National Academies Press: OpenBook

Chip Seal Best Practices (2005)

Chapter: Chapter Ten - Conclusions and Suggestions for Future Research

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Page 64
Suggested Citation:"Chapter Ten - Conclusions and Suggestions for Future Research." National Academies of Sciences, Engineering, and Medicine. 2005. Chip Seal Best Practices. Washington, DC: The National Academies Press. doi: 10.17226/13814.
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Page 64
Page 65
Suggested Citation:"Chapter Ten - Conclusions and Suggestions for Future Research." National Academies of Sciences, Engineering, and Medicine. 2005. Chip Seal Best Practices. Washington, DC: The National Academies Press. doi: 10.17226/13814.
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Page 65

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65 CONCLUSIONS Maintenance chip seals can play an important role 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 design life the chip seals extend. The most surprising finding of this study was that advances in the state of the art in chip seal design essentially ended in North America in the 1960s when N. McLeod proposed his design method based on F.M. Han- son’s work, and it was accepted by the Asphalt Institute and most North American departments of transportation as the theoretical basis on which chip seals would be delivered to the traveling public. The development further stalled as pub- lic agencies evolved a system whereby no design is per- formed and only empirical rates are used to develop esti- mated quantities for unit-price chip seal contracts. Five of the U.S. states that responded to this study’s sur- vey reported that they do not use maintenance chip seals. This is an indication that the value placed on chip seals by the states reporting excellent results from their programs is not shared across the nation. Some states rate their chip seal experiences as “unacceptable,” whereas neighboring states may rate their experiences as “good.” Such differences in practice are difficult to explain. It is likely that part of the decision not to use maintenance chips seals flows from the idea or experience that chip seal is an “art” that cannot be easily or predictably replicated. That finding is supported by this study’s literature review, survey responses, and interviews. However, this synthesis demon- strates that chip seals can be reliably designed and installed. Engineers in Australia, New Zealand, the United Kingdom, and South Africa did not allow their search for a better- engineered chip seal to cease in 1970. Because Australia, New Zealand, and South Africa literally depend on chip seals to maintain large percentages of their national transportation systems is evidence that chip seal pavement preservation technology can be reliably and predictably engineered. These countries, as well as Canada, routinely impose warranties on the performance of their chip seal projects, demonstrating that chip seal contractors can construct chip seals that per- form satisfactorily, and they can do so profitably without driving the price of chip seal projects to the point where hot- mix asphalt overlays become economically competitive. This study has found that maintenance chip seal practices can be instituted that will improve the reliability of mainte- nance chip seals. Many of the best practices identified fell in the areas of construction procedures and equipment manage- ment practice. This is not surprising, in that construction is the most critical portion of the chip seal project life cycle. The area that apparently has the greatest potential for enhancement is chip seal design. This is also the area in which advancements in technical understanding will have the greatest potential to dispel the view that the use of chip seals is merely an art. The major issue in chip seal design lies in accurately characterizing the surface on which the seal will be applied, through using engineering measurements of macrotexture and hardness. Such knowledge allows engi- neers in Australia and New Zealand to select both binder types and aggregate gradations that are compatible with the surface on which they will be applied. This discussion leads to the overall conclusion of this synthesis. Americans and Canadians can learn from the procedures that are used in Australia and New Zealand. Those countries, whose highway authorities have joined forces under the name Austroads, have built on the legacy of Hanson and McLeod and have kept advancing chip seal state of the art to the point where they have developed specialized equipment and scientific design and quality control methodologies. They use performance-based contracts for chip seals in which final payment is based not on quantities and unit prices but on the 12-month performance of the completed seal. Perhaps most surprising of all, this has been achieved in partnership with the construction industry through the use of alternative project delivery methods. This circumstance then leads to a discussion of where the U.S. transportation industry needs to proceed with its pavement preservation research. SUGGESTIONS FOR FUTURE RESEARCH That some of the references cited in this study date back to the 1950s and 1960s (and even earlier) indicates the need for new research in pavement preservation. A number of areas from this study deserve further research and study. Research is needed to base chip seal design methods on sound engineering principles and scientifically measured design input data. The methods used in Australia, New Zealand, and South Africa may be adapted to U.S. and Cana- dian chip seal projects. The tests for macrotexture and sur- face hardness may be adapted for road conditions in North America. Output from these surface condition tests may then CHAPTER TEN CONCLUSIONS AND SUGGESTIONS FOR FUTURE RESEARCH

be used as input for the chip seal design process. A robust design process such as that used in those three countries might lend itself to the development of chip seal projects whose inherent variability is reduced and whose potential for replicable success is greatly increased. These assumptions could be investigated by research and field tests. Two other design features that could be investigated for use in the United States and Canada are the inverted seal and the racked-in seal. The inverted seal is used to correct a bleed- ing pavement. In the United States, chat or fine aggregate is used to temporarily restore the skid resistance on a failed, bleeding chip seal. All that the addition of fines does is to effectively change the gradation of the chip seal aggregate on the surface of the road. If a contractor had proposed to install an aggregate with the gradation that exists after fines are spread, that aggregate probably would have failed the sieve analysis and been rejected. Experience has shown that this measure merely exacerbates the bleeding after a period of time as the fines work their way down into the seal and flush more binder to the surface. The racked-in seal shows good promise for use in those areas of road to receive a chip seal that experience a large amount of turning and stopping. The “sacrificial” stone should lock in the primary chips and allow them to resist rolling and dislodging. Both types of chip seals deserve to be tested and evaluated in the United States to determine if they can adequately address these problems. In line with studying international chip seal design meth- ods, another suggestion pertains to transferring chip seal con- struction technology to North America. Of particular interest are the methods for retexturizing the road’s surface. If retex- turizing can be economically performed in the United States, it could eliminate the single greatest source of uncontrolled variation in chip seal construction: the need to adjust binder and aggregate rates on the fly during construction. Through application of a constant rate of binder and aggregate, it would give the public agency the ability to require the chip seal con- tractor to comply with the design shown on the plans and spec- ifications. Furthermore, retexturizing would allow designers to observe and adjust the next design based on actual perfor- mance of the rates and materials called out in the contract. Also of interest is the special-purpose equipment devel- oped to precoat aggregate as it is being installed and the low- drop chip spreaders used in Australia and New Zealand. These types of equipment were developed to minimize the degradation of the chips by reducing the number of times they are handled. Furthermore, these pieces of equipment are highly touted by their users in Australia and should be eval- uated to determine the potential for improving the perfor- mance of chip seals in North America. Australia and New Zealand cooperate under the umbrella of Austroads and have developed a national guide to chip sealing. Each state-level highway agency then develops its own adaptation to fit its climatic, legislative, and business 66 environment. This situation could provide a model to study for use in the United States. Both the literature review and the survey responses showed that the importance of the roller in achieving chip embedment is not well understood. Therefore, it is suggested that a com- prehensive study of chip seal rolling practice be conducted in both the laboratory and, most important, in the field. All types of rollers, including rubber-covered drum vibratory rollers and vibratory pneumatic tire rollers, should be tested. Optimum size and weight for chip seal rolling should also be investi- gated. The result of such a study would be a comprehensive guide specification for chip seal rolling methods. To implement chip seal warranties, research is needed to develop both performance-based specifications and end- product specifications. The finding that most of the non-U.S. responding agencies use some form of chip seal warranty makes this type of research both timely and important. One major disadvantage of chip sealing cited in both the literature review and the survey responses was the increase in road noise caused by chip seals. Research is needed on the relationship between chip seal macrotexture and noise emis- sions. This research should quantify the expected level of noise based on aggregate nominal size and type of chip seal to furnish chip seal designers with engineering guidance when deciding on the aggregate size. Data from Australia show that agencies in that country are willing to pay a premium to guarantee the quality of their chip seal aggregate. They justify the expense by using life-cycle cost analysis. A study on the life-cycle cost implications of paying the additional costs to transport high-quality aggregate is recommended to justify investing in chip seal materials, to maximize their abilities to extend pavement service life. Montana’s sweep test specification, to identify the quantity of excess aggregate left on the road, shows great promise for minimizing the negative effects of loose stone on newly chip sealed roads. Research could be undertaken to quantify the benefits of implementing this specification across the nation. There is a strong need to be able to tie the construction process to the ultimate performance of the chip seal. The lit- erature review indicated that a lot of the uncertainty associ- ated with the forensic analysis of chip seal failures is the result of the inability of investigators to determine the exact rates of binder and aggregate installed at the failed section, as well as their being unable to determine the condition of the underlying surface. Therefore, a study of chip seal construc- tion record keeping and performance monitoring is in order to provide a guide to agencies on this critical issue. Finally, it is suggested that a uniform glossary of chip seal terms be developed and distributed throughout the nation. The effect would be to standardize the technical communi- cation within agencies in the field.

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TRB’s National Cooperative Highway Research Program (NCHRP) Synthesis 342: Chip Seal Best Practices examines ways to assist in the development and implementation of pavement preservation programs by identifying the benefits of using chip seal as part of a preventive maintenance program and by highlighting advanced chip seal programs in use around the world. The report includes approximately 40 best practices in the areas of chip seal design methods, contract administration, equipment practices, construction practices, and performance measures. According to the report, the increased use of chip seals for maintenance can be a successful, cost-effective way of using preventive maintenance to preserve both low-volume and higher-volume pavements.

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