LONG TERM STUDY OF SKID RESISTANCE ON IN-SERVICE ROADS IN ENGLAND.

Chris Kennedy, John Donbavandand 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 2002.

• 4 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 is1 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

– 1st May to 20th June; early– 21st June to 10th August Mid– 11th August to 30th September Late

Seasonal Shape

0.45

0.48

0.51

0.54

0.57

0.6

april may june july august sept oct

Skid

Res

ista

nce

Month

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 30th 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 4th run

• The survey periods in the UK skid standard have subsequently been changed because of these findings

Survey Period Dates for Previous Standard

Dates in the current HA Contracts

Early 1st May to 20th June

1st May to 27th June

Middle 21st June to 10th August

28th June to 24th August

Late 11th August to 30th September

25th August to 20th 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.

0.30

0.40

0.50

0.60

0.70

0 1 2 3 4 5

TIME (years)

SK

ID R

ES

IST

AN

CE

new surface

initial polishing

polishing duringspring and summer

rejuvenation duringautumn and winter

equilibriumsummer level

overallequilibrium level

The experimental evidence

y = -0.7914x + 72.16R² = 0.7318

y = -0.8196x + 64.015R² = 0.687

y = -0.7795x + 57.254R² = 0.7689

45.0

50.0

55.0

60.0

65.0

70.0

75.0

2002 2003 2004 2005 2006 2007 2008 2009

Ave

rag

e S

CR

IM R

ea

din

g

Survey Year

>70 >60 >50

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:

Monthly measurements of skid resistance by TRL over 11 years (LR 738)

0.30

0.40

0.50

0.60

0.70

0.80

0 12 24 36 48 60 72 84 96 108 120 132

TIME (months)

SK

ID R

ES

IST

AN

CE

• But can meteorological data be used to explain general trends in skid resistance?

Evidence of a link between SC and rainfall

Year May June July August Sept October

2006 100.9 24.1 41.8 88.4 73.6 106.5

2007 107.1 146.0 125.1 54.3 44.1 45.0

2008 62.1 59.1 100.5 106.0 96.3 94.9

2009 54.2 55.1 128.6 58.5* 32.5 67.0

Av 02-09 70.4 64.4 84.8 74.4 53.8 90.8

Year

2006 63.4 57.0 55.8 62.6

2007 63.4 59.8 55.3 63.4

2008 58.9 58.1 60.7 64.0

2009 63.3 59.9 59.9 59.9 58.3

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

0.0

20.0

40.0

60.0

80.0

100.0

120.0

10

60

11

0

16

0

21

0

26

0

31

0

36

0

41

0

46

0

51

0

56

0

61

0

66

0

71

0

76

0

81

0

86

0

91

0

96

0

10

10

10

60

11

10

11

60

12

10

SR

(5

0)

Metres

Site 4

Early

VL08

Mid

Late

very late

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 40 2008 and 2009 Runs

No signs of seasonal effects

Site 41 2008 runs

Concrete Surface 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+ 0.06 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