Finite Element Analysis of Hydration Heat Temperature Distribution in Dumbbell-Shaped CFST Sections

Literature Overview

The paper by Lin Chunjiao, Zheng Jialian, and Qin Rong from Guangxi University and the Guangxi Zhuang Autonomous Region Department of Transportation, published in China and Foreign Highway (Vol. 27, No. 4, 2007), presents a finite element analysis of the temperature field distribution in dumbbell-shaped concrete-filled steel tube (CFST) arch ribs under hydration heat effects. The study compares the thermal behavior of dumbbell-shaped sections with conventional circular sections and investigates the influence of section composition, steel tube diameter, web height, and wind speed on the temperature distribution. The research addresses a critical practical concern in large-span bridge construction where hydration heat can induce significant thermal stresses and deformations.

Technical Background

Hydration Heat in Mass Concrete

The hydration heat generated during concrete curing represents a major concern in mass concrete structures, including large CFST arch ribs used in bridge construction. The exothermic chemical reactions between cement and water release heat over time, with peak temperatures typically occurring within the first few days of curing. In CFST structures, the steel tube acts as a heat sink, absorbing and dissipating some of the hydration heat, but the confined geometry can also trap heat and create significant temperature gradients.

Dumbbell-Shaped Section Characteristics

The dumbbell-shaped CFST section consists of two circular steel tubes connected by a flat web plate, creating a distinctive cross-sectional geometry. This configuration offers structural advantages in terms of bending resistance and weight efficiency, but it also introduces complex thermal behavior due to the variation in concrete volume and steel-to-concrete interface along the section. The thermal analysis of such sections is more complex than for simple circular sections due to the non-uniform geometry.

Finite Element Analysis Methodology

Model Development and Assumptions

The finite element model was developed to simulate the three-dimensional temperature field distribution within the dumbbell-shaped CFST section under hydration heat loading. The model incorporated the geometric details of the section, including the circular tube diameters, web plate dimensions, and concrete fill geometry. Material properties for both steel and concrete were defined with temperature-dependent thermal conductivity and specific heat capacity to capture the nonlinear thermal behavior.

Boundary Conditions and Loading

The thermal analysis incorporated appropriate boundary conditions representing the environmental conditions during construction. External surface convection was modeled with a heat transfer coefficient that varied with wind speed, reflecting the enhanced cooling effect of wind on exposed surfaces. The initial temperature was set to ambient conditions, and the hydration heat was applied as a volumetric heat source with a time-dependent rate function based on cement hydration kinetics.

Parametric Study Results

The parametric study revealed several important findings regarding the thermal behavior of dumbbell-shaped CFST sections. The maximum temperature achieved in the dumbbell section was found to be higher than that in an equivalent circular section, attributed to the increased concrete volume and reduced surface-to-volume ratio in the web region. The steel tube diameter was identified as a significant factor influencing the temperature field, with larger diameters providing greater concrete volume and consequently higher peak temperatures.

Comparison with Circular Section

The comparison between dumbbell-shaped and circular CFST sections revealed distinct differences in temperature distribution patterns. The circular section exhibited more uniform radial temperature gradients, while the dumbbell section showed complex three-dimensional temperature fields with hot spots developing in the web region where heat dissipation is most restricted. The peak temperature in the dumbbell section exceeded that of the circular section, indicating a greater risk of thermal cracking in the web region.

Engineering Practice Implications

Thermal Stress and Cracking Risk

The elevated temperatures observed in dumbbell-shaped CFST sections have direct implications for thermal stress development and cracking risk. The temperature gradients between the core and surface of the section, as well as between the web and tube regions, generate differential thermal expansion that can induce tensile stresses exceeding the tensile strength of early-age concrete. Engineers must account for these thermal effects in the design and construction planning of large CFST arch ribs.

Construction Sequence Optimization

The findings from this study inform the optimization of construction sequences for CFST arch ribs. Timing of concrete placement, consideration of ambient conditions, and implementation of cooling measures such as embedded cooling pipes or surface cooling systems can be guided by the thermal analysis results. The identification of critical parameters such as tube diameter and wind speed allows for targeted mitigation strategies focused on the most influential factors.

Design Considerations for Dumbbell Sections

When designing structures with dumbbell-shaped CFST sections, engineers should incorporate thermal effects into the structural analysis, particularly for large-span bridges where the section dimensions are substantial. The thermal stresses may need to be considered in combination with structural loads, and the design may need to account for potential thermal cracking and its effects on long-term durability. The use of supplementary cementitious materials with lower hydration heat, such as fly ash or slag, can be considered as a material-based mitigation strategy.

Study Insights and Outlook

This paper addresses a practical and important aspect of CFST construction that is often underappreciated in design practice. The thermal behavior of CFST sections, particularly non-circular configurations, significantly influences construction quality and long-term performance. The finite element methodology presented provides a systematic approach to predicting temperature distributions and identifying critical parameters for thermal control. For engineers involved in the design and construction of large CFST structures, this research underscores the importance of integrating thermal analysis into the design process and implementing appropriate construction measures to mitigate thermal effects. The continued development of large-span bridge structures with innovative cross-sectional geometries will require ongoing research into the thermal-mechanical behavior of CFST sections under various environmental and construction conditions.