After recent guidance from the Road Surface Treatments Association (RSTA) and ADEPT provided new evidence on the long-term cost effectiveness of high friction surfacing (HFS), Mike Harper, director at Stirling Lloyd, shares his views on the subject, which still leaves some highways experts divided.
HFS has had a significant role to play in reducing casualties in road traffic accidents since it was introduced in the UK in 1968. Other factors have also had an impact, including better car design, better passive engineering measures and better emergency response to accidents.
However, over the last three years, KSI (Killed and Seriously Injured) accident statistics on UK roads have started to rise again (1). Simultaneously, the use of HFS across the UK is declining. Estimates from the Road Surface Treatments Association (RSTA) HFS committee suggests that usage in the UK has dropped from five million to around one million square metres per year over 10 years. High Polished Stone Value (PSV) aggregates in asphalt are being used as a direct replacement on some sites, but there is little published evidence about the retained skid resistance of such systems and whether they can provide an equivalent result to HFS.
So, after 50 years of saving lives, what is causing this decline in the use of HFS? With many highway engineers estimating a life span of just 3-4 years, the perceived short life of HFS systems seems to be a major factor, leading some road authorities to view it as a maintenance liability. For HAPAS approved systems, which should last at least five years, this would be a poor result if true. The reality is that experience varies widely, with some systems lasting less than three years, and others lasting up to 20.
Exploring the evidence
The British Board of Agrément (BBA) carried out a study investigating over 200 historical HFS sites to see if the perception of short life was supported by the empirical data (2). The findings showed that HFS systems generally wear well and the RSTA advice on service life was revised to increase the average life expectancy of HFS to eight years for hot applied systems and 12 years for cold applied systems (3).
Nonetheless, the perception of HFS having a shorter service life, combined with the increasing availability of high PSV aggregates in asphalt materials, has led to a shift toward the use of high PSV asphalt as an alternative to HFS on some sites. Experience of this has varied, with some engineers reporting good results over the long term with SCRIM values remaining above investigatory levels (ILs), while others report a rapid decline in skid resistance after initial good results leading to subsequent HFS treatment to keep SCRIM levels up.
Until recently, there was little independent research on this subject, but in 2016 Highways England published a report by TRL to address this issue (4). The report revealed that Alternative HFS materials (AHFS as they have been termed in this report) – or high PSV aggregates used in asphalt mixes – do not ‘consistently achieve skid resistance levels similar to traditional calcined bauxite HFS’ and further that, ‘use of calcined bauxite aggregates for HFS should be continued for high risk sites’. It also concludes: ‘AHFS materials may be suitable for some high risk sites but a better understanding of the parameters affecting skid resistance performance is needed.’
The data from this report also suggests that high PSV aggregates in asphalt may provide skid resistance above the IL in year one, but that this rapidly declines, with the vast majority of sites having fallen below the IL within the four years that data was available for. By comparison, it concludes that traditional calcined bauxite HFS (CHFS as it is termed in this report) may have a life span of around eight years, but that ‘with increasing age and trafficking, no clear reduction in skid resistance was shown. It adds: ‘This suggests that the failure mechanism of CHFS is not due to polishing of the aggregate.’
This confirms views widely held by industry practitioners that bauxite ‘does not wear out’ during the service life of HFS – it is about finding a better solution to keep the bauxite bonded to the road surface.
If we consider what HFS is ultimately for – saving lives – the decision-making process should not be about whether we can scrape a pass above the IL for a short period of time – it should be about achieving the shortest stopping distance of a vehicle in an emergency situation.
We need to differentiate between HFS systems
If we accept that bauxite reaches an equilibrium point after wear that keeps it in the HFS zone and above the IL for all situations, then the aggregate choice is not the problem. It is the apparent variability in service life that brings HFS into question.
Research in the 1950’s and 60’s concluded that calcined bauxite out-performed all other aggregates – a conclusion that has been revisited of late (5). The original long-lasting HFS system was also a cold applied thermoset resin – a modified epoxy at the time. Today, however, a wide range of HFS systems are available based on a range of binder technologies – the essential part of the system for keeping the bauxite on the road – and I believe it is these variations that contribute to the variable service life.
Over the last decade or so, thermoplastic resins have become the predominant HFS binder type in the UK. Here the bauxite is bound up in a relatively soft thermoplastic resin screed, so the tyres are not just acting on bauxite (which we know doesn’t wear) but also on the binder that holds it in place. Thermoplastics are attractive in that they have a faster return to service than epoxies and are often suitable for use all year round. However, average service lives are shorter than for thermoset resins.
Traffic levels in the UK have increased by 300% (6) since the 1960s, putting added strain on road surfacing systems. This means that, in order to create minimum disruption to heavier flows of traffic, bauxite needs to be applied in a shorter time frame, often at low temperatures at night, and it needs to be as hardwearing and long-lasting as possible to deliver the lowest whole life cost, under unprecedented volumes of traffic.
This is where modern thermoset resins come in. They take the best of the original systems (i.e. resin between the bauxite and the road surface providing 100% bauxite contact with the tyres combined with very high bond strengths to the asphalt) and combine it with low temperature working capability. This provides both longevity and a fast return to service for night and winter working. Safetrack HW, for example, is based on Stirling Lloyd’s unique Esselac resin technology, which is the only MMA-containing system that has been in service across the UK since 1999 and offers a five-year guarantee for HFS applications for peace of mind. There are original applications still in service today 18 years after application, providing skid resistance above the IL and retaining the bauxite. Such applications provide the lowest whole life cost for HFS, while providing the highest levels of skid resistance and minimum disruption – requiring no intervention in the short or medium term.
It is possible to choose the required levels of skid resistance, without creating a maintenance liability. HFS performance depends on selecting the right system for the application. There will be situations where a thermoplastic system will provide adequate performance – particularly in light traffic situations. But for long-term skid resistance retention under the highest traffic conditions, a modern thermoset resin will provide maximum service life and lowest whole life cost.
Let’s work to reduce road accident statistics by getting the right solutions in the right places on the UK road network.
- The trend is upwards since a low point in 2013, and in 2016 KSI’s were up 7% compared to the 2010-2014 average: Department for Transport, ‘Reported road casualties in Great Britain: Quarterly provisional estimates year ending September 2016’, Statistical Release.
- BBA, BBA/HAPAS Approved High-Friction Surfaces, A study of the visual condition of BBA HAPAS High-Friction Surfacing (HFS) Sites, December 2015
- RSTA Service Life Document
- Militzer & Premathilaka, TRL published project report PPR789, 2016.
- Woodward & Friel, ‘Predicting the Wear of High Friction Surfacing Aggregate’, Coatings, May 2017
- Department of Transport, ‘Road Traffic Estimates 2016’, Statistical Release