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FEATURE High Friction Surfacing On a Slippery Slope? High PSV
Asphalt v High Friction Surfacing
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. Recently there have been moves to design this out of Irish road- building projects and replace with an unproven alternative called High PSV Asphalt (HPSVA). Council Journal is aware of at least two major projects completed where this has occurred. John Cagney, Chartered Civil Engineer, explains.
Typical locations for HFS
resistance over the service life of the road. The writer believes that this practice is short term thinking with long term adverse consequences, is extremely dangerous and will most likely result in multiple preventable skidding accidents and loss of life.
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. The aggregate 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.
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installation are high risk locations including approaches to roundabouts,
pedestrian crossings, junctions, sites with steep gradients and dangerous bends.
HFS with Calcined Bauxite has been in commercial use for over 50 years & has a proven track record of significant reductions in skid related accidents.
Why HFS Works
HFS has been found to reduce wet weather accidents in UK by 57% (TRL Research – BRF Road fact 1997).
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.
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.
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.
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