Report on the Diagnosis, Prognosis, and Mitigation of Alkali-Silica Reaction (ASR) in Transportation Structures
4.0 ASR Investigation Program Level 2: Preliminary Studies for the Diagnosis of ASR
The objective of this part of the program is to confirm the results of the condition survey, i.e., determine whether or not ASR is a contributing factor in the deterioration observed, and to generate preliminary quantitative assessment of the extent of damage. It consists in a limited number of field and desk/laboratory activities that will generate information leading to the selection of immediate remedial actions or that will open the door to a more detailed investigation program (ASR Investigation Program Level 3).
Any documents (i.e., testing of materials, construction, and inspection reports) related to the structure examined should be gathered and examined as they may provide valuable information in the appraisal process. This activity could also be carried out either in preparation for the condition survey or following it (as in Figure 1), i.e., for structures where some signs of deterioration potentially indicative of ASR have been noticed. Useful information could include the following (CSA A864, FHWA-HRT-04-113):
- Type and location of the structure and, hence, its likely exposure conditions due to its nature of operation and geography.
- Age of the structure and details and dates of any modifications or repairs. ASR may take from 3 to even more than 25 years to develop significantly in concrete structures depending on factors such as the nature (reactivity level) of the aggregates used, the moisture and temperature conditions, and the concrete alkali content.
- Plans, drawings, and specifications.
- Details of concrete mixes used, particularly mix proportions, source of cement and aggregates, and details of any analyses or tests carried out on concrete materials. The availability of samples of these materials should also be checked; some agencies store samples of cements and aggregates used in major projects.
- Previous inspection/testing reports, especially dates when deterioration was first observed.
- Information from other structures in the area that may have been constructed with the similar materials, especially if these structures are exhibiting signs of deterioration typical of ASR.
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. It is recognized that information of this nature is often not available or lacks specific detail in the case of many structures; however, it is important to collect whatever data is available.
4.2. Measurement of the Cracking Index (CI)
The development and the extent of surface cracking on concrete structures or members exposed to the elements is a function of many factors. In the case of concrete members undergoing internal expansion due to ASR and subjected to wetting and drying cycles (cyclic exposure to sun, rain, wind, or portions of concrete piles in tidal zones, etc.), the concrete often shows surface cracking because of induced tension cracking in the "less expansive" surface layer (because of variable humidity conditions and leaching of alkalis) under the expansive thrust of the inner concrete core (with more constant humidity and pH conditions). The extent of surface cracking on those elements is thus somewhat related to the overall amount of expansion reached by the affected concrete member.
The Cracking Index (CI) is a crack mapping process that consists in the measurement and summation of crack widths along a set of lines drawn perpendicularly (i.e., parallel and perpendicular to the main restraint(s)) on the surface of the concrete element investigated. The method gives a quantitative assessment of the extent of cracking in structural members.
The CI is introduced here, in combination with petrographic examination of cores (see Section 4.4) to provide decisionmaking criteria for the early detection of ASR and the selection of further actions right in this early stage of the global investigation program. Details on the CI method, the type of and conditions for readings, and reporting of the data are given in Appendix B and are summarized below.
4.2.1. Number and location of the CI reference grids
In order to generate a statistically representative assessment of the extent of cracking through the CI method, a minimum of two CI reference grids, 0.5m (20 in) in size, should be drawn on the surface of the most severely cracked structural components (see Appendix B). Those components generally correspond to those exposed to moisture and severe environmental conditions, as well as those where ASR should normally have developed to the largest extent.
4.2.2. Timing of the readings
Because of the significant effect of temperature and humidity on crack widths, CI readings should be carried out and repeated under very similar conditions of sun exposure, outdoor temperature, and outdoor humidity conditions (see Appendix B for details).
4.2.3. Measurements and calculation of the CI
The width of each crack crossing the four lines drawn on the surface of the element investigated is measured using a magnifying lens with internal gradations (e.g., 0.05 mm (0.002 in)) or a plastic crack comparator. As described in Appendix B, the CI is calculated and expressed in mm/m (in/yd).
The following cracking criteria, which are obtained from the crack mapping survey performed as part of the Cracking Index method, are proposed to identify an extent of cracking that should justify more detailed investigations.
|CI > 0.5 mm/m (0.018 in/yd), and/or Cracks of width > 0.15mm (0.006 in)|
The estimate of the expansion attained to date by the structural member is a critical parameter in the evaluation of its current condition in view of selecting appropriate remedial actions. This is, however, not a parameter easy to determine in most cases. Section 5.2.2 indicates that the Cracking Index measured on the most exposed concrete component (which is most of the time the one showing the most severe signs of deterioration due to ASR), can give a rough estimate of the expansion reached to date by the element under investigation. The Institution of Structural Engineer (ISE 1992) suggests that expansions in structural members in excess of 0.5mm/m (0.018 in/yd) should warrant further investigations and that the potential consequences of such expansions should be assessed. In addition, cracks in excess of 0.15mm (0.006 in) in width could start to be a source of concerns, especially in the case of prestressed concrete members, and should also justify further investigations (ISE 1992). Table 3 gives ACI 224 committee guide to reasonable crack width for structural concrete members under service loads (ACI 224R-01). The limits range from 0.41 mm (0.016 in) in the case of members exposed to dry air or protected by a membrane to 0.10 mm (0.004 in) for water-retaining structures. In the case of highway bridge structures and pavements, cracks of 0.15mm (0.006 in) in size are large enough to raise attention and justify some investigations aiming at identifying their cause, thus potentially allowing an early detection of ASR cracking and some early remedial actions.
|Exposure conditions||Crack widths|
|Dry air or protective membrane||0.41||0.016|
|Humidity, moist air||0.30||0.012|
|Seawater and seawater sprays, wetting and drying||0.15||0.006|
4.3. Preliminary Sampling Program
4.3.1. Nature and extent of sampling
Sampling is carried out from a limited number of components of the structures, essentially to determine whether or not the concrete contains petrographic evidence of ASR. A minimum of two cores will thus be collected from each of those components showing typical to more severe signs suggestive of ASR (see Appendix A), which will or should most often be structural components exposed to a constant or renewable supply of moisture, with/without cycles of wetting and drying. For comparison purposes, it will also be appropriate to collect a few cores from structures that are less deteriorated than the structure in question, not deteriorated, or not exposed to the environment (i.e., to environmental elements).
4.3.2. Type and size of samples
Core samples, typically 100 mm (4 in) in diameter, are most suitable; however, special circumstances, i.e., where larger size aggregates or closely spaced reinforcement, may require cutting larger or smaller cores. Smaller cores are more susceptible to cracking during coring, which may be misleading regarding the severity of internal cracking. Also, the use of larger size cores (e.g., 150 mm (6 in) in diameter) will be beneficial in the case of expansion testing on cores at 38°C (100°F) and R.H. > 95 percent as it will contribute at reducing leaching of alkalis during the test, thus generating more reliable test data for estimating the potential for future expansion (see Section 5.3.4).
As petrographic symptoms of ASR are known to vary from the surface to the interior part of the affected element, cores should be as long as possible to provide a profile of the element sampled. In the case of massive concrete elements such as abutment walls, reinforced concrete columns, and beams, the cores should be at least 30 cm (12 in) long. In the case of thinner concrete elements such as pavements, bridge decks, and parapet walls, cores should pass through the whole element.
4.3.3. Treatment of samples and information collected
A detailed record of all sampling operations should be made on site. The use of a sampling form or "Site Core Record" accompanied by pictures showing the characteristics of the components sampled is most appropriate. The information recorded on site should include the following:
- Sketch showing location of core.
- Photograph of core location.
- Size (diameter and total length) and orientation.
- Record of any features that may be indicative of ASR, such as damp patches on core surfaces, gel in cracks and voids, or reaction rims around aggregate particles.
The samples collected should be labeled carefully, photographed and, immediately after their preliminary examination, wrapped in a plastic film and sealed in a plastic bag to prevent alkali-silica gel and surfaces to carbonate, become contaminated, or dry out during subsequent transport and storage.
4.4. Petrographic Examination
As mentioned before, this ASR Investigation Program Level 2 consists in identifying the presence (or not) of petrographic signs of ASR in the cores sampled from those components showing visual signs of deterioration most suggestive of ASR. Although not necessarily exclusive to ASR, petrographic signs of ASR generally consist of:
- microcracking in aggregates and/or cement paste;
- reaction product "gel";
- reaction rims; and
- loss of the cement paste-aggregate bond.
Detailed information and photographs illustrating petrographic signs of ASR are given in Appendix C of this document, including a discussion on the use of the uranyl acetate test for the identification of alkali-silica gel in concrete2. Appendix C also includes a table that lists features to look for from petrography as a function of the method of examination used.
4.4.1. Macroscopic description of the core (as received)
The cores are first examined and photographed in an 'as-received' condition. If the surfaces of the cores are dry, they should be dampened and replaced in a plastic bag for an additional 24 hours before examination. The "macroscopic" description of the core is generally performed with the naked-eye and with a magnifying lens (7-10x) or a stereo-binocular (generally up to 60x). Certain features may be highlighted by rewetting the core surfaces and making observations as the core dries. In addition to observations normally made on core samples (e.g., size and distribution of aggregate, compaction, void content, and presence and condition of reinforcement), observations regarding the following features may assist in the diagnostic process, and their presence should be noted:
- Cracking location (e.g., around or through aggregate particles, etc.), associated gel exudation, crack width, depth of surface cracking, etc.
- Presence of gel (and other secondary reaction products - e.g., ettringite) in voids, cracks, around aggregate particles, or exuding from the core.
- Damp patches on the concrete surface.
- Reaction/weathering rims around aggregate particles.
- Any signs of concrete disintegration.
- Presence, size, position, and condition of reinforcement.
4.4.2. Microscopic description of the core
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 AAR. The examination of thin sections (magnifications of up to 250x) will allow to further positively identify 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.
As mentioned before, Appendix C provides a description (with photographs) of the various petrographic features on ASR that can be obtained from the examination of polished slabs and thin sections. In summary, the following information may be obtained from the above examination (CSA 2000):
- Description of aggregates (for further information on the identification of rock types and potentially reactive components, see ASTM C 295).
- General characterization of microcracking, including intensity, size range of cracks, apparent association with particular aggregate type, cracking in or around aggregate particles, and presence of gel or any other deposits in cracks.
- Presence of reaction and/or alteration rims around aggregate particles ("reaction" rim types to be distinguished from "weathering" rims that are sometimes observed surrounding weathered gravel particles).
- Presence of gel or other deposits in voids.
- Sites of expansive reaction-occurrences of features that provide evidence of reaction and emanation of expansive forces, e.g., reactive aggregate particles showing cracking internally or at the cement/aggregate interface, with cracks propagating into the surrounding matrix and cracks filled or partially filled with gel.
4.4.3. Interpretation of the findings from petrographic examination
Petrographic examination on polished and thin sections, when conducted by a qualified petrographer experienced in the examination of concrete affected by ASR, is the most powerful tool to confirm the occurrence of that deleterious phenomenon. Table 4 classifies the occurrence of the features obtained from the petrographic examination as indicative of low, medium, and high probability of ASR.
As indicated in Figure 1, if the probability of ASR from petrographic examination is low, it may be advisable to investigate for other mechanisms to explain the deterioration observed. However, when the probability of ASR is medium to high, further work may be required.
Some cases may, however, justify special attention. It would be the case, for instance, where some petrographic signs of ASR would be identified in a fairly young but critical structure. Such observations may not at that stage justify any immediate remedial actions; however, it would contribute to raising attention and justifing accelerating the frequency of investigations (condition survey and petrographic examination).
|Probability of ASR||Nature and Extent of Features|
|Low||No potentially reactive rock types (from petrographic examination of thin sections):
|Medium||Presence of some features generally consistent with AAR:
|High||Presence of extensive signs of ASR (as described in the previous section but observed in larger frequency), for instance:
4.5. Assessment of the Results from CI and Petrographic Examination
As illustrated in Figure 1, the assessment of the results from the crack mapping measurements in the field and from the petrographic examination in the laboratory will lead to options described in Table 5.
|Cracking||Probability of ASR||Findings||Recommended Actions|
|Cracking < agency specified criteria||Low probability of ASR (from petrography)||Although cracking is noted in the element examined, the extent of cracking is still limited; there is no conclusive evidence of ASR in the concrete (based on petrography).||Monitor the progress in cracking by repeating the crack mapping (i.e., CI) process as part of the next routine inspection survey. If evidence of progress in cracking is noted, further coring and petrographic examination is recommended to evaluate the progress in internal distress due to ASR.|
|Medium to high probability of ASR (from petrography)
(situation not in Figure 1 flow chart)
|This a fairly unlikely situation as ASR, when present to a significant extent in concrete, generally leads to noticeable cracking at the surface of at least on the most severely exposed affected elements. It may however correspond to a relatively early stage of ASR. Also, some signs of ASR may be observed in the case of some reactive aggregates such as opal-bearing or cherty that may react close to the surface (thus producing pop outs) or that may dissolve in the concrete without necessarily inducing significant cracking in the concrete element as a whole.||Initiate further investigations on other members of the structure (e.g., assess effect of exposure conditions, look for signs of expansion, coring of other members for petrography).|
|Cracking > agency specified criteria||Low probability of ASR (from petrography)||Significant cracking is affecting the element investigated. On the other hand, there is no conclusive evidence of ASR in the concrete (based on petrography).||Initiate further investigations for other mechanisms of deterioration.|
|Medium to high probability of ASR (from petrography)||Presence of significant to extensive signs of ASR, both in-situ (cracking) and internally (petrography).||Additional investigations may be required to establish the expansion reached to date and the potential for further expansion, leading to the selection of the most appropriate remedial action. Some immediate remedial actions (e.g., application of sealers) may already be a possibility at this stage (i.e., without any further investigations).|
4.6. Necessity to Pursue Investigations or Not
A decision should now be made as to whether or not a remedial action could or should be selected/implemented at this stage, in other words whether or not additional investigations should be carried out before any remedial actions could be implemented.
Based on the results of the field (condition survey and cracking index) and laboratory (petrographic examination) investigations carried out so far, we should be in the presence of concrete structures showing noticeable signs of deterioration. The latter is likely to range from mild to severe, and be totally or at least partially associated with ASR. The review of documentation may also have contributed useful information supporting a low to high probability of ASR [e.g., concrete structure "sufficiently" old for ASR distress to develop (i.e., generally > 10 years), previous inspection reports suggesting ASR as a possible cause of distress, materials testing reports showing that potentially reactive aggregates have been used, mix design information indicating that fairly high concrete alkali loadings have been used, etc.].
The field and laboratory investigations performed so far may have identified cases where some remedial actions could/should already be implemented. Examples of such cases are given in Table 6.
|Type of Structure (Table 7)||Damage||Signs of ASR||Rationale for Implementing Immediate Remedial Action|
|S1 and S2||Mild to moderate||Mild to moderate||
|S1||Severe||Mild to moderate||
|S3 and S4||Mild to moderate||Mild to moderate||
|S2, S3, and S4||Severe||Mild to severe||
Section 6.0 discusses some remedial measures that could be implemented in such cases, with the limitations involved.
Critical information leading to a more complete assessment of ASR in the concrete structure and the selection of most appropriate remedial measures is, however, still missing. Such information is related to the following questions:
- What is the current condition (or the degree of damage) of the concrete element?
- What stage of the ASR-deteriorating process or what level of expansion has been reached to date?
- Is the structural integrity in danger (e.g., relating to the stability of the reinforcing steel and of the steel-to-concrete bonding, development of extensive spalling at joints in pavements)?
- How much more expansion could be expected?
- How much additional deterioration can be expected (impact on long-term durability and service life)?
The ASR Investigation Program Level 3 described in Section 5 aims at answering the above questions, which will be especially important in the case of critical structures such as Class S3 and S4 structures in Table 7.
|Class||Consequences of ASR||Acceptability of ASR||Examples|
|S1||Safety, economic, or environmental consequences small or negligible.||Some deterioration from ASR may be tolerated.||
|S2||Some safety, economic, or environmental consequences if major deterioration.||Moderate risk of ASR is acceptable.||
|S3||Significant safety, economic, or environmental consequences if minor damage.||Minor risk of ASR is acceptable.||
|S4||Serious safety, economic, or environmental consequences if minor damage.||ASR cannot be tolerated.||
†Note: this table does not consider the consequences of damage due to ACR. This report does not permit the use of alkali-carbonate aggregates.
2ASTM C 856 outlines procedures for the petrographic examination of samples of hardened concrete, while The Petrographic Manual (Walker 1992, Walker et al. 2006, FHWA-06) is a valuable source of procedures for petrographic examinations related to ASR.