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Why High Friction Surfacing with Calcined Bauxite Provides The Best

Safety Surface for Skid Resistance


 

 

High Friction Surfacing – Definition & Applications

High Friction Surfacing (HFS), commonly termed Anti-skid, refers to a surface treatment approximately 3-5mm thick which 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. The aggregate used 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.

Buff HFS at a Dangerous Bend

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’s are set out in Table 4.1. 

 Table 4.1 HD28/15

 

HD 36/06 provides guidance on the site categories where HFS should be installed. Table 3.1a from IAN 156/16 rev. 1 October 2016 provides updated guidance from HD 36 on the minimum required PSV for a given IL & traffic level and also where HFS should be installed.

By way of explanation, let’s consider an approach to a pedestrian crossing as an example. HD 28/15 table 4.1 tells us that it is site category K with an IL of 0.5 or 0.55 based on a judgement of the degree of risk. Turning to the updated Table 3.1a from HD 36, we see that where the IL is 0.55 and the cv/lane/day is > 500, then HFS is required.

 

Why is Calcined Bauxite Used for HFS?

Calcined Bauxite is the perfect aggregate for HFS in that it has a high PSV and low AAV typically between 1 & 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 should be cubical (block) shape, very clean with a 1-3mm grading as recommended by the RSTA HFS Code of Practice document.

1-3mm Buff & Grey Bauxite

Why are these properties so important?

It is necessary to ensure that, over the required service life, a correctly installed HFS will retain adequate microtexture, macrotexture and SCRIM values (> 0.55), whilst being durable enough to withstand crushing & traffic wear.

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 and Macrotexture (Nicholls, 2002)


Are High PSV Natural Aggregates a Viable Alternative to Calcined Bauxite in HFS?

Given the high cost of importing Calcined Bauxite, it seems reasonable to ascertain whether locally available high PSV aggregates can fulfil the same purpose. The writer would urge great caution in this regard. Research has proven that PSV is gained at the expense of almost every other property such as strength and durability (Woodward, 1995). For example chalk has a high PSV but would not make a very good aggregate.

 What high PSV natural aggregate can match the unique properties & performance of Calcined Bauxite?

 In NCAT report 15-04 July 15, the US Federal Highway Administration carried out a major lab & field study to examine the surface friction performance of seven natural alternative aggregates in comparison with Calcined Bauxite. The study concluded that the measured friction using these aggregates was not equal to Calcined Bauxite.

 Closer to home, the University of Ulster (Woodward & Friel) carried out lab trials to determine whether there is potential for natural aggregates that are commonly available in the British Isles to be used for HFS. Nine natural aggregates plus Calcined Bauxite were tested. The PSV of the natural aggregates varied from 40 (Limestone) to 70 (Sandstone). After a 100,000 wheel passes, there was a large variation in the Pendulum Tester Value (PTV) and retained texture depth of the natural aggregates. Not surprisingly Limestone was the worst performer and Calcined Bauxite had the highest PTV & retained texture after 100,000 wheel passes.

 The writer has recently had the opportunity to review two HFS road sites laid with a natural high PSV aggregate. The system in question was being marketed as equivalent to HFS with Calcined Bauxite. Both surfaces were reported to have been laid approximately two years. The writer’s observations were that the surfaces seemed far smoother than a standard HFS (no sandpaper effect). The aggregate seemed to have worn down and there was a perceived loss of texture. Sand patch tests were carried out on one of the sites and confirmed the loss of texture, with the reported texture depth only at 0.65mm, substantially below the HFS minimum of 1mm. PTV tests were also carried out and the ‘natural aggregate HFS’ showed results which were no better than the adjacent uncovered HRA. At the same time, PTV tests carried out on a newly installed HFS with Calcined Bauxite demonstrated a marked improvement in PTV.

Loss of Texture on HFS with Natural Aggregate

The following questions must be asked where natural aggregates are proposed:

  • Will the aggregate wear significantly or crush over time?
  • Will the texture be retained?
  • Will the stone polish?
  • Most importantly, what will the SCRIM value be over time?

It is the writer’s view that any proposed HFS system with natural aggregate should be monitored on the road for texture depth & SCRIM over three to five years before it can be considered as a viable alternative to traditional systems with Calcined Bauxite.

 

What About High PSV Asphalts in Lieu of HFS?

HFS is perceived to have a high cost per square meter. Cash strapped local authorities have looked for cheaper alternatives at high skidding risk locations. One such practice has been to replace the 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.

 Does this approach provide an acceptable solution and a better whole life cost option?

 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. 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, although 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. 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.

 

 

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.

HFS with natural aggregate is unlikely to provide the same performance. Any proposed natural aggregate HFS system should be monitored in road trials over three to five years before it can be carefully considered for further use.

 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. 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.