Computational Road Mechanics - with Emphasis on FE-analysis of Rutting
Doctoral thesis, 2003
A road is a very complex structure in which a large number of different structural phenomena occur simultaneously that effects its expected lifetime. The structure itself is of a multi-layer type, in which the different layers consist of materials that exhibit vastly different physical behaviors when subjected to various kinds of loads. In addition to the mechanical loads originating at the surface the response of the material in the different layers depends on, for example, the temperature and the water present in the structure. This coupling between different physical phenomena, environmental as well as mechanical, suggests that the structure at hand can be classified as a mechanical, hydrological and thermal type. Solving a problem of this type requires that a number of mathematical models have to be formulated in a consistent way in order to insure compatibility as well as numerical stability.
This thesis focus on the impact of mechanical loads with emphasis on permanent deformation in the road surface. The complexity of the problem makes it necessary to identify and use a tool based on coherent formulations, which can better describe the physical phenomena that occur in the structure. This is in contrast to the design procedure used today, which often consists of a number of separate parts based on empirical knowledge. Simulating these types of problems requires powerful numerical tools such as FEM, which can be used for analysis of both coupled and uncoupled sets of differential equations describing the physical phenomena that occur in a road structure. A main task of the thesis is to explain the application of Burger's material model for asphalt concrete and the Boyce model for unbound material and to suggest alternatives. The selected material models, both for bitumen and unbound materials, have been implemented in the Finite Element software ABAQUS in order to further investigate their characteristics in an overall analysis model.
It was found that the Burger's material model chosen for asphalt concrete describes the behavior of bitumen bound material in an adequate way in cases where the speed of the surface load is low but that it has properties that can make it expensive, i.e. time consuming. The material model suggested for unbound materials in road design, i.e. the Boyce model that has been developed for use in other design tools such as elastic multi-layer methods, agrees well with tests. However it can be expensive in terms of computer time in certain situations in which strong non-linearity occurs especially in larger numerical simulations i.e. full 3D. An alternative material model is suggested for some of these cases. The importance of utilizing 3D analysis in the overall model in order to capture the impact of moving loads on the surface of a road is discussed and exemplified.