A. Scarpas

Spectral element technique for efficient parameter identification of layered media. I. Forward calculation

Abstract This contribution deals with the use of spectral analysis as a means of analysing the dynamic behaviour of the axially symmetric multi-layered systems as a result of a transient force. The objective of this research work is to develop an accurate and computationally efficient forward tool suitable for solving inverse problems. The spectral element technique is utilized. Details of the mathematical derivation, implementation and verification of newly developed axi-symmetric and half-space spectral elements are presented. It is shown that the suitability of the spectral element method to such a problem encompasses in its ability to model a whole layer without the need for subdivisions. As a consequence, the size of the modelled structure becomes as large as the number of the layers involved. This reduces the computational requirements substantially and hence enables efficient utilization of the method in iterative algorithms for solving inverse problems.

First Observation of Blending-Zone Morphology at Interface of Reclaimed Asphalt Binder and Virgin Bitumen

One of the challenges in designing recycled asphalt mixtures with a high amount of reclaimed asphalt pavement (RAP) is estimating the blending degree between RAP binder and added virgin bitumen. The extent of blending is crucial because asphalt concrete response is influenced by the final binder properties. This paper focuses on the evaluation of interaction and extent of blending between RAP binder and virgin bitumen by studying the microstructures of the blending zone with atomic force microscopy (AFM). AFM is used to probe the change of microstructural properties from a RAP binder and virgin bitumen to the blending zone of these two. Averaged microstructural properties have been observed in thin-film blends of RAP binder and pure bitumen. The morphology of the blending zone (spatial extent of about 50 μm) exhibits domains of a wide range of microstructure sizes (160 nm to 2.07 μm) and can be considered to be a completely blended new material that has been observed directly for the first time. The fully blended binder properties are found to be between those of the two individual binders, as could be inferred from the averaged microstructural properties derived from AFM images of the blending zone. This finding is also consistent with the results of mechanical tests by dynamic shear rheometer on the same materials. Finally, a design formula is proposed that relates the spatial dimensions of the blending zone to temperature and mixing time, which will eventually allow the results of this study to be extended from small-length scales up to the engineering level.

Spectral element technique for efficient parameter identification of layered media. Part III: viscoelastic aspects

This article addresses the issues of wave propagation in elastic–viscoelastic layered systems and viscous parameter identification from non-destructive dynamic tests. A methodology that combines the spectral element technique, for the simulation of wave propagation, with the differential operator technique, for stress–strain relationship in viscoelastic materials, is adopted. The compatibility between the two techniques stems from the fact that both can be treated in the frequency domain, which enables naturally the adoption of Fourier superposition. The mathematical formulation of spectral elements for Burger’s viscoelastic material model is highlighted. Also, an inverse procedure for the identification of the material’s Young’s moduli and complex moduli for layer systems is described. It is shown that the proposed methodology enables the substitution of an expensive laboratory testing procedure for the determination of material complex moduli with non-destructive dynamic testing. 2002 Elsevier Science Ltd. All rights reserved.

Verification of falling weight deflectometer backanalysis using a dynamic finite elements simulation

Falling weight deflectometer (FWD) testing is probably the most widely used device for the non-destructive testing of pavements. In many cases, engineers use the FWD test data for the estimation of pavement material properties, assuming a quasi-static approach. Since the verification of such results is often necessary, a dynamic finite element (FE) approach can be used towards this goal. Analytically, data provided by static backanalysis is used as input for 2D and 3D dynamic FE modeling of the FWD test in order to calculate the expected deflection bowl. Comparison of the measured with the calculated deflection bowl may provide a strong indication for the validation of the backanalysed data. Investigation of the application of this approach using field data from new and existing asphalt pavements with two different FWDs and several backanalysis programs as well as data of laboratory tested cores is also presented and discussed.

Finite element modelling of field compaction of hot mix asphalt. Part II: Applications

A constitutive model is developed and implemented in the finite element system three-dimensional computer-aided pavement analysis for the simulation of hot mix asphalt field compaction. The details of this model are presented in a companion paper (Masad et al., Finite element modelling of field compaction of hot mix asphalt. Part I: Theory, International Journal of Pavement Engineering, Accepted, 2014). This model is based on nonlinear viscoelasticity theory and can accommodate large deformations that occur during the compaction process. The model was used to study the influence of frequency and amplitude of vibratory compaction rollers on the level of compaction. In addition, it was used to analyse the influence of various methods for compacting longitudinal joints on the percent air voids near these joints. The model was used to simulate the compaction of asphalt pavements with different structures and compacted using various equipment and patterns. The finite element results of the level of compaction and percent air voids were in reasonable agreement with the measurements. The model offers the opportunity to simulate and predict the compaction of asphalt mixtures under various rolling patterns and for different pavement structures.

Experimental Calibration Of A Viscoplastic-Fracturing Computational Model

An extensive experimental and analytical investigation is currently being carried out on the mechanisms leading to the initiation and propagation of damage in viscoplastic materials. One of the major goals of the investigation is the development and the finite elements implementation of a generalised triaxial, strain rate sensitive, history and temperature dependent constitutive model. Explicit procedures have been formulated for the experimental determination of the model parameters. As a minimum, only uniaxial test results are needed for determination of the basic parameters. The model has been implemented in the finite element code CAPA-3D. Results of the utilization of CAPA-3D for the investigation of the dynamic non-linear response of a road pavement are reviewed in the last part of this contribution.

Influence of Temperature on Tire–Pavement Friction: Analyses

Past experimental studies show that tire–pavement friction values are related to conditions surrounding the tire such as pavement temperature, ambient temperature, contained air temperature, and surface characteristics of the pavement. For measurements taken in different temperature conditions, road agencies generally apply correction factors. These correction factors are based primarily on experience and previous field test measurements that have very limited transferability under different conditions. This paper studies frictional behavior of test tires under different surrounding temperature conditions using finite element analysis. The scope of this research is to analyze the effect of pavement temperature, ambient temperature, and contained air temperature on frictional measurements. Finite element analysis of fully and partially skidding tires over different asphalt pavement surfaces, namely, porous asphalt, ultrathin surface, and stone mastic asphalt, is considered. Observation showed that a higher pavement temperature, ambient temperature, and contained air temperature resulted in a lower hysteretic friction for a given pavement surface and a given tire slip ratio. In contrast, a lower tire slip ratio and a pavement with higher macrotexture resulted in higher friction. This study highlights that a critical combination of these factors will decrease friction significantly.

Contribution of Hysteresis Component of Tire Rubber Friction on Stone Surfaces

This research examines the hysteresis friction of a sliding elastomer on various types of stone surfaces. The hysteresis friction is calculated with an analytical model that considers the energy spent by the local deformation of the rubber due to surface asperities of the stone surface. By establishing the fractal character of the stone surfaces, one can account for the contribution to rubber friction of stone roughness at different length scales. A high-resolution surface profilometer is used to calculate the three main surface descriptors and the minimal length scale that can contribute to hysteresis friction. The rubber is treated as a Zener visco-elastic material model. Modeling of the contact between the elastomer and the stone surface is based on the analytical model of Klüppel and Heinrich, which is a generalization of the Greenwood and Williamson theory of contact between spheres that are statistically distributed about a mean plane. The results show that this method can be used in order to characterize in an elegant manner the surface morphology of various stone surfaces and to quantify the friction coefficient of sliding rubber as a function of surface roughness, load, and speed.

The Influence of Air Void Content on Moisture Damage Susceptibility of Asphalt Mixtures: A Computational Study

Because of the difficulties associated with the generation of finite element meshes based on X-ray computed tomography scans and with the extraordinary computational demands in performing three-dimensional (3-D) finite element analyses, past modeling efforts have focused primarily on two-dimensional representations of asphalt mixtures and have placed no emphasis on the inclusion of the air voids network in the body of an asphalt concrete specimen. A 3-D micromechanical moisture damage model has been developed and implemented in the finite element system CAPA-3D capable of addressing individually the three main phases of asphalt concrete: aggregate, mastic, and air voids. The 3-D finite element meshes of different types of asphalt mixtures were generated on the basis of X-ray scans. By means of CAPA-3D, the significance of the air voids structure in the development of moisture damage in asphalt concrete specimens was demonstrated. Availability of the model enables evaluation and ranking of the contribution of the characteristics of the individual mixture components to the overall mixture moisture resistance.

Geogrid Reinforcing of Recycled Aggregate Materials for Road Construction: Finite Element Investigation

Economic advantages and environmental benefits encourage the use of recycled materials for road construction. Usually, however, these materials have lower stiffness and strength characteristics than typically used natural materials. Because of these inferior material properties, use of recycled aggregate materials for unbound road base construction may result in increased rutting, differential settlement, and reflective crack propagation. Placement of reinforcement between the underlying soil layer and the aggregate layer has been proposed to improve the load-carrying capacity of road bases constructed from recycled aggregate materials. To investigate the viability of such an approach, finite element analyses were performed of asphalt concrete pavements with base layers consisting of reinforced unbound recycled aggregate materials. The response of such pavements was compared with that of pavements consisting of unreinforced natural aggregates. The criteria chosen for comparison were the influence of the material characteristics of the recycled aggregate and the reinforcement on the development and speed of propagation of reflective cracking in the body of the pavement. Various combinations of reinforcement and aggregate material characteristics were simulated. It was concluded that the placing of reinforcement can reduce the speed of crack propagation into the top layer, improve load spreading in the unbound base layer, and prolong the economic life of the construction.