High-friction surfacing (HFS), commonly termed ‘anti-skid’, refers to a surface treatment approximately 3-5mm thick that provides enhanced surface skid-resistance for drivers to brake under emergency conditions at hazardous locations.
HFS is defined by industry as having a minimum skid-resistance value (SRV) of 65, measured using the portable skid-resistance pendulum tester (as defined in TRL Report 176, Appendix E). Typical locations for HFS installation include approaches to roundabouts, pedestrian crossings, junctions, sites with steep gradients and dangerous bends.
Chapter 9 in the Transport Infrastructure Ireland (TII) document DN-PAV-03024 (formerly NRA HD 37) provides information regarding HFS. Numerous studies have demonstrated that HFS typically reduces accidents by 35% (TRL MOLASSES: Monitoring Of Local Authority Safety Schemes database, 2001)
Typical breaking distance comparison of HFS v standard surface course
[caption id="attachment_30709" align="alignnone" width="663"] Car travelling at 90km/h, with braking distance in metres in wet conditions (RSTA, 2014)[/caption]
Car travelling at 90km/h, with braking distance in metres in wet conditions (RSTA, 2014)
HFS using calcined bauxite as aggregate provides high-contact pressure between pavement and tyre at interface, whilst also providing hydraulic conductivity (drainage) – hence also reducing aquaplaning risk.
Microtexture of HFS aggregate, gauged by its polished stone value (PSV), is the dominant contributor to skidding resistance at lower speeds, less than 50km/h (i.e. urban Ireland). Macrotexture, which is 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. It contributes best in wet skidding resistance at higher speeds i.e. Irish national road network.
There are two primary HFS systems, a cold applied ‘thermosetting’ (resin bonded) system and a hot applied thermoplastic (pre-mixed) system. This article will mostly discuss the cold applied system, as predominantly used in high traffic count locations in Ireland.
Cold applied systems involve application of an accredited factory production controlled (FPC) resin of either epoxy, bitumen-extended epoxy, polyurethane, polyurea or methyl methacrylate (MMA) compliant per TII document CC-SPW-00900_Table 23a. These systems comprise a resin layer, dressed with a ’scatter coat’ of aggregate, typically graded 1-3mm.
To attain long-standing texture depth, a strong and flexible resin with good adhesion properties is paramount to ensure a bond to the underlying surface and to hold the aggregate firmly in place. The aggregate is very hard with a low aggregate abrasion value (AAV) and with a PSV+70, compliant per TII document CC-SPW-00900_Table 23b or c.
Once the resin has cured (between two and six hours, pending ground temperate), a suction sweeper is used to remove the excess aggregate. In essence, a durable aggregate is effectively glued to the road by a very strong binder. Cold systems are typically applied from April to October due to moisture and temperature constraints. For night works (normal practice in Dublin), the laying season is shorter: realistically from May to September.
This proven system type originated in the USA in the 1950s. Calcined bauxite is the perfect aggregate for HFS in that it has a high PSV and a low AAV typically of 2. It has a Moh’s hardness of 9, compared to diamond being the hardest with a Moh’s value of 10. Calcined bauxite is produced in two colours: buff and grey. It is important that the aggregate is not dusty, and is cubical with a compliant flakiness Index. The HFS aggregate chosen must retain adequate micro and macrotexture for skid resistance, whilst being durable enough to withstand crushing and traffic wear for the service life demanded.
Care must be taken in use of high PSV natural aggregate, as research has proven PSV is gained at the expense of almost every other property such as strength 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).
There is potential for mid-range PSV natural aggregate to produce adequate in-service skid resistance, and due to their relatively higher durability, provide an economical and optimal HFS system regarding whole-life cost. The authors believe that further research is required and that potential ‘natural aggregate HFS systems’ should be monitored over a five-year period for retained texture and SRV.
TII SPW-00900 section 7.3 superseded the old NRA Clause 924 HFS specification in March 2015. It is a performance specification that requires the contractor to ensure the HFS system to be used exceeds the contract-specific demanded ‘service life’ for detailed site categories and traffic counts. HFS systems on offer from contractors can cover a range of prTAIT family, which are trial evidence proven per the following parameters:
TII document DN-PAV-03075 (formerly NRA HD 3-1) - Annex 2C
Table 2C.1 – Limiting number of prTAITs and defining families for high-friction surfacing