Gripping stuff – benefits and specification of high-friction surfacing
28 June 2016
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
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
|Parameters||Site Categories per NRA HD28|
(i) prTAIT family 2 may be proposed for use on site categories Q, G1 & S1 with traffic category ≤ 1000
(ii) prTAIT family 4 may be proposed for use on site categories G2 & S2 with traffic category ≤ 750
(iii) prTAIT family 6 may be proposed for use on site category K with traffic category ≤ 500
Management of skid resistance is controlled by TII document AM-PAV-06045 (formerly NRA HD 28), and determines the investigatory level (note this is not an intervention level) for different Site categories and traffic levels. TII document DN-PAV-03023 (formerly NRA HD 36). Table 4.1 below defines the five site categories where HFS is only permitted to be installed (Clause 4.12), due to its whole-life cost compared to conventional surface coarse materials.
It is worth noting Site Category ‘Q – Approaches to and across major and minor Junctions’ is not included. It is the author’s view, as this TII Standard is only for the National Roads Network, that there should be allowance for installation of HFS on Site Category ‘Q’ on both national and non-national roads. Installations of HFS should be over the entire width on all lanes of the same traffic direction, and for a minimum of 50m. This may be extended where queuing traffic or sightlines indicate that 50m may not be sufficiently long (Clause 4.10 and 14).
HFS colour is at the discretion of the highway engineer, either buff, grey or the natural aggregate colouration if calcined bauxite is not used. Industry guidelines are:
• Grey – recommended for use on gradients, bends and approaches to roundabouts so that driver behaviour is not influenced & Road markings remain very visible;
• Buff – used to highlight a significant hazard. Recommended for approaches to pedestrian crossings and other high risk situations.
Colour pigmentation of HFS systems is possible, but adds significant cost, and colour retention is unsatisfactory. Pigmentation usually results in ‘grinning/shadow’ in time within the wheel tracks as the pigment wears off and the base aggregate is exposed. It is not standard practice.
N-PAV-03023 (formerly NRA HD 36) only covers national roads. Until there is a skid-resistance policy for non-national roads, direction can be extracted somewhere between ‘Guidance on the consistent installation of HFS – Jacobs, 2009’ and the Road Safety Authority’s ‘Rules of the Road’ document, as tabulated below.
|Speed||Proposed length of HFS (m); [Jacobs]||RSA – Rules of The Road|
|MPH||Km/h||Km/h||Min. stopping distance in wet conditions (m)|
Note 1. HFS should only be provided on 30Km/h(20mph) sites when there is a very hazardous location and evidence of skidding accidents
The UK Clause 924 is a performance based HFS specification, which differs considerably from the new TII specification. Systems must comply with British Board of Agrément/Highway Authorities’ Product Approval Scheme (BBA/HAPAS) and be installed by a BBA-approved contractor that is certified to install the particular system. BBA/HAPAS provide an independent external product and system accreditation scheme for the highway sector in the UK. It is important to note that both system and installer must be approved to comply with BBA requirements. For clarity, a BBA/HAPAS system installed by a non-approved contractor does not meet the requirements.
Difference between HFS and coloured demarcation
There has been confusion within the industry between HFS and coloured demarcation such as for central hatching areas, traffic calming, cycle lanes and advance stop lines (ASLs)/stacking locations. Over the past number of years, Northstone has received many enquiries with descriptions such as the following: ‘Resin based, high-skid-resistant surface treatment, red coloured surface treatment’ or ‘Red cycle-lane surfacing to NRA Clause 924’.
Demarcation surface guidance is found in the TII document DN‐PAV‐03023 clause 4.10, which states it must be a maximum difference of 5 PSV points from adjacent pavement. Coloured demarcation surfaces are typically PSV+55 with SRV+65, available mostly in red, green and blue, providing a more cost-effective solution than HFS.
Typical uses include demarcation such as central hatching areas, village gateways, cycle tracks/lanes, ASLs and speed-reducing bars (rumble strips). There is no BBA/HAPAS accreditation system for coloured demarcations. HFS is solely for the locations stated in DN-PAV-03023_Table 4.1.
Service life and whole-life cost
A 2011 study carried out by the Association of Directors of Environment, Economy, Planning and Transport and the Road Surface Treatments Association concluded that HAPAS-accredited cold-applied HFS should last between five and 11 years with a service life (mid-point) of eight years. Additionally, in 2013, the BBA commenced a study on the performance of HFS. The study was initiated by a general drop in demand for HFS and a perception that HFS offered inferior durability and no longer provided value for money, compared to alternative high PSV surfacing.
The report was published in December 2015. After assessing 272 HFS locations, the conclusion was that 95% of sites were performing after five years, with failed sites condemned due to poor substrate and/or installation workmanship. Conversely, high-PSV asphalt surfacing has been indicated to be performing poorly, with TRL in 2014 experiencing marginal Scrim values within 12 months (The Embankment, London 2012) and returning to HFS at safety-critical areas.
Northstone’s own experience locally is that HFS does offer excellent whole-life cost. Recent site follow-up assessments in Dublin indicate systems performing very well after five years. The installed systems were HAPAS-accredited cold-applied HFS systems incorporating calcined bauxite, installed by HAPAS-accredited operatives.
This recently photographed five-year-old surface at Castleknock would appear to have several years of service life remaining, before it needs renewal.
In 2008, Northstone installed HFS for Dublin City Council on the Swords Road at the Collins Avenue junction outbound and at the Iveragh Road junction inbound.
As can be seen from these recent pictures, both locations have met their service life expectations and would appear to have several years of useful life remaining. It is quite possible that the HFS will only fail due to the failure of the underlying substrate.
Northstone has recently completed over 3,500m2 at Chapelizod Bypass and Con Colbert Road, including the junction with South Circular Road.
The project involved the installation of 15 lane-junction approaches of HFS plus some infill sections on the South Circular Road. Roadstone, which was the main contractor to Dublin City Council, had resurfaced the carriageway in October 2015. Given the time of year, HFS works had to be deferred and were completed in May 2016.
Traffic-management (TM) plans were submitted and approved in advance by Roadstone’s specialist TM sub-contractor. Works had to be carried out on a nightshift basis per the council’s ‘Directions for the Control & Management of Roadworks in Dublin City’ guidelines. Permissible working hours were 19:30-06:00.
A HAPAS-approved cold-applied (resin bonded) HFS system incorporating calcined bauxite was installed in a very impressive six nights. This was aided by co-operation of all the stakeholders to ensure optimal safety, extensive TM planning and facilitate maximum productivity. As summer temperatures had not yet fully arrived, it was critical to have the HFS laid by midnight as there was a four- to five-hour set time on the resin. Sufficient time had to be allowed to suction sweep the excess calcined bauxite, allow the TM to be removed and reopen the road by the 6am deadline.
John Cagney, ROI special products manager, Northstone Materials
Paul McBride, NI special products manager, Northstone Materials
(Northstone Materials, a CRH company)