Zhuojin Pipe Fitting Co., LtdBond-Slip Constitutive Relationships in Textured Steel Tube Concrete Composites
Literature Overview
The study by Chen Lihua, Dai Jixiang, Jin Qiliang, Wang Bei, and Sheng Benlin (Hefei University of Technology, 2015), published in the Journal of Hefei University of Technology (Vol. 38, No. 8, pp. 1091-1096), investigates the bond-slip behavior of textured (corrugated) steel tube concrete composite systems through push-out tests. Funded by the Ministry of Housing and Urban-Rural Development Science and Technology Program (2013-k2-1), this research addresses a critical interface behavior that governs load transfer between steel tubes and concrete in composite structural members.
Core Technical Content
Textured steel tubes — steel tubes with surface corrugations or protrusions — are designed to enhance mechanical interlock between the steel tube and surrounding concrete, thereby increasing the bond strength and improving the composite action of the structural member. The study conducted push-out tests on nine specimens with varying corrugation heights and concrete strengths to establish empirical relationships for characteristic bond strength and to propose a τ-Sf (shear stress versus slip) constitutive model.
Test Parameters and Specimen Configuration
| Parameter | Variable Range | Number of Levels |
|---|---|---|
| Corrugation height | Multiple values | 3 levels |
| Concrete compressive strength | Multiple grades | 3 levels |
| Total specimens | — | 9 |
The push-out test configuration involves a steel tube embedded in a concrete cylinder, with the tube loaded axially while the concrete is restrained. The relative displacement (slip) at the steel-concrete interface is measured, and the resulting shear stress distribution along the interface is calculated from equilibrium conditions.
Interpretation of Technical Points
Characteristic Bond Strength
The characteristic bond strength (τ_c) is defined as the maximum shear stress that develops at the steel-concrete interface before significant slip occurs. The authors established statistical regression formulas for τ_c as a function of corrugation height and concrete compressive strength. The key finding is that corrugation height has a significant positive effect on characteristic bond strength, while concrete strength also contributes positively but with diminishing returns at higher grades.
From a metallurgical and materials science perspective, the bond strength at the steel-concrete interface is governed by three mechanisms: (1) chemical adhesion between the steel surface and the cement paste, (2) friction between the steel surface and hardened concrete, and (3) mechanical interlock provided by the corrugations or ribs. For smooth steel tubes, chemical adhesion and friction dominate, yielding characteristic bond strengths typically in the range of 1.0-2.0 MPa. Textured tubes with corrugation heights of 2-4 mm can achieve characteristic bond strengths of 3.0-6.0 MPa, representing a 2-3 fold improvement.
τ-Sf Constitutive Model
The proposed τ-Sf constitutive model divides the bond-slip curve into three distinct stages:
| Stage | Slip Range | Behavior Description | Governing Mechanism |
|---|---|---|---|
| Stage 1 | 0 – 0.1 mm | Linear elastic | Chemical adhesion and elastic interlock |
| Stage 2 | 0.1 – 0.5 mm | Non-linear hardening | Progressive mechanical interlock engagement |
| Stage 3 | > 0.5 mm | Softening / plateau | Interlock failure and friction-dominated behavior |
Stage 1 represents the initial loading where the corrugations engage the concrete elastically. The stiffness in this stage is proportional to the concrete elastic modulus and the geometric interlock ratio. Stage 2 involves progressive crushing of the concrete in front of the corrugations and engagement of additional corrugation surfaces. Stage 3 represents the post-peak behavior where the mechanical interlock has been partially or fully destroyed, and load transfer continues through residual friction.
Process and Standards Analysis
The bond-slip behavior in textured steel tube concrete composites has direct implications for several design standards and specifications:
- GB 50017-2017 (Standard for Design of Steel Structures): Provides design bond strength values for smooth steel tube concrete members but does not explicitly address textured tubes.
- CEB-FIP Model Code 1990: Offers general bond-slip formulations for reinforced concrete that can be adapted for composite members.
- Eurocode 4 (EN 1994-1-1): Specifies minimum bond strength requirements for composite members but does not distinguish between smooth and textured tubes.
The engineering significance of textured tubes is substantial. In CFST (Concrete-Filled Steel Tube) columns and beams, the bond strength at the steel-concrete interface governs: (1) the effectiveness of composite action in bending members; (2) the confinement pressure distribution in compressed members; and (3) the overall ductility and energy dissipation capacity under cyclic loading. Insufficient bond strength leads to premature debonding, reduced effective confinement, and potential brittle failure.
Welding Considerations for Textured Tubes
From a welding and fabrication perspective, textured steel tubes present unique challenges:
- Surface preparation: The corrugations must be accounted for when designing weld joints. Welding directly over corrugations can lead to inconsistent penetration and weld quality.
- Heat input control: The increased surface area of textured tubes affects heat dissipation during welding, potentially requiring higher heat input to achieve full penetration.
- Weld procedure qualification: Standard weld procedure specifications (WPS) for smooth tubes may not be directly applicable to textured tubes, and new qualification testing is recommended per ASME Section IX or ISO 15614.
Integration with Engineering Practice
In practical applications, textured steel tubes have been used in: (1) composite columns in high-rise buildings where enhanced confinement is required; (2) bridge piers subject to seismic loading where improved energy dissipation is critical; (3) industrial plant structures where ductility under accidental loading is paramount. The push-out test data and constitutive model presented in this paper can be directly implemented in finite element analysis software (such as ABAQUS or ANSYS) using cohesive zone elements or contact interfaces with frictional and interlocking behavior.
For quality control purposes, the bond strength of textured steel tube concrete members can be verified through: (1) pull-out tests on representative specimens during construction; (2) ultrasonic testing of the concrete-steel interface to detect voids or incomplete filling; and (3) strain measurement during loading tests to verify composite action.
Key Questions and Reflections
A critical question that arises from this study is the long-term durability of the bond-slip behavior in textured steel tube concrete. The mechanical interlock provided by corrugations is effective under short-term loading, but under sustained loading or cyclic loading, the concrete in front of the corrugations may undergo progressive crushing, leading to bond degradation. This phenomenon is analogous to bearing capacity degradation in bolted connections under cyclic loading and warrants further investigation.
Another important consideration is the interaction between bond strength and corrosion. In aggressive environments, corrosion of the steel tube surface can reduce the effective corrugation height and degrade the chemical adhesion component of bond strength. The use of duplex stainless steel tubes (e.g., SAF 2205 per ASTM A790) or protective coatings can mitigate this issue but introduces additional complexity in the design and fabrication process.
Study Insights and Implications
This paper makes a valuable contribution to the understanding of interface behavior in composite steel-concrete structures. The three-stage τ-Sf model is physically meaningful and can be readily implemented in numerical analyses. The finding that corrugation height is the dominant parameter for bond strength enhancement provides clear guidance for the selection of textured tube specifications in design.
For the practicing engineer, the key takeaway is that textured steel tubes offer a practical and effective means of enhancing composite action without the need for external shear connectors or mechanical interlocks. However, the benefits must be weighed against the additional fabrication complexity, the need for specialized welding procedures, and the potential for long-term bond degradation. A comprehensive design approach should consider the entire service life of the structure, including the effects of cyclic loading, environmental exposure, and maintenance access.