Analysis of asphalt concrete fatigue through energy methods

Abstract

The fatigue life of asphalt concrete is an important parameter in the functional design of a road construction. In the Netherlands it is determined by the four Point Bending (4PB) test on laboratory prepared specimen. During the past years there is an increase in desire to validate the laid down asphalt pavement to the established functional properties. The most practical test setup applicable is theCyclic Indirect Tensile Test (CY-ITT). Both fatigue tests differ on many fronts from each other. The resulting traditional fatigue line between them is therefore not coherent. A solution is previously sought in applying the energy based method of the Ratio of Dissipated Energy Change (RDEC). Developedby Shen & Carpenter. This research is a continuation of those works. The objective of this research is therefore formulated as follows: Establishing a fatigue life relation through the energy methods, which couples the fatigue life results of both the CY-ITT and 4PB fatigue tests and preserve the asphalt mixture characteristics.In total 12 different mixtures were used to establish a wide field of mixture variation. Differences in RAP content, bitumen, PEN-grade and max aggregate size were used. Each mixture was tested on both force controlled CY-ITT and displacement controlled 4PB. Additional 4PB force controlled and Uni-axial displacement controlled fatigue tests were conducted on a single mixture.The results were analysed by the two main energy methods: The RDEC and the Viscous-Elastic Continuum Damage (VECD) model of Kim. The RDEC method is based on the slope ratio of the dissipated energy. The dissipated energy is calculated by the hysteresis loop. The VECD method is based on thelinearization of the strain by the pseudostrain energy function. The rate of the pseudostrain energy function (GR) is then used for the fatigue relation.For the RDEC is concluded that the method is a single line relationship. Independent of frequency, temperature, mixture type, mixture density and stress strain relation for each mode of loading. The test setup independence between the CY-ITT and 4PB was found for 6 out of the 12 mixtures. The 4PB force controlled test did not deliver an evident result. The Uni-axial displacement controlled fatigue test validated the RDEC independence of test setup. It is therefore not recommended to use the RDEC in the current configuration as a practical application for validation between laboratory prepared specimen and the road constructed asphalt concrete layer.For the VECD is concluded that the CY-ITT and 4PB did not form a single coherent fatigue line in the log(GR − NfatSN) relation. The relation is for a single test setup however mixture type dependent. The hypothesis that the fatigue line is dependent on the linear stiffness from the frequency sweep and could be shifted similar as on the mastercurve, proved preliminary to be true for the same test setup. However to be untrue between the CY-ITT and 4PB GR fatigue relation. The used relation between log(GR−NfatSN) is therefor not applicable in linking the CY-ITT and 4PB fatigue test setups.