Advancing Design and Detailing of Axial UHPC Columns for Wide-Range of Mixtures and Construction Types
Authors
Cimesa, Milana
Issue Date
2023
Type
Dissertation
Language
Keywords
Alternative Title
Abstract
Ultra-high performance concrete (UHPC) shares common constituents with normal strength concrete (NSC), such as cement, water, fine aggregates (sand), steel fibers, and admixtures. However, unlike NSC, UHPC typically excludes coarse aggregates. The inclusion of steel fibers is a crucial element in UHPC, enhancing its tensile strength (approximately two time higher when compared to NSC) and effectively bridging microcracks, providing micro-level confinement. Additionally, UHPC may incorporate carbon nanofibers for nanoconfinement, further refining its nanostructure. Despite its numerous advantages and growing markets for larger UHPC applications, a comprehensive understanding of the behavior of complete UHPC structural components, especially columns, is lacking. This knowledge gap impedes the establishment of design codes and standards for UHPC. Therefore, the goal of this doctoral research is to significantly fill this gap by contributing comprehensive insight into the axial behavior of UHPC columns of a wide-range of mixtures as well as different construction techniques, and in turn, provide, for the first time, axial design and detailing guidelines for future UHPC columns. As such, the outcome of this doctoral work promotes the use of robust UHPC materials as integral part of full structural components and systems at large through either revisiting existing or developing new UHPC sensible design tools. To further generalize such tools, the focus of this work spans both well-established commercial UHPC technologies as well as emerging variants, including carbon nanofiber (CNF)-reinforced UHPC and economically viable UHPC mixtures that utilizing locally sourced materials.
In order to properly fill the identified knowledge gap and achieve the overarching goal of this doctoral study, the following specific research objectives are defined and addressed in this study: (1) establish fundamental understanding of material behavior of emerging UHPC mixtures with focus on experimental and analytical assessment of unconfined and confined behavior of CNF-enhanced UHPC cylinders; (2) investigate the axial compressive behavior of 18 large- or full-scale UHPC building columns of four different UHPC mixtures to generalize behavior trends and understanding considering various design parameters; (3) demonstrate production-scale UHPC mixing at actual precast plant by fabricating and testing four full precast circular UHPC bridge columns with varying spirals and hoops confinement; (4) leverage all tested cylinders and columns data to assess code-based and existing modulus of elasticity (MOE) prediction equations and propose new modifications for UHPC, and (5) develop general design and detailing guidelines for axial UHPC columns using a comprehensive database of the tested columns and others
available in the literature, with focus on revisiting the ACI-based capacity design equation and transverse reinforcement requirements for axial UHPC columns.
A mostly-experimental approach was complemented with empirical analytical approach to address the research objectives. The extensive experimental campaign employed in this study involved the fabrication and testing of hundreds of UHPC cylinders and 22 full-scale columns under axial loading. In the first part of this study, the focus was detailed behavior analysis at both material and structural levels and enriching the literature with unique and exclusive datasets towards establishing comprehensive UHPC structural databases in the future. The second part of the study leveraged all tests data, along with extended database based on current literature, to assess behavior trends and existing design tools. To be specific, this part of the study was concerned with understanding transverse reinforcement detailing and confinement on axial behavior of UHPC columns to develop UHPC sensible
design guidelines.
To generalize the results of this doctoral study and facilitate immediate implementation, all experimental tests and analytical assessments considered different UHPC mixtures along large number of other design variables such as transverse and longitudinal reinforcement ratios (spacing, bar diameter, and configuration), steel fiber percentage (1% versus 2% by volume), production methods (small scale in a lab environment versus large scale in precast plants), and columns geometry and cross-sections area (square, rectangular, and circular). Accordingly, the developed understanding and design guidance is sought to widen and deepen the knowledge body of UHPC columns, and directly contribute to the development of future design specifications, codes, and standards for future structural
applications of UHPC.