Coupled System Pavement - Tire - Vehicle

In this chapter, the tire-pavement system as one subsystem of the complex vehicle-tire-pavement system is investigated in detail. As basic framework, the finite element method (FEM) is used for both, tire and pavement simulation, to obtain a detailed representation of the dynamic system, where the special case of steady state motion of the rolling tire is considered. The finite element (FE) discretization further enables to study the tire-pavement interface in terms of transmitted stresses and friction characteristics for different tire and surface properties. For the modeling of this complex subsystem, new FE based analysis methods have been derived using the Arbitrary Lagrangian-Eulerian (ALE) framework for tire and pavement. With the help of the ALE framework, the relative motion of tire and pavement is captured in a computationally efficient way. Friction in the tire-pavement interface is numerically represented by a homogenization approach of the friction interface over several length scales. With the help of a time homogenization technique, spatially detailed long-term predictions regarding rutting of the pavement become feasible by considering different time scales of the thermo-mechanical investigation.

Asphalt pavement compaction is important, and it can determine the service quality as well as durability of pavement. In recent years, numerical methods have been extensively used to simulate and study the construction process of asphalt pavement and mechanical properties of asphalt mixtures. In the following sections, the compaction process, considering the interaction between the materials and the equipment, is simulated, and the influence of different compaction methods on the mechanical performance of asphalt mixtures is investigated. To achieve this goal, a pre-compaction model is developed using the Discrete Element Method (DEM), and the models of both materials and the paving machine are generated separately. After the pre-compaction simulation, the theory of bounding surface plasticity is combined with the theory of Finite Element Method (FEM) as well as with a kinematic model of a roller drum to simulate the asphalt mixture behavior during a roller pass. In order to ensure consistency both in the laboratory compaction and in-situ compaction, the Aachen compactor has been developed. The effect of different compaction methods (Field, Aachen and Marshall Compactions) on the asphalt specimens is compared and evaluated using the microscale FEM.

Different computational methods dealing with functional properties of road surfaces are presented. Drainage and skid resistance as functional properties are treated. A special focus is set on the relationship between functional properties and asphalt structures, examples of important connecting aspects are described with regard to drainage of porous pavements and relevant void structures. Furthermore, in that context, analyses of the inner structure of asphalt are performed with XRCT scanning methods in order to develop a better understanding of asphalt structures and their implications on functional properties. In addition, the deformation behavior in before/after comparisons of XRCT images after uniaxial load tests are investigated.

This chapter presents a comprehensive characterization of asphalt concrete at different scales of observation. State-of-the-art characterization procedures for bitumen, mastic, mortar and asphalt are described in detail. The procedures were envisaged to provide experimental data for parameter identification and validation of constitutive numerical models. The validation of numerical models against experimental data is a prerequisite for the use in any application. The temperature-frequency dependency of the rheological properties of bitumen and mastic was characterized using temperature sweeps in the dynamic shear rheometer. The results showed that the bitumen provenance and the filler’s mineralogy have a major impact on the rheological response of the asphalt. A new rheometer, known as Dresden dynamic shear tester, was developed with the aim of characterizing the thermo-viscoelastic properties of mortar. This novel equipment was also used to identify the stiffening effect of the aggregates by comparing the results of bitumen, mortar and asphalt. Finally, the short and long term behavior of different asphalt mixtures were characterized with the repeated load triaxial tests and with the indirect tensile tests. The results showed that the performance of asphalt is highly affected by the bitumen type and the aggregate gradation.

This subproject of the research group FOR2089 focuses on the vehicle-induced road load and the interaction between vehicle, tire and road. The here presented measurement methods are not only used to validate the different simulation models established by the research group members, but also to parametrize and optimize physical tire models for the application to real road topology as well as asphalt texture models. In comparison to models with a single-point road contact, a discretized tire footprint interacts locally with the road, which allows the investigation of ground pressure and shear stress distribution on varying surfaces. In previous studies, this local road load has not been validated at this level of detail. By a holistic analysis of the tire’s influence on the vehicle and the road at the same time, a more realistic vehicle-tire-pavement behavior can be predicted by the simulation models. This chapter is separated into two parts. The first part mainly focusses on methods regarding vertical forces. As heavy-duty vehicles cause the highest loads on the main traffic routes, the methods for vertical dynamics are applied for heavy-duty purposes. The influence of different component model approaches on the road load are presented and validated using a hydraulic axle test rig. The second part presents methods for analyzing horizontal forces in the tire-road interface on a passenger car level to take advantage of specialized measurement systems. The influence of asphalt modulation on the tire force transmission mechanisms and the vehicle dynamics are presented. Furthermore, the friction characteristics on asphalt is investigated with a special regard to future tire measurement on artificial surfaces with asphalt texture.

Microstructural analyses of asphalt mixtures are described in this chapter using the X-Ray computer tomography (X-Ray CT) and related digital image processing (DIP) approaches. Different parameters of single elements like aggregates or air voids as well as characteristics of the whole grain and void structure, which can be determined, are introduced. These features can be linked to different mechanical, structural and functional properties. Changes of certain parameters by load application (tensile or compressive stress) or by artificial soiling (with porous structures), e.g. in before-and-after studies, are observed and certain conclusions are drawn. Fatigue and deformation of asphalt pavements and related deterioration effects in the microstructure like cracking are presented as exemplary use cases in this chapter as well as drainage and sound absorption of porous asphalt. The virtual reconstruction of asphalt structures based on three-dimensional (3D) images of real asphalt samples is also of great relevance for computational studies, e.g. for finite element modeling, and is shown exemplarily in this chapter. Simplification approaches of the microstructure are discussed in that context briefly as well.

Friction between tire and pavement surface—also referred to as skid resistance in pavement engineering—is a complex phenomenon depending on many influencing parameters like speed, load or wetness of the surface as well as different effects like hysteresis and adhesion. Two different friction model approaches are used in this chapter, a microscale analytical model with special focus on microtexture influence and a multi-scale FE model considering both micro- and macrotexture wavelengths. Both approaches employ a generalized Maxwell model as material formulation for the tire rubber. Real and virtual textures of asphalt surfaces are replicated by 3D SLM printing on stainless steel plates. The virtual texture samples—which are still based on real asphalt surfaces—comprise pure microtextures (without macrotexture elements after filtering) and artificial combinations of sinusoidal waves with two different wavelengths. The printed surfaces are investigated by texture measurements for printing discrepancies with respect to the templates. Friction is measured with a linear friction test rig on these printed samples as well as on a real asphalt surface in dry and wet conditions. The measurements are used for calibration and validation issues by comparing them to the model calculations in wet and dry surface conditions.

An innovative consistent simulation chain is used in this chapter for the combination of the advantages of a microstructure finite element (FE) model of asphalt composites with a macrostructure FE model of pavement under tire rolling load. For this study, an existing microstructural FE model of a Stone Mastic Asphalt including coarse aggregates, asphalt mortar, and air voids was parameterized and validated beginning with experimental tests of asphalt mortar. In order to identify the macroscopic (homogenized) material properties of the asphalt mixture for use in the FE computations of two pavement structures under rolling tire load, this validated microstructural model is applied. These calculations are then evaluated using a new macro-micro-interface, which represents the rolling tire loading conditions for the microstructural model by generating time-dependent displacement boundary conditions. The results indicate that the introduced simulation chain allows for the investigation of the processes, stresses and strains inside the asphalt composite at realistic loading conditions. The experimental tests on the component level can be improved and a better comprehension of the interacting processes in asphalt mixtures under rolling tire load can be obtained by using the results.

In this chapter, a simulation chain is described and applied to an asphalt test track and a highway pavement structure. The simulation chain consists of different modules reaching from the experimental identification of the asphalt material, its numerical modeling on the material scale via adequate models to finally the structural scale of the pavement and the vehicle-tire system, which is numerically assessed in the framework of a coupled vehicle-tire-pavement system. Relative dynamic effects of vehicle-tire-pavement interaction have been investigated based on a multibody analysis of the vehicle driving on a rough pavement surface (external stimulus). For the pavement simulation, equivalent tire loads are used in an arbitrary Lagrangian Eulerian framework for tire and pavement. Finally, the rut formation is computed by varying different influence factors (climate temperature, vertical tire force, type of asphalt material of the surface layers etc.). With the help of the simulated deformed pavement geometry (whole service life), surface drainage characteristics are finally analyzed and assessed via a surface drainage module, e.g. to compute and predict the alteration of the pavement runoff during the service life of the pavement.