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CHAPTER 6 ECONOMIC CONSIDERATIONS OF USING LITHIUM COMPOUNDS

6.1 INTRODUCTION

This chapter discusses some of the economic considerations of using lithium compounds to control ASR-induced expansion in new concrete (when used as an admixture) or existing structures (when used as a post-treatment). Because of limited field applications of lithium to date, it is not possible to perform a comprehensive, quantifiable cost analysis of using the compounds to control ASR. However, some of the important factors to consider are discussed below.

6.2 ECONOMICS OF USING LITHIUM COMPOUNDS IN NEW CONCRETE

When considering using lithium nitrate as an admixture for concrete, the economics of this approach is often compared to other mitigation measures. The cost of lithium is probably higher than that of other technologies (e.g., fly ash, silica fume, low-alkali cement, etc.) typically used to control ASR-induced expansion. However, the delivered cost of concrete is just a part of the in-place cost of concrete, with the in-place cost depending on the type of structure, the amount of reinforcing steel, construction method, and other factors. For example, the in-place cost of concrete for a bridge deck may be as high as $450/m3. Thus, one should regard direct comparisons of raw materials costs with caution because they do not reflect total delivered concrete or in-place concrete costs.

It is clear that adding lithium to concrete increases the cost of the raw materials and delivered concrete, and in many cases, other less-expensive alternatives are selected, such as using appropriate amounts of SCMs. However, when considering the use of lithium in new concrete, other factors must be taken into account:

  • If the alternative is transporting nonreactive aggregates or low-alkali cement over a long distance, or if high-quality SCMs are not locally available, lithium becomes much more competitive.
  • For some highly reactive aggregates, relatively high dosages of fly ash or slag may be required to control expansion, but these higher replacement levels would have a significant effect on early strength gain and related constructability issues. Using lower dosages of fly ash or slag, in combination with lithium, can then improve the early strength properties, which improve the economics of the situation.
  • Some agencies and organizations have limited the maximum amount of SCMs mainly because of perceived concerns with salt scaling, and using lithium in these instances in combination with lower dosages of lithium becomes a viable alternative
  • For certain concrete structures (i.e., dams or airfield pavements), very little expansion can be tolerated before the expansion impacts performance or function of the structure. Using lithium in such structures, preferably in conjunction with SCMs, is a mitigation method worthy of consideration. Those designing and constructing these types of important or sensitive structures are generally more willing to spend additional money up front to ensure the desired function of the structure for the desired service life.

A critical factor identified above is the impact of materials selection on service life. For example, nondurable concrete that suffers from ASR (or other durability problems) may require significant repairs or even total replacement, and this has a major effect on the life-cycle cost of the structure. Recently, models have been developed to predict service life of reinforced concrete structures suffering from corrosion. These models can be used to predict impact of different mitigation options (i.e., SCMs, corrosion inhibitors) on the service life and life-cycle cost of structures. However, models of this type are not currently available to predict the service life of structures suffering from ASR-induced damage. Nevertheless, it is clear that using lithium compounds, SCMs, or combinations of those will prolong the life of structures containing reactive aggregates significantly, thereby reducing the impact of initial material costs. As new models are developed that specifically address ASR, it will be possible to integrate life-cycle costs into initial strategies for controlling ASR, making the use of lithium compounds more attractive and competitive with other materials.

6.3 ECONOMICS OF TREATING EXISTING CONCRETE WITH LITHIUM

As discussed in sections 4.3 and 5.3 of this report, the effectiveness of treating existing ASR-affected concrete with lithium has not yet been established. Therefore, it is not possible to provide information on the economic viability of using this form of treatment. However, some discussion of the relevant economic considerations is warranted.

Lithium treatment of ASR-affected concrete is unlikely to be a lasting and complete solution to the problem. At best, such treatment may retard the process of deterioration and delay the time until more permanent repair or replacement becomes necessary. Also, lithium treatment will almost certainly only be considered when some level of deterioration is already present, and additional strategies may have to be considered to improve the existing condition of the concrete. However, extending the time to a more expensive repair or replacement option still may be a viable alternative. For example, consider the case of a pavement suffering from ASR. If it is predicted that, left untreated, the pavement will require some level of major rehabilitation (e.g., overlay or repair) at time T1 with a cost of R1, then the present worth of this option, P1, is given by:

P sub one equals R sub one over parenthesis one plus i T sub one (8)

Where: i = discount rate for the financial analysis

If the cost of applying a topical lithium treatment is R2, and it is predicted that the lithium treatment will extend the time to major rehabilitation to time T2, then the cost of the lithium treatment can be estimated as:

P sub two equals R sub two plus R sub one over parenthesis one plus i T sub one (9)

Both R1 and R2 should include the full cost to the user of implementing the rehabilitation strategy.

The comparative costs of the two options, P1 versus P2, are clearly a function not only of the cost of the lithium treatment, but also of the difference in the timing of the major rehabilitation, T1 versus T2. Without reliable information to predict how lithium will impact the timing of the repair schedule, it is not possible to perform an economic analysis. It is anticipated that an analysis of this type will be performed in the near future, using data obtained from the lithium treatment of pavement sections in Delaware.


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Updated: 04/07/2011
 

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