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24 chapter six CONCLUSIONS The objective of this synthesis is to review the current state of the practice and research efforts on the use of thin asphalt con- crete overlays for pavement maintenance, rehabilitation, and preservation. This synthesis was performed by conducting a literature review and a survey of state departments of transpor- tation (DOTs) and selected agencies to determine the current usage of thin asphalt concrete overlays. Information has also been gathered from selected individuals and private industry representatives who have experience with thin asphalt con- crete overlay. Responses to the survey were received from 47 of 52 U.S. agencies (90%) and eight companies from the private industry (total, 55 of 60 or 92%). Case examples from agencies that have had successful experiences with thin over- lays are also included. SUMMARY OF FINDINGS From information gathered during this synthesis, it is evident that thin asphalt overlays are being used more frequently as a routine maintenance/pavement preservation tool. Thin over- lays have several benefits that make them attractive for most agencies, and in numerous cases it was found that thin over- lays have been standard practice for many years. For example, in Georgia and Alabama, placement of surface courses no more than 1.5 in. (38 mm) thick has been performed routinely for more than 40 years, even on highâtraffic-volume and heavy-truck routes, such as interstates. Alabama has success- fully used 9.5-mm stone matrix asphalt (SMA) mixes on high- traffic facilities to enhance performance of thin overlays. The revision of AASHTO specifications in 2002 to include grada- tion parameters for a 4.75-mm mix has also expanded the use of such overlays. Respondents report that one of the appealing factors for thin overlays is that these overlays are more economical than thicker dense-graded layers. The thin layers allow pavement managers to overlay more lane-miles with the same tonnage. As a result, thin overlays are often shown to have lower life- cycle costs than do other types of pavement preservation treat- ments (Newcomb 2009). It was also shown that the benefit of thin overlays in improving ride quality is highly significant because ride quality is the most noted factor by the traveling public for evaluating quality of performance. Ohio DOT deter- mined that thin overlays perform as long as 16 years before smoothness levels digress to the point they were at before the overlay (Chou et al. 2008). Thin open-graded friction course (OGFC) overlays can also extend the life of concrete pavements. By acting as an insulation layer, OGFC reduces the curling stress in concrete pavements caused by large temperature differentials from the top to the bottom of the pavement structure. The degree to which thin asphalt overlays are successful depends in large part on the project selection and amount of distress in the existing pavement. Pavements that are failing, or have failed, cannot be treated successfully with a thin overlay alone; they must be repaired so that a stable foundation is provided before the thin overlay is placed. When agencies were asked where they would not recom- mend placing thin overlays, the largest response was not to use thin overlays on projects that have moderate to severe cracking. As with thicker layers and coarser mixtures, the decision to use modified asphalt binders for thin overlays is generally based on traffic volume or axle loading. Many of the parame- ters used for Superpave, SMA, and OGFC in the past are also applicable to mixtures for thin overlays. Although increasing proportions of reclaimed asphalt pavement (RAP) are being researched and implemented in Superpave mixtures, there are only a few instances in which RAP is allowed in SMA and OGFC courses. The finer aggregate also has a potential to have higher mois- ture content as a result of the increased surface area of fine particles. Contractors have found that substantial savings can be realized by placing stockpiles of fine aggregate and RAP under shelters to minimize the amount of moisture absorbed during rainfall. Contractors have also discovered the thin overlays cool much faster than do thicker layers. For that reason, and to help with drying issues, the contractor may need to produce the mix at a higher temperature to maintain workability until it can be adequately compacted. The use of warm mix asphalt technology may allow the contractor to lower production temperatures and still maintain workability. An adequate and uniformly applied tack coat is essential to the success of thin overlays. Raveling and slipping of the surface course at the interface with the existing pavement are common problems when tack coats are insufficient or when they are applied in streaks.
25 modified asphalt binder in thin asphalt overlays to determine the cost-benefit in terms of initial construction cost and in rela- tion to improvements in performance. Any improvements in performance or service life from the use of modified asphalt in thin overlays could be validated for various geographical and climatic areas. One of the main concerns with the use of thin overlays is the potential for reduced performance as a result of reflective cracking from underlying layers. Research might compare the various laboratory crack prediction tests under a variety of simulated field conditions. In this manner, one crack initiation test may be found to better correlate laboratory performance to field performance. A cracking test might be developed for thin overlays that can assess the potential for a mix to resist cracking at the mix design stage so crack susceptible mix- tures can be identified before the mixture is placed on the roadway. Some of the laboratory test procedures used for mixtures placed in thick layers are not reliable for mixtures placed in thin layers. A research investigation might be conducted that relates test results for specimen size and thickness used in the laboratory to the specimen thickness that would be encountered on field projects. In many cases, results of density and tensile strength tests performed on thin layers are not reliable. To further reduce the cost of thin overlays, research might investigate the use of RAP and/or recycled asphalt shingle (RAS) material in SMA and OGFC mix designs. Because the asphalt binder is the most expensive component of the mix, the binder contribution from RAP and RAS may reduce the cost of thin overlays considerably. Although the use of RAP and RAS is common in typical dense-graded mixtures, it is not as common in SMA and OGFC mixtures. Additional research might address durability concerns regard- ing the use of high RAP proportions in thin layers. It is difficult to determine accurate density on thin layers, especially those less than 1 in. thick. Normally, a core would be taken and the layer in question would be sawed off, but for such thin layers this process may distort the sample and make the test results unreliable. Therefore, several agencies have provisions designating that the degree of compaction is not measured, but rolling is left up to the satisfaction of the project engineer. In other cases, the type of roller and number of roller passes are specified. Agencies reported the service life of thin overlays is gen- erally within 7 to 11 years, although some states reported ser- vice life beyond 12 years. The reasons most often expressed for the variability are differences in traffic, weather, existing pavement condition (and level of distress) at the time of the overlay, and the use of different quality standards when thin overlays are placed on interstate projects versus secondary and local roads. An Ohio study (Chou et al. 2008) determined that thin overlays cost only 40% of the expense of a minor rehabilita- tion on primary system projects and 60% of the minor reha- bilitation expense on general system routes. Not only do thin overlays save on materials costs, but there also is a signifi- cant reduction in time delays for motorists and disruption to traffic flow. SUGGESTIONS FOR FUTURE RESEARCH Although modified asphalt has been used in thicker courses placed on more severe traffic and load conditions, the use of modified asphalt with thin overlays was inconsistent from state to state. It was noted during the survey that several agen- cies do not use modified asphalt in thin overlays in an effort to minimize the cost of the mixes. Meanwhile, other agencies use modified asphalt as a way of extending the life and improving performance of the mixes. Research could evaluate the use of
