|FHWA > Engineering > Pavements > Concrete > HRT-06-073 > Chapter 5|
CHAPTER 5 APPROACH FOR USING LITHIUM IN NEW AND EXISTING CONCRETE STRUCTURES
This chapter provides interim recommendations for the use of lithium compounds to inhibit expansion due to ASR in both new and existing concrete. These recommendations are based on a review and synthesis of laboratory studies, field applications, and existing specifications. The intention of these recommendations is to 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.
5.2 USING LITHIUM COMPOUNDS IN NEW CONCRETE
This section focuses primarily on performance-based recommendations for using lithium compounds to mitigate ASR in new concrete. After presenting a method of evaluating and selecting lithium dosage based on laboratory testing, some information on a prescriptive approach is presented. However, it is not possible to propose the prescriptive dosages of lithium needed to control ASR due to the lack of data required to link aggregate mineralogy to requisite lithium dosage. This gap in understanding, described in chapter 4, extends beyond just aggregate-related issues, but also to combining lithium with SCMs. It is hoped that ongoing research on lithium technology will aid in the development of prescriptive guidelines for using lithium as an admixture in new concrete.
This chapter does not provide comprehensive recommendations regarding all the options available to mitigate ASR (i.e., SCMs, low-alkali concrete), as summarized earlier in Chapter 1, but rather provides practitioners who are interested in using lithium compounds with technical guidance on the topic.
5.2.1 Performance-Based Recommendations for Using Lithium in New Concrete
This section provides guidance on how to evaluate lithium compounds through laboratory performance tests. Emphasis is placed on 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 previous lithium report (Guidelines for the Use of Lithium to Mitigate or Prevent ASR, Folliard, et al., 2003) recommended either the CPT (ASTM C 1293) or a modified version of the accelerated mortar bar test (ASTM C 1260) as tests to determine the requisite lithium dosages to suppress ASR-induced expansion. However, research since the time of the aforementioned report has shown that the proposed, modified version of ASTM C 1260 (where lithium-based admixtures is added to the soaking solution) tends to overpredict the efficiency of lithium in actual concrete mixtures. Research under Federal Highway Administration (FHWA) funding is currently assessing the mortar bar test and other rapid tests to determine if a quick test is able to accurately predict lithium behavior in field concrete. Until this has been achieved, only the CPT is being recommended as an accurate predictor of lithium performance for use in concrete.
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 alkalis 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 only reliable standard 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 as the basis.
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 previously mentioned, this updated recommendations report does not recommend ASTM C 1260, or modifications to the test, as a method of testing and selecting requisite dosages of lithium. Matching the [Li]/[Na+K] ratio of the mortar bar to the host solution tends to overestimate the beneficial effects of using lithium to control ASR-induced expansion. This overprediction of the efficacy of lithium would translate into inadequate dosages in field concrete.
More work is needed to develop a rapid test that best predicts the performance of lithium compounds in laboratory concrete (as per ASTM C 1293) or in concrete exposed to field conditions. This rapid test may be based on the accelerated mortar bar test, or may be entirely different. Regardless of the nature of the test, it should provide an accurate assessment of how lithium would perform in actual concrete mixtures.
5.2.2 Prescriptive Guidelines for Lithium in New Concrete
As previously mentioned, it is not possible at this time to provide prescriptive guidelines for using lithium compounds in new concrete. The previous version of this guideline report (Guidelines for the Use of Lithium to Mitigate or Prevent ASR, Folliard, et al., 2003) had recommended a minimum lithium dosage of 4.6 L of 30 percent lithium nitrate solution for every kilogram of Na2Oe contributed by the portland cement per cubic meter of concrete. However, since that time, research has shown that this dosage may not be sufficient for all aggregate types (Tremblay, et al., 2004). Furthermore, it is not possible to estimate how much additional lithium is needed for a given aggregate type as the relationship between inherent aggregate reactivity (e.g., expansion in ASTM C 1293) and requisite lithium dosage is not currently known. More research is needed to establish this relationship for a range of aggregate types. Furthermore, work is needed to determine appropriate lithium dosages when used in conjunction with SCMs. Ongoing FHWA-funded research is aimed at addressing these issues and developing prescriptive guidelines for lithium usage in new concrete.
5.3 USING LITHIUM IN EXISTING 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., 2000b), 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 effective depth of penetration of the lithium.
Laboratory research has indicated 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 nitrate 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 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., 2000b).
With regard to the application rate, there are two things to consider: (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 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 State Route 1 in Delaware involved a total of six 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 to treatment will more readily absorb 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 by lithium compounds. However, these conditions clearly cannot be controlled.
The extent of 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 will clearly facilitate ingress of the solution. However, if the 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 the schematic shown in Figure 40.
Figure 40. 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 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
Since the writing of the first guideline document (Guidelines for the Use of Lithium to Mitigate or Prevent ASR, Folliard, et al., 2003), the Pennsylvania DOT and Texas DOT led efforts to vacuum impregnate bridge elements with lithium nitrate. Because there have only been limited field efforts involving vacuum impregnation, it is not possible at this time to provide specific guidelines for application. However, as long-term data become available from these two DOT trials and as more field structures are treated via vacuum impregnation, it is expected that specific guidelines will be developed and published.