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SUMMARY A chip seal consists of a layer of asphalt binder that is overlaid by a layer of embedded aggre- gate that furnishes, among other things, protection to the asphalt layer from tire damage and a macrotexture that creates a skid-resistant surface on which vehicles may safely pass. A chip seal's main purpose is to seal the fine cracks in a pavement's surface and prevent water intru- sion into the base and subgrade. The use of chip seals and similar surface treatments began in the 1920s, and consisted primarily of providing one or more wearing courses in the con- struction of low-volume gravel roads. Since then, the use of chip seals as preventive mainte- nance (PM) treatments has been a successful surface treatment on both low- and high-volume pavements. Their popularity owes to their favorable cost in comparison with thin asphalt and other factors as a technique to extend the life of the underlying pavement structure. A chip seal is a frequently used PM treatment on flexible pavements. It must be recog- nized that successful chip seals are a function of their application on underlying pavements that have not suffered structural failure. To achieve the pavement preservation benefits of a chip seal, an agency must apply it on roadway surfaces when the level of pavement distress is low. Thus, pavement selection becomes the first and perhaps the most critical factor in an agency's chip seal program. Many agencies base their chip seal procedures on local anecdotal experience rather than on engineering principles and have limited knowledge of what other agencies may be doing to achieve success. Design and installation of chip seals involve a significant degree of "art." Also, much engineering information is available in the literature of transportation agencies, organizations, and academia. Technical information on good practice for materials, design, construction techniques, and effectiveness of chip seals is available and will be summarized in this synthesis. The project limits its focus to single- and double-course PM chip seal sur- face treatments. This synthesis study was initiated with a comprehensive review of the literature on the subject to explore the theoretical foundations for chip seal practices and experiences as well as to set the stage for the identification of chip seal best practices. The review uncovered a large body of technical information. Nearly 80 years of research that included more than 120 published articles on chip seals and pavement PM were identified and reviewed to create the foundation for this synthesis. Special consideration has been paid to the highly technical process used to design and build chip seals in Australia, New Zealand, South Africa, and the United Kingdom. These countries indicated that they consistently achieve chip sealing performance excellence on both low- and high-volume roads. Additionally, the chip seal design and construction man- uals from Australia, New Zealand, South Africa, the United Kingdom, and a number of U.S. state departments of transportation (DOTs) were reviewed. The findings can be divided into seven basic categories: 1. Design methods, 2. Contract administration, 3. Material selection, 4. Equipment practices,
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2 5. Construction practices, 6. Performance measures, and 7. Case studies in excellence and innovation. A survey of U.S., Canadian, and other international public highway and road agencies that potentially used chip seals as a part of their roadway maintenance programs was developed and conducted. The survey was initially directed to the chief maintenance engineer in each state DOT, as well as to points of contact at federal, municipal, and county levels. It was also sent to international public highway agencies in Canada, Europe, Africa, and the Pacific. Appen- dix A contains a copy of the survey, and the summary of survey responses is shown in Appen- dix B. A total of 92 individual responses were received from 42 states, 12 U.S. cities and counties, 10 Canadian provinces, 1 Canadian territory, 2 Canadian cities, 4 Australian provinces, 2 New Zealand provinces, 2 public agencies in the United Kingdom, and 1 from South Africa. Multiple responses were received from a number of agencies, and because each response represented the practices in that local area (which varied significantly in terms of climate and level of urbanization), no effort was made to consolidate them into a single statewide response. The survey responses indicated that the United States and Canada have very sim- ilar practices and that they are quite different from those employed internationally. The distribution of states and provinces that reported that they are achieving good results with their chip seal program is fairly evenly distributed across the continent. Also, there is not much difference in the average daily traffic levels at which U.S. and Canadian DOTs choose chip seal treatments. This supports the finding in the literature review that the experience of highway agency personnel appears to be the major factor for achieving chip seal success. Because the literature review showed that climate has a large impact on chip seal per- formance, the survey responses were categorized by AASHTO climatic region and, sur- prisingly, no trend was evident. Agencies were able to achieve good results in all sorts of climates. The other surprising result was the almost total reliance on asphalt emulsion binders. Only three responding state DOTs indicated that they used hot asphalt cement binders in their maintenance chip seal program, and emulsions were used exclusively in Canada and overseas. The study resulted in several significant findings. The first is that maintenance chip seals play an important part in the nation's pavement preservation program; therefore, they deserve the same level of technical engineering rigor that is reserved for the hot-mix asphalt pavements whose service life the chip seals extend. There was no trend in state population, urbaniza- tion, or climatic region as to whether or not maintenance chip seals were employed by a pub- lic highway agency. The survey showed that in some cases, local entities were successfully using chip seals when the state DOT indicated that it did not. The view that a chip seal is an art, not a science, may be held by the agencies that do not use it in their pavement preservation programs. Another view is that maintenance chip seals can be successfully applied only to low-volume roads, despite evidence to the contrary. These views persist because the development of chip seal design methodology essentially ceased in 1970 in North America with the introduction of the McLeod method, subsequently adopted by the Asphalt Institute. This method relied on qualitative design input and field adjustment of the design rates of binder and aggregate. Although this approach has been successfully used since its introduction, it requires experienced personnel on both the agency and con- tractor teams, introducing another aspect of variability into an already highly variable tech- nology. When one considers that the only really effective solution for an improperly installed chip seal is to mill and overlay the failed surface, one can understand why some agencies in this country have abandoned the technology. The international community, on the other hand, has been aggressively advancing the state of the practice in chip seals and has developed a number of highly engineered advancements that appear to hold great promise. Therefore, this report recommends that future research be directed at importing for North America the chip seal practices found in Australia, New Zealand, and South Africa.
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3 The remaining findings dealt with construction and construction equipment. The report identifies the need to complete a definitive study of chip seal rolling requirements, because rolling is critical to chip seal performance, and to furnish specific guidelines for implemen- tation in general specifications and chip seal quality control manuals. The report also rec- ommends that special purpose equipment that is used overseas be investigated for use in North America. As can be seen, the findings relate to removing the art from the chip seal process and replacing it with solid engineering science. Additionally, 13 respondents indicated that they routinely achieve excellent results from their chip seal programs. These responses were separated from the general survey population and analyzed as a group to identify trends associated with attaining excellence in chip seals. From that group, six case studies that illustrate the trends found in best practices from the chip seal excellence group are presented in chapter nine. Finally, there were two innovative and emerging technology cases provided by survey respondents that appeared to speak to areas of great concern for future implementation of chip seal techniques. Those cases are included and analyzed in Appendix D. The combination of immediately implementable best practices and recommendations for the future research on chip seal technology in the United States and Canada make up the final results of this study. The research team was gratified by the strong and completely sincere response received to its "overly long and complicated" questionnaire and the willingness of engineers all over the world to share their experience and expertise. One unexpected by- product of the study was the establishment of an informal, international network of chip seal experts who can be called on in the future to assist in the revitalization of the pavement preser- vation technology in North America. All of the best practices that were identified in the synthesis are organized in logical groups and cite only the best practice as identified. The definition of a best practice for this synthe- sis is a method or procedure that was found in the literature and confirmed as applicable through survey responses. The literature review and survey responses identified 38 specific best practices in the sev- eral categories mentioned earlier. They ranged from packaging chip seal contracts in large enough volume to attract the most competent contractors to specific recommendations with regard to roller linger times. The best practices came from across the United State, as well as from international respondents. They are summarized in the following lists, and each is dis- cussed in detail in the chapter indicated. Best practices in pavement selection, design, and material selection are as follows: 1. View chip seals as a preventive maintenance tool to be applied on a regular cycle to reinforce the pavement preservation benefits of the technology (chapter nine). 2. Chip seals perform best on roads with low underlying surface distress that will bene- fit from this technology (chapter three). 3. Chip seals can be successfully used on high-volume roads if the agency's policy is to install it on roads before pavement distress becomes severe or the structural integrity of the underlying pavement is breached (chapter nine). 4. Characterize the underlying road's texture and surface hardness and use that as a basis for developing the subsequent chip seal design (chapter three). 5. Try using the "racked-in seal" as the corrective measure for bleeding instead of the North American practice of spreading fine aggregate (sometimes called "chat") on the bleeding surface (chapter three). 6. Conduct electrostatic testing of the chip seal aggregate source before chip design to ensure that the binder(s) selected for the project is compatible with the potential sources of aggregate (chapter five). 7. Use life-cycle cost analysis to determine the benefit of importing either synthetic aggregate or high-quality natural aggregates to areas where the availability of high- quality aggregate is limited (chapter five).
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4 8. Specify a uniformly graded, high-quality aggregate (chapter five). 9. Consider using lightweight synthetic aggregate in areas where post-construction vehi- cle damage is a major concern (chapter five). 10. Use polymer-modified binders to enhance chip seal performance (chapter five). 11. Recognize that both hot asphalt cement and emulsified asphalt binders can be used successfully on high-volume roads. The selection of binders modified by polymers or crumb rubber seems to reinforce success (chapter nine). Best practices in contract administration, warranties, and performance measures are as follows: 1. Award chip seal contracts in time to permit early season construction (chapter four). 2. Time the letting of the contract to allow sufficient time for the curing requirements of preconstruction pavement preparation activities (chapter four). 3. Package chip seal contracts in jobs large enough to attract the most qualified contrac- tors (chapter four). 4. In-house maintenance personnel are best used to install chips seals in areas where the greatest care must be taken to achieve a successful product (chapter nine). 5. Use warranties for chip seal projects only when the contractor is given the latitude to determine the final materials and methods used to achieve a successful chip seal (chap- ter four). 6. The sand patch method to measure chip seal macrotexture can serve as an objectively measured chip seal performance indicator (chapter eight). 7. The use of the chip seal deterioration model expressed in the New Zealand P17 specifi- cation can furnish an objective definition of chip seal performance based on engineering measurements (chapter eight). 8. The two previously described practices can be supplemented with continued visual dis- tress rating based on the Ohio DOT chip seal performance criteria shown in the text in Table 13 (chapter eight). Best practices in construction are as follows: 1. For optimum performance, apply all types of chip seals in the warmest, driest weather possible, for optimum performance (chapter seven). 2. Ambient air temperature at the time of application should be a minimum of 50°F (10°C) when using emulsions, and 70°F (21°C) when using asphalt cements, with a maximum of 110°F (43°C) (chapter seven). 3. When using emulsions, the temperature of the surface should be a minimum of 70°F (21°C) and no more than 140°F (54°C) (chapter seven). 4. Complete patches at least 6 months before and crack seals at least 3 months before the application of chip seals (chapter seven). 5. Variable nozzles permit the application of a reduced rate of binder in the wheelpaths and help combat flooding in the wheelpaths, a defect that makes chip seals prone to bleeding. Conversely, the Australian use of prespraying is another method for adjust- ing the transverse surface texture of a pavement surface before applying a chip seal (chapter seven). 6. A drag broom fitted on those rollers doing the initial roller pass corrects minor aggregate spread deficiencies such as corrugation, uneven spread, or missed areas (chapter seven). 7. Apply the aggregate as quickly as possible to both emulsified and hot asphalt binders (chapter seven). 8. The Montana field-sweeping test curtails the bias to spread excess aggregate created by a unit-price contract (chapter seven). 9. Have the most experienced inspector predrive each shot and paint binder rate adjust- ments on the pavement to facilitate field rate adjustments (chapter seven). 10. In areas where extensive stopping and turning movements take place, the application of a small amount of excess aggregate may reduce scuffing and rolling. The use of a racked-in seal may be a viable engineered solution for determining the precise amount of aggregate for these problematic areas (chapter seven).
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5 11. Furnish and enforce rolling guidelines and specifications for roller coverage, rolling patterns, and minimum rolling time to achieve full lane coverage and a similar number of passes for all areas of the lane (chapter seven). 12. The required number of rollers is a function of desired distributor production and required rolling time for each shot width on the project (chapter seven). 13. Have rolling follow as closely as practical behind the chip spreader (chapter seven). 14. Maintain traffic control for as long as possible to give the fresh seal the maximum amount of curing time (chapter seven). Best practices in chip seal equipment and quality assurance and quality control are as follows: 1. Require chip seal contractors to use state-of-the-art equipment and to control the rolling operation to enhance chip seal success (chapter six). 2. Use computerized distributors (chapter six). 3. Require preproject analysis of the ability of the chip seal equipment spread to keep up with the production rate of the distributor (chapter six). 4. Use variable nozzles to reduce the amount of binder that is sprayed in the wheelpaths (chapter six). 5. Plastic bristles for rotary brooms minimize aggregate dislodgment during brooming (chapter six). 6. An aggressive quality control testing program combined with close inspection gener- ates chip seal success (chapter seven). 7. Assign experienced personnel who understand the dynamics of chip seal construction as field quality control and quality assurance persons (chapter seven). 8. Regularly calibrate both the distributor and the chip spreader (chapter seven). 9. Evaluate aggregatebinder compatibility tests shown in the text in Table 12 for local appropriateness and use in the field (chapter seven). 10. Field test both binders at the distributor and aggregate stockpiles to ensure that material has not degraded owing to handling during transportation (chapter seven).