Report on the Diagnosis, Prognosis, and Mitigation of Alkali-Silica Reaction (ASR) in Transportation Structures
3.0 ASR Investigation Program Level 1: Condition Survey
The condition survey is generally carried out to provide data on: 1) the nature, the extent, and the progress (when damage ratings are established and repeated as part of the survey program) of any distresses and deterioration affecting the concrete structure; and 2) identify areas that may need further investigations and/or immediate action (i.e., repair).
With the special purpose of diagnosing for AAR in the concrete structure, special attention should also be given at assessing the exposure conditions to which the structure (or parts of it) is (are) subjected. ASR typically develops or sustains in concrete elements with internal relative humidity > 80-85 percent. ASR is not expected to develop to a significant extent in a dry environment, which corresponds to an average ambient relative humidity lower than 60 percent, normally only found in buildings. For intermediate conditions; i.e., between 60 and 80 percent, the extent of AAR will depend on factors such as the nature and reactivity level of the aggregate; however, the rate of expansion will be reduced compared to higher humidity conditions.
Expansion and cracking due to ASR is generally most severe in concrete elements subjected to an external and constantly renewable supply of moisture. Surfaces of concrete elements affected by ASR and exposed to sun, wetting and drying cycles (e.g., splash zones on bridge parapet/abutment walls), frost action (freezing and thawing cycles), saline water (e.g., tidal zones for structures exposed to sea water, splashing zone on the abutment walls of bridges or jersey barriers exposed to deicing chemicals), usually show more extensive/severe cracking and deterioration; although the above conditions are not necessarily promoting expansion due to AAR, they are exacerbating its effects and the damage it generates.
Common visual symptoms of ASR have been described in numerous documents since Stanton identified and reported the first case of ASR in concrete structures in the late 1930s (Stanton 1940)1. Although not necessarily exclusive to ASR, they generally consist of:
- Expansion causing deformation, relative movement, and displacement.
- Surface discoloration.
- Gel exudations.
- Occasional pop-outs.
Detailed information and photographs illustrating the above defects in highway structures affected by ASR are given in Appendix A of this document.
As part of the condition survey, each component of the structure should be examined separately and observations on the type, extent (severity), and location of the defects, recorded in a consistent manner (e.g., using a condition survey form). Typical examples of the distress(es) observed should be photographed (including an indication of scale); this will help compare severity ratings of the damage between various parts of a single component, various components of the structure, as well as between conditions surveys. Sketch(es) and/or picture(s) of the structural members should be used to locate the areas of low, medium, and high damage severity, as well as any evidence of a potential relation between the damage observed and features such as the presence of physical restraints and the availability of moisture (exposure to rain, poor or defective drainage systems, etc.).
A field test to detect the presence of ASR silica gel by using uranyl acetate fluorescence was developed under the Strategic Highway Research Program (SHRP) in the United States (D. Stark 1991; Natesaiyer, et al.). Care should be taken in interpreting the results (see ASTM C 856 and section C2.2 - Appendix C).
Also, if the condition survey points out issues that can impair the stability/integrity of the structure or public safety, related or not to ASR, immediate action should be taken in consultation with experts in the respective fields.
3.2. Interpretation of the Findings from the Condition Survey
Table 2 classifies the occurrence of the features obtained from the condition survey as indicative of low, medium, and high potential of ASR contribution in the deterioration observed. It is often difficult to determine from field observations only whether ASR is the only/main factor responsible for the observed distresses since some of the visual signs of deterioration generally associated with ASR may have been caused by other processes such as internal sulphate attack, or plastic or drying shrinkage.
|Feature||Potential for ASR|
|Expansion and/or displacement of elements||None||Some evidence (e.g., closure of joints in pavements, jersey barriers, spalls, misalignments between structural members)||Fair to extensive signs of volume increase leading to spalling at joints, displacement and/or misalignment of structural members|
|Cracking and crack pattern||None||Some cracking pattern typical of ASR (e.g., map cracking or cracks aligned with major reinforcement or stress)||Extensive map cracking or cracking aligned with major stress or reinforcement|
|Surface discoloration||None||Slight surface discoloration associated with some cracks||Many cracks with dark discoloration and adjacent zone of light colored concrete|
|Exudations||None||White exudations around some cracks; possibility of colorless, jelly-like exudations||Colorless, jelly-like exudations readily identifiable as ASR gel associated with several cracks|
The assessment of the exposure conditions should also contribute to support the observations of the symptoms of distress listed in Table 2, as follows:
- Low potential for ASR: element in dry and sheltered environment.
- Medium potential for ASR: element exposed outdoors but sheltered from constant wetting.
- High potential for ASR: Parts of components frequently exposed to moisture-e.g., rain, groundwater, or water due to natural function of the structure (e.g., hydraulic dam or bridge).
As indicated in Figure 1, if the potential for ASR contribution is low (i.e., no conclusive evidence of AAR-related distress is noted), further work is postponed until the next condition survey. However, when the potential for ASR contribution is medium to high, further work is required, which will be carried out as part of a preliminary investigation program for the diagnosis or ASR (Figure 1).
1The following publications provide additional information on the topic: Stark (1991), BCA (1992), Farni and Kosmatka (1997), ACI (1998), LCPC (1999), CSA (2000), Fournier and Bérubé (2000), Van Dam et al. (2002), and Folliard et al. (2006).