Performance of Cladding-Formed Sleeve Grouting Connections Under Unidirectional Tensile Loading

Research Background and Engineering Significance

This 2022 study published in the Journal of Building Structures investigates the mechanical performance of cladding-formed sleeve grouting connections (CFSGC) under unidirectional tensile loading. The research, conducted by Gao Qiang and Zhao Weijian from Shenyang Jianzhu University and Zhejiang University respectively, represents an innovative application of cladding technology in structural engineering—a domain where I have found particularly interesting parallels to traditional pressure vessel cladding practice.

The CFSGC concept involves using a cladding-formed steel sleeve to connect reinforced concrete or steel members through a grouting mechanism. The sleeve is manufactured by cladding a hardfacing or transition layer onto a base steel tube, creating a composite structure with enhanced mechanical properties. This is conceptually similar to the bimetallic construction used in pressure vessels, where a corrosion-resistant cladding layer is bonded to a structural base material.

Technical Design of the Cladding-Formed Sleeve

The CFSGC system consists of the following components:

The cladding process creates a metallurgically bonded interface between the base tube and the overlay layer, providing superior load transfer compared to mechanical fastening or adhesive bonding. The overlay layer serves multiple purposes: it enhances the local bearing capacity at stress concentration points, provides wear resistance during assembly, and can improve the fatigue performance of the connection.

Experimental Program and Key Findings

The study employed a systematic experimental program to characterize the tensile behavior of CFSGC connections:

1. Specimen geometry: Sleeves with outer diameters of 60-120 mm, wall thicknesses of 8-15 mm, and lengths of 200-400 mm were tested.
2. Loading conditions: Uniaxial tensile loading at quasi-static rates (typically 1-5 mm/min) to characterize the full load-displacement response.
3. Variables studied: Sleeve diameter, wall thickness, cladding layer thickness, grout strength, and rebar-to-sleeve ratio.

Key findings from the experimental program include:

• The cladding layer significantly improves the ultimate tensile capacity of the connection by providing enhanced bearing resistance at the critical interface between the sleeve and the grout.
• The failure mode transitions from grout shear failure (in unclad sleeves) to sleeve yielding or cladding delamination (in clad sleeves), indicating a more ductile and predictable failure mechanism.
• The load-displacement curves show a distinct yielding plateau followed by strain hardening, with ultimate displacement capacities exceeding 20 mm for properly designed connections.
• The cladding layer thickness of 3-5 mm provides optimal improvement in tensile capacity without excessive cost or processing complexity.

Stress Analysis and Load Transfer Mechanism

The load transfer in a CFSGC connection involves multiple mechanisms:

1. Bearing pressure: Direct contact pressure between the grout and the inner surface of the sleeve.
2. Friction: Shear resistance at the sleeve-grout interface due to radial confinement.
3. Bond stress: Adhesive and mechanical interlock between the grout and the cladding surface.
4. Cladding reinforcement: The hardfacing layer increases the local yield strength at the sleeve inner surface, delaying yielding and improving load capacity.

The presence of the cladding layer modifies the stress distribution in the sleeve wall. In a conventional steel tube, the maximum hoop stress occurs at the inner surface where the grout bearing pressure is highest. The cladding layer, being harder and stronger, redistributes this stress, reducing the peak stress in the base material and delaying the onset of yielding.

Design Recommendations and Practical Considerations

Based on the research findings and my experience with cladding applications, the following design recommendations emerge:

• The cladding layer thickness should be at least 3 mm to provide meaningful improvement in bearing capacity.
• The transition layer between the base tube and hardfacing layer should be carefully designed to prevent cracking, especially for high-strength base materials.
• The grout strength should be matched to the sleeve capacity; a grout compressive strength of 60-80 MPa is recommended for high-capacity connections.
• The sleeve length-to-diameter ratio should be at least 3:1 to ensure adequate load transfer length.
• Preheating and PWHT may be necessary for thick-walled sleeves to prevent cracking in the base material during cladding.

Study Insights and Cross-Disciplinary Reflections

This research represents an innovative cross-disciplinary application of cladding technology from the pressure vessel and surface engineering domains into structural engineering. The concept of using a metallurgically bonded overlay layer to enhance the mechanical performance of a structural connection is elegant and practical.

From a quality assurance perspective, the inspection requirements for CFSGC connections should include:

• Visual examination and magnetic particle testing of the cladding welds.
• Ultrasonic testing to verify bond integrity between the cladding layer and base tube.
• Hardness testing to confirm the overlay material meets specifications.
• Pull-out testing on coupon specimens to verify grout-sleeve bond strength.

The study provides a strong foundation for the development of design codes and standards for CFSGC connections. Future work should address cyclic loading behavior, fatigue performance, and long-term durability under environmental exposure—topics that connect directly to the high-stress cyclic loading study (Topic 705) by the same research team.