Report on the Diagnosis, Prognosis, and Mitigation of Alkali-Silica Reaction (ASR) in Transportation Structures
Appendix C Diagnosis of Alkali-Silica Reaction (ASR) Petrographic Symptoms of ASR
Petrographic examination is a very powerful technique in the diagnosis of the cause of its deterioration. ASTM C 856 outlines procedures for the petrographic examination of samples of hardened concrete. Interesting information regarding petrographic features of ASR-affected concrete can be found in several publications, including Bérubé and Fournier 1986, Hobbs 1988, Walker 1992, BCA 1992, St. John et al. 1998, CSA 2000, and Walker et al. 2006.
This appendix is meant to provide information on typical petrographic features of AAR, as observed on polished concrete sections, broken surfaces, and thin sections.
C.1 Macroscopic Observations on Cores (as Received)
Several macroscopic signs of concrete deterioration, some of which are related to ASR, can be observed by examining the cores immediately after the extraction or in the laboratory in an as received condition. The observations can be made with naked eye or aided by a small magnifying lens (up to 10X magnification) that can be easily used in the field. Such features can consist of:
- Macrocracks penetrating at different depths in the concrete member (Figure C1-A); fine to-medium size cracks will stay damp while rewetting (Figure C1-C); macrocracks can be due to several mechanisms other than ASR.
- Gel staining surrounding surface cracks (Figure C1-B).
- Dark reaction rims at the periphery of reacted aggregate particles (Figure C1-D); dark rims may appear at the periphery of weathered gravel particles and are consequently not fully indicative of ASR in the case of gravel aggregates.
- Cracks within reactive aggregates (Figure C1-E), which extend sometimes in the cement paste, with/without reaction products gels.
- Alkali-silica gel in voids of the cement paste (Figure C1-F).
- Deposits of reaction products on the cracked surfaces of cores (Figure C1-G).
Figure C1. (A). Cores extracted from a concrete pavement affected by ASR and showing macrocracks penetrating from the upper and lower parts of the pavement. (B). Gel staining surrounding cracks and gel exudations at the surface of a core extracted from a sidewalk section affected by ASR (C). Fine cracking pattern showing up after re-wetting of the core. (D). Dark rim around the periphery of reactive aggregate particles. (E). Macrocracks in reactive coarse aggregate particles. (F). Deposits of alkali-silica gel in a void on core surface. (G). Deposits of alkali-silica gel on the broken surface of a core extracted from a highway bridge structures affected by ASR.
C.2 Microscopic Description of the Cores
Following the macroscopic description of the cores, various types of specimens may be prepared from the drilled cores. These mainly consist of polished sections or slices, broken (fresh) surfaces, and thin sections. The examination of polished surfaces with the naked eye and low-powered (stereo-binocular; up to 60x magnification) microscopy is an efficient method for studying large areas of concrete and determining the presence, distribution and extent of macroscopical features of ASR. The examination of thin sections will allow further positively identifying diagnostic features of ASR (e.g., sites of expansive reactions, reaction products). To maximize information generated through petrographic examination, polished slabs and thin sections can be prepared from various depths along the core sample. Table C1 lists features obtained from petrography as a function of the various methods of examinations mentioned above. Although not necessarily exclusive to ASR, petrographic signs or features of ASR generally consist of the following:
- Microcracking in aggregates and/or cement paste.
- Reaction product "gel".
- Reaction rims.
- Loss of the cement paste-aggregate bond.
|Methods of Examination||Features|
|Megascopic examination (using a 10x lens)||
|Microscopic examination on polished sections (using a stereo-binocular microscope)||
|Microscopic examination of thin sections (petrographic microscope, scanning electron microscope)||
|Examination of broken concrete pieces (fracture surfaces)(using a stereo-binocular and/or SEM)||
BCA (1992) and St. John et al. (1998) compare "idealized" cracking patterns in concrete specimens affected by various deleterious mechanisms (Figure C2). In the case of cracking induced by drying shrinkage (Figure C2-A), cracks are running fairly randomly through the cement paste, connecting aggregate particles through the interfacial zone with the cement paste. In the case of concrete affected by internal sulfate attack (e.g., Delayed Ettringite Formation (DEF)), cracks are found surrounding the coarse aggregate particles and filled with ettringite (Figure C2-B); when internal cracking is induced by shrinkage of the coarse aggregate particles, the cracks are similarly preferentially found surrounding the aggregate particles; however, they are empty (Figure C2-C). In the case of distress generated by frost attack, a random cracking pattern develops in the inner portion of the concrete, together with cracks developing parallel to the exposed surface of the affected concrete member; cracks will be often filled with secondary products such as calcite, portlandite, and/or ettringite (BCA 1992) (Figure C2-D). In the case of concrete affected by ASR when the reactive fraction in part either of the fine aggregate (Figure C2-E) or coarse aggregate (C2-F) fraction, a network of microcracks develops in the inner part of the concrete, with only a few "macrocracks" being observed in its outer portion (i.e., close to the surface). The microcracks are found connecting the aggregate particles; when the reactive material is found in the coarse aggregate particles, cracks typically run through the particles (Figure C2-F). The pattern will be affected by applied stresses/restraint and will develop preferentially parallel to the direction of the main compressive stress(es). Finally, the cracks will be often filled with secondary reaction products (i.e., the alkali-silica gel), which will be found showing a range of microtextures and chemical compositions. This will be discussed in more details hereafter.
Figure C2. From BCA (1992) (Note: This was taken directly from the BCA publication and should be for internal use / discussion only at this stage, i.e. until copyright issues be resolved) (A). Internal crack pattern which can be induced by drying shrinkage. (B). Internal crack pattern which can be caused by internal sulfate attack for delayed ettringite formation (DEF), or from sulfates derived from the aggregates. (C). Internal crack pattern which can be induced by shrinkage of the coarse aggregate. (D). Internal crack pattern which can be induced by frost attack. (E). Internal crack pattern which can be caused by ASR: reactive silica in the sand fraction. (F). Internal crack pattern which can be caused by ASR: reactive silica in the coarse aggregate.
Microcracking due to ASR is generated through forces applied by the expanding aggregate particles and/or swelling of alkali-silica gel within and around the boundaries of reacting aggregate particles. The extent of ASR-related microcracking in a deteriorated concrete specimen depends on many factors such as the amount of reaction/expansion undergone by the concrete-which in turn depends on the inherent reactivity of the aggregate, the moisture conditions, the alkali content of the mix, etc.-and the total restraint to which the concrete member is subjected.
The proportion of aggregates showing internal cracking generally increases with progressing ASR. In the early stages of the reaction, microcracks are generally limited to the reacting aggregate particles and the cement paste-aggregate interface (Figure C3-A). With the progress of expansion, microcracks, more or less filled with alkali-silica gel, will extend from the aggregate particles into the cement paste; depending on the extent of expansion, the cracks will cover considerable distances through the paste where they are often filled with secondary reaction products (Figure C3-B to C3-E). In badly deteriorated concrete specimens, cracks-even filled with gel-may run through non-reactive aggregate particles. Consequently, great care should be taken to correctly identify the sites (or the aggregate particles) that have generated the expansive forces.
The examination of epoxy-impregnated polished slabs or thin sections are commonly used methods for the examination of microcracks in concrete; the incorporation of a UV tracer in the epoxy resin allows better detection of microcracking under UV illumination (Figure C3-F).
C.2.2 Reaction product "gel"
The ASR generates secondary reaction products containing silica, alkalis and calcium as typical constituents. The so-called "alkali-silica gel" will be found filling cracks within the aggregate particles (Figure C4-A), lining or filling voids and fractured surfaces of the cement paste and the aggregate particles (Figure C4-B to C4-E). These deposits will cover more or less important surfaces depending on many factors, such as the extent of the reaction-expansion processes that have occurred, the availability of water, etc. The above reaction product-which can be observed under the petrographic microscope-the stereo-binocular, and the scanning electron microscope-is a characteristic feature of ASR.
However, the abundance of gel deposits is not necessarily indicative of the magnitude of any resultant expansion and cracking (BCA 1992). Large amounts of gel in a concrete specimen do not necessarily indicate that large expansion or extensive cracking have occurred in the structure. On the other hand, cracking due to ASR has been observed in many concrete structures while very little gel was found in concrete specimens taken from the affected members.
The confirmation of the presence and the nature of reaction products is not always easy. Great care should be taken when preparing polished or thin sections from affected concrete specimens to avoid "leaching" of the alkali-silica gels. This could be achieved using a non-aqueous lubricant to avoid dissolution of the water-soluble compounds (BCA 1992).
Figure C3. (A to C): Sections of concrete cut and polished from concrete prisms subjected to the Concrete Prism Test and incorporating a highly-reactive limestone coarse aggregate. (A and B). The proportion of cracked aggregate particle and the extent of ASR cracking increase with increasing expansion (A: expansion 0.065 percent; B: 0.149 percent). (C). Extensive cracking both in the aggregate particles and the cement paste; also, void filled with alkali-silica gel (Expansion > 0.25 percent). (D). Cracking due to ASR extending from one aggregate to another through the cement paste. (E). Polished concrete section incorporating a reactive volcanic aggregate and showing reaction rims and cracking filled with gel in the reactive aggregate particles. (F): Polished section of ASR-affected concrete impregnated with a fluorescent dye to help identify the presence and distribution of cracks and voids (left: impregnated; right: natural light).
Figure C4. (A). Polished concrete slab showing cracks filled with gel inside reactive limestone aggregate particles. (B&C). Broken surfaces of concrete cores showing deposits of alkali-silica gel on the cracked surfaces of the aggregate particles and in voids of the cement paste. (D). Desiccated alkali-silica gel lining a crack in the cement paste of a concrete core affected by ASR. (E). Micrograph showing desiccated gel filling voids of the cement paste of concrete affected by ASR (F). Broken concrete core after treatment with uranyl acetate solution under UV illumination; the alkali-silica gel, which offers a greenish-yellow staining color, surrounds reacted particles of siliceous sandstone.
Staining techniques have been proposed to facilitate identification of the reaction product gel in concrete affected by ASR (Natesaiyer et al. 1991, Stark 1991, Guthrie and Carey 1997). A technique developed at Cornell University (Natesaiyer et al. 1991) consists in applying an uranyl acetate solution on polished or fresh broken surfaces of concrete specimens to be examined followed by a visual observation of the section under a UV light; the technique has even been used on field structures (Stark 1991, AASHTO 1993, ASTM C 856-02). Stark (1991) indicated that "by applying the 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" (Figure C4-F, C5-A, and C5-B). This technique should be used with great care following appropriate health and safety procedures because of the potentially hazardous nature of the product. Technically speaking, the results of the test should be interpreted with great care. Some aggregates fluoresce naturally, which can incorrectly suggest the presence of alkali-silica gel through macroscopic or microscopic examinations; also, although the technique may help locate the presence of gel in the concrete, it will not differentiate between a harmless presence of gel and that which is detrimental (the source of the observed distresses). Actually, any concrete incorporating silica-bearing aggregates may show traces of gel in air voids or surrounding aggregate particles, which is not necessarily an indication of deleterious ASR. On the other hand, some concrete experiencing ASR expansion sometimes contain limited amounts or "visible" gel (i.e., gel visible under commonly used stereobinocular or petrographic microscope); in such cases the uranyl acetate test could be very useful in detecting the presence and the distribution of gel. Overall, when used with care, the method can efficiently support conventional petrographic examination procedures and physical tests for investigating causes of concrete expansion.
The examination of ASR-affected concrete in thin section under the petrographic microscope often allows better locating the presence and the distribution of the reactive aggregates and the secondary reaction products, as illustrated in Figure C5-C to C5-E.
Finally, confirmation of the presence and, to certain degree, of the extent of alkali-silica reaction in the concrete, can be done through examination of polished (Figure C5-F) or broken concrete fragments (Figure C6-A to C6-E) under the scanning electron microscope. This approach allows to precisely identify the presence and distribution of alkali-silica gel through its typical textural and chemical characteristics.
C.2.3 Reaction rims
Dark reaction rims are observed at the internal periphery of a number of alkali-silica reactive aggregates in deteriorated concrete specimens. These are particularly evident on polished sections or slabs of affected concrete cores (Figure C6-F). However, these rims must not be mixed up with pre-existing (e.g., before the introduction of the aggregate particle in the concrete) "weathering" rims that are often found in the outer (but also internal) portion of weathered gravel particles.
Figure C5. (A, B). Polished concrete sections treated with uranyl acetate solution to enhance the presence of alkali-silica gel (A. natural light; B. under UV illumination showing the gel in greenish-yellow staining color filling cracks in the cement paste in the vicinity of reactive aggregate particles). (C-E). Thin sections micrographs showing cracks filled with gel (desiccation texture) and extending from reactive aggregate particles through the cement paste. (F). SEM micrograph of polished concrete section showing cracks filled with gel and extending from reactive aggregate particles through the cement paste.
Figure C6. (A). SEM micrograph showing deposits of alkali-silica gel on the surface of a broken concrete core affected by ASR. (B). SEM micrograph showing alkali-silica gel lining voids of the cement paste in a concrete sample affected by ASR. (C). Broken surface of a concrete cores showing deposits of alkali-silica gel on the cracked surface of a reactive aggregate particle and in voids of the cement paste. (E). SEM micrograph showing the layer of gel forming the dark rim on Figure (C). (E). SEM micrograph of the crystalline products showing a rosette-like microtexture corresponding to the white deposits inside the aggregate particle of Figure (C). (F). Polished sections of concrete cores showing dark reaction rims around reacted aggregate particles.
When concrete cores are fractured for examining "fresh" broken surfaces, cracks that have formed within the aggregate particles and the cement paste, due to the ASR processes, will form zones of weakness where the core will preferentially break. The fractured surfaces thus created (which in many cases correspond to "ASR cracking surfaces") often show a dark rim surrounding internal deposits of whitish color (Figure C6-C). Such a feature does not correspond to a reaction rim per se; it actually corresponds to a typical arrangement of reaction products deposited on the cracking surface, i.e., 1) a dark rim covering the immediate internal periphery of the particle, and 2) white deposits going through the central portion of the particle showing a powdery aspect. Examination under the scanning electron microscope (SEM) confirms the dark rim to be a layer of calcium-rich alkali-silica gel (Figure C6-D), while the whitish deposits are formed by a rosette-like crystalline product (Figure C6-E). Those are typical products of ASR.
C.2.4 Loss of the cement paste-aggregate bond
The interfacial region between the cement paste and the aggregate particles certainly represents, because of its nature and the arrangement of hydrates that form herein, a preferential zone of weakness where cracks will initiate and run. Loss of the cement paste-aggregate bond has been reported as a petrographic consequence but is not necessarily indicative of AAR.
C.3 Quantitative Petrographic Assessment - The Damage Rating Index
Grattan-Bellew (1992) and Dunbar and Grattan-Bellew (1995) described a method to evaluate the condition of concrete by counting the number of typical petrographic features of ASR on polished concrete sections (16x magnification). A grid is drawn on the polished concrete section, which includes a minimum of 200 grid squares, 1 cm by 1 cm (0.4 by 0.4 in) in size. The Damage Rating Index represents the normalized value (to 100 cm2) (16 in2) of the presence of these features after the count of their abundance over the surface examined has been multiplied by weighing factors representing their relative importance in the overall deterioration process (Table C2) (Shrimer 2000). Rivard et al. (2000) used the method to estimate the amount of expansion reached by concrete specimens cored from a large concrete dam affected by ASR; the authors also found that the relative importance of the different petrographic features of ASR, in terms of how they correlate with the measured expansion due to ASR, can vary significantly from one aggregate to another (Rivard et al. 2002). Figure C7 illustrates petrographic observations performed under the DRI method, as obtained by Fournier et al. (2007). Figure C7A to C7D illustrates the various petrographic features of ASR that are quantified as part of the process; the results are then compiled as a function of those various features, as illustrated in Figure C7E, to obtain the DRI for the sections examined. The DRI results in Figure C7E can be seen to correlate with the expansion of the test samples examined.
|Petrographic feature||Weighing factor|
|Coarse aggregate with cracks||x||0.25|
|Coarse aggregate with cracks and gel||x||2.0|
|Coarse aggregate debonded||x||3.0|
|Reaction rims around aggregate||x||0.5|
|Cement paste with cracks||x||2.0|
|Cement paste with cracks and gel||x||4.0|
|Air voids lined or filled with gel||x||0.50|
Figure C7. Micrographs showing petrographic symptoms of ASR as quantified as part of the DRI method. The polished concrete sections were cut from C 1293 prisms having reached different amounts of expansion (Spratt limestone reactive aggregate); a reference grid with 1cm (0.4 in) squares was then dawn at the surface of the polished section for DRI measurements. (A). Concrete prisms having reached an expansion of 0.020 percent. (B). Concrete prisms having reached an expansion of 0.066 percent. (C). Concrete prisms having reached an expansion of 0.111 percent. (D). Concrete prisms having reached an expansion of 0.283 percent. (E). Compilation of DRI values for the test prisms examined
More recently, the DRI method was used to quantify the condition of laboratory-made concrete specimens incorporating various aggregate types and subjected to accelerated test conditions in the laboratory (38°C (100°F) and R.H. > 95 percent) (Smaoui et al. 2004a). The same method was used to determine the condition of concrete cores sampled from blocks exposed at the CANMET-MTL outdoor exposure site in Ottawa (Canada) and incorporating similar aggregates (Smaoui et al. 2004b). All of the above specimens were examined at known expansion levels due to ASR. They used the reasonable correlations thus obtained to estimate the amount of expansion reached by concrete cores extracted from structures affected by ASR incorporating siliceous limestone aggregates (Bérubé et al. 2004). The authors concluded that, unfortunately, the DRI method could not differentiate between concretes affected most and least visually/mechanically by ASR. High DRI values and consequently exaggerated estimated expansion to date were obtained for all concretes investigated (Bérubé et al. 2005). For that reason, the method is not recommended for evaluating the expansion to date of concrete affected by AAR.
Despite the above comments, the DRI method can represent a useful tool for the quantitative assessment, based on petrography, of internal damage in concrete due to ASR or other mechanisms. However, as the results are very much related to the experience of the petrographer and since there is currently no standard test procedure available, the method is fairly subjective and the results can be quite variable from one petrographer to another. Consequently, it is impossible, at this stage, to identify limits of DRI numbers that would be indicative of low, mild, and severe AAR ratings. This is even more the case knowing that the relative importance of the different petrographic features of ASR, in terms of how they correlate with the measured expansion due to ASR, can vary significantly from one aggregate to another, which can result in fairly different DRIs for similar level of expansion depending on the reactive aggregate present in the concrete (Rivard et al. 2002). Despite all of that, the DRI method can provide very useful relative information when the examination of a set of cores from the same structure (presumably incorporating the same aggregate) will be carried out by the same experienced petrographer. The method would also identify differences in damage ratings between members of a single structure that would incorporate different reactive or non-reactive aggregates.
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