Bond-Slip Performance of Square Steel Tube Recycled Concrete Interface

Literature Overview

This study, published in the Journal of Huaqiao University (Natural Science Edition, 2016, Vol. 37, No. 1, pp. 115–119) by Zhao Qiang from the Shanxi Provincial Institute of Building Science, investigates the bond-slip behavior at the interface between square steel tubes and recycled aggregate concrete (RAC). The research was supported by the National Natural Science Foundation of China (Grant 51308371) and the Shanxi Provincial Natural Science Foundation (Grant 2014011033-1). Eight square steel tube RAC columns were tested to evaluate the effect of recycled aggregate replacement rate and RAC strength on the interfacial bond performance.

Core Technical Points

The use of recycled aggregate concrete in concrete-filled steel tube (CFST) structures is an attractive option for sustainable construction, as it diverts construction and demolition waste from landfills and reduces the demand for natural aggregates. However, the bond between the recycled concrete and the steel tube is a critical factor in the structural performance of CFST members, as it governs the load transfer between the two materials and the composite action of the section.

The experimental program consisted of eight square CFST columns with varying recycled aggregate replacement rates and RAC strengths. The bond-slip behavior was evaluated through direct shear tests on the interface, and the results were analyzed in terms of three distinct stages:

1. No-slip stage: At low load levels, the interface behaves as a monolithic unit with no relative displacement between the steel tube and the RAC.
2. Rising stress stage: As the load increases, micro-cracks begin to form at the interface, and the bond stress increases until it reaches a peak value.
3. Falling stress stage: After the peak bond stress is reached, the bond stress decreases as the cracks propagate and the interface separates.

The key findings of the study are summarized as follows:

The study finds that the recycled aggregate replacement rate has a significant negative effect on the interfacial bond strength. This is attributed to the weaker and more porous nature of recycled aggregates, which results in a weaker interfacial transition zone (ITZ) between the recycled aggregate particles and the cement paste. The ITZ is a critical region for bond development, and its degradation due to the presence of recycled aggregates directly reduces the overall bond strength at the steel tube-concrete interface.

Interpretation of Technical Points

From a materials science perspective, the bond between the steel tube and the concrete infill is a complex phenomenon that involves both mechanical interlock and chemical adhesion. In the case of recycled aggregate concrete, the chemical adhesion component is likely to be reduced due to the presence of adhered old mortar on the recycled aggregate particles, which acts as a weak layer between the new cement paste and the recycled aggregate. This weak layer can propagate into the interface between the RAC and the steel tube, reducing the overall bond strength.

The finding that increasing the RAC compressive strength improves the bond strength but with diminishing returns is consistent with the general behavior of concrete interfaces. At higher concrete strengths, the ITZ becomes more critical to the overall bond strength, and the improvement in bond strength is limited by the properties of the ITZ rather than by the bulk concrete properties. This suggests that for high-strength RAC, the focus should be on improving the quality of the ITZ rather than on further increasing the concrete strength.

The load-slip curves obtained from the tests provide valuable information for the development of constitutive models for the steel tube-RAC interface. The three-stage behavior (no-slip, rising stress, falling stress) is consistent with the behavior observed for conventional concrete-filled steel tubes, which suggests that existing constitutive models can be adapted for use with RAC by modifying the peak bond stress and the slip at peak bond stress.

Integration with Engineering Practice

The practical implications of this study are significant for engineers considering the use of recycled aggregate concrete in CFST structures. The reduction in bond strength due to recycled aggregates must be accounted for in the design of CFST members, particularly for connections and joints where the bond is critical for load transfer.

From a construction quality control perspective, the bond between the steel tube and the RAC can be verified through several methods:

• Pull-out tests: Direct measurement of the bond strength at the interface.
• Ultrasonic testing (UT): Non-destructive evaluation of the interface quality by measuring the acoustic impedance mismatch between the steel tube and the RAC.
• Visual inspection: Examination of the interface after cutting or breaking a test specimen.

For engineering applications where high recycled aggregate replacement rates are desired for sustainability reasons, the bond strength reduction must be compensated for by other means, such as the use of shear connectors (e.g., welded studs or helical reinforcement) or by increasing the concrete cover thickness.

Key Questions and Reflections

A significant question raised by this study is the long-term durability of the steel tube-RAC interface, particularly in aggressive environments where the recycled aggregates may be more susceptible to chemical attack. The porous nature of recycled aggregates can facilitate the ingress of chlorides and sulfates, which can lead to corrosion of the steel tube and degradation of the bond over time.

Another consideration is the effect of the recycling process on the properties of the recycled aggregates. Different recycling processes (e.g., mechanical crushing, washing, and sorting) can produce recycled aggregates with different surface properties, particle shapes, and contaminant levels, all of which affect the bond strength. The study does not address the variability in recycled aggregate properties due to different recycling processes, which is a significant source of uncertainty in practice.

Study Insights and Implications

This study provides valuable experimental data on the bond-slip behavior of square steel tube recycled concrete interfaces and highlights the significant influence of recycled aggregate replacement rate on bond strength. The key insight is that while recycled aggregate concrete can be used in CFST structures, the bond strength reduction must be carefully accounted for in the design, and additional measures may be necessary to ensure adequate composite action.

The methodology of direct shear testing is a well-established approach for evaluating interface bond behavior, and the results obtained in this study can be used to calibrate and validate numerical models for CFST members with RAC infill. The consistency of the three-stage load-slip behavior across different replacement rates is a positive finding, as it suggests that the fundamental bond mechanism is not altered by the use of recycled aggregates, even though the bond strength is reduced.

For engineers involved in the design and construction of CFST structures using recycled materials, this study reinforces the importance of understanding the material-level properties that govern the composite behavior of the structure. The bond between the steel tube and the concrete infill is not a trivial detail, but rather a critical factor that can significantly affect the load capacity, ductility, and durability of the structure.