The following article was published in the Fall 2019 edition of the Ohio Asphalt magazine and is presented here courtesy of Flexible Pavements of Ohio. There are several references to Ohio Department of Transportation specifications and mix classifications; however, the information and basic concepts are applicable across the United States.
The Federal Highway Administration (FHWA) has issued a new (June 2019) TechBrief (FHWA-HIF-19-053) on “The Use of Thin Asphalt Overlays for Pavement Preservation,” which agencies may find useful in understanding the concepts of pavement preservation and the most advantageous use of thin asphalt concrete overlays (Thinlays) as a pavement preservation treatment (PPT) within such a strategy.
The TechBrief contains guidance on:
• The benefits and limitations of thin overlays
• When to use thin overlays, including a discussion of factors to be considered in preservation project development
• Mix design considerations
• Construction practices and quality control, including discussions of expected performance (life) of properly used thin asphalt overlays
Sections 1, 3, and 5 of the TechBrief explain the best applications of a PPT. This guidance may be helpful to agencies in selecting an appropriate treatment for their pavement conditions, traffic, and climate. In general, the TechBrief stresses the importance of restricting PPTs to pavements that are in good structural condition in order to get optimum performance.
Section 2 discusses the benefits of using a thin asphalt overlay as a PPT:
• Improve surface smoothness and achieve a better ride
• Reduce wheel path rutting to improve safety
• Reduce water intrusion to maintain the pavement structure
• Restart the surface aging process (i.e. refresh the wearing surface) and slow asphalt binder property changes in the existing asphalt pavement (e.g. stiffness that can result in cracking)
• Decrease pavement surface noise
The TechBrief acknowledges that because of the difficulty of quantifying the smoothness and the ride and noise from the surface texture, the agency/agencies considering those levels of service benefits are in a better position of making PPT decisions. It is important to remember that ride and texture are the most-important factors to the road users. The TechBrief states that research is developing analysis procedures to quantify and compare service value benefits, which agencies can use to improve the cost-benefit ratio.
While Section 4, which deals with asphalt mix design, may be of interest to mix design technicians, it perhaps won’t be very helpful to pavement managers. Fortunately, in Ohio we have a choice of mixes that have already been developed specifically for thin overlays. The premium material is ODOT Item 424, Fine-Graded Polymer Asphalt Concrete. We also have available Thinlay Asphalt Concrete, ODOT Item SS 860 (or FPO’s Thinlay Specification) and FPO Specification 404LVT.
Section 5 discusses project development and pavement structure design. The TechBrief explains the need for adequate evaluation of a candidate project before selecting a PPT. The TechBrief explains that thin asphalt overlays used as PPT are not normally selected to strengthen a pavement. However, when applied appropriately to a pavement in good structural condition, a thin overlay does add strength to the pavement.
Section 6 of the FHWA TechBrief pertains to construction practices and quality control, which should be of interest to owners and inspectors as well as paving contractors. Of particular interest are the cautions about ensuring proper compaction. The TechBrief emphasizes that density measurement cannot be used on thin lifts, and as a consequence, the development and consistent use of an adequate rolling pattern is necessary. The other critical issue mentioned is that thin lifts have very little time available for compaction before they cool, and so the rolling must be completed quickly.
Section 7 presents information on the expected performance of thin asphalt overlays used as pavement preservation treatments.
The list of references in the TechBrief contains a wealth of information for those designing a pavement preservation program. Of particular local interest is the study performed for ODOT/FHWA by Dr. Chou at the University of Toledo, “Effectiveness of Thin Hot Mix Asphalt Overlay on Pavement Ride and Condition Performance.”
The TechBrief can be downloaded at https://www.fhwa.dot.gov/pavement/pub_details.cfm?id=1093.
You may learn more about Flexible Pavements of Ohio by visiting www.FlexiblePavements.org.
