Didier Bodin

Experimental and Numerical Analysis of Crack Initiation and Growth in Thin Film of Bitumen

ABSTRACT In order to estimate the influence of bitumen on the resistance to cracking and fatigue behaviour of bituminous mixes, a specific test has been developed to estimate fracture properties. A thin film of binder between two steel protuberances, simulating the pseudo contact of two aggregates, is subjected to successive tension loads with a constant strain rate during the test. Formation and evolution of the crack size during the test was investigated using numerical methods. This paper deals with the combination of the experimental evolution of the sample stiffness and modelling of the considered test by finite element method. The modelling method, described here, integrates both viscoelasticity with Prony series and the sample geometry. A quantitative method using stress intensity factor and displacement intensity factor is also presented to study the effect of strain rate on propagation of crack.

Non Linearity in Bituminous Materials during Cyclic Tests

ABSTRACT In the recent years rheology has been taking an increasing importance in the characterization of bituminous materials. Rheological parameters are suggested by many studies as good indicators of the performance of these materials and tools to efficiently predict the performance and durability of asphalt pavements.Bituminous materials exhibit a non-linear mechanical response beyond a certain level of solicitation. This behaviour is important as it is a source of error in linear stiffness measurements. Fatigue tests on bituminous materials are typically performed in the non-linear region. In the present study, non-linearity of bituminous binders and mixtures was investigated through strain sweeps and fatigue tests. The development of non-linearity was described and quantified. It was found the strain level effect has the same impact on mechanical properties as the accumulation of cycles. A phenomenological description of this behaviour is proposed, based on the destruction of supra-molecular structures.

Size effect regarding fatigue evaluation of asphalt mixtures

ABSTRACT Experimental results for fatigue tests on three size geometrically similar asphalt concrete specimen are presented. Effect on fatigue life is analyzed and compared to trends given by a fatigue damage model. This innovative laboratory campaign points out a size effect on fatigue assessment of asphalt mixture. In a second part, the application of a nonlocal fatigue damage model for asphalt mixtures is presented. A qualitative agreement is obtained between experimental results and non local damage fatigue predictions trends. This agreement validates the nonlocal modelling that allows to model the influence of the heterogeneous nature of the material on fatigue failure. Differences observed between laboratory data and model predictions may be attributed to viscoelasticity induced thermal softening which had to be quantified and coupled to damage in order to properly study these fatigue test data.

Comparison of small and large-scale wheel tracking devices

ABSTRACT This paper presents a comparison of results from two wheel tracking devices. It is part of a larger on-going collaborative research project between the University of Nottingham in the UK, Delft University of Technology in The Netherlands and LCPC in France on the development and Finite Element (FE) implementation of a non-linear elasto-visco-plastic constitutive model for asphalt. Two devices, a large scale and small scale device, encompassed by the European Standard and the former British Standard were used. Rutting evaluation at the same temperature, on two different materials for pavement surface layers was tested under the same conditions allowing the evaluation of the rutting rate to be determined which was found to be very close from one test to another but sensitive to mixture type. In addition to rutting results, the contact dimensions and pressures from the two devices are evaluated in both the static case and during motion using a flat force transducer on the surface of the slab. The stresses induced in the slabs were calculated using finite element simulations and the measured contact conditions. For different pressure distributions under the wheel the isotropic and deviatoric stresses at the centre of the slab were compared which showed the importance of a good understanding of the wheel/slab contact conditions. The paper provides experimental rutting results obtained at the same temperatures for two characteristic materials on two devices. For future mechanical modelling the wheel print was found to be smaller during the motion than static prints. For wheels using inflated tyres a constant pressure over and elliptic area was found to be appropriate. For hard rubber wheels a rectangular area with an elliptic pressure distribution is more realistic.

Mechanical Testing of Bituminous Mixtures

This chapter focuses on permanent deformation, which is at the origin of pavement rutting. First experimental laboratory tools and associated analyses related to permanent deformations are presented. It is either material tests, that can be homogeneous or giving a simple empirical characteristic, or non-homogeneous Structural/Specimen tests.

Cracking in Asphalt Materials

This chapter provides a comprehensive review of both laboratory characterization and modelling of bulk material fracture in asphalt mixtures. For the purpose of organization, this chapter is divided into a section on laboratory tests and a section on models. The laboratory characterization section is further subdivided on the basis of predominant loading conditions (monotonic vs. cyclic). The section on constitutive models is subdivided into two sections, the first one containing fracture mechanics based models for crack initiation and propagation that do not include material degradation due to cyclic loading conditions. The second section discusses phenomenological models that have been developed for crack growth through the use of dissipated energy and damage accumulation concepts. These latter models have the capability to simulate degradation of material capacity upon exceeding a threshold number of loading cycles.