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Development and Characterization of Green Ultra-High Performance Fiber-Reinforced Concrete (G-UHP-FRC) for Structural and Non-Structural Applications

  • Author(s): Aghdasi, Parham
  • Advisor(s): Ostertag, Claudia P
  • et al.
Abstract

This research first focused on developing a green ultra-high performance fiber-reinforced concrete (G-UHP-FRC) to reduce its cement content, and thus its carbon footprint and environmental impact. In this study, 50% of Portland cement by weight was replaced by 25% fly ash (FA) class F and 25% ground granulated blast-furnace slag (GGBFS). A comprehensive study was undertaken to achieve high workability while ensuring a minimum compressive strength of 125 MPa after 28 days of moist curing. The best performing G-UHP-FRC mixtures were selected for flexural as well as large-scale tensile testing. Ductility enhancement was achieved by optimizing a hybrid mechanism through combining three different types of micro and macro fibers.

Ideal lightweight structures, such as façade and flooring systems provide strength as well as thermal and acoustic insulation while keeping weight to a minimum. Highly porous concrete can be lightweight and have excellent insulating properties; however, increasing porosity, if geometrically disordered, results in rapid strength loss, with strength generally scaling as the cube of the solid volume fraction. Furthermore, recent advances in the development of Ultra-High Performance Fiber-Reinforced Concrete (UHP-FRC) with very high compressive strength (120 to 210 MPa) inspired the development of a lightweight structure by engineering the void spaces in the material, thus taking advantage of porous concrete’s thermal insulating properties while maintaining strength and stiffness. This engineered material is here referred to as Octet-Truss Engineered Concrete (OTEC). To make OTEC structures, UHP-FRC and G-UHP-FRC mixtures were developed and used. 50.8-mm side-length OTEC unit cell specimens with various element diameters as well as 5×1×1-cell OTEC flexural specimens with 8 mm-diameter elements were cast and tested under uniaxial compression and four-point bending, respectively. The compressive strength of the OTEC unit cell specimens with various element diameters (8, 10, and 11 mm resulting in 66.4, 53.6, and 47.5% porosity, respectively) is mainly stretching-dominated, and hence considerably surpasses that of the control foam Green Ultra-High Performance Concrete (G-UHPC [no fibers]) specimens with random pore orientations (by about 180, 290, and 400%, respectively). Furthermore, the flexural performance of polylactic acid (PLA) octet-lattice reinforced Ultra-High Performance Concrete (UHPC) with three different UHPC volume fractions (37.5, 75, and 100%) was briefly investigated. All these results indicate a promising application of UHP-FRC and G-UHP-FRC OTECs as well as PLA octet-lattice reinforced UHPC for lightweight structures, such as façade, flooring, and membrane systems.

UHPC composites, with compressive strengths exceeding 125 MPa, carry tension to much higher levels compared to conventional concrete or High-Performance Fiber-Reinforced Cement-Based Composites (HP-FRCC). This improved strength of UHPC composites, however, leads to a brittle behavior under both tension and compression. Therefore, through addition of fibers in UHP-FRC composites, ductility and strain-hardening is achieved for such materials. During the last few years, however, while many researchers have focused on developing UHP-FRC composites with higher tensile strain values than 0.2–0.4% (strain value of steel rebar at its yielding point), the interaction and synergy between the composite and steel reinforcing bar to large deformations up to fracture remains unknown. In this research, eleven G-UHP-FRC as well as one Hybrid Fiber-Reinforced Concrete (HyFRC) dogbone-shaped specimens reinforced with a single deformed steel rebar in the middle were tested under uniaxial tension up to fracture of the steel reinforcing bar. After the yielding of the steel rebar, a dominant crack is formed at the location of which early strain-hardening and ultimately, fracture of the rebar can lead to an overall brittle failure at much lower strain values than the bare rebar for under-reinforced specimens. Therefore, a minimum longitudinal reinforcing ratio of ~4.0% (A706 Grade 60 mild steel) is recommended for UHP-FRC and G-UHP-FRC composites to ensure multiple macro cracking, uniform bar yielding throughout the specimen, and an overall ductile behavior. For all specimens, Digital Image Correlation (DIC) techniques were utilized based on which different crack formations were recorded, and concrete, rebar, and bond stresses were calculated. Such information then provides for the estimation of flexural strength of reinforced G-UHP-FRC components, which could be later verified through flexural experiments.

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