Axial behavior of UHPC columns with new emerging mixtures and varying confinement

Using advanced materials to optimize designs and make structures resilient is becoming of crucial importance. Advanced rapidly growing materials with superior mechanical properties, like ultra-high performance concrete (UHPC), can make structures better in terms of strength and service life when compared to conventional concrete. Behind the excellent performance of UHPC stands its dense packing theory and usage of steel fibers or even carbon nanofibers (CNF) in emerging UHPC mixtures. UHPC is mostly used in small-scale applications like bridge field joints and overlays, but research is extending UHPC to full structural applications such as girders and columns. To expand existing knowledge on UHPC columns and add new results from emerging and recently commercialized UHPC mixtures, this study uses, for the first time, two new UHPC mixtures with white cement and CNF enhancement to investigate axial behavior of nine full-scale columns with varying confinement. An extensive experimental program considers two groups of columns from two different mixtures with several other variables such as transverse reinforcement detailing, cross-section, and percentage of steel fibers. The paper presents results from companion material tests along with a detailed discussion of the global and local behavior of the columns, i.e. force, axial strains, reinforcement strains, and stiffness. Test results are interpreted in light of existing ACI 318 Code provisions and guidance is provided for axial design capacity estimation and transverse reinforcement detailing.