Seismic Performance of Square Steel Tube Recycled Concrete Columns

Literature Overview

Zhang Jicheng, Lv Xing, Fan Qifeng, Wu Sheng, and Wang Jingfeng (2016), from Yangtze University, published a study in Concrete investigating the seismic performance of square steel tube recycled concrete (RSC) columns. Funded by the National Natural Science Foundation of China (Grant 51378077) and the Hubei Provincial Department of Education, this research employed orthogonal design to prepare nine specimens varying in steel tube wall thickness, recycled aggregate replacement ratio, and axial compression ratio. Pseudo-static cyclic loading tests were conducted to evaluate hysteretic behavior, ductility, energy dissipation, and stiffness degradation.

Core Technical Findings

The experimental results revealed that square steel tube recycled concrete columns exhibit seismic performance comparable to conventional steel tube concrete columns:

Performance Indicator	Key Finding
Failure mode	Bulging and local buckling of steel tube at column base, similar to conventional SC columns
Hysteretic loops	Full and plump with no significant pinching, indicating good deformation capacity
Displacement ductility	Primarily governed by steel tube wall thickness and axial compression ratio; recycled aggregate ratio has minimal effect
Energy dissipation	Equivalent viscous damping coefficient exceeded 0.2 for all specimens
Stiffness degradation	Mainly influenced by steel tube wall thickness and axial compression ratio; recycled aggregate ratio effect is small
Skeleton curve model	Three-line P-Δ model provides good fit to experimental data after normalization

Orthogonal Design Parameters

Parameter	Levels	Values
Steel tube wall thickness	3 levels	2.0 mm, 3.0 mm, 4.0 mm
Recycled aggregate replacement ratio	3 levels	0%, 50%, 100%
Axial compression ratio	3 levels	0.3, 0.5, 0.7

Interpretation of Technical Points

Recycled Aggregate Impact on Seismic Behavior

The finding that recycled aggregate replacement ratio has minimal effect on ductility and stiffness is significant for sustainable construction. Recycled concrete typically exhibits lower elastic modulus and compressive strength compared to natural aggregate concrete, which could theoretically reduce seismic performance. However, the steel tube confinement compensates for these deficiencies by maintaining lateral constraint even as the recycled concrete core degrades under cyclic loading.

Failure Mechanism Analysis

The bulging failure mode at the column base indicates that the steel tube provides effective confinement throughout the loading history. The absence of significant pinching in hysteretic loops suggests that the steel tube-concrete interface maintains bond integrity even under large cyclic deformations. This is attributed to the triaxial stress state induced by the steel tube, which prevents concrete spalling and maintains load transfer capacity.

Integration with Engineering Practice

For engineers considering recycled concrete in seismic structures, this study provides the following guidance:

Steel tube confinement effectively mitigates the adverse effects of recycled aggregate on seismic performance, making recycled concrete viable for use in seismic zones.
Steel tube wall thickness should be the primary design variable for controlling ductility, with minimum thicknesses of 3.0 mm recommended for seismic applications.
Axial compression ratio should be limited to 0.5 for seismic design to maintain adequate ductility margins.
The three-line skeleton curve model can be used for pushover analysis and nonlinear time history analysis of recycled SC columns.
Quality control of recycled aggregate (cleanliness, particle size distribution, absorption rate) remains important for long-term durability, even though seismic performance is not significantly affected.

Design Recommendation	Basis
Minimum steel tube thickness: 3.0 mm	Ensures adequate confinement and ductility
Maximum axial compression ratio: 0.5	Maintains displacement ductility coefficient > 4.0
Recycled aggregate ratio: up to 100% acceptable	Minimal effect on seismic performance with steel tube confinement
Concrete strength: ≥ 30 MPa	Ensures adequate energy dissipation capacity
Column slenderness ratio: ≤ 15	Prevents flexural buckling failure mode

Study Insights and Reflections

This research contributes meaningfully to the advancement of sustainable structural engineering by demonstrating that recycled concrete can be safely used in seismic applications when confined within steel tubes. From a materials science perspective, the recycled aggregate introduces heterogeneity into the concrete matrix, with interfacial transition zones (ITZ) between recycled aggregate particles and cement paste being potential weak links. However, the steel tube's continuous confinement effectively bridges these micro-scale weaknesses, transforming potential failure points into non-critical features. Engineers should note that while seismic performance is not compromised, long-term durability considerations—such as chloride permeability, carbonation depth, and freeze-thaw resistance—may still be affected by recycled aggregate content and should be evaluated separately. The study's methodology, combining orthogonal design with pseudo-static testing, provides an efficient framework for investigating multi-parameter structural behavior and is directly applicable to other composite structural systems.