Olivier Chupin

Viscoroute 2.0 : a tool for the simulation of moving load effects on asphalt pavement

ABSTRACT As shown by strains measured on full-scale experimental aircraft structures, traffic of slow-moving multiple loads leads to asymmetric transverse strains that can be higher than longitudinal strains at the bottom of asphalt pavement layers. To analyze this effect, a model and software called ViscoRoute have been developed. In these tools, the structure is represented by a multilayered half-space, the thermo-viscoelastic behaviour of asphalt layers is accounted for by the Huet-Sayegh rheological law, and loads are assumed to move at constant speed. The paper presents a comparison of results obtained with ViscoRoute to results stemming from the specialized literature. For thick asphalt pavement and several configurations of moving loads, other ViscoRoute simulations confirm that it is necessary to incorporate viscoelastic effects in the model to predict the pavement behaviour and to anticipate possible damages in the structure.

Mixed FEM for solving a plate type model intended for analysis of pavements with discontinuities

This paper aims at presenting the development of a numerical tool dedicated to the computation of the mechanical response of pavements incorporating vertical cracks and/or interlayer debonding. In this tool, the structure is modelled as a pilling of “plate” elements of type M4-5n (Multi-Particle Model for Multilayer Materials) which considers 5 equilibrium equations per layer (n stands for the number of layer). Here we focus on the development of a mixed Finite Element (FE) method dedicated to the solving of M4-5n. This method relies on the derivation of a variational principle based on the complementary energy theorem. Expressing stationarity of the functional obtained with respect to all its fields leads to the mixed formulation. Special attention is paid to the discretization process of this formulation in order to avoid ill-conditioned system of algebraic equations after discretization and to insure stability of the solution. The developed method is implemented in a FreeFem++ script. The advantage of the method is twofold: (i) the initial 3D problem can be handled through 2D FE simulations and (ii) finite values of the generalised efforts are obtained at crack and interlayer debonding locations. This approach is thus particularly adapted to parametric studies and, in the future, might be considered for crack growth in layered structures such as pavements. This paper ends with the analysis by means of M4-5n of a 3D structure incorporating cracks, representative of a pavement tested under full-scale conditions during an accelerated fatigue test performed at IFSTTAR. Several scenarios of cracking are analysed and compared to experimental results.

M4-5n Numerical Solution Using the Mixed FEM, Validation Against the Finite Difference Method

The final aim of this work is to build a tool dedicated to the calculation of the mechanical fields in pavements incorporating possible vertical cracks in some layers or partial debonding at the interface between layers. The development of this tool is based on a specific layer-wise modeling of the structure so-called M4-5n. In this model the stress fields are approached through polynomial approximations in the vertical direction for each layer. Its construction is based on the Hellinger-Reissner (H-R) variational principle of continuum mechanics. One advantage of the M4-5n is to reduce by one the dimension of the problem. Moreover this model leads to finite values of the generalized interface stresses at the crack lips of the structures studied. This approach is thus particularly adapted to parametric studies and might be considered for analyzing crack growth in layered structures such as pavements. The contribution of the present paper to this model is focused on the computation of its numerical solution by means of the mixed Finite Element Method (FEM). The developed method is based on the maximum of the complementary energy theorem using Lagrangian multipliers to ensure the equilibrium equations. The resulting formulation is equivalent to the H-R variational principle applied to the generalized displacement and stress fields. This approach is applied to a beam structure composed of four elastic homogenous layers resting on Winkler’s springs. Vertical cracks across some layers are introduced. The results obtained are compared with those from an earlier approach using the Finite Difference Method (FDM).

Experimental evidence of the viscoelastic behavior of interfaces in bituminous pavements: an explanation to top-down cracking?

Top-down cracking is known to be initiated near the surface of the asphalt layers. The aim of this paper is twofold: (i) to show experimental evidence of the viscoelastic behavior of interface in asphalt pavements under some temperature conditions, (ii) to show that the integration of such a behavior could provide an explanation of the mechanism involved in the initiation of bottom-up and top-down cracking. This paper documents the methodology used to investigate the behavior of the upper interface from experimental tests carried out at the IFSTTAR’s track facility. The mechanical behavior of the experimental pavement is evaluated using three different models: (1) elastic model, (2) Huet-Sayegh viscoelastic model to account for the behavior of asphalt layers, and (3) same as 2, but with additional very thin viscoelastic layers to represent interfaces between the asphalt layers. Computations are performed for two load configurations using software ViscoRoute 2.0© to evaluate the stresses and strains at different depths of the pavement for the 3 models. The comparison between the test track results and the models clearly show that model 3 is that yielding the best fit. This model shows that significant tensile stresses and strains occur near the surface and at the interface between two asphalt layers. The transposition of the viscoelastic behavior of interfaces to real traffic conditions could explain top-down cracking as one of the modes of failure of asphalt pavements.

Highlighting of the viscoelastic behaviour of interfaces in asphalt pavements – a possible origin to top-down cracking

Top-down cracking (TDC) is known to initiate and remain near the surface of bituminous pavements. The aim of this paper is twofold: (i) show experimental evidence of the viscoelastic behaviour of interface in asphalt pavements under some temperature conditions and (ii) show that taking into account such a behaviour could provide an explanation to the mechanism involved in the initiation of TDC. This paper documents the methodology used to investigate the behaviour of the upper interface from experimental tests. The mechanical response of the experimental pavement is evaluated using three models: (1) the elastic model, (2) the Huet–Sayegh viscoelastic model to account for the behaviour of asphalt layers and (3) same as 2, but with additional very thin viscoelastic layers to represent interfaces between the asphalt layers. The software ViscoRoute 2.0© is used to evaluate the stresses and the strains at different depths of the pavement. The comparison between the experimental results and the models clearly shows that model 3 is that yields the best fit. This model shows that significant tensile stresses and strains occur near the surface and at the interface between two asphalt layers. The transposition of the viscoelastic behaviour of interfaces to real traffic conditions could explain TDC as one of the damaging modes of asphalt pavements.

Effect of Temperature and Traffic Speed on the Asphalt Moduli for Fatigue Cracking and Pavement Structural Design Considerations

Asphalt pavements exhibit strongly temperature-dependent viscoelastic behaviour resulting in response to load varying with both temperature and traffic speed. To design against fatigue cracking for structural pavement design applications the linear-elastic material response is typically assumed to simplify the calculation of the critical strains in the pavement structure layers. To be representative the simplified pavement model needs to be defined in such a way that it accurately reflects the effect of both temperature and traffic speed on the critical strains used to compare with the material’s tolerable strains. Against this background, this paper presents a method to determine an equivalent asphalt modulus (EAM) for the asphalt layer which represents the effect of temperature and loading speed on the critical tensile strains. The EAM is determined from viscoelastic modelling. Two thick asphalt pavement configurations representative of typical French pavement designs are considered. Results show expected trends of the equivalent asphalt modulus increasing with increasing traffic speed and decreasing with increasing temperature. The application of the asphalt material shift factors allowed building pseudo-master curves for the EAM dataset. Finally, the effect of temperature and pavement structure on the relationship between traffic speed and complex modulus frequency is examined. Results support the use of 10 Hz for 70 km/h at intermediate temperatures currently used for pavement designs in France.

Force Chain Lifetimes in Shear Bands in Sands

We present imaging‐based experimental data of the meso‐scale kinematics associated with force chain buildup and collapse within shear bands in sands. Dense sand specimens are subjected to plane strain compression in an apparatus in which the zero‐strain conditions are enforced by glass walls which permit imaging if in plane deformations. Grain‐scale displacements are quantified continuously through the experimental technique of Digital Image Correlation (DIC). We evaluate the meso‐scale kinematics within the shear bands, the patterns in which have been shown to be strongly indicative of force chain buildup and collapse. We follow the change in these kinematic patterns from global softening to critical state deformation, and discuss the implications of the observed microstructural changes with regard to the evolution in macroscopic response. The kinematics suggest the transition from softening to critical state to be defined microstructurally by a distinct collapse event of the force chains formed at shear...