Asphalt internal structure characterization with X-Ray computed tomographyDenis Jelagin, Ibrahim Onifade, Alvaro Guarin and Nicole Kringos KTH, Highway and Railway Engineering
OutlineUnderstanding of asphalt mixtureproperties based on constituentmaterials spatial distribution and theirmechanical properties: - Determination of quantitative parameters to describe asphalt internal structure. - Mechanical modeling with finite element method to quantify the impact the constituent material parameters have on mixture mechanical behavior
Asphalt mixture internal structure andits effect on field performance Asphalt consists of three main phases: stones, binder and air voids; their spatial distribution and properties have a major impact on asphalt performance: • Stones and stone-to-stone contacts provide a primary load carrying mechanism in compression and shear, especially at high temperatures • Bitumen-based binder and its distribution control tensile stiffness and fracture resistance • Air void structure controls mixture permeability, resistance to bleeding and ageing Deficient internal structure of asphalt results in pavement failures
X-Ray computed tomography (CT) characterization of asphaltX-Ray CT system to acquire images Avizo® Fire to segment CT data and to obtainwith spatial resolution of 5-100 µm quantitative parameters for specimens structure Preprocess for FEAUse mechanical testing to investigate FEM modeling to quantify thethe impact of the observed internal effect of different micromechanicalstructure on materials performance and geometrical parameters on materials performance
X-Ray CT characterization of asphalt • Porous (“quiet”) asphalt - Cylindrical core 80 mm high x 100 mm diameter - High air voids (20%) to facilitate drainage and noise damping • CT data with 59x59x59 µm voxel size is acquired • Analysis is performed on a rectangular volume (60x60x40 mm) in the center of the specimen
Analysis procedure X-Ray CT slice before post-processing: • Significant density variation within phases (stones and binder) • Considerable amount of beam hardening • Image noise
Analysis procedure Corrected image: • Histogram equalization to improve contrast • Noise reduced with median filter (3x3 kernel) and edge preserving smoothing filter • Beam hardening corrected based on background flat field correction - Illumination profile:
Analysis procedure Segmented image: • Phase (air voids and stones) identification with threshold-based segmentation • Binder is defined as the difference between total volume, stones and air voids • Stones are separated based on the distance map with watershed segmentation • Stones smaller than 2.34 mm are filtered out and replaced with binder
Results Stone skeletonReconstructed stone surfaces Parameters describing stone size distribution, their shape , roughness and orientation in the material are obtained. These parameters define to a great extent the stone skeleton strength and its susceptibility to aggregate breakage.
ResultsStone skeleton (contact regions) During separation based on the distance map, the contact regions between stones are identified: • Regions where the separation lines intersect with the segmented stone phase represent contact regions • A sensitivity range for contact detection is defined presently as 108 µm (2 pixels)
ResultsStone skeleton (contact regions) The stone contact regions provide a primary load transferring mechanism in compression and shear. In several recent studies contact zones geometry and orientation have been correlated with asphalt compactability and rutting performance.
Results Air voidsReconstructed air voids surfaces CT data is analyzed in order to evaluate if the air void distribution and connectivity in the specimen agree with the design parameters of the mixture. Reduced air void content at the bottom of the specimen results in compromised permeability and noise damping capabilities.
Micromechanical analysis with FEMFEM simulations based on structural information obtained withthe X-Ray CT allow to:• Improve our understanding of the mechanical behavior of asphalt and its degradation processes.• Quantify the effect of using constituent materials with improved (or worsened) characteristics.• Develop a “virtual specimen” type of approach for asphalt mixture design. This will provide a cost effective way to optimize different asphalt mixture parameters, e.g. binder type, air void contents and stone size distribution for better field performance.Analysis results illustrate the capability of this method tocapture stress concentrations and strain localization arisingdue to differences in mechanical and thermal properties of thephases.
Uniaxial tension and thermal stresses(2D)h=0.1 mm • Reconstructed surfaces and volumes are exported to COMSOL Multiphysics package • Mechanical and thermal properties representative for each phase are assigned to stone and binder regions in the model • 2D plane strain analysis for: - Uniaxial tension - Thermally induced stresses (temperature at the air void boundary is reduced at a rate of 10ºC/hour)
Uniaxial tension (2D) • Strains are localized in the binder phase • Strains up to 12% are observed as compared to approx. 0.2% predicted for homogeneous material case • The information obtained with this type of modeling can be used to identify representative stress and strain levels for binder testing
Uniaxial tension (2D) • Load transfering chains can be seen in the material • Only main load transfering regions in the binder are subjected to a tensile stress >10MPa (as compared to the uniform tension of 19 MPa for the uniform material case)
Thermal stresses (2D) • Temperature variation of approx. 1.5ºK can be seen. The temperature gradient would increase with increasing cooling speed and decreasing air void content. • As the specimen is not constrained, this type of thermal loading would result only in negligible stresses in the homogeneous material.
Thermal stresses (2D) • Stones are subjected to higher stresses due to their higher stiffness • Regions of localized tension are formed in the binder due to difference in thermal contraction properties between phases. • Maximum tensile stresses in the binder reach approx. 2.5 MPa
Uniaxial compression (3D) Analysis of small regions around stone-to-stone contact zones to get insight into the local degradation mechanisms: - Work in progress…
Uniaxial compression (3D) Von Mises stress localized in stones around contact pointsUnderstanding the mechanisms controlling:• stone breakage and polishing during asphalt compaction• Micro-fracture initiation in binder films Compressive strains localized in the binder