Long-life asphalt roads can be designed to perform well for 40 years and more. The structural design of a road pavement is a balance of a number of requirements based on predictions of future traffic volumes, axle loads and tyre pressures:

Ease of maintenance Flexibility and speed of construction Safety Durability Recyclability Predicted Growth Index of HGV Traffic, in Tonne-kilometres

The client or road authority has to take all of these factors into account whilst minimising the impact of the road on the environment. Heavy Duty Asphalt Pavements offer pavement design engineers a proven technology to help maintain the balance in an era of a dramatic growth of traffic, particularly Heavy Goods Vehicles.

Effect of Tyre Type / Configuration on Road Wear	Reduction in User Cost over Life of Pavement

Why do we need Heavy Duty Pavements?

Although the demands on European roads increase year by year asphalt roads continue to give excellent service. By the year 2000 the total number of vehicles on our roads will have increased by 20% to over 27 million with heavy goods vehicles (HGVs) growing by 15% over the same period. Currently, approximately 90% of all the UK's inland freight is moved by road. In France and Germany the number of HGVs has grown by 25% in the last 5 years and it is anticipated that growth will continue into the next century. The need for heavy duty asphalt pavements is clear.

There are substantial challenges for asphalt roads in the future:

Greater numbers of HGVs and increased HGV axle loadings. Greater stresses applied to pavements by slower moving HGVs (as a result of higher traffic densities increasing congestion). Increased stresses due to higher tyre pressures and 'super single' tyres. Greater polishing of the road surface.

U.K. M25 London Orbital Motorway

How will asphalt meet these challenges?

In response to these challenges, the European asphalt industry, supported by EAPA, has developed the 'The Heavy Duty Asphalt Pavement' (HDAP).

This type of pavement is designed to carry the heaviest predicted traffic both in terms of the total number of vehicles and, more importantly, the extremely high axle loadings applied by HGVs. In terms of structural impact, cars are insignificant compared to HGVs, so pavements are designed on the basis of their ability to withstand a cumulative load (expressed as the equivalent number of standard axles) related to HGV axle loadings, usually 80 kN, 100 kN or 130 kN, depending on the country.

Different countries define heavy duty pavements in different ways, so it is not possible to give a single definition. However, HDAPs are generally designed for heavily trafficked roads which carry at least 10% to 20% HGVs and HGV lanes which carry 3000 to 5000 HGVs/day, crawler lanes carrying 0.3 million 80 kN standard axles/year or sites that have static loads of over 1 N/mm2.

The ability of HDAPs to carry heavier axle loads is achieved by improving the structural performance of the road, by:

• Increasing the stiffness of the asphalt in the structure, primarily the roadbase, and/or
• Increasing the thickness of the roadbase layer.

By doubling the stiffness of the roadbase the predicted number of standard axles the pavement can carry is increased by a factor of between 2 and 5. Similarly, increasing the roadbase thickness by say 10%, can double the number of standard axles that the pavement can carry.

Alternatively, these techniques can produce thinner and therefore more economical road structures without compromising the life expectancy of the road.

The strength, which determines the load-bearing capacity of the road, can be calculated using analytical pavement design procedures. The industry has the knowledge and technology to design and construct stronger, more durable roads, increasingly to meet the demands of tomorrow's traffic.

In addition to raise safety and environmental standards, and to reduce traffic disruption, there have been substantial improvements in the surfacings used on asphalt pavements, eg:

• Alternative asphalt mix designs - advanced mix design to optimise blend proportions such as that used in stone mastic asphalt (SMA) and in thin surfacings.
• High performance polymer modified bitumens - to improve characteristics such as deformation resistance, flexibility and adhesion.
• Porous asphalt - to improve the environmental characteristics of the road by reducing the noise generated between the tyre and the road surface and also reduce spray in wet conditions. In terms of noise, the substitution of porous asphalt for a conventional asphalt surfacing has the same effect as moving the road twice as far away or halving the volume of traffic.

The Nottingham Asphalt Tester (courtesy Cooper techn. Ltd)	Wheel Tracking Test Facility at the LCPC in Nantes

Are Heavy Duty Asphalt Pavements cost-effective?

HDAPs can carry substantially greater numbers of HGVs and higher axle loadings, although it is likely that HDAPs will have a higher initial cost. However, HDAPs are very cost-effective when evaluated on their 'whole-life cost' rather than the traditional 'lowest initial cost' basis. Reduced maintenance costs over the lifetime of HDAPs and particularly the dramatic savings in reduced user delay (traffic disruption) and accident costs clearly produce large economic benefits.

Methods of road procurement are radically changing in Europe from traditional approaches, to now include Design and Build (D & B); Design, Build, Finance and Operate (DBFO) and functional contracts.

The Boulevard Peripherique in Paris, France

These new approaches to road construction focus far more attention on building and managing the road to meet the needs of the user at minimum whole-life cost.

However, road costs are only one element in transport economics. In future, governments may have to balance the benefits of higher axle loads and tyre pressures against increased road wear.

What is the life expectancy of a Heavy Duty Asphalt Pavement?

Traditionally asphalt pavements have been designed to have a life of 20 years. However, with relatively modest increases in the structural strength or thickness of the roadbase a life of 40 years is undoubtedly achievable using a HDAP.

A5 Frankfurter Kreuz Interchange in Germany

The cost-benefit of asphalt pavements designed to meet specific needs, lasting 40 years or more, is fully demonstrated by prediction and by the performance of HDAPs in service around Europe

Are Heavy Duty Asphalt Pavements currently used in Europe?

Yes. HDAPs in various forms are already providing substantial economic and other advantages throughout Europe.

For example:

In the UK Heavy Duty Macadam (HDM) is now widely used in the design of heavily trafficked roads. In particular, significant stretches of the M25 (which at some locations carries 140,000 vehicles/day in each direction) have been built using this material. The Westbound carriageway of M4 in Wiltshire, laid over 10 years ago, carries over 4,000 HGVs per day in each direction (12% of the total traffic) and is a good example of an HDAP in service.

In 1977 the A5 "Frankfurter Kreuz" interchange (Germany) was constructed using a HDAP consisting of 37 mm of Gussasphalt, 200 mm of asphaltic concrete and 150 mm stabilised subbase. This interchange carries a total of 152,000 vehicles/day in both directions of which 10% to 20% are HGVs. After 17 years, and approximately 125 million 80 kN standard axles, the pavement continues to give excellent performance.

In Paris the Periphérique carries a total of 200,000 vehicles/day in both directions of which 10% are HGVs. This is the equivalent of 10 million 80 kN standard axles/year. To meet this loading a new HDAP was laid in l993, using 220 mm of a high-stiffness asphaltic concrete, surfaced with 40 mm of porous asphalt.

Harbour Road in the Rotterdam Area in the Netherlands

Can Heavy Duty Asphalt Pavement technology be applied to strengthen existing pavements?

Applying relatively sophisticated evaluation techniques the residual life of an existing asphalt pavement can be determined. Then by selecting materials with a greater structural strength, pavement life can be extended without increasing the thickness of the structure. This may be a requirement because of height restrictions, eg under bridges. However, even a modest increase in thickness, say 25 mm, can double the life expectancy in terms of the number of standard axles the pavement will carry.

Road-widening schemes will also benefit from the application of HDAP techniques.

Further detailed information is given in the EAPA technical document "Asphalt - Meeting the performance demands of Heavy Duty Pavements" (1996).

EAPA. European Asphalt Pavement Association, P.O. Box 175, 3620 AD Breukelen, The Netherlands