COST 334

Chapter 4• Molzer used worst-case assumptions for Zahnmesser’s data and for inflation pressuredifferences,• all calculations are based on the worst case with the maximum compression difference(one tyre at the bottom of the rut and the other high up).Therefore, practical average values of R 1 /Q are almost certainly below 60%, probablyeven below 55%.The centre distance between the tyres was derived from extensive measurements by Blab(1995), yielding the distribution in Table 4.41.Table 4.41 - Distribution of centre distances between dual tyres (Blab 1995)Distance class 0.24-0.29 m 0.29-0.34 m 0.34-0.39 m 0.39-0.44 mOccurrence 8 % 29 % 53 % 10 %Molzer used this distribution, together with R 1 /Q=75%, to calculate distress factors, andused a characteristic (distress-weighted average) value of 0.34 m for the centre distancebetween the tyres. Generally, a decrease in centre distance will produce higher maximumstresses and strains at the bottom of the asphalt, under the same total wheel load, andhence greater pavement wear.Finally, Molzer calculated distress factors for the three chosen pavement types, using 0.34m tyre centre distance and R 1 /Q=75%, for axle loads between 20 and 115 kN. Heconcluded that:• thick pavements (0.23 m asphalt) were about twice as sensitive to changes in axle loadas thin pavements (0.09 m asphalt),• thin pavements were about twice as sensitive to changes in contact stress as thickpavements.Concluding can be stated that Molzer gives very comprehensive analyses of all relevantparameters and he provides a good methodological framework for such analyses.However, his results cannot be used directly to answer the TG3 research questions, as hecompounds worst-case assumptions about the actual conditions. Hence, his results presenta very extreme condition, a deliberately conservervative estimate for design purposes.Furthermore, his results are based on numerical modelling only. Therefore, TG3 decidedto perform actual measurements of pavement response under unequally inflated tyres, andto do some modelling work with input values which are representative for in-serviceconditions. This is described in the following sections.4.6.3 Unequal load sharing due to differences in inflation pressure, British responsetestingFull scale pavement response and performance tests were carried out using TRL’sPavement Testing Facility, as part of the British contribution to COST 334 (Blackman etal. 2000). Two pavement structures were tested, one comprising an asphalt thickness of100 mm, the other with an asphalt thickness of 200 mm. Subgrade strains were measuredunder six different tyre configurations at several wheel loads and inflation pressures.The tested pavements and instrumentation were already described in section 4.5.4. Thedual tyres assemblies were tested at combinations of 10 & 8 bar, 10 & 6 bar and 8 & 6 bar,besides being tested at equal inflation pressures for both tyres of 6, 8 and 10 bar. For allversion 29 November 2001113