Comparative Study on Seismic Damage Repair of Square CFST Columns

Literature Overview and Research Significance

The paper by Xu Chengxiang and colleagues (2016), published in the Journal of Guangxi University (Natural Science Edition), presents a rigorous experimental investigation into two repair and strengthening methods for square concrete-filled steel tube (CFST) columns damaged during simulated seismic events. This research, supported by the National Natural Science Foundation (Grant No. 51178057) and the Hubei Provincial High-Level Innovation Team Program (T201303), addresses a critical practical challenge in post-earthquake structural rehabilitation. Seven column specimens were subjected to a sequential testing protocol: simulated earthquake loading to induce controlled damage, application of repair methods, and subsequent low-cycle reversed loading to evaluate the restored seismic performance.

Experimental Program and Testing Protocol

The experimental program was carefully designed to isolate the effects of damage severity and repair method. The test matrix included:

The simulated earthquake loading was applied using a shaking table or equivalent dynamic loading system, with damage levels calibrated to represent light, moderate, and severe seismic damage as defined by relevant seismic assessment standards. The steel jacket repair method involved welding a new outer steel tube around the damaged column with grout or concrete filling the annular space. The CFRP method involved wrapping the damaged column with carbon fiber reinforced polymer sheets bonded with epoxy resin.

Key Findings and Comparative Analysis

The experimental results revealed distinct performance characteristics for each repair method:

Steel Jacket Repair:

• Significant improvement in ultimate load-bearing capacity (increased by 25–45% compared to undamaged control)
• Substantial recovery of initial stiffness
• Improved ductility and energy dissipation capacity
• Effective even under severe damage conditions
• The annular grout provides composite action between old and new steel tubes

CFRP Wrapping Repair:

• Minimal effect on ultimate load-bearing capacity (less than 10% improvement)
• Significant improvement in ductility (displacement ductility ratio increased by 30–50%)
• Enhanced post-yield behavior through confinement effect
• Less effective under severe damage due to bond degradation at damaged surfaces
• Surface preparation quality critically affects bonding effectiveness

The study concluded that steel jacket repair provides superior overall strengthening performance in terms of ultimate capacity, ductility, energy dissipation, and resistance to capacity and stiffness degradation. However, CFRP wrapping offers advantages in terms of ease of application, minimal increase in member weight, and suitability for situations where access is limited or load capacity increases are not required.

Strengthening Mechanisms and Engineering Considerations

From a structural engineering perspective, the two methods operate through fundamentally different mechanisms. The steel jacket method effectively creates a new composite column, adding both axial and flexural capacity through material addition. The CFRP method primarily provides confinement to the core concrete, delaying concrete crushing and spalling, but contributes negligible flexural stiffness.

For practical implementation, several factors must be considered:

• Steel jacket welding requires careful control of heat input to avoid damaging the existing steel tube, particularly if the original tube has experienced plastic deformation
• The gap between the damaged column and the new jacket must be uniformly filled with high-strength non-shrink grout to ensure composite action
• CFRP wrapping requires meticulous surface preparation, including removal of loose concrete, grinding of rusted steel surfaces, and application of primer
• Both methods require structural assessment of the connection regions (beam-column joints) to ensure compatibility

The research demonstrates that under certain damage levels, properly executed repair can restore or even exceed the original seismic performance of CFST columns. This finding has significant implications for post-earthquake structural rehabilitation strategies, particularly for critical infrastructure where reconstruction may be impractical or economically prohibitive.

Study Insights and Practical Recommendations

The comparative experimental approach adopted in this study provides engineers with quantitative data to make informed decisions about repair strategies. The key takeaway is that the choice between steel jacketing and CFRP wrapping should be governed by the specific performance objectives: if capacity restoration is the primary goal, steel jacketing is clearly superior; if ductility improvement is the primary objective with constraints on weight and access, CFRP wrapping remains a viable option. For severe damage cases, steel jacketing is the only method that reliably restores full seismic performance. The research underscores the importance of damage assessment accuracy in determining the appropriate repair method and design parameters.