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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 ).
aggregate in the UK & Ireland. There is a misconception that high PSV equates to good skid resistance.
FEATURE High Friction Surfacing Skid Resistance of High PSV
In the UK, the service life of a HAPAS accredited cold HFS system for asset management purposes is considered to be 12 years (ADEPT/RSTA 2017).
High PSV is gained at the expense of other aggregate properties such as strength, abrasion and durability. (Woodward, WDH, Woodside, AR and Jellie, J.H). Research by Woodward et al suggests that standard PSV test is simply a ranking tool that offers limited prediction of the complex development of skid resistance during the life of modern asphalt materials.
In 2012 the London Skid Project commissioned a review of the use of HFS at pedestrian crossings in London (Stephenson, Hodgson & Premathilaka) .
Aggregates for High PSV Asphalt
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.
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.
This research has shown that the standard PSV test should not be considered as a measure of an ultimate state of polish, nor should it be the sole basis of surfacing aggregate selection (Woodward, WDH, Woodside, AR and Jellie, J.H).
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.
Different aggregate sources with the same PSV can perform differently under traffic and deliver different levels of skid resistance.
In late 2016, TRL in a study commissioned by Highways England released their project report PPR789.
Management of Skid Resistance
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). HD 36, although a useful guide to PSV requirements, does not take account of the aggregate source. Local knowledge of aggregates is required to ensure that they will provide the required skid resistance performance over the service life.
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.
In the UK & Ireland, management of skid resistance on principal roads is controlled by HD 28/15 (AM-PAV-06045), 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. HD 36/06 (DN-PAV-03023) 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.
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).
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.
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.
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).
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.
This is frequently not the case.
Asphalt
Individual aggregates can deliver a range of skidding resistances, even at the same traffic level.
This was a performance review of the use of High PSV Asphalt (PSV ≥ 68) to replace HFS.
The RSTA in the UK have recently published guidance documents on road skid resistance strategy, policy & procedures.
Under the UK HAPAS scheme, 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).
Council Journal 43
Aggregate Properties & Skid Resistance
PSV is typically the main property used when selecting surface course
Typical Motorway Interchange With HFS Shown in Red (Atkins). It has become industry practice to install HPSVA in this type of application.
Product Approval