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7 CHAPTER ONE INTRODUCTION DEFINITION AND HISTORY A comprehensive review of the literature on this subject has been completed to provide a solid theoretical as well as A chip seal (also called a "seal coat") is essentially a single anecdotal foundation for the review of chip seal construction layer of asphalt binder that is covered by embedded aggre- practices. The literature review furnished a global perspec- gate (one stone thick), with its primary purpose being to seal tive for identifying successful chip seal programs. Particular the fine cracks in the underlying pavement's surface and pre- attention was paid to the sophisticated chip seals of Australia, vent water intrusion into the base and subgrade. The aggre- New Zealand, South Africa, and the United Kingdom, as gate's purpose is to protect the asphalt layer from damage these nations consistently confirm chip sealing benefits and and to develop a macrotexture that results in a skid-resistant successful results on both low-volume and high-volume surface for vehicles. Chip seals and similar surface treatment roads with routine service lives that are nearly double those use originated in the 1920s (Hinkle 1928). These early uses assumed in North America, as shown in Figure 2. Addi- were predominantly as wearing courses in the construction tionally, the subjects of end-product and performance spec- of low-volume gravel roads. In the past 75 years, chip seals ifications, emerging construction methods and trends, and have evolved into maintenance treatments that can be suc- advanced design methodology have been covered to ensure cessful on both low-volume and high-volume pavements. that the latest developments in this field are considered as The popularity of chip seals is a direct result of their low ini- possible candidates for use in future research. tial costs in comparison with those of thin asphalt overlays and other factors influencing treatment selection where the A comprehensive survey was developed and North Amer- structural capacity of the existing pavement is sufficient to ican and international participation alike was encouraging. sustain its existing loads. The survey emphasized identifying any critical and emerg- ing best practices. The survey was developed to focus on Historically, most transportation agencies in North Amer- the specifics of design, contracting procedures, construc- ica would allow their pavements to deteriorate to fair or poor tion methods, and performance measures. Figure 3 illus- condition (Beatty et al. 2002). As a result of the national trates the amount of chip sealed surfaces that are under pavement preservation initiative, funding agencies are becom- authority of the agencies that responded to the survey. Note ing familiar with the cost-effectiveness of using preventive that international responses were tailored to reflect actual maintenance (PM) to preserve the infrastructure, and they are lane miles sealed. finding that chip seal research is worth the investment. Fig- ure 1 illustrates the concept of PM, whereby each dollar spent Survey analysis has been directed toward identifying on maintenance before the age of rapid deterioration saves $6 practices that are likely to contribute to both successful and to $10 in future rehabilitation costs (Hicks et al. 1999) and unsuccessful projects. After identification of such practices, could conceivably save even more when user delay and traf- a structured case study format was developed to clarify fic control costs are added to the bottom line. trends in the features of the best practices. The case study process illustrated lessons learned from highly successful The focus of this synthesis was on summarizing the practices. Considerable constructability improvements may research and practices that point toward consistently suc- be possible through emulating practices that have been iden- cessful chip seal projects. For this synthesis, a "best practice" tified as critical to project success. is defined as any superior planning, design, or construction method that was found in the literature review and confirmed by survey responses. The project's objective was to assist in CHIP SEALS AS A TOOL FOR PREVENTIVE MAINTENANCE the development and implementation of pavement preserva- tion programs by identifying the benefits of using a techno- Definition logically advanced chip seal as part of a PM program. A great deal of research on chip seals has been performed in Aus- Pavement preservation is the long-term goal for most high- tralia, Canada, New Zealand, South Africa, the United King- way agencies. Such action must be taken to not only protect dom, and the United States. Innovative and advanced chip the capital investment made when a roadway is constructed seal programs have been identified with respect to critical but also to maximize its ultimate useful life. Pavement factors that can be incorporated by other agencies. preservation has its own unique definition that must be
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8 FIGURE 1 Preventive maintenance concept (Source: Galehouse et al. 2003). (PCI = pavement condition index.) understood for one to put it in context with PM. The pave- The AASHTO definition of PM is a "planned strategy of ment preservation definition developed by the FHWA Pave- cost-effective treatments that preserves and maintains or ment Preservation Expert Task Group states that improves a roadway system and its appurtenances and retards deterioration, but without substantially increasing structural Pavement preservation is a program employing a network capacity" (Pavement Preservation . . . 1999). Thus, one can level, long-term strategy that enhances function pavement see that planned PM actions are actually a part of a much performance by using an integrated, cost-effective set of broader pavement preservation program. practices that extend pavement life, improve safety, and meet motorist expectations (Pavement Preservation . . . 1999). Furthermore, it is important that the reader understands that Benefits Pavement preservation is not a maintenance program, but an Chip seals are most frequently used as PM treatments on agency program. Almost every part of an agency should be flexible pavements. The ideal benefits of applying a chip seal involved. Success depends on support and input from staff in are obtained if the chip seal is applied early in a pavement's planning, finance, design, construction, materials, and main- tenance. Two other essentials for an effective program are life (i.e., before it exhibits a great degree of distress) and long-term commitment from agency leadership and a dedi- within the context of a PM program (Wade et al. 2001). A cated annual budget (Galehouse et al. 2003). strict PM program whereby the roads are sealed at the end of every PM cycle may require several chip seals to be applied to the pavement's surface for that pavement to reach its ser- 9.60 136,416 139,713 5.76 5.33 106,575 United States Canada AU, NZ, UK, 39,482 35,950 SA Years FIGURE 2 Survey respondents' chip seal service life (Source: Galehouse et al. 2003). (AU = Australia, NZ = New Australia Canada NZ UK US Zealand, UK = United Kingdom, SA = South Africa). FIGURE 3 Lane miles of chip seal service life.
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9 vice life. The main concept behind implementing a PM cycle good condition is not recommended practice and will likely is to maintain the desired quality of the pavement without the be more expensive in the long run. Therefore, chip seal appli- development of major distresses. In contrast with routine cations should not be applied on badly cracked or weathered maintenance, which is a reactive approach to repair pavement pavement surfaces where reconstruction, rehabilitation, or a distresses, PM is a proactive approach to preserve and extend conventional overlay is needed. a pavement's life (Seal Coat and Surface Treatment Manual 2003). In PM programs that use chip seals, practitioners believe that chip seals provide economically justifiable life Process extension benefits if applied at the correct time (Wade et al. 2001). Figure 4 shows the distribution of respondent agencies The chip seal process begins in the planning stage when the that strive to use chip seals as a PM practice. pavement surface is analyzed to determine if a chip seal is an appropriate PM treatment. Surface characterization may con- Chip seals are expected to provide at least 5 years of ser- sist of assessing the hardness, texture, and other measures of vice; therefore, three or four chip seals may be necessary for the structural condition of the pavement surface. If a chip seal a pavement to reach its design life. When applied on an exist- is determined to be an appropriate treatment for the pave- ing flexible pavement, a chip seal will provide a surface ment, various surface preparation techniques are then per- wearing course, seal the underlying pavement against water formed on the surface. Crack repair, selected patching, lev- intrusion, enhance or restore skid resistance, and enrich the eling, presealing, and/or texturizing can be used to prepare pavement surface to prevent the distresses caused by oxida- the surface before chip sealing. These treatments should nor- tion. Chip seals are generally effective in sealing fine cracks mally be performed 6 to 12 months before the chip seal to on the roadway surface, unless the cracks are indications of allow sufficient time for curing. The surface needs to be free structural distresses. of foreign materials before material application. The applica- tion of the chip seal involves essentially four pieces of equip- Chip seals are not expected to provide additional struc- ment: the binder distributor, aggregate spreader, rollers, and tural capacity to the pavement, although it appears to be a brooms. common practice to apply chip seals to pavements that have structural distresses. Justification for chip sealing as a stop- The binder distributor provides application of the binder gap procedure is straightforward; it is believed that the chip to the pavement surface. A chip spreader immediately applies seal will reduce the rate of further deterioration until funds a uniform, predetermined rate of aggregate onto the binder. are made available for a conventional overlay. However, as These two operations are at the heart of constructing a sur- a PM treatment, chip sealing on pavements that are not in face that is one stone thick and has enough binder to retain the aggregate, but not an excess amount of binder that causes the surface to bleed. Depending on the binder, aggregate, and actual type of chip seal being constructed, various rollers will 20 be used to orient the aggregate to achieve appropriate embed- 15 YES ment. Pneumatic rollers are typically found on all chip seal projects. The rollers are followed by the brooms that remove NO 10 excess aggregate from the finished surface. 5 This report will cover the aforementioned process in detail. The results of the literature review have been corre- 0 United AU, NZ, lated with the survey responses to identify best practices. States Canada UK, SA These best practices will be discussed in detail. Specific best FIGURE 4 Preventive maintenance practice by survey practice case studies will also be presented and their essence respondents. discussed.