Interface Bond Performance of Steel Fiber Toughened Micro-Expansion Concrete-Filled Steel Tubes

Literature Overview

This research, published in Journal of Functional Materials (2013, Vol. 44, No. 6, pp. 809-813) by Ding Qingjun et al. from the State Key Laboratory of Silicate Materials for Architectures, Wuhan University of Technology, and the Sichuan Provincial Highway Planning, Survey and Design Research Institute, investigates the interface bond performance between core concrete and steel tube wall in steel fiber reinforced micro-expansion concrete-filled steel tubes (CFST). The study is supported by the National Basic Research Program of China (973 Program, 2009CB623201) and the Ministry of Transport Science and Technology Project (2009318000105). Thirty push-out test specimens were prepared and tested to systematically evaluate the interface bond behavior.

Experimental Configuration and Key Parameters

The push-out test is the standard method for evaluating the interface bond between concrete and steel tube in CFST members. The test involves casting concrete inside a steel tube and then pushing a concrete plug out of the tube to measure the bond strength and slip at failure. The micro-expansion concrete used in this study is designed to develop internal compressive stresses upon setting, which can enhance the confinement effect between the concrete core and the steel tube wall.

Core Technical Findings

The study demonstrates that steel fiber reinforced micro-expansion CFST exhibits significantly higher interface bond strength compared to conventional CFST members. The steel fibers bridge micro-cracks at the concrete-steel interface, enhancing the interfacial shear resistance and delaying debonding. The micro-expansion property of the concrete creates a pre-compressive state at the interface, which further improves the bond performance by increasing the normal pressure between the concrete core and the steel tube wall.

Two primary factors govern the interface bond strength: the diameter-to-thickness ratio (D/t) and the steel fiber volume fraction. The D/t ratio affects the confinement effectiveness, as larger D/t ratios result in reduced lateral confinement pressure from the steel tube on the concrete core. The steel fiber volume fraction influences the crack-bridging capacity and the overall toughness of the interface zone.

However, the study reveals an important finding that the addition of steel fibers leads to a reduction in the micro-expansion CFST interface bond performance. This counterintuitive result is attributed to the disruption of the micro-expansion mechanism by steel fibers, which interfere with the expansive aggregate particles and reduce the effective expansion pressure at the interface. The recommended maximum steel fiber volume fraction of 0.75% represents an optimal balance between crack-bridging benefits and expansion preservation.

Critical Threshold Behavior

A particularly noteworthy finding is that when the steel fiber volume fraction reaches 22%, the effects of both steel fiber content and interface length on the interface bond performance become insignificant. This threshold behavior suggests that beyond a certain fiber content, the interface bond is governed by other factors such as the concrete-steel adhesion mechanism and the confinement pressure rather than the fiber bridging effect. This finding has direct implications for practical design, as it establishes an upper limit beyond which increasing steel fiber content provides no additional benefit for interface bond performance.

Engineering Practice and Quality Control Considerations

From a manufacturing perspective, the interface bond performance of CFST members is critical for ensuring the composite action between the steel tube and concrete core. Poor interface bonding can lead to premature debonding under load, reducing the overall capacity of the member. The push-out test results provide quantitative data for design purposes, allowing engineers to predict the bond strength under various loading conditions.

For quality control in CFST production, the following considerations are important: the steel fiber volume fraction must be carefully controlled during concrete mixing to stay within the optimal range of 0.5% to 0.75%. The micro-expansion agent dosage must be calibrated to account for the presence of steel fibers, as the fibers can reduce the effective expansion. The D/t ratio of the steel tube should be selected to ensure adequate confinement pressure, typically requiring D/t values below certain limits depending on the application.

The interface bond slip behavior is also important for the analysis of composite members. Under cyclic loading, the interface may undergo repeated slip and re-engagement, which affects the fatigue performance and energy dissipation capacity. The push-out test data can be used to develop constitutive models for the interface bond, which are essential for accurate finite element analysis of CFST structures.

Study Insights and Implications

This research highlights the complex interaction between steel fiber reinforcement and micro-expansion in CFST members. The finding that steel fibers can reduce the micro-expansion interface bond performance is particularly important for engineers who may be tempted to increase fiber content without considering this trade-off. The recommended maximum of 0.75% steel fiber volume fraction provides a clear design guideline. The threshold behavior at 22% volume fraction, where further increases in fiber content have no effect, suggests that the interface bond mechanism transitions from fiber-bridging dominated to concrete-steel adhesion dominated at high fiber contents. This understanding is valuable for optimizing the material composition of CFST members for specific performance requirements, balancing bond strength, toughness, and constructability.