Zhang, Jun (2017-11). FRACTURE AND FATIGUE CHARACTERIZATION OF STABILIZED SOILS AND SULFUR-EXTENDED ASPHALT USING MONOTONIC AND CYCLIC SEMI-CIRCULAR BENDING TESTS. Doctoral Dissertation.
The primary goal of this research is to develop test methodologies to characterize fracture and fatigue performances of chemically stabilized soils and asphalt mixtures. The fracture resistance of a chemically stabilized base or subbase layer is important for durability and sustainability of the pavement structure. Thus, an appropriate test protocol to characterize the fracture resistance of the stabilized bases, subbases and subgrade soils is essential for design of pavement materials and structures. This research proposed a protocol based on the semi-circular bending (SCB) test to measure fracture resistance (i.e., fracture energy and fracture toughness) of the chemically stabilized material (CSM). The effects of three test variables including specimen thickness, notch length and loading rate on fracture properties were investigated, and appropriate values for these test variables were selected for the SCB test protocol. The proposed SCB test method was successful in characterizing the fracture resistance of three different CSMs. In order to more definitively address fracture properties of the CSM three-dimensional cohesive zone modeling was used and the simulations agreed very well with the experimental results. Both of the fracture properties obtained from the experiment and the cohesive zone modeling indicated that polymer-stabilized limestone exhibited a much higher fracture resistance than cement-stabilized limestone and cement-stabilized sand. In order to characterize crack growth of CSMs, a compliance method based on the cyclic SCB test was proposed, which was successfully used to characterize crack growth rate of cement stabilized materials. This method is promising as it shows much higher coefficients of correlation when fitting the data to the Paris' law equation. Characterizing fracture behavior and crack propagation of asphalt mixtures is helpful for optimizing mixture design and predicting cracking performance of asphalt pavements. This research used a digital image correlation (DIC) system to measure the horizontal strain field of a crack tip, which is consistent with the SCB fracture test results. It is observed that the horizontal strain field is more localized at a lower testing temperature and a higher peak load. In addition, this research proposed a new method based on the cyclic semi-circular bending test to characterize crack growth rate of asphalt mixtures. To accurately capture crack length for determining crack growth rate, a DIC is used, and crack mouth opening displacement (CMOD) is measured by linear variable differential transformers. Correlations between crack length and CMOD are established, which are used to determine crack lengths corresponding to loading cycles over the testing process. The proposed cyclic semi-circular bending test successfully characterizes the Paris's law coefficients of sulfur-extended asphalt mixtures. The cyclic semi-circular bending test provides substantially lower coefficients of variance in terms of cycles to fatigue failure compared with other traditional fatigue tests. Test results were used to determine the impact of sulfur content on fatigue life with the conclusion that a low level of sulfur added in the case of 15% change the rheology (softener) of the asphalt to the degree that more damage is caused in a controlled-stress mode loading. However, an increase in sulfur content (30% and 45%) apparently produces a stiffer mixture that is more resistant to damage and is comparable to the control mixture.