Fatigue behavior of conventional and rubberized asphalt mixes

Abstract

One of the main distress modes of flexible pavements is the fatigue cracking of the asphalt concrete surface layer. The addition of crumb-rubber modifier (CRM), obtained from scrap tires, to asphalt-aggregate mixtures has shown promise in enhancing their fatigue behavior. In this study, conventional unmodified and CRM modified asphalt-aggregate mixtures are evaluated in terms of their fatigue behavior. Controlled-strain flexural beam fatigue tests are conducted in the laboratory over a wide range of temperatures. Experimental results are compared in terms of flexural, tensile and compressive stiffnesses, phase angle, fatigue life and cumulative dissipated energy. Results showed that CRM mixes are more flexible than unmodified mixes, and that mix fatigue resistance is enhanced by the addition of CRM. Furthermore, a method of converting controlled-strain test data into equivalent controlled-stress behavior is presented. Experimental results revealed the existence of two types of controlled-strain stiffness-ratio variations. For each type of variation, an equivalent controlled-stress stiffness-ratio variation with cycles is derived. Using the predicted variations, fatigue lives for both modes of loading are determined. Predictions showed that, at a given temperature, controlled-stress mode of loading yields, as expected, shorter fatigue lives than its controlled-strain counterpart. An implicit validation of the proposed conversions revealed that fatigue equation parameters K and n for the different mixes fit within the range of values obtained from the literature for controlled-stress conditions. In addition, a fatigue life model, applicable to the haversine pattern of loading used in this study, is presented. The model takes into account the cumulative dissipated energy to failure, mode-of-loading, and initial phase angle, strain and stiffness of the mix. Analogy with the traditional strain-based fatigue equation revealed that K is a temperature-dependent parameter, whereas n and m are independent of mix temperature. A decrease in K is associated with an increase in temperature. The newly developed model is then used to predict fatigue lives of conventional and CRM mixes in typical pavement structures. For this purpose, a finite element-based mechanistic analysis is used. Results revealed the enhanced fatigue resistance of CRM mixes in comparison to unmodified conventional mixes.