Optimization of thin asphalt layers, OPTHINAL. Final report

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Jörgen Kragh
Erik Nielsen
Erik Olesen
Luc Goubert
Stefan Vansteenkiste
Joëlle De Visscher
Robert Karlsson

ERA-NET ROAD initiated a transnational research project titled "Optimization of thin asphalt layers". Thin asphalt layers have been used extensively and with promising results for more than 15 years in several countries in Europe and abroad. They seem to be cost effective, fast to build and may have good surface properties. In recent years thin asphalt layers have been shown to imply reduced traffic noise levels, increased traffic safety (skid resistance and forward visibility during wet condition) and to be durable compared with traditional alternatives.

The DRI-BRRC-VTI Consortium was trusted with carrying out the ERA-NET ROAD project and began with a State-of-the-Art report covering, among other things, a literature study and an inventory of experience with using thin asphalt layers. The results of this phase of the project were documented in a separate project report.

The main conclusions were that the application of thin asphalt layers is certainly worthwhile, in particular as a renewable "skin" of a stable road construction having sufficient bearing capacity. The skin serves road users' need for skid resistance and other important functions. Compared with more conventional and traditional surfacing such as dense asphalt concrete 0/11 or stone mastic asphalt 0/11, thin asphalt layers in general come out somewhat better in most respects; for example concerning cost, use of nature resources, rolling resistance, and traffic noise emission. However, there are also drawbacks or problems under special traffic that need to be handled, for example TAL durability when exposed to wear from studded tyres. The availability of premium quality aggregate is a prerequisite for applying thin asphalt layers, and good quality aggregate may be difficult to procure.

The present report looks at possibilities to optimize the application of thin asphalt layers, including an analysis of the cost of applying thin asphalt layers compared with the cost of applying more conventional solutions. The study has been limited to thin asphalt layers with a maximum thickness of 30 mm.

Recommendations are given concerning the best practice in applying thin asphalt layers and suggestions are given for future research needed to fill the gaps in available knowledge.

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