Zhuojin Pipe Fitting Co., LtdAxial Compression Performance of RC Square Columns Strengthened with Square Steel Tubes and Self-Compacting Concrete
Literature Overview
The paper by Liao Jiancong, Lu Yiyao, Li Shan, and Liang Hongjun, published in the journal Concrete (2019, Issue 8, pages 47–53), presents both experimental and finite element studies on the axial compression behavior of reinforced concrete (RC) square columns strengthened with externally bonded square steel tubes filled with self-compacting concrete (SCC). The research was funded by the National Natural Science Foundation of China (Grant No. 51078294) and the Hubei Provincial Science and Technology Innovation Special Major Project (2016AAA025). Seven specimens were tested: five strengthened columns and two unstrengthened reference columns.
Core Technical Content
The study addresses the strengthening of existing RC columns using a composite approach that combines a square steel tube jacket with self-compacting concrete infill. This technique offers advantages over traditional strengthening methods such as steel jacketing alone or concrete encasement, as the steel tube provides lateral confinement while the SCC ensures complete filling of the interface without vibration, thereby improving bond quality between the steel tube and the existing column surface.
Load-Bearing Mechanism Analysis
The finite element analysis performed using ABAQUS revealed important insights into the load-sharing behavior among the original RC column, the steel tube, and the newly added concrete. The key findings are summarized as follows:
| Parameter | Effect on Load Distribution |
|---|---|
| Increased SCC strength | Increases load share of new concrete; decreases load share of original column and steel tube |
| Increased steel tube wall thickness | Increases load share of steel tube; decreases load share of original column and SCC |
| Increased steel tube yield strength | Increases load share of steel tube; decreases load share of original column and SCC |
| Loading stage | Load-sharing ratio varies significantly across elastic, yielding, and post-peak stages |
A critical observation is that the steel tube yields before the composite strengthened column reaches its ultimate load. This means that the steel tube enters its plastic stage while the overall structure still has reserve capacity, which is a favorable characteristic from a ductility perspective.
Load-Displacement Behavior
The comparison between finite element predictions and experimental results showed excellent agreement in both ultimate load and the overall shape of the load-versus-longitudinal-displacement curves. This validation confirms that the numerical model, including the constitutive relationships for the steel tube, SCC, and the interface bond, accurately captures the structural behavior. The load-sharing ratio among the three components changes with the loading stage: in the initial elastic stage, the original column carries a larger proportion of the load; as loading progresses, the steel tube and SCC progressively assume greater shares.
Engineering Practice Implications
This research has direct relevance to retrofitting existing reinforced concrete structures that require increased load capacity. The use of self-compacting concrete in the strengthening process is particularly advantageous in congested reinforcement zones where conventional concrete placement with vibration is impractical. The finding that steel tube parameters (wall thickness and yield strength) significantly influence the load-sharing ratio provides engineers with clear design levers for optimizing strengthened column performance.
Design Recommendations
Based on the parametric study results, the following recommendations emerge for practical application:
- Steel tube selection: Increasing wall thickness or using higher-grade steel provides more effective load redistribution than relying solely on higher-strength SCC.
- Interface preparation: The bond between the steel tube and the existing column surface is critical; surface roughening and the use of bonding agents are recommended.
- SCC specification: SCC with a compressive strength of at least 40 MPa is advisable to ensure adequate confinement and load-sharing contribution.
Study Insights and Reflections
The research contributes valuable data to the growing body of knowledge on composite strengthening techniques. The observation that the steel tube yields before the ultimate load is reached is particularly important for seismic retrofit applications, as it indicates that the strengthening system can sustain large deformations without catastrophic failure. The parametric analysis provides a systematic understanding of how material and geometric parameters influence performance, which is essential for cost-effective design. In my professional experience with structural retrofitting, the integration of steel tubes with self-compacting concrete represents a practical and efficient solution, and the quantitative load-sharing data presented here will help engineers make informed design decisions.