Key Construction Technologies for Large-Span CFST Rigid Skeleton Arch Bridges

Literature Overview

This technical review paper by Zhao Jianye, Yuan Maojun, Zhou Xiaohang, and Meng Xin, published in Railway Engineering in 2024, summarizes the key construction technologies for large-span concrete-filled steel tube (CFST) rigid skeleton arch bridges. The authors, affiliated with the China Academy of Railway Sciences and several universities, provide a comprehensive overview of the construction challenges and solutions for this bridge type, which has seen continuous innovation and breakthroughs in span length in China.

Core Technical Content

CFST rigid skeleton arch bridges represent a significant advancement in bridge engineering, combining the advantages of steel tube construction (high strength, rapid erection) with the benefits of concrete filling (increased stiffness, improved fire resistance, enhanced load capacity). The rigid skeleton, formed by the steel tubes, provides the primary structural framework, while the concrete fill enhances the overall structural performance.

Construction Challenges

The authors identify four major construction challenges for large-span CFST rigid skeleton arch bridges:

Rigid Skeleton Assembly

The assembly of the rigid skeleton is the first critical construction phase. The steel tubes are typically erected in segments using a gantry crane or launching system. The assembly process requires:

• Precision measurement: The position and orientation of each segment must be controlled to within ±5 mm to ensure proper alignment and fit-up.
• Temporary bracing: Temporary bracing must be installed to stabilize the structure during assembly and to transfer loads to the temporary supports.
• Welding or bolting: Segment connections can be welded or bolted. Welded connections provide higher stiffness but require careful control of welding quality. Bolted connections allow for easier adjustment but may have lower stiffness.

From a welding perspective, the assembly of the rigid skeleton involves the welding of thick-walled steel tubes, which presents several challenges:

1. Thick section welding: Steel tubes with wall thicknesses of 30-50 mm require multi-pass welding with careful control of heat input and interpass temperature.
2. Positional welding: Welds may be in any position (flat, horizontal, vertical, overhead), requiring versatile welding procedures and skilled welders.
3. Residual stress control: The welding sequence must be planned to minimize residual stresses and distortion. Symmetrical welding from the center outward is recommended.
4. NDT requirements: 100% UT of all welds is mandatory, with acceptance criteria per ASME B31.3 or equivalent standards.

In-Tube Concrete Pumping

The pumping of concrete into the steel tubes is one of the most challenging aspects of CFST arch bridge construction. The challenges include:

• Long pumping distances: Concrete may need to be pumped over distances of 200-500 m, requiring high-pressure pumps and large-diameter hoses.
• High pumping pressures: The pumping pressure may exceed 10 MPa, requiring robust equipment and careful monitoring.
• Confined space: The steel tube provides a confined space that limits access for inspection and maintenance.
• Segregation risk: The high pumping pressure and long pumping distance increase the risk of concrete segregation, leading to non-uniform strength distribution.

To address these challenges, the following measures are recommended:

• Concrete mix design: The concrete mix should be designed for high workability and low segregation risk. Self-compacting concrete (SCC) is often used, with flow values of 280-320 mm and no slump.
• Pumping equipment: High-capacity pumps with flow rates of 100-200 m³/h and pressures of 15-25 MPa are typically used.
• Pumping sequence: The pumping sequence should be planned to ensure uniform concrete placement and to minimize segregation. Sequential pumping from the lowest point upward is recommended.
• In-process monitoring: The pumping process should be monitored using pressure gauges, flow meters, and temperature sensors to detect anomalies and ensure quality.

Outer Concrete Pouring

The outer concrete, which encases the steel tubes to provide additional stiffness and fire protection, is typically a large-volume pour that requires careful planning:

• Volume: The outer concrete volume can be in the range of 10,000-50,000 m³ for large-span bridges.
• Placement rate: The placement rate may need to be 100-200 m³/h to ensure continuous placement and avoid cold joints.
• Thermal control: The heat of hydration can cause internal temperatures to exceed 70°C, leading to thermal cracking. Thermal control measures include cooling pipes, retarders, and staged placement.
• Curing: The large volume of concrete requires extended curing periods, typically 14-28 days, with continuous moisture maintenance.

Curing and Quality Control

The curing of both in-tube and outer concrete is critical for achieving the design strength and durability. The authors emphasize the importance of:

• Temperature monitoring: Internal temperature sensors should be installed to monitor the temperature profile and detect anomalies.
• Moisture maintenance: The concrete surface should be kept moist for a minimum of 14 days to prevent drying shrinkage and cracking.
• Thermal control: Cooling pipes, retarders, and staged placement should be used to control the temperature rise and prevent thermal cracking.
• Quality verification: Non-destructive testing (NDT) methods such as UPV, rebound hammer, and GPR should be used to verify concrete quality.

Construction Quality Control Measures

The authors propose a comprehensive quality control framework for CFST rigid skeleton arch bridge construction:

Key Questions and Reflections

A significant question arising from this paper is how to ensure the quality of in-tube concrete in large-span CFST arch bridges. The authors acknowledge the challenges of pumping and curing but do not provide specific solutions for quality verification. In practice, engineers have relied on a combination of in-place testing, comparison specimens, and load testing, but the uncertainty associated with these methods can be significant.

Another reflection is that the construction of large-span CFST rigid skeleton arch bridges is a multi-disciplinary challenge that requires the integration of steel pipe manufacturing, welding, concrete technology, structural engineering, and construction management. The authors' framework of construction challenges and solutions provides a useful starting point, but the successful execution of these projects requires close collaboration between all disciplines.

Study Insights and Implications for Practice

This paper provides a valuable overview of the construction technologies for large-span CFST rigid skeleton arch bridges. The identification of key construction challenges and the proposal of corresponding quality control measures give engineers a structured approach to project planning and execution. The emphasis on welding quality, concrete quality, and thermal control highlights the critical factors that determine the success of these projects.

For steel pipe manufacturers and welding engineers, the key takeaway is that the quality of the steel tubes and their connections is a prerequisite for achieving the design performance of the bridge. The steel tubes must be manufactured to tight dimensional tolerances, welded to high quality standards, and assembled with precision to ensure proper alignment and fit-up.

The paper also underscores the importance of innovation in construction technology. The authors mention the potential applications of automated welding, self-deforming concrete, and multi-parameter monitoring as future directions for CFST arch bridge construction. Engineers should embrace these innovations and contribute to the development of new technologies that can improve the quality, efficiency, and sustainability of CFST arch bridge construction.