Pavement - High Friction Surfacing (HFS)

Skid resistance is widely acknowledged to be very important in terms of safety. It is a term used to describe the combined ability of a road surfacing material and the vehicle tyre to deliver the required performance in terms of grip. The provision and maintenance of skid resistance are closely inter-related with other road surfacing properties such as noise and rolling resistance.

The friction available on a road is dependent on the condition of the surfacing materials i.e., how polished the road surface has become under traffic. This depends primarily on the aggregate type (its resistance to polishing), the amount of heavy traffic and the type of traffic movement (free flow, braking, turning etc.).

Lack of friction can be important contributory factor in road accidents. So, there is a dire need of providing high friction surfacing particularly on high risk locations.

High friction or ‘anti-skid’ surfacing is surfacing that makes use of aggregates with better skid-resistance properties than normal. They minimise the risk of skidding accidents and vehicles overshooting stop or give way lines.

It is well known fact that the choice of surfacing material/system plays a vital role in providing roads that meets the needs of the user, are safe and give value for money.

Micro-texture of the surface aggregate is the main contributor to sliding resistance and is the dominant factor in determining skid resistance. The micro-texture characteristics of a particular stone depend on its polishing susceptibility under the action of tyre forces. It is measured by Polished Stone Value (PSV).

The following locations have the potential of posing high risk:

• Approaches to pedestrian crossings
• Approaches to junctions
• Roundabout circulation areas
• Bends (radium < 500m) on all type of roads.
• Sites with gradient greater than 5%.

The normal surfacing materials cannot provide the required friction (or PSV) for the said locations. So, there is a need of high friction surfacing on these locations (reference DMRB Standard CS228). (The safety auditors always suggest provision of high friction surfacing (HFS) on these locations). The high friction surfacing is generally an artificial aggregate produced out of the ‘residue’ from aluminium called calcined bauxite. In order for the aggregate to be retained on the surface with high stresses expected, the binder needs to be stronger than can generally be achieved with conventional materials. The use of epoxy-resin as the binder in surface treatment has increasingly been adopted across the UK and other countries on approaches to pedestrian crossings and roundabouts.

There are two main processes employed to produce HFS surfaces. Both – ‘hot’ and ‘cold’ applied – use calcined bauxite as the high PSV aggregate. The choice of system is based on site characteristics, weather and ambient temperature at the time of laying, plus personal preferences and experience often influence the type of system laid.

With the hot system the materials come ready blended in polythene bags. The resin used is thermoplastic, i.e., it is plastic and workable when hot, and then solidifies on cooling to ambient temperature. Application, after the material has been heated in a thermostatically controlled ‘pot’ is via on open bottomed rectangular ‘shoe’ and finished with a screed. Thickness should be approximately equal to the nominal size of the coarse aggregate.

This system is less durable, but it is not as weather dependent and can be opened to traffic within 15 minutes of the last screed being laid. Due to the method of application, ridges are sometimes formed perpendicular to the direction of travel than can cause driver discomfort, and a thicker layer than required can occur, causing durability issues.

With regard to cold system there are a number of proprietary resin-based formulations on the market. The resins are usually epoxy or polyurethane based. Once the resin is mixed there is a set time for it to be spread and the surface aggregate applied before an initial set takes place.

The resin should be applied – usually by hand – at the appropriate spread rate, as directed by the manufacturer, with the high PSV aggregate spread (by hand) to slightly exceed the amount required to ensure complete coverage of the resin. Excess stone is swept off after the resin has set and bonded to the aggregate in contact.

Use of cold mix HFS on freshly laid asphalt surfaces can cause problems and it is often necessary to allow the asphalt temperature to cool sufficiently and the bitumen to reduce on the aggregate before applying a HFS, to ensure a good bond can be achieved.

With careful consideration and good workmanship, both types of HFS should last the life of the pavement. If appropriate standards are used, there should be no concern over misuse of the products and minimal maintenance for highway authorities.

In most of the developed countries high friction surfacing is used extensively on the approaches to almost all the roundabouts (and/or pedestrian crossing). Some of the advantages of high friction surfacing (in addition to providing enhanced skid resistance) are:

• Unlike bitumen-based surfacing, resins tend to be colourless and can be coloured, and can thus be coloured much more easily to make a visual impact.
• The high friction surfacing are relatively expensive surface treatments, but the skid resistance is durable, and they have proved very cost effective in accident reduction.
• The products with reflective aggregates are proving effective at segregating traffic on unlit roads and improving the visibility in urban areas where street lighting is being removed or reduced to lower the consumption of electricity with the aim of reducing the carbon footprints.

The use of HFS is quite important for ensuring safety of road users. It is quite important to set the standards for it, introduce them on critical locations of existing road network. In the conditions where over-speeding is a norm and where skid resistance is likely to be reduced due to appearance of bitumen film on the surface of roads (i.e., bleeding) during summer, the application of HFS on sensitive locations will go a long way towards safer conditions.

Specifications of HFS used in the UK

The HFS systems are proprietary material. They shall have current British Board of Agreement HAPAS Roads and Bridges Certificates.

A HFS system with a current British Board of Agreement HAPAS Roads and Bridges Certificate shall only be installed by a Contractor approved by the BBA and the Certificate Holder as an Approved Installer for that system.

Aggregate used in HFS systems shall have a minimum declared PSV category in accordance with BS EN 13043, clause 4.2.3. The resistance to abrasion of coarse aggregate shall have a maximum AAV in accordance with BS EN 13043, clause 4.2.2. The Contractor shall provide, before work commences, test certificates, issued by an appropriate organisation accredited in accordance with sub-clauses 105.3 and 105.4 for those tests, not more than six months previously, showing conformity with the requirements.

For each location where HFS is applied, the total quantities of each system component used, the measured area of the surface treated and the calculated coverage rate in kg/sq.m shall be reported to the Overseeing Organisation within three days of completion at that location. For system in which aggregate is broadcast over a film of binder applied to the surface, the calculated coverage rate shall be that of the binder film and shall not include the mass of the aggregate.

The Contractor shall guarantee the high friction surfacing materials and workmanship for a period of two years for the date of opening the surfacing to traffic. This guarantee shall exclude defects arising from damage caused by settlement, subsidence or failure of the carriageway on which the surfacing has been applied but shall cover failure to meet the minimum requirements set out in BBA/HAPAS ‘Guidelines Document for the Assessment and Certification of High Friction Surfaces for Highway’.

Note: The site category/character and maximum traffic levels (CVs per lane per day) define the necessity of HFS. Moreover, expected service life of HFS (5-10 years) depends on the traffic levels.