The placing of hot mix asphalt (HMA) overlays thinner than the usual 1.5-in. minimum often raises eyebrows. But it’s commonly being done nationwide, and many agencies are totally sold on the concept.
“Thinlay” is the general term used for this strategy. It involves special small-aggregate HMA mix in thicknesses only ¾ in. to 1 in. placed as surface treatment on structurally sound pavement. These mixes are produced in hot mix plants and placed with conventional pavers and rollers.
In California, typically only ope- graded friction courses (OGFC) and thin-bonded overlays are placed in thickness less than 1 in. But based on national experience, thinlays might be thought of as another tool in the bag of surface treatments, usually applied as preventive maintenance for pavements still in good condition.
In a recent webinar sponsored by the Asphalt Pavement Alliance, a panel of road managers from across the nation, including officials New Jersey, Ohio, Wisconsin, Oregon and Mississippi discussed their experiences and best-practice tips for thinlay technology.
Based on their experience, thinlays have a lot of potential benefits and uses, including sealing, smoothing, improving skid resistance, noise reduction, x-slope restoration, minor rut leveling and raveling repair. Smaller thinlay projects are often done in-house with agency crews and equipment.
As for the mix itself, thinlay HMA mixes generally use smaller top-size aggregate (3/8 in.) and a harder polymer-modified asphalt binder (e.g. PG 76-22). Specifications often require 100% crushed aggregate for maximum stability – especially in hotter climates. Because of the rapid cooling of thinlays, warm mix asphalt (WMA) technology can be used to aid in proper compaction. Thinlay best practices include:
- Targeting pavements with PCI > 65
- Pre-repair of localized defects
- Pre-milling of rougher pavement (e.g. distortions around intersections)
- Placing an asphalt tack coat
- Using riser rings on manholes and other ‘iron’ in the street.
Thinlay HMA paving involves conventional paving equipment and is dust free with minimal traffic interruption. Life expectancies are reported to be 10-plus years over old asphalt pavement and 6 to 10 years over concrete pavement. For more information go to: www.roadresource.org or http://store.asphaltpavement.org/index.php?productID=696