Zhuojin Pipe Fitting Co., LtdBond-Slip Behavior of Square Steel Tube UHPC Composite
Literature Overview
This paper by Wang Qiwei and colleagues from Xi'an University of Architecture and Technology, published in the Journal of Hunan University (Natural Science Edition) in 2022, investigates the interfacial bond-slip behavior between square steel tubes and ultra-high performance concrete (UHPC). Supported by two National Natural Science Foundation grants (51878543 and 51878540), the study designed 18 push-out specimens with varying steel tube width-to-thickness ratios, height-to-width ratios, and UHPC strength grades. The specimens were subjected to static push-out tests to characterize the bond-slip performance comprehensively.
Core Technical Findings
The failure mode of the push-out specimens provides the first insight into the bond mechanism. After the push-out test, the specimens remained largely intact, with no visible bulging or local buckling of the steel tube. However, the concrete at the loading end edge exhibited some degree of damage. This observation is significant because it indicates that the bond failure is primarily an interfacial phenomenon rather than a steel tube failure or a bulk concrete failure. The absence of steel tube bulging confirms that the UHPC provides adequate confinement to prevent local buckling of the steel tube walls during the push-out process, which is a favorable outcome for the structural integrity of the composite member.
The load-slip curves at both the loading end and the free end exhibit fundamentally similar shapes, which can be classified into two distinct categories: a weakening type with a clear peak point, and a strengthening type without a pronounced peak. The weakening type behavior is characteristic of lower bond strength conditions where the interface reaches its maximum resistance and then degrades as slip increases. The strengthening type behavior, on the other hand, suggests that the interface continues to develop resistance even after the initial slip, likely due to progressive engagement of mechanical interlock mechanisms and increased frictional resistance as the interface settles into a more stable configuration.
| Parameter | Effect on Bond Strength | Notes |
|---|---|---|
| Increasing width-to-thickness ratio | Decrease | Thinner tubes provide more confinement |
| Increasing height-to-width ratio | Decrease | Longer tubes have more uniform strain distribution |
| Increasing UHPC strength (high width-to-thickness ratio) | Significant increase | Effective only when tube is relatively thick |
| Increasing UHPC strength (low width-to-thickness ratio) | Limited increase | Confinement already maximized |
The bond strength decreases with increasing width-to-thickness ratio and height-to-width ratio. The width-to-thickness ratio effect is consistent with the confinement mechanism: thinner tubes generate higher radial pressure on the UHPC, increasing the normal stress on the interface and thereby enhancing frictional resistance. The height-to-width ratio effect is more nuanced; longer tubes develop a more uniform longitudinal strain distribution, which means that the differential slip between the steel tube and the UHPC is more evenly distributed along the height, reducing the peak bond stress at any single location.
A particularly important finding is the interaction between UHPC strength and width-to-thickness ratio. When the width-to-thickness ratio is large (thick-walled tube), increasing the UHPC strength can significantly improve the bond strength. However, when the width-to-thickness ratio is already small (thin-walled tube), the confinement effect is already maximized, and further increases in UHPC strength yield diminishing returns on bond strength. This interaction effect has important implications for material selection: high-strength UHPC is most beneficial when paired with thicker-walled steel tubes, whereas for thin-walled tubes, the bond strength is already near its practical maximum regardless of UHPC strength.
Interpretation of Technical Points
The longitudinal strain distribution in the steel tube provides additional insight into the bond mechanism. The strain at the loading end is smaller than at the free end, and the strain varies approximately exponentially along the height direction. This exponential distribution is consistent with the classical bond-slip theory, where the bond stress is highest near the loading end and decreases exponentially toward the free end. The fact that the loading end strain is lower than the free end strain may seem counterintuitive, but it can be explained by the fact that the loading end is where the push-out force is applied, and the steel tube is directly loaded there, while the free end relies on bond stress to transfer load from the UHPC to the steel tube, resulting in higher strain at the free end.
The proposed bond strength calculation model, which decomposes the total bond strength into interfacial friction stress and mechanical interlock stress while neglecting chemical adhesion, represents a rational simplification for UHPC-steel tube interfaces. The chemical adhesion between steel and UHPC is relatively small compared to the friction and interlock components, particularly when the UHPC has a high compressive strength and the steel tube surface has adequate roughness. The model's performance under two different curing conditions demonstrates its robustness, although the specific curing conditions (standard curing versus steam curing) would influence the surface roughness and porosity of the UHPC, thereby affecting the mechanical interlock component.
Engineering Practice Integration
For engineers designing square steel tube UHPC composite members, the key design implications are clear. The width-to-thickness ratio should be minimized to maximize bond strength, subject to local buckling constraints. When high bond strength is required and the tube wall thickness is limited, the height-to-width ratio should be kept small to avoid the bond strength reduction associated with longer tubes. The selection of UHPC strength should be made in conjunction with the tube geometry: high-strength UHPC (f'c > 120 MPa) is most effective when the width-to-thickness ratio is relatively large, while for thin-walled tubes, moderate UHPC strength (f'c = 80-100 MPa) may be sufficient and more cost-effective.
The curing condition should be carefully controlled to ensure adequate surface roughness of the UHPC for mechanical interlock. Steam curing, while accelerating strength development, may produce a smoother surface that reduces mechanical interlock. Engineers should specify the curing protocol in conjunction with the bond strength requirements, and consider surface treatments such as roughening or the use of mechanical anchors if the bond strength is found to be insufficient.
Key Questions and Reflections
One important question that this study raises is the applicability of the bond-slip model to fatigue loading conditions. The push-out tests were conducted under static loading, and the bond behavior under cyclic or fatigue loading may be significantly different. In practical applications, such as bridge decks or industrial floors, the composite members are subjected to repeated traffic or machinery loads. The progressive degradation of bond strength under cyclic loading could lead to unexpected slip and loss of composite action. Future research should investigate the fatigue bond-slip behavior of square steel tube UHPC composites.
Another consideration is the effect of temperature on the bond-slip behavior. UHPC is sensitive to high temperatures, and in fire conditions, the bond strength may degrade significantly due to thermal expansion mismatch between the steel tube and the UHPC. The differential thermal expansion could generate additional slip at the interface, reducing the bond strength and potentially leading to premature failure. Engineers should consider the fire resistance requirements when designing UHPC composite members and apply appropriate fire protection measures to the steel tube.
Study Insights and Implications
This study provides a comprehensive characterization of the bond-slip behavior between square steel tubes and UHPC, offering valuable data and a practical calculation model for design engineers. The identification of the two bond-slip curve types (weakening and strengthening) is particularly useful for predicting the post-peak behavior of composite members. The interaction between UHPC strength and tube geometry highlights the importance of integrated material-geometry optimization in composite design. The study reinforces the growing recognition that UHPC, despite its high cost, offers superior structural performance in composite applications where high bond strength and ductility are required.