Refining Conditions of Fatigue Testing of Hot Mix Asphalt

The beam fatigue test of hot-mix asphalt (HMA) has been used for nearly a half century. However, several conflicting results have been recently reported. This study attempts to refine test conditions such as waveform type (haversine versus sinusoidal), incorporating rest periods between loading cycles, and the effect of rest period on the healing of the HMA to minimize (eliminate) gross errors in the data analysis of the fatigue test results. In the deflection-controlled haversine test [ASTM D7460, 2010, "Standard Test Method for Determining Fatigue Failure of Compacted Asphalt Concrete Subjected to Repeated Flexural Bending,"Annual Book of ASTM Standards, Vol. 04.03, ASTM International, West Conshohocken, PA, pp. 1-14] permanent deformations lead to a new equilibrium neutral position of the beam and the force output follows a sinusoidal waveform. This tends to bend the beam in both directions similar to the deflection-controlled sinusoidal test. This would produce erroneous fatigue results since the test assumptions do not match the actual test conditions. In contrast, the deflection-controlled sinusoidal test [AASHTO T-321, 2012, "Standard Method of Test for Determining the Fatigue Life of Compacted Hot Mix Asphalt (HMA) Subjected to Repeated Flexural Bending,"Annual Book of AASHTO Standards, Vol. 32, Washington, DC, pp. T321-1-T321-11] is more consistent than the deflection-controlled haversine test [ASTM D7460, 2010, "Standard Test Method for Determining Fatigue Failure of Compacted Asphalt Concrete Subjected to Repeated Flexural Bending,"Annual Book of ASTM Standards, Vol. 04.03, ASTM International, West Conshohocken, PA, pp. 1-14].When tests, with and without rest periods, are compared for healing studies, it is even more important to use a deflection-controlled sinusoidal test in order to obtain a fair comparison and accurate healing results. Since neither the haversine waveform nor the sinusoidal waveform in the lab exactly simulates field conditions, it is important to use a sinusoidal waveform in order to obtain consistent results. It is recommended that ASTM changes the ASTM D-7460 designation and test procedure to require a deflection-controlled sinusoidal waveform instead of haversine. Implementing the recommended test conditions is a crucial step in studying the concept of HMA healing and; as a result, estimating the endurance limit which plays an important role in designing sustainable pavements.