Normal Bond Strength Test Study of Steel Pipe Concrete Interface

Literature Overview

The paper by Liu Zhenyu and Chen Baochun, published in Journal of Guangxi University (Natural Science Edition) (Vol. 37, No. 4, 2012, pp. 698-705), presents a pioneering experimental study on the normal bond strength between steel and concrete at the interface of concrete-filled steel tubes (CFST). The research was supported by the Fujian Provincial Natural Science Foundation and the Huaqiao University Research Startup Fund. This study is of fundamental importance because the steel-concrete interface bond is a critical factor governing the composite action of CFST members, and insufficient understanding of this interface behavior has historically limited the full exploitation of CFST technology.

Experimental Methods and Test Procedures

The authors employed two test methods to measure the normal bond strength between steel and concrete: the tensile pull-off method and the flexural pull-off method. The tensile pull-off method involves directly pulling the steel and concrete apart in the normal direction to the interface, while the flexural pull-off method subjects the specimen to bending to induce normal separation at the interface.

Test Method	Description	Evaluation
Tensile pull-off method	Direct normal separation of steel and concrete	Less reliable due to stress concentration
Flexural pull-off method	Bending-induced normal separation	Better method for normal bond strength measurement

The flexural pull-off method was identified as the superior method for measuring normal bond strength between steel and concrete. This conclusion is supported by the more uniform stress distribution and more controlled failure mechanism achieved through bending, as opposed to the stress concentration and potential shear failure that can occur in direct tensile pull-off tests.

Key Experimental Findings

The experimental results revealed several important characteristics of the steel-concrete interface bond:

Concrete strength has no significant effect on the normal bond strength, which is a surprising finding that challenges conventional assumptions about the role of concrete strength in interface bonding.
The surface condition of the steel plate has a definite influence on bond strength. Rusty steel surfaces exhibit higher bond strength than smooth surfaces, which can be attributed to the increased surface roughness providing mechanical interlock.
The normal bond strength between steel and concrete was determined to be 0.86 MPa based on the flexural pull-off test results.
Under temperature changes and concrete shrinkage effects, the CFST arch rib interface is highly susceptible to debonding.

The finding that concrete strength does not significantly affect normal bond strength is particularly noteworthy. This suggests that the bond strength is governed by mechanical interlock and surface adhesion rather than by the chemical bonding or frictional resistance that would be expected to correlate with concrete strength. This has important implications for the design of CFST members, as the normal bond strength can be assumed to be relatively constant regardless of the concrete grade used.

Debonding Analysis and Engineering Implications

The authors conducted a debonding analysis of CFST cross-sections under temperature variation and core concrete shrinkage effects. The analysis revealed that the interface is highly susceptible to debonding under these conditions, which is a critical concern for CFST arch bridges and other CFST structures subjected to thermal cycling and long-term shrinkage.

The debonding risk under temperature changes is particularly relevant for CFST arch bridges, which are exposed to significant diurnal and seasonal temperature variations. The differential thermal expansion between steel and concrete generates interfacial stresses that can lead to progressive debonding. Similarly, concrete shrinkage, which occurs over time as the concrete cures and ages, generates tensile stresses at the interface that can initiate and propagate debonding.

Factor	Effect on Debonding Risk	Mitigation Measure
Temperature variation	High debonding risk	Thermal expansion compensation design
Concrete shrinkage	High debonding risk	Shrinkage-compensating admixtures
Concrete strength	No significant effect	Standard concrete grades acceptable
Steel surface condition	Rusty surface improves bond	Controlled surface roughening
Concrete strength grade	No significant effect	Cost-effective concrete selection

Study Reflections and Recommendations

This study represents the first systematic experimental investigation of normal bond strength at the steel-concrete interface, filling a significant gap in the understanding of CFST composite action. The determination of a normal bond strength value of 0.86 MPa provides a quantitative basis for interface modeling in finite element analysis and analytical design methods.

For steel pipe manufacturers, the finding that surface condition affects bond strength has direct implications for surface preparation specifications. While rusty surfaces provide higher bond strength, this is not a desirable fabrication condition. Instead, controlled surface roughening through mechanical methods such as shot blasting or brushing should be specified to achieve adequate bond strength while maintaining acceptable surface quality.

The debonding risk under temperature and shrinkage effects highlights the importance of proper construction sequencing and curing practices for CFST members. The concrete should be placed under controlled conditions to minimize shrinkage, and the steel tube should be properly heated or cooled to match the concrete temperature to reduce thermal mismatch stresses.

This research provides a valuable foundation for further studies on interface behavior under various loading conditions, including cyclic loading, impact loading, and fire exposure. The test methodology developed in this study can be adapted for these advanced investigations, and the quantitative bond strength data can serve as benchmark values for model validation.