Seismic Performance Testing of Steel Tube Recycled Concrete Columns

Literature Overview

This paper by Zhang Xianggang, Chen Zongping, Xue Jianyang, and Su Yisheng from Guangxi University and Henan Polytechnic University presents a comprehensive experimental investigation into the seismic performance of circular steel tube recycled concrete (CTRC) columns. Published in China Civil Engineering Journal in 2014, the study was supported by the National Natural Science Foundation of China and multiple Guangxi provincial research programs. The research addresses an important sustainability question: whether recycled coarse aggregate concrete can be used in steel tube-concrete composite columns for seismic regions without significantly compromising structural performance. Ten circular specimens were designed and tested under quasi-static cyclic loading, varying four parameters: recycled coarse aggregate replacement rate, slenderness ratio, axial compression ratio, and steel ratio.

Experimental Program and Specimen Design

The test matrix was designed to systematically investigate the influence of each parameter on the seismic behavior of CTRC columns. The following table presents the specimen parameters:

The quasi-static cyclic loading protocol simulated seismic loading by applying lateral displacement to the specimen while maintaining a constant axial load. The loading was applied in displacement-controlled mode, with displacement amplitudes increasing incrementally until failure. This testing method, while not replicating the dynamic characteristics of actual earthquakes, provides reliable data on the hysteretic behavior, energy dissipation capacity, and ductility of the specimens, which are the primary indicators of seismic performance.

Key Experimental Results

The experimental results revealed several important findings regarding the seismic behavior of CTRC columns:

The finding that the failure mode of CTRC columns is similar to that of normal CTC columns is particularly significant. The primary failure mechanism is the local buckling (bulging) of the steel tube at the base of the column, where the bending moment and shear stress are highest. This indicates that the recycled aggregate replacement does not fundamentally alter the structural behavior or failure mechanism of the composite column.

The displacement ductility coefficient exceeding 3 is an important criterion for seismic design, as it indicates that the column can undergo significant inelastic deformation without losing its load-carrying capacity. The equivalent viscous damping coefficients ranging from 0.305 to 0.460 indicate that the CTRC columns possess adequate energy dissipation capacity through hysteretic damping, which is essential for reducing seismic forces in the structure.

Materials and Welding Considerations

The steel tubes used in the CTRC columns are typically seamless steel pipes conforming to GB/T 8162 or GB/T 8163, or welded steel tubes conforming to GB/T 9711. The steel grade is usually Q235B or Q345B per GB/T 700 and GB/T 1591, respectively. The following table summarizes the material specifications:

The quality of recycled coarse aggregate is a critical factor influencing the concrete properties and, consequently, the seismic performance of the CTRC columns. Recycled aggregate typically has higher water absorption, lower density, and more adhered mortar than natural aggregate, all of which can reduce the concrete's compressive strength and elastic modulus. However, the steel tube confinement mitigates these effects by providing lateral support to the concrete, which is why the experimental results show minimal degradation in seismic performance even at 100% replacement.

Standards and Design Recommendations

The study recommends the use of DL/T 5085-1999 for the design calculation of the flexural-compressive bearing capacity of circular CTRC columns under cyclic loading. This recommendation is based on the comparison between the calculated and experimental bearing capacities, which showed good agreement. Other relevant standards include GB 50011 for seismic design of buildings, GB 50010 for concrete structure design, and JGJ 24 for the application of recycled aggregates in concrete.

The seismic design of CTRC columns should consider the following key factors:

Key Reflections and Study Insights

The experimental results of this study are encouraging for the promotion of recycled aggregate concrete in seismic structural applications. The finding that the replacement rate has minimal influence on the hysteretic behavior of CTRC columns within the tested range suggests that the steel tube confinement effectively compensates for the reduced concrete quality associated with recycled aggregates. This is a significant finding because it demonstrates that structural sustainability goals (recycled material usage) can be achieved without compromising seismic safety.

From a materials science perspective, the steel tube-concrete composite action is the key mechanism that ensures the seismic performance of CTRC columns. The steel tube provides lateral confinement to the concrete, preventing spalling and maintaining the concrete's compressive strength even after cracking. Simultaneously, the concrete core prevents local buckling of the steel tube by providing a continuous lateral support. This mutual reinforcement is the fundamental principle behind the composite behavior, and it appears to be robust enough to accommodate the variability introduced by recycled aggregates.

The quasi-static testing methodology, while widely accepted for evaluating seismic performance, does have limitations. It cannot capture the strain-rate effects on material properties, the inertial effects during dynamic loading, or the progressive damage accumulation under multiple earthquake events. Future research should complement the quasi-static testing with shake table tests on full-scale CTRC specimens to validate the findings under more realistic seismic loading conditions.

Summary and Outlook

This research provides valuable experimental evidence supporting the use of recycled aggregate concrete in steel tube-concrete composite columns for seismic regions. The consistent seismic performance indicators, including ductility, energy dissipation, and bearing capacity, across different replacement rates demonstrate that the composite system is resilient to the material variability introduced by recycled aggregates. The recommended design methodology based on DL/T 5085-1999 offers a practical framework for engineers to design CTRC columns for seismic applications. As the global construction industry moves toward sustainability and circular economy principles, the findings of this study contribute to the technical basis for wider adoption of recycled materials in structural applications without compromising safety or performance.