Inelastic Behavior of UHPC Material and Structures

Abstract

The superior mechanical and durability properties of ultra-high performance concrete (UHPC) can be utilized to design structures with enhanced load bearing capacity, great damage resistance and a prolonged service life. However, its widespread application is constrained by a lack of comprehensive knowledge regarding its inelastic behavior, and the absence of standardized design guidelines for steel bar reinforced UHPC that are essential for safe and reliable structural design of buildings and bridges. This dissertation employs a broad range of experimental and computational techniques to delve into the inelastic response of both plain and steel bar reinforced UHPC structures. It primarily aims to explore the inelastic behavior of UHPC material under monotonic and cyclic loading, and the strength, ductility, and collapse response of steel bar reinforced UHPC. A secondary goal is to scrutinize the crack localization strain that dictates the failure of reinforced UHPC components under tension, covering evaluations of plain UHPC under direct tension, alongside reinforced UHPC under both direct tension scenarios and in beams and slabs subjected to flexure.

The inelastic behavior of UHPC is evaluated through a battery of experimental studies including single fiber pullout (SFPT), direct tension (DTT), four-point bending (4PBT), and cube compression tests (CCT). A significant discovery is the exponential tension softening curve from DTT, illustrating the impact of fiber types and dosage. This curve's predictive accuracy is confirmed through finite element (FE) analysis of 4PBT. The study further investigates the stochasticity of UHPC's tensile stress-strain quantities, particularly at a 2.0% fiber volume fraction, and its performance under cyclic loading, leading to the development of cyclic damage constitutive models for UHPC, based on extensive empirical data. Additionally, the dissertation explores the effect of reinforcement ratios on UHPC’s crack localization strain, which is necessary for designing reinforced UHPC flexural elements. It scrutinizes how steel reinforcement affects this strain under direct tension and in 4PBT, linking data from various tests to highlight a novel connection between the material and structural ductility of UHPC.

The research culminates by extending its focus to the inelastic and collapse response of UHPC structures, specifically investigating the performance of R/UHPC subjected to flexure. These tests are crucial for evaluating critical performance indicators such as cracking initiation, load-bearing capacity, ductility, and failure modes of flexural members. Using calibrated finite element (FE) models, the research conducts a detailed parametric study that elucidates the relationship between the material ductility of UHPC and the ductility observed in structural elements. In addition, the investigation includes experimental and FE simulation studies of one-way waffle slab strips, a design innovation with the potential to significantly reduce the weight of the superstructure. This approach is further leveraged to examine the development of yield lines and tensile membrane action, also known as catenary action, in two-way UHPC waffle slabs. The study meticulously documents and analyzes the crack formation, yielding behavior, and ultimate failure patterns that define the structural response of UHPC systems.

By deepening the collective understanding of UHPC, this dissertation makes important contributions to the technical foundation necessary for formulating new design guidelines and will therefore ultimately promote the construction of a more resilient and lasting concrete infrastructure made of UHPC.