Experimental characterization and analytical assessment of compressive behavior of carbon nanofibers enhanced UHPC

With superior mechanical and durability properties, ultra-high performance concrete (UHPC) is making strides towards revolutionizing concrete structures and the construction industry. Even though UHPC is widely known, it is still used only in small-scale elements. Recently, more advanced commercial and non-proprietary UHPC mixtures, e.g., nano-enhanced UHPC, have emerged timely to support a rapidly growing market and increased interest in expanding UHPC applications to full structural components. Thus, there is a need to comprehensively characterize the mechanical properties of such emerging mixtures using more experimental data and assess how such mixtures compare to traditional UHPC. This paper focuses on the uniaxial compression behavior of carbon nanofibers (CNF) enhanced UHPC. The literature is currently lacking any data on the combined effects of steel fibers and traditional reinforcement confinement in CNF-enhanced UHPC. The objective of this study is to experimentally characterize and analytically assess the uniaxial compressive behavior of CNF-enhanced UHPC at different ages and with varying steel fibers content and confinement reinforcement. In this study, more than 230 cylinders with 0–4 % varying steel fiber ratio and 0–4 % varying steel spiral confinement ratio were tested. Experimental results of the CNF-enhanced UHPC cylinders were assessed using established analytical models for traditional UHPC for Young's modulus and confinement effects on both stress and strain. The study first demonstrates that using accelerator in UHPC mixtures can help achieve about 80 % of the 28-day-strength only in three days after casting. Overall, the results suggest that the combined effect of steel fibers and steel spirals increases the ductility of CNF-enhanced UHPC relative to traditional UHPC. The study also concludes that current analytical models do not precisely capture the compressive and confinement behavior of this emerging type of UHPC.