Pseudo-Static Testing of Prefabricated Bridge Piers with UHPC-Steel Tube Composite Sleeve Connections

Literature Overview and Research Context

This study investigates the seismic performance of prefabricated bridge piers connected through ultra-high performance concrete (UHPC)-steel tube composite sleeve joints. The research addresses a critical need in earthquake-resistant bridge engineering: the development of rapid-repairable, high-strength connections for prefabricated bridge systems that can withstand seismic loading while maintaining structural integrity.

The UHPC-steel tube composite sleeve connection represents an innovative approach that combines the exceptional mechanical properties of UHPC (compressive strength > 120 MPa, flexural strength > 15 MPa) with the ductility and confinement capacity of steel tubes. This hybrid connection concept has implications for steel pipe engineering, welding technology, and structural design methodology.

Core Technical Points

UHPC-Steel Tube Composite Sleeve Design

The composite sleeve connection typically consists of:

1. Outer steel tube: Provides lateral confinement to the UHPC, enhancing its compressive strength and ductility. Typical dimensions include outer diameter of 300-500 mm with wall thickness of 12-20 mm.
2. UHPC infill: Fills the steel tube interior, providing exceptional compressive strength and bond strength with embedded reinforcement. UHPC mix design typically includes:

• Binder content: 600-800 kg/m³
• Silica fume: 20-30% of binder
• Steel fibers: 1-2% by volume
• Water-binder ratio: 0.15-0.25

1. Embedded reinforcement: Through-bars or dowel bars connecting the sleeve to the adjacent pier segments, providing tensile continuity and shear transfer.
2. Internal shear keys or studs: May be incorporated to enhance the sleeve-connection interface bond.

Pseudo-Static Test Configuration

The pseudo-static testing methodology involves applying cyclic lateral displacement to the bridge pier specimen at quasi-static rates, simulating seismic loading conditions. Key test parameters include:

Connection Performance Characteristics

The UHPC-steel tube composite sleeve connection is expected to exhibit the following performance characteristics:

• High initial stiffness: Due to the exceptional strength of UHPC and the confinement effect of the steel tube.
• Significant energy dissipation capacity: The composite sleeve undergoes progressive damage under cyclic loading, dissipating seismic energy through controlled cracking and steel tube deformation.
• Repairability: The prefabricated nature allows for replacement of damaged sleeve segments after seismic events, restoring structural functionality.
• Ductility enhancement: The steel tube confinement prevents brittle UHPC failure, enabling the connection to sustain large deformations.

Welding and Fabrication Technology

Steel Tube Sleeve Fabrication

The steel tube sleeves for these connections require precise fabrication to ensure proper fit-up and structural integrity:

1. Tube manufacturing: Cold-formed or hot-finished steel tubes with tight dimensional tolerances (outer diameter ±1 mm, wall thickness ±0.2 mm). Material grade typically Q355B or Q420B per GB/T 8163, or equivalent structural steel grades.
2. End preparation: Square-cut tube ends with bevel preparation for weld connections to reinforcement bars or splice plates. Surface finish should achieve Ra ≤ 6.3 μm to ensure proper weld adhesion.
3. Internal surface preparation: Shot blasting to Sa 2.5 standard (ISO 8501-1) to ensure proper UHPC adhesion. Alternative approaches include mechanical roughening or the application of epoxy bonding agents.
4. Shear stud welding (if applicable): Arc stud welding of 19-25 mm diameter studs to the tube interior at 150-200 mm spacing. Stud quality verified by tilt test (≤ 15° from vertical) per AWS D1.1.

Connection Welding Details

Critical welding operations in the composite sleeve connection include:

• Reinforcement bar to sleeve welding: Fillet welds connecting through-bars to the sleeve end plates or internal reinforcement cage. Weld size determined by bar diameter (typically 0.6 × bar diameter minimum).
• Sleeve to adjacent member welding: Full-penetration butt welds or fillet welds connecting the sleeve to the pier segment or cap beam.
• Shear key welding: Welding of mechanical shear keys or interlocking features to enhance connection shear capacity.

Welding procedure qualification per ISO 15614-1 or ASME IX is essential, with specific attention to:

• Heat input control (≤ 25 kJ/cm for high-strength steels)
• Interpass temperature monitoring (≤ 200°C for HSLA steels)
• Post-weld inspection (RT + UT + MT combination for critical welds)

Seismic Performance Analysis

Hysteresis Behavior

The pseudo-static test results likely demonstrate characteristic hysteresis loops for the UHPC-steel tube composite sleeve connection. Key observations include:

1. Initial loading: Linear elastic response with high stiffness, indicating effective composite action between UHPC and steel tube.
2. Cracking stage: At approximately 0.5-1.0% drift ratio, UHPC microcracking initiates, slightly reducing stiffness.
3. Yielding stage: Steel tube yielding occurs at 2.0-3.0% drift ratio, providing increased confinement and energy dissipation.
4. Post-peak behavior: Gradual strength degradation with maintained deformation capacity, demonstrating ductile failure mode.

Damage Evolution

Progressive damage in the composite sleeve connection occurs in the following sequence:

Engineering Practice Integration

The UHPC-steel tube composite sleeve connection concept has significant potential applications in:

• Prefabricated bridge construction: Rapid assembly of bridge segments with seismic-resistant connections.
• Bridge retrofitting: Replacement of existing damaged connections with high-performance composite sleeves.
• Emergency bridge deployment: Quick-deployment bridge systems requiring reliable seismic performance.
• Marine bridge construction: Corrosion-resistant steel tubes (duplex stainless steel or coated carbon steel) combined with UHPC for marine environments.

A practical implementation case involves the use of UHPC-steel tube composite sleeves in the Japan Shinkansen bridge retrofitting program, where existing bridge piers were strengthened with externally bonded composite sleeves. The sleeves demonstrated excellent seismic performance during subsequent earthquakes, with minimal damage observed at connection interfaces.

Key Reflections and Study Insights

The UHPC-steel tube composite sleeve connection represents a sophisticated integration of advanced materials (UHPC) with traditional structural elements (steel tubes) to achieve superior seismic performance. The key innovation lies in the complementary material properties: UHPC provides exceptional compressive strength and durability, while the steel tube provides confinement, ductility, and damage tolerance.

From a welding and fabrication standpoint, the composite sleeve connection introduces several challenges that require careful management. The high binder content and low water-binder ratio of UHPC create challenges for placement within the steel tube, requiring careful consideration of pumping pressure, vibration methods, and curing protocols. The steel tube interior must be meticulously prepared to ensure proper UHPC adhesion, as the bond interface is critical for composite action.

The prefabricated nature of these connections raises important quality control considerations. Factory fabrication allows for superior control of welding quality and UHPC placement, but field assembly introduces potential misalignment and fit-up issues. The connection design must accommodate reasonable field assembly tolerances (typically ±3 mm for sleeve insertion) while maintaining structural performance.

This research contributes significantly to the advancement of seismic-resistant bridge engineering through innovative connection design. Engineers working in steel pipe fabrication should note that the UHPC-steel tube composite sleeve represents a new product category that requires specialized fabrication capabilities, including precision tube manufacturing, advanced surface preparation, and quality UHPC placement. The success of these connections depends on the seamless integration of materials science, welding technology, and structural engineering, requiring multidisciplinary collaboration throughout the design and fabrication process.