Interfacial Bond Strength Between Steel Tubes and Reactive Powder Concrete

Literature Overview

The study by Yan Zhigang, An Mingzhe, Wu Pengpeng, Li Jinsong, and Zhang Lanqing, published in China Railway Science (2009, Vol. 30, No. 6, pp. 7-11), investigates the interfacial bond strength between steel tubes and reactive powder concrete (RPC). This research was supported by the National Natural Science Foundation of China (Grant No. 50508005), the Doctoral Program Special Research Fund (Grant No. 200800041019), and the Beijing Jiaotong University Science and Technology Fund (Grant No. 2007XM031). The work is significant because RPC, with its ultra-high compressive strength and excellent durability, represents a next-generation concrete material for steel tube concrete (CFST) applications, yet its interfacial bond characteristics with steel tubes have not been well characterized.

Experimental Design and Key Results

Two groups of steel tube RPC column specimens were designed and tested using the push-out test method, following established design theories and test methods for conventional steel tube concrete. The authors optimized the RPC mix design and curing conditions to balance construction practicality with economic considerations.

The load-slip curves obtained from the first axial push-out test showed behavior similar to that of conventional steel tube concrete specimens. However, the repeated push-out test curves exhibited notable differences from conventional CFST behavior. Specifically, the odd-numbered push-out curves and even-numbered push-back curves of the steel tube RPC specimens did not exhibit symmetry, indicating a fundamentally different interfacial damage mechanism compared to normal concrete.

Technical Interpretation

The interfacial bond strength in CFST members is governed by three mechanisms: chemical adhesion, friction, and mechanical interlock. For RPC, the extremely low porosity and high density of the concrete matrix alter all three mechanisms:

• Chemical adhesion: The denser microstructure of RPC may reduce the chemical bonding area at the steel-concrete interface, as there is less capillary porosity for hydration products to interlock with the steel surface.
• Friction: The high compressive strength of RPC increases the confining pressure on the steel tube, which in turn increases the frictional resistance at the interface. This is consistent with the Cai Shaohuai bond strength formula, which relates bond strength to the concrete compressive strength.
• Mechanical interlock: The absence of aggregate in RPC eliminates the aggregate interlock mechanism that contributes significantly to bond strength in conventional concrete. This may explain the observed asymmetry in repeated push-out tests.

The validation of the Cai Shaohuai bond strength formula against the experimental results is a positive finding, suggesting that existing design approaches for conventional CFST can be adapted for RPC with appropriate modifications to the concrete strength input.

Welding and Steel Tube Considerations

From a steel pipe fabrication and welding perspective, several issues arise:

1. Surface preparation: The bond strength at the steel tube interior is sensitive to surface condition. For RPC applications, where the concrete has extremely high strength, the steel tube inner surface should be roughened (e.g., by shot blasting or mechanical profiling) to enhance mechanical interlock.
2. Welding of steel tube components: Steel tubes used in CFST columns often have longitudinal or spiral welds. The HAZ of these welds, particularly for high-strength steels, may exhibit reduced toughness and altered microstructure. When filled with RPC, the high confining pressure from the concrete may partially restore the ductility of the HAZ, but this benefit should be verified experimentally.
3. Thermal effects during concrete placement: The hydration heat of RPC, which has a high cementitious content, can be substantial. This heat can induce thermal stresses in the steel tube, particularly if the concrete is placed in stages. The thermal expansion mismatch between steel and RPC during the hydration period should be considered in the design of the steel tube fabrication tolerances.

Engineering Practice Recommendations

For engineers designing RPC-filled steel tube structures:

• Natural curing, while producing lower bond strength than steam curing, is often the only practical option for field construction. The 88% strength ratio should be incorporated into design calculations.
• The asymmetric behavior observed in repeated push-out tests suggests that RPC-filled steel tubes may exhibit different post-peak behavior under cyclic loading compared to conventional CFST. This is particularly relevant for seismic applications where ductility is a design requirement.
• Interface bond strength testing should be included in the quality control plan for RPC-filled steel tube projects, with acceptance criteria derived from the load-slip curve characteristics.

Study Insights and Conclusions

This study provides the first systematic experimental data on the interfacial bond between steel tubes and RPC, filling an important gap in the literature on high-performance CFST members. The confirmation that existing bond strength formulas can be applied to RPC, with appropriate adjustments, is encouraging for the practical adoption of this technology. However, the asymmetric behavior under repeated loading warrants further investigation, particularly for applications involving cyclic or seismic loading. The optimization of RPC mix design for bond strength, rather than solely for compressive strength, should be a focus of future research.