Seismic Performance of Circular Steel Tube Recycled Aggregate Concrete Columns

Literature Overview

This experimental study investigates the seismic performance of circular steel tube columns filled with recycled aggregate concrete (RAC), examining their behavior under cyclic lateral loading that simulates earthquake conditions. The research addresses the growing need for sustainable structural systems that can withstand seismic events while utilizing recycled construction materials. Circular steel tube columns provide superior confinement compared to rectangular sections, and the combination with recycled aggregate concrete aims to demonstrate that environmental sustainability does not necessarily compromise seismic resilience.

Core Technical Points

The seismic performance evaluation typically involves low-cycle reversed loading tests conducted on column specimens with varying parameters such as recycled aggregate replacement ratio, steel tube diameter-to-thickness ratio, concrete strength grade, and axial compression ratio. The key performance indicators include peak load capacity, ductility coefficient, energy dissipation capacity, cumulative hysteretic energy, and degradation rate of strength and stiffness.

The confinement effect of the circular steel tube is particularly effective because the radial confinement pressure is uniformly distributed around the circumference, preventing premature concrete crushing and maintaining column integrity even after significant lateral displacement. The recycled aggregate concrete, despite having lower initial elastic modulus and strength compared to conventional concrete, benefits from the confinement in terms of ductility enhancement.

The hysteretic behavior of these columns is characterized by full and stable hysteresis loops, indicating good energy dissipation capacity. The stiffness degradation pattern typically follows a gradual decline in the elastic range followed by a more rapid decrease after concrete cracking and steel tube yielding.

Process and Standards Analysis

The circular steel tubes used in these columns are typically manufactured by cold-drawing or cold-rolling processes from seamless steel pipes or cold-formed welded tubes. For seismic applications, the steel grade must meet ductility requirements specified in GB 50011 (Chinese seismic design code) or equivalent international standards such as AISC 341 for structural steel for seismic applications. The carbon equivalent should be limited to ensure weldability and ductility.

The welding of end connections and any internal stiffening elements must comply with seismic welding requirements that mandate full-penetration welds with 100% non-destructive examination. The weld metal should have ductility properties compatible with or exceeding the base metal to ensure that fracture initiates in the base metal rather than at the weld. Post-weld heat treatment may be required for high-strength steels to relieve residual stresses that could initiate premature cracking under cyclic loading.

The concrete placement inside the circular steel tube requires careful attention to ensure complete filling without voids. For recycled aggregate concrete, which may have different flowability characteristics due to the higher water absorption of recycled aggregates, modified placement procedures including the use of tremie methods or specialized vibrators may be necessary.

Integration with Engineering Practice

The experimental findings have direct implications for the design of seismic-resistant structures in regions where recycled construction materials are mandated or preferred. The research demonstrates that recycled aggregate concrete-filled circular steel tube columns can achieve seismic performance comparable to conventional concrete-filled steel tube columns when properly designed with appropriate confinement parameters.

From a fabrication and construction quality perspective, the seismic performance is critically dependent on the integrity of the steel tube–concrete interface. Any delamination or void between the tube wall and the concrete core reduces the confinement effectiveness and can lead to premature failure. Construction quality control measures should include ultrasonic testing of the filled tube to detect voids, and visual inspection of the tube interior before concrete placement to ensure cleanliness and proper surface preparation.

The welding connections between columns and beams or foundations must be designed with consideration for the potential for plastic hinge formation within the column. The design should ensure that plastic deformation is concentrated in designated zones away from the welds, which requires careful detailing of the connection geometry and potentially the use of reduced section plates or moment frames with special moment-resisting connections.

Key Questions and Reflections

An important consideration is the long-term durability of recycled aggregate concrete in seismic applications. The recycled aggregate concrete may be more susceptible to carbonation and chloride ingress due to the higher permeability of the recycled ITZ, which could lead to reinforcement corrosion in reinforced versions of these columns. For steel tube columns without internal reinforcement, the corrosion of the steel tube itself in aggressive environments reduces the confinement capacity over time, potentially compromising seismic performance during the service life.

Another reflection concerns the scale effect. Laboratory specimens are typically limited to practical test sizes (e.g., 200–400 mm diameter, 1.5–3.0 m height), and the seismic performance of full-scale columns may differ due to size effects on concrete fracture energy, steel tube buckling behavior, and strain rate effects during actual seismic events. The research should ideally include scale model testing or numerical validation at full structural scale to ensure the laboratory results are transferable to real structures.

Study Insights and Implications

This research provides valuable experimental evidence that recycled aggregate concrete-filled circular steel tube columns are viable seismic-resistant structural elements. For steel pipe engineers, the key takeaway is that the geometric precision and material quality of the steel tube are paramount for seismic performance. The steel tube must maintain its circularity under loading without premature local buckling, which requires adequate wall thickness, smooth surface finish, and freedom from fabrication defects such as wrinkles, dents, or weld imperfections. The welding of any end connections or stiffeners must be performed with procedures that minimize residual stresses and ensure full fusion, as these welds are critical load paths during seismic events. The research reinforces the principle that sustainable construction through material recycling is compatible with high seismic performance when proper engineering design and quality control are implemented.