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65 CHAPTER TEN CONCLUSIONS AND SUGGESTIONS FOR FUTURE RESEARCH CONCLUSIONS nance chip seals. Many of the best practices identified fell in the areas of construction procedures and equipment manage- Maintenance chip seals can play an important role in the ment practice. This is not surprising, in that construction is nation's pavement preservation program. Therefore, they the most critical portion of the chip seal project life cycle. deserve the same level of technical engineering rigor that is reserved for the hot-mix asphalt pavements whose design life The area that apparently has the greatest potential for the chip seals extend. The most surprising finding of this enhancement is chip seal design. This is also the area in study was that advances in the state of the art in chip seal which advancements in technical understanding will have design essentially ended in North America in the 1960s when the greatest potential to dispel the view that the use of chip N. McLeod proposed his design method based on F.M. Han- seals is merely an art. The major issue in chip seal design lies son's work, and it was accepted by the Asphalt Institute and in accurately characterizing the surface on which the seal will most North American departments of transportation as the be applied, through using engineering measurements of theoretical basis on which chip seals would be delivered to macrotexture and hardness. Such knowledge allows engi- the traveling public. The development further stalled as pub- neers in Australia and New Zealand to select both binder lic agencies evolved a system whereby no design is per- types and aggregate gradations that are compatible with the formed and only empirical rates are used to develop esti- surface on which they will be applied. This discussion leads mated quantities for unit-price chip seal contracts. to the overall conclusion of this synthesis. Five of the U.S. states that responded to this study's sur- Americans and Canadians can learn from the procedures vey reported that they do not use maintenance chip seals. that are used in Australia and New Zealand. Those countries, This is an indication that the value placed on chip seals by whose highway authorities have joined forces under the the states reporting excellent results from their programs is name Austroads, have built on the legacy of Hanson and not shared across the nation. Some states rate their chip seal McLeod and have kept advancing chip seal state of the art to experiences as "unacceptable," whereas neighboring states the point where they have developed specialized equipment may rate their experiences as "good." Such differences in and scientific design and quality control methodologies. practice are difficult to explain. They use performance-based contracts for chip seals in which final payment is based not on quantities and unit prices but It is likely that part of the decision not to use maintenance on the 12-month performance of the completed seal. Perhaps chips seals flows from the idea or experience that chip seal is most surprising of all, this has been achieved in partnership an "art" that cannot be easily or predictably replicated. That with the construction industry through the use of alternative finding is supported by this study's literature review, survey project delivery methods. This circumstance then leads to a responses, and interviews. However, this synthesis demon- discussion of where the U.S. transportation industry needs to strates that chip seals can be reliably designed and installed. proceed with its pavement preservation research. 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, SUGGESTIONS FOR FUTURE RESEARCH New Zealand, and South Africa literally depend on chip seals to maintain large percentages of their national transportation That some of the references cited in this study date back to systems is evidence that chip seal pavement preservation the 1950s and 1960s (and even earlier) indicates the need for technology can be reliably and predictably engineered. These new research in pavement preservation. A number of areas countries, as well as Canada, routinely impose warranties on from this study deserve further research and study. the performance of their chip seal projects, demonstrating that chip seal contractors can construct chip seals that per- Research is needed to base chip seal design methods on form satisfactorily, and they can do so profitably without sound engineering principles and scientifically measured driving the price of chip seal projects to the point where hot- design input data. The methods used in Australia, New mix asphalt overlays become economically competitive. Zealand, and South Africa may be adapted to U.S. and Cana- dian chip seal projects. The tests for macrotexture and sur- This study has found that maintenance chip seal practices face hardness may be adapted for road conditions in North can be instituted that will improve the reliability of mainte- America. Output from these surface condition tests may then
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66 be used as input for the chip seal design process. A robust environment. This situation could provide a model to study design process such as that used in those three countries for use in the United States. might lend itself to the development of chip seal projects whose inherent variability is reduced and whose potential for Both the literature review and the survey responses showed replicable success is greatly increased. These assumptions that the importance of the roller in achieving chip embedment could be investigated by research and field tests. is not well understood. Therefore, it is suggested that a com- prehensive study of chip seal rolling practice be conducted in Two other design features that could be investigated for both the laboratory and, most important, in the field. All types use in the United States and Canada are the inverted seal and of rollers, including rubber-covered drum vibratory rollers and the racked-in seal. The inverted seal is used to correct a bleed- vibratory pneumatic tire rollers, should be tested. Optimum ing pavement. In the United States, chat or fine aggregate is size and weight for chip seal rolling should also be investi- used to temporarily restore the skid resistance on a failed, gated. The result of such a study would be a comprehensive bleeding chip seal. All that the addition of fines does is to guide specification for chip seal rolling methods. effectively change the gradation of the chip seal aggregate on the surface of the road. If a contractor had proposed to install To implement chip seal warranties, research is needed to an aggregate with the gradation that exists after fines are develop both performance-based specifications and end- spread, that aggregate probably would have failed the sieve product specifications. The finding that most of the non-U.S. analysis and been rejected. Experience has shown that this responding agencies use some form of chip seal warranty measure merely exacerbates the bleeding after a period of makes this type of research both timely and important. time as the fines work their way down into the seal and flush more binder to the surface. The racked-in seal shows good One major disadvantage of chip sealing cited in both the promise for use in those areas of road to receive a chip seal literature review and the survey responses was the increase that experience a large amount of turning and stopping. The in road noise caused by chip seals. Research is needed on the "sacrificial" stone should lock in the primary chips and allow relationship between chip seal macrotexture and noise emis- them to resist rolling and dislodging. Both types of chip seals sions. This research should quantify the expected level of deserve to be tested and evaluated in the United States to noise based on aggregate nominal size and type of chip seal determine if they can adequately address these problems. to furnish chip seal designers with engineering guidance when deciding on the aggregate size. In line with studying international chip seal design meth- Data from Australia show that agencies in that country are ods, another suggestion pertains to transferring chip seal con- willing to pay a premium to guarantee the quality of their chip struction technology to North America. Of particular interest seal aggregate. They justify the expense by using life-cycle are the methods for retexturizing the road's surface. If retex- cost analysis. A study on the life-cycle cost implications of turizing can be economically performed in the United States, paying the additional costs to transport high-quality aggregate it could eliminate the single greatest source of uncontrolled is recommended to justify investing in chip seal materials, to variation in chip seal construction: the need to adjust binder maximize their abilities to extend pavement service life. and aggregate rates on the fly during construction. Through application of a constant rate of binder and aggregate, it would Montana's sweep test specification, to identify the quantity give the public agency the ability to require the chip seal con- of excess aggregate left on the road, shows great promise for tractor to comply with the design shown on the plans and spec- minimizing the negative effects of loose stone on newly chip ifications. Furthermore, retexturizing would allow designers to sealed roads. Research could be undertaken to quantify the observe and adjust the next design based on actual perfor- benefits of implementing this specification across the nation. mance of the rates and materials called out in the contract. There is a strong need to be able to tie the construction Also of interest is the special-purpose equipment devel- process to the ultimate performance of the chip seal. The lit- oped to precoat aggregate as it is being installed and the low- erature review indicated that a lot of the uncertainty associ- drop chip spreaders used in Australia and New Zealand. ated with the forensic analysis of chip seal failures is the These types of equipment were developed to minimize the result of the inability of investigators to determine the exact degradation of the chips by reducing the number of times rates of binder and aggregate installed at the failed section, they are handled. Furthermore, these pieces of equipment are as well as their being unable to determine the condition of the highly touted by their users in Australia and should be eval- underlying surface. Therefore, a study of chip seal construc- uated to determine the potential for improving the perfor- tion record keeping and performance monitoring is in order mance of chip seals in North America. to provide a guide to agencies on this critical issue. Australia and New Zealand cooperate under the umbrella Finally, it is suggested that a uniform glossary of chip seal of Austroads and have developed a national guide to chip terms be developed and distributed throughout the nation. sealing. Each state-level highway agency then develops its The effect would be to standardize the technical communi- own adaptation to fit its climatic, legislative, and business cation within agencies in the field.