The UK has experienced three heatwaves already this summer, with air temperatures rising into the mid-thirties. And these high temperatures are predicted to continue into the future.
There have been reports that these temperatures have caused stress on some UK roads, including some surface dressed sites, with binder bleeding experienced (often described in the media as ‘melting roads’).
In direct sunlight, roads absorb solar radiation and road surface temperatures can reach temperatures significantly higher than the surrounding air temperature (these were likely to have exceeded 50° C during the recent 35oC heatwave). There are maximum surface temperatures at which surface dressing should be installed (as documented in the TRL Road Note 39 Design Guide for Road Surface Dressing) which sit well below these kinds of temperatures.
Therefore, due to extreme temperatures, the Association urges flexibility in local authority permitting, to accommodate already challenging seasonal conditions, to secure the completion of quality surface dressing installations. This may include early starts to avoid mid day higher temperatures, for example.
Where binder bleeding is experienced; this issue should be managed by spreading of a hardstone aggregate, for example; granite dust (0-4mm). This will reduce material pick up from vehicle tyres, which otherwise left untreated, causes further damage.
Dusting of roads includes only minimal and short term exposure to granite dust. Guidance provided by the Health and Safety Executive suggest that risks of exposure is increased by heavy and prolonged exposure.
Mike Hansford Chief Executive at 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 adaption is increasingly important, and we encourage local highway authorities to discuss with their contractor about a collaborative approach to being more prepared for summer dusting of roads, that makes use of local authority gritting resources where available.”
Surface dressings in some cases are being applied to legacy sites which might already be very binder rich, therefore consideration of ‘right treatment, right place, right time’ should be factored into decision making into appropriate treatments.
Roads with excess binder, which has bled through onto the road surface, may need further management in respect of skid resistance. Hydro retexturing could be a potential longer term treatment option to strip away any excess binder, exposing the aggregate, and restoring skid resistance.”
In terms of future climate adaptation and resilience; the industry has developed harder binders with increased polymer modification, which are more resilient to the effects of both higher and lower temperatures, and could be considered in surface dressing designs on appropriate sites.
The hot weather does not completely undermine the valuable work being carried out by highway authorities on our road networks. However, it is important such factors affecting the performance of surface dressings are managed, to provide a resilient and safe dressing.