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Author(s): Mahmoud Aboukifa;Mohamed A. Moustafa

Publication: CRC



Appears on pages(s):

Keywords: ultra-high, performance, concrete, durability, UHPC, columns, slender, buckling,


Date: 6/11/2021

Ultra-high performance concrete (UHPC) is considered to be a relatively new generation of cementitious materials with multiple times higher strength and durability than conventional concretes. The global market size of UHPC is increasing exponentially and is expected to reach USD 1.9 billion by 2025 due to the increasing demand from the construction industry. The use of UHPC is currently expanding worldwide from bridge deck joints and connections to full components and larger applications. With the superior mechanical properties and durability of UHPC, one potential application is the use of UHPC columns in buildings and bridges to reduce the members’ cross-sections and footprint or increase resiliency against extreme events. Thus, the main goal of this research study is to provide experimental demonstration and reliable datasets of UHPC columns to validate current ACI 318 analysis and design procedures and inform future designs. This was achieved through a large experimental testing campaign of nine full-scale UHPC columns. In fact, this study provides results from the largest axially tested UHPC columns to-date anywhere in the world, where a 4000-kip testing machine at the University of California Berkeley was used. Due to the UHPC high compressive strength, more slender columns are expected for same applications relative to conventional reinforced concrete (~40% decrease in cross-sections can be achieved based on previous research by the authors). Thus, the first objective of this study is concerned with exploring the experimental behavior of slender UHPC columns under concentric axial loading and evaluate current ACI procedure for including slenderness effects and applying the moment magnification method used to quantify the 2nd order moments resulting from column buckling. The second objective of this study is to analyze the experimental behavior of slender UHPC columns of minimal or negligible slenderness effects but with varying reinforcement details (i.e. different longitudinal, transverse, and fiber reinforcement ratios) under concentric axial loading, and to inspect the validity of the ACI 318 equations for estimating the UHPC columns axial strength. According to the conducted assessment and evaluation, the study provides design guidance and recommendations for the UHPC columns to be incorporated into future design codes. For an overview of the key findings of this study, the results indicated that using the actual material properties of UHPC and longitudinal bars for ACI 318 equations will overestimate the axial load capacity of columns with different reinforcement details by approximately 13% on ii average. However, using nominal values will still lead to a factor of safety of about 2.1. The study also recommended a slenderness lower limit to include 2nd order effects of 23.5 or 32 for braced UHPC columns based on the ACI 318 criteria of 5% reduction in column axial strength or the Eurocode 2 criteria of 10% reduction in column axial strength, respectively. Furthermore, a new strength reduction factor of 0.75 is suggested (instead of 0.85) for estimating axial capacity of UHPC columns with slenderness limit less than 30.