Is High PSV Asphalt a Safe Alternative to High Friction Surfacing?

 

 Introduction

In recent years, the use of HFS has reduced considerably in UK & Ireland. There is a view in some quarters that conventional surfacing materials with a high PSV aggregate can offer a better whole life cost option to deliver a high level of skid resistance. Is there merit to this viewpoint and does High PSV Asphalt offer a safe alternative to HFS?

Management of skid resistance is controlled by HD 28/15, and determines the investigatory level IL (note this is not an intervention level) for different Site categories and traffic levels. The IL for a high risk site with heavy traffic would be 0.55. HD 36/06 provides guidance on the minimum PSV requirements for a given IL and defines the site categories where High Friction Surfacing (HFS) should be installed.  

Table 3 1a HD36

HFS is perceived by some to have a high cost per square meter and offer poor value for money. Local authorities have looked for cheaper alternatives at high skidding risk locations and in some locations have replaced HFS with asphalt incorporating high PSV natural aggregate. The practice has also extended to DBFO Contractors, where perceived savings are made by replacing HFS at high risk locations with high PSV (typically 68+) asphalt. 

 

High Friction Surfacing

HFS or Anti-skid Surfacing is a proven road surface treatment that increases skid resistance and reduces braking distance, thereby reducing the potential for accidents and saving lives. It has a long history of proven use having first been trialled in the UK in the late 60’s. The aggregate used in HFS is Calcined Bauxite (normally buff or grey), which is usually imported from either China or India. Typical locations for HFS installation include approaches to roundabouts, pedestrian crossings, junctions, sites with steep gradients and dangerous bends. Numerous studies have demonstrated HFS typically reduces accidents by 35% (TRL MOLASSES: Monitoring Of Local Authority Safety Schemes database, 2001). 

Calcined Bauxite is the perfect aggregate for HFS in that it has a high PSV and low AAV typically less than 3. It has a typical relative density >3 and a Moh’s hardness of 9, compared to diamond being the hardest with a Moh’s value of 10. It is a very hard wearing and abrasion resistant aggregate. The aggregate has a cubical (block) shape, is very clean with a 1-3mm grading as recommended by the RSTA HFS Code of Practice document. A Clause 924 BBA/HAPAS Type 1 HFS should retain adequate microtexture, macrotexture and SCRIM values (> 0.55) on high stressed sites over its service life, whilst being durable enough to withstand crushing & traffic wear.

Indian Grey Bauxite

1-3mm Indian Grey Bauxite

Microtexture of HFS 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 HFS system, 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&Macro

Micro and Macrotexture (Nicholls, 2002)


High PSV Asphalts

Thin Surface Course Systems or SMA derivatives with a high PSV natural aggregate (≥ 68) have been used at some locations where HFS would previously have been applied. A number of proprietary and hybrid mixtures have been developed for such applications. Also HRA with a high PSV precoated chip (≥ 68) has been used at some locations. Can these surfaces achieve a similar skid resistance performance to HFS and can they retain their skid resistance over time? This writer believes that the practice is flawed, creates unnecessary safety hazards and increases risk of skidding accidents at high risk locations.

 

PSV has a history of over emphasis amongst Highway Engineers, to the detriment of other properties required for HFS. The PSV test, although an indicator does not provide an accurate prediction of the in service skid resistance. Research has proven that PSV is gained at the expense of almost every other property such as strength, resistance to abrasion and durability (Woodward, 1995). But research also shows that the PSV test should only be regarded as a ranking test and not as a method to predict in-service skid resistance. The PSV test is not the ultimate state of polish for an aggregate but rather produces a result that is dependent on the test conditions (Friel; Woodward, Engineers Journal Feb 2016). A natural aggregate will not have the same resistance to abrasion as Calcined Bauxite. Typical AAV’s for a UK high PSV Sandstone or Gritstone would be around 8, as against Calcined Bauxite with a typical AAV of 3 or less.

 

Different aggregate sources with the same PSV can perform differently under traffic and deliver different levels of skid resistance (TRL Report 322, 1998). This conclusion is further supported by a more recent study carried out by WDM for Somerset County Council (Stephenson, Premathilaka, Jones, Davies) regarding the performance of a range of different aggregates & surfacing materials after 2-8 years of trafficking. HD 36 while 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.

 

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 high PSV surface types as well as HFS at 23 locations over a 3-4 year period. It found that whereas some high PSV surface types may initially provide acceptable SCRIM values, these values reduce after 2-3 years of traffic. The report concluded that the probability of a high PSV asphalt exceeding the 0.55 IL beyond 3 years of service is low, whereas all the HFS surfaces appear to provide skid resistance over 0.55 over the same period.

 

 

Conclusions

  • For high risk sites (IL > 0.55), the use of HFS with Calcined Bauxite is the only surface that can give a high confidence of performance. 
  • 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. 
  • The source of the high PSV aggregate can have a major bearing on the skid resistance. This is not taken into account in HD 36 or generally by local authorities. 
  • Whereas High PSV asphalt may be appropriate for some lower risk locations, it is not a better whole life cost option than HFS at high risk locations.