Sudhir Varma

Backcalculation of viscoelastic and nonlinear flexible pavement layer properties from falling weight deflections

Appropriate characterisation of individual layer properties is crucial for mechanistic analysis of flexible pavements. Typically in inverse analyses, pavements are modelled as elastic or nonlinear elastic to obtain layer material properties through non-destructive falling weight deflectometer (FWD) testing. In this study, a layered viscoelastic–nonlinear forward model (called LAVAN) was used to develop a genetic algorithm-based backcalculation scheme (called BACKLAVAN). The LAVAN can consider both the viscoelastic behaviour of asphalt concrete (AC) layer and nonlinear elastic behaviour of unbound layers. The BACKLAVAN algorithm uses FWD load-response history at different test temperatures to backcalculate both the (damaged) E(t) and |E*| master curve of AC layers and the linear and nonlinear elastic moduli of unbound layers of in-service pavements. The BACKLAVAN algorithm was validated using two FWD tests run on a long-term pavement performance section. Comparison between the backcalculated and measured results indicates that it should be possible to infer linear viscoelastic properties of AC layer as well as nonlinear elastic properties of unbound layers from FWD tests.

Viscoelastic genetic algorithm for inverse analysis of asphalt layer properties from falling weight deflections

The falling weight deflectometer (FWD) is a nondestructive test whose results are typically used for backcalculating in situ layer properties of pavements. Most backcalculation methods assume the pavement to be a layered elastic half-space. However, asphalt pavements behave more like multilayered viscoelastic systems, especially in response to small or short-duration load applications. Hence, although elastic analysis is computationally efficient and well accepted in the engineering community, the theory cannot produce the viscoelastic properties of the asphalt concrete (AC) layer. In this study, a new inverse analysis method is proposed to backcalculate both linear elastic and viscoelastic properties of pavement layers as well as the AC time–temperature shift factor. In this method, the FWD load–response history of a single FWD drop and variation in temperature along the depth of the AC layer during the drop are used for performing the computations. The underlying (viscoelastic) forward solver is approximate and disregards dynamic effects; these factors, in return, make it computationally efficient. A genetic algorithm–based optimization scheme is offered to search for the pavement properties. As an example, two sections from the long-term pavement performance study were selected for investigation. The back-calculation results were positive, which indicates that unless a stiff layer exists close to the surface, it should be possible to infer linear viscoelastic properties from a single FWD drop.

Viscoelastic Nonlinear Multilayered Model for Asphalt Pavements

AbstractFlexible pavements are multilayer structures, typically with a viscoelastic asphalt layer followed by nonlinear unbound/bound layers. Conventionally, multilayered elastic analysis is performed to obtain the response of flexible pavements for design and inverse analyses; however, assuming asphalt pavement to be a linear elastic material is an oversimplification of its actual behavior. It is well known that the responses of asphalt pavements are both rate and temperature dependent. In the present work, a computationally efficient model has been developed to analyze flexible pavements, considering the top layer of linear viscoelastic asphalt concrete (AC), followed by a stress-dependent (nonlinear) base layer, and an elastic subgrade. Constitutive equations are formulated for layered viscoelastic–nonlinear axisymmetric systems. It is shown that the developed model can be used to simulate pavement response under stationary or transient loading. A comparison between the model responses and results obta...

Data requirements from falling weight deflectometer tests for accurate backcalculation of dynamic modulus master curve of asphalt pavements

Dynamic modulus (|E*|) master curve is a fundamental material property for an asphalt pavement. It is also a key input to DARWin-METM, a pavement design and analysis software that can predict progression of distresses. Backcalculation of |E*| master curve of an in-service pavement using Falling Weight Deflectometer (FWD) data can lead to more accurate estimation of its remaining service life (with the aid of DARWin-METM). However, backcalculation of the entire |E*| master curve, including the time-temperature superposition shift factor coefficients, requires more data than the surface deflection time-histories of a single FWD drop. This paper presents a novel |E*| backcalculation algorithm that is based on a genetic optimization methodology and a layered viscoelastic forward solution. The main objective of this work was to estimate a set of temperatures where FWD tests should be conducted, in order to maximize the portion of the |E*| master curve that can be accurately backcalculated. The results indicate that there exists a range of temperatures (of FWD testing) at which the FWD response leads to better inverse solutions. CE Database subject headings: Asphalt Pavements, Dynamic Modulus, Relaxation Modulus, Backcalculation, Viscoelasticity, Falling Weight Deflectometer, Nondestructive tests, Temperature effects, Optimization.

Backcalculation of Swollen Crumb Rubber Modulus in Asphalt Rubber Binder and Its Relation to Performance

A method is presented to estimate the engineering properties of swollen rubber particles within crumb rubber–modified (CRM) asphalt binders. It was observed that the backcalculated modulus of swollen rubber within the CRM asphalt binder was related to performance. Thus improper mixing temperatures as well as interaction times could lead to the insufficient swelling of rubber and to a performance worse than anticipated in CRM asphalt mixtures. The backcalculated modulus of the rubber within the CRM asphalt could be used as a quality control tool, which could be measured during construction. An experimental approach was taken to investigate the mechanisms of interaction between the asphalt binders and crumb rubber and their effect on the modulus of the rubber within the CRM asphalt binder. The experimental program included dynamic shear modulus (|G*|) as well as linear amplitude sweep tests. These tests were performed on neat binder, CRM binders mixed at three temperatures, neat binder mixed at these three temperatures (without the rubber), and the residual binder obtained through the drainage of the CRM binders (i.e., by filtering out the rubber particles). In addition to the experimental approach, a three-dimensional finite element (FE) based micromechanical model of the |G*| test was developed with ABAQUS software. The FE model provided insight into the relationship between the microscale and macroscale material behavior (i.e., the stiffening and softening effects of rubber in CRM asphalt binders).

Development of Predictive Models for Low-Temperature Indirect Tensile Strength of Asphalt Mixtures

AbstractThermal cracking is the predominant flexible pavement distress in northern climates, causing transverse cracks perpendicular to the direction of traffic. The indirect tensile (IDT) strength test is currently the most widely used method to characterize thermal cracking susceptibility and is required in mechanistic empirical pavement design. When laboratory IDT strength testing data are not available, it is predicted by pavement design software using mixture volumetrics and Superpave performance grade (PG) of the binder. The primary purpose of this study was to examine the IDT strength characteristics of asphalt mixtures commonly used by the Michigan Department of Transportation (MDOT) and to develop improved prediction methods for IDT strength. Laboratory testing of 62 unique MDOT mixtures (a total of 201 samples with replicates) showed that the pavement design software predicted the IDT strength very poorly. Three models were developed to improve the accuracy of IDT strength prediction. First, the...

A micromechanical model to create digital microstructures of asphalt mastics and crumb rubber-modified binders

Abstract This paper presents a micromechanical model to develop digital microstructures of asphalt mastics and crumb rubber-modified binders. The micromechanical model was based on the dissipative particle dynamics (DPD) model that is typically used for modelling suspensions. This paper utilises X-ray tomography images of particles to create microstructures. First, a database of 3D images of crumb rubber particles were generated by scanning specimens using X-ray computed tomography and X-ray microtomography. Once the 3D images were generated for individual particle shapes, a series of spherical harmonic (SH) functions were fitted to the surface of the particles and SH coefficients were determined. Utilisation of SH coefficients for each particle (rather than the actual 3D image voxels) allowed efficient numerical DPD simulations performed to generate microstructures. Example simulations were performed to generate microstructures and used to generate finite element meshes and exported to ABAQUS. The linear viscoelastic responses of the microstructures were simulated using ABAQUS and compared to the measured values.

A framework based on engineering performance and sustainability to assess the use of new and recycled materials in pavements

Growing need for increased use of recycled and new materials in road construction has emerged due to the continuous depletion of natural resources and increased impact of the current state of practice on the environment. Sustainable construction practices have been favoured by Federal and State Departments of Transportation as well as the industry. However, the impacts of using new and recycled materials in pavements, particularly on long-term pavement durability and performance, are often unknown. A comprehensive procedure for evaluating these “proposed materials” in terms of engineering performance and sustainability is very significant for making appropriate decisions on whether to use them for road construction. The research is performed based on whether the material is proposed to be used in asphalt, concrete, or in unbound layers. An analysis framework and, a software (called NewPave) was developed to help the Michigan Department of Transportation (MDOT) identify the impacts of new and recycled materials on pavement performance and the environment. The analysis framework included two basic components; (i) engineering performance, and (ii) sustainability. Engineering performance evaluation included several options for each material type to be used in different pavement layers. The sustainability analysis included three basic components; environmental, economic and social analyses. Finally, the scores obtained from the engineering evaluation are combined with those based on sustainability to obtain an overall score. The overall score can be used to accept/reject the trial use of the new and recycled materials in MDOT administered roads. While the framework presented herein was developed for MDOT, it can easily be adapted by other DOTs.