compounds in controlling expansion because, unlike the other lithium salts, LiNO3 does not increase pore solution pH.
- A substantial portion of lithium is absorbed in early hydration products, thereby requiring higher dosages of lithium compounds to offset this loss and to control expansion adequately. The development of a lithium-bearing glass has been reported as a means of minimizing this uptake of lithium by hydration products, thereby resulting in more efficient use of the active lithium compound in controlling ASR-induced expansion.
- The combined use of lithium and SCMs (especially fly ash and slag) is recommended to reduce the economic impact of using lithium and to produce low-permeability concrete that is more resistant to ASR and other deterioration mechanisms.
- The test method most recommended for assessing lithium compounds in the laboratory is ASTM C 1293, with a test duration of 2 years. Other more rapid tests are not currently available as an alternative to ASTM C 1293.
- Lithium compounds have proven to be effective in post-treating hardened mortar or concrete in the laboratory that has already expanded from ASR, thereby reducing or eliminating future expansion.
7.2 RECOMMENDATIONS FOR FUTURE WORK
As the availability of high-quality, low-reactivity aggregates continues to dwindle, the use of alternative means of controlling ASR certainly will become even more important. Given the past successes of using lithium compounds to inhibit expansion, both in the laboratory and in the field, the future of using lithium in concrete construction appears bright. However, there are several technical and practical issues that deserve further investigation:
- More mechanistic research is needed to define better how lithium compounds suppress expansion due to ASR. Several theories have been proposed, as described in Chapter 3, but gaining a better understanding of the underlying mechanisms will result in more efficient and cost-effective applications of lithium compounds in concrete construction.
- The uptake of lithium by hydration products reduces the amount of lithium available for ASR suppression. Research is needed to overcome this problem, thereby reducing the dosage of lithium needed to control expansion. Lithium-bearing glass has been reported to address this issue, but additional work on this material and new lithium-bearing products is recommended.
- Research is needed to establish correlations between aggregate mineralogy (and reactivity) and requisite lithium dosage to control expansion. Although it was initially proposed (Guidelines for the Use of Lithium to Mitigate or Prevent ASR, Folliard, et al., 2003) that a 0.74 molar ratio of lithium (e.g., manufacturer's 100 percent dosage) adequately controls expansion for all aggregates, it has since been shown that some aggregate require significantly more lithium to control expansion below typically prescribed limits (e.g., 0.04 percent expansion at 2 years in ASTM C 1293).
- Research is needed to refine available test methods (such as ASTM C 1260) to provide a more rapid method of assessing lithium compounds in mortar and concrete. Appropriate (and accurate) tests to assess lithium compounds in mortar and concrete must also be proposed for adoption by ASTM, AASHTO, CSA, and other organizations. The availability of these standard tests will help spur the development of specifications related to using lithium in concrete construction.
- Case studies have shown that lithium compounds, when used in new concrete or as a post-treatment for existing structures, are effective in inhibiting ASR-induced expansion. Long-term monitoring of the structures described in Chapter 4, as well as future field applications of lithium will be essential in relating laboratory tests to field performance, refining and improving the guidelines and recommendations provided in Chapter 5, and understanding the benefits of using lithium on service life extension (including economic considerations).
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