Axial Compression Performance of BFRP-Strengthened Steel Tube Concrete Columns

Literature Overview

This 2014 paper by Zheng Hengzhong, Deng Jun, and Li Hui from Guangdong University of Technology investigates the axial compression behavior of steel tube concrete (STC) columns externally wrapped with basalt fiber reinforced polymer (BFRP) sheets. Published in Industrial Construction, Vol. 44, No. 4, the study demonstrates that BFRP wrapping is an effective retrofitting technique for enhancing the load capacity, ductility, and corrosion resistance of STC members. The research is particularly relevant for aging infrastructure where steel tube concrete columns require strengthening without major structural disruption.

Core Technical Content

The experimental program consisted of seven specimens: four STC columns wrapped with BFRP sheets and three control STC columns without BFRP strengthening. All specimens were subjected to monotonic axial compression loading until failure. The load-displacement curves and load-strain curves were recorded and analyzed to characterize the mechanical behavior of the strengthened and unstrengthened columns.

Failure Modes

The failure modes of the two groups of specimens were distinctly different. The control specimens (without BFRP) exhibited typical STC failure characterized by outward buckling of the steel tube accompanied by concrete crushing. The steel tube bulged outward as the confined concrete reached its ultimate compressive strength, and the failure was relatively sudden with limited post-peak deformation.

In contrast, the BFRP-strengthened specimens failed through a combination of BFRP tensile rupture and steel tube buckling. The BFRP wrapping provided lateral confinement to the steel tube, delaying outward buckling and allowing the concrete core to sustain higher compressive stresses. The ultimate failure occurred when the tensile strain in the BFRP sheets exceeded the material's ultimate strain capacity, leading to sudden rupture of the BFRP layer followed by steel tube buckling.

Load Capacity Enhancement

The experimental results showed that the BFRP-strengthened specimens achieved an average 17.3% increase in axial load capacity compared to the control specimens. This improvement is attributed to the lateral confinement effect provided by the BFRP wrapping, which increases the effective confining pressure on the concrete core and delays the outward buckling of the steel tube. The confinement effect also increases the post-peak deformation capacity, contributing to enhanced ductility.

The following table summarizes the key performance comparison:

Theoretical Validation

The authors compared the experimental axial load capacity results with theoretical predictions using established confinement models for FRP-strengthened concrete columns. The deviation between experimental and theoretical values was small, confirming the accuracy of the theoretical approach. This validation is important because it provides engineers with a reliable analytical tool for predicting the capacity of BFRP-strengthened STC columns in practical applications.

Material and Construction Considerations

BFRP is a promising alternative to glass fiber reinforced polymer (GFRP) and carbon fiber reinforced polymer (CFRP) for structural strengthening applications. BFRP offers several advantages: it is non-conductive (making it suitable for applications where electromagnetic interference is a concern), it has higher temperature resistance than GFRP, and it is generally more cost-effective than CFRP. However, BFRP also has lower tensile strength and modulus compared to CFRP, which limits the maximum confinement pressure achievable with a given number of wraps.

The following table compares the key mechanical properties of common FRP materials:

Engineering Practice and Limitations

The 17.3% capacity improvement demonstrated in this study is encouraging but should be interpreted with caution. The actual improvement in a real-world application depends on several factors including the quality of the BFRP-to-steel bond, the surface preparation of the steel tube, the number of BFRP layers applied, and the ambient temperature during installation. Surface preparation is particularly critical; any contamination, rust, or paint on the steel tube surface can significantly reduce the bond strength between the BFRP and the steel substrate.

The study also highlights an important limitation: the failure mode of BFRP-strengthened specimens involves sudden BFRP rupture, which can be a concern for progressive collapse scenarios. In practice, designers should consider the consequences of BFRP failure and ensure that the underlying STC column retains sufficient residual capacity after BFRP rupture to prevent catastrophic collapse.

The corrosion resistance benefit of BFRP wrapping is particularly valuable for STC columns in aggressive environments such as marine structures or chemical plants. The BFRP layer acts as a physical barrier against chloride ions, carbon dioxide, and other corrosive agents, extending the service life of the steel tube. However, the long-term durability of the BFRP-to-steel bond under cyclic thermal loading and moisture exposure requires further investigation.