2015 - NZ - Flushing in Chip Seal Pavements

Flushing is the process whereby chipseal texture depth is lost over time, resulting in a loss of skid resistance. It is the single most important reason for resealing on New Zealand state highways.

This report details research carried out from 2012 to 2015. In the first part of the work the aim was to identify and investigate the physical mechanisms causing flushing. The aim of the second part of the project, undertaken by researchers at the University of Auckland, was to use pavement condition data to develop a model to predict the rate of flushing progression in chipseals.

 

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.

The linear model was statistically strong (R2 of 0.445 for first-coat seals and 0.628 for second and higher generation seals).