A new type of ultrathin friction course has appeared recently. It consists of a layer of hot-mix asphalt placed over a heavy tack coat. Specifically designed machines spread both emulsion and hot-mix in a single pass and smooth the course. These machines comprise a built-in spray bar and a screed unit. They operate at high speed, up to 20 m/minute. The tack coat binder is a modified bitumen emulsion. The friction course hot mix consists of single size aggregate, coated with a mortar made of sand, filler and bitumen. The binder content ranges from 5.3 to 6.0%. The thickness is between 1 and 2 cm. The experience with these surfacings has now reached 4 years. They have been placed on various types of pavements. The resulting texture is open. They have shown some capacity of levelling existing (small) irregularities of the support. The in-place behaviour of these friction courses has been systematically monitored. High levels of surface macrotexture and skid resistance are readily obtained. The rolling noise is significantly lower than that of surface dressing with the same aggregate size. The field of application of ultrathin friction courses can now be accurately defined. Practical aspects are also addressed. (A) The German title of this article is: Ultradünne Rauhschichten : ein neues Verfahren der Fahrbahn-Oberflächenbehandlung.
Abstract