Characteristics of Ultra High Performance Concrete Subjected to Dynamic Loading.

Abstract

Ultra High Performance Concrete (UHPC) is among the most promising cementitious materials developed to date. It has the potential to be a viable solution for improving the resilience and sustainability of the built infrastructure because of its high strength, durability and energy absorption capacity. Pilot studies have shown that its impact resistance is particularly impressive, yet there is hardly any information about its dynamic behavior. The experimental effort focuses on investigating the strain rate sensitivity of UHPC as a function of fiber type (straight or twisted), characteristics and volume fraction. Low strain rate tests are conducted using a hydraulic actuator, whereas high strain rate tests are conducted using a new device that employs suddenly released elastic strain energy to apply an impact pulse. Developed and optimized through computational modeling, the new device permits accurate and practical testing of UHPC specimens in direct tension, under high strain rate, and can capture both hardening and post peak responses. It is compact compared to existing test methods and permits the use of specimens that are similar in size and geometry to the specimens tested with the hydraulic actuator. A key experimental observation is that UHPC becomes significantly more energy dissipative in tension under increasing strain rates, which highlights the material’s potential for use in impact- and blast-resistant applications. Although specimens with twisted steel fibers show somewhat better mechanical properties than specimens with straight fibers due to the untwisting mechanism, comparable benefits could be obtained by using straight fibers with higher aspect ratios. Crack propagation studies show that crack speed increases asymptotically as notch tip strain rate increases. The analytical portion of the study focuses on the source of strength enhancement for concrete materials under high rate tensile loadings, a topic of current controversy in the literature. Dynamic fracture models considering crack velocity dependency prove that strain rate sensitivity is strongly associated with the characteristics of dynamic crack growth, especially the asymptotic nature of crack speed versus strain rate and inertial effects at the crack boundaries. The theoretical observations are corroborated with experimental data in the literature and new data produced by this work.