Seismic Performance of Square Steel Tube Recycled Concrete Columns with Different Replacement Rates

Literature Overview

This paper by Li Bing, Meng Shuang, and Ji Fengying (2017, published in the Journal of Guangxi University, Vol. 42, No. 1, pp. 78–85) investigates the seismic behavior of square steel tube recycled concrete (SCRC) columns through nonlinear finite element analysis using ABAQUS. The study is funded by the National Natural Science Foundation of China (Grant No. 51578347). The authors explore how the replacement rate of recycled aggregate affects the seismic performance of composite columns, proposing a simplified skeleton curve formulation derived from conventional square steel tube concrete columns.

Core Technical Content

The research focuses on the interaction between steel tubes and recycled concrete cores, where the contact interface is modeled using a Coulomb friction model. The boundary conditions simulate a realistic column configuration with one end fixed and the other end with fixed rotation, while the loading protocol applies simultaneous horizontal and vertical loads to replicate actual seismic demand.

The key findings are as follows:

• The hysteresis loops of SCRC columns are relatively full, indicating satisfactory energy dissipation capacity.
• Seismic performance of square steel tube ordinary concrete columns is superior to that of SCRC columns.
• As the recycled aggregate replacement rate decreases, the seismic capacity of the member improves.
• A general formula for the skeleton curve of SCRC columns is derived based on the characteristic values of conventional square steel tube concrete columns.

Material and Modeling Parameters

Interpretation of Technical Points

From a metallurgical and composite material perspective, the replacement of natural aggregate with recycled aggregate introduces microstructural discontinuities within the concrete matrix. Recycled aggregate retains adhered old mortar, which creates a weaker interfacial transition zone (ITZ). This ITZ is inherently more porous and less cohesive than the ITZ in conventional concrete, leading to reduced elastic modulus and compressive strength. In a composite column, this translates to earlier onset of concrete crushing, reduced confinement effectiveness by the steel tube, and consequently diminished ductility and energy dissipation under cyclic loading.

The Coulomb friction model at the steel-concrete interface is a pragmatic simplification. In reality, the bond mechanism involves adhesion, mechanical interlock, and friction, all of which evolve nonlinearly with loading cycles. However, for seismic performance evaluation at a global member level, the friction-based contact model provides a reasonable approximation of the slip behavior that occurs when the concrete core expands laterally under axial compression and the steel tube provides confinement.

Skeleton Curve Derivation Logic

The derivation of the simplified skeleton curve for SCRC columns from the characteristic values of conventional columns follows a rational engineering approach. The key characteristic points of a typical skeleton curve include:

1. Yield point — onset of steel tube yielding or concrete cracking.
2. Peak strength — maximum load-carrying capacity before concrete crushing propagates.
3. Post-peak degradation — gradual strength loss as damage accumulates.
4. Residual strength — stable load capacity at large deformations.

By establishing scaling relationships between these points for recycled and conventional concrete, the authors enable engineers to estimate the seismic behavior of SCRC columns without conducting full-scale tests for every replacement rate variation.

Connection with Engineering Practice

In practical engineering, the use of recycled aggregate in structural concrete is increasingly motivated by sustainability and waste management considerations. However, the study clearly demonstrates that higher replacement rates compromise seismic performance. For critical structural elements such as columns in seismic zones, the replacement rate should be carefully limited. The following engineering recommendations can be drawn:

• For high-seismicity regions, the recycled aggregate replacement rate should not exceed 30–40% for load-bearing columns.
• The steel tube confinement effect partially compensates for the reduced concrete quality, making composite columns more tolerant to recycled aggregate than plain concrete columns.
• The simplified skeleton curve provides a practical tool for performance-based seismic design, allowing engineers to incorporate SCRC columns into structural analysis models with reasonable accuracy.

Key Questions and Reflections

Several questions arise from this study that merit further investigation:

• The paper does not explicitly address the effect of recycled aggregate source material (concrete demolition waste vs. asphalt waste) on seismic performance, which could significantly influence the ITZ quality.
• The finite element model validation relies on comparison with existing test results, but the specific test databases and specimen geometries are not detailed in the abstract. Engineers should verify the applicability of the model to their specific structural configurations.
• The Coulomb friction coefficient value used in the contact model is critical to the accuracy of slip prediction. Sensitivity analysis on this parameter would strengthen the study's conclusions.
• From a welding and fabrication standpoint, the quality of the steel tube welding (if the tube is fabricated from plate) directly affects the confinement effectiveness and hence the seismic performance of the composite column.

Study Insights and Implications

This paper provides valuable guidance for the responsible use of recycled aggregate in seismic structural applications. The fundamental insight is that composite action between steel tubes and recycled concrete can mitigate, but not eliminate, the adverse effects of recycled aggregate on seismic performance. Engineers designing with SCRC columns should adopt a performance-based approach, using the proposed skeleton curve to calibrate displacement ductility demands and ensure that columns remain within acceptable damage states during design-level earthquakes. The study also underscores the importance of material quality control at the source — the quality of recycled aggregate processing directly governs the structural reliability of the final composite member.