LONG TERM STUDY OF SKID RESISTANCE ON IN-SERVICE ROADS IN ENGLAND. Chris Kennedy, John Donbavand and Ramesh Sinhal

Benchmark Sites The Highways Agency in the UK has established a network of 43 skid resistance benchmark sites in England. 39 of these sites have bituminous surfacing and were established in more Sites added 2 in 2008 and 2 in 2009 –These 4 sites have mainly concrete surfacing

Sites are positioned throughout the English Trunk roads. Each site is 1 to 2km long Site locations

Reasons for the Benchmark Sites Establish any changes in the pattern of skid resistance within year and between years. Establish whether mean summer skid resistance levels in any year are significantly higher or lower than in an average year.

Reasons for the Benchmark Sites Provide information on the rate of initial polishing after a new surface has been applied. Function as dynamic calibration sites for checking the SCRIM machines. The reasons for the 4 newly added concrete sites was to see whether concrete surfaces reacted the same as bituminous surfaces to seasonal changes

Has the summer period remained the same? for each year up until 2005: 3 SCRIM surveys carried out for each site –1 st May to 20 th June;early –21 st June to 10 th AugustMid –11 th August to 30 th September Late

Seasonal Shape Historical experience More recent observations

Surveys 2006 Onwards In 2006 an additional survey was undertaken towards the end of October –(SCRIM test season finished on 30 th Sept) The additional survey was to determine if the site readings would return to those of the early run. It was found that they did so the additional survey was continued in 2007, 2008 and 2009 to confirm.

Impact of the 4 th run The survey periods in the UK skid standard have subsequently been changed because of these findings Survey PeriodDates for Previous Standard Dates in the current HA Contracts Early1 st May to 20 th June 1 st May to 27 th June Middle21 st June to 10 th August 28 th June to 24 th August Late11 th August to 30 th September 25 th August to 20 th October

Implications of the change The mean summer SCRIM coefficient will have been slightly low resulting in some sites being investigated unnecessarily; Greater machine productivity by adding 3 weeks to what was a 20 week survey period.

Does skid resistance plateau to an equilibrium value? Conventional approach says that a given aggregate under a given level of traffic will reach an equilibrium level of skid resistance after rejuvenation of the coarse aggregate each year during the winter/wet season.

The experimental evidence 25 sites over 8 years

Implications of no plateau Although rejuvenation of the coarse aggregate occurs some residual polishing remains; On the heavily trafficked network monitored in the UK the long term reduction was 0.05 over an 8 year period. When selecting aggregates they should be selected to provide a skid resistance at least 0.06 above the desired skid resistance.

Is there a link between rainfall and skid resistance? Many studies have demonstrated that seasonal variation occurs: But can meteorological data be used to explain general trends in skid resistance?

Evidence of a link between SC and rainfall YearMayJuneJulyAugustSeptOctober * Av Year National rainfall All site SCRIM average readings

The use of meteorological data? It can give broad indications but cannot be used to give seasonal correction factors because rainfall is often very localised.

Do testing conditions affect skid measurements? All skid testing equipment puts water on the road surface with the intention of providing a minimum water film thickness; Below the minimum water film thickness skid resistance can vary dramatically but above the minimum (0.9mm for SCRIM) water film thickness is not thought to be critical.

Line Plot for Site 4 The mid run is atypically high compared to the other readings

Photo of site 4 Early Run

Photo from Mid run Evidence of standing water

Implications of standing water On a smooth textured surface it is clear that testing when standing water is present will affect SCRIM measurements and therefore on this type of surface surveys should not be undertaken in these conditions; These results also emphasise the importance of using a water flow control for all testing to avoid excessive water being applied at low survey speeds.

Does concrete behave like a bituminous surface? New sites were introduced in 2008 and 2009 to investigate if concrete surfaces react to seasonal change the same way as bituminous surfaces.

Site and 2009 Runs No signs of seasonal effects

Site runs Concrete Surface Calcined Bauxite

Site 43 No signs of seasonal effects

Seasonal correction on concrete surfaces The evidence is that seasonal effects on concrete are significantly less than on bituminous surfaces; The current advice in the UK is NOT to correct skid resistance values measured on concrete.

Thing Learnt so Far In England SCRIM mean summer values can be obtained only if the survey season is extended into October. Surveying in standing water increases the SCRIM readings and must be avoided. There is no plateau and the skid resistance continually trends downwards requiring IL SC.

Thing Learnt so Far Broad features of the trends in skid resistance coincide with national rainfall patterns –This supports the general view that seasonal variation of skid resistance is strongly affected by rainfall. There appears to be far less seasonal variation on concrete surfaces than occurs on bituminous surfacings.

Thank you