Eccentric Compression Performance of Square Steel Tube Sandwiched Concrete Reinforced Damaged RC Columns

Literature Overview

This paper by Li Shan, Zhao Qin, Lu Yiyuan, and Xiao Lingwei, published in the Journal of Wuhan University of Technology in 2020 (Volume 42, Issue 8, pages 51-56), investigates the eccentric compression behavior of damaged reinforced concrete (RC) square columns reinforced with an external square steel tube and sandwiched concrete layer. The research was supported by the National Natural Science Foundation of China (Grant No. 51878520) and the Hubei Provincial Technology Innovation Program (Grant No. 2019ACA142). The study employs finite element analysis using ABAQUS to systematically evaluate the influence of key parameters on the structural performance of the reinforced columns.

Core Technical Content

The reinforcement method studied involves wrapping a damaged RC square column with an external square steel tube, with a concrete layer sandwiched between the original column surface and the steel tube. This creates a composite section that combines the existing RC column with the new steel tube and concrete jacket. The finite element model captures the nonlinear material behavior of concrete and steel, including cracking of concrete, yielding of steel, and the bond interface between the original column and the sandwiched concrete layer.

The authors conducted parametric studies varying three key parameters: the thickness of the external square steel tube, the eccentricity ratio of the applied load, and the degree of damage to the original RC column. The results demonstrate that this reinforcement method can substantially improve the eccentric compression load capacity and stiffness of damaged RC columns, while significantly enhancing their ductility.

The finite element model was validated against experimental data from the literature, ensuring the reliability of the numerical predictions. The model incorporated realistic material constitutive laws for concrete under confined and unconfined conditions, as well as the steel tube material model accounting for the strain-hardening behavior typical of structural steel.

Technical Analysis and Engineering Relevance

The key finding that the original column damage level has minimal influence on the performance of the reinforced column is particularly significant from an engineering perspective. This suggests that the external steel tube and sandwiched concrete system can effectively compensate for pre-existing damage, including concrete cover spalling, rebar corrosion, or partial section loss. This makes the method highly attractive for retrofitting damaged or deteriorated columns in existing structures.

The improvement in ductility is attributed to the confinement effect of the steel tube on the sandwiched concrete and the enhanced bond between the original column and the new concrete layer. The steel tube restrains lateral expansion of the concrete under compression, creating a triaxial stress state that increases both the compressive strength and the deformation capacity of the concrete core.

From a welding and fabrication standpoint, the external square steel tube reinforcement requires careful consideration of the connection details between the tube and the existing column. The sandwiched concrete layer must be properly bonded to the original column surface, which may require surface preparation, including cleaning, roughening, and the application of bonding agents or mechanical anchors. The steel tube itself must be fabricated with appropriate tolerances to ensure uniform sandwiched concrete thickness around the column perimeter.

Study Insights and Practical Implications

This research contributes to the growing body of knowledge on structural retrofitting techniques for damaged concrete structures. The external steel tube with sandwiched concrete method offers several advantages over traditional retrofitting approaches, such as jacketing with steel plates or external reinforcement with additional rebar and concrete. The method is relatively quick to implement, requires minimal disruption to the surrounding structure, and provides significant improvements in both strength and ductility.

For engineering practice, the parametric study results provide guidance for the design of retrofitting solutions. Engineers can use the findings to select appropriate steel tube thicknesses for target performance levels, and to assess the effectiveness of the method for columns with varying degrees of damage. The finite element approach demonstrated in this study can be adapted for specific project applications, allowing engineers to evaluate the performance of proposed retrofitting solutions before implementation.

The study also raises important questions about the long-term durability of the sandwiched concrete layer, particularly in environments subject to carbonation, chloride ingress, or freeze-thaw cycling. The bond interface between the original column and the new concrete layer represents a potential weak zone that may be susceptible to degradation over time. Future research should address these durability concerns to ensure the long-term reliability of the reinforcement method.

Overall, this work represents a valuable contribution to the field of structural retrofitting, providing both theoretical understanding and practical guidance for the use of external steel tube and sandwiched concrete reinforcement of damaged RC columns under eccentric compression.