Design of Long-life Flexible Pavements for Heavy Traffic

The pavement design method for fully flexible pavements that has been used in the United Kingdom since the mid 1980s was established by considering the performance of a wide range of experimental pavements which formed part of the trunk road network. Performance trends from these roads were extrapolated to provide a design life of 40 years, based on staged construction in which major strengthening in the form of an overlay is normally applied after 20 years to carry the traffic predicted over the next 20 years. Since this method was developed in 1984, traffic levels have increased, with the consequent increase in traffic disruption at roadworks. Recently the option of considering a 40 year design life for very heavily trafficked locations was introduced that would not require major structural strengthening during the design life. This option was developed by further extrapolating the design curves. Since these design curves were established, the most heavily trafficked roads have carried in excess of 100 million standard axles and these now provide the opportunity to confirm the validity of the initial extrapolations. Also, more information has become available on the performance of heavily trafficked roads and changes that occur in asphalt over the life of the road. This has indicated that deterioration, as either cracking or deformation, is far more likely to be found in the surfacing than deeper in the pavement structure, as assumed by the current design method. Also, it was found that the great majority of the thick pavements examined have maintained their strength or become stronger over time, rather than gradually weakening with trafficking. The overall conclusion of this project is that a well constructed pavement, built above a threshold strength, will have a very long structural service life provided that distress, in the form of cracks and ruts appearing at the surface, is treated before it begins to affect the structural integrity of the road. These roads are referred to as long-life roads. (A)