Surface dressing performs best when applied within a specific temperature window; installing it in excessive heat can affect how well it bonds and performs over the long term. With the UK having already experienced three heatwaves this summer, and air temperatures reaching the mid-thirties, the RSTA says local authorities should avoid requiring contractors to work in the middle of the day, when road surface temperatures are at their highest.
In direct sunlight, road surfaces absorb solar radiation and can reach temperatures far higher than the surrounding air – understood to have exceeded 50°C during the recent 35°C heatwave. This matters because there are maximum surface temperatures at which surface dressing should be installed, as set out in the TRL Road Note 39 Design Guide for Road Surface Dressing, and current conditions are pushing well beyond them.
These extreme temperatures have already put stress on some UK roads, including surface dressed sites, resulting in binder bleeding – often described in the media as ‘melting roads’.
With hotter summers predicted to become the norm, the RSTA is urging local authorities to build flexibility into permitting to accommodate already challenging seasonal conditions and secure the completion of quality surface dressing installations. This could include allowing contractors to start work earlier in the day, to avoid the hottest midday temperatures.
Mike Hansford, Chief Executive of the RSTA, said: “The seasonal challenges for surface dressing contractors are exacerbated by extreme high summer temperatures, and we ask that permitting authorities allow flexibility in the timing specified in permits, to help avoid the hottest road surface temperatures.
“Climate scientists are predicting future hotter summers, which means climate adaptation is increasingly important, and we encourage local highway authorities to discuss with their contractor a collaborative approach to being better prepared for summer dusting of roads, making use of local authority gritting resources where available.”
Where binder bleeding does occur, the RSTA recommends the issue is managed by spreading a hardstone aggregate, such as granite dust (0-4mm), across the affected area. This reduces material pick-up from vehicle tyres, which would otherwise cause further damage if left untreated.
Dusting of roads involves only minimal, short-term exposure to granite dust. Guidance from the Health and Safety Executive indicates that the risk of exposure increases with heavy and prolonged exposure.
The Association also notes that surface dressing is, in some cases, being applied to legacy sites that are already binder-rich, so a ‘right treatment, right place, right time’ approach should be factored into decisions on appropriate treatments. Where excess binder has bled through onto the road surface, further management may be needed to protect skid resistance; hydro retexturing is one potential longer-term option, stripping away excess binder to expose the aggregate and restore skid resistance.
Looking ahead, the industry has also developed harder binders with increased polymer modification, which are more resilient to both higher and lower temperatures and could be considered for surface dressing designs on appropriate sites, as part of longer-term climate adaptation and resilience planning.
The RSTA stresses that hot weather does not undermine the valuable work being carried out by highway authorities on the UK’s road network. However, it says the factors affecting the performance of surface dressings in extreme heat need to be actively managed, to ensure a resilient and safe road surface for all users.