The Swiss standard SN 640433b "Courses with porous asphalt" was adopted January 1st and specifies volumetric properties as well as the drainability of porous asphalt. However, water sensitivity and mechanical properties are not specified.
Especially on motorways but also on other roads, wearing courses of porous asphalt are subjected to the same stresses as any other types of bituminous layers. In addition to stresses common to all layers (permanent deformation, fatigue, thermal stress) there are stresses more specific to porous asphalts (water sensitivity and particle loss). To be consistent in striving for a fundamental approach, mechanical properties suitable for porous asphalts have to be evaluated.
The goal of the research project is the evaluation of performance orientated mechanical tests applicable for porous asphalts. Depending on the course (wearing course, binder course and base course for drainage), the relevant properties of the course will be identified and the sets of tests proposed to assets the courses. The sensitivity to water will be given special concern. The research will result in proposals for testing standards. It should suggest values for specification.
The research builds on existing routes and their behavior. The research institutes have identified:
- Module as a material characteristic value (all layers)
- Fatigue behavior (top layer)
- Low-temperature behavior (top layer)
- Stable speed (surface and binder)
- Abrasion resistance (top layer)
In the framework of this research programme a wide range of porous asphalt (PA) mixes with heterogeneous composition and performance have been studied. Porous asphalt is characterised by high porosity, consequently a high macro roughness, a lower thermal conductivity, a strong permeability to water, and less contact points between the stones in comparison to the traditional dense graded mixes.
The choice of a binder of excellent quality, aggregates and additives adapted to the conditions of traffic and to the situation of the road is imperative in the design of high performing mixtures. The key point is to ensure cohesion and adhesion of the surface particles subjected to the mechanical loads. To this end, polymer modified binders have been shown to improve the performance of PA.
Improvements in performance are however not limited to polymer modifications to the binder. The results in this research pro- gram indicate that in most cases standard EN recommended laboratory tests corroborated field performances. Specific recommendations for standardisation of mechanical tests for PA have been made. With design and maintenance adapted, current experience shows good long term behavior (mechanical, permeability, acoustical) with remarkable service life.
This research project demonstrated that with the use of proper mechanical tests the in situ performance of PA could be to a certain extent predicted and improved. The road administration recommends that with this type of pavement it is necessary to adapt the maintenance practices in order to retain the desired serviceability especially in winter.
The research project is to provide information on research methods with which the drain asphalt pavements can be evaluated appropriately. It provides suggestions for appropriate testing and requirement standards.