Fatigue Loading Effects on Steel Tube Concrete Interface Bond Performance

Literature Overview

This paper by Gu Zhangchuan, Yu Xianjiang, Chen Mengcheng, and Xu Kaicheng (Railway Construction, Vol. 53, Issue 9, 2013, pp. 127-130) investigates the degradation of the steel-concrete interface bond in circular steel tube concrete (CFT) members under fatigue loading. The study, supported by the National Natural Science Foundation of China (Grant No. 50968006), conducted fatigue tests followed by push-out tests on seven circular CFT specimens to characterize the evolution of load-slip behavior and bond strength under cyclic loading.

Core Technical Findings

The experimental program involved subjecting each CFT specimen to a prescribed number of fatigue cycles at a given stress ratio, followed by a push-out test to measure the residual bond performance. The key findings include:

• Fatigue loading causes significant degradation of the steel-concrete interface bond strength
• Both the number of fatigue cycles and the stress ratio have pronounced effects on bond performance
• The load-slip curves exhibit abnormal behavior after fatigue loading, indicating progressive interface damage
• The secant modulus of the bond strength-slip curve can effectively define a damage variable for the bond interface

The damage variable definition is particularly noteworthy. By defining the damage variable as the ratio of the secant modulus at a given slip to the initial secant modulus, the authors establish a quantitative relationship between fatigue loading history and bond degradation. This approach provides a practical framework for incorporating fatigue damage into the constitutive modeling of CFT members.

Technical Parameters and Test Configuration

The experimental setup and key parameters are summarized below:

The push-out test measures the force required to pull the steel tube out of the concrete core, providing a direct measure of the interface bond strength. The load-slip curve characterizes the bond behavior from initial elastic loading through peak bond strength to post-peak softening. After fatigue loading, the peak bond strength is reduced, and the slip at peak load is increased, indicating that the interface has accumulated damage.

Welding and Manufacturing Considerations

From a steel pipe manufacturing perspective, the quality of the steel-concrete interface bond is influenced by several fabrication factors. The internal surface condition of the steel tube, including surface roughness, cleanliness, and the presence of mill scale or rust, directly affects the mechanical interlock between the steel and concrete. The welding quality of any internal stiffeners or connection plates also influences the stress distribution at the interface.

In practice, the following measures are recommended to ensure adequate interface bond performance:

• Internal surface preparation of the steel tube to achieve a minimum roughness profile
• Removal of mill scale and contaminants prior to concrete placement
• Use of high-quality concrete with appropriate workability to ensure complete filling of the steel tube
• Proper concrete vibration to eliminate voids and ensure uniform confinement pressure

The fatigue degradation of the interface bond has important implications for the design of CFT members in fatigue-critical applications, such as railway bridges and wind turbine towers. The study's damage variable approach provides a basis for developing fatigue life prediction models that account for the progressive loss of interface bond strength.

Damage Variable Analysis

The authors define the bond interface damage variable as follows:

D = 1 - (E_sec / E_sec,0)

where E_sec is the secant modulus of the load-slip curve at a given slip after fatigue loading, and E_sec,0 is the initial secant modulus before fatigue loading. The damage variable ranges from 0 (undamaged) to 1 (fully damaged). The study demonstrates that the damage variable increases with both the number of fatigue cycles and the stress ratio, and that the relationship can be described by a power-law function.

This damage variable approach has significant implications for the finite element analysis of CFT members under fatigue loading. By incorporating the damage variable into the interface constitutive model, engineers can predict the residual bond strength after a given fatigue loading history and assess the remaining structural capacity. This is particularly important for the assessment of existing CFT structures that have been subjected to long-term fatigue loading.

Engineering Practice and Reflections

The study's findings have direct relevance to the design and assessment of CFT members in fatigue-critical applications. In railway bridges, CFT piers and arch ribs are subjected to repeated train loading, and the progressive degradation of the steel-concrete interface bond can lead to a significant reduction in the residual structural capacity. The damage variable approach provides a practical framework for incorporating fatigue damage into the structural assessment of existing CFT structures.

However, the study also highlights the complexity of the steel-concrete interface bond behavior under fatigue loading. The abnormal behavior observed in the load-slip curves after fatigue loading suggests that the damage mechanism involves not only the degradation of mechanical interlock but also the development of micro-cracks at the interface and the reduction of chemical adhesion. These mechanisms are influenced by the concrete mix design, the steel tube surface condition, and the loading history.

From a quality control perspective, the study underscores the importance of ensuring high-quality concrete placement within the steel tube. Any voids or incomplete filling at the steel-concrete interface will accelerate the fatigue damage process and reduce the residual structural capacity. Engineers should therefore implement rigorous quality control measures during concrete pumping, including the use of high-quality concrete with appropriate workability, proper vibration, and post-placement inspection.

Summary and Reference Value

This paper provides valuable experimental data and analytical tools for understanding the fatigue behavior of the steel-concrete interface bond in CFT members. The damage variable approach based on the secant modulus of the load-slip curve offers a practical framework for incorporating fatigue damage into structural analysis and assessment. Engineers working on CFT structures in fatigue-critical applications should consider incorporating these findings into their design and assessment procedures. The study also highlights the need for further research on the long-term fatigue behavior of the steel-concrete interface bond under realistic loading conditions, including the effects of environmental factors such as temperature cycling and corrosion.