Zhuojin Pipe Fitting Co., LtdUniaxial Tensile Performance of Overlay-Welded Grout-Filled Sleeve Connections
Literature Overview
This paper by Gao Qiang and Zhao Weijian (Shenyang Jianzhu University and Zhejiang University, 2022, Journal of Building Structures, Vol. 43, No. 4, pp. 208-219) investigates the mechanical behavior of steel bar sleeve grout connections fabricated using an overlay-welded sleeve manufacturing technique. Twenty-one specimens with varying geometric parameters were tested under uniaxial tension to establish design guidelines for this emerging connection technology.
Technical Context and Manufacturing Innovation
The overlay-welded sleeve manufacturing method represents a departure from traditional machining or rolling approaches for producing internal ribbed sleeves. By depositing rings of weld metal inside a tubular sleeve, internal ribs are created that mechanically interlock with deformed steel bars when grout is injected. This approach offers:
| Advantage | Description |
|---|---|
| Manufacturing flexibility | Rib geometry adjustable without tooling changes |
| Cost reduction | Reduced machining time for internal features |
| Material efficiency | Additive process minimizes waste |
| Scalability | Suitable for various sleeve diameters |
The overlay-welded ribs serve as mechanical interlock elements, replacing or supplementing traditional cold-formed or machined ribs. The weld metal composition and rib profile directly influence the bond strength between the steel bar and grout.
Experimental Design and Key Parameters
The study systematically varied three geometric parameters:
| Parameter | Range Studied | Number of Levels |
|---|---|---|
| Anchorage length (L) | 5d to 10d | 5 levels |
| Ring rib spacing (S) | 20-80 mm | 4 levels |
| Sleeve inner diameter (D) | ±5% of nominal | 3 levels |
Where d is the steel bar diameter. A total of 21 specimens were tested, with standard HRB400 deformed bars and cement-based grout as the bonding medium.
Key Findings and Failure Mechanisms
Critical Anchorage Length
The most significant finding is the identification of a critical anchorage length between 5d and 6d. Below this threshold, failure occurs through a scraping-out mechanism where the bar is pulled out of the grout-sleeve assembly. Above this threshold, the steel bar itself fractures in tension before the connection fails. This defines a clear design boundary:
| Anchorage Length | Failure Mode | Connection Efficiency |
|---|---|---|
| L < 5d | Bar pull-out (scraping) | < 80% of bar strength |
| 5d ≤ L ≤ 6d | Transition zone | 80-100% |
| L > 6d | Bar fracture | ≥ 100% |
Strain Distribution Characteristics
The strain measurement data reveals a non-uniform load transfer mechanism:
- Steel bar strain decreases from the sleeve end toward the middle, indicating progressive load transfer from the end region to the interior.
- Sleeve axial strain increases from the end toward the middle, reflecting the cumulative effect of bond stress on the sleeve wall.
- The sleeve end exhibits radial expansion (bulging) while the middle section shows slight radial contraction—a phenomenon attributable to the Poisson effect under axial tension.
Effect of Ring Rib Spacing
| Rib Spacing | Load Capacity | Behavior Characteristic |
|---|---|---|
| 20 mm | Highest | Stress redistribution to ribbed end zone |
| 40 mm | Moderate | Balanced load transfer |
| 60 mm | Lower | Reduced mechanical interlock |
| 80 mm | Lowest | Approaching smooth sleeve behavior |
A critical observation is the diminishing returns effect: reducing rib spacing below approximately 30 mm provides minimal additional strength improvement. This suggests an optimal spacing window of 25-40 mm for practical design.
Sleeve Inner Diameter Effect
The sleeve inner diameter variation within ±5% of nominal produced no statistically significant effect on connection capacity. This indicates that the bond mechanism is dominated by the rib geometry and grout-bar interaction rather than the overall sleeve diameter, which simplifies manufacturing tolerances.
Engineering Design Implications
Based on the experimental findings, the following design recommendations emerge:
- Minimum anchorage length: Adopt L ≥ 6d for ductile connection design to ensure bar fracture governs failure.
- Rib spacing optimization: Select S = 30-40 mm to balance strength and manufacturing cost.
- Tolerance control: Sleeve inner diameter tolerances can be relaxed to ±5% without performance penalty.
- End-zone attention: The sleeve end region carries disproportionate load; local reinforcement or geometric optimization may enhance durability.
Study Insights and Future Directions
This research demonstrates that overlay-welded sleeve manufacturing is a viable and potentially superior alternative to traditional machining methods for grout-filled sleeve connections. The key insight is that the critical anchorage length of 5-6d is remarkably short compared to conventional connections (typically 10-15d), suggesting that the overlay-welded ribs provide significantly enhanced mechanical interlock. However, the study's limitation lies in its static loading focus—cyclic and fatigue behavior remain unexplored, which is addressed in the companion paper (Topic 5). Engineers adopting this technology should ensure that weld quality of the internal ribs meets structural welding standards, as any weld defect (porosity, lack of fusion) could compromise the interlock mechanism.