Axial Compression Behavior of CFRP-Constrained Square CFST Columns with Circular Arc Mortar Interface

Literature Overview and Research Background

This study by Shi Yatao and colleagues from Jilin University, published in the journal "Concrete" (2021, Issue 1, pp. 1-5), addresses a critical engineering challenge in the reinforcement and retrofitting of square concrete-filled steel tube (CFST) columns. The core innovation lies in the introduction of a circular arc cement mortar layer (CAM) between the CFRP fabric and the square steel tube, creating a hybrid composite column designated as C-C-SCFST. The research is supported by the National Natural Science Foundation of China (Grant No. 41702300) and the University Student Innovation Training Program (2019A63242). The fundamental motivation stems from the well-documented observation that CFRP-wrapped square CFST columns suffer from premature CFRP debonding at the sharp corners due to severe stress concentration, which drastically reduces the effective confinement efficiency and limits the ultimate load capacity.

Key Technical Parameters and Test Configuration

The experimental program comprises 16 C-C-SCFST short column specimens subjected to axial compression loading. The primary variables investigated are the number of CFRP layers and the mid-section thickness of the external circular arc cement mortar layer. The following table summarizes the critical design parameters and their engineering significance.

The CAM layer serves a dual function: it provides a geometric transition from the square cross-section to a more circular profile, and it distributes the CFRP tension forces more uniformly along the steel tube perimeter. This is particularly important from a welding and fabrication standpoint, as the CAM layer effectively eliminates the need for complex corner radiating treatments on the steel tube itself, simplifying the prefabrication process.

Core Findings and Technical Interpretation

The experimental results reveal several important mechanical behaviors. First, increasing the mid-section thickness of the CAM layer leads to a substantial improvement in the vertical load-bearing capacity of the column. This finding is consistent with the theoretical understanding that a thicker mortar interface provides greater geometric compliance, allowing the CFRP to develop a more uniform hoop tension distribution. Second, the presence of the CAM layer significantly mitigates the stress concentration at the square tube corners, causing the overall force transmission characteristics of the C-C-SCFST column to resemble those of a CFRP-confined circular CFST column. This is a crucial insight because circular CFST columns are well-established in design codes and benefit from extensive experimental databases.

The failure mode observed under ultimate axial load is particularly instructive for practicing engineers. The internal square CFST column undergoes shear failure, which subsequently triggers the fracture of the external CFRP fabric. This sequence indicates that the CFRP system functions as a secondary confinement mechanism that becomes fully engaged only after the internal steel-concrete composite has reached its shear capacity. From a quality control perspective, this implies that the bond interface between the CAM layer and the CFRP fabric must be rigorously inspected, as any delamination would compromise the confinement effectiveness at the critical failure stage.

Proposed Design Formula and Code Comparison

Based on the experimental data and existing code provisions, the authors propose a load-bearing capacity calculation formula for C-C-SCFST short columns. The calculated results show good agreement with the experimental values, which validates the proposed design methodology. The following table compares the theoretical framework with conventional design approaches.

Engineering Practice Implications

For engineers involved in structural retrofitting and new construction, this study offers several actionable insights. The CAM layer approach provides a practical solution to the corner stress concentration problem that has long plagued CFRP-wrapped square sections. From a fabrication standpoint, the CAM layer can be cast in situ, eliminating the need for specialized corner radiating equipment and reducing construction costs. However, quality assurance measures must be implemented to ensure proper compaction of the mortar layer and adequate bond strength between the mortar and the CFRP fabric. The recommended inspection procedures include visual examination of the CAM surface profile, pull-off adhesion testing, and ultrasonic testing to detect voids within the mortar layer.

The study also highlights the importance of material compatibility in composite structural systems. The thermal expansion mismatch between CFRP, cement mortar, and steel tube must be considered in the design, particularly for structures subjected to temperature variations. Future research should extend the investigation to cyclic loading conditions, as seismic applications demand ductile behavior that may not be fully captured by monotonic axial compression tests.

Study Insights and Critical Reflection

After careful consideration of the experimental methodology and results, several points merit further attention. The test program of 16 specimens is adequate for establishing general trends but may not capture the full variability in CAM thickness effects, particularly at the lower and upper bounds of practical application. The study would benefit from additional tests with different CFRP layup angles, as the optimal angle for a square section with a CAM interface may differ from that of a circular section. Furthermore, the long-term durability of the CAM layer under environmental exposure, including moisture ingress and carbonation, remains an open question that could significantly affect the service life of the composite column. The proposed design formula should be validated against a larger database before being incorporated into design codes.