Armelle Chabot

Experimental and Theoretical Investigation of Three Dimensional Strain Occurring Near the Surface in Asphalt Concrete Layers

Several pavement failures have been observed to be initiated at or near the surface of the hot-mix asphalt layers and some of them propagate downward through the surface layer (top-down cracking). These modes of failure are affected by heavy vehicular loading configuration, pavement structure and their interaction at the tire-pavement contact. This paper documents an experimental investigation of surface strain induced under the entire tire by using specific instruments based on fiber optic sensors. Two innovative retrofit techniques which allow measuring strains in the upper parts of the asphalt layer have been used on the IFSTTAR’s test track facility. The association of these two techniques allows obtaining the strains, few centimeters below the surface, in three directions: longitudinal, transverse and vertical. Two pavement structures with two temperatures (moderate and hot) have been tested. Shape of the signal under the tire and magnitude of strain are compared with viscoelastic model pavement calculations.

A 4pt bending bond test approach to evaluate water effect in a composite beam

Abstract Considering that water may cause a separation of interfaces between layers of pavement structures, specific test on bi-layer specimens is performed in a water bath. For the study of the bond between layers made of cement concrete overlay on bituminous material, four-point bending results show a competition between different failure mechanisms. Actually a very good bond resistance between layers compared to the fracture tension resistance of the cement concrete layer is preliminary observed in dry conditions. In this work, first results of the water effect on the behaviour of such a material interface are presented. The final fracture length of the specimen and the corresponding curve of force-displacement highlight the influence of water immersion on the debonding failure mode. The field displacement measurement obtained by Digital Image Correlation is used to improve the understanding of the fracture scenario.

Mixed-Mode Debonding Approach to Evaluate Water Sensibility in Bi-Layer Composite Pavement

In order to evaluate water sensibility on the interface between layers of composite pavements, a four-point bending (4PB) test on bilayer structure in a water bath is proposed. Using the virtual crack closure technique, the individual strain energy release rates are calculated with a specific model. The debonding mode I should be recognized as the main failure mode. For bilayer specimens made of a cement concrete overlay on bituminous material, the specific test has shown a competition between the different failure mechanisms. A very good bond resistance between layers compared to the fracture tension resistance of the cement concrete layer is observed. In this work, first results of the effect of water on the behavior of such a material interface are presented. An aquarium is built in order to submerge under water the bi-layer specimen. The final fracture length of the specimens, curves of force-displacements and first digital image correlation results show the influence of water immersion on the debonding failure mode.

Full Scale Tests on Grid Reinforced Flexible Pavements on the French Fatigue Carrousel

Grids are increasingly used. They have proved their efficiency, but there is presently no widely accepted design method to predict the long term life of grid reinforced pavements. This paper describes a full scale experiment carried out on the large pavement fatigue carrousel of IFSTTAR, to test simultaneously 3 pavement sections with different types of grids, in comparison with an unreinforced pavement structure. The tests are carried out on typical French low traffic pavement structures. Results up to approximately 800 000 loads are presented. The experiment is planed to continue to load the test sections up to at least 1 million loads. During the experiment, the behaviour of the pavement sections has been followed by deflection and rut depth measurements, and surface distress analysis (observation of cracks and other degradations). As observed on the circular APT for low traffic pavements with thin bituminous layers, crack development was following a transversal orientation. This experiment shows the necessity to better understand the grid behaviour by means of modelling, experiments and use of new measurement techniques as planned in the new Rilem TC-SIB and TC-MCD.

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.

Characterization of the bond between asphalt layers and glass grid layer with help of a Wedge Splitting Test

With the aim of increasing the durability of pavements, the use of glass fiber grid to reinforce asphalt pavements has received a lot of attention in recent years. Performance of the bond between asphalt layers including a glass fiber grid at the interface is among the important parameters to be investigated. In this work, the Wedge Splitting Test (WST) developed by Tschegg (1986) is chosen to evaluate the bond between such material layers. The aim of this study is to characterize the fracture mechanical behavior of the interface through the specific fracture energy GF. Experiments are performed on specimens extracted from full scale pavement sections. These specimens of significant size (200 × 200 × 150 mm3) are prepared with a cylindrical groove instead of a habitual rectangular groove. As recommended in the literature, a slim wedge of 14° is used to ensure the characterization of the bond in a quasi-pure crack opening mode I. A notch of 5 mm thick is sawn at the interface between the two material layers to guarantee the initiation of the crack at the interface. Tests are controlled with a constant displacement speed rate of 2 mm/min and constant temperatures (~20°C). A Digital Image Correlation (DIC) technique is used to evaluate and compare the interface crack propagation in two different configurations with and without grid at the interface.

Mechanisms of cracking and debonding in pavements: Debonding mechanisms in various interfaces between layers

Editorial for the Special Issue RILEM MCD2016: The 8th Rilem International Conference on Mechanisms of Cracking and Debonding in Pavements (MCD2016) was held in Nantes (France) from June 7 to 9, 2016 (mcd2016.sciencesconf.org). The international journal, European Journal of Environmental and Civil Engineering (EJECE) published from MCD2016 a thematic issue entitled “Debonding Mechanisms in Various Interfaces between Layers”, related to the topic: “Debonding mechanisms between layers of asphalt and composite pavement”. From the 36 MCD2016 papers submitted on this topic, the five papers selected, revised and accepted for this thematic issue had all followed the standard EJECE review process. They present the main advances achieved in understanding pavement interface behaviour.

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).

Interface Debonding Behavior

The performance and durability of multi-layered pavements strongly depend on interlayer bonding between layers, especially for pavements with a thin or ultra-thin surface course. These pavements, comprised of several differing material layers, are often subjected to premature distresses (corrugation, peeling, slippage or fatigue cracking, etc.) caused by poor interface bonding. This chapter summarizes the different bond characterization tests available around the world (mostly in the laboratory) available to characterize the bond between pavement layers. Many of the tests can be performed on specimens prepared in the laboratory or on cores or slabs obtained from the pavement. Mostly, “pure” fracture mode test methods (opening mode I or in-plane, shear mode II or out-of-plane, shear mode III) are currently used worldwide for determining the interlayer bond of pavement layers. Most of the mixed-mode test methods (mainly for the combination of Modes I and II) were developed by a few research teams and there are therefore no standard tests. Although tack coat type and content are the main parameters studied by researchers and engineers, surface roughness, moisture, freezing, and presence of dust or debris on the interface are additional parameters that may decrease bonding performance.