Impact of Loading Frequency on the Fatigue behavior of SBS Modified Asphalt Mixtures

Fatigue cracking is considered as one of the most serious distresses associated with flexible pavements. Loading frequency has a great effect on fatigue behavior of asphalt mixes. Such that for standard mixes it is widely stated that the fatigue life decreases by increasing loading frequency when the test is conducted in deformation control mode. On the other side the effect of loading frequency on SBS modified asphalt mixes has not been investigated extensively. During fatigue tests on polymer modified mixes (PMA), temperature of the specimen increases. Higher temperature reduces the stiffness of asphalt concrete during the fatigue test and increases the fatigue life of the specimens and may affect the characterization of asphalt fatigue behavior. In this research an investigation was made to understand the fatigue behavior of SBS modified asphalt mixes and influence of loading frequency on PMA fatigue life in comparison with unmodified mixtures. The conventional bitumen with 60/80 penetration grade was used in this research and was modified with styrene-butadiene-styrene (SBS) at 5% by weight of the bitumen. Two different loading frequencies were applied and fatigue life for asphalt mixtures was determined using the four-point bending beam fatigue test based on phenomenological approach as well as dissipated energy concept. Results showed that the fatigue life of SBS-modified mixture is up to three times more than conventional mixtures. Temperature increase of PMA specimens during four-point bending beam test mainly affects the fatigue behavior in the test. Because of simultaneous effects of this temperature increase and healing phenomena, the difference between fatigue lives in two different loading frequencies is negligible, while this difference is significant in case of conventional mixtures.