Plastic-Buckling Fatigue Testing of Large Diameter Steel Reinforcing Bars
Critical regions of reinforced concrete elements designed for ductility and energy dissipation are required to sustain many large-amplitude strain cycles during rare and strong intensity earthquakes. Steel reinforcing bars in such critical regions often end up buckling and then fracturing in a mode of failure that defines the collapse prevention limit-state. While this failure mode is commonly misnamed low-cycle fatigue, it does not meet ASTM Manual on Low-Cycle Fatigue (1969) guidelines which require to avoid buckling or bending. Instead, the term Plastic Buckling-Straightening Fatigue (PBSF) is used to describe the fatigue testing where the effects of plastic buckling are included.
Historically, the longitudinal reinforcement used for ordinary large-diameter RC bridge columns has been limited to No. 11 and smaller bars. The combination of such longitudinal reinforcement and the closely spaced transverse reinforcement at the plastic hinge region results in result in heavily congested column cages that prove challenging to build and require large amounts of labor and materials. To help reduce the congestion, larger sized bars could be used to reduce the number of bars needed to provide the same longitudinal reinforcement ratio. This would reduce the amount of material, construction time, and amount of labor needed. Furthermore, in some particular cases, the use of mechanical splices for these large diameter bars would further accelerate bridge construction by allowing the use of precast concrete techniques. However, the PBSF life of large diameter bars and their mechanical splices has not been investigated. To date, research to investigate the effects of buckling in strain-controlled fatigue testing of longitudinal steel reinforcement have focused on No. 11 and smaller bar sizes. The experimental results presented here provide the first successful PBSF data for large diameter bars and are used to implement a Damage Index to quantify the fatigue life of a reinforcing steel bar.
The innovative design and implementation of a loading apparatus used to test large-diameter reinforcing bars and their mechanical splices for PBSF is described. Main features of the loading apparatus are the high rotational stiffness and the gripping method, which was successfully achieved by exploiting the thermoplastic properties of sulfur concrete.