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Multiscale Damage Characterization of Hybrid Fiber-Reinforced Concrete Composites under Seismic and Environmental Conditions

  • Author(s): Nguyen, Wilson Linh Nam
  • Advisor(s): Ostertag, Claudia P
  • et al.

The deteriorating quality of the United States’ infrastructure will result in the construction of new civil engineering structures, particularly bridges, in the coming decades. This future construction presents an opportunity to modify current designs with novel materials that are more damage-resistant and durable than conventional materials.

Hybrid fiber-reinforced concrete (HyFRC) is proposed for implementation in reinforced concrete structures. This material utilizes a multiscale crack control scheme to achieve greater ductility and toughness than plain concrete, allowing for progressive crack resistance from microcrack initiation to macrocrack propagation. In this dissertation, reinforced HyFRC (i.e. HyFRC with embedded steel reinforcing bars) was investigated to evaluate realistic expectations of mechanical and durability performance of constructed structural elements.

Two primary sources of damage in reinforced HyFRC bridge structures were considered for study. The first source relates to earthquake events while the second source relates to corrosion of embedded steel reinforcing bars. Regarding earthquakes, which induce high deformations on reinforced concrete, reinforced HyFRC was investigated for its fracture behavior under direct uniaxial tension. Prismatic samples with a low longitudinal reinforcement ratio were tested to evaluate the feasibility of reducing reinforcing bar congestion in construction through the use of fiber-reinforced concrete. The overall deformation capacity of the reinforced composite was largely dependent on HyFRC cracking characteristics, which generally consisted of localized dominant cracking and longitudinal splitting crack suppression. For ductility enhancement, sufficient load capacity from strain hardening of steel reinforcement is required to offset load softening from fiber pull-out processes at a dominant crack.

A 1:4.5 scale bridge column utilizing a precast HyFRC tube, which contained all of the column’s steel reinforcement, was laterally loaded to evaluate simulated seismic damage behavior. The experimental column was subjected to static, unidirectional, cyclic loading and utilized a base-rocking design for further ductility enhancement. At the conclusion of the test, the column reached a peak drift ratio of 13.1% and showed minor damage, including elimination of cover spalling. The HyFRC tube column design contributed effective resistance against longitudinal reinforcing bar buckling, allowing the column to maintain 93% of its peak load capacity at 9.5% drift. Compared to a monolithic HyFRC column, the precast HyFRC tube design showed no reduction in seismic performance despite being fabricated with 43% less HyFRC volume.

Steel reinforcement corrosion is an additional source of severe reinforced concrete bridge damage. Because cracking can occur in a structure prior to corrosion activity, as from seismic loads or other service loads, corrosion studies in this dissertation considered the effect of both preexisting (i.e. applied load-induced) and corrosion-induced cracks on corrosion activity. In a study focused on electrochemical polarization, samples with a single steel reinforcing bar were exposed to a chloride environment for 2.5 years while in a continuous tensile stress state or in a nonloaded condition, and were periodically monitored for electrochemical behavior. Electrochemical impedance spectroscopy (EIS) was additionally preformed at the conclusion of the test program. The severity of corrosion-induced splitting matrix cracks along the length of embedded steel reinforcing bars and subsequent formation of anodic surfaces were found to be the primary factors dictating the magnitude of corrosion current. Cathodic and anodic Tafel coefficients and Stern-Geary coefficients for passive and active samples are also reported.

Corrosion activity monitoring of reinforced HyFRC was further extended to beam elements containing a steel reinforcing bar of interest and a larger secondary reinforcing bar, which lumped the galvanic corrosion effects of electrically connected reinforcing bars expected in a steel reinforcement cage on the bar of interest. Samples were mechanically loaded to induce varied matrix cracking characteristics prior to a 2.2-year chloride exposure duration. When precracked, time to corrosion initiation was governed by flexural stiffness degradation. After corrosion initiation, HyFRC restricted corrosion-induced cracking, caused more extensive diffusion of corrosion products into the bulk cement paste, and lowered overall steel reinforcing bar mass loss. Any corrosion damage incurred by reinforced HyFRC had negligible effects on flexural stiffness.

To visualize the progressive in-situ corrosion damage of reinforced fiber-reinforced cementitious composites, X-ray micro-computed tomography (µCT), a non-destructive, 3D imaging technique, was performed. Similar to larger scale experiments, samples in this study were preloaded to induce cracking prior to environmental conditioning for 44 weeks. Corrosion damage of embedded steel bars was monitored at different environmental exposure times, with plain cementitious composites exhibiting splitting cracks that caused widespread corrosion of the steel bar. In contrast, the high splitting crack resistance of the fiber-reinforced cementitious composite restricted corrosion propagation to near the initial site of corrosion. The localized corrosion rate was determined based on actual steel mass loss and actual corroded surface area.

The experimental results and conclusions reported in this dissertation indicate that HyFRC composites can significantly enhance the damage resistance and durability of reinforced concrete structures, though HyFRC performance is most optimized under certain conditions.

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