A Rational Mix Design Method for Porous Asphalt
KHALID H and WALSH C, Liverpool University, UK
Porous asphalt is a type of coated macadam in which the aggregate skeleton is deliberately designed to contain, when compacted, a high air void content, usually in excess of 20%. The voids are interconnected to allow free flow of air and water through the
Porous asphalt is a type of coated macadam in which the aggregate skeleton is deliberately designed to contain, when compacted, a high air void content, usually in excess of 20%. The voids are interconnected to allow free flow of air and water through the material. The binder can be a conventional high penetration grade bitumen to which may be added synthetic fibres, or a polymer modified bitumen - which is becoming more common in current practice.
Porous asphalt is used as a wearing course on major roads, usually constructed as an approximately 50mm thick layer on an impermeable basecourse, and drains water out through the road edges into appropriately placed drainage pipes. Figure 1 shows a cross section of a typical porous asphalt pavement.
Porous asphalt descends from friction courses developed in the US in the 1950s for use on airfield runways to reduce aquaplaning. The erstwhile Property Service Agency was the first British organisation to import the technology from the US and adopt it on UK military airfields (PSA, 1979). It was later transferred to highway pavements to reduce splash and spray on high speed roads following performance monitoring in pilot-scale trials in the 1960s under the auspices of the Transport Research Laboratory (RRL, 1969).
Porous asphalt was included in the 1988 edition of BS 4987 when it used to be known as Pervious Macadam and was specified for the I0 and 20ram nominal maximum aggregate size recipes. Even though the 1993 version of BS 4987 (BSI, 1993) included both grading varieties, currently, only the 20ram nominal size porous asphalt is included in the Design Manual for Roads and Bridges (DTp, 1994) and was incorporated into the DTp Specifications for Highway Works in 1994.
Despite its inclusion in the national specifications, the use of porous asphalt on major roads in the UK remains almost non-existent. In a survey conducted by Fabb (1995), it was reported that the usage, in M.m 2, of porous asphalt in some European countries was as high as 20 in France, 15 in Holland, 8 in both Italy and Austria and 5 in Spain, whereas in the UK it was a mere 0.1 M.m 2. The main concerns that have led to such a limited use are those related to the durability of the material and its construction cost in contrast to other conventional surfacings such as Hot Rolled Asphalt. The open nature of the material renders it liable to degradation by the abrasive action of the vehicle tyres, as well as potentially accelerating the hardening/stripping of the binder due to the ingress of water and air. Added to that is the reportedly premature clogging of the voids which leads to ineffective drainage of surface water (Daines, 1992). As regards strength, traditionally porous asphalt has not been considered as a contributing layer to the overall structural integrity of the pavement. In France, Sainton (1990) reported a structural equivalency factor of 0.5 in relation to conventional dense asphalts. As a consequence of this, the overall pavement thickness will need to be enhanced to provide adequate structural support to carry the anticipated traffic volume if porous asphalt were to be used as a wearing course.
In spite of the aforementioned limitations, porous asphalt has been growing in prominence in Europe over the last decade due to the multitude of advantages it can provide. The main benefit that has contributed to the material's popularity in Europe is its noise and spray reduction characteristics. Compared to other surfacings, porous asphalt has been considered the "Rolls Royce" of noise and spray reduction (Daines, 1996). The main safety benefit of porous asphalt is that it dramatically reduces the risk of aquaplaning at high speeds, which is considered a major factor in motorway pile- ups. The reduction in aquaplaning combined with the suppression of splash and spray and the enhanced skid resistance in the wet hugely contribute to the reduction of accident risks, particularly on high speed roads with high volumes of commercial vehicles. Other benefits incurred as a result of the use of porous asphalt on major roads include reductions in fuel consumption for vehicles and in tyre wear (Fabb, 1992) and enhanced driver comfort (Lefebvre, 1993).
In the UK, porous asphalt specifications have been set by the DTp adopting the recipe approach (DTp, 1994) and is entirely based on experience gained from field trials. The only test advocated by the specifications to arrive at a Target Binder Content for porous asphalt mixtures is the Binder Drainage Test (Daines, 1992), carried out on uncompacted mix specimens. It is evident, therefore, that there is a need for a rational mix design method that addresses the critical service parameters which affect the material's performance, both in the short and long-term stages. Moreover, due to the variability of weather and traffic conditions, and the shortage of high quality aggregates to comply with the stringent specification requirements, the need for a design method based on performance criteria can be clearly seen. This paper describes the development and application of a rational design method for porous asphalt mixes that takes into account the material's overall performance requirements and failure criteria. The development of such a method is also intended to enhance the current level of knowledge and expertise in porous asphalt and enable its adoption on major road surfacing contracts in the UK at a level commensurate with that in other European countries.
Association for European Transport