In order to improve the efficiency of the road transport industry in New Zealand, a range of mass limit increases for heavy vehicles has been proposed. Some of the options for mass increases include increasing the axle load limit, which would inevitably lead to increased road wear. As New Zealand has a mass-distance road user charging regime, where the users pay for the road wear they generate, this is in itself not a problem provided that the charges accurately reflect the wear. At present (2001) road user charges are based on the fourth power law, which was developed from the AASHO road test in the United States in the 1950s. The pavements and vehicles used for that test differ considerably from those in use in New Zealand today.
In this study, carried out between 1999–2001, an accelerated loading test was undertaken at the Canterbury Accelerated Pavement Testing Indoor Facility (CAPTIF) to compare the wear generated by different levels of loading. The pavement consisted of four different segments that were subjected to one million load cycles in two parallel wheel paths. The axle load on one wheel path was 80kN while the load on the other was 100kN. As a result the compaction-wear model for VSD (Vertical Surface Deformation) has been developed, which provides a much better fit to the observed behaviour, than the conventional power law model, for thin-surface unbound pavement structures.