2015 - New Zealand - Flushing of Chip Seals

This report details research carried out from 2012 to 2015 into chipseal flushing. The physical mechanisms causing flushing were investigated and a model was developed to predict the growth of flushing over the New Zealand state highway network. Factors making a major contribution to flushing are: • aggregate abrasion and breakdown • compaction and reorientation of the seal layer under traffic • water venting and sub-surface stripping in seal layers. Factors having no or making only a minor contribution to flushing are: • thermal expansion of the bitumen • excess bitumen application • binder viscosity. Further work is needed to quantify the significance of chip embedment into the basecourse. A two-part model using parameters in the NZ Transport Agency Long-Term Pavement Performance database was developed. The first part uses a logistic model to predict the onset of flushing and an accuracy of 74% when used to predict the initiation of flushing on a separate data set. The second part uses a linear model to predict the rate of flushing progression. First-coat seals, and second and higher generation seals were modelled separately.