Interface Bond Performance of Steel Tube Reactive Powder Concrete Columns

Literature Overview

The paper by Yan Zhigang, Luo Hua, and An Mingzhe from Beijing Jiaotong University, published in the China Civil Engineering Journal (2010, Vol. 43, No. 8), investigates the bond characteristics at the interface between steel tubes and Reactive Powder Concrete (RPC) columns. The study employs push-out tests on two groups of steel tube RPC specimens, supplemented by nonlinear finite element analysis. The work was funded by the National Natural Science Foundation of China (50508005) and the Doctoral Program Foundation of Higher Education (201000041019). This research addresses a critical gap in the understanding of composite column behavior when ultra-high-performance concrete is used as the infill material.

Core Technical Content

Reactive Powder Concrete, distinguished from conventional concrete by its elimination of coarse aggregate and incorporation of silica fume and steel fibers, achieves compressive strengths exceeding 120 MPa. When confined within steel tubes, the interaction at the steel-concrete interface governs the overall load-bearing capacity and ductility of the composite system. The authors conducted axial push-out tests to quantify the bond strength, slip behavior, and failure modes. The push-out specimen configuration follows the standard approach where a concrete cylinder is embedded in a steel tube, and a steel rod is cast into the concrete core to pull it out axially.

The nonlinear finite element model was constructed using appropriate constitutive models for both the steel tube and RPC material. Contact elements were carefully selected to simulate the frictional and interlocking behavior at the interface. The model was validated against existing push-out test data from reference literature, confirming its predictive accuracy before being applied to the RPC composite system.

Key Findings and Technical Parameters

The critical finding is that despite RPC's superior material properties, the bond failure mechanism remains fundamentally similar to that of conventional steel tube concrete columns. This suggests that existing design codes and empirical formulas for bond strength may be partially applicable, though calibration for RPC-specific parameters is necessary.

Engineering Practice Implications

From a manufacturing and fabrication standpoint, several practical considerations emerge. The high compressive strength of RPC (typically 120-200 MPa) combined with steel tube confinement creates significant internal pressures during curing, which can lead to excessive hoop stress in the steel tube. For seamless steel tubes used in such applications, the minimum wall thickness must be calculated considering both the design loads and the curing-stage radial pressure. The interface bond performance directly affects whether the composite column can be treated as a monolithic member in structural analysis or whether slip-based interaction models are required.

In terms of quality control, the surface condition of the steel tube interior is critical. Any oxide scale, residual oil, or welding slag from the tube manufacturing process will significantly reduce bond strength. For welded tubes (ERW or HFW), the internal weld bead geometry must be evaluated for its influence on bond stress concentration. Surface preparation methods such as shot blasting or chemical treatment should be specified in fabrication procedures.

Study Insights and Reflections

This research demonstrates that the push-out test methodology, well-established for conventional concrete-filled steel tubes, remains valid for RPC systems. However, the high strength of RPC introduces a new concern: the bond strength may become the governing failure mode rather than concrete crushing, which would require different design considerations. The finite element approach provides a valuable tool for parametric studies that would be prohibitively expensive through experimentation alone. Engineers involved in the design of RPC-filled steel tube columns should pay particular attention to the interface treatment and ensure that the steel tube surface is properly prepared to maximize mechanical interlock and frictional resistance.