Zhuojin Pipe Fitting Co., LtdSeismic Performance and Damage Evolution of Steel Pipe Recycled Concrete Column-Recycled Concrete Short Beam Frames
Literature Overview
This paper by Meng Ercong, Yu Yalin, Zhang Xu, Su Yisheng, and Chen Zongping, published in Vibration and Shock (2019, Vol. 38, No. 20, pp. 188–195), investigates the seismic behavior and damage evolution of frames composed of steel pipe recycled concrete (RPC) columns and recycled concrete short beams. Three specimens with different recycled aggregate replacement ratios (0%, 50%, and 100%) were subjected to low-cycle reversed loading tests, and the Park-Ang two-parameter damage assessment model was applied to quantify damage progression. The study addresses a critical sustainability question: whether recycled aggregate can substitute natural aggregate in composite structural members without compromising seismic performance.
Core Technical Findings
The experimental results reveal several important characteristics of the RPC frame system under cyclic loading. All specimens exhibited brittle shear failure modes, which is a notable observation given that steel pipe confined columns are generally expected to show more ductile behavior. The hysteresis curves displayed full spindle-shaped loops, and the equivalent viscous damping coefficients at the end of testing all exceeded 0.3, indicating satisfactory energy dissipation capacity. The inter-story drift angles at yielding ranged between 1/107 and 1/95, satisfying serviceability comfort requirements, while the drift angles at failure were between 1/39 and 1/36, demonstrating good anti-collapse capability.
Effect of Recycled Aggregate Replacement Ratio
The replacement ratio exerted differential effects on various performance indicators:
| Performance Indicator | 0% Replacement | 50% Replacement | 100% Replacement | Relative Change (vs. 0%) |
|---|---|---|---|---|
| Average yield load fluctuation | Baseline | <5% | <5% | Negligible |
| Peak load fluctuation | Baseline | <5% | <5% | Negligible |
| Ductility coefficient | Baseline | Decreased | Decreased | -9.70% / -20% |
| Stiffness degradation curve | Baseline | Nearly coincident | Nearly coincident | Negligible |
| Damage at large displacements | Baseline | Higher | Highest | Progressive increase |
The most significant finding is that while load-carrying capacity and stiffness remain largely unaffected by the replacement ratio, the ductility coefficient drops by 20% at 100% replacement. This reduction in ductility is attributed to the higher porosity and lower interfacial transition zone (ITZ) quality of recycled aggregate, which reduces the concrete's tensile strain capacity and crack propagation resistance. The skeleton curve descending branch becomes steeper with increasing replacement ratio, reflecting accelerated post-peak strength loss.
Damage Evolution Analysis
The Park-Ang damage model was used to track damage progression throughout the loading history. The damage index D is defined as:
D = Σ(Δu_i / u_u) + γ(Δu_i / u_y)
where Δu_i is the displacement increment at cycle i, u_u is the ultimate displacement, u_y is the yield displacement, and γ is the energy dissipation coefficient.
The analysis revealed that during the initial loading stages, the replacement ratio had minimal influence on damage accumulation. However, as displacement amplitude increased beyond the yield point, specimens with higher replacement ratios exhibited progressively greater damage levels. This divergence is consistent with the known degradation of concrete tensile properties and bond strength in recycled concrete, where the recycled aggregate surface retains adhered old mortar that weakens the aggregate-matrix interface.
Engineering Practice Implications
From a structural engineering perspective, this study provides valuable guidance for the use of recycled concrete in seismic regions. The following observations are particularly relevant for engineering practice:
- Acceptable replacement threshold: A 50% replacement ratio appears to be a practical upper limit for seismic structural applications, as the ductility reduction remains within 10% while maintaining excellent energy dissipation capacity.
- Design considerations for 100% replacement: When full replacement is desired, additional confinement reinforcement or structural modifications may be necessary to compensate for the 20% ductility loss. The steel pipe confinement itself provides significant lateral restraint, which partially mitigates the adverse effects of recycled aggregate.
- Shear failure concern: The brittle shear failure observed in all specimens suggests that shear reinforcement design should be conservative, particularly for frames where recycled concrete is used in beam-column connections.
- Damage monitoring: The progressive damage acceleration at large displacements for high-replacement specimens implies that post-earthquake inspection protocols should be more stringent for structures utilizing recycled concrete.
Key Reflections
The study raises an important question about the trade-off between sustainability and structural performance. While the recycled aggregate replacement offers environmental benefits through waste reduction, the 20% ductility reduction at 100% replacement represents a meaningful degradation in seismic safety margin. The steel pipe confinement provides a substantial buffer, but engineers must recognize that the composite action depends on the concrete's ability to fill the pipe and transfer loads effectively. Recycled aggregate's higher porosity may reduce the concrete's ability to fully fill the pipe interior, potentially creating voids that compromise the composite mechanism.
The near-coincidence of stiffness degradation curves across replacement ratios is an encouraging finding, as stiffness governs inter-story drift under service and moderate seismic loads. This suggests that for design-level earthquakes, the structural response of RPC frames will be similar to conventional frames, with differences emerging primarily at extreme loading levels beyond the design basis.