Development, Characterization and Modeling of Ultra-high Performance Concrete (UHPC) with Locally Available Materials

Book excerpt: Ultra-high performance concrete (UHPC) is a new generation of advanced cementitious materials with superior mechanical properties that far surpass conventional concrete. However, UHPC is expensive and proprietary, and only a few field producers are available commercially in the United States. Moreover, its tensile behavior has not been well understood. This research aims to develop, characterize and model UHPC, specifically for its tensile behavior. Two viable UHPC mixes produced with locally available materials are developed, and they exhibit comparable mechanical properties to those of commercial products. An effective specimen for direct tension test (DTT) is designed and then used to evaluate both the short- and long-term tensile behavior of UHPC. The extended freeze-thaw actions have obvious impact on the tensile responses of UHPC, particularly over the post-cracking period, and the energy-based evaluation approach from DTT is more critical than the modulus-based approach to screen and evaluate material deterioration over freeze-thaw period. An analytical model is proposed to predict the tensile responses of UHPC from the bridging behavior of matrix and fibers. A fiber reinforcement efficiency function is derived to characterize the combined effects of fiber orientation, fiber snubbing and matrix spalling on the tensile behavior of UHPC. The fiber geometry and volume fraction, interfacial bond strength, and fiber reinforcement efficiency pronouncedly govern the tensile strength and hardening behavior of UHPC. The prediction capability of the analytical model is greatly improved by adjusting some coefficients that govern the hardening and softening branches of tensile stress-crack width curves. This model can effectively predict the tensile behavior of UHPC and its deterioration effect due to freeze-thaw actions. The non-contacted lap-splice pullout test is conducted to investigate the bond behavior of rebar in UHPC. The adopted analytical model can predict the bond stress-slip relation accurately. The critical embedment length and bond strength of rebar in UHPC mixes are recommended for bridge deck connection design. The comprehensive study on the tensile behavior of UHPC presented sheds light on better understanding UHPC performance in tension and provides both the experimental and analytical methods to effectively evaluate its tensile behavior.