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Publication Number: FHWA-HRT-04-113 Date: August 2004 |
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This document describes a protocol for evaluating damaged structures to determine whether they are suitable candidates for lithium treatment to address alkali-silica reaction (ASR). A major part of this report deals with the approach/tools that can be used to determine whether ASR is the principal cause or only a contributing factor to the observed deterioration (diagnosis), determine the extent of deterioration due to ASR in the structure, and evaluate the potential for future expansion due to ASR (prognosis). Finally, the report lists items to be included in the proposal that will be submitted for the selection of structures for lithium treatment.
T.Paul Teng, P.E.
Director, Office of Infrastructure Research and Development
Notice
This document is disseminated under the sponsorship of the U.S. Department of Transportation in the interest of information exchange. The U.S. Government assumes no liability for the use of the information contained in this document.
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Technical Report Documentation Page
1. Report No.
FHWA-HRT-04-113 |
2. Government Accession No. | 3 Recipient's Catalog No. | ||
4. Title and Subtitle
Protocol for Selecting ASR-Affected Structures for Lithium Treatment |
5. Report Date August 2004 |
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6. Performing Organization Code | ||||
7. Author(s)
M.D.A. Thomas, B. Fournier, and K.J. Folliard |
8. Performing Organization Report No.
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9. Performing Organization Name and Address The Transtec Group, Inc. |
10. Work Unit No. (TRAIS) |
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11. Contract or Grant No. DTFH61-02-00051 |
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12. Sponsoring Agency Name and Address
Office of Infrastructure Research and Development |
13. Type of Report and Period Covered
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14. Sponsoring Agency Code
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15. Supplementary Notes The Contracting Officer's Technical Representative is Fred Faridazer, HRDI-11. |
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16. Abstract This document describes a protocol for evaluating damaged structures to determine whether they are suitable candidates for lithium treatment to address alkali-silica reaction (ASR). A major part of this report deals with the approach/tools that can be used to determine whether ASR is the principal cause or only a contributing factor to the observed deterioration (diagnosis), determine the extent of deterioration due to ASR in the structure, and evaluate the potential for future expansion due to ASR (prognosis). Finally, the report lists items to be included in the proposal that will be submitted for the selection of structures for lithium treatment. Guidelines on evaluating and managing structures affected by ASR have been published by the Canadian Standards Association (CSA).(1) Pictures of field symptoms and petrographic features of ASR can be found in the documents from CSA, the British Cement Association, the American Concrete Institute, Stark, and Farny and Kosmatka. (See references 1, 2, 3, 4, and 5.) More recently, Folliard and Kurtis summarized such features as part of the Federal Highway Administration (FHWA) workshop material "Guidelines for the Use of Lithium to Mitigate or Prevent ASR in Concrete."(6,7) |
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17. Key Words
Aggregates, alkali-silica reaction, alkali-aggregate reaction, cracking, diagnosis of ASR, expansion, field inspection, gel, lithium, lithium treatment, petrographic examination, prognosis of ASR |
18. Distribution Statement
No restrictions. This document is available to the public through the National Technical Information Service, Springfield, VA 22161. |
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19. Security Classification Unclassified |
20. Security Classification Unclassified |
21. No. of Pages 23 |
22. Price |
Form DOT F 1700.7 | Reproduction of completed page authorized |
SI* (Modern Metric) Conversion Factors
LIST OF TABLES
Three conditions are necessary to initiate and sustain alkali-silica reaction (ASR) in concrete: (1) reactive siliceous phase(s) must be present in the aggregate; (2) the concentration of alkali hydroxides ([Na+, K+, -OH-]) in the concrete pore fluid must be high (which is generally a function of the alkali content of the cement used); and (3) sufficient moisture must be present. Concrete elements affected by ASR respond quite differently from one another, reflecting wide variations in the above conditions, especially in the type and degree of reactivity of the aggregates used, the mixture characteristics (e.g., type and composition of cement, concrete alkali content, water/cement ratio (w/c), and use of supplementary cementing materials (SCM)), the temperature and humidity exposure conditions, and mechanical restraints.
To reliably evaluate the efficacy of lithium in treating ASR-damaged concrete structures, the structures selected for field trials must meet the following general criteria:
The first phase in the evaluation procedure is to review all documents relating to the structure. Information that may assist in the appraisal of the structure includes:
Details regarding the concrete materials, especially the composition and proportion of the cement and the type of aggregate used, are most useful when assessing the likelihood of ASR at this stage. Information of this nature often is not available or lacks specific detail in the case of many structures; however, it is important to collect whatever data is available.
The evidence from field and laboratory investigations should be compared to establish a causal link between signs of reaction in laboratory examinations and the damage observed onsite.
Field inspection is a critical part of the diagnosis of ASR in concrete structures. Each major component of the structure should be examined separately, and observations of the type, extent (relative severity from one component to another and even within one component as a function of the exposure condition), and location of the defects should be recorded consistently. Examples of damage should be documented using color photographs that include an indication of scale. In addition, a sketch of the structure indicating the location of each component examined should be made. Particular attention should be paid to the following features:
Map cracking often is associated with, but not exclusive to, ASR; it develops when internal expansive forces occur in concrete components that are free of stress or restraint. In pavement and slabs on grade, cracking from expansive ASR often begins near free edges and joints where moisture is abundant. The ASR cracks are usually perpendicular to transverse joints and parallel to free edges along the roadside and against asphalt pavements, where there is less restraint. These cracks often progress to a map pattern. In reinforced concrete members or under stress and loading conditions, the ASR cracking pattern generally will reflect the arrangement of the underlying steel, or travel in the direction of the major stress fields. Longitudinal cracking is often observed in reinforced concrete columns and beams affected by ASR. Surface macrocracking due to ASR rarely penetrates more than 25 to 50 millimeters (mm) of the exposed surface (in rare cases, reaching depths of more than 100 mm), where they convert into microcracks. The width of surface macrocracks generally varies from 0.1 mm to 10 mm in extreme cases.
The site investigation report should include a description of the presence, distribution, and extent (severity) of the above features on the various components of the structures, with appropriate sketches and picture records. As mentioned previously, special attention should be given to the potential correlation between the development of the above features and the specific exposure conditions affecting the different components (such as the availability of moisture, exposure to sun, wind, etc.). Canadian Standards Association (CSA) A864 classifies the occurrence of the above features obtained from the field survey of concrete structures as indications of low, medium, and high ASR probability (see table 1).(1)
Feature |
Probability of ASR |
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---|---|---|---|
Low |
Medium |
High |
|
Expansion and/or displacement of elements |
None |
Some |
Structure shows symptoms of increase in concrete volume leading to concrete spalling, displacement, and misalignment of elements |
Cracking and crack pattern |
None |
Some cracking-pattern typical of ASR (i.e., map cracking or cracks aligned with major reinforcement or stress) |
Extensive map cracking or cracking aligned with major reinforcement |
Surface discoloration |
None |
Slight surface discoloration associated with some cracks |
Line or cracks having dark discoloration with an adjacent zone of light-colored concrete |
Exudations |
None |
White exudations around some cracks |
Colorless, jelly-like exudations readily identifiable as ASR gel associated with some cracks |
Environment |
Dry and sheltered |
Outdoor exposure but sheltered from wetting |
Parts of components frequently exposed to moisture such as rain, groundwater, or water due to natural function of the structure (e.g., hydraulic dam or bridge) |
For the purposes of selecting candidate structures (and appropriate components of structures) for lithium treatment, a full, detailed investigation of the structure is required. Samples, typically 100-mm diameter cores (although other sizes may be required where large aggregate or closely spaced reinforcement requires cutting larger or smaller cores, respectively), are to be taken from the major components of the structure and/or those showing the most typical signs of deterioration. In addition, parts of a single component subjected to different exposure conditions and exhibiting different degrees of damage should be sampled. Cores should be as long as possible to provide a profile of the concrete from the surface to the interior of the element. If the original documentation or subsequent reports show that different concrete mixtures were used, then the sampling program should ensure that each mix type is adequately represented.
The main objectives of the laboratory investigation are:
Petrographic Examination
The cores should be examined and photographed in "as-received" condition. The following macroscopic features may assist in the diagnostic process, and their presence should be noted:
Certain features may be highlighted by rewetting the core surfaces and making observations as the core dries. The visual examination of cores should include observations normally made on core samples, such as size and distribution of aggregate, compaction, void content, and presence and condition of reinforcement.
Polished surfaces and thin sections should be prepared from samples taken at various depths (including the surface) within the structure. When the core is taken from an area showing surface distress, the section for microscopic examination should be taken from a region of the core exhibiting damage. At depths below the original concrete surface, visible signs of deterioration may not be obvious, and suitable areas for examination may have to be chosen on the evidence of damp patches, reaction rims around aggregates, or the presence of gel on the surface of the core.
Examining polished surfaces with the naked eye and low-powered (stereo-binocular) microscopy are efficient methods for studying large areas of concrete and determining the intensity of certain macroscopical features. However, examining thin sections often is necessary to positively identify diagnostic features of ASR; these sections must be used to confirm the existence of features identified on polished surfaces.
Using polished surface and thin-section microscopy together, the information listed below may be obtained. Record the presence of these features and estimate their frequency of occurrence.
The uranyl-acetate treatment is a method that helps detect alkali-silica gel on polished and broken surfaces of concrete specimens; it also has been used to detect ASR gel in field structures.(4,8) By applying a uranyl-acetate solution to a surface containing the gel, the uranyl ion substitutes for alkali in the gel, thereby imparting a characteristic yellowish-green glow when viewed in the dark using short wavelength ultraviolet light. ASR gel fluoresces much brighter than cement paste due to the greater concentration of alkali and, subsequently, uranyl ion in the gel.
The uranyl-acetate treatment procedure requires experienced technicians for correct interpretation. The test does not differentiate between a harmless presence of gel or reactivity and one that is detrimental. Not all florescence indicates ASR gel. For example, some aggregates fluoresce naturally. In addition, uranyl ions can be absorbed on cement hydration products and appear as broad, faint areas of fluorescence. Neither of these conditions is an indication of ASR gel. Furthermore, positively identifying gel by this technique does not necessarily means that destructive ASR has occurred. The test is ancillary to more definitive petrographic examinations and physical tests to determine concrete expansion. The uranyl-acetate treatment procedure must not be used alone to diagnose ASR. Because of the potentially hazardous nature of the product, preparing, using, and handling the uranyl-acetate solution should be done cautiously, following appropriate health and safety procedures.
Petrographic examination of polished and thin sections is the most powerful tool in establishing whether ASR has occurred and whether the extent of the reaction is sufficient to cause the level of concrete deterioration observed onsite. If signs of damaging reaction cannot be found by such an examination, it may be reasonable to assume that ASR is not the main cause of damage, and other mechanisms should be sought. The petrographic examination must be conducted by a qualified petrographer who is experienced in examining concrete affected by ASR.
The laboratory investigation report should include a description, for the cores sampled, of the presence, distribution, and extent of the features listed previously in this document, with appropriate picture record. CSA A864 classifies the occurrence of features obtained from petrographic examination to give an overall assessment of the probability of ASR (see table 2).(1)
Probability of ASR | Nature and Extent of Features |
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Low |
No gel present, no sites of expansive reaction, presence of other indicative features rarely found |
Medium |
Presence of some or all features generally consistent with ASR, such as:
|
High |
Evidence of site of expansion reaction, i.e., locations within the concrete where evidence or reaction and emanation of swelling pressure can be positively identified, and/or presence of gel in cracks and voids associated with reactive particles and readily visible to the unaided eye or under low magnification |
Mechanical Testing
In addition to petrographic examination, some mechanical testing of cores can be performed; however, selecting the appropriate test methods is critical because ASR does not alter the engineering properties of concrete equally. The compressive strength generally is not sensitive to ASR until excessive expansions/cracking are reached; losses in tensile strength of 40 to 80 percent were reported, depending on the test method used and the expansion level. The tensile-to-compressive strength was found to be a good indicator of internal concrete damage due to ASR; this ratio typically varies from 0.07 to 0.11 for sound concrete, while values less than 0.06 would indicate internal damage due to ASR.(9) ASR deleteriously affects the elastic modulus of concrete, even at a low level of expansion or when compressive strength is still increasing.
The interpretation of the data collected from the investigation outlined here should be conducted by a professional concrete specialist with experience in evaluating concrete structures affected by ASR. CSA A864 analyzes the findings from both the site and laboratory investigations to determine the likely contribution of ASR to the overall observed deterioration (see table 3).(1)
Evidence of ASR |
Interpretation |
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---|---|---|
Site |
Laboratory |
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Low |
Low |
If neither the site nor laboratory investigations produce significant evidence of ASR, the reaction can be positively eliminated as a possible cause of damage, and alternative mechanisms must be sought. The presence of considerable displacement, movement, or cracking of the structure is not sufficient to suggest ASR if neither the type of damage observed onsite nor the results of laboratory examinations are consistent with ASR. |
Low |
High |
If the evidence from the site indicates a low probability of ASR but a high incidence of reaction observed in the laboratory, it is not possible to establish a causal link between the deterioration onsite and ASR. The most likely explanation for this result is that ASR has occurred, but the operation of other mechanisms has prevented typical manifestations of ASR in the structure. Other possible mechanisms must be sought and eliminated before ASR is implicated as the main or sole cause of damage. |
High |
Low |
If the evidence from the site indicates a high probability of ASR, but no evidence of reaction was observed in the laboratory examination, three possibilities exist:
A judgment must be made whether to carry out further sampling, seek the presence of alternative mechanisms, or both. |
Medium |
Medium |
If the evidence from both the site and laboratory investigations indicates a medium probability of ASR, then it may be concluded that ASR has occurred and may be a contributory cause of damage; however, it is likely that other mechanisms exist and have contributed to the overall deterioration of the structure. |
High |
High |
If the evidence from the site and laboratory investigations implies a medium-to-high probability of ASR, it may be concluded that ASR is at least a significant contributing cause of the damage to the structure. In the absence of any other mechanism, it may be reasonable to assume that ASR is the principal or sole cause of damage. |
Ideal candidate structures for lithium treatment are those for which laboratory testing or in situ monitoring indicate that potential for further expansion and damage due to ASR is significant if the structure is left untreated.
The most reliable method for determining the likelihood of further reaction and expansion is to instrument the structure and monitor its behavior for a period of time; the period of time required is usually at least 2 years to account for seasonal variations in measurements. This may not be practical (or desirable) in cases when a decision regarding lithium treatment must be made in a shorter timeframe.
There are several ways to monitor the rate of expansion. For example, the long-term change of length between reference points mounted on the concrete surface can be measured. The method most suitable for monitoring the expansion must be considered in each specific case. However, such observation should cover entire structural units. Crack mapping is an interesting visual tool for evaluating the progress of the expansion/deterioration. The measurements and summations of individual crack widths in concrete structures are too uncertain for this purpose, because shrinkage of the concrete between the cracks will contribute to the opening of the cracks. Measurements of crack widths may therefore give a false indication of the expansion in the concrete. Likewise, gathering sufficient data to be able to correct for the effects of variations in ambient temperature and humidity is important. Because these variations are often seasonal or more frequent, at least several years of measurements normally are necessary before definite conclusions can be reached about the rate of ASR-induced expansion in the structure.
Humidity and temperature measurements at different depths within the concrete elements can provide information that can help when interpreting seasonal fluctuations in the in situ expansion measurements.
Expansion tests (usually carried out at 38 ºC) on cores often are used to provide an indication of the potential for further expansion of the concrete. However, the initial volume and mass changes observed when the specimen is placed at high humidity (and temperature) may indicate the extent of ASR already in the concrete. This initial behavior should be interpreted with great caution, because it depends on factors other than ASR (e.g., moisture sensitivity of aggregates). A further complication arises from the leaching of alkalis from relatively small specimens stored at 100 percent relative humidity. This can lead to underestimating the residual potential for ASR. Indeed, cores taken from structures that clearly exhibit symptoms of continuing ASR often show little potential for further expansion in the laboratory.
It is possible to get an indication of the quantity of reactants (reactive silica and soluble alkali) remaining in the concrete separately. Expansions tests on cores immersed in alkali solution (1M NaOH at 38 °C or 80 °C has been used) can provide an indication of the amount of reactive aggregate remaining in the system. The water-soluble alkali content, on the other hand, can provide a measure of the alkalis that are still available for reaction. It is possible to combine these measurements to determine the potential for further ASR. A procedure for predicting the future risk of expansion of structures based on such measurements has been developed by Bérubé et al.(10)
The objective of the lithium implementation program is to evaluate the potential efficacy of lithium treatment for different types of concrete structures/elements affected by ASR in different environments (i.e., various regions across the United States), and of using various methods such as electrochemical extraction, vacuum impregnation, and topical treatment. Although specific requirements may be identified depending on the method of treatment to be used for particular affected components, ideal structures for lithium treatment will be those for which, in general:
For the purpose of selecting structures for this Federal Highway Administration-sponsored study, proponents are asked to prepare submission files reporting findings from site inspection and laboratory investigations of the proposed concrete structures in accordance with the recommendations described in this protocol. In summary, the proposal will include the following information on the candidate structure (see previous sections for detailed information):
Assistance can be provided to the State departments of transportation in developing the proposal, especially in the analysis of the field evidence of the ASR, the evaluation of the petrographic features of the ASR, and the mechanical testing of samples taken from candidate structures.
For additional information, contact Fred Faridazar at fred.faridazar@fhwa.dot.gov.
The following references provide further information that may be useful in evaluating a concrete structure for the purposes of determining the presence and extent of ASR.
Federal Highway Administration, 2002, "Guidelines for Detection, Analysis, and Treatment of Materials-Related Distress in Concrete Pavements"
(Available online at http://www.fhwa.dot.gov/pavement/pub_details.cfm?id=81.)
Federal Highway Administration, 1997, "Petrographic Methods of Examining Hardened Concrete: A Petrographic Manual," FHWA-RD-97-146 (available online at http://www.fhwa.dot.gov/pavement/pccp/petro.cfm).
Guthrie, G.G. Jr. and Carey, J.W., 1997, "A Simple, Environmentally Friendly, and Chemically Specific Method for the Identification and Evaluation of the Alkali-Silica Reaction," Cement and Concrete Research, Vol. 27, No. 9, pp. 1407-1417.