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Evaluation of Internal Resistance in Asphalt Concretes
Yousef Zandi
Muhammet Vefa Akpinar
Composites are somewhat more difficult to model than an isotropic material such as iron or steel due to the fact that each layer may have different orthotropic material properties. In finite element literature the asphalt mixes are represented by using rectangular meshes, not the actual picture of their cross-sections. Asphalt aggregate size and distribution in the asphalt concrete sample, aggregate shape, and fractured surface effects are ignored. In this research, the actual image of the sample including all these effects were directly considered in the finite element. The samples, were cut into cross-sections and were scanned. The image-processing toolbox of Labview was utilized in obtaining the rectangular gray images of the scanned images. In the rectangular sample the aggregates were white and the asphalt binders were black. The grayscale images were converted by LABVIEW into the format required by ANSYS as an input file, with the same dimensions. The nodes at the bottom of the model were constrained in both x and y directions. Left and right edges were symmetry and top was free. Certain amount of pressure was applied along the top surface to simulate the tire pressure.
1. Introduction
A major concern in asphalt pavement roads is excessive
permanent deformation (rutting) resulting from heavy truck
loads. Rutting appears as longitudinal depressions in the
wheel paths and increases with increasing numbers of load
applications. Safety concerns such as steering problems and
decrease in bearing capacity on asphalt pavements are the
results of rutting. The asphalt pavement layer must exhibit
high resilient moduli and show low permanent deformation
in order to reduce and rutting in the asphalt pavement.
Rutting (permanent deformation) in an asphalt-concrete
layer is caused by a combination of densification (volume
change) and shear deformation. Shear deformations caused
primarily by large shear stresses in the upper portions of the
asphalt-aggregate layer(s) are dominant.
Granular materials have a major function in the structural
capacity of a highway pavement. Because approximately
85 % of the total volume of asphalt concrete mixtures
consist of aggregates, the performance of the asphalt concrete is
greatly affected by the properties of the aggregate blend
(Ahlrich 1996; Ahlrich 1995; Zakaria and Less 1996). They
provide a foundation that supports asphalt concrete layers
and helps to support the pavement. One of the most
important properties of an aggregate blend is its size
gradation, which defines the percentages of different particle
sizes that are present in the blend. Gradation affects almost
all the asphalt concrete properties, including durability,
stability, stiffness, permeability, workability, fatigue resistance,
frictional resistance and moisture damage.
The primary objective of 2002 AASHTO (American
Association of State Highway and Transportation Officials)
was to advance the state-of-the-practice from empirical to
mechanistic related design procedure. AASHTO Joint Task
Force on Pavements (JTFOP) to initiated an study objective
of developing mechanistic pavement analysis suitable for
use in future versions of the AASHTO guide.
Rutting associated test techniques for asphalt pavement
materials are empirical. It is important that a mechanistic
procedure be developed which will reasonably predict the
permanent deformation under loading by heavy traffic.
Finite element (FE) analyses provide significant basis for
the development of mechanistic analysis. Available FE
programs are powerful tools for studying stressstrain
analysis in pavement structure. Pavement rutting cannot be
estimated with sufficient accuracy and reliability using
current mechanistic procedures which are based on either (1)
linear viscoelastic models or (2) layer-strain algorithms.
However, FE techniques are now available that are well
adapted to the analysis of permanent deformation in
pavement structures. They can effectively handle complex
constitutive relationships as well as the transverse distribution of
traffic.
2. Objective
In order to determine the rutting of the asphalt pavement
layer for a given mix type, permanent deformation
parameters of the mix can be developed from laboratory testing.
These parameters can then be used to predict the permanent
deformation of the material taken in the field and from the
image and FEM analyis is the optimum use of unbound
granular materials as a structural layer in pavements can be
determined. In this research, the actual image of the sample
were directly considered in the FE. In the FE literature the
asphalt mixes are represented by using rectangular meshes,
not the actual picture of their cross-sections.
3. Materials and Methods
3.1 Sample Preparation
The preparation of the samples were followed according to
superpave protocols. Fifteen centimeter diameter samples
were prepared by a superpave gyratory compactor.
Aggregate gradations use (...truncated)