U.S. Department of Transportation
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Federal Highway Administration Research and Technology
Coordinating, Developing, and Delivering Highway Transportation Innovations
|This report is an archived publication and may contain dated technical, contact, and link information|
Publication Number: FHWA-RD-03-047
Date: July 2003
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 posttreatment). 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.
The cost of lithium is inherently quite high, compared to other concrete materials. Typical unit costs of selected raw materials are shown below:
Basic Materials -Approximate Cost
Type I portland Portland cement: $80/ton
Aggregates: approximately $7-10/ton
Selected aAdmixtures and SCMs -A approximate C cost
Fly ash: $25-30/ton
Silica fume: $600/ton
Calcium nitrite corrosion inhibitor: $1.50/L liter
High-range water reducer: approximately $3.25 per m3 of concrete
LiNO3 (30 percent% solution): ~ $3.50/L liter
The above estimates are based on typical values but do not reflect availability, differences in freight cost, local market trends, or other factors. The raw materials cost typically constitute approximately 50 to 75 percent of the delivered cost of concrete. For reference, a typical cubic meter of delivered concrete costs anywhere from around $60 to $120.
Assuming a delivered cost of concrete of $90/m3, a cement content of 389 kg/m3, and a cement alkali content (Na2Oe) of 0.6 percent, the incremental cost of adding a typical LiNO3 dosage (4.6 L of 30 percent solution per kg Na2Oe) to this concrete would be approximately $38, resulting in a final delivered cost of approximately $128/m3. This increase in cost is considerable, but can be somewhat offset by combining LiNO3 with fly ash. Using the same example as above, but replacing 25 percent of the cement with low-lime fly ash and reducing the molar ratio (based on cement) to 0.54 (see section 5.2), the total cost of the concrete would be approximately $105/m3. The above is just an example using assumed materials, mixture proportions, and costs. Specific cases should be evaluated independently to determine the potential economic impact of using lithium compounds, with or without SCMs. It is almost always the case that combining lithium with SCMs will make lithium more cost effective and will also produce higher quality, low-permeability concrete. If laboratory testing (based on ASTM C 1293) shows that the required lithium dosage, used in conjunction with an SCM, is even less than that recommended in section 5.2, it may be possible to reduce the lithium dosage, thereby further decreasing the cost of the mixture.
The delivered cost of concrete is also 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, direct comparisons of raw materials costs one should be regarded direct comparisons of raw materials costs with caution, as these comparisonsy 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:
A critical factor identified above is the impact of materials selection on service life. For example, non-durable 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 theose 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 are developed, 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.
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 deterioration process of deterioration and delay the time until more permanent repair or replacement becomes necessary. Also, lithium treatment usually will almost certainly only be considered only 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:
where i = the 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:
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, is 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 performedis anticipated in the near future, using data obtained from the lithium treatment of pavement sections in Delaware.