Thin and Ultra-Thin Whitetopping (2004)

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34 EXPECTED PERFORMANCE AND MODES OF DETERIORATION Since whitetopping was first used, certain predominant dis- tresses have been observed in each class, including TWT and UTW. Research projects, such as the investigation of UTW overlays at the ALF pavements at the Turner–Fairbank High- way Research Center, have provided invaluable information and data toward determining the predominant distresses in UTW overlays (141,142). Similar observations have been made of other controlled UTW and TWT overlays nation- wide (18,20,41,42,51). Understanding the failure mode of any pavement or over- lay type is critical to proper design. One method to accomplish this is using “fault trees.” These trees include pathways indi- cating the steps and processes that eventually lead to observed distresses. Figure 28 is an example of a fault tree for UTW overlays. In this example, several of the common distresses that have been observed are listed at the top of the figure, and the process leading to them is followed by a chain of events. The reader is encouraged to review the survey responses under “Performance, Repair, and Rehabilitation” in Appen- dix A. The observations of the various respondents are sum- marized, and can be reviewed to provide a better understand- ing of the actual performance of UTW and TWT overlays in the field. In the following sections, the various types of distress documented in the literature for both UTW and TWT over- lays are discussed. As much as possible, the distress types are categorized into early-age and long-term distress types. Early- age distresses are often related to the concrete mix properties, the environmental conditions that occur during construction (hourly effects), and the various techniques employed dur- ing construction. Long-term distresses are attributed to the structural design, traffic loading, concrete quality, and envi- ronmental conditions that occur after construction (seasonal effects). Distress Types in UTW • Early age – Uncontrolled cracking as a result of late sawing (48), restrained shrinkage, and/or thermal movement (12); – Plastic shrinkage cracking (10); and – Joint raveling and spalling as a result of early sawing or opening to traffic too early (12). • Long term – Corner cracking (12,48), commonly found on sec- tions with large joint spacing and/or significant traf- fic (12). Although this distress is dominant where slab corners are in the wheelpath (70), many sections have shown reflective corner cracks on adjacent panels that are not loaded (48,77); – Debonding and delamination of the PCC from the underlying HMA owing to environmental and/or traf- fic loading (10,48). This has been reported by some to be aggravated by stripping of the asphalt binder from the underlying HMA layer (2,54); – Longitudinal cracking, especially in the wheelpath (48); – Transverse cracking on slabs with longer joint spac- ings (70); – Fractured and shattered slabs (48); – Surface wear (12); – Failure in support layers (48); – Faulting (48); and – Spalling (48). Distress Types in TWT • Early age – Uncontrolled cracking; – Plastic shrinkage cracking; and – Joint edge distresses including spalling and raveling. • Long term – Longitudinal cracking (143); – Transverse cracking (143); – Corner cracking (143); and – Faulting (4). REPAIR AND REHABILITATION SELECTION PROCESS There are several steps involved in determining appropriate rehabilitation alternatives for a specific UTW or TWT over- lay, including the following (57): • If it is not already known, determine the overlay classi- fication—UTW or TWT. • Evaluate distresses. CHAPTER SIX PERFORMANCE, REPAIR, AND REHABILITATION

35 • Determine possible alternatives. • Develop traffic management plans. • Conduct a cost analysis. • Select the most appropriate process. • Execute fieldwork for localized repair, rehabilitation, or replacement activity. Each of these steps will be discussed in the following sec- tions. The general decision process is described by the flow- chart in Figure 29. The major decisions and analyses are included in this flowchart and can help in determining the most appropriate action. In some cases, the most appropriate action will be to do nothing at that time. In other cases, replac- ing an entire panel may be the best decision. Whether con- sidering the thickness of the whitetopping overlay, the cost analysis, the importance of quickly reopening the facility to traffic, and other factors, the flowchart will lead the evalua- tion to the recommended rehabilitation activity. Determine Classification The first step in this process is to determine the classification of the whitetopping overlay to be rehabilitated—UTW or TWT. The repair of UTW overlays is unique owing to their geometry and degree of bond with the underlying HMA. However, rehabilitation options for TWT will usually be more numerous than for UTW and they will depend on the project specifics. A decision must be made on how to treat it: as a conventional concrete pavement or as being similar to a UTW. These decisions should be made based on the agency’s expe- rience and attitudes toward pavement rehabilitation, as well as on engineering principles. Transverse Cracks Longitudinal Cracks Joint Spalling Corner Cracks “ Surface Wear” Shrinkage Thermal Contraction In UTW concrete layers with longer joint spacings Permanent strain in HMA layer beneath UTW in wheelpath due to repeated loading Crack initiation at bottom of UTW concrete layer Crack propagation through UTW layer in wheelpath Intersection with transverse joints and cracks Y - Cracking or Diamond Cracking Corner breaks on each of the four corners of the intersection pushing the intersection downward Water penetrates intersection and softens HMA Y - cracking or diamond cracking deteriorates Top 3/4 - 1 in. of concrete is finished to produce a mortar with larger shrinkage and thermal contraction properties than the concrete below Early -age shrinkage creates a horizontal delamination crack about 3/4 - 1 in. below surface Early -age traffic traction stresses cause “ half-moon ” shaped joint spalls Corners curl and warp while being restrained in horizontal shear from contracting by the underlying HMA layers Traffic traveling along the edge of the pavement loads the unsupported corner and cracks the UTW in tension within 3 ft from the corner Crazing and map cracking due to the over - finishing of the concrete surface FIGURE 28 Fault tree indicating distress development for UTW (2).

36 Once the whitetopping classification has been determined, the next step is to evaluate the distresses, their severity, and their effect on the structural and functional performance of the overlay. Evaluate Distresses It is important to determine the nature of the distresses observed from the UTW or TWT before moving forward with a predetermined rehabilitation strategy. To make preventive maintenance decisions before a condition becomes critical and warrants expensive measures, it is important to evaluate the condition of the overlay at regular intervals and to make note of actions that could prolong the life of the pavement. Once distresses have developed in the pavement surface, further evaluation should be conducted to assess the effect of those distresses on the structural and functional performance of the overlay (1). For example, it has been found that a crack in a concrete surface will not necessarily impair the perfor- mance of the pavement structure (132). Furthermore, it is hypothesized that it may be as expensive to seal the crack, with all the associated costs such as traffic control, as it is to wait for the panel to become a performance problem and then to replace the entire panel. The remainder of this section describes a number of typi- cal distresses that have been observed in UTW and TWT over- lays. These distresses include the following: • Corner cracking, • Midpanel cracking, • Shattered slab, • Joint or crack spalling, • Joint or crack faulting, • Surface wear, • Permanent deformation of support layers, • Durability, and • Corner and panel debonding. Several publications are available to aid in developing a maintenance and rehabilitation strategy (57,144). Although many of these publications are intended for conventional PCC pavements, the concepts contained in them can be applied to developing strategies for TWT and some UTW overlays as well. Corner Cracking Corner cracking can occur as a result of either wheel loading alone or in combination with another distress. Examples are wheel loading and permanent deformation of layers below the corner or wheel loading and debonding of the whitetopping overlay from the existing HMA layer (48). According to survey respondents, this is the most commonly observed dis- tress on UTW, and the second most commonly observed distress on TWT overlays. Figure 30 shows a typical corner crack shortly after developing. Figure 31 shows how a corner crack can further deteriorate under repeated traffic loading. A corner crack may or may not affect the performance of the overlay, depending on the severity and cause of the crack. If the corner crack results from deformation of the underlying layer, or from debonding, then it may be necessary to repair the crack with a full-depth repair (145). If the cracked corner is not faulted or debonded, it could be left alone until it becomes a ride problem or hazardous debris problem. Some agencies may wish to repair any corner crack, given its poten- tial to debond and become a hazard to the driving public. FIGURE 30 Typical corner crack on a UTW. Collect Project Information Collect Distress Information Evaluate Distress Information Considering Classification (UTW or TWT) Preliminary Selection of Alternatives Preliminary Design and Cost of Traffic Management Estimate User Costs for Preliminary Alternatives Select Preferred Alternative Prepare Plans, Specifications and Estimates Execute Fieldwork FIGURE 29 Flowchart for evaluating recommended rehabilitation activity (57).

37 Midpanel Cracking Midpanel cracking can occur together with other distresses, similar to the situation for corner cracking. Midpanel cracking usually does not warrant repairs, especially in UTW (146). However, on some TWT overlays, crack sealing may be an appropriate method of repair. Figures 32 and 33 show typi- cal midpanel cracks on TWT and UTW projects, respec- tively. Figure 32, provided by the Montana DOT, shows an isolated failure that developed as the result of several factors. These factors were reported as including a poor PCC–HMA bond owing to the presence of dried sawcutting slurry, late sawing, and late curing. Shattered Slab As discussed previously, the thinner the whitetopping layer, the more economical it can become to replace an entire panel than to perform routine preventive maintenance. In such cases, the panels are small and thin, and the bond between the con- crete and the underlying HMA prevents pieces of concrete from being ejected onto the surface of the pavement. It is important, however, to replace the panel before the first piece of broken concrete is thrown onto the surface, because the effect of traffic is to loosen these pieces after the panel has shattered. Figure 34 shows a slab with several cracks that could be considered to be shattered. Figure 35 shows several slabs that are conclusively shattered, and pieces of concrete FIGURE 33 Typical midpanel crack on a UTW. FIGURE 31 Deteriorated corner crack on a TWT. FIGURE 32 Typical midpanel crack on a TWT. FIGURE 34 Typical shattered slab on a UTW.

38 attached to some HMAs have been ejected from the pave- ment structure onto the surface. Joint or Crack Spalling A joint or crack can spall under wheel loads or incompress- ibles between the joint or crack faces. This can indicate a prob- lem such as poor support from one of the underlying layers or a need for joints and cracks to be sealed, depending on the type of whitetopping (13). According to survey responses, this dis- tress type is commonly observed, ranking number one for TWT distresses. In UTW, spalled joints or cracks may not cause deterioration of ride quality, but if they do, the panels may be candidates for replacement. Figure 36 shows a severely spalled crack. Spalling of this severity should be repaired as part of the maintenance program. Joint or Crack Faulting This type of distress can also be caused by poor support from the HMA or other underlying layer. It can be aggravated by debonding of the PCC from the HMA layer (48). This effect has been cited by survey respondents and in the literature. Joints or cracks with severe vertical movement (e.g., fault- ing) are likely to cause ride quality problems and they should be repaired. As with many distresses, there are multiple ways of addressing faulting, and a cost analysis as well as a repair performance analysis should be performed. Surface Wear Surface wear does not necessarily cause ride quality problems, but it should be investigated, for it often is present in more than a few panels. This distress indicates a more systemic problem perhaps related to the construction practices, concrete mix design, or climatic conditions during construction of the over- lay. An example of surface wear is shown in Figure 37. Permanent Deformation of Support Layers Although it is not usually a cause of distress for PCC pave- ments, permanent deformation of the support layers has been reported as a potential contributor to whitetopping distress, especially for UTW overlays (2,48,49). Permanent deforma- tion of the HMA and other underlying layers can lead to other distresses in the PCC layer, but it is not easily detected unless the PCC distresses are severe. When a slab is replaced or when other rehabilitation is performed that exposes the underlying HMA layer, any permanent deformation in that layer should be addressed before replacing the slab. If this type of distress is suspected, it may be confirmed before construction by trenching or coring. In this way, the rehabilitation activities and construction schedule can be adjusted, as well as the cost estimate for the activity. Durability As with surface wear, concrete durability usually indicates a systemic problem, related to the materials used, mix propor- FIGURE 35 Severely shattered slabs on a UTW. FIGURE 36 Severely spalled crack. FIGURE 37 Typical surface wear.

39 tioning, construction practices, or other conditions during or after overlay construction. Further evaluation should be conducted to estimate the extent of the problem and the poten- tial effects if no action is taken at a given time. Durability problems are very diverse. There are mechanisms by which chemicals and freeze–thaw action can break down the struc- ture of the PCC. Corner and Panel Debonding An important characteristic of UTW and most TWT overlays is that they are well bonded to the underlying HMA layer. Sometimes, however, this bond either does not develop dur- ing construction or it deteriorates from traffic loading or environmental influences (2,96). The result is that the stiff- ness of the overall pavement structure is compromised and many of the previously discussed distresses can occur. This type of distress can be repaired, but often it is more econom- ical to replace the panel, especially with UTW. Determine Alternatives On the basis of the type of distresses observed in the UTW or TWT overlay, a set of possible repair or rehabilitation alternatives can be assembled and ultimately the most appro- priate process can be selected. This section describes some of these alternatives. UTW The only recommended rehabilitation option for UTW over- lays is full panel replacement. Survey respondents reported this as the most commonly used technique. Because UTW panels are small, thin, and easily removed, they do not take much time to replace (2,54). UTW overlays are too thin for many types of standard PCC pavement repairs. If a panel is cracked or otherwise distressed, but the ride quality of the pavement is not compromised, the recommended action is to leave the panel in place until a ride quality problem develops (2). The panel should be replaced before the first piece of concrete has been ejected from the overlay. Several problems can affect the ride quality characteristics of a panel in a UTW overlay. As a panel becomes cracked and eventually shattered, it is more likely that pieces of the con- crete will be ejected from the pavement structure under the force of vehicular traffic. The presence of moisture increases this likelihood. Stripping can result, whereby moisture breaks down the HMA and thus the bond with the whitetopping con- crete. As more cracks develop in the concrete, it is easier for water to infiltrate into the HMA, further accelerating this process. In most cases, as long as the bond between the two layers remains strong, the panel can be left in place. Pavements on a 3% grade or more are reportedly less susceptible to the effects of moisture, because the slope causes water to run off and it is less likely to remain in the pavement structure (13). A UTW panel can typically be replaced very quickly (2,54). Figures 38 through 41 illustrate a process undertaken by the Montana DOT for removing and replacing UTW pan- els, using readily available equipment (54). The remove-and- replace-only policy is cost-effective only if the repairs can be made quickly and with minimal impact to traffic. It has been reported that UTW panels can be removed and fresh con- crete placed and finished within 2 h. It is recommended that fast-setting concrete be used, which can allow the affected lane to be opened in approximately another 4 h. Thus, the total time required for repairing a panel is only 6 h. Given this time frame, four such panels could be replaced per crew, and pave- ment can be opened to traffic in only 12 h (2). One suggestion that has been advanced is for agencies to consider developing a fully equipped UTW replacement truck (2). A dedicated crew cab pickup truck and trailer can carry all materials required for UTW removal and replace- ment. Such an arrangement would include a concrete screed, vibrator, jackhammer, compressor, generator, steel mesh for the new panel, and other necessary equipment. The vehicle would also carry four workers who would meet a concrete truck at the site. The trailer also could carry away the old concrete. This configuration has been developed and used by the Colorado–Wyoming chapter of ACPA, and it has been reported to work very well. Finally, the technique of epoxy injecting has been reported to effectively stabilize loose UTW panels (147,148). Although FIGURE 38 UTW repair step 1—Outline sawing.

40 effective, it is also expensive and should be considered only if full panel replacement is not an option. TWT For TWT overlays, the rehabilitation recommendations begin to resemble those for conventional concrete pavements. This is especially true for TWT thicker than 150 mm (6 in.) or where a bond with the HMA was not intentionally con- structed. In these cases, most of the standard concrete pave- ment repairs can be performed. In addition, as the panel size and thickness increase, those factors influence the decision whether or not to replace the entire panel. As a result, TWT overlays that are thinner, have short joint spacings, and are bonded to the HMA can be rehabili- tated following the guidelines recommended for UTW. Mean- while, recommended repairs for thicker TWT overlays can include joint resealing, partial-depth repairs (where the area to be repaired is small and shallow), and crack sealing. The prevalence of these repair types was queried in the user sur- vey and the results are summarized in Appendix A. Where a panel has failed, shattered, or has several distresses that should be repaired it may be more cost-effective to replace the entire panel than to repair individual distresses. Develop Traffic Management Plans Once the potential maintenance and rehabilitation alterna- tives have been selected through the engineering analysis, the required traffic management schemes for each method of repair or rehabilitation should be developed. The cost of the traffic management plans for the alternatives must be consid- ered in the cost analysis, described in the next section. There are many publications to aid in developing traffic management plans for pavement maintenance and rehabilitation activities, with the primary source being the Manual on Uniform Traf- fic Control Devices (149). Others have been developed that have been tailored for pavement maintenance and rehabili- tation (131,150–152). Conduct a Cost Analysis To develop an effective maintenance and rehabilitation pro- gram an agency should conduct a cost analysis, in addition to the engineering feasibility analysis. An appropriate cost analy- sis should consider both agency and user costs in the evalu- ation of rehabilitation strategy alternatives (62). Agency costs usually consist of the actual construction cost, including labor, materials, and traffic management. However, user costs are more difficult to quantify. The two most prominent and most easily calculated are time motorists spend in traffic that is attributable to work zones and extra fuel expended in work zone traffic (153,154). Costs to local businesses, vehicle main- FIGURE 39 UTW repair step 2—Removing panels. FIGURE 40 UTW repair step 3—Place new PCC. FIGURE 41 UTW repair step 4—Finish and cure PCC.

41 tenance and depreciation, and effects of poorly maintained pavement on vehicles and public perception are also important points to consider (155). Other costs are more often assigned to “societal costs,” or costs borne by society as a whole. They include the effects of air quality owing to work zone traffic, traffic and construction noise, and other intangible costs. Each of these costs, both agency and user, should at least be con- sidered in an evaluation of rehabilitation strategies. The following sections describe the nature of these costs and give recommendations for evaluating them with respect to rehabilitation strategies. Often, whitetopping repair or reha- bilitation strategies that result in lower agency costs can also bring about lower user costs in regard to traffic delay and the impacts associated with delay. Agency Costs When an analysis of agency costs is done, only those costs directly paid by the agency should be included. Included are the cost of materials, labor, traffic management, etc. Other items that should be considered, if technically feasible, are rapid-repair materials and techniques. User Costs Direct costs to users of a facility are often called user costs. These costs can include tangible items such as extra fuel consumed, oil, and maintenance; they can also pertain to time expended while negotiating a work zone and the associated traffic congestion that often accompanies lane closures. Sev- eral studies have been conducted to quantify user costs in var- ious situations and work zone configurations (64,153,154). Other costs that are not as easily quantified include vehic- ular accidents, public perception, local business costs, and the societal costs mentioned previously. The costs of traffic con- gestion owing to street and highway maintenance construc- tion, which are borne by society as a whole, include poor air quality and noise. Select Appropriate Process Because many UTW and TWT overlays are constructed in urban areas and on highway access ramps, it is imperative that the highway agency allow lane closures judiciously, to minimize the impact to the public. Rapid-repair techniques for panel replacement operation can greatly reduce traffic delays and thus improve public perception about highway maintenance and rehabilitation construction (155). Localized repairs can also be done quickly with rapid-repair materials and techniques. Research has been done to identify those materials and techniques that can greatly decrease the time required for construction and the associated lane closures. Execute Fieldwork for Localized Activities When engineering and economic analyses show that the best alternative is to perform localized maintenance and rehabilita- tion, possible alternatives shown in Table 1 are recommended for various distresses (2). Each of the recommended rehabil- itation alternatives is described in detail in the referenced pub- lications. Although these alternatives are more commonly used for conventional concrete pavements, they may be useful for TWT overlays. It should be stressed that these techniques are not typically used for UTW overlays. Distresses Recommended Rehabilitation Alternatives Corner cracking Crack sealing (156) Epoxy injection (109,147) Cross stitching (132) Midpanel cracking Crack sealing (156) Shattered slab Slab replacement (145,146) Joint spalling Partial-depth repair (157) Joint or crack faulting Slab stabilization (148) Load transfer retrofit (158) Surface grinding (153,159–161) Surface wear (poor skid resistance) Surface grinding (153,159–161,*) Permanent deformation of support layers Full-depth repair (145) Corner debonding Epoxy injection (109,147) Panel debonding Full-depth repair (145) Slab replacement (145,146) *J. Norris, personal communication (Information on UTW Projects in Tennessee) to T. Ferragut, Sep. 4, 2000, TABLE 1 POTENTIAL DISTRESSES AND RECOMMENDED REHABILITATION ALTERNATIVES FOR TWT