Seismic Strength and Stiffness Test Research of Steel Tube Recycled Concrete Frames

Literature Overview

This study by Zhang Xianggang, Chen Zongping, and Xue Jianyang, published in the Journal of Disaster Prevention and Mitigation Engineering (Vol. 35, No. 6, 2015, pp. 799-806), presents quasi-static (low-cycle cyclic) testing of steel tube recycled concrete (SRC) frames. The research was supported by the Henan Provincial Key Scientific Research Project (15A560008) and the Henan Polytechnic University Doctoral Fund (B2015-72). The study addresses the seismic performance of SRC frames using recycled concrete—a material made by incorporating crushed recycled aggregate from demolished concrete structures into new concrete mixes—as an environmentally sustainable alternative to conventional natural aggregate concrete.

Core Technical Content and Key Findings

The authors conducted quasi-static tests on two single-story, single-bay SRC frame specimens: one with circular steel tube columns and one with square steel tube columns, both using recycled concrete for the column cores and recycled concrete for the beam sections. The tests measured the failure mechanisms, hysteresis curves, strength degradation, and stiffness deterioration under cyclic loading.

Seismic Performance Evaluation

The test results demonstrate that the SRC recycled concrete frames satisfied the seismic design principles of "strong column, weak beam" and "strong shear, weak bending," which are fundamental to ductile seismic design. The hysteresis curves were essentially symmetric, indicating good energy dissipation capacity and stable cyclic behavior.

Strength Degradation and Stiffness Deterioration Patterns

A notable finding is that the strength degradation and stiffness deterioration under cyclic loading follow a non-monotonic pattern: the degradation rate first increases, then decreases, and then increases again. This three-phase behavior can be interpreted as follows:

1. Phase 1 (Increasing degradation): Initial cracking and micro-damage accumulation in the recycled concrete and at the steel-concrete interface. The recycled aggregate-concrete interface transition zone (ITZ) is a critical weak link due to the porous surface of recycled aggregate particles.
2. Phase 2 (Decreasing degradation): The structure reaches a quasi-stable state where the damage has stabilized. The steel tube provides confinement to the recycled concrete, preventing further rapid deterioration. The bond between the steel tube and concrete stabilizes the load path.
3. Phase 3 (Increasing degradation): Progressive damage accumulation leads to accelerating deterioration, culminating in the ultimate failure. This phase is characterized by significant concrete crushing, steel tube yielding, and debonding at critical interfaces.

Interstory Shear Capacity and Stiffness Design Methods

The authors propose three methods for calculating the interstory shear capacity of single-story, single-bay SRC recycled concrete frames:

The study also proposes a method for estimating the initial elastic interstory stiffness of SRC recycled concrete frames.

Material and Welding Considerations for Recycled Concrete SRC Members

The use of recycled concrete in SRC members introduces specific material and fabrication challenges that require careful attention:

Recycled Concrete Properties

Welding Considerations for Recycled Concrete SRC Frames

The welding requirements for SRC frames using recycled concrete are similar to those for conventional SRC frames, but with additional considerations:

• Concrete shrinkage: Recycled concrete typically exhibits higher shrinkage than conventional concrete due to the higher water absorption of recycled aggregate. This increased shrinkage can create additional tensile stresses at the steel-concrete interface during the early curing period, potentially leading to premature debonding.
• Bond strength: The bond strength between the steel tube and recycled concrete may be lower than with conventional concrete, particularly at the interface transition zone where recycled aggregate particles are located. The porous surface of recycled aggregate creates a weaker ITZ that can reduce the effective bond strength.
• Weld HAZ and concrete interaction: The welding residual stresses in the steel tube can interact with the concrete shrinkage stresses, creating a complex stress state at the interface. For recycled concrete with higher shrinkage, this interaction is more pronounced.

FMEA for Recycled Concrete SRC Frame Seismic Performance

Study Insights and Conclusions

This research makes a significant contribution to the sustainable construction field by demonstrating that SRC frames using recycled concrete can achieve satisfactory seismic performance, including compliance with the fundamental "strong column, weak beam" and "strong shear, weak bending" design principles. The non-monotonic strength degradation pattern observed in the tests provides valuable insight into the progressive damage mechanism of recycled concrete SRC members and should be incorporated into performance-based seismic design procedures. The proposed interstory shear capacity calculation methods offer practical tools for structural engineers. For welding and fabrication engineers, the key takeaway is that the use of recycled concrete does not fundamentally alter the welding requirements for the steel tube components, but it does necessitate enhanced attention to the steel-concrete interface quality, given the potentially reduced bond strength associated with recycled concrete. The environmental benefits of recycled concrete must be balanced against the slightly reduced mechanical properties, and the optimal aggregate replacement ratio should be determined through a comprehensive life-cycle assessment that considers both structural performance and environmental impact.