Zhuojin Pipe Fitting Co., LtdEccentric Compression Performance of Damaged RC Columns Strengthened with Square Steel Tube Sandwich Concrete
Literature Overview
Published in 2020 in the Journal of Wuhan University of Technology (Vol. 42, No. 8), this paper by Li Shan, Zhao Qin, Lu Yiyang, and Xiao Lingwei from Wuhan University investigates the eccentric compression behavior of damaged reinforced concrete (RC) square columns strengthened by an external square steel tube with sandwich concrete infill. Supported by the National Natural Science Foundation of China (Grant No. 51878520) and Hubei Province Innovation Program (2019ACA142), the study employs finite element analysis using ABAQUS to evaluate the effectiveness of this strengthening method.
Core Technical Content
Strengthening Methodology
The proposed strengthening technique involves wrapping a damaged RC square column with an external square steel tube and filling the gap between the original column and the steel tube with concrete. This creates a composite cross-section consisting of three layers: the original damaged RC core, the sandwich concrete layer, and the outer square steel tube. The method offers several practical advantages for retrofitting existing structures:
- Minimal disruption to surrounding structural elements
- Applicability to severely damaged columns where internal reinforcement is not feasible
- The steel tube provides both lateral confinement and additional axial load capacity
- The sandwich concrete layer bonds the original column to the steel tube, ensuring composite action
Parametric Study Results
The finite element analysis examined three primary parameters: outer steel tube thickness, eccentricity ratio, and the degree of damage to the original RC column.
| Parameter | Range Studied | Effect on Load Capacity | Effect on Stiffness | Effect on Ductility |
|---|---|---|---|---|
| Steel tube thickness | 4–12 mm | Increases with thickness | Increases with thickness | Increases with thickness |
| Eccentricity ratio (e/h) | 0–0.3 | Decreases with eccentricity | Decreases with eccentricity | Minimal influence |
| Original damage level | 0%–60% capacity loss | Minimal influence | Minimal influence | Minimal influence |
The key finding is that the strengthening method is remarkably effective even for severely damaged columns. The external steel tube and sandwich concrete system effectively "isolates" the composite behavior from the original damage state, as the load is predominantly carried by the new steel tube and sandwich concrete rather than the damaged core.
Failure Modes and Load-Displacement Behavior
The composite strengthened columns exhibit progressive failure modes:
- Initial elastic stage: Linear load-displacement relationship with composite action between all layers
- Steel tube yielding: Local yielding of the outer steel tube at the compression zone
- Concrete crushing: Progressive crushing of the sandwich concrete in the compression zone
- Post-peak behavior: The steel tube continues to confine the concrete, providing significant post-peak ductility
The load-displacement curves demonstrate that the strengthened columns achieve ultimate loads substantially higher than the original undamaged columns, with ductility ratios (displacement at peak load divided by displacement at first yielding) significantly improved.
Implications for Steel Pipe and Tube Fabrication
From a steel tube manufacturing standpoint, this research has direct relevance to the production of square and rectangular steel tubes for structural retrofitting applications:
- Material requirements: The square steel tubes used for column strengthening typically require structural steel grades such as Q235B, Q345B, or Q390B per GB/T 1591. The material must exhibit adequate elongation (≥ 20%) and impact toughness (≥ 27 J at 20°C) to ensure ductile behavior under eccentric loading.
- Dimensional tolerances: For the sandwich concrete method to work effectively, the square tube dimensions must be precise to ensure uniform concrete cover. Deviations in tube dimensions can lead to voids or excessive concrete thickness in certain regions.
- Welding of tube segments: When square tubes must be fabricated from flat plate sections (due to length constraints), the longitudinal and transverse welds must achieve full-strength fusion. The weld metal must match the strength of the parent material, and post-weld heat treatment may be required for thicker sections.
- Surface preparation: The external surface of the steel tube should be roughened or coated with bonding agents to ensure adequate bond between the steel tube and the sandwich concrete. Surface treatments such as shot blasting or epoxy primer application are common practice.
Critical Analysis
The finite element study provides valuable insights, but several limitations should be noted for practical application. The study relies on numerical simulation rather than experimental validation, which introduces uncertainty in the material constitutive models and interface behavior assumptions. In practice, the bond between the original damaged column surface and the sandwich concrete is often the weakest link, particularly if the original concrete surface is contaminated or if there is significant corrosion damage. Field implementation requires careful surface preparation, including removal of loose concrete, cleaning of rebar, and application of bonding agents. Furthermore, the long-term durability of the sandwich concrete layer, particularly in aggressive environments, warrants further investigation through accelerated corrosion testing and exposure studies.