Friction measurement methods and the correlation between road - VTI

0,70,60,50,40,30,20,10Dense asphalt Surface dressing Porous asphalt197519841998Figure 3.2 Friction coefficient measured with three different batches of PIARCtest tyres [10]Friction values are very sensitive to the tyre used and even apparently identicalsamples from two different batches of a standardised tyre can produce differentfriction results, maybe not large differences but significant. The reasons for thatare for example small differences in the tyre rubber composition and maybe alsosmall differences in tyre geometry. For most manufacturers the market forstandardised test tyres is very small compared to their other products and thereforetest tyres are normally produced in larger batches say every fourth year and alsooften by different producers. KOAC, WMD (Dutch Road Research Laboratories)[10] has compared PIARC test tyres from the batches 1975, 1984 and 1998 andfound differences in the friction values partly explained by differences in thecompound chemistry, see figure 3.2. In the case of the 1975 tyres the difference isquite large which in the Netherlands also led to the introduction of a frictioncorrection formula.According to investigations by Henry [22] and Bachmann [12] the ribbed andsmooth standard test tyres (ASTM and PIARC) have a lower friction coefficienton a normal wet road than a commercial passenger car tyre. As water depth isincreased, the ribbed test tyre increases in performance and was found to givehigher friction values than a slightly worn normal passenger car tyre.Friction of the road pavement is usually evaluated in the summer period. Thereis a seasonal variation that has to be considered. The wet friction of a specificpavement is normally higher in the spring than in the autumn [23; 27] due to adepolishing effect from the snow removal activities and the studded tyres.During the winter period there is a risk that the water spread during the wetfriction measurement freezes. The pavement can also be covered with snow or ice.Friction measurements during winter are therefore typically aimed at wintermaintenance evaluation.VTI meddelande 911A 11