Cyrille Chazallon

Discrete element approach in brittle fracture mechanics

Purpose – The purpose of this paper is to use the discrete element method (DEM) to model the fracture behaviour of brittle materials in 2D.Design/methodology/approach – The material consists of a set of particles in contact with a close‐packed structure. It allows the derivation of an expression for the stress intensity factor as a function of the contact forces near the crack tip. A classical failure criterion, based on the materials toughness, is then adopted for the analysis of crack propagation, represented by the contact loss between particles.Findings – The DEM approach is compared to two tensile cases (mode I); both presenting a monotonous convergence towards classical solutions for more precise discretization.Originality/value – The paper proposes a DEM approach in fracture mechanics of isotropic brittle materials entirely compatible with continuous classical theory. Hence the toughness value is directly introduced as a parameter of the material without any previous calibration of the DEM.

Molecular weight distribution of asphaltic paving binders from phase-angle measurements

Based on previous works on polymers and regular bitumens, a new method for determining the molecular weight distribution (MWD) of asphaltic paving binders, using the phase angle () of the complex modulus measured in the linear viscoelastic domain, has been developed in this paper. MWDs of pure petroleum bitumens, natural bitumens, modified bitumens and artificially aged bitumens are calculated by this method and compared with MWDs obtained by gel permeation chromatography. The two methods give comparable results, but the δ-method developed here is more sensitive to the molecular weight of species. This method is very simple to implement, and seems a powerful tool in the determination of MWD of bitumens, in the visualisation and repartition of modifiers in the bitumen matrix and in the ageing monitoring and quantification.

Recycling of road materials into new unbound road layers - main practice in selected European countries

Most European countries are active in the field of recycling road materials, but knowledge and practice differ between countries. The European project DIsmantling and RECycling Techniques for road MATerials – Sharing knowledge and practices aims at sharing knowledge and practice in this field among the 15 participating countries, with the view of drafting European best-practice guidelines. This paper reports on the first step towards this goal, which consists of summarising documented practices within these countries concerning demolition and recycling of road materials back into new unbound road layers. Common documented practice and major differences between European countries are highlighted and put in perspective, thanks to a broader international document review.

Water Influence on Mechanical Behaviour of Pavements: Constitutive Modelling

This chapter deals with the effects of water on the mechanical behaviour of pavements. The analyse is based on constitutive considerations. Constitutive models devoted to both routine and advanced pavement analysis and design are introduced and both the resilient behaviour as well as the long term elasto-plastic approaches are presented. As soon as the approach considers the material as a two phase (solid matrix and a fluid), the introduction of the effective stress concept is required. In the last section an analysis is made on the extension of the constitutive models to the characterisation of partially saturated materials.This chapter deals with the effects of water on the mechanical behaviour of pavements. The analysis is based on constitutive considerations. Constitutive models devoted to both routine and advanced pavement analysis and design are introduced and both the resilient behaviour as well as the long term elasto-plastic approaches are presented. As soon as the approach considers the material as a two phase (solid matrix and a fluid), the introduction of the effective stress concept is required. In the last section an analysis is made on the extension of the constitutive models to the characterisation of partially saturated materials

Laboratory characterisation of the fatigue behaviour of a glass fibre grid-reinforced asphalt concrete using 4PB tests

This paper presents a complete experimental study of the fatigue behaviour of a glass-fibre-reinforced asphalt concrete, respecting EN 12697-24:2012, Annex D. The study is based on 38 four point bending tests, performed on 18 non-reinforced asphalt specimens and 20 geogrid-reinforced asphalt specimens. Both non-reinforced and reinforced asphalt specimens are tri-layered beams. The reinforced specimens contain two glass fibre grids, each one placed at the interface between two asphalt concrete layers. The objectives of the study were: (1) to characterise the fatigue behaviour of reinforced asphalt concrete, and to determine fatigue parameters, which can be used for pavement design, following the French pavement design method, (2) to show that glass fibre grid delays fatigue crack propagation and (3) to estimate the gain in fatigue due to the geogrid. The comparison of the fatigue curves of non-reinforced and reinforced asphalt concrete indicates that the geogrid increases fatigue life for the testing conditions, which were: fully reverse loading, displacement control, sinusoidal waveform, T = 10°C and f = 25 Hz.

Effective stress concept for the effect of hydraulic hysteresis on the resilient behaviour of low traffic pavements

This paper presents the influence of the unsaturated state and especially the different hydraulic behaviours during the wetting and drying cycles (hysteresis phenomenon) on the resilient behaviour of unbound granular materials for roads. In this context, the soil water retention curve (SWRC) of a compacted clayey sand was initially obtained for both wetting and drying paths. These experimental results were then complemented by a series of direct shear tests. Finally, repeated load triaxial tests (RLTTs) were also performed at the same water contents on both wetting and drying paths. It can be stated that the samples prepared on the wetting path showed higher resilient volumetric strains compared to those prepared on the drying path, especially at low water content. However, the influence of the hysteresis phenomenon is less evident for the resilient shear strains. Different effective stress concepts based on the SWRC and shear test results were also added to the classical Boyce model generally used to estimate the resilient behaviour based on the total stress concept. The estimated effective stresses of the RLTT give the best correlation coefficient for all water contents of the wetting and drying paths. Finally, finite element calculations, carried out with CAST3M, were performed to compare the deflection of a low traffic pavement at saturated/dry conditions for the subgrade soil. The asphalt concrete layer and the base layer were taken the same for both conditions. The influence of the hysteresis phenomenon on the deflections is more pronounced at the dry condition.

Influence of fine content and water content on the resilient behaviour of a natural compacted sand

Granular materials are often used in pavement structures. The influence of fine content and water content on the mechanical behaviour is very important. In this work, first, we present the resilient behaviour of compacted clayey natural sands with two different fine contents using the repeated load triaxial tests. The samples were compacted at different water contents with the same dry density. It can be stated that an increase in the fine content (from 4% to 7.5%) causes an increase in the volumetric strains and shear strains for all the water contents. The influence of water content can be considered negligible comparing to the influence of fine content. Then, the experimental results were simulated using the nonlinear elastic model (modified Boyce model) generally used for the resilient behaviour of the unsaturated granular soils. The results show that the model can correctly predict the variation of the volumetric strains and the shear strains with the mean stress for all the stress paths. Finally, finite element calculations carried out with CAST3M were performed to compare the deflection of a low traffic pavement with sub-base layers containing two different fine contents. The surface asphalt concrete layer and the base layer were taken the same for both structures. The modelling results show that the sub-base soil with higher fine content produces 37% higher deflection in the whole structure and that the effect of the water content is lower than that of the fine content.

Ageing Performances of Asphaltite Modified Bitumens; Comparisons with Equivalent Petroleum Bitumens

This work focuses on the ageing behavior of asphaltite modified paving bitumens. The addition of asphaltites in soft petroleum bitumens gives harder binders which are suitable for the production of high modulus bituminous mixes used in pavements construction. Such techniques are already used in several projects. However it is not known how asphaltite modified binders behave during ageing. In general, bitumens get harder, more elastic and consequently more brittle during ageing. In order to evaluate the ageing incidence we have studied the evolution of several mechanical properties of asphaltite modified bitumens during ageing. The evolutions of asphaltite modified bitumens during aging are compared with the evolutions of pure petroleum bitumens of equivalent grade. The petroleum bitumens of equivalent grade, used for comparison purposes, are produced in refinery in France by the same fabricant as the soft petroleum bitumen which has been selected to be modified by asphaltites. All the petroleum bitumens satisfy the European Norms and are used currently in France and elsewhere. The present study shows that the asphaltite modification gives hard paving binders which have better performances, regarding to the ageing evolution, compared to hard petroleum bitumens produced in refinery. The asphaltite appears to be an ageing inhibitor.

A Size and Boundary Effects Model for Quasi-Brittle Fracture

The fracture behaviors of quasi-brittle materials are commonly specimen size (size effect) and crack size (boundary effect) dependent. In this study, a new failure model is developed for characterizing the size and boundary effects. The derivative of the energy release rate is firstly introduced to predict the nominal strength dominated by the strength mechanism. Combined with the energy criterion for the energy mechanism, an asymptotic model is developed to capture the effect of any crack size on the nominal strength, and its expression for geometrically similar specimens is also established, which is able to characterize the size effect. Detailed comparisons of the proposed model with the size effect law and the boundary effect model are performed, respectively. The nominal strength predictions based on the proposed model are validated with the experimental results of cracked three-point bending beam specimens made of concrete, of limestone and of hardened cement paste and compared with the model predictions given by the size effect law and the boundary effect model.

Effect of dry density, soil texture and time-spatial variable water content on the soil thermal conductivity

Study of the heat transfer process in saturated and unsaturated soils requires, basically, a relationship between thermal conductivity and the characteristics of the soil, such as water content, dry density and texture of the soil. This study intends to produce a generic model that can predict soil thermal conductivity with the help of easily measurable parameters. The proposed model is first calibrated using measured thermal conductivities from literature data. In order to validate the proposed model the predicted thermal conductivity of this proposed model as well as existing ones are compared with the measured thermal conductivity in literature for different soils. Validation of the proposed model was also performed on our experimental results obtained for a compacted Misillac sand and in-situ clay loam soils. The results show an average of 15% improvement in prediction accuracy for the proposed model compared to the existing models, considering all soil textures. Moreover, we perform a model to estimate thermal conductivity over time throughout the profile of soil in the context of seasonal variation of temperature. The proposed model shows an important effect of heterogeneity on the thermal conductivity variations of a double layered soil.