AASHTO TP 125:16(2020) Standard Method of Test for Determining the Flexural Creep Stiffness of Asphalt Mixtures Using the Bending Beam Rheometer (BBR).
4.4. The load and deflection at 0.0 and 0.5 s are reported to verify that the full-testing load (4000 ± 100 fiN) during the test is applied within the first 0.5 s. They are not used in the calculation of stiffness and rn-value and should not be considered to represent material properties. The rise time of the load (time to apply full load) can be affected by improper operation of the pressure regulators, improper air bearing pressure, malfunctioning air bearing (friction), and other factors. By reporting the 0.0 and the 0.5 s signals. the user of the test results can determine the conditions of the loading.
The test temperature for this test is related to the temperature experienced by the pavement in the geographical area for which the asphalt binder is intended. For quality control purposes the single test temperature shall be 10°C above the specified binder grade required for the mixture. For performance prediction at least 3 temperatures shall be used at 6°C intervals. The test temperatures of 4°C, 10°C, and 16°C above the specified binder grade used in the mixtures have been successfully used. Other temperatures can also he used depending on the project requirements.
5.2. The flexural creep stiffness or flexural creep compliance, determined from this test, describes the low-temperature, stress—strain—time response of asphalt mixtures at the test temperature within the linear viscoelastic response range.
5.3. The low-temperature thermal cracking performance of paving mixtures is related to the creep stiffness and the slope of the logarithm of the creep stiffness versus the logarithm of the time curve of the asphalt mixture.
5.4. The creep compliance is used in the low temperature algorithm of the AASHTO MechanisticEmpirical Pavement Design Guide to calculate thermal stresses used in predicting pavement performance.
Bemiding Beam Rheometer (BBR) Test Svstem—A bending beam rheometer (BBR) test system consisting of (1) a loading frame which permits the test beam, supports, and the lower part of the test frame to be submerged in a constant temperature liquid bath, (2) a controlled temperature liquid bath which maintains the test beam at the test temperature, and (3) a computer-controlled automated data acquisition component, and (4) items needed to standardize the BBR, verify it, or both.
Note 1—The buoyant force in the liquid of the bath provides partial counterbalance of the weight of the mixture beam. The remainder of the weight is approximately equal to 90 rnN, which can be neglected for a testing load of 4000 mN.AASHTO TP 125 pdf download.