Surface properties of Stone Mastic Asphalt

The driver of a car is exclusively interested in the impact of the surface properties on the road. He or she expects a high level of driving comfort and safety from the road surface. These expectations call for a whole series of requirements regarding the surface properties:

• roughness
• skid resistance
• evenness (longitudinal and transversal evenness)
• acoustic properties (effects on the residents living near roads).

The “Guideline for the assessment of road skid resistance when wet" - M BGriff 2003 - defined "roughness" as the geometric character of the road surface in a wavelength range of a very few micrometers to several decimeters. There are differentiations between micro, macro and mega roughness; the latter includes the wavelength of approx. 50 mm to 500 mm and has a significant effect on the tire noise.

According to the same guideline, "skid resistance" is the effect of roughness and the material composition of the road surface on the frictional connection tires / road , identified by the frictional connection between index tires and wet road surfaces under established conditions. To assess the skid resistance of road surfaces with the measurement test SCRIM, the following values are given in the guideline:

	Measured speed  (km/h)	Skid resistance value
Warning value	80	0,39
	60	0,44
	40	0,49
Threshold value	80	0,32
	60	0,37
	40	0,42

With "evenness", there is longitudinal and transversal unevenness. Both criteria are measured specially (for purposes of assessing the condition) with contact-free sensors (usually, lasers). In asphalt road pavements, the transversal evenness in the form of ruts, has played an especially large role in the past few years. However according to "Huschek (Bitumen 4/97)", effects of ruts on accidents have presumably been overrated when compared with the significance of skid resistance".

The differing forms of the roughness of the road surface affect the "acoustic effects".

With respect to the construction method "Stone Mastic Asphalt", this means:
The roughness of Stone Mastic Asphalt-wearing courses comes from the high chippings content and especially from the high proportion of the respective coarsest particle size group. It approaches that of porous asphalt. As everyone knows, this is used specifically for reduction of tire / road noise. This phenomenon has been proven undeniably through numerous noise level measurements of Stone Mastic Asphalt wearing courses.

The skid resistance of Stone Mastic Asphalt wearing courses is – as already mentioned previously – low at the time of opening for traffic and continues to be low until the mastic film on surface of the chippings particles lying on the uppermost layer of the road is worn off by traffic friction. That is why it is provided for and usually specified in the contract that the finished, compacted Stone Mastic Asphalt-wearing course must be spread with high quality chippings aggregate 2/5 mm or 1/3 mm before the last roller pass. After the wearing off of the mastic film, the skid resistance of the micro roughness , i.e. the polishing resistance of the chippings used is determined. Further details are listed in the section Minerals.

The requirement for transversal evenness i.e. for the avoidance of ruts, depends greatly on the quality of the wearing courses. Stone Mastic Asphalt wearing courses has proved this requirement optimally. The last doubt was removed in a German Asphalt Institute-sponsored study carried out by Professors Pätzold and Steinhoff which will be talked about in closer detail in Section Long term reliability.

In conclusion, it can be said that the surface properties of Stone Mastic Asphalt wearing courses are optimal for driving comfort and traffic safety.