A three year research study at Transit New Zealand's pavement testing facility, CAPTIF was undertaken to assess the relative effect of increases in wheel loads to unbound granular pavements. CAPTIF is a circular track where the wheel paths of the two vehicles can be separated to assess the relative damaging effect of the pavement and surfacing. Tests were conducted comparing the damaging effect of 40 kN (equivalent 8.2 tonne dual tyred axle) compared with 50 kN (equivalent 10 tonne dual tyred axle) and 60 kN (equivalent 12 tonne dual tyred axle) on the pavement with four different aggregate types, two thicknesses and one subgrade type. For each load comparison a total of 1 million wheel passes were applied. During the testing falling weight deflectometer (FWD), and rut measurements were taken in both wheel paths. It was found that the pavement structural number was a good predictor of relative damage compared to the standard load. Relative damage was defined in terms of damage law exponent akin with the fourth power law used to calculate the design traffic loading in pavement design. A range of damage law exponents was obtained from 1 to 4 that could be related to the pavements structural number. Higher exponent values were obtained for the weaker pavements (i.e. low pavement structural number).
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