Step-change approach for Scotland’s trunk roads

New and more durable surface course specification developed by Transport Scotland and TRL

A NEW surface course specification developed by Transport Scotland and TRL for the Scottish trunk road network, should help deliver significant improvements in material durability, which ultimately will result in less disruption to road users generated by road construction and maintenance.

The new specification follows on from European developments in asphalt technology in the mid-1990s, when thin surfacing was introduced across the UK. Initially welcomed by both the road industry and road users in Scotland, by 2006 incidents were being reported of surface deterioration after only short periods in service. Safety concerns were also expressed due to a possible loss of skid resistance.

In response, Transport Scotland commissioned a review of the performance of thin surfacings as laid on the Scottish trunk road network by TRL. The review encompassed site inspections to identify and quantify surfacing defects, as well as a series of workshops to discuss the findings and to find possible remedies to improve the performance of the surfacing.

Stemming from these workshops, the Transport Scotland Pavement Forum (TSPF) was established to implement annual monitoring of the surfaces in order to provide an estimate of service life based on performance. This resulted in a major recommendation that efforts should be made to reduce the open nature of surface materials by specifying denser binder-rich mixes and smaller stone sizes.

A series of road trials followed using different sized aggregates based on specifications and experiences in Germany, where smaller aggregate sizes and higher binder content are used to enhance material durability. Study visits were also made to Germany to gain knowledge and help refine the development of a new surface course specification for Scotland.

As a result of this work, Transport Scotland has given final approval to a new surface course specification for the country’s trunk road network, which is said to mark a step change in approach for specifying surfacing material and should provide key benefits in terms of material durability and value for money.

Key elements of the new specification are: strict grading requirements to ensure the correct extent of gap grading; high polymer-modified binder content; maximum air void content; gritting of all new surfaces; the addition of cellulose fibres; and performance-based in-service skid-resistance requirement.