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21 chapter five CASE EXAMPLES Several state transportation agencies indicated in the sur- vey that they had successful experiences with the use of thin overlays. Some of those same states also had experi- ences in which the thin overlays did not perform as well. NCAT has also placed several thin asphalt sections 0.75 in. (19 mm) thick as part of a pavement preservation experi- ment. This chapter discusses some of those experiences to determine under what conditions thin overlays are most successful. OHIO Ohio DOT has been using thin overlays for maintenance and preservation of its highway system for many years. The agency has found that one of the most significant factors for achieving successful performance with thin overlays is project selection criteria. It is not cost-effective to place thin overlays on existing surfaces that have high levels and high severity of distress. The level of distress is reflected in the pavement condition rating used by most agencies to assist in planning and prioritizing resurfacing and rehabilitation needs. Eltahan (1999, in Chou et al. 2008) found that when the existing pave- ment was structurally sound and in good condition, pavement life expectancy of a thin overlay was 7.5 years as opposed to only 2.5 years when the existing pavement was in poor condi- tion. The study also concluded that applying thin overlays on surfaces that are in poor condition increases the risk of failure by two to four times. The amount of annual snowfall in cold geographical areas is also partially responsible for lower life expectancy in those areas. Ohio DOT has developed a decision tree that helps ensure the right treatment is used on the right project depending on the current pavement condition rating. Thin overlays are considered as an alternative for any project with a condition rating less than 80, depending on traffic level and whether structural deductions are noted. In doing so, Ohio DOT has been able to obtain 10 to 12 years of service from their thin overlays. It was also found that comparing service life of a pavement can be misleading because some projects may be resurfaced when the pavement is in better condition. Thus, deciding not to resurface a thin overlay project until it is in poor condition may artificially indicate a long service life (Chou et al. 2008). The terminal threshold for resurfacing with thin overlays is 65 for primary routes and 60 for general system routes. Figure 15 shows actual service life of two- lane general system Ohio projects at the time they were ter- minated or resurfaced. Ohio DOT also verified the importance of thin overlays on improvement in smoothness. For flexible pavements, it takes nearly 16 years for the smoothness level of a thin over- lay to return to the same International Roughness Index of the existing pavement prior to the overlay (Chou et al. 2008). The cost of a thin overlay is only about 40% of the cost of a minor rehabilitation project on a primary route and about 60% of minor rehabilitation cost on general system routes. Not only are considerable funding and natural resources saved, but the time required for disrupting traffic flow is greatly reduced. TEXAS Texas DOT has had considerable success in using thin over- lays and has worked with the Texas Transportation Institute (TTI) to develop fine-graded dense and open-graded mixtures as well as a fine-graded SMA. These mixes may be placed at 1 in., or less, in thickness and can result in savings of 30% compared with the cost of traditional mixes (Dennis 2013). The thin (¾ in.) porous friction course (PFC) was recently applied on US-183 in Stephens County, Texas (Figure 16) to combat a problem with a âbleedingâ surface from a prior chip seal application and reduce road noise. The dense-graded mix was recently placed at 0.5 in. thick on a project near Austin. Such a thin layer makes an overlay cost competitive with chip seals and microsurfacing options. The SMA was placed on a busy intersection project on State Route 6 in the Bryan District. Texas DOT is expecting 10 to 12 years of ser- vice life from this thin SMA layer. All three mix types passed Texas DOT requirements for Hamburg testing for moisture and rutting resistance, and the Texas OT, which indicates resistance to reflective cracking. Specifications for these mixes can be found at www.dot. state.tx.us/business/specifications.htm. LOUISIANA Louisiana placed its first UTBWC in 1997. The process, also known as Novachip and Paver-Laid Surface Treatment, uses a special paver that applies a polymer-modified emulsion
22 the section needed rehabilitation or reconstruction (Ruranika and Geib 2007). After 7 years, the UTBWC had some trans- verse reflective cracking but the cracks were still tight. GEORGIA For years, Georgia DOT has made efficient use of thin over- lays. A long-time goal of resurfacing 10% of its paved roads each year has allowed Georgia DOT to keep its highway sys- tem in good condition by applying thin overlays relatively early in the pavement deterioration cycle. In the 1990s, Georgia DOT also implemented the use of SMA surface courses on interstate projects. During that time, the agency learned that the use of polymer modified asphalt binder and fiber stabilizers to improve durability and eliminate drain-down in SMA mixtures could improve OGFC mixtures as well. This combination of dense SMA surface course with an OGFC overlay to provide water drainage that reduces the potential for hydroplaning and back-spray has proven to be a success- ful combination. This SMA/OGFC combination was used on a major resurfacing project on I-75 and I-85 in Atlanta just before the 1996 summer Olympic Games. A portion of the project through the heart of downtown Atlanta car- ries approximately 300,000 vehicles/day and was resurfaced after 16 years (Figure 17). Another portion of the project is being scheduled for resurfacing after 18 years. During the mid-1990s, Georgia DOT began to use a coarser- graded OGFC mixture (referred to as porous European mix, or PEM) based on European specifications, and increased the tack coat immediately ahead of the mixture. In doing so, the tack coat is applied at a higher rate than conventional tack applications so it can seal the surface and ensure adequate bond to the existing surface. The typical application range is 0.23 ± 0.07 gal/yd2. The project is just more than 5 miles in length on State Route 308 north of Raceland and com- pared the UTBWC with conventional hot mix asphalt. The UTBWC layer was placed at 0.75 in. thick, whereas the con- ventional layers were 3.5 in. thick. A life-cycle cost analysis was performed that assumed the UTBWC would need to be overlaid a second time in the same 20-year period that the conventional mixes were expected to perform (Cooper and Mohammad 2004). Even so, it was determined that the Louisiana DOT would save approxi- mately $3.34/yd2 using the UTBWC. After more than 6 years, the UTBWC was still performing well. As a result, it was rec- ommended that UTBWC be considered on all new or surface rehabilitation projects as an alternative surface for concrete overlays and as an alternate to mill-and-fill operations so long as there is a stable base foundation. MINNESOTA Minnesota DOT placed UTBWC on US-169 in Princeton in 1999/2000 at an average of ³/8 in. thick and compared perfor- mance to a control section on the same project that consisted of the existing pavement with crack seal and annual mainte- nance. The project has average annual daily traffic of 15,900 vehicles, including 4% trucks. Cracks in the existing surface were sealed before the overlay. After 6 years, the ride quality of the UTBWC layer was still in good condition, whereas the control section had deteriorated five times faster to the point FIGURE 16 Thin PFC overlay on US-183 in the Brownwood District of Texas. (Source: Cindy Estakhri, TTI.) FIGURE 17 I-75/85 Connector in downtown Atlanta with OGFC. (Source: Georgia DOT.)
23 to the option of doing nothing (Hunley 2013). Performance curves may also be developed based on the time it takes for each section to deteriorate to the same condition level it was at before the treatment. One of the sections is a UTBWC mix- ture placed with a spray paver at 0.75 in. (19 mm) thick. The same thickness is used for comparison on seven test sections with various 4.75-mm NMAS mixes. The 4.75-mm mix sec- tions include variations in surface preparation, asphalt binder grade (including a highly modified binder), and use of RAP and RAS. One of those sections has as much as 50% RAP, and another has a high polymer concentration (PG 88-22). This research project will continue to be monitored for sev- eral years to document service life of various treatments and determine cost-benefit scenarios. layer thickness from 0.8 in. to 1.25 in. The changes were made to further increase the drainage capacity of the porous surface course across multiple lanes. NATIONAL CENTER FOR ASPHALT TECHNOLOGY NCAT, with significant support from several sponsors, has constructed special test sections for 4.75-mm mix on a local county road near Auburn, Alabama. Trucks from a rock quarry and an asphalt plant use the road, so the heavy load- ing will be a challenging test for each of the sections. Control sections with no treatment are also included, so there will be the capability to compare the cost-benefit of each treatment
