Zhuojin Pipe Fitting Co., LtdMechanical Behavior Analysis of CFT Push-Out Test with Bonded Interface
Literature Overview
This paper by Liu Jiemin, Li Ning, and Sun Yazhen from Shenyang Jianzhu University, published in the Journal of Shenyang Jianzhu University (Natural Science Edition) (2011, Vol. 27, No. 5, pp. 882-885), presents a theoretical analysis of the mechanical behavior of the bonded interface in concrete-filled steel tube (CFT) push-out tests. The study employs the micro-element analysis method to derive approximate differential equations for the bond shear stress within the elastic range. The analysis provides explicit solutions for the bond shear stress, axial force-displacement relationships, and the relative axial displacement between the concrete core and the steel tube under given boundary conditions.
Core Technical Content
The push-out test is a fundamental experimental method for characterizing the bond behavior between the steel tube and the concrete core in CFT members. The test involves applying an axial load to the concrete core while restraining the steel tube, thereby generating shear stress at the interface. Understanding this bond behavior is essential for the accurate analysis and design of CFT structures, as the bond strength determines the degree of composite action between the two materials.
The micro-element analysis method decomposes the interface into infinitesimal elements and considers the equilibrium of each element. Under linear elastic assumptions, the bond shear stress satisfies an approximate differential equation that can be solved analytically. The solution reveals that the bond shear stress follows a hyperbolic sine function distribution along the interface length, with stress concentration occurring at the loading end.
Key Analytical Results
| Parameter | Mathematical Behavior | Engineering Implication |
|---|---|---|
| Bond shear stress distribution | Hyperbolic sine function | Maximum stress at loading end; stress concentration risk |
| Effect of shear modulus | Stress concentration decreases with increasing modulus | Higher modulus adhesives reduce peak stress |
| Axial relative displacement | Positive at loading end; may become negative further away | Complex displacement field requires careful measurement |
| Steel tube axial force | Increases with distance from loading end | Progressive load transfer from concrete to steel tube |
The analysis reveals a critical finding: the bond shear stress undergoes stress concentration at the loading end, and the severity of this concentration decreases with increasing shear modulus of the adhesive. This means that using a stiffer adhesive layer can help distribute the shear stress more uniformly along the interface, reducing the risk of premature debonding at the loading end.
Relevance to Steel Pipe Fabrication and Welding
The push-out test results have direct implications for the design and fabrication of CFT members. The bond interface between the steel tube and concrete is influenced by several factors related to steel pipe manufacturing:
- Steel tube surface condition: The internal surface roughness of the steel tube affects the mechanical interlock with the concrete. Seamless steel pipes typically have smoother internal surfaces than welded pipes, which may result in lower mechanical bond strength. Surface treatments such as shot blasting or chemical etching can be applied to enhance the bond.
- Steel tube geometry: The diameter-to-thickness ratio (D/t) of the steel tube influences the confinement pressure and, consequently, the bond shear stress distribution. For large-diameter steel tubes (D > 1000 mm), the bond stress distribution may be more uniform due to the larger interface area, but the absolute bond capacity may be lower due to the reduced confinement pressure.
- Welding effects: For CFT members fabricated from welded steel tubes (such as LSAW or SAW pipes), the longitudinal weld seam creates a geometric discontinuity that can affect the local bond behavior. The weld toe and weld cap geometry may create stress concentrations that initiate debonding under cyclic loading. The weld HAZ, with its potentially altered microstructure and residual stresses, may also exhibit different bond characteristics compared to the base metal.
The analytical solution provided in this study can be used to validate experimental push-out test results and to develop design equations for the bond strength of CFT members. The hyperbolic sine function distribution of bond shear stress provides a basis for determining the development length required to fully transfer the load between the steel tube and the concrete core.
Engineering Practice Considerations
In practical CFT fabrication, the concrete is typically poured into the steel tube through a vertical or inclined access hole. The concrete flow and consolidation process can affect the bond quality at the interface. For large-diameter steel tubes, internal vibrators are used to ensure proper concrete compaction, but the vibration energy must be carefully controlled to avoid damaging the steel tube surface or creating voids at the interface.
The presence of an adhesive layer between the steel tube and concrete, as considered in this study, is not common in standard CFT fabrication. However, in repair and retrofit applications, or in specialized applications requiring enhanced bond strength, adhesive layers may be used. The analytical results provide guidance for selecting the appropriate adhesive modulus to optimize the bond stress distribution.