Asphalt Pavement Analyzer (APA): Rut Test Basics
The asphalt pavement analyzer, or APA, is a lab machine that shows how a mix will rut. Loaded wheels roll back and forth over pressurized rubber hoses sitting on top of the specimens, imitating truck tires. After thousands of passes at summer temperatures, the depth of the groove tells the designer whether the mix is stiff enough.
Asphalt tonnage calculator →Where it came from
The APA grew out of the Georgia Loaded Wheel Tester, built in the 1980s to screen mixes for rutting. Pavement Technology Inc. developed the commercial version, which can test several specimens at once and control temperature and water. The rut procedure is standardized as AASHTO T 340.
How the rut test runs
- Specimens: gyratory cylinders (150 mm by 75 mm) or beams, usually compacted to about 7 percent air voids.
- Temperature: commonly the high PG temperature for the project, such as 58, 64 or 70 °C.
- Load: wheels apply about 100 lb on hoses pressurized to roughly 100 psi.
- Duration: 8,000 cycles, where one cycle is a forward and back pass.
- Result: average rut depth in millimeters at the end of the run.
Reading the results
Agencies set their own rut limits, often tighter for heavy-traffic surface mixes than for low-volume or base mixes. A shallow rut at the end of 8,000 cycles means the stone skeleton and binder are carrying load well. A deep rut points to soft binder, too much binder, rounded or weak fine aggregate, or too much dust.
Example (made-up numbers): two specimens of a 12.5 mm surface mix finish at 3.8 and 4.4 mm, averaging 4.1 mm. If the spec limit for that traffic level is 5.0 mm, the mix passes.
What changes APA rut depth
- Binder grade: a higher high-temperature grade or a polymer modified asphalt cuts rutting.
- Gradation: angular, crushed aggregate with good stone-on-stone contact resists better than rounded sand-heavy blends.
- Binder content: excess binder lubricates the skeleton and deepens ruts.
- Air voids: specimens far off the target compact further under the wheel and confuse results; see air voids in asphalt.
APA vs Hamburg
| APA | Hamburg | |
|---|---|---|
| --- | --- | --- |
| Wheel | Aluminum wheel on pressurized hose | Steel wheel directly on specimen |
| Environment | Usually air, can run submerged | Submerged in hot water |
| Main use | Rutting | Rutting and moisture stripping |
Many agencies now prefer the Hamburg wheel tracking test because it catches stripping as well, but the APA remains in use in several states and labs.
Why it matters for buyers
A mix that passed a loaded-wheel test is less likely to shove or groove at intersections, loading docks and steep driveways. For ordering, the asphalt tonnage calculator turns your area and thickness into tons.
Questions people ask
What does the asphalt pavement analyzer measure?
Mainly rut depth: how far loaded wheels push into an asphalt specimen after a set number of passes at a high temperature. It can also be run with specimens under water to look at moisture damage.
What standard covers the APA rut test?
AASHTO T 340, the method for rutting susceptibility of hot mix asphalt using the asphalt pavement analyzer.
How many cycles is the APA test?
The standard rut test runs 8,000 cycles, with rut depth recorded along the way and at the end.
What temperature is used in the APA?
Usually the high-temperature PG grade of the binder for the project, for example 64 °C for a PG 64-22 area.
Where did the APA come from?
It grew out of the Georgia Loaded Wheel Tester developed in the 1980s and was commercialized by Pavement Technology Inc.
How is the APA different from the Hamburg?
The Hamburg uses steel wheels running directly on submerged specimens. The APA uses wheels riding on pressurized rubber hoses laid over the specimens, usually in air.
Related guides
- Hamburg wheel tracking test
- Superpave mix design
- Polymer modified asphalt
- Asphalt testing basics
- Air voids in asphalt
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