Interface Bond Strength Between Steel Pipe and Reactive Powder Concrete

Literature Overview

The research by Yan Zhigang, An Mingzhe, Wu Pengpeng, Li Jinsong, and Zhang Lanqing (2009), published in China Railway Science (Vol. 30, No. 6, pp. 7–11), examines the interfacial bond strength between steel pipes and Reactive Powder Concrete (RPC), a high-performance concrete without coarse aggregate that achieves compressive strengths exceeding 120 MPa. Funded by the National Natural Science Foundation of China (Grant No. 50508005), the Doctoral Point Special Research Fund (200800041019), and the Beijing Jiaotong University Science Fund (2007XM031), this study fills an important gap in the design theory of steel tube RPC columns, for which no established bond strength data existed at the time of publication.

Experimental Design and Methodology

The investigators designed two groups of steel tube RPC column specimens and conducted push-out tests following the methodology traditionally used for conventional steel tube concrete (SRC) bond strength evaluation. Key aspects of the experimental design include:

• RPC mix design optimization: Considering construction practicality and economic constraints, the RPC mix proportions and curing conditions were optimized to balance performance with feasibility.
• Repeated push-out testing: Unlike conventional push-out tests that are performed only once, this study subjected specimens to multiple cycles of axial push-out loading to capture the bond behavior under repeated loading, which simulates seismic or cyclic loading conditions.
• Load-slip curve recording: Detailed load-slip relationships were measured for each cycle, enabling characterization of the bond degradation mechanism.

Key Findings and Technical Analysis

The principal results of the study can be summarized in the following comparative framework:

The first-cycle axial push-out curve of the steel tube RPC specimens exhibits behavior similar to that of conventional SRC specimens, suggesting that the fundamental bond mechanism—comprising chemical adhesion, mechanical interlocking, and friction—is preserved. However, the repeated push-out behavior diverges significantly from conventional SRC, indicating that the ultra-high-strength RPC introduces unique interfacial failure mechanisms. The observed asymmetry between odd-numbered push-out and even-numbered pull-back curves implies that the bond interface undergoes irreversible damage accumulation during each cycle, with the fracture surface and contact conditions changing progressively.

The Cai Shaohuai bond strength formula, originally developed for conventional SRC, was applied to the RPC specimens and yielded results reasonably close to the experimental values. This suggests that the formula may be applicable to RPC-filled steel tubes with appropriate modification factors, providing a practical design tool for engineers.

Curing Condition Effects

A notable finding concerns the influence of curing conditions on both RPC strength and bond performance:

The 88% ratio between natural and steam curing conditions for RPC cube strength is consistent with the known sensitivity of RPC to early-age hydration conditions. RPC contains high volumes of silica fume and superplasticizers, which accelerate early-age reactions that are most effective under elevated temperatures. For steel tube RPC columns fabricated in the field, this finding has direct implications: if steam curing is not feasible, designers should either specify a higher strength grade or apply a reduction factor to the bond strength and overall column capacity.

Engineering Practice Implications

From a steel pipe and welding engineering perspective, this study carries several important implications:

1. Surface treatment of steel pipes: The bond strength between the steel pipe inner surface and the RPC fill is critical for composite action. Surface roughening (e.g., through controlled oxide scale retention or mechanical profiling) can enhance mechanical interlocking. For welded steel pipes, the internal weld bead geometry should be considered—smooth ERW welds provide less interlocking than rough submerged arc welds.
2. Welded connections in composite columns: Steel tube RPC columns often require welded connections to beams or other structural elements. The high strength of RPC means that the connection welds may become the weak link. Welding procedures should be qualified for the specific steel grade, and weld design should account for the reduced ductility of the overall column due to the brittle nature of RPC.
3. Quality control during fabrication: The sensitivity of RPC properties to curing conditions means that batch-to-batch variability must be tightly controlled. Engineers should require compressive strength testing of field-cured specimens and apply acceptance criteria that reflect the actual curing environment rather than laboratory steam-cured values.

Study Insights and Outlook

This study represents an important early contribution to the understanding of steel tube RPC systems. The observation that repeated push-out behavior differs fundamentally from conventional SRC is a cautionary note for designers who might naively extrapolate from conventional SRC design codes. The asymmetry in cyclic bond behavior suggests that under seismic loading, steel tube RPC connections may exhibit different energy dissipation characteristics than conventional SRC connections, potentially affecting drift demands and damage patterns in buildings.

Future research should address the effect of steel pipe surface condition (mill finish, coated, roughened) on bond strength, the influence of confinement ratio on interface behavior, and the long-term durability of the bond under sustained loading and environmental exposure. For practitioners, the key takeaway is that RPC-filled steel tubes offer superior strength but require careful attention to curing, surface preparation, and connection design to realize their full potential.