What Thickness of a Course?

In bituminous mixtures, the adopted rule used to be that the thickness of a course should not be less than 3.5 times the nominal maximum aggregate size (NMAS) in a given mixture. However, due to problems with compaction, usually 3.5-4.0 times the NMAS in a mixture is normally suggested as the appropriate thickness of an SMA course. A course that is too thin in comparison with the maximum particle size causes the following:

• Tearing the mat during laydown and cracking during rolling

• Problems with compacting the course

• Breaking of weaker particles during rolling

• Problems with the appropriate arrangement of particles and weakening of the aggregate structure

Many SMA guidelines provide for a range of thicknesses for each particular mix­ture. Some regulations from the old German guidelines ZTV Asphalt-StB 01 and from the new ZTV Asphalt-StB 07 on the thickness of the SMA 0/11 course are noteworthy because of an exceptionally narrow range of placement thickness for this mixture, specifically 35-40 mm (Table 6.1).

In some countries, a mass criterion on 1 m2 of the surface area (spread rate) is specified instead of designating the recommended thickness of a course. Knowing the SMA bulk density, the thickness of a layer of that mixture may be calculated. The German ZTV Asphalt-StB 01 and ZTV Asphalt-StB 07 and Finnish regulations PANK 2008 are examples of such approaches (Table 6.2). Table 6.1 shows particle sizes and corresponding courses according to various guidelines.

Finally, it is worth noting that a thin course gets cold much faster than a thick one, which has a significant effect on compaction during the cooler periods of the year.

table 6.2

The Mass of sMA Wearing courses, depending on the Maximum particle

sizea