Axial Compression Performance of CFRP-Wrapped Square Steel Tube Confined Concrete Short Columns with Circular Arc Mortar Layer

Literature Overview and Research Context

The study by Shi Yatao, Hu Zhongjun, Tan Mingrui, Li Xin, and Song Xuejiao, published in the journal "Concrete" in 2021, investigates the axial compression behavior of a novel composite column system that combines carbon fiber reinforced polymer (CFRP) sheets, a circular arc cement mortar layer (CAM), and a square steel tube confined concrete (SCFST) core. This research was supported by the National Natural Science Foundation of China (Youth Fund, Grant No. 41702300) and the Undergraduate Innovation Training Program (2019A63242). The study is motivated by the recognition that square CFST columns, while offering excellent confinement and load-bearing capacity, suffer from stress concentration at the corners when externally reinforced with CFRP sheets, which can lead to premature debonding and reduced strengthening efficiency.

The C-C-SCFST Column System

The proposed column system, designated as C-C-SCFST, consists of three layers: an inner square steel tube filled with concrete, an intermediate circular arc cement mortar layer that smooths the sharp corners of the square tube, and an outer CFRP sheet wrap. The CAM layer serves a dual purpose: it provides a transition in curvature from the flat faces of the square tube to the circular cross-section of the CFRP wrap, and it reduces the stress concentration at the corners of the square tube. This is analogous to the fillet effect in pressure vessel design, where smooth transitions between sections of different curvature reduce peak stresses.

The following table summarizes the test matrix of the 16 specimens:

Experimental Results and Key Findings

The experimental results demonstrated that the axial load-bearing capacity of the C-C-SCFST columns increases significantly with both the number of CFRP layers and the thickness of the CAM layer. The following table presents the key quantitative findings:

The most important finding is that the CAM layer fundamentally changes the stress distribution in the CFRP wrap. Without the CAM layer, the CFRP sheets experience severe stress concentration at the corners of the square tube, leading to premature debonding at relatively low loads. With the CAM layer, the stress distribution becomes more uniform, and the CFRP wrap behaves more like a circular confinement system, which is the ideal condition for CFRP strengthening.

The failure mode of the C-C-SCFST columns was found to be characterized by shear failure of the inner SCFST core, which triggers the rupture of the outer CFRP wrap. This is a desirable failure mode because it indicates that the full strength of the CFRP reinforcement is being utilized. The failure sequence is: (1) concrete cracking and steel tube yielding in the mid-height region, (2) progressive lateral expansion of the SCFST core, (3) increasing tensile stress in the CFRP wrap, and (4) sudden CFRP rupture accompanied by shear failure of the SCFST core.

Constitutive Model and Design Formula

Based on the experimental results and existing design codes, the authors proposed a load-bearing capacity formula for C-C-SCFST short columns. The formula accounts for the contributions of the concrete core, the steel tube, and the CFRP wrap, with correction factors for the CAM layer geometry and the number of CFRP layers. The predicted values showed good agreement with the experimental results, with deviations typically within 10%, which is considered acceptable for preliminary design purposes.

The following table presents a comparison of predicted and experimental peak loads:

Engineering Practice and Application Considerations

From a practical standpoint, the C-C-SCFST column system offers a promising approach for strengthening existing square CFST columns or for designing new columns with enhanced load-bearing capacity. The CAM layer is a relatively simple intervention that can be applied during construction, and the CFRP wrap can be installed either at the time of construction or as a retrofit. The system is particularly attractive for applications where space constraints prevent the use of larger cross-sections, such as in urban infrastructure or existing building retrofits.

However, several practical considerations must be addressed. The CAM layer must be properly bonded to the steel tube surface, which requires surface preparation and the use of compatible adhesives. The CFRP wrap must be applied with careful attention to avoid wrinkles and voids, as these can significantly reduce the effective reinforcement capacity. The long-term durability of the CFRP-CAM-SCFST system, particularly in corrosive environments, requires further investigation, as the CFRP sheets may be susceptible to environmental degradation over time.

Study Insights and Reflections

The most innovative aspect of this research is the introduction of the circular arc cement mortar layer as an intermediary between the square steel tube and the CFRP wrap. This simple yet effective approach addresses the fundamental incompatibility between the sharp corners of square tubes and the optimal curvature for CFRP confinement. The concept is analogous to the use of transition pieces in pressure vessel design and demonstrates the value of cross-disciplinary thinking in structural engineering.

The research also highlights the importance of understanding the interaction between different materials in composite structural systems. The performance of the C-C-SCFST column is not simply the sum of the individual contributions of the concrete, steel tube, CAM layer, and CFRP wrap; rather, it is determined by the complex interaction between these components, which is governed by the geometric compatibility and the stress distribution at their interfaces.

In conclusion, this paper presents a novel and practical approach to strengthening square CFST columns using CFRP sheets and a circular arc mortar layer. The experimental results demonstrate significant improvements in load-bearing capacity and ductility, and the proposed design formula provides a practical tool for engineering applications. The research opens new possibilities for the design of high-strength, ductile composite columns that combine the advantages of steel, concrete, and polymer materials.