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Report on the Diagnosis, Prognosis, and Mitigation of Alkali-Silica Reaction (ASR) in Transportation Structures

6.0 Mitigation Measures for AAR-Affected Structures

Report on Determining the Reactivity of Concrete Aggregates and Selecting Appropriate Measures for Preventing Deleterious Expansion in New Concrete Construction (Thomas, et al., 2008) is the first of this two-part series of reports, and summarizes the various options for preventing AAR in new construction. The options are numerous, and the end result, namely avoidance of deleterious AAR reaction, has been borne out by years of laboratory and field experience. With this knowledge in hand, there is no reason that concrete structures constructed in the future should suffer from AAR. However, there are literally thousands of structures already in service that were built before this knowledge was applied in practice, and unfortunately, there are still new structures being built that will ultimately suffer from AAR-induced expansion and distress because guidance, such as that contained in the aforementioned report by Thomas, et al. 2008, was not followed. As such, AAR-affected structures will continue to plague our infrastructure for years to come. Unfortunately, the mitigation options available for existing structures suffering from AAR are no where near as numerous as those available for preventing AAR in new concrete, and there is a major shortage of information/data documenting the effectiveness of the mitigation measures that have been implemented in the field. The majority of the work to date on treating existing structures has focused on ASR-affected structures, as opposed to ACR-affected structures, and there are by far many more ASR-affected structures worldwide. As such, the focus of this portion of this report is aimed at ASR-affected structures. However, some of the mitigation measures, particularly those aimed at drying the concrete, would be helpful whether it is ASR or ACR that is impacting the structure.

This section briefly describes the mitigation measures that are available for ASR-affected structures, identifies options that have been used the most, discusses those whose effectiveness has been proven in the laboratory and field, and describes those that remain experimental in nature due to a lack of data/information proving their merit in real-world applications. The main objective of this section is to provide guidance on means of extending the service life of ASR-affected structures. The terms "remediation" and/or "mitigation" are used in lieu of "repair" because the methods described herein are generally not able to, nor are they intended to, repair or restore the original properties or integrity to the ASR-affected structure. Rather, the intention is to reduce future expansion of the structure or to lessen the detrimental impact of future expansion.

6.1. Decision Factors When Considering Mitigation Options

Once the decision has been made that mitigation or remediation measures are necessary, the owner/agency who has jurisdiction over the structure must carefully consider all available options before deciding upon and implementing the selected measure(s). This section identifies some of the critical decision factors that must be considered during this process. After discussing the decision factors, presented as a series of questions to an owner/agency, the various mitigation options that are available to an owner/agency are described in Section 6.2, and lastly, Section 6.3 describes how the decision factors can be coupled with the available mitigation measures to select the mitigation measure(s) for a given structure.

Figure 3 summarizes the decision factors that should be considered (or questions that should be answered, for that matter) for a given structure that has been deemed in need of immediate mitigation or remediation due to ASR-affected expansion and distress. The relevance of these factors, or questions, may not be obvious at this point, but Sections 6.2 and 6.3 of this report will describe the importance of these factors as they relate to potential mitigation measures. For example, if a reinforced concrete structure is exposed to both chlorides and freeze-thaw cycles, the ingress of water and chlorides must be restricted and maximum crack widths must be closely scrutinized - this situation would thus trigger specific measures aimed at these concerns, such as caulking of cracks and improvements in drainage to limit moisture ingress.

Figure 3. Decision factors when considering mitigation option(s) for structure that has been deemed in need of immediate mitigation due to ASR-affected expansion and distress.

What is the criticality of the structure?

  • Can ASR be tolerated?
  • What are the consequences should ASR occur?
  • What is the intended service life of the structure?

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What are the exposure conditions for the structure?

  • Is the structure directly exposed to moisture, and if so, is it constant exposure or cyclic exposure?
  • Is the structure exposed to freezing and thawing cycles?
  • Is the structure reinforced with steel and is it exposed to chlorides?
  • Is the structure exposed to an external source of alkalies (deicing salts, etc.)?
  • Is the structure exposed to external sulfates or other aggressive ions?

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How well-reinforced (or confined) is the structure?

  • Is the affected structure internally reinforced and detailed such that the ASR is sufficiently confined in the "cage" (see ISE (1992) for classification of structures based on level of reinforcement detailing)?
  • Is the affected structure confined externally (i.e., by adjacent structures, etc.)?
  • What specific type of reinforcement is used in the structure (black steel, epoxy-coated steel, prestressed/post-tensioned, etc.)

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What are the geometric configuration and accessibility of structure or structural element?

  • What are the dimensions, shape, aspect ratio of the structure or structural element (relevant for strengthening/confining by FRP, etc?)
  • How accessible is the structure or structural element (relevant for certain mitigation options that need a certain degree of access)
  • Is the structure on grade or is there backfill on one more sides?

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Are there any serviceability issues due to current or projected ASR damage?

  • Is the serviceability affected by ASR (i.e., ride quality for bridge deck, watertightness for containment structure, etc?)
  • Is the intended use of the structure jeopardized by ASR-induced damage?
  • Are adjacent structures/elements being adversely affected?

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Is future expansion anticipated for the structure?

  • What is the estimated magnitude (and direction) of future expansion (especially relevant if external confinement is being considered as mitigation option)?

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How important is the aesthetics of the structure?

  • Is it a "historic" structure?
  • Can mitigation options that impact aesthetic nature of structure be tolerated?

6.2. Overview of Mitigation Measures for ASR-Affected Structures

Figure 4 summarizes the various mitigation options that have been used or proposed for use in field structures. These are grouped according to whether they are intended to treat the causes of ASR or the symptoms of the deleterious reaction. This section will briefly discuss each of the options shown in Figure 4 and will then focus on those that have the greatest potential for effectively treating ASR-affected structures. For each of these options, the merits will be discussed, as well as inherent shortcomings, both in terms of general applicability to field structures and specific application to certain structures.

When attempting to treat the causes of ASR, it is worth noting that three conditions must be satisfied for ASR to occur: 1) Sufficient alkalies must be present within the concrete; 2) sufficient reactive silica within the aggregates must be present in the concrete; and 3) sufficient moisture must be present within the concrete to sustain the reactions. As such, when attempting to mitigate active ASR by treating the underlying causes, actions should be taken to reduce or eliminate the above factors.

Figure 4. Potential options for mitigating ASR - treating the cause vs. treating the symptom
Treat the cause Treat the symptom

Chemical Treatment/Injection

  • CO2
  • Lithium Compounds

Drying

  • Sealants
  • Cladding
  • Improved Drainage

Crack Filling

  • Aesthetics
  • Protection (e.g., from Cl- ingress)

Restraint

  • Prevent Expansion
  • Strengthen/Stabilize

Relieve Stress

  • Saw Cutting/Slot Cutting (accommodate movement)

With regard to addressing the first of the above conditions, it has been proposed that the alkali content (or pH) could be reduced by injecting ASR-affected concrete with CO2, which would have the impact of lowering the pH and carbonating the ASR gel (Cavalcanti and Silveira, 1989). However, there are several technical and practical limitations to this method. First, carbonating reinforced concrete can substantially increase the rate of corrosion. Second, injecting gas under high pressure can lead to significant distress in concrete already suffering from microcracking due to ASR. Although it has not been reported in literature, this technique would likely have little impact on ACR as it tends to be driven by a minimal quantity of alkalies, and these alkalies tend to be recycled in the process. Either way, whether it is ASR or ACR, injecting concrete with CO2 to attempt to reduce the alkali content appears to hold little or no promise in field structures.

There have been several laboratory-based publications related to using lithium compounds to treat concrete already suffering from ASR-induced expansion. Research by Stark et al. (1993), Stokes et al. (2000), and Barborak et al. (2004) have shown that lithium compounds can reduce future expansion of small, ASR-affected concrete specimens in accelerated laboratory tests. Although the mechanism is not fully understood, it is generally believed that lithium compounds enter into the existing gel and change the nature and behavior of the gel from expansive to essentially non-expansive. Because of these positive results in laboratory-based work, there has been considerable interest in treating ASR-affected field structures, especially in recent years under FHWA-funded research (FHWA Project DTFH61-02-C-00097). A detailed review of past field trials using lithium compounds can be found in Folliard et al. (2006), and several field trials are still being monitored under current FHWA projects (East 2007). The most common method of applying lithium compounds in field trials has been via topical application, primarily for pavements (see Figure 5) and bridge decks. There have also been a handful of field trials where lithium was applied either by vacuum or through electrochemical means, both aimed at increasing the depth of penetration of lithium.

Photograph showing topical application of 30 percent-LiNO3 solution to concrete pavement in Idaho. This photo shows a multi-nozzle sprayer mounted on the back of a truck applying a solution to the pavement. A fine misty spray is being ejected into the pavement and the pavement behind the nozzles is shown to be wet.
Figure 5. Photograph showing topical application of 30 percent-LiNO3 solution to concrete pavement in Idaho.

Although lithium compounds have been found to be effective in laboratory-based research, which has focused on treating small specimens affected by ASR, there is, unfortunately, very little, if any, documentation that lithium is effective in reducing ASR-induced expansion in actual structures in the field. Part of this is due to the general lack of monitoring of field trials in which lithium compounds have been applied to structures (or pavements). However, in recent FHWA-funded field trials (under FHWA Project DTFH61-02-C-00097) in Idaho, Massachusetts, and Texas, which have likely been the most instrumented and monitored lithium-based field trials to date, the depths of penetration of lithium have been measured to be quite minimal, especially for topical applications. In fact, depths of penetration for topical applications in an Idaho pavement were found to be only a few millimeters, with dosages of lithium necessary to suppress expansion measured only down to the first 2 to 3 mm, even after three treatments in heavily cracked pavements. These results are consistent with laboratory evaluations performed under the same project, and when taken as a whole, it appears that due to an inherent lack of penetration, the topical application of lithium compounds shows little, if any, promise of mitigating ASR in structures and pavements, with perhaps the exception where ASR is being exacerbated in the outer surface by an external source of alkalies (e.g., deicing salts). However, research is needed to determine if lithium compounds, applied topically, show merit in this specific case where pavements or bridge decks are exposed to deicing salts that are driving the ASR process. It remains to be seen if topical lithium treatments would be able to reduce future expansion in this situation, given the shear quantities of deicing salt applied annually.

Because of the documented lack of penetration in field and laboratory trials in which lithium compounds have been applied topically, recent focus has shifted towards more aggressive means of driving lithium into ASR-affected concrete, specifically through vacuum impregnation and electrochemical methods. Unfortunately, in research performed under FHWA Project DTFH6102-C-00097, vacuum impregnation was not found to be effective in the laboratory or in field structures in Texas and Massachusetts. For example, for ASR-affected bridge columns in which lithium nitrate was applied via vacuum, the depths of lithium penetration were found only to be in the present in the outer 9 to 12 mm, drawing into question whether such an elaborative and expensive vacuuming technique is justified. Substantially higher depths of penetration were observed in the same study when lithium nitrate was electrochemically driven into bridge columns, with dosages sufficient to reduce ASR measured all the way down to the reinforcing steel (50 mm from outer surface). However, one "side effect" of the latter process must be addressed. Lithium ions were clearly driven to the reinforcing steel, as was the intention, but because the steel serves as a cathode in the electrochemical process, hydroxyl ions are produced at the surface of the reinforcing steel. To maintain charge neutrality and to offset the production of hydroxyl ions at the reinforcing steel surface, sodium and potassium ions from within the concrete migrated towards the steel surface. This creates an increase in the hydroxyl ion concentration and a subsequent increase in alkali (sodium and potassium) concentration near the surface of the reinforcing steel may exacerbate ASR-induced expansion and cracking in this region. Future monitoring of these columns (expansion, cracking, microstructural evaluations aimed at regions near the concrete/steel interface) should help to determine if the potentially detrimental side effects of electrochemical impregnation outweigh the benefits of the significant lithium penetration. Information on the specific details of the electrochemical method used for this bridge structure can be found in East (2007).

However, one major concern with this technique is that the electrochemical process itself tends to drive alkalies already present in the concrete towards the rebar, which may be a significant obstacle to this technology. It appears that as a whole, this technique is quite powerful in driving external lithium into the concrete, but the rearrangement of internal alkalies and accumulation of sodium and potassium (which in turns leads to an augmented pH near the rebar) is a serious concern that deserves further attention. More work is in progress to evaluate this treatment technique and to quantify the benefits (and downsides) of this approach.

Despite the general lack of penetration observed in laboratory and field structures in which lithium was applied topically or by vacuum, it is hoped that data will be generated from other field trials, thereby increasing the state of knowledge and expanding the database of depth of penetration data. It will especially be quite useful to determine if lithium can help to extend the service life of structures exposed to external deicing salts. Lastly, the success in driving lithium all the way to the reinforcing steel is encouraging, but the adverse effects of pushing sodium and potassium to the vicinity around the steel deserve further attention. Given that lithium compounds have clearly been shown to be effective in reducing future expansion in ASR-affected concrete in the laboratory, and given that the options for treating the cause of ASR in the field are limited, it is hoped that additional lithium-based field trials be conducted and monitored, thereby helping to quantify the effects of lithium application on remaining service life.

Moisture is an essential component of ASR-induced expansion and cracking. Stark (1991) and Pedneault (1996) showed that below a relative humidity (RH) of 80 percent, ASR-induced expansion is significantly reduced or suppressed. As such, any methods that can be applied in the field to reduce the internal RH are worthwhile. This can involve improving drainage for a given structure, for example, by diverting drainage from a bridge deck away from an ASR-affected column. The application of exterior cladding that prevents the ingress of additional moisture may be beneficial, but it should be noted that the moisture already present within the concrete may be sufficient for ASR to remain active, and this fact must be considered when contemplating a cladding as a mitigation measure. A more sound solution with regard to reducing internal moisture is to apply a coating or sealer that prevents external water from penetrating into the concrete, but also allows water vapor from within the concrete to exit, thereby resulting in an overall decrease in the internal relative humidity. A coating or penetrating sealer that will trigger this reduction in internal relative humidity must provide the following characteristics (after CSA864-00):

  • Be resistant to water absorption.
  • Penetrate to a measurable depth.
  • Resist deterioration from ultraviolet (UV) radiation.
  • Possess long-term stability in an alkaline environment.
  • Be of long-term stability in an alkaline environment; and
  • Allow vapor transmission.

Siloxanes and silanes, tend to be most suitable and are, as a result, the most commonly used as mitigation measures for not only ASR, but also to help reduce the ingress of water (to enhance frost resistance) and external chlorides (to reduce the rate of corrosion of reinforcing steel). In recent years, silanes have become the most important and most widely used product for these purposes. There are a variety of silane products available, varying primarily based on the concentration of silane in the specific formulation (ranging from 20 percent to close to 100 percent) and based on the type of carrier with which the silane is combined (either water-based or solvent-based). More stringent restrictions regarding VOC emissions have resulted in more water-based silanes or solvent-based silanes with higher silane contents (and thus lower solvent content and reduced VOCs). Silanes are almost always applied topically (see Figure 6), at a coverage rate similar or slightly higher than that of lithium-based products, and depths of penetration are in the same order of magnitude-a few millimeters. However, unlike lithium compounds, which must penetrate into concrete to reach ASR gel, thereby rendering the gel less expansive, silanes or similar penetrating sealers must only penetrate into the concrete in the outer region to form a functional barrier preventing water from entering but allowing moisture vapor to escape.

Photograph showing topical application of 40 percent-Silane Solution (solvent-based) to ASR-affected highway barrier in Massachusetts. This photo shows a stretch of concrete barriers along the median of a highway with extensive cracking. A worker is using a hand-held compression sprayer to apply a solution to the map-cracked surface of a highway barrier.
Figure 6. Photograph showing topical application of 40 percent-Silane Solution (solvent-based) to ASR-affected highway barrier in Massachusetts.

Depths of penetration of silane are typically measured to be less than 5 or 6 mm, which is generally adequate for these materials to serve their purpose-preventing liquid water from entering the concrete while allowing internal water vapor to escape, thereby lowering the internal relative humidity of concrete. Deeper penetration of silanes is desirable as it aids in the providing longevity to the treatment as sealers present at or near the surface can be removed with time due to surface abrasion (e.g., bridge decks) or by UV radiation.

Producers and distributors of silanes or similar penetrating sealers generally recommend that the products not be applied when the concrete surface is damp/wet (after rainfall, for example), and some State Highway Agencies, such as New York and Texas, specify maximum surface moisture contents of concrete, as measured by commercially-available moisture meters. A single application of a silane-based product is not generally believed to be a "permanent fix," as its effectiveness will tend to dissipate with time, especially due to abrasion and UV effects, and as such, it is generally believed that re-applying silanes every 5- years or so is prudent. Carter (1994) reported that the re-application of silane over a surface previously treated with silane resulted in deeper penetrations and enhanced ability to reduce internal moisture. When reapplying silanes or other materials to concrete, manufacturers' recommendations should be followed to ensure that they are applied properly as there have been some concerns raised in the field on the efficacy of re-applications of silanes or similar products (e.g., water- vs. solvent-based, etc.).

There have been several studies that have confirmed the benefits of applying siloxanes, and especially silanes, to field structures to reduce future ASR-induced expansion (Bérubé et al. 2002a and 2002b; Grabe and Oberholster 2000). The research by Bérubé et al. (2002a and 2002b), illustrated in Figure 7, was particularly encouraging as it showed that applying silane to highway barriers heavily damaged by ASR resulted in a dramatic reduction in future expansion.

Photograph showing benefits of silane treatment of highway barriers in Canada (Bérubë et al. 2002a). The photograph, which shows control section on left and silane-treated section on right, was taken three years after treatment. This photo shows two concrete barriers side by side, and contrasts the heavily map-cracked control surface with the adjacent mostly crack-free silane-treated surface of a highway barrier.
Figure 7. Photograph showing benefits of silane treatment of highway barriers in Canada (Bérubé et al. 2002a). The photograph, which shows control section on left and silane-treated section on right, was taken three years after treatment.

As described, silanes can be quite effective in the field in reducing ASR-induced expansion, but there are certainly limitations. For pavements, slabs on grade, wingwalls, or other applications where moisture is available from below (or beneath), silanes will not be as effective as their benefits are only realized from the treated surface. Also, for applications where concrete will be fully submerged or not allowed to dry, silanes will likely not work well because wetting and drying cycles are needed to reduce internal concrete moisture.

Lastly, it should be noted that the application of silanes will not be effective in concrete with large crack widths. For these larger cracks, flexible caulking or similar products should be used to seal the larger cracks. There have been recent developments, including the use of "high-build" paints or elastomeric coatings that may show promise in bridging larger cracks and avoiding the need for caulking of individual cracks. The need to seal larger cracks becomes critical when reinforced concrete is exposed to external chlorides or in regions exposed to cycles of freezing and thawing.

Rather than addressing the underlying cause of ASR, one can attempt to minimize or manage the symptoms or manifestation of the deleterious reaction. In essence, this approach allows ASR to continue but focuses on lessening the impact on the performance or service life of the structure. Following are brief discussions on measures that can be taken to address ASR-induced cracking, primarily through crack filling to minimize ingress of water, chlorides, and other aggressive ions, and ASR-induced expansion, either by confining the expansion or allowing for expansion through slot cutting/concrete removal.

Cracking due to ASR may not only have an impact on the performance of a given structure, but the cracks serve as access points by which water, external alkalies, chlorides, and sulfates can enter the concrete, exacerbating ASR and potentially leading to other forms of distress, such as frost attack, salt scaling, corrosion of reinforcing steel, and sulfate attack. In cases where cracking has become excessive and/or for structures exposed to aggressive environments, crack filling is often the option of choice. In this report, crack width thresholds of 0.15 mm for reinforced members of bridges and 0.30 mm for pavements and non-reinforced members of bridges were proposed, with crack caulking/filling recommended if these limits are exceeded. However, different crack width thresholds can be defined on a structure-by-structure basis, depending on the details of the structure, loading conditions, and surrounding environment.

Flexible grouts or caulking tend to be more effective in filling cracks and keeping water and other entities from entering through the cracks. Although rigid polymer- and cement-based grouts may help to stabilize cracks initially, their rigid nature and strong bonding with the substrate concrete often forces cracks to appear adjacent to the grouted area.

Numerous studies and field trials have shown that physical restraint or confinement (e.g., encapsulation of the affected member by a surrounding non-reactive concrete, applied stress or reinforcement) can significantly reduce deleterious expansion due to ASR in the direction of restraint (Fournier et al. 2004). Because of the unique nature of this mitigation approach and the fact that the structural response is impacted, it is imperative that a structural engineer play the leading role in specifically designing the methodology for a given ASR-affected structure. A detailed structural evaluation is essential, and care must be taken to select and implement this type of mitigation option. Because every structure is different and because a structural engineer is required for this process, no firm guidelines are available herein that can be universally applied. However, some general discussion is provided to shed some light on available options for structural engineers.

Post tensioning in one or two dimensions, or by encasement in conventional reinforced concrete, is currently used as a mean to restore the integrity of the structure; however, it should generally be restricted to relatively small masses of structural concrete because of the huge forces that may result from the expansive process due to ASR (Rotter 1995, CSA 2000). Post-tensioned tendons or cables are considered to be an effective solution for thin arch dams (Singhal and Nuss 1991) or structural members of bridge/highway structures; however, they may be less attractive for large concrete structures because of the necessity of periodic destressing (Rotter 1995). Strengthening by introducing reinforcement with straps, steel plates and tensioning through bolts was also found to be effective in providing containment for selected ASR-affected concrete members (Wood and Angus 1995). Methods to restrain expansion and movement in mass concrete foundations such as tower bases have also included rock anchors and/or encapsulation (Bérubé et al. 1989). Figure 8 illustrates a case in South Africa in which an ASR-affected bridge was remediated by removing damaged concrete, encapsulating the section with new reinforced concrete, and installing prestressing cables through the repaired section.

Care should be taken in designing the encapsulating element because, if sufficient reinforcement is not provided to control stresses due to ASR expansion, the only beneficial effect of encapsulation may be to limit the ingress of moisture (CSA 2000). Strapping or encapsulation of ASR-affected reinforced concrete columns by or with composite materials may be an interesting solution provided that sufficient structural strengthening is assured (Carse 1996).

For certain applications, such as a pavement suffering from ASR-induced expansion, a viable option to extend the service life is to remove sections of concrete near the joints by saw cutting. Removing these sections is helpful in eliminating joint-related failures and minimizing ride quality issues. The sections that have been removed can be replaced by sound concrete, with careful attention paid to restoring the intended joint details (opening, dowel bars, etc.). This approach has been done on a much larger scale for concrete dams, where large slots have been cut to accommodate future expansion. It should be noted that this approach (saw cutting/joint cutting) only relieves stresses but does nothing to address the root cause of the expansion. It is common for this method to be performed repeatedly as expansion continues and negates the benefits achieved from the previous concrete removal.

ASR-affected pavements are frequently overlaid, mainly with hot-mix asphalt (HMA) but sometimes with portland cement concrete (PCC). Asphalt overlays will likely have little, if any, positive impact in terms of reducing ASR-induced expansion. In fact, internal moisture can be trapped within (and still available from below), helping to promote ASR, and temperatures within the ASR-affected concrete will tend to increase with the dark color of the HMA layer attracting heat. PCC overlays have been used to a lesser extent than HMA overlays but may show promise for unbounded overlays, where the ASR-affected pavement will not reflect cracking into the overlay.

(A) Series of photographs showing strengthening of ASR-affected bridge in South Africa (after Fournier et al. 2004). (B) General view of a highway structure affected by ASR in South Africa. Cracking due to ASR was observed in the pile caps supporting reinforced concrete columns. The cracked concrete was first removed. (C) Additional steel reinforcement was added around the pile cap. (D) External strengthening was provided by means of prestressed cables. (Photos courtesy of R.E. Oberholster, PPC Technical Services, Cleveland, South Africa). Photo A shows the underside of a bridge deck with columns in the distance. Photo B shows a concrete pile cap covered in plastic sheeting. Photo C shows a concrete pile cap surrounded by steel reinforcement. Photo D shows a cracked concrete pile cap with several square cuts on the side with reinforcing steel protruding from the cuts.
Figure 8. (A) Series of photographs showing strengthening of ASR-affected bridge in South Africa (after Fournier et al. 2004). (B) General view of a highway structure affected by ASR in South Africa. Cracking due to ASR was observed in the pile caps supporting reinforced concrete columns. The cracked concrete was first removed. (C) Additional steel reinforcement was added around the pile cap. (D) External strengthening was provided by means of prestressed cables. (Photos courtesy of R.E. Oberholster, PPC Technical Services, Cleveland, South Africa)

6.3. Selection of Mitigation Measures

This section provides guidance on how an owner/agency can select and implement mitigation options for ASR-affected structures. As mentioned throughout this section, it is not possible at this point in time to develop a definitive, step-by-step methodology for selecting mitigation measures for ASR for several reasons, including:

  1. There are few documented case studies, accompanied by adequate monitoring, to quantify accurately the benefits of various mitigation options.
  2. Some of the available technologies are still in their infancy and can only be considered experimental at this point in time.
  3. Every structure is different, and as such, there are no "one size fits all" approaches that will work across the gamut of structures being used in transportation applications.
  4. When ASR-induced expansion has affected the structural integrity of a structure and/or when strengthening or confinement are being considered, a structural engineer is required to design and implement the selected option. As such, it is not possible to provide broad guidance on such complex situations.

Having stated the above challenges, this section will attempt to provide a framework by which an owner/agency can make an informed decision on potential mitigation measure(s) for a given structure. The ultimate decision on what option to select, if any, will be the responsibility of the owner/agency, and this decision may be affected by many factors, including technical, economic, and others. Lastly, it should be reiterated that none of the options described herein are expected to be permanent in their effects, but rather they should realistically be viewed as methods of extending the service life of a structure. As such, selecting and implementing mitigation option(s) is just one step in the process. Monitoring of the performance of the mitigated structure is essential in terms of quantifying the efficacy of the treatment and in terms of planning future actions.

Figure 9 illustrates the overall approach to selecting mitigation measure(s) for a given ASR-affected structure. This approach combines an integration of information gathered under Section 6.1 (decision factors for selecting mitigation options) and Section 6.2 (discussion on available mitigation options) to select and implement the mitigation measure(s). The figure also illustrates the critical role of monitoring (discussed briefly in Section 6.4) whatever mitigation measure is implemented, and to evaluate the output from the monitoring program as part of the future management of the structure.

To aid in linking the structure-specific decision factors to the selecting of mitigation measure(s), Table 10 is provided, which highlights the key features of the various mitigation option(s) as they relate to specific applications in transportation structures. The output from this overall approach may actually be a combination of mitigation measures, which can then be evaluated for economic and practical feasibility. Certain mitigation options, such as attempting to improve drainage or increasing focus on routine maintenance of the structure, are universally recommended for all ASR-affected structures as the benefits are high and the costs are typically low. Other mitigation options, such as external confinement or strengthening, are reserved for more extreme cases and only when performed under the supervision of a qualified structural engineer. Lastly, some potential mitigation measures, such as CO2 injection, are not included in the table as they are not deemed to be viable from a technical, safety, or practical perspective.

Table 10 also highlights some of the gaps in our current understanding of certain mitigation measures. For example, there have been very few applications of electrochemical methods in driving lithium into ASR-affected structures. The results of an ongoing field study in Houston (under FHWA Project DTFH61-02-C-00097) are quite encouraging in terms of lithium penetration, as described earlier, but concerns have also been raised because sodium and potassium already present in the concrete have been drawn to the reinforcing steel, potentially increasing the potential for ASR in this locality. This table, in essence, is a work in progress, and it is hoped that field trials conducted in the future will be well documented and monitored; helping to close the gaps in our understanding and to provide more definitive guidance in future reports.

Global flow chart for selecting, implementing, and monitoring mitigation measures for ASR-affected structures. This flowchart shows the overall approach to selecting mitigation measures for a given ASR-affected structure, combining the information gathered in Sections 6.1 and 6.2, decision making in Section 6.3 and monitoring actions in Section 6.4.
Figure 9. Global flow chart for selecting, implementing, and monitoring mitigation measures for ASR-affected structures.

Table 10. Summary of mitigation options and applicability to transportation structures
Mitigation measure Applicability to specific transportation structure Positive attributes of mitigation measure Negative attributes of mitigation measure Other relevant information
Improved drainage and enhanced maintenance All structures benefit from less contact with water. Benefits are most obvious where drainage problems are most pronounced. Water is essential to ASR process - lowering RH below 80 percent leads to suppression of ASR-induced expansion. May not be as effective when source of moisture is from below (e.g., pavement) or from behind (wing wall in bridge structure). Should be included in overall management strategy for ASR-affected structure, due to high benefit/cost ratio.
Application of penetrating sealers (silanes, etc.) Most applicable to bridge structures, highway barriers, etc. Proven to reduce internal RH in laboratory and field trials. Best when element is easily accessible (e.g., highway barrier) and is not in direct contact with water or saturated soil. Benefits may not be observed when element is directly or permanently exposed to moisture (sealers need wet/dry cycles for internal RH to decrease). Must apply to dry surface. Typically need to re-apply every five years, sooner when surface is subject to abrasion or UV radiation. Sealer must be breathable.
Application of cladding Applicable to certain bridge elements. Can be effective in reducing ASR, provided that concrete below cladding is not saturated and able to sustain ASR. Can trap in moisture. Also, it is difficult to inspect element after cladding is placed on top of it. Should take measures to dry out concrete before applying cladding layer.
Application of lithium compounds Applicable to certain bridge elements and pavements. Lithium has been shown in laboratory studies to suppress ASR in small specimens. Electrochemical methods appear effective in increasing depth of penetration. Effectiveness in topical applications is minimal, due to lack of penetration. May be helpful if ASR is exacerbated at surface due to external source of alkalies (yet to be proven). Electrochemical methods cause K+ and Na+ to migrate to steel, possibly exacerbating ASR in this region. Minimal depths of penetration when applied with vacuum. Although optimistic results have been found in laboratory, this technology remains experimental in field applications, due to lack of monitoring/documentation proving its long-term efficacy. Ongoing research under FHWA funding should help to quantify the effects of lithium treatment on transportation structures.
Crack filling Applicable to most structures. Flexible caulking or crack fillers work best (as opposed to rigid polymer- or cement-based products). Can be effective in reducing ingress of water and Cl. Only provides benefit in slowing down ingress of water, chlorides, etc. Does not increase structural integrity or restore mechanical properties. Flexible caulking is especially beneficial when crack widths are large and structure is still expanding.
Application of restraint to confine/ strengthen structure Most applicable to columns (especially circular). Applying sufficient confinement can help to manage stresses generated by ASR .Can use FRP, internal/external reinforcement, etc. Difficult to confine many structural elements (e.g., square columns). Qualified structural engineer required to design and implement. Qualified structural engineer must design and implement selected technique, and he/she must monitor subsequent strains to ensure that mitigation is effective and safe.
Saw cutting/slot cutting Most applicable to pavements and bridge decks (at joints). Can help to accommodate stresses and joint-related failures. Does not address underlying causes of ASR, and in fact, allows it to continue unimpeded. Must ensure proper joint details (dowel bars, opening, etc.) when removing concrete near joints of pavements or bridge decks.

6.4. Monitoring of Structures After Mitigation/Remediation

It is absolutely imperative that structures in which mitigation measures are applied be monitored to quantify the long-term effects on the future progression of ASR. It is the current lack of this type of data and information that makes it so difficult to provide definitive guidance on appropriate mitigation measures for ASR-affected structures. Detailed information on methods to instrument and monitor structures affected by ASR is contained elsewhere in this report and is not repeated herein, for conciseness. However, the type of information and data needed includes:

  • Rates of expansion- using imbedded gauge studs and appropriate gauge (e.g., DEMEC).
  • Cracking- using cracking index (CI) approach.
  • Relative humidity-using portable humidity meters and imbedded measurement ports, or equivalent method. For mitigation measures aimed at reducing internal RH, it is important to establish baseline RH values prior to application of mitigation measures, thereby enabling one to quantify subsequent reductions in internal moisture.
  • Petrographic evaluations- including damage rating index to track the progression of internal distress after application of mitigation measure.
  • Visual inspections-including photographs taken regularly to track overall appearance of structure and specific manifestation of cracking, etc.
  • Strain measurements on external confinement- specifically when FRP wraps are applied to attempt to confine expansion due to ASR. Data can be used to determine if excessive expansions are developing and nearing limits of confining wrap.
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Updated: 06/13/2012