Fatigue of Asphalt Mixes - Theory of Viscoelasticity and Continuum Damage Mechanics Applied to Uniaxial Fatigue Data from Laboratory Tests

Abstract: The advance of analytic (mechanistic) design methods increases the need for fundamental material properties. Fatigue cracking due to traffic loading, in combination with permanent deformation (rutting), thermal cracking, irregularities, frost heave and reflective cracking are some of the major criteria used when designing an asphalt concrete pavement. The objective of this Ph.D. project is to evaluate existing material models used to evaluate the fatigue-life of an asphalt concrete pavement, e.g. ATB VÄG, the Shell Criterion, the Asphalt Institute Criterion etc. and, eventually, to find an improved material model that can be used in a structural response model. Different Continuum Damage Mechanics (CDM) models are also evaluated, and the results are compared with results obtained with traditional methods for fatigue-life evaluation, i.e. linear logarithmic relations between initial strain and number of cycles to failure. Uniaxial laboratory fatigue testing will also be evaluated in this project. Three types of asphalt mixes were used in this research: a standard Swedish base-course mixture called AG16, 4.8 %, 160/220 together with two alternative mixes called Inorbind16, 5.3 %, 70/100 + fiber and Durabind16, 4.3 %, 70/100 + polymer. Frequency sweep tests, constant strain rate monotonic tests and cyclic fatigue test were performed on specimens cored from laboratory-manufactured plates. Ten Ø 75-mm specimens were cored from each plate using portable coring equipment. After coring, specimens were cut, resulting in specimens with a height equal to 150 mm (2:1 ratio). The tests were performed using a dynamic servo hydraulic test machine, UTM-25 (Universal Testing Machine-25 kN) supplied by Industrial Process Controls Ltd (IPC), Australia. The model that has the largest potential for the future is the one based on uniaxial constitutive modeling using viscoelasticity and continuum damage mechanics based on Schapery’s theories. By using the pseudostrain concept and separating the viscoelastic effects from damage accumulation in the specimen, it was possible to eliminate the rate dependency for the material. By using the time-temperature superposition principle and the same shift factors that were used when the mastercurve was developed, it was possible to shift the characteristic material functions between different temperatures. It was also possible to describe monotonic and cyclic tests of both the controlled-strain and controlled-stress type, using one single characteristic material function. The tests were performed using different loading rates for monotonic tests and different stress and strain levels as well as frequencies for the cyclic tests. This observation makes it possible to reduce the number of specimens tested and time-consuming fatigue tests are no longer necessary, since the same information can be obtained from monotonic test data.