2015 - Australia - Seal Design Improvement for Low Volume Roads

The current version of the Austroads sprayed seal design method was published in 2006 and has been in use for the last nine years. During recent times, a number of sprayed sealing practitioners have indicated that the basic voids factor for single/single seals in the current Austroads sprayed seal design method is too high for low volume roads. To address these concerns, a questionnaire was initially distributed to jurisdictions and local councils to determine the extent of the concerns. The results obtained during a literature review of the derivation of Austroads basic voids factor for single/single seals, and the equivalent factor in the New Zealand seal design method, were used as a basis to propose changes to the Austroads basic voids factor.

The responses to the questionnaire indicated that the main issue associated with the Austroads sprayed seal design method was that the basic voids factor for single/single seals was too high for low volume roads. The results obtained during the literature review indicated that the Austroads basic voids factor has developed empirically over time based on observations by sealing experts and practitioners, while the current New Zealand equivalent factor is based on quantitative measurements obtained during an extensive series of road trials. As the New Zealand equivalent of the basic voids factor was based on quantitative measurements, the New Zealand seal design approach was used to propose changes to the Austroads basic voids factor.

A new version of the Austroads basic voids factor has been proposed for single/single seals which will require lower basic voids factors to be used for low volume roads. The new version of the Austroads basic voids factor was compared with recent Australian seal data/observations in order to investigate whether its use would result in issues with sprayed seals. Based on these comparisons, it appears unlikely that use of the new version of the Austroads basic voids factor will result in issues with sprayed seals.