Report on the Diagnosis, Prognosis, and Mitigation of Alkali-Silica Reaction (ASR) in Transportation Structures
9.0 Appendices - Appendix A Diagnosis of Alkali-Silica Reaction (ASR)
Visual Signs of ASR
A.1 Introductory Remarks
Symptoms of ASR affecting concrete structures generally consist of (1) expansion causing deformation, relative movement and displacement, (2) cracking, (3) surface discoloration, (4) gel exudations and, occasionally, (5) pop-outs; however, the presence of one or many of these features is not necessarily an indication that ASR is the main factor responsible for the damage or distress observed.
A.2 Expansion Causing Movements and Deformations
The extent of ASR often varies between or within the various members/parts of an affected concrete structure, thus causing distresses such as relative movement of adjacent concrete members or structural units, deflection, closure of joints with associated squeezing/extrusion of sealing materials and, ultimately, spalling of concrete at joints. There are a number of reasons that can explain such a variation, including:
- Variations in the proportioning and composition of the concrete [cement and alkali contents, nature and proportion of aggregate material (e.g. concrete provided by different suppliers and incorporating aggregates of different reactivity levels, cements of different alkali contents, etc.), localized cement-to-aggregates ratio, etc.].
- Variations in the exposure conditions and loading regimes between and within the various parts/members of the structure.
- Differences in the size and geometry of different parts/members of the structures.
- Variations in the reaction rates between different parts of the structure because of differences in the alkali concentration through alkali migration processes, leaching, dilution, external applications of salts, etc.
- Variations in the structural restraints within (reinforcement detailing density and design, prestressing, postensioning, etc.) and between (confinement, etc.) the different parts/members of the structure.
- Variations in the age of the different structural members (e.g. structure built in different stages).
Staff performing condition surveys should be aware of the factors listed above and of their potential impact as just described so visual evidence of the presence of such conditions should be noted as part of the condition survey reports. However, it is important to remember that deformations in concrete structures may be caused by a range of different mechanisms, such as loading, thermal or moisture movements, differential shrinkage, gravity and foundation effects, hydraulic pressure, creep, impact, and vibrations (BCA 1992).
Examples of expansion causing deformation, relative movements and spalling in transportation structures are illustrated in Figure A1.
Figure A1. (A). Relative movement of abutting sections of parapet wall in a bridge structure affected by ASR (Stark 1991). (B). Expansion of bridge girder leading to loss of clearance between the girder and embankment and eventually crushing of the girder end with localized spalling. (C). Expansion causing spalling at joints in a concrete pavement incorporating a highly-reactive aggregate; also noted longitudinal cracking in the middle part of the pavement sections. (D). Relative movement between a pier block showing ASR cracking and an adjacent deck slab causing spalling of concrete and extrusion of sealing material along the joint (CSA 2000). (E). Relative movements between pier blocks of a concrete bridge structure affected by ASR (CSA 2000) (F). Expansion with associated severe spalling in abutting jersey barrier sections affected by ASR.
The pattern of cracking due to ASR is influenced by factors such as the shape or geometry of the concrete member, the environmental conditions, the presence and arrangement of reinforcement, and the load or stress fields (restraint) applied to the concrete.
Cracking is usually most severe in areas of structures where the concrete has a constantly renewable supply of moisture, such as close to the waterline in piers, from the ground behind retaining walls, beneath pavements slabs, or by wick action in piers or columns. Concrete members undergoing ASR and experiencing cyclic exposure to sun, rain and wind, or portions of concrete piles in tidal zones often show severe surface cracking resulting from induced tension cracking in the "less expansive" (due to alkali leaching/dilution processes, variable humidity conditions, etc.) surface layer under the expansive thrust of the inner concrete core (Stark and Depuy 1987, ACI 1998). Cracking due to ASR will preferentially (or more rapidly) develop in the exposed portions of concrete structures (Figure A2-A).
Figure A2. (A). Bent cap in relatively new bridge structure showing signs of ASR, primarily where direct access to moisture/rain prevails. (B). ASR cracking in drilled shaft supporting high-mast illumination pole. (C). Gel staining around cracks in the parapet wall of a bridge structure affected by ASR. (D). Pop-out created by the expansion of a frost-susceptible porous coarse aggregate particle. (E). ASR-induced pop-out in a concrete pavement incorporating highly-reactive aggregates; also noted pattern cracking (F). Efflorescence and exudations of alkali-silica gel at the surface of the concrete foundation of 25-year-old highway bridge affected by ASR.
"Map" or "pattern" cracking is often associated with, but not exclusive to, ASR (Figure A3-A to A3-C); it is often observed in AAR-affected concrete members free of major stress or restraint. Drying shrinkage, freezing/thawing cycles and sulfate attack can also result in a pattern of cracks showing a random orientation. In reinforced concrete members, or under stress and loading conditions, the ASR cracking pattern will generally reflect the arrangement of the underlying steel or the direction of the major stress fields. For instance, in pavements and slabs on grade, ASR cracking usually develops perpendicular to transverse joints and parallel to free edges along the roadside (Figure A3-E), and against the asphalt pavements where is less restraint; these cracks often progress to a map pattern (Figure A3-E). Longitudinal cracking is often observed in reinforced concrete decks, columns and beams affected by AAR (Figure A2-B, A3-D, A3-F, A4A to A4-D). Concrete members affected by ASR may show more than one pattern of cracking at a time; common associations are predominant longitudinal cracks interconnected by a finer, randomly oriented, cracking pattern (Figure A3-E).
Surface macrocracking due to ASR rarely penetrates more than 25 (approximately 1 inch) to 50 mm (approximately 2 inches) of the exposed surface (in rare cases reaching depths >100 mm or 4 inches) where they convert into microcracks. The width of surface macrocracks generally varies from 0.05 mm (approximately 0.002 inch) to 10 mm (approximately 0.40 inch) in extreme cases. The measurements of crack widths on deteriorating concrete members can be used to monitor the progress of damage between visual condition surveys. This is discussed in further details in the Appendix B of the document.
Cracking will develop in concrete members wherever the tensile strain from the combined effects of internal expansive or shrinkage mechanisms, structural loads and reinforcement restraints exceed the tensile strength of the concrete (ISE 1992). Improper mixture proportioning, poor workmanship or inadequate curing may also cause concrete to crack.
A.4 Surface Cracking
Cracks caused by ASR are often bordered by a broad brownish zone, giving the appearance of permanent dampness (Figure A2-C, A3-B). Sections of concrete members that are badly damaged may develop a patchy surface staining; however, this is not necessarily an indication of ASR.
The expansion of individual unsound or frost-susceptible aggregate particles [such as laminated, schistose and argillaceous, clayey or porous particles or certain varieties (porous) of chert, ironstones] at or near the concrete surface due to frost-action is likely to be the main factor for the development of pop-outs in northern countries (Figure A2-D). Pop-outs can also be caused by a poor bond between the cement paste and dusty coarse aggregate particles.
Figure A3. (A). Map-cracking in parapet walls of a bridge structure affected by ASR. (B). Severe map-cracking and associated gel staining around cracks in a median highway barrier affected by ASR. (C). Severe map-cracking in the wing wall of a 30-year-old bridge structure affected by ASR (CSA 2000). (D). Longitudinal cracking on the deck soffit of a 20-year-old highway bridge affected by ASR (CSA 2000). (E). Well-defined crack pattern associated with the development of ASR in highway pavement; the orientation of predominant cracks is longitudinal, while map- or pattern-cracking is also identified. (F). Longitudinal cracking in a precast, reinforced concrete beam affected by ASR. The edge beams, i.e., those exposed to the action of wetting and drying, typically show a more advanced stage of ASR deterioration than the interior beams.
Figure A4. (A). Fine longitudinal cracking in a reinforced concrete column of a 20-year-old highway bridge structure affected by ASR. (B). Longitudinal cracking on the edge of the deck and in the column of a 20-year-old highway bridge affected by ASR. The presence of reinforcement and related stress fields has influenced the pattern of cracking. (C). Pattern-cracking associated with main longitudinal cracking along the prestressed cable in a reinforced concrete beam of a bridge structure affected by ASR. (D). Cracking in the beam (upper part) and the reinforced concrete column of a bridge structure affected by ASR.
Alkali-silica reactive aggregates undergoing expansion near the concrete surface may induce the detachment of a portion of the skin of concrete leaving the reactive aggregate in the bottom (Figure A2-E). Any particular visual observations that could help understand the expansive process observed, such as traces of rust (reacting ironstone), alkali-silica gel (reactive aggregate), laminated aggregate particle in the bottom part of the "crater" (could suggest frost action), should be noted.
A.6 Surface Deposits (Gel Exudation vs. Efflorescence)
Although surface gel exudation is a common and characteristic feature of ASR, the presence of surface deposits is not necessarily indicative of ASR as other mechanisms (such as frost action or the movement of water through cracked concrete members) can also cause surface deposits called efflorescence (without the present of ASR gel) (Figure A2-F). It is good practice during the condition survey to record the extent and location of surface deposits along with their color, texture, dampness, and hardness. A sample of the surface deposit can be taken and submitted to and/or X-ray analysis to help determine if ASR gel is present.
A field test to detect the presence of ASR silica gel by using uranyl acetate fluorescence was developed under the SHRP program in the United States (D. Stark 1991, Natesaiyer, et al.1991). Care should be taken in interpreting the results (see ASTM C 856).
American Concrete Institute (ACI), "State-of-the-Art Report on Alkali-Aggregate Reactivity," ACI 221.1R-98, 1998.
American Standards for Testing and Materials (ASTM), "Standard Practice for Petrographic Examination of Hardened Concrete," ASTM International, ASTM C856-02, Annual book of ASTM Standards 2003, Section Four, Vol. 04.02 Concrete and Aggregates, pp. 434-450, 2003.
British Cement Association (BCA), "The Diagnosis of Alkali-Silica Reaction - Report of a Working Party," Wexham Springs, Slough (UK), SL3 6PL, 44p., 1992.
Canadian Standards Association (CSA), "Guide to the Evaluation and Management of Concrete Structures Affected by Alkali-Aggregate Reaction," CSA A864-00, Canadian Standards Association, Mississauga, Ontario, Canada, 2000.
Institution of Structural Engineers (ISE), "Structural Effects of Alkali-Silica Reaction - Technical Guidance Appraisal of Existing Structures," Institution of Structural Engineers, 11 Upper Belgrave Street, London SW1X 8BH, 45p., 1992.
Natesaiyer, K., Stark, D. and Hover, K.C., "Gel Fluorescence Reveals Reaction Product Traces," Concrete International, pp. 25-28, January 1991.
Stark, D., "Handbook for the Identification of Alkali-Silica Reactivity in Highway Structures," SHRP-C/FR-91-101, TRB National Research Council, 49p., 1991.
Stark, D. and Depuy, G., "Alkali-Silica Reaction in Five Dams in Southwestern United States," Proceedings of the Katherine and Bryant Mather International Conference on Concrete Durability, Atlanta, Georgia (USA), ACI SP-100, pp. 1759-1786, April 1987.