Austroads has released two new research reports that together provide transport agencies with clearer evidence and practical guidance to support safer, more reliable use of thin asphalt‑surfaced unbound granular pavements across Australia and New Zealand.
Originally posted here
Thin asphalt surfacings are sometimes used as an alternative to sprayed seals where traffic loading, noise, turning stresses or site constraints require another surfacing solution. While asphalt‑surfaced unbound granular pavements can perform satisfactorily or offer benefits in some situations, agencies have reported variable performance, especially under changing moisture and temperature conditions.
To investigate if improved guidance could be provided, Austroads commissioned a coordinated research program examining both how thin asphalt pavements perform under controlled loading and how in‑service performance compares with existing design assumptions.
The first report, Performance Evaluation of a Thin Asphalt Surfaced Unbound Granular Pavement Under Accelerated Pavement Testing (AP‑T389‑26), documents full‑scale accelerated testing of asphalt‑surfaced unbound granular pavements undertaken in a controlled environment. The study assessed the test pavements under dry and higher moisture conditions, finding that cracking of the asphalt did not occur until the pavement had undergone significant deformation. The findings highlight the criticality of the support provided by the unbound granular base, and highlighted importance having a sound base with appropriate moisture levels in order to reduce the risk of early failures.
The companion report, Fatigue Performance Evaluation and Design of Thin Asphalt Surfaced Granular Pavement (AP‑T390‑26), draws on analysis of extensive field performance data from the Queensland road network for asphalt‑surfaced unbound granular pavements. A survival‑probability analysis, to assess how existing Austroads design methods for these pavements reflect in‑service behaviour, was completed using available data. The assessment found that observed pavement performance in warm climates generally exceeded current design predictions. Consequently, potential adjustments to design parameters and a framework to better align fatigue predictions with field outcomes have been identified.
Together, the reports link experimental evidence with observed (i.e. field or “real-life”) network performance for asphalt‑surfaced unbound granular pavements, providing enhanced guidance about when thin asphalt surfacings may be appropriate, how construction quality and support conditions can be verified in practice, and how design methods could potentially be adjusted to reflect real-world performance.
Ross Guppy, Austroads Transport Infrastructure Program Manager, said the research responds directly to questions raised by road agencies.
“Practitioners told us they needed clearer evidence on why some thin asphalt pavements perform well while others don’t, and how to reduce the risk of early cracking,” said Ross.
“The accelerated testing shows that loss of support and deformation are key triggers for distress, while the field analysis helps us understand how design assumptions can be better aligned with real‑world performance, particularly in warmer climates,” he said.
The work is not intended to replace sprayed seals as the default treatment for unbound granular pavements.
“These reports don’t suggest thin asphalt is suitable everywhere,” said Ross. “They provide clearer boundaries, checks and design insights so agencies can make informed, evidence‑based decisions where this treatment is being considered.”
For transport agencies and councils, the research supports more confident planning and delivery of thin asphalt treatments by clarifying performance risks, highlighting the importance of moisture and support control, and providing guidance to improve alignment between design and in-field performance outcomes. The findings can help avoid early failures, minimise unplanned maintenance interventions and support more cost‑effective asset management.
Download Fatigue Performance Evaluation and Design of Thin Asphalt Surfaced Granular Pavement
