Hydration Heat Temperature Field and Temperature Stress Analysis of CFST Arch Bridge Ribs

Literature Overview

This paper by Xuan Jiming, Xiang Huawei, and Lu Keqi, published in Bridge Construction (2010, Vol. 40, No. 3, pp. 29-32), presents a detailed thermal analysis of concrete-filled steel tube (CFST) arch ribs during construction. The study focuses on the Changshan Nanmenxi Bridge, a CFST basket-handle arch bridge where the arch rib construction was conducted during winter. The authors used LUSAS finite element software to analyze the hydration heat temperature field and temperature stresses in the arch rib, which employs a bundled CFST structural system with a high concrete-to-steel ratio and relatively thin steel tubes.

Core Technical Analysis

The hydration heat problem in CFST arch ribs is particularly critical for several reasons. First, the large volume of concrete in the bundled CFST system generates significant heat during hydration, creating a steep temperature gradient between the interior and exterior of the section. Second, the thin steel tubes provide limited thermal mass to moderate the temperature rise. Third, the winter construction conditions introduce additional thermal gradients from ambient temperature differences.

The finite element analysis considered the coupled thermal-mechanical behavior of the CFST arch rib, accounting for:

• The time-dependent heat generation from concrete hydration, which follows an exponential decay pattern with the majority of heat released in the first 72 hours after placement.
• The thermal conductivity and specific heat of both concrete and steel, which differ significantly and create complex heat transfer patterns.
• The boundary conditions imposed by the winter ambient temperature, including wind effects and solar radiation.
• The mechanical response of the steel tube and concrete to the thermal gradients, including differential thermal expansion.

Temperature Field and Stress Results

The analysis revealed several critical findings regarding the temperature field and stress distribution in the CFST arch rib:

The temperature gradient in the arch rib concrete was found to be significantly larger than in conventional reinforced concrete structures. This is attributed to the bundled CFST configuration, where multiple steel tubes are arranged within a larger concrete-filled section, creating a geometry that traps heat and creates steep thermal gradients. The thin steel tubes, while providing structural confinement, do not contribute sufficient thermal mass to mitigate the temperature rise.

Construction Process Considerations

The winter construction conditions posed additional challenges. The ambient temperature during the construction period was low, creating a large temperature differential between the freshly placed concrete and the environment. This differential exacerbated the temperature gradient and increased the risk of thermal cracking in the concrete.

From a steel pipe manufacturing and welding quality control perspective, several important considerations emerge:

• The steel tubes used in the bundled CFST system must have uniform wall thickness to ensure consistent thermal and mechanical behavior. Variations in wall thickness would create localized stress concentrations under thermal loading.
• The welding quality of the steel tube joints is critical because weld defects would act as stress concentrators under the combined action of structural loads and temperature stresses. Any lack of fusion, porosity, or crack in the weld would be particularly detrimental under thermal cycling.
• The surface quality of the steel tubes should be controlled to ensure proper bonding with concrete. Surface irregularities or mill scale would reduce the interfacial bond strength and potentially allow for differential thermal movement between steel and concrete.

Engineering Practice Implications

The study's findings have direct implications for the construction of CFST arch bridges:

1. Temperature monitoring: Real-time temperature monitoring should be implemented during concrete placement and curing to track the temperature field evolution and detect potential thermal cracking risks.
2. Thermal management measures: Insulation blankets, cooling pipes, or controlled curing methods should be employed to reduce the temperature gradient and prevent thermal cracking.
3. Construction sequencing: The concrete placement sequence should be optimized to minimize the thermal effects, such as placing concrete in layers with controlled intervals to allow heat dissipation between layers.
4. Steel tube quality control: The steel tubes should be inspected for dimensional accuracy, surface quality, and weld integrity before installation to ensure they can withstand the combined structural and thermal loads.
5. Post-construction monitoring: The temperature stresses induced during construction may persist as residual stresses in the completed structure, affecting the long-term structural behavior and fatigue life.

Key Reflections and Insights

The thermal analysis of CFST arch ribs reveals a fundamental challenge in the construction of large-scale CFST structures: the need to manage thermal effects that are inherent to the construction process but are often overlooked in design. The bundled CFST configuration, while structurally efficient, creates a thermal environment that is significantly more challenging than conventional reinforced concrete construction.

I find particularly noteworthy the interaction between the steel tube geometry and the thermal behavior. The thin steel tubes, which are economical and lightweight, create a geometry that is thermally unfavorable because they do not provide sufficient thermal mass to moderate the concrete temperature. This creates a design dilemma: thicker steel tubes would improve thermal performance but increase material costs, while thinner tubes reduce costs but increase thermal risks. The optimal solution requires a careful balance between structural efficiency, thermal management, and economic considerations.

The study also highlights the importance of construction sequencing and scheduling in CFST bridge construction. The choice of construction season has a profound impact on the thermal behavior of the structure. Winter construction, as in the case study, introduces additional thermal challenges that must be addressed through appropriate construction measures. Summer construction would present different challenges, including higher ambient temperatures and potential thermal cracking from excessive heat.

Study Value and Outlook

This research provides a valuable framework for the thermal analysis of CFST arch bridge ribs during construction. The developed finite element model can be adapted to different bridge geometries and construction conditions, providing a practical tool for thermal management in CFST bridge construction. For steel pipe manufacturers, the study underscores the importance of providing steel tubes with consistent quality and dimensional accuracy, as variations in tube properties would directly affect the thermal performance of the completed structure. Future research should extend this work to include long-term thermal cycling effects, the interaction between thermal stresses and fatigue, and the development of optimized thermal management strategies for different construction seasons and climates.