Characterization of Ultra-High Performance Concrete Tension Behavior Using Different Test Methods

Ultra-high performance concrete (UHPC) is a relatively new type of cementitious construction material known for its superior mechanical and durability properties, which has contributed to its increasing popularity and usage in the construction industry. Unlike conventional concrete, UHPC possesses a sustained post-cracking tensile strength resulting from the presence of discrete steel fibers in its composition. Furthermore, it exhibits exceptional compressive strength and durability. These remarkable properties are a result of the optimized gradation of cementitious materials, chemical admixtures, and steel fibers that compose UHPC. However, the standard test methodologies for accurately describing the tensile behavior of UHPC are still in the process of being developed and standardized. Direct tension (DT) and four-point bending (4PB) test methods are two commonly used methods in research and practice to assess the tension behavior of fiber-reinforced concrete. This thesis work provides a comprehensive analysis of the experimental investigation, test methodologies, calculations, and the correlation between the tension behavior computed from both test methods. For this, a total of 50 UHPC specimens were constructed, including 10-2 in. square prisms for direct-tension tests and 40 four-point bending beam specimens with different cross-sections. The experimentally obtained moment vs. curvature responses from four-point bending tests were used to calculate the tensile stress-strain response of UHPC. These results were then compared to the ones obtained experimentally from direct tension tests. Additionally, to investigate the impact of beam size on the measured tension behavior in the four-point bending method, four different sizes of specimens were fabricated for testing. It was observed that the UHPC tension stress-strain behavior obtained from the inverse analysis of 4PB tests and direct tension test methods were similar. The average tension behavior obtained from 2 by 4 in. bending specimens especially matched closely with results from direct tension tests. Ultimately, all 4PB and DT beams were cut into evenly spaced cuboids to examine differences in the number of fibers in the cross-section, dispersion, and orientation throughout the length of the beam. To also analyze the fibers present in the cross-section of the failure region, the specimens were cut around the location of critical crack propagation. The observed variability in the measured tensile strength can then be explained by the fiber count and orientation factor.