Long term reliability of Stone Mastic Asphalt wearing courses

At the end of the 1980's and start of the 1990's, more than 20 years after the paving of the first Stone Mastic Asphalt-wearing courses or rather, its predecessor, there was no doubt that Stone Mastic Asphalt was an excellent, long-lasting useful construction method - almost all wearing courses paved in the early days, were excellent even under extreme traffic demands. The pre-requisite was, however, that the "rules" for the building materials, their composition and the paving of the mixture were followed.
After the reunification, increased efforts were undertaken in the new federal states to bring the road network which during the German Democratic Republic had been neglected, up to modern standards for traffic as soon as possible. There were however some slip ups. The necessary qualified mineral aggregate was not available in adequate quantities. The available plants and equipment did not meet the western standards for the most part and in a great rush, the needed care was often not exercised.

For asphalt road courses or even only making wearing course restorations, Stone Mastic Asphalt was chosen frequently in an effort to do something beneficial. But due to the reasons mentioned above, many mixtures were used which had nothing in common with "classic" Stone Mastic Asphalt and which did not deserve the name Stone Mastic.

As can be expected: many of these pseudo Stone Mastic Asphalt wearing courses failed miserably under traffic loads and effects. There were numerous rut formations and the experts who were not very experienced with the special features of this construction method, put the blame on the Stone Mastic Asphalt construction method. The result was a "General Circular of the Federal Minister of Transportation No. 5/96" January 1996 with the title "Criteria for the Choice and Assessment of different Construction Methods for the Pavement of Federal Motorways with separated Directional Lanes ". This contained the regulation that asphalt road pavements made with Stone Mastic Asphalt as a wearing course, had to cost around 1.80 €/m² less than the other comparable road pavements (e.g. asphalt with wearing course made with gussasphalt), in order to be considered for in the awarding of bid.

This drastic measure gave raise to much discussion and activity with the goal of rescinding the discrimination against the Stone Mastic Asphalt construction method. Because this was not possible by negotiation, the German Asphalt Institute felt bound to commission a large-scale project to document the long-term reliability of Stone Mastic Asphalt wearing courses on roads with separate directional lanes. Prof. Dipl. Eng. Heinz Pätzold of the Technical College in Nienburg and Prof. Dr. Eng. Gerd Steinhoff of the University of Siegen undertook the project beginning with a pilot study that included the federal states of Baden Wurttemberg, Bavaria and Lower Saxony.

The focus of this study, which was done with in cooperation with the federal minister for transportation, was the deformation or deformation damage in the road transversal track (transversal unevenness), or, in other words, ruts. Collection of data was done centrally by the state road construction administration or by the autobahn head office in the selected states. As many new and restoration projects (complete wearing course restoration/repair) that used Stone Mastic Asphalt wearing courses as possible were to be taken from the documentation available. Basically areas were considered which were produced after the introduction of the ZTV bit 84 - meaning after a point in time when the Stone Mastic Asphalt construction method had been legitimized through description in an official technical regulation.

Between 700-800 sets of data were taken from control checks for each state- a total of more than 2,000 sets of data were available. When the focus was placed on the Stone Mastic Asphalt and sets of data from the first lane (main lanes), the useable number of data was reduced to about 250 per state. The broadly differing compositions of Stone Mastic Asphalt variations in the 3 states did not lead to any systematically differing results with regard to the deformation behavior (rut formation).

Building materials assessment showed that within the parameters of the existing regulations, it is possible to produce Stone Mastic Asphalt-wearing courses with high resistance to deformation.

The study led to the following results (Pätzold/Steinhoff, Strasse + Autobahn 8/99):

• The measured rut depths are, for the recorded time period (around 10 years) on average between 4 and 5 mm with a deviation of approx. 1.5 mm. At the same time the stresses from heavy traffic and temperature (summer 1995) lay in part in extremely high ranges.
• In more than 75 % of the assessed data records, the traffic loads of the heavy traffic (DTV (SV) 1995) exceeded the value of 4,500 vehicles in 24 hours (highest value 18,000 vehicles/24h). Therefore no reason for a limiting of the construction method to a maximum traffic load can be identified.
• There is a trend that suggests that transversal unevenness is increasing during the useful life. The magnitude lies in the period observed here of a max. of 10 years, at approx. 2 mm (i.e. approx. 0.2 mm/pa).
• With regard to the minimal spreading of the transversal unevenness, no connection could be detected between material data and size of deformation.

So it can be absolutely proven that the reservations of the federal minister for transportation with regard to the limitations of the Stone Mastic Asphalt construction method, are irrelevant - provided that the general valid "rules" are followed.