Zhuojin Pipe Fitting Co., LtdInterface Bonding Strength Calculation Method for Steel Tube Reactive Powder Concrete Composite Columns
Literature Overview
The paper by Wang Qiuwei, Liu Le, Shi Qingxuan, and Wang Peng (2020), published in Engineering Mechanics, Volume 37, Issue 4, presents a study on the interface bonding strength calculation method for steel tube reactive powder concrete (RPC) composite columns. The research, supported by the National Key R&D Program of China (Grant 2017YFC0703406), was conducted at Xi'an University of Architecture and Technology, specifically at the State Key Laboratory of Western Green Building jointly constructed by the Ministry of Education and Shaanxi Province.
Experimental Programme
The study involved uniaxial push-out tests on 10 steel tube confined RPC specimens to investigate the bonding-slip mechanism at the steel tube-RPC interface. The experimental parameters and their effects are summarised below.
| Specimen Parameter | Range Studied | Effect on Bonding Strength |
|---|---|---|
| Length-to-diameter ratio (L/D) | Multiple values | Bonding strength increases with increasing L/D |
| Diameter-to-thickness ratio (D/t) | Multiple values | Bonding strength increases with decreasing D/t |
| RPC compressive strength | Multiple grades | No significant effect on bonding strength |
| Confinement coefficient | Variable | Governs the presence of descending branch in load-slip curve |
The push-out test is a standard method for evaluating the bond performance between steel and concrete in composite members. In this test, the steel tube and concrete core are pushed apart relative to each other, and the bond stress is calculated from the measured load and the contact area between the tube and concrete.
Bonding Mechanism and Failure Modes
The study identified two distinct failure modes depending on the confinement coefficient (the ratio of steel tube cross-sectional area to concrete core cross-sectional area):
| Failure Mode | Confinement Coefficient | Load-Slip Curve Characteristic |
|---|---|---|
| Mode 1 | Small | Obvious descending branch after peak load |
| Mode 2 | Large | No descending branch; plateau or gradual softening |
The key mechanistic finding is that when the bond stress reaches the bonding strength, the lateral deformation coefficient of the steel tube exceeds Poisson's ratio, indicating that the confining action of the steel tube begins to engage. This is a critical transition point: below this threshold, the bond is governed by adhesion and friction alone; above it, the steel tube's confinement effect provides additional resistance to slip.
The bonding strength calculation model proposed in the paper establishes a relationship between the internal pressure of the steel tube and the bond stress, and was calibrated through regression of the experimental data. The model showed good agreement with the test results.
Welding Quality and Interface Integrity
From a fabrication and welding perspective, the quality of the steel tube-RPC interface is influenced by several welding-related factors:
| Factor | Influence on Bonding | Control Measure |
|---|---|---|
| Steel tube surface condition | Directly affects adhesion | Mill scale removal or controlled roughness |
| Weld seams on tube surface | Create localised stress concentrations | Orient welds away from high-stress zones |
| Internal stiffeners | Enhance shear transfer but complicate concrete placement | Weld quality verified by PAUT |
| Tube end preparation | Affects concrete flow and void formation | Chamfered ends, no burrs |
| Residual stresses from welding | Influence the onset of confining action | PWHT for thick-walled tubes |
The presence of longitudinal weld seams (in ERW, HFW, or LSAW pipes) on the outer surface of the steel tube does not directly affect the internal bond interface, but the weld-induced residual stresses can influence the overall tube behaviour under compression. For the internal bond interface, the critical factor is the cleanliness and surface roughness of the inner tube surface. Any residual welding spatter, flux residue, or oxide scale on the inner surface can create weak zones that reduce the effective bonding area.
For pipes manufactured by longitudinal welding processes (ERW, HFW, LSAW), the inner surface is typically smooth and free of weld seams, which is advantageous for bonding. However, for pipes manufactured by transverse welding or with internal stiffeners welded to the inner surface, the welding consumables must be selected to avoid introducing contaminants that could impair the concrete bond.
Bonding Strength Model and Design Application
The proposed bonding strength calculation model relates the internal pressure of the steel tube to the bond stress through a mathematical relationship derived from equilibrium and compatibility conditions. The model accounts for:
- The geometric parameters (L/D, D/t) that govern the stress distribution at the interface.
- The material properties of both the steel tube and the RPC that determine the deformation compatibility.
- The confinement effect that activates when the lateral deformation exceeds Poisson's ratio.
The finding that RPC compressive strength has no significant effect on bonding strength is somewhat counterintuitive but can be explained by the fact that the bond strength is governed by the adhesion and mechanical interlock at the interface, which depends more on the surface condition and geometric parameters than on the bulk compressive strength of the concrete. This is an important insight for design engineers who might otherwise assume that using higher-strength RPC would automatically improve the bond performance.
Study Insights and Reflection
This study addresses a critical but often overlooked aspect of steel tube concrete composite design: the interface bonding behaviour. In my experience with composite member design, the bond strength between the steel tube and concrete core is frequently assumed to be adequate based on empirical rules, without rigorous verification through testing or detailed analysis. The proposed calculation model provides a more rational basis for design, particularly for applications involving reactive powder concrete, which is increasingly used in high-performance structural applications. The distinction between the two failure modes based on the confinement coefficient is practically important: designers should aim for a sufficiently high confinement coefficient to ensure Mode 2 behaviour, which provides a more predictable and stable bond performance without the sudden loss of load-carrying capacity associated with the descending branch of Mode 1. The welding and fabrication aspects of this study, while not the primary focus of the paper, are nonetheless essential for ensuring that the theoretical bond performance is realised in practice, and any welding-related defects at the interface can significantly reduce the effective bond strength.