The objective of this research (undertaken 2008–2011) was the development of an improved method of modelling the decision to rehabilitate a typical New Zealand thin-surfaced unbound granular pavement. This was driven by previous research that had found a poor correlation between the data recorded in the road asset maintenance management (RAMM) database and the decision to rehabilitate. It had been hoped that by talking to local engineers and examining pavements proposed for rehabilitation, distress not currently recorded may be identified. This would have then driven the development of better models and may also have expanded the detail collected in the visual surveys. The research found, however, that the drivers are not obvious and that the decision maybe being based on factors such as the engineers’ assessment of the risk of rapid failure.
The conclusions from this research are:
• According to a visual engineering inspection, many pavement sections require rehabilitation.
• In many cases, a significant quantum of deferred maintenance needs to be performed for the do-minimum option. This maintenance is not necessarily obvious from the data in RAMM or visual observations of the high-speed data videos.
• The methods used to determine future maintenance costs vary widely. This ranges from including the deferred maintenance cost into one year and extrapolating from this cost, to ignoring the deferred maintenance cost in the analysis.
• The timeframe for assessing maintenance cost history is variable.
• The net present value (NPV) calculation can be very sensitive to assumptions made on future maintenance and seal lives. This includes assuming that higher priced polymer-modified seals need to be used.
• Rutting and flushing at 88% and 80% of the surveyed pavements are the two most commonly quoted distress mechanisms. These do not appear in a proposed rehabilitation algorithm.
• Digouts are a factor mentioned in 55% of justifications.
• The inspection length associated with the visual pavement inspection did not reflect the treatment section length in 40% of sites.
• The influence of non-engineering factors, such as concerns over ‘consuming the asset’ and fears of rapid pavement failure, need to be investigated.
• The difference in condition between the typical pavements in a network and those chosen for rehabilitation can often be minor and thus very difficult to quantify.
• Better guidelines should be developed to assist and standardise the decision process. These guidelines need to be based on a risk and consequence approach, which, it is believed, will better reflect the engineers’ approach.