Hard roads for Scotland

Major roads in Scotland may benefit from road building experience gained in Germany, as the Scottish government looks for surfaces with a longer life.

Improvements in the durability of road material are being claimed as a result of a new surface course specification developed by Transport Scotland and the UK's Transport Research Laboratory (TRL) for the Scottish Trunk Road Network.

TRL said that the new specification would ultimately result in less disruption to road users through road construction and maintenance.

Following European developments in asphalt technology in the mid 1990s, a change in the type of road surfacing, known as thin surfacing, was introduced across the UK.

TRL said these proprietary surfacing systems came with a range of benefits, which included improved deformation resistance, speed of application, noise and spray reduction, and changes to traffic management practices.

While this was welcomed initially by both the road industry and road users in Scotland, TRL said that by 2006, incidents were being reported of surface deterioration after only short periods in service. Safety concerns were also being expressed as a result of a possible loss of skid resistance.

Transport Scotland - the Scottish government's national transport agency - commissioned a review of the performance of thin surfacings, including site inspections to identify and measure surfacing defects. It held a series of workshops to discuss the findings and to find possible ways of improving the performance of the surfacing.

From these workshops, the Transport Scotland Pavement Forum (TSPF) was established. Its annual monitoring of the surfaces has resulted in a recommendation that efforts should be made to reduce the open nature of the materials by specifying denser, binder-rich mixes and smaller stone sizes.

A series of road trials followed using different sized aggregates based on German specifications and experience, where the use of smaller aggregate sizes and higher binder content were used to enhance material durability.

As a result, Transport Scotland has been able to give final approval to a new surface course specification for the Trunk Road Network in Scotland.

Key elements of the new specification include strict grading requirements to ensure the correct extent of gap grading; high polymer modified binder content; and maximum air void content. It also calls for gritting of all new surfaces and the addition of cellulose fibres, plus a performance-based in-service skid resistance requirement.