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Shear Modulus Simulation of Asphalt Mixtures

Overview

The main objective of this study was to develop 2D and 3D micromechanical finite element (FE) models to predict the shear modulus of two asphalt mixture types. The internal microstructure of the asphalt mixtures were determined by X-ray computed tomography (CT) imaging. X-ray CT images were processed to create 2D and 3D meshed asphalt mixture FE model structures using the software Simpleware. Meshed model structures were exported to the FE modeling software Abaqus®. Elastic and viscoelastic assumptions were used for modeling aggregate and mastic subdomains, respectively. Shear modulus predictions for 2D and 3D models were compared to the measured values to determine the effectiveness of each model type for the simulation of the shear frequency sweep at constant height (FSCH) test. It was concluded that the 3D micromechanical FE model is capable of predicting shear modulus at relatively high test temperatures with high accuracy across a range of loading frequencies.

Characteristics:

  • 2D and 3D micromechanical FE models from X-ray CT images
  • Automatic segmentation using Threshold tool
  • Triangle elements are perfectly bonded
  • Shear modulus predictions in Abaqus®
  • Comparison with lab results to determine effectiveness

Thanks to

Department of Civil and Environmental Engineering, University of California Davis & Department of Radiology, University of California Davis: E. Coleri • J.T. Harvey • K. Yang • J.M. Boone

Image Processing

Simpleware software was used in this study to create masks for each domain based on measured intensity groups. The total volume of the asphalt mixture sample was calculated for the complete intensity range, 0 to 255. The upper limit for the air void intensity range was determined by trial and error to match the measured air void contents.

After the thresholds for each domain were determined, masks with different colors were assigned to clearly visualize the boundaries between each domain.

Mesh Generation

The mask created for the air void domain was excluded from the material structure because it has no stiffness. Before meshing, both the aggregate and the mastic domains were slightly smoothed while preserving the element quality and volume. This presmoothing stage allows the software to create more realistic meshes. High quality (triangles that are “as equilateral as possible") tetrahedral meshes were created for the aggregate and mastic domains while preserving the empty air void domain. It can be observed that triangle elements in the mastic and aggregate domains are perfectly bonded on the boundaries. The meshed structures were exported to FE analysis software ABAQUS for modeling.

Simulation

Micromechanical viscoelastic FE simulations were conducted on meshed 2D and 3D samples that were exported to FE analysis software ABAQUS. Comparing the measured and predicted 2D numerical values, it can be observed that the 2D models always underpredicted the shear modulus values. In contrast, the shear modulus values predicted by the 3D numerical model are close to the laboratory measured shear modulus values and the prediction of shear modulus with the developed 3D model is reasonable. It can be concluded that inability to capture the 3D microstructure with the 2D models decreases the accuracy of numerical predictions.