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Rethinking Reinforced Concrete Ductility in Compression

  • Author(s): Williams, Ian David
  • Advisor(s): Ostertag, Claudia P
  • et al.
Abstract

Concrete is the most-used construction material in the world, yet on its own it is not ductile. It relies on carefully detailed steel reinforcing bar (rebar) cages to provide the tensile capacity and confinement needed to behave in a ductile manner. Building codes supply research-backed provisions that dictate how a reinforced concrete (RC) structure should be designed in order to meet ductility demands. Observations from seismic events within the last ten years reveal deficiencies in the code provisions in cases where large compressive strains are required. Subsequent experimental investigations found that in certain cases no practical amount of reinforcing steel could prevent brittle failure. To meet these demands it is necessary to stray from conventional paths and search for creative solutions to provide the requisite ductility.

Research presented in this document explores an alternative approach to provide compressive ductility in reinforced concrete elements using Hybrid Fiber Reinforced Concrete (HyFRC). This material utilizes fiber hybridization to achieve deflection-hardening and a small amount of tensile strain-hardening. The goal is not to completely replace the steel reinforcing cages embedded in RC elements, but to enhance it in such a way that it maintains load-carrying capacity at large deformations. Response of experimental specimens is examined at a global and local level in order to provide a comprehensive understanding of the interactions and overall performance. One large-scale and one small-scale experiment is performed, and these data are used to construct and validate a predictive computational model that can be used to evaluate the behavior in other scenarios.

Results of a large-scale compression test on a high aspect ratio column representative of a special shear wall boundary element demonstrate improved performance when constructed with HyFRC in place of conventional concrete. Control specimens lose confinement due to rebar buckling and opening of the lateral reinforcement which initiates upon cover spalling. Cover in the HyFRC specimen does not exhibit spalling, but instead shows a stable crushing behavior while leaving the cover material intact. This retained cover material delays the onset of longitudinal rebar buckling and provides additional confinement to the core. Due to this behavior toughness up to failure for the HyFRC specimen is, on average, 1.9-times higher than that of control specimens constructed with conventional concrete.

Rectangular column specimens with embedded rebar are tested to isolate the behavior of longitudinal rebar buckling when embedded in HyFRC. Specimens are built with tie spacings of four-, six-, and twelve-bar diameters (db) and tested in uniaxial compression until failure. Results show that HyFRC slows the progression of buckling compared to conventional concrete and yields a more ductile, predicable behavior. At 2% normalized axial displacement the HyFRC specimens with 6-db tie spacing exhibited, on average, 34% higher toughness than the control specimens with the same spacing. At the same level of displacement, HyFRC specimens with 12-db tie spacing had, on average, 8% higher toughness than the control specimens with 6-db¬ tie spacing. These results provide evidence that replacing conventional concrete with HyFRC could enable minimum lateral tie spacing requirements to be relaxed up to 12-db while still maintaining a ductile response with respect to buckling.

A computational model is developed to predict the buckling behavior of longitudinal rebar in compression. This model accounts for the effects of transverse reinforcement and HyFRC cover. Experimental results from the large- and small-scale experiments are used to validate the model, which is then used to evaluate test cases with other geometries and materials. These results show that HyFRC improves the compressive response of large diameter rebar and high strength rebar with a nominal yield strength of 689-MPa (100-ksi).

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