Title:
STRUCTURAL RESPONSE AND BUCKLING BEHAVIOR OF SLENDER ULTRA-HIGH PERFORMANCE CONCRETE (UHPC) COLUMNS
Author(s):
Mahmoud Aboukifa;Mohamed A. Moustafa
Publication:
CRC
Volume:
Issue:
Appears on pages(s):
Keywords:
ultra-high, performance, concrete, durability, UHPC, columns, slender, buckling,
DOI:
Date:
6/11/2021
Abstract:
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.