Hot Mix Asphalt Technology - May/June 2011 - (Page 40)

NCAT Explains the by the National Center for Asphalt Technology (NCAT) Staff Asphalt Mixture Performance Tester Dynamic modulus and flow number data can help you design high-performing, cost-effective pavements. Here’s how the experts at NCAT use the AMP Tester to get fast, accurate data. T he Asphalt Mixture Performance Tester (AMP Tester or AMPT) can be used to perform both dynamic modulus and flow number testing. Dynamic modulus testing is primarily used to characterize asphalt materials for pavement structural design using the Mechanistic-Empirical Pavement Design Guide, while flow number testing can be used as an indicator of rutting resistance. The dynamic modulus is a measure of material stiffness at different temperatures and loading frequencies. The flow number is the point at which the material goes from a low rate of deformation under a specific type of load to an accelerated rate of deformation. Both tests are performed on compacted asphalt specimens using the AMPT (Figure 1) or a servo-hydraulic testing machine capable of producing a controlled sinusoidal compressive loading. The AMP Tester includes highly accurate systems for loading, measuring deformations and controlling test temperatures. Dynamic Modulus Testing Because asphalt is a composite material that exhibits complex behavior, its modulus is highly dependent on temperature and rate of loading. For example, the stiffness of a mixture may change 100-fold over the temperature range from summer to winter. For this reason, a cyclic loading test is used to characterize how the properties of a given mix are affected by loading. In the dynamic modulus test, repeated axial loads are applied to a cylindrical specimen. Stress and strain are recorded, as shown in Figure 2, and dynamic modulus is calculated by dividing the peak stress amplitude by the peak strain amplitude. Another important property obtained from the dynamic modulus test is the phase angle, which is essentially the time lag between stress and strain. There are two standard test methods for determining dynamic modulus of asphalt mixtures, AASHTO TP 62 and AASHTO TP 79. AASHTO TP 62 requires testing at six loading frequencies between 0.1 and 25 Hz, and five temperatures between –10 and 54.4°C (14 and 130°F). However, since the Asphalt Mixture Performance Tester cannot maintain a test temperature of –10°C, dynamic modulus testing with this machine is conducted in accordance with AASHTO TP 79. Figure 1. Asphalt Mixture Performance Tester (AMP Tester) 40 • View past issues of HMAT online at

Table of Contents for the Digital Edition of Hot Mix Asphalt Technology - May/June 2011

Hot Mix Asphalt Technology - May/June 2011
Chairman’s Commentary: Who Knew Asphalt Could Be So Green?
Industry News
Warm-Mix Asphalt: Best Practices
States Stride Ahead With Warm Mix
Roads to Rubble: De-Paving a Half-Century of Progress
NCAT Explains the AMP Tester
Tools for the Trade
Calendar of Events
Index of Advertisers/

Hot Mix Asphalt Technology - May/June 2011