Understanding the Correlation Between APA Results and Permanent Deformation in HMA Mixes at NCAT Test Track

Hot Mix Asphalt (HMA) pavements face constant stress from traffic loads, leading to permanent deformation or rutting over time. Predicting how different HMA mixes perform under these conditions is crucial for designing durable roads. One key test used in this prediction is the Asphalt Pavement Analyzer (APA), which simulates traffic loading to assess rutting potential. At the National Center for Asphalt Technology (NCAT) Test Track, researchers have studied how APA results relate to actual field performance, providing valuable insights for pavement engineers.

This post explores the connection between APA test outcomes and permanent deformation observed in HMA mixes at the NCAT Test Track. Understanding this relationship helps improve mix design and pavement longevity.

What is the APA Test and Why It Matters

The APA test measures the rutting resistance of asphalt mixtures by applying repeated loads to a compacted specimen under controlled temperature and pressure. It simulates the effect of vehicle tires rolling over pavement, causing deformation.

Key features of the APA test include:

• Controlled temperature to mimic field conditions, usually around 64°C (147°F)

• Repeated loading cycles to simulate traffic stress

• Measurement of rut depth after a set number of cycles

  
  

The APA test provides a rut depth value that indicates the mix’s susceptibility to permanent deformation. Lower rut depths suggest better resistance to rutting.

The NCAT Test Track: Real-World Pavement Performance

The NCAT Test Track in Alabama is a 1.7-mile oval track designed to evaluate pavement materials and designs under actual traffic conditions. It hosts various HMA mixes subjected to millions of load repetitions from heavy trucks.

Researchers monitor rutting and other distress types over time, providing a rich dataset to compare laboratory test results like APA with field performance.

NCAT Test Track asphalt lanes under traffic loading

NCAT Test Track lanes showing different HMA mixes under heavy truck loading

Correlating APA Results with Field Rutting at NCAT

Studies at NCAT have shown a clear correlation between APA rut depths and permanent deformation measured on the test track. Mixes with lower APA rut depths generally exhibit less rutting in the field.

Key findings include:

• Strong correlation for dense-graded mixes: APA results closely predict rutting for dense-graded HMA mixes, which are common in heavy traffic areas.

• Variability for modified binders: Mixes with polymer-modified binders sometimes show less rutting in the field than APA results suggest, indicating the test may be conservative for these materials.

• Importance of aggregate properties: Aggregate shape and gradation influence both APA and field rutting, highlighting the need to consider these factors in mix design.

  
  

For example, a dense-graded mix with an APA rut depth of 5 mm after 8,000 cycles typically showed less than 6 mm of rutting after millions of load repetitions at NCAT. This close match supports using APA as a predictive tool.

Practical Implications for Pavement Design

Understanding the APA-field rutting relationship helps engineers make better decisions:

• Selecting mixes with low APA rut depths can reduce the risk of early rutting.

• Adjusting mix design parameters such as binder type and aggregate gradation based on APA results improves durability.

• Using APA as a quality control tool during production ensures consistent performance.

  
  

At NCAT, the APA test has become a standard part of evaluating new HMA mixes before full-scale construction.

APA machine testing asphalt specimen for rutting resistance

APA machine applying repeated loads to asphalt specimen to measure rutting potential

Limitations and Considerations

While APA provides valuable data, it has limitations:

• Test conditions differ from field: The APA uses a fixed temperature and load, which may not capture all field variations.

• Not all mix types respond equally: Some innovative or modified mixes may not be fully represented by APA results.

• Supplement with other tests: Combining APA with tests like the Hamburg Wheel Tracking or flow number tests gives a fuller picture.

  
  

Engineers should interpret APA results alongside field data and other laboratory tests for best outcomes.

Summary

The correlation between APA test results and permanent deformation at the NCAT Test Track confirms the APA’s value in predicting rutting resistance of HMA mixes. Dense-graded mixes with low APA rut depths tend to perform well under heavy traffic, making APA a useful tool for mix design and quality control.