Evaluation of Sample Orientation in Flexural Stiffness and Temperature-Induced Damage in Fatigue Life of Asphalt Concrete

Flexural stiffness and fatigue life of asphalt concrete are determined by a four-point bending test using AASHTO T321 test protocol. A sample is prepared using a kneading compactor and is usually placed in the loading frame regardless of the direction of compaction. That means that the sample may be placed upside down (flipped) rather than placing the sample as it is compacted. To evaluate the flipping effect, a number of beam samples (375 mm x 63 mm x 50 mm dimension) are prepared and tested in two different ways to determine the flexural stiffnesses. First, these samples are tested in the way they are compacted. Secondly, the tests are conducted on flipped samples. Results show that the flipped samples always have greater stiffness (varies 3-26%) than the usual samples. In addition, based on field instrumentation on Interstate 40 (I-40) in New Mexico, it is revealed that thermal contraction and expansion due to daily and yearly temperature fluctuations at the bottom of asphalt concrete produce a relatively large amount of strain. Analyzing with Mechanistic-Empirical Pavement Design Guide (MEPDG), it is found that vehicle strain, daily and yearly thermal strains cause 5%, 0.01% and 95% fatigue damage at the bottom of HMA based on field measured strain and MEPDG fatigue model. Therefore, damages due to thermal expansion and contraction should be considered to determine the fatigue life of asphalt concrete.