Temperature Influence on Skid Resistance Measurement Ed Baran Principal Engineer (Pavement Testing) Network Services

Temperature Influence on Skid Resistance Measurement* By skid resistance measure, the paper is referring to the measurement of Braking Force Coefficient at a slip speed of 60 km/h, that is, F60 the friction component of International Friction Index. * Braking Force Coefficient at a slip speed of 60 km/h

Introduction Project Aims Examine temperature influence on skid resistance Determine the appropriate reference temperature for use in Queensland Develop F60 temperature correction factors

Temperature Effects on Skid Resistance “Increase in temperature reduces skid resistance” What temperature ? Air, tyre, water and pavement surface temperatures. Water temperature is not significant. Tyre temperature is a function of air and surface temperature while, Other factors (load, tyre pressure & vehicle speed) are controlled by test procedures, Making surface temperature the biggest contributor to tyre temperature. These two players effectively control the temperature game, during skid testing, at the tyre road interface. Most organisations use surface temperature for temperature correction of skid resistance results Water temperature is not significant – refer Lou 2003 the Virginia Smart Pavement skid trials

Air – Surface Temperature Relationship Dickinson (1981) Temperature Conditions - Typical Cold Season Days - Canberra Even on the coldest days, surface temperature is greater than air temperature. Why ? Radiation heats the surface during the day and “pavements” store heat.

Air – Surface Temperature Relationship Daytime Air vs Surface Temperature Relationship – Brisbane DTMR (1985) A poor relationship exists between air and surface temperature because air temperature is not the only contributor to surface temperature. Have to also include solar radiation. Oliver (1980)

Air – Surface Temperature Relationship Summary Asphalt stores heat Solar radiation plays a big part Even under cold conditions, surface temperature is unlikely to fall as low as air temperature

Typical Australian Temperature Correction Procedures Use a reference temperature of 20oC surface temperature or, 23oC ambient air temperature while, 30oC surface temperature is used in Queensland. How appropriate is 30oC for QLD ? 30 is used in Queensland as the reference temperature for correcting Pendulum results since 1982 at least.

Temperature Regimes in Australia (Asphalt) Cumulative Surface Temperature Distributions (% less than) A 20 oC reference is quote appropriate for Melbourne and Canberra however in Brisbane, this temperature only occurs for 10% of time. Dickinson (1981)

Temperature Regimes in Australia (Asphalt) Cumulative Surface Temperature Distributions (% greater than) In Brisbane the surface temperature is 30oC or higher for 50% of the time. I suspect that it was based on this original data that Queensland adopted 30oC as the reference temperature for correction of Pendulum skid test results. Dickinson (1981)

Temperature Regimes in Australia (Asphalt) Temperature Distributions – Comparison with Darwin and Townsville Vertical axis is % greater than. Note the Townsville data was supplied post publication of Dickinson (1981). We probably have the only copy. Townsville and Brisbane have similar surface temperature conditions hence the 30oC reference temperature is appropriate for Townsville as well. In Darwin, the surface temperature is greater than 30oC for 70% of the time. Why would you adopt a 20oC reference temperature here. A reference temperature of around 34oC would seem more appropriate. Dickinson (1981)

Temperature Regimes in Australia Annual Average Daily Solar Exposure The solar exposure map reflects the different asphalt temperature regimes reported by Dickenson. Therefore, the temperatures in north west Queensland would be closer to Darwin’s surface temperature than to Brisbane of Townsville. Hence 35oC (rounding of Darwin’s 34oC) would be more appropriate for this area.

Temperature Regimes in Australia (Seals) Asphalt vs Spray Seal (Sydney) Asphalt vs Spray Seal (Perth) Seal temperatures are similar to asphalt temperatures. How is heat stored in thin seals ? Must be stored in the upper pavement granular base. Dickinson (1981)

Temperature Regimes in Brisbane (Asphalt vs Concrete) Temperature Conditions - Typical Hot Season Day – Brisbane Concrete surface temperatures are only marginally lower than asphalt temperatures. Therefore the 30oC reference temperature is close enough for use with concrete pavements not that we have many in Qyeensland. Even a night of heavy rain followed by an overcast day with frequent showers was not enough to cool surface temperatures down to air temperature. Concrete surface is only marginally cooler than an asphalt surface. “Water cooling the pavement during skid testing ?” – took two rainy days for the surface temperature to even get close to the air temperature. DTMR (2010)

Surface Temperature – F60 Relationship Field Trial Nudgee Beach Road Test Site

Surface Temperature – F60 Relationship Field Trial F60 vs Temperature Results The cooler surface temperatures were achieved by starting testing well before dawn on the first day. On the second day, testing extended into the afternoon to ensure that the surface was tested at its hottest. Conducting the trial over two nearly adjacent days ensured that seasonal effects minimised or eliminated. At high slip speeds, temperature effects on skid resistance results are minimal.

Surface Temperature – F60 Relationship F60(30) = F60(t) + 0.0015(t) – 0.045 where t = the surface temperature at time of test, F60(t) = the recorded F60 at t oC and F60(30) = the corrected F60 value for a 30oC reference temperature F60(35) = F60(t) + 0.0015(t) – 0.0525 This relationship can be adjusted for any reference temperature and is valid for skid testers using the ASTM E-1551 tyre and operating at slip speeds of around 60 km/h. It needs to be validated for seals and concrete surfaces.

Comparison of Temperature Correction Relationships Our relationship agrees with Lou (2003) and the high temperature end of the old TRL Pendulum Relationship. The Ekin relationship, which was based on testing at similar slip speeds does not agree with our findings. However, their trial was based on monthly testing over a year to obtain the temperature range and would have included some seasonal influences. We support Lou (2003)’s findings that temperature effects at high slip speeds are not as significant as the are at low slip speeds.

Conclusions There is no unique relationship between air and surface temperature. The reference temperature of 30oC is appropriate for the south east and coastal strip of Queensland. For the north west of the state, the reference temperature should be raised to 35oC. There is minimal difference in surface temperature between asphalt, concrete and seals. The developed temperature correction relationship can apply to any skid testers that operate at approximately 60 km/h slip speeds and use the ASTM E-1551 standard tyre.