Flow Number Test for Asphalt: AMPT Rutting Check
The flow number test tells you how many load pulses an asphalt mix can take before it starts to flow like a rut-prone material. It runs in the Asphalt Mixture Performance Tester (AMPT), the same machine used for dynamic modulus, and it gives a single number that rises with rutting resistance.
Asphalt tonnage calculator →How the test works
- A gyratory specimen is cored and sawn to 100 mm diameter by 150 mm tall, usually at about 7 percent air voids.
- It is brought to the test temperature, often the high design pavement temperature for the site.
- The AMPT applies a repeated haversine pulse: 0.1 second of load followed by 0.9 second of rest.
- The machine records permanent axial strain after each cycle.
- The test stops once the specimen reaches tertiary flow or a cycle limit, often 10,000.
The three stages of deformation
- Primary: fast initial strain as the specimen seats and densifies.
- Secondary: a steady, slower strain rate.
- Tertiary: the rate climbs again as the specimen begins to shear and flow.
The flow number is the cycle where the strain rate is at its minimum, the point between secondary and tertiary. Software fits the strain curve and finds it automatically.
Suggested minimums
NCHRP Report 673, the manual for mix design practice that came out of NCHRP project 9-33, suggested flow number minimums for hot mix that increase with design traffic:
| Design traffic (million ESALs) | Suggested minimum Fn (HMA) |
|---|---|
| --- | --- |
| Under 3 | None |
| 3 to under 10 | 50 |
| 10 to under 30 | 190 |
| 30 and over | 740 |
Warm mix has its own, lower suggested values because of reduced aging. Agencies that use flow number often adopt their own temperature and stress settings, so check the spec. For a refresher on traffic loading, see equivalent single axle load.
Example (made-up numbers)
A surface mix for a 15 million ESAL route reaches its minimum strain rate at cycle 312. The flow number is 312, above a 190 minimum. A second trial with 0.4 percent more binder flows at cycle 165, which would fail the same target and hint that the extra binder costs rut resistance.
What raises the flow number
- Stiffer or polymer-modified binder.
- Crushed, angular coarse and fine aggregate.
- A gradation with solid stone-on-stone contact.
- Avoiding excess binder or dust.
Flow number vs wheel tests
Wheel tests such as the Hamburg and the asphalt pavement analyzer roll a wheel over the specimen. The flow number test squeezes it in one direction with no confinement in the common version. It needs more equipment but ties into mechanistic pavement design because the same specimens and machine produce dynamic modulus data.
Practical takeaway
Flow number is a research-grade and highway-grade test; driveway mixes are not designed with it. Your order quantity comes from area and thickness, which the asphalt tonnage calculator works out.
Questions people ask
What is the flow number in asphalt?
The load cycle at which a specimen under repeated compression stops deforming at a steady rate and starts to deform faster, the start of tertiary flow.
What machine runs the flow number test?
The Asphalt Mixture Performance Tester (AMPT), a servo-hydraulic or pneumatic frame with a temperature chamber.
What standard covers flow number?
AASHTO T 378, which covers dynamic modulus and flow number in the AMPT. It replaced the earlier provisional method TP 79.
What is a good flow number?
Higher is better. NCHRP Report 673 suggested minimums that rise with traffic, for example about 50 for 3 to 10 million ESALs and about 190 for 10 to 30 million ESALs on hot mix.
What temperature is the flow number test run at?
Near the high design temperature for the site, often figured from LTPPBind for a depth of about 20 mm below the surface.
How long does a flow number test take?
Usually under an hour per specimen once it is conditioned, since the test stops at flow or at a maximum cycle count such as 10,000.
Related guides
- Asphalt pavement analyzer
- Hamburg wheel tracking test
- Superpave mix design
- Equivalent single axle load
- Asphalt testing basics
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