Flow number test using the AMPT

The specimens, a minimum of three per type of mix, are 150 mm high and 100 mm in diameter and are obtained by coring gyratory-compacted samples. 

Each load cycle consists of a pulse load applied for 0.1 s, followed by a rest period of 0.9 s. The test is typically conducted at the average 7-day maximum pavement temperature 20 mm below the surface for a given location, at 50% reliability as determined by the LTPPBind (LTPP Products Online 2013) software. 

The test is an unconfined test but there is continuing research to evaluate whether unconfined or confined testing simulates the field conditions better. 

The permanent axial strain is measured during load cycles and the strain rate is calculated (microstrain/cycle). Initially, the sample experiences high strain, as the material is seated at the beginning of the test. Then, the strain rate settles down to a constant value. Later, as the material becomes unstable, the strain rate increases again. The flow number is defined as the number of load cycles corresponding to the minimum permanent strain rate. A higher flow number indicates a longer time for the material to become unstable and therefore a more rut-resistant bituminous mixture. 

The average value of three tests is taken as the representative flow number for the mix tested. 

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