Effect of specimen geometry and test configuration on the fracture process zone for asphalt materials

Asphalt concrete;Fracture mechanics;Digital image correlation

Rivera-Perez, José J. ; Al-Qadi ; Imad L. ; Ozer ; Hasan

关键词: Asphalt concrete;    Fracture mechanics;    Digital image correlation;   

Others  :  https://www.ideals.illinois.edu/bitstream/handle/2142/98417/RIVERA-PEREZ-THESIS-2017.pdf?sequence=1&isAllowed=y

美国|英语

来源: The Illinois Digital Environment for Access to Learning and Scholarship

【 摘 要 】

In the United States and elsewhere in the world, recycled materials are commonly incorporated into asphalt mixtures, to provide environmental and economic benefits by decreasing the use of virgin materials, such as natural or quarried aggregates and asphalt binder, in newly designed asphalt mixtures. However, recycled materials such as reclaimed asphalt pavement (RAP) and recycled asphalt shingles (RAS) have resulted in asphalt mixtures prone to early cracking. In addition, Superpave volumetric design requirements are no longer sufficient to design asphalt mixtures because of the inconsistent properties of RAP or RAS. Consequently, agencies have been adopting performance tests to assess the cracking and rutting vulnerability of asphalt mixtures to achieve a balance mix design.The Illinois Flexibility Index Test (I-FIT) protocol was developed at the Illinois Center for Transportation (ICT) and published as American Association of State Highway and Transportation Officials provisional standards (AASHTO TP 124) to evaluate the cracking vulnerability of asphalt concrete. The test consists of a semi-circular asphalt concrete sample that has a vertical notch loaded along the symmetric axis resulting in mode-I type of fracture similar to, the typical three-point bending beam tests. The I-FIT global response refers to the load-versus-displacement curve that is characterized using the fracture energy (FE), strength, and post-peak slope. The microstructural response refers to the deformation occurring in the fracture process zone (FPZ). FPZ is the region surrounding the notch tip that develops micro cracks before a macro crack is observed. Hence, the energy dissipation due to the deformation that occurs in this region eventually controls the global response of the specimen.I-FIT outputs are influenced by specimen geometry and test conditions (e.g., thickness, and loading rate, as well field aging). For this reason, there is a need to understand the effect of test parameters on global and microstructural responses to calibrate the I-FIT results when test parameters are altered. Therefore, this work investigates the effect of the notch length, specimen thickness, loading rate, air void content, and asphalt binder on I-FIT global results and microstructural response. Multiple samples with varying notch lengths, thicknesses, and loading rates were evaluated to observe the effect of the test parameters. Then, samples with varying air void content and asphalt binder were tested to observe the mixture properties effect. The tests were recorded with high-resolution cameras to allow for digital image correlation (DIC) measurements. DIC measured the strain and displacement fields at a resolution of 8 microns/pixels. The resolution allows to evaluate the local characterization of fracture mechanisms and the interaction between the asphalt mastic and aggregate phases. It was found that an increase in the thickness or loading rate resulted in an increase of the post-peak slope without affecting the FE. On the other hand, an increase in the notch length or air voids content resulted in lower post-peak slope values. The FE was affected by the notch length and the loading rate. From, DIC results, it was seen that a decrease in the FPZ area correlated to a decrease in the FE and lower post-peak slope. The results from varying the notch length did not follow this trend because, as the notch length gets longer, the compressive strain (not included in the FPZ definition) interacts as an energy dissipation mechanism at failure. It can be concluded that existing correction factors that address the variation due to specimen thickness and air void content are appropriate. A new correction factor to account for the notch length is proposed. Finally, the specimen properties affect the microstructural response of the specimen. As the one of the test parameter (thickness, notch length, loading rate, or air voids) is modified, the size of the FPZ changed.