CHAPTER 7 CONCLUSIONS
AND RECOMMENDATIONS FOR FUTURE WORK
7.1 CONCLUSIONS
This report has reviewed the basics of ASR, summarized past research
and field applications of lithium compounds in concrete construction,
and presented guidelines (including economic considerations) for using
lithium in new concrete and as a posttreatment for existing structures.
Some of the main findings and conclusions from this report include:
- ASR is a significant problem in the United States and elsewhere,
but there are several available methods available forof preventing ASR-induced
expansion, including the use of non-reactive 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 non-expansivenonexpansive.
ASR gels still form in the presence of lithium, but the ir 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.
- Adding lithium to concrete at a molar ratio Li:(Na + K) of 0.74 is
adequate to suppress expansion for most aggregates, although certain
aggregates may require more, and others may need less.
- 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.
A modified version of ASTM C 1260 (modified by adding lithium to the
soak solution) has shown promise as a rapid test for assessing lithium-aggregate
combinations, but further research is recommended to correlate this
rapid test with ASTM C 1293 and field performance.
- Lithium compounds have proven to be effective in posttreating hardened
concrete 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 refine available test methods (such as ASTM
C 1260) to provide a more rapid method of assessing lithium compounds
in mortar and concrete. Modifying ASTM C 1260 by adding lithium to
the soak solution appears promising, but more work is needed to correlate
this version with the concrete prism test, and more importantly, with
field performance. ASTM, AASHTO, CSA, and other organizations must
propose for adoption aAppropriate (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 posttreatment 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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