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Publication Number: FHWA-RD-03-047
Date: July 2003
This chapter provides guidelines for the use of lithium compounds to inhibit expansion due to ASR in both new and existing concrete. The guidelines are based on a review and synthesis of laboratory studies, field applications, and existing specifications. The intention of these guidelines is to help assist practitioners in testing, specifying, and using lithium to control ASR-induced expansion and damage efficiently. Relevant information on the economics of using lithium compounds is presented in chapter 6.
Performance-based guidelines are presented first in this section, followed by prescriptive guidelines for using lithium admixtures in new concrete. Specific guidelines and details are provided on what laboratory tests and expansion limits should be considered when testing lithium compounds.
This chapter does not present comprehensive guidelines regarding all the options available to mitigate ASR (i.e., SCMs, low-alkali concrete), as summarized earlier in 0, but rather to provide practitioners who are interested in using lithium compounds with technical guidance on the topic. The performance-based guidelines allow for testing any lithium compound in the laboratory. The prescriptive guidelines only involve the use of LiNO3, because it is the lithium compound most commonly used, does not exhibit a pessimum effect at lower dosages, and is safer to handle than other forms of lithium.
5.2.1 Performance-based Guidelines for Using Lithium in New Concrete
This section provides guidance on how to evaluate lithium compounds through laboratory performance tests. It emphasizes determining the appropriate dosage of lithium to use in combination with a given reactive aggregate, either with or without the combined use of SCMs, especially fly ash and slag.
The two performance tests discussed in this section are ASTM C 1293 (concrete prism test) and ASTM C 1260 (accelerated mortar bar test). ASTM C 1293 is recognized as the best indicator of field performance and is preferred over ASTM C 1260 when assessing aggregate reactivity or the efficacy of SCMs in controlling expansion. In addition, based on past research, including the works reviewed in chapter 3, ASTM C 1293 is recommended as the best available test for assessing lithium compounds. Because ASTM C 1260 has several limitations, and because only limited testing has been performed using lithium compounds, caution should be taken when using it as the sole method of assessing a specific lithium/aggregate combination. Only limited testing has been performed to date to correlate ASTM C 1260 tests using lithium compounds to either ASTM C 1293 results or field performance. Whenever possible, ASTM C 1293 should be performed to gain the most reliable index of lithium performance.
ASTM C 1293
ASTM C 1293 generally has been recognized as the most accurate test in predicting field performance of aggregates (1-year test) and SCMs (2-year test). The test is less severe than ASTM C 1260 or other rapid tests, uses concrete (rather than mortar), and results in less leaching of alkalies than other tests. Its main drawback has been of the practical sort, rather than technical. Specifically, the long duration of the test has been the major criticism. However, this longer test time provides a more realistic environment and results in better correlation with field performance.
Based on a review of published literature and a survey of current practice, ASTM C 1293 is recommended as the preferred test for assessing lithium compounds. However, ASTM C 1293 does not currently provide guidance on using the test for this purpose. Following are recommendations for testing lithium compounds, by themselves or in combination with SCMs, using ASTM C 1293.
To test lithium compounds (without SCMs) using ASTM C 1293:
To test lithium compounds in combination with SCMs using ASTM C 1293:
ASTM C 1260
As described in chapter 3, the use of a modified version of ASTM C 1260 has shown promise in assessing the efficacy of lithium compounds in reducing expansion due to ASR. The modification entails the adding tion of lithium to the soak solution to minimize or prevent lithium leaching of lithium from the mortar bars. Although this approach appears to be technically sound, there are limited data correlating the test to either ASTM C 1293 or field performance. Therefore, caution should be exercised when interpreting the results from the modified version of ASTM C 1260, and if testing has been conducted on the same lithium-aggregate combination in ASTM C 1293, the data obtained from ASTM C 1293 should govern.
Following are recommendations for testing lithium compounds, by themselves or in combination with SCMs, using ASTM C 1260 (modified by adding lithium to the soak solution).
To test lithium compounds (without SCMs) using ASTM C 1260 (modified):
To test lithium compounds in combination with SCMs using ASTM C 1260 (modified):
5.2.2 Prescriptive Guidelines for Using Lithium in New Concrete
Prescriptive guidelines are intended to provide specific recommendations that can be followed without the need for performance tests. Examples of this approach when considering ASR would include specifying a minimum of 20 percent Class F fly ash or a minimum slag content of 35 percent. Prescriptive guidelines or specifications for SCMs are widespread and are used by many agencies and organizations with confidence because of the wealth of laboratory data and field-performance records available.
Because there is not the same magnitude of laboratory data and documented field performance for lithium compounds, recommending similar prescriptive guidelines is challenging. There are also sIn addition, some unresolved technical issues that have not been resolved that make it even more difficult to develop prescriptive guidelines for using lithium to control expansion due to ASR. Because of these limitations and concerns, the recommendations provided in this section tend to be more conservative. As the use of lithium compounds in concrete construction continues to evolve, more data and performance histories will become available, making future guidelines more directed and, perhaps, less conservative.
Before summarizing the prescriptive guidelines for using lithium compounds in new concrete, some background information on the factors influencing the performance of lithium compounds in concrete (e.g., effects on ASR-induced expansion) is presented. This information is intended to serve as a foundation for prescriptive guidelines and also is provided to show that the gaps in understanding and lack of field data warrant a conservative approach at this time. Of course, the performance testing described in the previous section, especially ASTM C 1293, provides practitioners with the necessary tools to determine lithium dosages required to suppress expansion and represents an alternative to prescriptive recipes.
As described in chapter 3, the efficacy of lithium in suppressing expansion due to ASR is dependent on the nature or reactivity of the aggregate, the form of lithium, and the availability of alkalies. Generally, aggregate reactivity is assessed by tests such as ASTM C 1260 and ASTM C 1293. The level of mitigation or protection against ASR is often based on the values obtained in these tests, with higher expansion values requiring higher dosages of fly ash or slag, for instance. The difficulty in applying this same approach for lithium compounds is that a clear correlation between a given aggregate's reactivity, as defined by ASTM C 1260 or ASTM C 1293, and the response of that aggregate to lithium (as a function of dosage) does not exist. For instance, research at BRE (Blackwell et al., 1997; Thomas et al., 2000) has demonstrated that some aggregates require more than a 0.74 molar ratio to control expansion, but some of these aggregates (i.e., siltstone) were not as reactive (as defined by ASTM C 1293) as other aggregates that responded more favorably to lithium compounds. In addition, research at the University of Texas at Austin (Folliard, unpublished data) has shown that an extremely reactive chert aggregate, which had an expansion greater than 0.6 percent in ASTM C 1260 and exceeded the 0.04 percent expansion limit in ASTM C 1293 in less than a month, responded very favorably to LiNO3. Using LiNO3, a molar ratio of 0.55 was found to suppress expansion with the highly-reactive chert aggregate, yet this aggregate was extremely difficult to mitigate using SCMs.
Clearly, there is not a trend between aggregate reactivity, as it is typically defined, and the response of an aggregate to lithium treatment, making it difficult to integrate aggregate reactivity into prescriptive guidelines. Based on a review of literature, a molar ratio Li:(Na + K) of 0.74 has been found to suppress expansion for the majority of aggregates, and a wider range of molar ratios, from 0.60 to 0.90, essentially brackets the range of lithium dosages needed to control expansion for the aggregate-lithium combinations reported in literature. For a given aggregate, a lower dosage of LiNO3 is needed to control expansion, compared to other forms of lithium. As previously mentioned, the prescriptive guidelines provided later in this section only cover the use of LiNO3. Because there are not enough data available to link LiNO3 dosage or to show that dosages lower than 0.74 (molar ratio) are required to suppress expansion, a minimum LiNO3 dosage of 0.74 will be recommended for all aggregates. Dosages higher or lower than this value may be used, provided that previous testing was performed using ASTM C 1293, as described under section 5.2.1.
Another issue that deserves attention is the role that alkalies present in SCMs should play in determining prescriptive dosages of lithium. As described in chapter 3, the BRE counts the alkalies present in SCMs to a varying degree, depending on the dosage of that SCM, and uses this alkali contribution in calculating the total alkali loading of the concrete. This alkali loading is then used to prescribe a lithium dosage (see section 3.3.4). The CSA guidance on using SCMs to control ASR actually does not actually use alkali contribution from SCMs in calculating the total alkali loading of the concrete, but CSA does recommend a tiered approach, where an SCM (i.e., fly ash) with higher alkali content requires a higher dosage of that SCM to adequately suppress expansion. There have been very few studies on the use of lithium combined with SCMs, and of these studies, the main focus has been on using SCMs with relatively low alkali contents. Because of the lack of available data on higher-alkali SCMs, the guidance provided in this section will only be valid for fly ashes with a total alkali content less than 3 percent or for slag with an alkali content less than 1 percent. If the alkali content of a specific fly ash or slag exceeds these limits, testing is required using ASTM C 1293 to determine the adequate lithium dosage. Prescriptive guidelines are not included in this section for SCMs other than fly ash and slag because of a lack of published information.
When combining the use of high-calcium fly ash (> 15 percent CaO) and slag with LiNO3, the lithium dosage recommended as part of these prescriptive guidelines will be based on the alkali contribution from portland Portland cement only (e.g., 0.74 molar ratio based on Na2Oe from cement). Therefore, the actual amount of lithium added to the concrete will be reduced, based on the replacement level of the fly ash or slag for portland cement, thereby treating the SCM essentially as a dilutent. No other reductions in lithium dosage, beyond this dilution effect, are prescribed for these higher-lime ashes and slag, because there are no published data to support such a reduction. It has been shown in chapter 3 that the beneficial effects of using low-calcium fly ash and lithium together to control ASR are cumulative. Thus, the guidelines provided later in this section allow for reduced lithium dosage (beyond the dilution effect) when combined with fly ash with a CaO content less than 15 percent. Although some studies have shown that a 50 percent reduction in lithium dosage (based on alkalies in the portland cement) may be possible with low-lime ashes (< 8 percent CaO), only moderate reductions in lithium dosage are proposed in these guidelines to be conservative.
Before summarizing the prescriptive guidelines, one last issue to be addressed is the total alkali loading of the concrete. Because most studies have dealt with moderate alkali loadings, the guidelines proposed in this section will be limited to concrete mixtures with alkali contents (based on alkalies from portland cement) of less than 5 kg/m3, a limit that is consistent with the BRE guidelines. As previously described, the BRE guidelines include anywhere from zero to one hundred percent of the alkalies present in SCMs when calculating the total alkali loading for a given concrete mixture. However, because limits have been imposed on the alkalies in SCMs in the current recommendations (3 percent for fly ash, 1 percent for slag), and because relatively high dosages of fly ash and slag will be recommended in most cases, the contribution of alkalies from SCMs will not be included in calculating the alkali loading of concrete.
When considering the developing ment of prescriptive guidelines, the CSA approach is the most comprehensive and is highly recommended for assessing aggregate reactivity and the need for preventive measures (CSA, 2000a). The CSA guidelines cover a range of mitigation options (see table 3), including various SCMs and ternary blends, but they do not yet include guidance for lithium compounds. Consultation with the CSA guidelines is urged when seeking general guidance on testing, specifying, and using concrete that does not contain lithium. When considering the use of lithium, the guidelines presented below are recommended and can be compared to other available mitigation methods by following CSA A23.2-27A.
The ideal approach to developing prescriptive guidelines for lithium would be to follow the same methodology as the CSA guidelines. However, because of the limitations described in this section, including the difficulty in relating aggregate reactivity to lithium dosage, a more simplistic approach is necessary. CSA A23.2-27A dictates different levels of prevention based on aggregate reactivity, defined by ASTM C 1293 or ASTM C 1260, but as previously cited, this approach may not be valid for lithium. However, it is still possible to use ASTM C 1293 or ASTM C 1260 to determine if an aggregate even requires any preventive measure; that is, if the expansion in ASTM C 1293 is less than 0.04 percent (at 1 year), or if the expansion is less than 0.10 (after 14 days), no mitigation (i.e., SCMs, lithium) is required. As always, if data are available for a given aggregate from both ASTM C 1293 and ASTM C 1260, the data obtained from ASTM C 1293 should govern. Beyond determining whether an aggregate requires mitigation or not, it is not possible at this time to link aggregate reactivity or expansion values to required lithium dosages.
Because there is a general lack of field experience using lithium compounds, it is not currently feasible to adopt the CSA approach of selecting levels of protection (e.g., lithium dosages) based on the size of the concrete element and the nature of the surrounding environment. The approach currently proposed is to remain conservative in terms of specifying lithium dosage (either with or without SCMs).
The following are the proposed prescriptive guidelines for using lithium compounds in concrete, which summarize the key issues already discussed, including limitations and assumptions. For materials that are not explicitly covered in these prescriptive guidelines, such as lithium compounds other than lithium nitrate or SCMs other than fly ash or slag, the performance-based guidelines (section 5.2.1) should be followed.
Prescriptive Guidelines for Using Lithium Compounds in Concrete
Although the parameters that affect the efficiency of lithium-based compounds as chemical admixtures for controlling expansion due to ASR in new concrete have been established by laboratory studies and confirmed by field evaluations, the efficacy of these products in terms of treating existing ASR-affected concrete have not. It is clear from laboratory studies that treating small samples with lithium can reduce expansion (e.g. Stokes et al., 2000), but data are lacking from field cases. Although a number of structures have been treated in the field (especially topical applications of lithium to pavements and bridge decks) there is no unequivocal evidence to demonstrate that these treatments have been successful, or indeed, that the treatment has had any beneficial impact. In most cases, this is due to the failure to implement appropriate monitoring programs to evaluate the effect of the treatment. Consequently, it is not possible to develop guidelines for treating existing structures at this stage. However, certain procedures have been established empirically, and these are discussed below.
5.3.1 Topical Applications
The success of topical treatments of pavements and bridge decks with lithium compounds is likely to be influenced by a number of parameters, including:
Many of these factors will, of course, affect the amount of lithium that infiltrates the concrete and the lithium's effective penetration depth of penetration of the lithium.
Laboratory research has indicatesd that certain forms of lithium may exhibit a pessimum effect when used as an admixture for new concrete. This takes the form of increased expansion when an insufficient level of lithium is used, and the effect is ascribed to the increased pH resulting from the use of LiOH or other lithium compounds that react with Ca(OH)2 to increase the concentration of OH- ions in solution (e.g., 2LiCO3 + Ca(OH)2 —› 2LiOH + CaCO3). If such compounds are used to treat concrete, there is a possibility that the treatment may exacerbate the reaction if only a small amount of the compound penetrates the concrete. Lithium nitrateLiNO3 generally does not generally result in an increased pH due to the relatively high solubility of the calcium analogue i.e., the nitrate ions stay in solution to balance the lithium rather than reacting with Ca(OH)2 to release OH- OH ions. For this reason, it is recommended that only solutions of LiNO3 be used for topical treatments of existing ASR-affected concrete. Increased penetration may be achieved by incorporating a suitable surfactant into the solution (e.g. Stokes et al., 2000).
There are two things to consider about the application rate: (1) if the application rate is too high, the solution may run off the surface to be treated, resulting in waste (and possibly increasing environmental concerns), and (2) under certain conditions, solution ponded on the surface may evaporate, precipitating LiNO3 salt on the treated surface, which may lead to reduced surface friction of trafficked surfaces. Experience has indicated that the optimum application rate for most cases is in the range of 0.16 to 0.40 liters L of 30 percent LiNO3 solution per square meter of concrete surface. A number of other controls may be necessary, and the following guidelines are reproduced from a recent specification to serve as an application:
The number of individual treatments that can be applied to a structure will be governed by economics and other aspects of the repair strategy. For example, if the structure is being treated prior to the application of a concrete or asphalt overlay, there may only be time for a single treatment. For pavements or bridge decks that remain exposed after treatment, additional treatments may be considered at appropriate intervals. For example, the treatment of S.R. 1 in Delaware involved a total of 6 individual treatments over a 3-year period.
It is expected that concrete with a high permeability that has been subjected to prolonged periods of drying prior tobefore treatment will more readily absorb more readily the solution applied to the surface. Also, a thin section with a high surface- area-to-volume ratio will permit a greater volume of the concrete to be infiltrated. However, these conditions clearly cannot be controlled.
The extent of concrete deterioration of the concrete at the time of treatment will have an impact on the ease with which the lithium solution can penetrate the concrete. Cracking clearly will clearly facilitate ingress of the solution. However, if the concrete deterioration of the concrete has proceeded too far, it may be too late to treat the affected concrete. Johnston et al. (2000) suggested that there is an optimum time to treat the concrete, in terms of the amount of deterioration, and explained the trade-off between cracking and lithium penetration by means of in the schematic shown in figure 39.
Figure 39. Optimal Time for Lithium Treatment Applied Topically (Johnston et al., 2000).
Lithium treatment will only address the problems related to future ASR deterioration. Clearly, it will not reinstate the concrete to its original condition, and if the deterioration present is likely to contribute to other deterioration processes, such as freeze-thaw or corrosion (by allowing access to chloride ions), then these problems have to be addressed separately. This may be achieved by sealing the cracks or applying an overlay.
5.3.2 Electrochemical Migration
Electrochemical repair techniques used to drive lithium in concrete usually have been applied with the additional aim to remove chlorides (or realkalize the concrete). Regardless, these techniques are highly specialized, and it is likely that every case will likely involve design considerations specific to the individual job. As such, it is not possible to produce generic guidelines for the procedure. However, it is recommended that such techniques only be carried out by contractors with high levels of expertise and, if possible, with previous experience in using lithium-based materials such as the electrolyte.
5.3.3 Vacuum Impregnation
At the time of writing, there were are no known cases involving the vacuum impregnation of lithium into concrete. Therefore, it is not possible to develop guidelines for this process.