On the Slippery Slope? High PSV Asphalt & HFS
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No liability is accepted for errors or inaccuracies in any of information contained on this website. All views or opinions expressed are personal to the author who does not purport to speak for, or represent, any professional body or organisation. Reliance should not be placed on any material contained in this webpage without first seeking and obtaining further professional advice.
High Friction Surfacing (HFS) refers to a surface treatment approximately 3-5mm thick which provides very high skid resistance for drivers to brake under emergency conditions at hazardous locations. Typical locations for HFS installation are high risk locations including approaches to roundabouts, pedestrian crossings, junctions, sites with steep gradients and dangerous bends. HFS has been in commercial use for over 50 years & has a proven track record of significant reductions in skid related accidents. HFS has been found to reduce wet weather accidents in UK by 57% (TRL Research – BRF Road fact 1997).
Grey HFS at Dangerous Bend
For the purposes of this article High PSV Asphalt (HPSVA) refers to either a Thin Surface Course with a +68 PSV natural aggregate or a traditional HRA with +68 PSV natural aggregate pre-coated chips. In recent years, there has been an increasing trend in UK & Ireland to use HPSVA at locations where HFS would have been previously applied. This practice has been common on DBFO / PPP schemes, motorways, trunk roads, national roads & local authority schemes. HPSVA is believed by many to offer a more cost effective mechanism than HFS of maintaining appropriate skid resistance over the service life of the road. The writer believes that this practice is short tern thinking with long term adverse consequences, is extremely dangerous and will most likely result in multiple preventable skidding accidents and loss of life.
Why HFS Works
Microtexture of an aggregate, gauged by its Polished Stone Value (PSV), is the dominant contributor to skidding resistance at lower speeds, less than 30 mph. Macrotexture (texture depth) created by the positive texture of the surface offers rapid drainage routes between tyre and pavement, and also allows air trapped beneath the tyre to escape. Macrotexture contributes best in wet skidding resistance at higher speeds.
Micro and Macrotexture (Nicholls, 2002)
The hardness & strength of an aggregate will be an important component of its performance within a bituminous surface course. Although an aggregate may be strong, it may not be resistant to the abrasive action of vehicle tyres. The Aggregate Abrasion Value (AAV) test provides an estimation of surface wear of an aggregate under traffic. Lower numerical values indicate more resistance to abrasion.
used in HFS is 1-3mm Calcined Bauxite (normally buff or grey), which is usually
imported from either China or India. Calcined Bauxite has a high PSV (>70)
and low AAV, typically less than 3. It has a typical relative density >3 and
is particularly hard having a Moh’s hardness of 9, compared to diamond being
the hardest with a Moh’s value of 10. Because Calcined Bauxite is a very hard
wearing and abrasion resistant aggregate, it wears away very slowly under
traffic within a robust HFS binder system, while maintaining macrotexture,
microtexture and SCRIM values. Calcined Bauxite retains its sharp edges,
thereby providing a bite at the tyre interface. Durable Micro-texture, reduced
contact points, high contact pressure points and improved hydraulic
conductivity are all critical for high skid resistance and reduced braking
distances ( Parry, TRL 1996 ).
The service life of a HAPAS accredited cold HFS system for asset management purposes is considered to be 12 years (ADEPT/RSTA 2017).
Aggregates for High PSV Asphalt
There is a very limited supply of high PSV aggregates. A desktop study has indicated that there are circa 8-10 quarries in UK & Ireland that supply +68 PSV aggregates for surfacing. Areas of supply include the south of Scotland, Cumbria, mid & south Wales, west Kerry and north east Down. The predominant rock type for these aggregates is the Sandstone/Gritstone family. The reported & typical AAV’s for these aggregates are in the range of 6-9. When incorporated in a Thin Surface Course, the most common aggregate sizes are 10mm & 14mm. For HRA with pre-coated chips, the chip size is generally 20mm.
Management of Skid Resistance
In the UK, management of skid resistance on principal roads is controlled by DMRB CS 228 (formerly HD 28), which determines the investigatory level (IL) for different Site categories and traffic levels using the Mean Summer SCRIM Coefficient (MSSC). The typical IL for a high risk site with heavy traffic would be 0.55. DMRB CD 236 (formerly HD 36) provides guidance on the minimum PSV requirements for specific site categories and where HFS should be installed. A typical application for HFS would be on roundabout approaches at motorway interchanges. This is now frequently changed to HPSVA. The approach in Ireland is similar to the UK approach.
Typical Motorway Interchange With HFS
Shown in Red (Atkins)
For non-principal, regional & local roads, Local Authorities have a duty to maintain them & ensure that they are not dangerous for traffic. A Skid Resistance Policy will be an important component of this obligation. The RSTA in the UK have recently published guidance documents on road skid resistance strategy, policy & procedures.
Aggregate Properties & Skid Resistance
PSV is typically the main property used when selecting surfacing aggregate in the UK & Ireland. As outlined in a previous article , there is a misconception that high PSV equates to good skid resistance.
This is frequently not the case as High PSV aggregates can frequently have inferior other properties such as strength, abrasion and durability. (Woodward, WDH, Woodside, AR and Jellie, J.H). Different aggregate sources with the same PSV can perform differently under traffic and deliver different levels of skid resistance. Individual aggregates can deliver a range of skidding resistances, even at the same traffic level. In locations where additional braking or deceleration can be expected, for example on approaches to slip roads, lower values of MSSC can occur than on the remaining mainline carriageway (TRL Report 322, 1998).
AAV values range from 1 for hard flints to over 15 for aggregates which would normally be considered too weak for use in road surfacing (Hawkes & Hosking 1972). Inadequate resistance to abrasion of road surfacing aggregates causes a loss of the texture depth that is necessary to maintain high speed skid resistance (Hawkes & Hosking 1972).
Skid Resistance of High PSV Asphalt
In 2012 the
London Skid Project commissioned a review of the use of HFS at pedestrian
crossings in London (Stephenson, Hodgson & Premathilaka).
One of the criteria was to assess the performance of the alternatives to HFS. SCRIM data was collated on a range of HPSVA surface types as well as HFS at 23 locations over a 3-4 year period. It found that whereas some HPSVA’s may initially provide acceptable SCRIM values, these values reduce after 2-3 years of traffic. The report concluded that the probability of a HPSVA exceeding the 0.55 IL beyond 3 years of service is low, whereas all the HFS surfaces appeared to provide skid resistance over 0.55 over the same period.
late 2016, TRL in a study commissioned by Highways England released their
project report PPR789.
This was a performance review of the use of High PSV Asphalt (PSV ≥ 68) to replace HFS. The study examined the relative performance of both material types over a range of sites with different traffic levels, road types etc. The report found that, in line with other studies, currently available HPSVA’s are not able to consistently achieve skid resistance levels similar to traditional HFS with Calcined Bauxite. The report recommended that the use of traditional HFS should be continued for high risk sites.
The findings of both reports are not surprising. A 1-3mm Calcined Bauxite with high resistance to abrasion is far less likely to polish & wear away than the softer larger size Sandstone aggregates, with lower abrasion resistance used in HPSVA. As previously stated good macrotexture is important for high speed skid resistance. Various researchers have demonstrated that there is a general increase in skid resistance for smaller surface aggregate sizes. The 1-3mm aggregate used in HFS is smaller than the aggregate used in the usual HPSVA’s.
Under the UK HAPAS scheme (soon to be superceded by BS 8870), a HFS must undergo a rigorous assessment procedure before it can be certified for use. There are six stages to the procedure (BBA Guideline Document March 2015)
Stage 1 – Assessment of applicant's data
Stage 2 – Assessment of factory production control
Stage 3 – Laboratory testing (and optional tests)
Stage 4 – System installation trial
Stage 5 – System performance trial (if applicable)
Stage 6 – Certification
The criteria for laboratory performance testing include wear, scuffing & tensile adhesion. A two year system performance trial must be undertaken on site. At the end of the two years, the system must meet minimum criteria including Skid Resistance Value (SRV), texture depth, tensile adhesion, erosion index & visual assessment.
In Ireland, Transport Infrastructure Ireland (TII) introduced a new HFS specification in 2015. A proposed Contractor system has its performance evidenced by a prTAIT (provisional Type Approval Installation Trial). The prTAIT consists of a defined section where high friction surfacing has been installed using Factory Production Control (FPC) and which has been subjected to in-service performance annually over a 5yr period. Testing following installation and one year later include SRV, texture depth, Pull Off Test, Shear Strength Test & visual assessment.
There is currently no product approval system in UK or Ireland for the use of HPSVA in situations where HFS would normally be required.
It has become industry practice in many areas to use HPSVA at locations where HFS would have been previously applied. The principal reasons are the perceived cost saving and the mistaken belief the high PSV equates to long term skid resistance. There is no accreditation system for HPSVA installed in lieu of HFS. Notwithstanding this roads authorities, PPP/DBFO companies, motorway maintenance companies & local authorities are sanctioning the use of HPSVA at high skidding risk locations.
Surely there needs to be a similar accreditation system for HPSVA as there is for HFS? A particular aggregate & binder HPSVA should have to undergo a rigorous performance evaluation before it can be approved for use at high skidding risk locations. What follow up SCRIM testing has been carried out on previously laid HPSVA sites?
As stated previously there can be major performance differences for surface courses with aggregates of similar PSV but from different sources. High PSV asphalt is not a like for like replacement for HFS and does not provide a guarantee of skid resistance over the service life. Whereas it may be appropriate for some lower risk locations, it is neither a better whole life cost option or an equivalent to HFS at high risk locations.
It is the author’s belief that the current widespread practice will result in numerous preventable skidding related accidents and consequential loss of life. The author believes than many high risk locations where HPSVA has been installed will subsequently be overlaid with HFS to prevent & reduce further accidents.
Roads authorities have an obligation to take such steps that are reasonably required to ensure the safety of the network. Will the current practice mean that roads authorities, PPP/DBFO contractors, designers, road safety auditors & local authorities become embroiled in litigation because of a short term decision to replace HFS with HPSVA at high risk locations?