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Publication Number: FHWA-RD-01-164
Date: March 2002

Section 3

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This chapter presents the third and final guideline prepared as part of FHWA Project No. DTFH61-96-C-00073 entitled Detection, Analysis, and Treatment of Materials-Related Distress in Concrete Pavements. It consists of two major sections. The first section addresses treatment and rehabilitation of materials-related distress (MRD) affected pavements and the second section discusses the prevention of MRD in new concrete pavement construction. In the case of the former, it is assumed that the first two guidelines were used to evaluate the pavement under consideration. The latter section assumes that the State highway agency (SHA) has conducted a thorough investigation to determine the types and extent of MRD commonly affecting their pavements, and wants to prevent similar occurrences in new construction.

Treatment and rehabilitation of MRD-affected pavements

This section presents information on selecting the most appropriate treatment or rehabilitation options to address MRD in concrete pavements. In addition, means of evaluating and selecting the proper repair materials and techniques are also presented. The process involves consideration of a large number of technical, economical, and practical factors, as well as coordination with the overall pavement condition and future rehabilitation plans. For example, serviceable concrete repairs can result only if the MRD is correctly identified, the proper materials and methods selected, and good construction practices followed. The proposed solution must also be economically feasible in that it should provide a cost-effective solution in comparison to other potential alternatives. Finally, the proposed solution must be practical, being able to be performed using available materials, techniques, and equipment.

Figure III-1 presents a flowchart for selecting the preferred treatment or rehabilitation option. Although the overall process is the same regardless of the type of MRD exhibited, portions of the guidelines — such as the selection of feasible alternatives and repair materials — are further broken out by MRD type. The overall process also considers means to address pavements exhibiting multiple distress types, including distresses that are not caused by durability problems.

The first step is to identify the extent and severity of MRD in the existing pavement as described in the first two guidelines in this series. The identification and analysis of MRD is a critical step in the selection of the most appropriate treatment or rehabilitation option. Not only is it important to identify the existing distress, but it is also important to understand the cause of the distress to prevent its recurrence. Assessing the rate of deterioration is also important to determine the progression in the deterioration process.

Once the type, extent, and severity of the MRD are characterized, the next step involves the selection of feasible alternatives, which differ depending on the type(s) of MRD identified. Although the overall process is the same, the feasible alternatives will be different for each MRD. These differences are addressed through corresponding tables that are referenced for each MRD.

Figure III-1. Flowchart for selecting preferred treatment and rehabilitation options.
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Text Box: Figure III-1.  Flowchart for selecting preferred treatment and rehabilitation options.


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The final step is to select the most appropriate option from the list of feasible alternatives. This process involves an evaluation of many considerations, including potential constraints, future rehabilitation activities, expected performance, and life cycle costs.

Techniques and Materials for Treatment and Rehabilitation

Methods to address existing MRD in concrete pavements are divided into treatment methods (those designed to prevent further development of the distress or to reduce its rate of deterioration) and rehabilitation methods (those designed to remove deteriorated areas and to maintain or restore pavement serviceability). For the sake of avoiding redundancy, a brief overview of all potential treatment and rehabilitation alternatives for addressing MRD is first presented. The practical alternatives for each MRD are then presented later in this guideline under separate headings.

Available treatment methods include chemical treatments, joint and crack sealing, crack filling, surface sealing, and retrofitted drainage. Drying and restraint are two other treatment methods that show promise in some applications, but are not practical for pavement applications, and are thus not discussed. The purpose of the various treatments is to arrest the mechanisms that cause distress or at least delay the progression of the distress mechanism. The different treatment methods work in a variety of different ways. For example, chemical treatments affect the chemical reactions that cause the distress. Retrofitted drainage, on the other hand, can be effective if it limits the amount of available water in the pavement system that would otherwise exacerbate the distress.

Rehabilitation methods follow a totally different approach to addressing MRD. Rather than altering the development or progression of MRD, their purpose is merely to repair deteriorated areas to maintain serviceability and possibly extend the life of the pavement. Rehabilitation methods include partial- and full-depth repairs, slab replacement, diamond grinding, reconstruction, and recycling.

The most economical solution to MRD is to address the durability problem in design and thereby prevent the distress from ever occurring. This is discussed in the last section of this guideline. However, durability problems do occur, as evidenced by the many miles of existing pavement exhibiting MRD. Therefore, there remains a primary need to apply methods that can address durability problems that exist in the field.

Overview of Treatment Methods

Chemical Treatments

Chemical treatments are often used in fresh concrete to prevent deleterious reactions. In some cases, they can also be effective if applied to the surface of existing concrete pavements. The main shortcoming in this approach is obtaining significant penetration of the chemical treatment through the depth of the slab. Several applications of the treatment can help increase the depth of penetration.

Chemical treatments can work in several ways, as follows:

Treatment with lithium salts is an example of a chemical treatment that arrests an existing reaction. Lithium salts prevent abnormal expansion due to alkali–silica reactivity (ASR). Other types of treatments are discussed as appropriate for each distress type.

Joint and Crack Sealing

Moisture is a key ingredient to the development of many durability-related distress types. As a result, these distress types are often observed to be much more severe at joints and cracks, where moisture can penetrate the concrete from several surfaces. Sealing joints and cracks can help reduce the amount of infiltrated water. However, the effectiveness of sealing depends on several issues:

It is impossible to completely eliminate the intrusion of moisture at joints and cracks, especially for an extended period of time. A Minnesota study found that the amount of infiltrated water (measured using tipping buckets at drainage outlets) returned to the same levels within 2 weeks after resealing (Hagen and Cochran 1995). This confirms that moisture can continue to penetrate the concrete at joints and at other locations, making this approach a short-term solution under the best of circumstances.

Crack Filling

Crack filling must first be disassociated from crack sealing. In the context of concrete pavements, crack filling refers to the filling of surface cracks (e.g., map cracking) with a material that penetrates into cracks, not the sealing of individual full-depth transverse and longitudinal cracks. The primary purpose of crack filling is to strengthen the concrete pavement. Crack fillers penetrate into surface cracks and effectively “glue” the concrete pieces together. However, crack filling provides the added benefit of partially sealing the surface to prevent infiltration of moisture and other harmful constituents. An example of a crack filling material used by highway agencies on MRD pavements is high molecular weight methacrylate (HMWM).

Surface Sealing

Surface sealing or coating helps prevent the ingress of moisture into the pavement, which can help deter the initiation or alleviate the extent of moisture-induced distress. However, sealers only treat the surface, so concrete pavements are still exposed to moisture from the sides and bottom. Consequently, their effectiveness is limited, especially on slab-on-grade designs in which the slab is in direct contact with the subgrade. Sealers can also reduce or prevent the ingress of oxygen, carbon dioxide, chloride ions, sulfate ions, and other constituents that contribute to damaging reactions.

Concrete sealers can be divided into two categories—coatings and penetrants. Coatings form a film on the pavement surface, whereas penetrants are designed either to fill the pores or to line them with a water-repellent substance (Campbell-Allen and Roper 1991). Penetrants can be considered a misnomer, because they do not really penetrate very far beneath the surface. Like all sealers, they are thus subject to being worn off by traffic. Examples of surface sealers include silane sealants, penetrating oils, and two-part resins.

Sealers have proven to be effective in laboratory testing where concrete samples can be sealed on all sides. However, as with chemical treatments, sealers can only be applied to the surface of existing pavements. Moisture and other constituents can still penetrate the concrete vertically through the bottom and laterally through the sides of the slab. Consequently, sealers are most effective at limiting constituents that infiltrate from the pavement surface, such as water and chloride ions from deicing salts.

Retrofitted Drainage

The addition of retrofitted drains will, in theory, remove moisture from under the slab and at joints and cracks, which would assist in slowing or delaying MRD, since most deleterious expansion involves water (e.g., expansion due to ice formation, ASR gels imbibing water, etc.). In reality, however, water cannot readily move through a dense-graded base (typically found in many older, deteriorating pavements) to the retrofitted drains at the edge of the pavement. Therefore, the effectiveness of retrofitted drains is reduced.

Moisture-induced distresses generally initiate and progress more quickly at the bottom of the slab, which is exposed to moisture for prolonged periods. Because the bottom of the slab is not exposed to the drying effects of the environment (such as the sun and wind), even a light rain can saturate the underlying layers, which may remain saturated for prolonged periods. Providing a means to remove moisture at the slab-base interface will help shorten the time the pavement is exposed to moisture. In order to be effective, retrofitted drainage must be applied during the early stages of deterioration. By the time moisture-related distresses are apparent on the surface, deterioration at the bottom of the slab has often progressed to the point where retrofitted drainage will no longer be effective.

Overview of Rehabilitation Methods

Partial-Depth Repairs

Partial-depth repairs are one rehabilitation method that can be used to repair localized deteriorated areas caused by MRD. These repairs consist of the removal of concrete near the surface and replacement with an acceptable patch material, usually a rapid-setting material to limit closure time. However, their effectiveness is limited to smaller areas where the deterioration is confined to the upper one-third of the concrete slab. Partial-depth repairs are most commonly performed along transverse and longitudinal joints.

Partial-depth repairs are not an ideal repair for many MRDs because the deterioration is often worse at the bottom of the slab, where moisture and deleterious chemicals are more readily available. In such cases, partial-depth repairs are not a practical treatment because they do not address the root cause of the deterioration, and the patch itself will likely become debonded and quickly deteriorate.

Full-Depth Repairs

Full-depth repairs are generally a better alternative than partial-depth repairs for addressing pavement deterioration caused by MRD concentrated at joints or cracks. These repairs consist of the removal of isolated deteriorated areas through the entire depth of the slab and replacement with a high-quality material. Full-depth repairs are a widely used means of repairing localized deterioration at joints or cracks. As previously noted, most types of MRD are generally more severe along joints and cracks due to increased exposure to water and deleterious substances, which makes full-depth repairs an appropriate repair method.

As with all repair methods, full-depth repairs should be viewed not as a solution to a durability problem but rather as a means to extend the life of the pavement. Because the problem is materials related, it cannot be completely remedied by replacing a portion of the pavement. However, full-depth repairs of badly deteriorated areas can improve the serviceability and buy additional life for the pavement.

Slab Replacement

For deteriorated areas that are not isolated along joints or cracks or for large areas of deterioration, slab replacement may be a better alternative than full-depth repairs. This is particularly true if there are isolated instances where distress is not confined to the joint area. The problem is that MRD generally occurs throughout the entire project and is not likely to be limited to an isolated number of slabs. Thus, it should be recognized that slab replacement does not completely address the durability problem (unless all slabs are replaced). Slab replacement can be used in conjunction with other rehabilitation techniques to restore the pavement condition to an acceptable level.

Diamond Grinding

Diamond grinding uses closely spaced diamond saw blades to remove a thin layer of the concrete pavement surface. This corrects surface irregularities, provides a smooth riding surface, and improves the frictional characteristics. For most durability problems, the purpose of diamond grinding is simply to “buy some time” until more permanent rehabilitation can be conducted. Scaling of the concrete surface caused by deicing salts is an example of a condition that can be corrected by diamond grinding. Diamond grinding is also an effective method in conjunction with partial-depth and full-depth repairs to restore the ride quality. As with other rehabilitation methods, diamond grinding does not directly address the cause of the durability problem unless the problem was isolated to the pavement surface as can occur when poor finishing practices have contribute to the distress.

Overlay

The feasibility of an overlay for addressing MRD depends largely on the type and extent of MRD. An AC overlay or unbonded PCC overlay can provide improved service and extended life if used under the right circumstances. An AC overlay requires that the existing concrete pavement not exhibit substantial areas of deterioration that will quickly reflect through the overlay; otherwise, extensive preoverlay repairs are required. Unbonded PCC overlays are more forgiving but are also more expensive and create additional concerns such as grade changes and overhead clearances. Bonded PCC overlays are not recommended for concrete pavements exhibiting MRD.

Pavement Reconstruction

The most extreme alternative is total reconstruction of the pavement. This solution corrects an MRD problem and will prevent its recurrence if deleterious materials are not used again or are accounted for in the mix design. Because durability problems are not often limited to isolated areas within the pavement, reconstruction is often the only practical alternative, especially where the MRD is extensive and has progressed to high-severity levels. In these cases, the guidance provided in the latter portion of this guideline should be used to ensure that a durable pavement is constructed.

Pavement Recycling

Another form of reconstruction is pavement recycling, which involves removal and crushing of the existing concrete for use as aggregate in the reconstructed pavement. Recycling offers several benefits over reconstruction with all new aggregate, including reduced cost and conservation of materials. Recycling has been a viable option for decades but has only recently gained acceptance for pavements exhibiting durability problems. In such instances, adjustments to conventional mix design procedures (e.g., crushing coarse aggregate to smaller size to prevent freeze-thaw deterioration of aggregate, addition of pozzolans to mitigate ASR, etc.) are required to prevent or limit the recurrence of durability problems in the recycled pavement. With these adjustments, however, recycling of concrete pavements with MRD can provide performance comparable to that of conventional mixes (Wade et al. 1997).

Feasibility of Available Techniques

A “feasible” alternative must address both the current condition and future performance of the pavement. The identification of feasible alternatives varies significantly depending on the type(s) of MRD occurring. The following sections provide guidelines, broken out for each particular MRD type, for selecting feasible alternatives. For each MRD type, the following issues are discussed:

It is not uncommon that more than one type of MRD may be at work within a given pavement. Fortunately, many of the treatment and rehabilitation alternatives are equally effective for a number of different MRD types. But it is important to consider the feasibility of the alternatives based on all the mechanisms at work if multiple mechanisms are observed.

Freeze-Thaw Deterioration of Aggregate

Freeze-thaw deterioration of aggregate, commonly known as D-cracking, is caused by freezing and thawing of water absorbed into susceptible coarse aggregate particles. The treatment/rehabilitation method must focus on eliminating one or more of the conditions that causes freeze-thaw deterioration, specifically susceptible aggregates, freezing temperatures, or available moisture. Treatment of in-place susceptible aggregates is not a feasible alternative. And although placement of an overlay can reduce the number of freeze-thaw cycles by reducing the exposure to severe conditions, it is impossible to eliminate freeze-thaw cycling in harsh climates. Thus, treatment methods focus on eliminating the amount of available moisture in the pavement. Rehabilitation methods are generally considered a temporary fix to extend the life of the pavement until major rehabilitation is performed.

Available Treatment Methods for Freeze-Thaw Deterioration of Aggregate

Most treatment methods are designed to reduce the amount of moisture in the pavement system. Such methods include sealing joints and cracks, sealing the pavement surface, and retrofitting drains. Many question the effectiveness of any method designed to eliminate moisture. Although the amount of water from surface infiltration can be reduced, it can never be completely eliminated. In addition, the underlying subgrade commonly remains continually moist, even when the most elaborate drainage system is used. Furthermore, water can enter the pavement from other sources, such as laterally from ditches and upward from the groundwater table. Nonetheless, there is still some value in taking measures to reduce the amount of available moisture. However, such methods should be used with caution and are recommended only under certain conditions.

Freeze-thaw deterioration of aggregate typically initiates at joints and cracks where water is allowed to infiltrate, so any measures that prevent the intrusion of water can be effective. Sealing joints and cracks will limit the amount of infiltrated water, although it will not completely eliminate infiltration. Moisture can still penetrate through well-sealed joints as well as from other sources.

Similar concerns are also valid for surface sealers. Their usefulness is questioned because freeze-thaw deterioration of aggregate typically initiates at the bottom of the slab and propagates upward. Nonetheless, surface sealers have been used to combat the effects of freeze-thaw deterioration. Laboratory testing found that water-based and solvent-based silane sealers slowed the rate of deterioration, whereas penetrating oils and two-part resins were not as effective (Janssen and Snyder 1994). Similar results were obtained in the field. Another field experiment using silane sealers indicated mixed results, although they were found to be more effective on pavements with less deterioration (Engstrom 1994).

Another treatment method designed to reduce the amount of available moisture is the addition of retrofitted drainage, although its effectiveness is also questionable. An important consideration is the permeability of the layers beneath the slab. Studies have shown that if water cannot move within the underlying layers to the edgedrains, the drainage system will be only marginally effective at removing water and even less effective at reducing moisture-related distress (Smith et al. 1996). When used solely to address freeze-thaw deterioration, retrofitted drainage provides limited benefits and is not cost effective, mainly due to the inability to move infiltrated water to the drains. However, if other moisture-related distresses are present, such as pumping or corner breaks, retrofitted drainage is more cost effective as an all-inclusive treatment method.

Another treatment method is filling cracks with a bonding material such as an HMWM. This type of treatment strengthens the concrete by effectively “gluing” the cracked concrete pieces together. Such treatments have been most effective when applied to cracks that are wide enough for the material to penetrate (Engstrom 1994). On the same note, HMWM should only be applied at joints and cracks; there are no benefits to applying it to the entire pavement surface unless cracking is present throughout the pavement. Field experiments found that HMWM were effective for up to 18 months, which implies that reapplication at such intervals may be required.

Available Rehabilitation Methods for Freeze-Thaw Deterioration of Aggregate

Rehabilitation methods for addressing freeze-thaw deterioration of aggregate involve the removal and replacement of distressed concrete areas. Feasible repair methods include partial-depth repairs, full-depth repairs, overlays, reconstruction, and recycling. Some pavement engineers believe that repair methods are not effective means to repair freeze-thaw deteriorated areas because they do not address the cause of the distress. However, depending on the objective (such as to extend the life of the pavement), the repair methods can be effective, especially in the short term.

Freeze-thaw deterioration is often confined to transverse joints, and is typically more severe at the slab surface and bottom. In isolated cases, it has been treated using partial-depth patching in which the deteriorated surface is removed using cold milling. This procedure does nothing to restore the deteriorated bottom of the slab, but does restore rideability, at least in the short term. More commonly, full-depth repairs are used as an effective method of rehabilitation. However, they also create an additional joint, where deterioration is likely to appear within 5 years. Treating the joint and possibly the adjacent concrete with a surface sealer can help delay the onset of the deterioration. Even with preventive measures, full-depth repairs should be viewed only as a means to extend the life of the pavement.

The placement of an asphalt concrete (AC) overlay has been the most common form of rehabilitation on concrete pavements exhibiting freeze-thaw deterioration of aggregate (Schwartz 1987). One philosophy is that by covering the concrete, it will not be directly exposed to air and deicers, thus effectively reducing the number of freeze-thaw cycles. But an AC overlay can never completely eliminate freeze-thaw cycling in harsh climates. Previous studies have shown that in order to stop the progression of freeze-thaw deterioration, freezing must be completely prevented. Even though an overlay decreases the number of freeze-thaw cycles, its placement can actually accelerate the rate of deterioration by increasing saturation (Janssen 1985; Janssen et al. 1986). Consequently, AC overlays may not be very effective when placed over a pavement affected by freeze-thaw deterioration and should be used cautiously unless the underlying concrete is rubblized.

An unbonded portland cement concrete (PCC) overlay is another rehabilitation option. Again, the overlay will not prevent freeze-thaw cycling in the underlying pavement. However, an unbonded PCC overlay can be more effective because its performance depends less on the condition of the underlying pavement; providing uniform support is the most important consideration. Rubblizing the existing pavement before placing the overlay will prevent continued deterioration from freeze-thaw deterioration but will likely result in the need for a thicker overlay.

The final alternative is removal and reconstruction of the pavement. Of course, this alternative is most cost effective on badly deteriorated concrete pavements, where treatment methods are ineffective and repairs are too numerous and costly. Recycling of the deteriorated concrete pavement as aggregate for the new pavement offers a variation within the reconstruction alternative. With special design considerations to prevent the recurrence of freeze-thaw deterioration (e.g., using a smaller top aggregate size and limiting the amount of recycled fines), recycling has proven to be an effective rehabilitation method.

Selection Guidelines for Freeze-Thaw Deterioration of Aggregate

As with any distress, the most appropriate treatment or rehabilitation method for pavements exhibiting freeze-thaw deterioration of aggregate depends on the extent and severity of deterioration. Table III-1 provides some basic guidelines for selecting feasible repair alternatives based on the extent and severity of deterioration observed during the field data collection process described in guideline I. These should be viewed as rough guidelines for selecting potential feasible alternatives. Using the data recorded in Guideline I in figures I-8 through I-11 and the photo log, the severity of the distress can be assessed using the following guidance:

In general, pavements exhibiting low-severity freeze-thaw deterioration of aggregate can be addressed through treatment methods because the deterioration is not creating a serviceability problem. Such methods may prevent further progression of freeze-thaw deterioration to higher severity levels and extend the pavement life. High-severity freeze-thaw deterioration, on the other hand, generally requires removal and repair of the distressed area. At this point, the pavement is too deteriorated, and treatment methods no longer offer an effective means of addressing the problem. For pavements exhibiting moderate-severity deterioration, the preferred method—treatment or rehabilitation—will further depend on the extent of deterioration.

The extent of deterioration also influences the specific treatment or rehabilitation method to be used. Freeze-thaw deterioration of aggregate typically initiates at the slab corners and progresses along the transverse joint. If deterioration is limited to these areas, the most effective methods are those that reduce the amount of available moisture at joints (for low-severity) or localized repairs (for higher severity). As freeze-thaw deterioration begins to progress along longitudinal

Table III- 1. Selection of feasible alternatives to address freeze-thaw deterioration of aggregate.

Severity

Extent

Feasible Alternatives

Comments

Low

Corners

Seal the pavement

Seal joints and cracks

Treatments should be aimed at limiting the amount of available moisture and delaying the progression to higher severity levels.

Joints and cracks

Seal the pavement
Seal joints and cracks

Moderate

Corners

Seal joints and cracks
Partial- or full-depth repairs
Overlay

Limit moisture to prevent further deterioration.
Full-depth repairs should be considered a temporary fix (extend life about 5 years).
HMWM requires reapplication to be most effective (about every 18 months).

Transverse joints and cracks

Seal joints and cracks
Apply HMWM
Partial- or full-depth repairs
Overlay

Longitudinal joints

Seal joints
Apply HMWM
Overlay
Recycling
Reconstruction

Damage is too widespread to repair each area.
Recycling of deteriorated pavements has proven to be a feasible alternative.

High

Corners

Full-depth repairs

Deterioration is too severe for treatment; deteriorated areas must be removed and replaced.

Transverse joints and cracks

Full-depth repairs
Recycling
Reconstruction

Longitudinal joints

Recycling
Reconstruction

Deterioration is too severe and too extensive for treatment or restoration.

joints and further into the slab interior, localized methods become less and less effective. Methods need to be employed that address the entire pavement, such as a surface sealer (low severity) or reconstruction (high severity). In the end, it is more cost effective to reconstruct the entire section than it is to conduct extensive full-depth patching (Hoerner et al. 2001).

Regardless of the extent of freeze-thaw deterioration, cores should be taken at representative locations throughout the pavement to examine the extent of deterioration through the depth of the slab. At least two cores should be taken along transverse joints, longitudinal joints, cracks, and midslab locations. The extent of deterioration at the bottom of the slab is just as important as the surface condition when selecting the most appropriate treatment method.

For low-severity freeze-thaw deterioration of aggregate, treatment methods designed to limit the exposure to moisture will be most effective. An important point to remember is that freeze-thaw deterioration of aggregate commonly initiates at the bottom of the slab, where moisture is more readily available, and propagates upward, so methods designed to prevent surface infiltration will only be partially effective. In other words, they may reduce the rate of freeze-thaw deterioration but will never fully arrest its development.

With that said, the best treatment method to address low-severity freeze-thaw deterioration is a combination of sealing/resealing joints and cracks and the application of a surface seal. Sealing joints and cracks will prevent excess moisture from infiltrating into the pavement and saturating the underlying layers. Meanwhile, the surface sealer will prevent the absorption of water through the surface, although it may require reapplication due to traffic wear. The surface sealer should be placed over the entire pavement surface to help slow the rate of deterioration and to prevent further deterioration. A water-based or solvent-based silane sealer is recommended.

For moderate-severity freeze-thaw deterioration of aggregate, the application of an HMWM has shown favorable results. To be effective, the cracks should be wide enough for the material to penetrate, which is the reason HMWM is not recommended for low-severity deterioration. Where the deterioration is limited to transverse and longitudinal joints and cracks, the application of an HMWM is recommended. The material should be applied only to cracked areas and will need to be reapplied at approximately 18-month intervals. The only other potential treatment method is sealing joints and cracks. The use of surface sealers is not recommended for deterioration that has progressed to moderate-severity levels. In terms of rehabilitation methods, full-depth repairs have been found to be the most cost-effective method for addressing moderate-severity freeze-thaw deterioration that is confined to transverse joints and cracks (Schwartz 1987). Full-depth repairs are also recommended in conjunction with treatment methods.

Where freeze-thaw deterioration has progressed to longitudinal joints or throughout the entire slab, full-depth repairs will no longer be cost effective. In such cases, methods that address the entire pavement need to be employed (e.g., overlay, reconstruction, or recycling). An AC overlay will provide a smooth-riding surface and years of service but will not prevent the further progression of freeze-thaw deterioration. This continued deterioration must be addressed in design by either placing a thicker overlay than normal or accepting a shorter service life. Other overlay options, such as rubblizing or recycling, are generally not cost effective for moderate-severity deterioration (Hoerner et al. 2001). A more effective solution is to allow the pavement to deteriorate (i.e., wait until distress progresses to high severity) before conducting such extensive rehabilitation.

For high-severity freeze-thaw deterioration of aggregate, treatment methods are not effective. The only applicable methods are those involving the removal and replacement of badly deteriorated areas, of which full-depth repairs are the most effective method. However, this method should be viewed as a temporary fix to extend the life of the pavement and must consider the overall pavement performance. For example, it is impractical to conduct extensive full-depth patching when the entire pavement will require reconstruction within a few years.

On pavements with freeze-thaw deterioration at nearly every joint (some of which is high severity), the most effective alternative is to either reconstruct or rubblize and overlay the entire pavement. At this point, treatment and restoration methods will no longer be effective. As a form of reconstruction, recycling of the damaged pavement as aggregate for the new pavement can be effective if appropriate measures are taken to prevent the recurrence of freeze-thaw deterioration. Another alternative is to rubblize the deteriorated pavement for use as a base course for a new pavement. This alternative involves other considerations, such as grade changes and overhead clearances, that must be addressed as part of the overall selection process.

Freeze-Thaw Deterioration of Cement Paste

Freeze-thaw deterioration of cement paste is caused by repeated freeze-thaw cycles of the saturated cement paste. Freezing of moisture within the concrete produces internal stresses that lead to deterioration if an adequate air-void system does not exist. Thus, as with freeze-thaw deterioration of aggregate, treatment methods focus on eliminating, or at least reducing, available moisture or freezing temperatures. Similarly, rehabilitation methods to address freeze-thaw deterioration of cement paste are considered temporary fixes to restore serviceability and extend the life of the pavement until more permanent rehabilitation techniques are conducted.

Available Treatment Methods for Freeze-Thaw Deterioration of Cement Paste

Treatment methods to address freeze-thaw deterioration of cement paste focus on reducing the amount of moisture or the number of freeze-thaw cycles. If neither moisture nor freeze-thaw cycles are present, the deterioration process will not take place. However, moisture is available from a variety of sources, so eliminating moisture in the pavement is difficult, if not impossible. Likewise, freezing temperatures are present throughout most of the United States. Feasible treatment methods include sealing joints and cracks and applying a surface seal. To be effective, these measures need to be performed during the early stages of the deterioration process.

Sealing joints and cracks is one means of reducing the amount of infiltrated water. Although this method will never be totally effective, it can reduce the amount of excess moisture that is allowed to infiltrate into the pavement system. The effect (if any) that this reduction in moisture infiltration will have on reducing freeze-thaw deterioration is debatable.

Another option is the application of a surface sealer. This treatment method forms a penetration barrier on the pavement surface, which expels moisture much like wax on a car. Freeze-thaw deterioration of cement paste is most extensive along the concrete surface, such as the top and bottom of the slab, where the concrete is subjected to greater exposure to the environment. Surface sealers can be effective for addressing freeze-thaw deterioration at the pavement surface, and may be very effective if the air-void system is compromised only at the pavement surface due to poor finishing during construction.

Available Rehabilitation Methods for Freeze-Thaw Deterioration of Cement Paste

The purpose of rehabilitation methods for addressing freeze-thaw deterioration of the cement paste is to restore the serviceability and extend the life of the pavement. Because of the difficulties in eliminating either moisture or freezing temperatures, rehabilitation methods are likely the best alternative on pavements in which the overall air-void system is inadequate. Feasible rehabilitation methods include partial-depth repairs, full-depth repairs, diamond grinding, overlays, reconstruction, and recycling.

Partial-depth and full-depth repairs are candidates where freeze-thaw deterioration is confined to joints and cracks. The determining factor between the two approaches is the depth of deterioration. For partial-depth patching to be effective, the deterioration needs to be limited to the upper one-third of the slab. Cores taken at representative areas of freeze-thaw deterioration can help determine the extent of deterioration and the potential for future deterioration. If only isolated slabs are affected by surface scaling due to a poor air-void system at the surface, partial-depth inlays may be effectively used. When damage is found during construction to be more extensive than anticipated, the repair area should be made wider and converted to a full-depth patch, if necessary. The repair will not perform adequately if the deterioration is not completely removed.

If the freeze-thaw deterioration is more extensive, diamond grinding may be a more cost-effective alternative. To be feasible, the deterioration must be limited to the pavement surface. Scaling and map cracking isolated to the surface are examples of distress types that are good candidates for diamond grinding. Although diamond grinding will remove deteriorated areas from the pavement surface, it must be recognized that a new layer of concrete will be exposed and will also deteriorate with time if the air-void system is not adequate. Diamond grinding is also an effective method to restore ride quality in conjunction with other repair methods such as partial-depth and full-depth repairs.

Another rehabilitation option is to construct an overlay. Either an AC overlay or an unbonded PCC overlay is feasible. Given that an overlay covers the pavement surface and prevents direct exposure to the underlying pavement (reducing the depth of freezing temperatures), an overlay could also be considered a treatment method. However, its effectiveness in this regard is limited. Temperature simulations in moderate climates found that a 150-mm overlay was not sufficient to prevent freezing in the underlying concrete pavement (Janssen and Snyder 1994).

In general, bonded PCC overlays are not recommended over pavements with durability problems because of the strong likelihood of the layers becoming debonded at the interface if the deterioration continues. On the other hand, in the special case where poor finishing techniques are solely responsible for the deterioration (e.g., over-finishing negatively affected the air-void system, water was added during finishing, bleed water was trapped, etc.), the complete removal of the susceptible layer and replacement with a bonded overlay may be a viable option.

Selection Guidelines for Freeze-Thaw Deterioration of Cement Paste

A summary of the feasible alternatives for pavements exhibiting freeze-thaw deterioration of cement paste is presented in table III-2. The selection of feasible alternatives depends on the extent and severity of distress observed during the field data collection process described in guideline I. As described in table I-1, paste freeze-thaw deterioration is most often manifest as map cracking, scaling, and/or spalling. Using the data recorded in figures I-8 through I-11 and the photo log, the severity of the distress can be assessed using the following guidance:

The final selection of the preferred alternative will depend on other factors as well, such as the cost of the alternative and its compatibility with the overall plans for the pavement section.

The severity of deterioration largely determines whether the most appropriate alternative will be a treatment or rehabilitation method. As with any distress, treatment methods are more effective for addressing distress in the early stages of deterioration (generally, low-severity distress). These methods are designed to prevent or arrest the development of freeze-thaw deterioration. Conversely, rehabilitation methods are more effective for addressing distress in the later stages of deterioration. These methods concede to the deterioration mechanism by removing and replacing the distressed area.

The extent of deterioration is also a controlling factor in the selection process. Freeze-thaw deterioration of cement paste can be worse at the bottom of the slab, which is exposed to moisture trapped in the underlying layers, or at the slab surface, which is exposed to harsher freeze-thaw conditions. In addition, poor finishing may compromise the air-void system only at the surface, whereas it may be adequate through the rest of the slab depth. The deterioration can also be worse near joints and cracks, where moisture is allowed to infiltrate and saturate the concrete. To assess the extent of deterioration, cores should be taken and analyzed at representative distressed and nondistressed areas.

For pavements with low-severity freeze-thaw deterioration of the cement paste, treatment methods are recommended. Treatment methods can stop or at least slow the progression to moderate-severity levels, thus extending the life of the pavement. The recommended method for low-severity deterioration, especially for deterioration concentrated at the pavement surface, is the application of a surface sealer. The sealer should be applied to the entire pavement surface to slow the rate of deterioration and prevent further progression. Water-based and solvent-based silane sealers have been used effectively for such applications. Another feasible method is sealing joints and cracks. However, this method can not completely eliminate the intrusion of water into the pavement system and is therefore limited in its effectiveness. Sealing joints and cracks can be more effective when used in combination with surface sealing.

For moderate-severity freeze-thaw deterioration, the best treatment method depends on the extent of deterioration. If the deterioration is limited to the pavement surface, such as exhibited by scaling or map cracking, only surface repair methods are necessary. Where the deterioration is limited to small areas along joints or other isolated locations, partial-depth or full-depth repairs are recommended. Diamond grinding can be used after any repairs to remove surface irregularities and to improve the serviceability of the pavement. The application of a surface sealer after diamond grinding should be considered to prevent the recurrence of deterioration.

Partial-depth repairs and diamond grinding are not recommended on pavements where freeze-thaw deterioration is not confined to the pavement surface. In such cases, rehabilitation methods that address the deterioration through the entire depth of the slab are required. For deterioration that is limited to transverse joints or other small isolated areas, full-depth repairs are recommended. For larger isolated areas, slab replacement is more cost effective. These methods address the full extent of deterioration. Where these methods are believed to be ineffective or where the number of repairs will be too costly, an alternative is to overlay the pavement.


Table III- 2. Selection of feasible alternatives to address freeze-thaw deterioration of cement paste.

Severity

Extent

Feasible Alternatives

Comments

Low

Corners

Seal the pavement
Seal joints and cracks

Treatments should be aimed at limiting the amount of available moisture and delaying the progression to higher severity levels.

Transverse and longitudinal joints

Seal the pavement
Seal joints and cracks

Entire slab

Seal the pavement

Moderate

Corners

Seal joints and cracks
Full-depth repairs

Limit moisture to prevent further deterioration.

Joints and cracks

Seal joints and cracks
Partial-depth repairs
Full-depth repairs
Diamond grinding

Isolated areas

Partial-depth repairs
Full-depth repairs
Inlay1
Slab replacement

Scaling of surface requires repair; cannot be treated.

Entire slab

Diamond grinding
Overlay

Damage is too extensive to repair each area.

High

Corners

Partial-depth repairs1
Full-depth repairs

Deterioration is too severe for treatment; deteriorated areas must be removed and replaced.

Joints and cracks

Partial-depth repairs1
Full-depth repairs
Diamond grinding
Recycling
Reconstruction

Isolated areas

Partial-depth repairs1
Full-depth repairs
Inlay1
Slab replacement

Scaling of surface requires repair; cannot be treated.

Entire slab

Diamond grinding
Overlay
Recycling
Reconstruction

Deterioration is too severe and too extensive for treatment or restoration.

1 Appropriate if laboratory analysis confirms that poor air-void system is isolated at the surface due to poor finishing during construction.

Treatment methods should not be used on pavements with high-severity deterioration. For such deteriorated areas, rehabilitation methods that involve removal and repair of the deteriorated area are required. Deterioration confined to transverse joints and cracks can be repaired effectively with full-depth repairs. Full-depth repairs can also be used to repair small isolated areas; slab replacement is more cost effective for repairing larger isolated areas. When repairs become too abundant and costly, reconstruction is the only remaining alternative. Recycling of the pavement for aggregate offers potential savings and conservation of resources.

Deicer Scaling/Deterioration

Deicer scaling/deterioration is caused by the repeated application of deicing chemicals, which accentuate many of the same stresses generated by cyclic freezing and thawing. High thermal strains produced when the deicing chemical melts the ice and increased ozmotic pressures are also thought to contribute to the problem. The problem is more likely on concrete pavements that have been overvibrated or improperly finished, creating a weak layer of cement paste over the surface (Mindess and Young 1981). Deicers also contribute to the dissolution of some hydrated paste components that are more soluble at temperatures at or below freezing.

Available Treatment Methods for Deicer Scaling/Deterioration

The application of deicing chemicals is a necessity in cold climates to prevent accidents. Elimination of the destructive component of the deterioration is therefore not an option. Elimination of freezing temperatures is also not possible. Treatment methods must therefore focus on preventing direct exposure between the concrete and the deicing chemicals.

The only feasible treatment method for deicer scaling is the application of a surface sealer. Surface sealers reduce the ingress of moisture and the chloride ions into the pavement without disrupting the frictional characteristics of the pavement. This method should only be used for pavements exhibiting limited amounts of low-severity scaling.

Available Rehabilitation Methods for Deicer Scaling/Deterioration

Deicer scaling is limited to exposed surfaces, mainly along the pavement surface. Diamond grinding can effectively remove the deteriorated concrete from the surface and provide a smooth-riding surface. It is most cost effective when the deterioration is widely distributed. Although diamond grinding removes the existing deterioration, it also exposes another layer of concrete to the deicing chemicals. If the underlying concrete is of higher quality because the surface was compromised due to poor finishing during construction (overfinishing or excess water on the surface during construction), this is a satisfactory long-term alternative. If on the other hand, the mass of concrete has a poor air-void distribution, deicer scaling will likely occur in the newly exposed surface.

Because scaling is confined to the surface, partial-depth repairs and full slab inlays can be an effective rehabilitation method. To be cost effective, the number and size of the repair areas should be limited. Small depressions and poorly finished areas are examples of good candidates for partial-depth repairs. If the area to be repaired is large or if multiple partial-depth repairs are required, full-depth repairs or slab replacements are more cost effective.

Other feasible alternatives are placement of an overlay, recycling, and reconstruction. These alternatives all address the entire pavement area. Overlays are likely to last longer on pavements exhibiting deicer scaling as compared to other MRD if the deterioration is limited to the surface because of poor finishing. Milling of the deteriorated surface before placing the overlay will help improve the support and bonding conditions. Because the distress is limited to the surface, recycling and reconstruction of the pavement will not be as cost effective as with other MRD types that effect the concrete through the entire slab depth.

Selection Guidelines for Deicer Scaling/Deterioration

Table III-3 provides a list of feasible alternatives and some general guidelines for selecting the preferred alternative to address scaling due to attack from deicing chemicals. The selection is based on the severity and extent of distress observed during the field data collection process described in guideline I. As described in table I-1, deicer scaling/distress is most often manifest as scaling or crazing with possible staining of the surface. Using the data recorded in figures I-8 through I-11 and the photo log, the severity of the distress can be assessed using the following guidance:

For low-severity scaling, the most cost-effective option depends on the extent and expected progression of scaling. For the most part, sealing the pavement with a silane sealer is the best method. This method may prevent further deterioration or prevent the development of scaling in other areas. For scaling that is limited to isolated areas and appears to be the result of improper finishing or standing water in depressions (i.e., scaling is not likely to become widespread), partial-depth patching and/or inlays can be effective alternatives.

The same guidelines for partial-depth repairs apply to moderate-severity deicer scaling if it is isolated and not believed to be progressive. Otherwise, the recommended alternative, especially when the deterioration is extensive, is to diamond grind the concrete surface to remove all deteriorated areas. After diamond grinding, consideration should also be given to the application of a surface sealer to delay the recurrence of scaling.

For high-severity scaling due to deicing chemicals, diamond grinding may not be effective because it only removes about 3 mm of concrete from the surface. For pavements with isolated areas of high-severity scaling, partial-depth repairs can be used. For larger areas, full-depth repairs, inlays, or slab replacement may be more cost-effective. If the deterioration is too widespread to repair isolated areas, the recommendation is to construct an AC overlay or bonded concrete overlay. Milling of the existing concrete surface to remove scaled areas is suggested to improve the bonding and support conditions. Recycling and reconstruction are also options but are unlikely to be cost effective except in extreme circumstances.

Table III- 3. Selection of feasible alternatives to address deicer scaling/deterioration.

1 Appropriate if laboratory analysis confirms that poor air-void system is isolated at the surface due to poor finishing during construction.

Alkali–Silica Reactivity

ASR is a reaction between alkalis in the cement paste and reactive silica found in some aggregate sources. The reaction forms a gel product that expands in the presence of moisture. The expansion initially appears on the pavement surface as irregular, map-like cracking and can ultimately lead to joint spalling, blowups, and other pressure-related distresses. ASR is rarely confined to isolated areas such as joints and cracks but rather occurs throughout the entire pavement. Consequently, treatment and rehabilitation methods must also address the entire pavement area.

Available Treatment Methods for Alkali–Silica Reactivity

The factors controlling ASR are the amount and properties of reactive silica, the amount of available alkali, and the amount of available water (Mindess and Young 1981). The amount and properties of reactive silica and available alkalis (assuming no external sources) are controlled by the constituent materials (namely the aggregate and cement) and cannot be altered. As discussed for other MRD, methods designed to limit the amount of available water will have limited effectiveness. The use of such methods for controlling ASR is even more questionable because studies have shown that even water in the vapor phase (relative humidity greater than 80 percent) is sufficient to cause swelling of the gel product (Stark et al. 1993). In one study, the use of a silane surface sealer was found to have little to no meaningful effect (Stark et al. 1993). Although the surface sealer did prevent moisture transfer in the liquid phase, it did not prevent moisture transfer in the vapor phase.

A promising alternative for addressing ASR is treatment with lithium salts. Lithium salts were first found to be an effective treatment in fresh concrete to prevent abnormal expansion due to ASR (Stark et al. 1993). They have since been found to be effective in laboratory testing of concrete samples but have yet to receive widespread use in the field. There are currently several on-going experimental projects being conducted to evaluate their effectiveness, and early results have been favorable (Stark et al. 1993; Johnston 1997). The major limitation for field applications is achieving penetration of the lithium solution through the depth of the slab.

One study tested a series of specimens, which included variations in the amount of expansion allowed before treatment, the type of treatment solution, and patterns of soaking and drying (Stark et al. 1993). The addition of lithium solutions into hardened mortar exhibiting large expansion due to ASR was found to reduce further expansion, whereas the control specimens continued to expand. Of the treatment solutions, LiOH solutions were more effective in controlling expansion than Li2CO3 and LiF solutions. However, the long-term effects of lithium salts have not been studied; the laboratory tests were only 25 months long. Preliminary results from another study also show signs that lithium salts are effective (Johnston 1997). That study avoided the use of LiOH due to safety concerns and ability of OH ions to accelerate the reaction.

Another treatment method is the application of HMWM, which is designed to fill and bond cracks in order to strengthen the pavement. Although HMWM can only penetrate surface cracks, the map-like cracking pattern produced by ASR typically extends only 50 to 75 mm below the surface. Studies have shown that HMWM penetrates cracks up to 50 mm deep (maximum depth of the surface cracks) and can initially reduce midslab and decrease joint deflections (Stark et al. 1993).

Available Rehabilitation Methods for Alkali–Silica Reactivity

ASR does not require complete saturation of the concrete to produce swelling and expansion; even high relative humidity levels (greater than 80 percent) have been shown to produce cracking from expansive ASR. Map cracking exhibited in localized areas of the pavement will eventually progress throughout the entire pavement area. Therefore, restoration techniques that only address a specific area are not effective for pavements exhibiting ASR. Localized repair methods such as full-depth repairs will only provide temporary solutions to the problem. Therefore, restoration techniques are only recommended to repair isolated areas of severe deterioration to maintain serviceability and smoothness (i.e., buying time until more extensive rehabilitation efforts). An AC overlay is more applicable for extensive ASR. An overlay can help slow the deterioration rate by decreasing the moisture gradient (between the top and bottom of the slab), which promotes more uniform expansion due to ASR through the slab depth.

The only applicable rehabilitation methods for fully addressing ASR distresses are rubblizing, recycling, and/or reconstruction. These methods involve the complete destruction of the slab and, in the case of the latter two alternatives, removal of the pavement. Other methods do not address the problem, and ASR damage will continue to progress in areas that are not repaired. With special considerations during mix design, such as the use of pozzolans, recycling of ASR-damaged concrete has been used successfully as aggregate for new concrete.

Selection Guidelines for Alkali–Silica Reactivity

Table III-4 presents general guidelines for selecting feasible alternatives to address ASR. Unlike other MRD, these guidelines are based solely on the severity of the distress. The extent of deterioration is not included because ASR affects the entire pavement area and not just isolated areas such as joints and low-lying areas (although the damage can be worse in isolated areas). ASR initially appears as map cracking and the presence of an exudate is possible. Over time, exudate is almost always observed and expansion-related distress becomes evident. Treatment methods are best suited for pavements exhibiting low- to moderate-severity distress. Using the data recorded in figures I-8 through I-11 and the photo log, the severity of the distress can be assessed using the following guidance:

Rehabilitation methods that address isolated areas are generally ineffective and are only recommended on pavements in which the deterioration is creating a safety problem. Otherwise, treatment methods should be used. If treatment methods are determined to be ineffective, then the best option is to let the pavement live out its life and then reconstruct the pavement.

A variety of feasible treatment methods are available for addressing low-severity ASR. Due to the relatively recent understanding of ASR, the long-term field performance of the treatment methods is uncertain. However, several methods have been found to be effective in laboratory testing and in short-term field experiments. Two of the more promising methods are the application of lithium salts and the application of HMWM. Either of these methods is recommended for low-severity ASR. With both methods, the key to success is to achieve penetration through cracks and into the pavement.

The recommended treatment method for moderate-severity map cracking due to ASR is the application of HMWM. HMWM penetrates cracks in the pavement and strengthens the concrete. The benefits are not reduced due to the higher severity cracks, as the wider cracks will allow easier access and penetration into the cracks. However, the effectiveness can be reduced due to traffic wear and environmental exposure, so reapplication at about 18-month intervals is often necessary. The use of lithium salts may also be effective. And although an AC overlay does not directly address the problem, it can be an effective method of improving serviceability.

Table III-4. Selection of feasible alternatives to address ASR.

Severity*

Feasible Alternatives

Comments

Low

Apply lithium salts
Apply HMWM

Objective is to prevent or delay further deterioration.
Measures designed to prevent access to moisture will likely be ineffective, especially in wet climates.

Moderate

Apply lithium salts
Apply HMWM
Overlay

Delay or slow the progression of deterioration.

High

Rubblization
Recycling
Reconstruction

Deterioration is too severe for treatment or restoration.

* The extent of deterioration is not considered because ASR generally occurs throughout the entire slab.

For pavements that exhibit high-severity damage caused by ASR, the only viable alternative is to reconstruct the pavement. At this point, the deterioration is too severe for treatment methods, and because ASR affects the entire pavement area, the deterioration is too extensive for localized repairs. With special considerations, the use of crushed concrete from pavements with ASR can be reused successfully.

Alkali–Carbonate Reactivity

Alkali-carbonate reactivity (ACR) involves a reaction between the alkalis in the cement and certain carbonate aggregates. The reaction is complex, and not well understood, but it is thought to involve a process called dedolomitization, in which dolomite is converted into calcium carbonate and magnesium hydroxide. The reaction produces expansive pressures that result in map-like cracking on the pavement surface and possibly spalls and blowups.

Available Treatment Methods for Alkali–Carbonate Reactivity

The deterioration due to ACR typically occurs throughout the entire pavement area, so treatments must address the entire pavement. No chemical treatments are currently available. The expansion does require the availability of moisture, and some research suggests that the presence of deicing salts exacerbates the distress. Consequently, the application of a surface sealer is a possible treatment method, although it has not been tested. A surface sealer will help prevent the penetration of moisture and limit the exposure to deicing salts.

Available Rehabilitation Methods for Alkali–Carbonate Reactivity

Rehabilitation methods to address ACR must also take into account the full extent of the deterioration. One possible method is diamond grinding, which will remove map cracking and scaling at the pavement surface. However, it will also expose another layer of concrete to moisture and deicing chemicals. Another possibility is an overlay, which will prevent direct exposure to harmful constituents while restoring ride quality. As always, rubblization or reconstruction are available methods for pavements that exhibit high-severity deterioration over a widespread area.

Selection Guidelines for Alkali–Carbonate Reactivity

As mentioned, the treatment and rehabilitation methods to address ACR are limited. Some possible methods are provided in table III-5. The selection is based solely on the severity and not the extent of the distress because ACR typically occurs over the entire pavement area. ACR initially appears as map cracking, sharing many physical manifestations with ASR. Over time, the cracking becomes much more severe and expansion-related distress becomes evident. A notable difference between ACR and ASR is that the presence of a gel reaction product is not a feature with ACR. Using the data recorded in figures I-8 through I-11 and the photo log, the severity of the distress can be assessed using the following guidance:

For low-severity ACR, the only feasible treatment method is the application of a surface sealer. A surface sealer will help reduce the penetration of moisture, which contributes to swelling in the concrete. In addition, a surface sealer will limit the exposure to deicing salts, which can further exacerbate the deterioration associated with ACR. However, the effectiveness of surface sealers on pavements affected with ACR has not been studied. Therefore, agencies should experiment with the use of surface sealers before application on a wide-scale project.

For moderate-severity ACR, diamond grinding is a viable alternative for addressing map cracking caused by ACR if the distress is limited to the pavement surface. Representative cores should be taken to ensure that the distress has not progressed beyond the surface. An overlay is another feasible alternative. This option will improve the serviceability as well as inhibit further deterioration by reducing the exposure to moisture and deicing salts. If the pavement is badly deteriorated, rubblization with an overlay or reconstruction are the best options.

Sulfate Attack

Deterioration due to sulfate attack is generally attributed to chemical decomposition of certain cement hydration products and/or the formation of an expansive reaction product, ettringite (DePuy 1994). The development of additional ettringite, which is considered the main destructive force in sulfate attack, can result in significant volume expansion and cracking. Deterioration due to sulfate attack first appears as cracking near joints and slab edges that can also progress to fine longitudinal cracking throughout the slab. In same cases, sulfate attack has been characterized by a series of closely spaced, tight map cracks with wider cracks appearing at regular intervals. The variation in manifestations makes it easy to confuse sulfate attack with other MRD types.

Table III-5. Selection of feasible alternatives to address ACR.

Severity*

Feasible Alternatives

Comments

Low

Seal the pavement

Sealing will reduce exposure to moisture and salt and thus limit further deterioration.

Moderate

Diamond grinding
Overlay

These methods will temporary correct the problem but will not prevent further development.

High

Overlay
Rubblization
Reconstruction

Deterioration is too severe for treatment or restoration.

* The extent of deterioration is not considered because ACR generally occurs throughout the entire slab.

Sulfate attack is commonly subdivided according to the source of the sulfate ions. External sulfate attack results from the penetration of sulfate ions from outside sources (e.g., groundwater, seawater, soil, or impurities in chemical deicers) into the concrete. Internal sulfate attack occurs when the source of the sulfate ions is internal, either from one of the constituents or due to the decomposition of primary ettringite due to high curing temperatures. Although the source of the sulfate ions differs, the mechanisms and the treatment methods are the same for internal and external sulfate attack.

Available Treatment Methods for Sulfate Attack

There are still many unknowns as to the cause and impact of sulfate attack. Likewise, methods for effective treatment have not developed to the level of methods used to address other MRD. Treatment methods for sulfate attack must either prevent sulfate ions from penetrating into the concrete or disrupt the reaction to limit expansion and/or decomposition. The first method, preventing the penetration of sulfate ions, is only viable if the sulfate ions are from an external source that can be stopped. Nothing can be done if the sulfate ions are contained within the concrete.

Like most MRD types, sulfate attack requires the presence of moisture to transport sulfate ions to the reaction sites, so methods designed to remove excess water from the pavement also offer feasible alternatives. However, the reaction does not require complete saturation of the concrete; a relative humidity of 80 to 90 percent is all that is required to fuel the reaction (Thaulow et al. 1996). As a result, methods to limit the amount of available water in the pavement system, such as sealing joints and cracks, are not very effective.

Sealing the pavement can help prevent the penetration of sulfate ions from external sources. Practically, however, only the pavement surface can be sealed, and the sources of sulfate ions (groundwater, seawater, and soils) more often than not penetrate from the bottom of the slab. For this reason, the benefits of sealing are questionable.

The addition of chloride ions is a possible treatment method. Ettringite has been found to dissolve in the presence of chloride ions, particularly NaCl (Attiogbe et al. 1990; Marks and Dubberke 1996). Laboratory testing of concrete cores containing ettringite confirmed that treatment with NaCl can dissolve ettringite. However, this process initially involves further expansion of ettringite before it dissolves. Further investigations into this initial expansion are currently being conducted. The potential for ASR should be investigated before using NaCl to treat sulfate attack, as the NaCl can further increase ASR potential and contribute to corrosion of reinforcement. The excess alkali can increase hydroxyl ion concentrations and possibly convert an otherwise innocuous cement-aggregate combination into a deleterious one (Stark 1994).

Available Rehabilitation Methods for Sulfate Attack

Rehabilitation methods for addressing damage due to sulfate attack involve the removal and replacement of the material. These methods are most applicable for repair of areas of moderate- to high-severity distress. Where the deterioration is confined to corners or along transverse joints and cracks, full-depth repairs offer a feasible alternative. Partial-depth repairs are not recommended because the deterioration is often worse at the bottom of the slab. Although effective, full-depth repairs should be viewed as temporary fixes (about 5 years). Placement of full-depth patches creates two new joints where there was only one joint previously, thus creating another avenue for water to infiltrate into the pavement.

When the damage becomes more extensive, rehabilitation methods that address the entire pavement area must be employed. One option is the placement of an overlay to cover the extent of the deterioration. Either an AC overlay or an unbonded PCC overlay is acceptable; bonded PCC overlays are not recommended over pavements exhibiting distress caused by sulfate attack. In either case, the design of the overlay should consider the continued deterioration of the underlying pavement because the overlay will not stop the mechanism of sulfate attack. Designers should take extra measures to ensure the expected life is achieved or must accept the fact that the overlay will not provide the normal expected life.

When the damage becomes too severe, overlays will no longer be effective because the cost of preoverlay repair becomes too great. If the deteriorated areas are not repaired, the overlay will fail prematurely. At this point, the alternatives are limited to rubblize and overlay, recycling, or reconstruction. When constructing the new pavement, consideration should be given to limit the intrusion of sulfate ions from groundwater or soil sources.

Selection Guidelines for Sulfate Attack

Table III-6 provides a summary of the feasible alternatives to address sulfate attack. Sulfate attack has many manifestations, but most commonly first appears on the pavement surface as fine cracking near joints and slab edges or as map cracking over the entire surface. As sulfate attack progresses, spalling will ensue and in some cases, a complete disintegration of the mortar fraction occurs. Distress manifestations for sulfate attack are similar to other distresses and thus an important step in the selection process is ensuring that the pavement is indeed experiencing sulfate attack based on the proper application of laboratory procedures presented in guideline II. Using the data recorded in figures I-8 through I-11 and the photo log, the severity of the distress can be assessed using the following guidance:

Table III-6. Selection of feasible alternatives to address sulfate attack.

Severity

Extent

Feasible Alternatives

Comments

Low

Corners

Seal pavement (external)
Seal joints and cracks

Reaction does not require complete saturation, so means to reduce the amount of moisture are not as effective.

Sealing the pavement is an alternative if source of sulfate ions is external.

Transverse and longitudinal joints

Seal pavement (external)
Seal joints and cracks

Entire slab

Seal pavement (external)

Moderate

Corners

Full-depth repairs

Full-depth repairs should be considered a temporary fix; deterioration may continue adjacent to the patch.

Transverse joints and cracks

Full-depth repairs

Longitudinal joints

Seal joints

Damage is too widespread to repair each area.

Entire slab

Overlay

High

Corners

Full-depth repairs

Deterioration is too severe for treatment; deteriorated areas must be removed and replaced.

Transverse joints and cracks

Full-depth repairs

Longitudinal joints

Rubblize and overlay
Recycling
Reconstruction

Deterioration is too severe and too extensive for treatment or restoration.

Entire slab

Rubblize and overlay
Recycling
Reconstruction

Methods for treating low-severity deterioration are sealing the pavement and sealing joints and cracks. The effectiveness of these methods is limited because none will completely eliminate the progression of sulfate ions into the pavement surface, nor address the influx from beneath the slab.

For moderate-severity deterioration, where cracking and spalling are limited to transverse joints and cracks, full-depth repairs are a cost-effective means to restore serviceability and extend the life of the pavement. If the deterioration is more extensive, an overlay will be more cost effective, as the number of full-depth repairs required will become too costly.

If the deterioration has progressed to high severity, only methods that involve repair of the deteriorated areas will be acceptable. Full-depth repairs are recommended for deterioration confined to transverse joints and cracks. Otherwise, recycling or reconstruction will be the most cost-effective alternative.

Corrosion of Embedded Steel

Corrosion of embedded steel occurs when chloride ions penetrate into the concrete, attacking the passive oxide film that helps protect reinforcing steel. The chloride ions can come from a calcium chloride accelerator, deicing salts, or seawater. The best approach to addressing corrosion is to either provide a protective coating on the reinforcing steel or use reinforcement that is not susceptible to corrosion (e.g., stainless steel, fiber reinforced polymers, etc.). This is not an option in an existing structure so most treatment methods focus on controlling the availability of moisture, oxygen, or chloride ions.

Available Treatment Methods for Corrosion of Embedded Steel

Corrosion of embedded steel can be limited by controlling the availability of moisture, oxygen, or chloride ions. The available treatment methods focus on ways to limit the amount of moisture and chloride ions into the pavement, or at least to reduce the exposure time to these harmful components. The available methods include sealing the pavement with a surface sealer and sealing joints and cracks. These methods have shortcomings that must be recognized. None will completely eliminate the exposure to moisture and chloride ions; sealing joints and cracks may be more effective at preventing salt from entering the pavement. Nonetheless, water containing chloride ions from the dissolved salt deposits can still infiltrate into the pavement.

Available Rehabilitation Methods for Corrosion of Embedded Steel

Rehabilitation methods that are available to address corrosion of embedded steel are full-depth repairs, overlays, recycling, and reconstruction. Joints that have locked-up and spalled due to corrosion of dowel bars are ideal candidates for full-depth repairs. Full-depth repairs not only remove the damaged areas but also restore joint load transfer. Corrosion of reinforcing steel at wide cracks and punchouts are also good candidates for repair.

Overlays should be used cautiously over pavements experiencing corrosion of embedded steel. If not repaired prior to overlaying, locked-up joints can lead to blowups in the concrete that will be just as damaging to the overlay. Temporary repairs to maintain serviceability until the pavement can be reconstructed may be a better alternative in this case.

Selection Guidelines for Corrosion of Embedded Steel

General guidelines, including a list of feasible alternatives to address corrosion of embedded steel, are presented in table III-7. Corrosion of embedded steel is initially manifest through cracking and staining on the pavement surface, followed by spalling. Using the data recorded in figures I-8 through I-11 and the photo log, the severity of the distress can be assessed using the following guidance:

Table III-7. Selection of feasible alternatives to address corrosion of embedded steel.

Severity

Extent

Feasible Alternatives

Comments

Low

Isolated areas

Seal the pavement
Seal joints and cracks

Limit the infiltration and availability of moisture and chloride ions.

Entire project

Seal the pavement

Moderate

Isolated areas

Seal joints and cracks
Partial depth patches1
Full-depth patches

 

Entire project

Overlay

 

High

Isolated areas

Partial depth patches1
Full-depth repairs

In most cases deterioration is too severe for treatment; deteriorated areas must be removed and replaced.

Entire project

Overlay
Recycling
Reconstruction

Deterioration is too severe and too extensive for treatment or restoration.

1 Partial depth patching is only appropriate if the embedded steel is not a load transfer device and is located in the top third of the slab. It must be ensured that the corrosion product be completely removed from the embedded steel during the patching process.

The available treatment methods for low-severity distress are limited in terms of their effectiveness. Sealing the pavement with a water-based or solvent-based silane sealer or sealing joints and cracks can provide some benefit. A similar approach is applicable to moderate-severity distress, although some patching may be required.

As the deterioration progresses to high severity, including joint spalling and possibly blowups, treatment methods become even less beneficial. Full-depth repairs are recommended for repairing spalling and blowups at transverse joints and cracks. The areas requiring repair should be isolated; repairing every joint is not cost effective. AC overlays also have their shortcomings. An AC overlay will not prevent blowups and badly spalled joints and cracks will quickly reflect through the overlay.

Partial-depth patching may offer a feasible alternative for isolated moderate- to high-severity distress over embedded reinforcing steel located in the top third of the slab depth at mid-panel locations.

High-severity spalls and blowups can also be repaired using full-depth concrete patches. However, the number of repairs must be limited if they are to be cost effective. Otherwise, reconstruction is the only viable alternative.

 

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The Federal Highway Administration (FHWA) is a part of the U.S. Department of Transportation and is headquartered in Washington, D.C., with field offices across the United States. is a major agency of the U.S. Department of Transportation (DOT).
The Federal Highway Administration (FHWA) is a part of the U.S. Department of Transportation and is headquartered in Washington, D.C., with field offices across the United States. is a major agency of the U.S. Department of Transportation (DOT). Provide leadership and technology for the delivery of long life pavements that meet our customers needs and are safe, cost effective, and can be effectively maintained. Federal Highway Administration's (FHWA) R&T Web site portal, which provides access to or information about the Agency’s R&T program, projects, partnerships, publications, and results.
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