Thin and Ultra-Thin Whitetopping (2004)

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42 As shown from the information reviewed for this synthesis, both thin whitetopping (TWT) and ultra-thin whitetopping (UTW) overlays have been used successfully on hundreds of projects worldwide. When designed and constructed prop- erly, these types of overlays serve as an important option for hot-mix asphalt (HMA) rehabilitation that is readily available to state departments of transportation and other agencies. Although TWT and UTW overlays cannot be used everywhere, they are a viable alternative for roads of light- to-moderate traffic, which still constitute a large percentage of the nation’s highways. Since whitetopping overlays were first used more than 80 years ago, a knowledge base has been developing. Through field trials under a wide range of conditions, best practices have emerged for design and construction of these overlays. This synthesis has identified many of these best practices and it has touched on the state of the art with respect to these overlays. Whitetopping overlays, including UTW and TWT, have proven to be successful rehabilitation methods when used properly. Although this document has identified numerous practices that can be adopted to ensure success in design, construction, and maintenance, a few of the more important conclusions are as follows: • The performance of UTW and some TWT overlays are closely correlated with the characteristics of the support layers, especially the HMA layer. If specific and care- ful consideration is not made toward characterizing the existing pavement system, the whitetopping overlay may be designed or constructed improperly. • For UTW and TWT overlays to continue to be consid- ered a viable rehabilitation alternative, specifiers and designers should recognize its limitations. As with other portland cement concrete pavements, UTW and TWT overlays have their inherent benefits. However, if they are applied improperly, their reputation as an engineer- ing solution can be tarnished. It needs to be recognized that whitetopping overlays are not a cure-all. • With respect to whitetopping design, a balance must be struck between accuracy (reliability) and simplicity. Although an overlay design of high accuracy is pre- ferred, the designer must avoid making the product too difficult to use. That is, the designer should be cognizant of the cost and difficulty in collecting information about the existing pavement and balance those findings with the added benefits that would be gained if that informa- tion were available. Examples are a lower variability and a factor of safety, leading to a more economical design. • Although whitetopping overlays have been used for many years, many questions remain about their proper use. This synthesis may serve as a tool to better under- stand the various issues, although the value of local expe- rience cannot be overstated. Whitetopping projects can be used with increasing success as engineers draw from the experience of the design, construction, and perfor- mance of existing overlays in their areas. Finally, there are some key factors to consider when select- ing, designing, and constructing a UTW or TWT project, including • Distress mode and severity of the existing HMA pavement, • Stiffness of the existing HMA pavement, • Proper thickness and joint design for the UTW or TWT, • Surface preparation of the HMA before overlay (com- monly milled and cleaned), • Fiber reinforcement for UTW and possibly TWT concrete, • Proper joint sawing depth and timing, and • Proper curing practices. From the synthesis survey results, it is clear that there are a number of issues related to UTW and TWT that warrant additional investigation. The following list describes some of the more pressing issues identified during this effort. • Design and construction standards—It has been asked if thinner concrete overlays should be designed and con- structed using the same high-quality standards as for more conventional concrete pavements. If the expected life of the overlay is shorter, can some of these standards be relaxed accordingly? If so, which standards, and to what degree can the quality be reduced without unex- pected consequences? • Joint spacing—Although there have been a number of recent studies to develop mechanistic–empirical models for whitetopping overlays, unanswered questions remain about the optimum joint spacing. Given the numerous competing economic and performance issues, research may be needed to address such questions. CHAPTER SEVEN CONCLUSIONS AND FUTURE RESEARCH NEEDS

43 • Preoverlay repair—Whitetopping overlays are com- monly used as a rapidly constructed but long-lasting rehabilitation method. With respect to expectations about longevity, questions remain about the effects of pre- overlay repairs. The types and quantities of various repairs might be explored with respect to cost first and then the effect on long-term performance of the overlay. • Bond—Whereas recent studies have revealed the need for adequate bond between the HMA and the white- topping overlay, little has been done to quantify these effects. More specifically, the relationship between the quality (properties) of the HMA and the bond strength should be explored. Furthermore, the effect on bond from various surface preparation techniques (e.g., milling) should be quantified. • Rehabilitation—Because many of the UTW and TWT overlays in service are yet to reach their terminal condi- tions, questions still remain about what techniques are available and cost-effective to rehabilitate or reconstruct these overlays. For example, can a second overlay be used? Can the concrete be milled and replaced? A study that synthesizes and builds on the limited field experi- ence in this area could be beneficial to the industry. • Performance—Modeling pavement performance under varying traffic, environmental, and other conditions con- tinues to evolve. At the same time, performance data on in-service whitetopping overlays continue to be col- lected. Research to further the ability to reliably predict UTW and TWT performance could take priority if this overlay type is to continue to be used cost-effectively.