Zhuojin Pipe Fitting Co., LtdHydration Heat Temperature Field and Temperature Stress Analysis of CFST Arch Bridge Ribs
Literature Overview
This paper, published in Bridge Construction in 2010 by Xuan Jiming, Xiang Huawei, and Lu Keqi from Zhejiang University of Technology, focuses on a practical engineering problem encountered during the construction of the Changshan Nanmenxi Bridge, a CFST suspension arch bridge. The research specifically addresses the thermal effects of concrete hydration in the arch ribs during winter construction, when the temperature gradient between the hydration heat and the ambient environment is at its maximum. The authors employed the LUSAS finite element software to analyze the hydration heat temperature field and resulting temperature stresses in the arch ribs, which feature a bundled CFST structural configuration with a relatively high concrete proportion and thin steel tubes.
Core Technical Content
The Changshan Nanmenxi Bridge presents a unique engineering challenge due to the combination of several factors: the bundled CFST arch rib design, the high concrete volume fraction within the section, the relatively thin steel tubes, and the winter construction conditions. Each of these factors contributes to the severity of the thermal problem.
The hydration heat generated during concrete curing is a well-known phenomenon in mass concrete construction. For ordinary Portland cement, the peak hydration heat can reach approximately 150-200 J/g, with the peak temperature occurring 2-3 days after concrete placement. In CFST arch ribs, the steel tube acts as a heat sink, absorbing some of the hydration heat and reducing the peak temperature in the concrete core. However, the steel tube also creates a thermal barrier that can lead to significant temperature gradients between the concrete core and the steel tube surface.
Thermal Analysis Parameters
| Parameter | Typical Value | Influence on Temperature Stress |
|---|---|---|
| Concrete hydration heat rate | 150-200 J/g (OPC) | Higher rate increases peak temperature and gradient |
| Ambient temperature (winter) | -5 to 5°C | Larger temperature difference increases thermal stress |
| Steel tube thickness | 6-12 mm | Thinner tubes reduce heat absorption capacity |
| Concrete volume fraction | 60-80% of section | Higher fraction increases total heat generation |
| Concrete thermal conductivity | 1.7-2.5 W/(m·K) | Lower conductivity increases internal gradient |
| Steel thermal conductivity | 45-60 W/(m·K) | High conductivity creates thermal bridge effect |
| Adiabatic temperature rise | 40-60°C | Primary driver of thermal stress |
Finite Element Analysis Methodology
The LUSAS finite element software was employed to model the three-dimensional temperature field within the arch rib during the concrete curing process. The analysis considered the time-dependent heat generation from hydration, the heat conduction through the concrete and steel tube, and the heat exchange with the ambient environment through the steel tube surface.
The temperature field analysis revealed that the maximum temperature gradient occurs between the concrete core and the steel tube surface, particularly in the early stages of curing when the hydration heat generation rate is highest. In winter construction conditions, the ambient temperature is significantly lower than the concrete core temperature, creating a large temperature difference across the steel tube wall.
The temperature stress analysis indicated that the combination of the internal temperature gradient and the external temperature difference creates complex stress states within the arch rib. The concrete core experiences compressive stresses due to thermal expansion, while the steel tube experiences tensile stresses due to the constraint imposed by the expanding concrete. These stresses can be significant enough to cause cracking in the concrete or local buckling of the steel tube.
Engineering Practice Implications
The findings of this research have direct implications for the construction of CFST arch bridges, particularly in cold climate regions. Several construction measures can be implemented to mitigate the thermal effects:
- Concrete mixture optimization: Using low-heat cement, supplementary cementitious materials (fly ash, slag), and controlled concrete mixing temperature to reduce the peak hydration heat.
- Steel tube preheating: Preheating the steel tube before concrete casting to reduce the initial temperature difference between the steel tube and the concrete.
- Insulation of the steel tube exterior: Applying thermal insulation to the exterior of the steel tube to reduce heat loss to the ambient environment and minimize the temperature gradient.
- Controlled curing rate: Using controlled curing methods to manage the rate of hydration heat generation and reduce the peak temperature.
- Temperature monitoring: Implementing real-time temperature monitoring systems to track the temperature field within the arch rib and adjust construction measures as needed.
Construction Sequence Considerations
The construction sequence of the arch ribs is also critical in managing thermal effects. For bundled CFST arch ribs, the concrete casting sequence should be planned to minimize the temperature differential between adjacent ribs. Sequential casting of the ribs, with sufficient curing time between each rib, allows the earlier-cast ribs to cool before the subsequent ribs are filled, reducing the thermal interaction between ribs.
The timing of steel tube installation relative to concrete casting is another important consideration. If the steel tubes are installed and the ribs are filled with concrete in a single operation, the thermal effects are contained within each rib. However, if the steel tubes are installed first and the concrete is cast later, the steel tubes may experience thermal cycling from the ambient environment before concrete filling, potentially affecting their dimensional accuracy and residual stress state.
Study Insights and Reflections
From a metallurgical perspective, the thermal effects of concrete hydration on the steel tube are of considerable interest. The steel tube undergoes significant temperature changes during the concrete curing process, which can induce thermal stresses and potentially alter the microstructure of the steel near the concrete-steel interface. For high-strength steel tubes, the thermal exposure during concrete curing may affect the mechanical properties of the steel, particularly if the temperature exceeds the critical range for microstructural transformation.
The research also highlights the importance of considering the interaction between the steel tube and the concrete in thermal analysis. The steel tube acts as both a heat sink and a thermal conductor, and its behavior depends on the steel tube thickness, the thermal contact resistance at the steel-concrete interface, and the ambient temperature conditions. Accurate modeling of these interactions is essential for predicting the temperature field and thermal stresses within the arch rib.
The practical significance of this research extends beyond the specific case of the Changshan Nanmenxi Bridge to all CFST bridge applications in cold climate regions. As the demand for CFST bridge structures continues to grow, particularly in high-speed rail and highway infrastructure, the thermal effects of concrete hydration must be carefully considered in the design and construction of these structures.
The research also underscores the importance of construction monitoring in CFST bridge projects. Real-time temperature monitoring, combined with predictive thermal analysis, enables engineers to identify potential thermal problems before they cause structural damage. This proactive approach to construction management is essential for ensuring the quality and durability of CFST bridge structures.
Summary
This research by Xuan Jiming, Xiang Huawei, and Lu Keqi provides valuable insights into the thermal effects of concrete hydration in CFST arch bridge ribs, particularly under winter construction conditions. The finite element analysis using LUSAS software demonstrated that significant temperature gradients and thermal stresses can develop within the arch ribs, with the combination of high concrete volume fraction, thin steel tubes, and cold ambient temperatures exacerbating the problem. The research offers practical guidance for construction measures to mitigate thermal effects, including concrete mixture optimization, steel tube preheating, insulation, and controlled curing. The findings are of broad applicability to CFST bridge construction in cold climate regions and emphasize the importance of thermal analysis and construction monitoring in ensuring the quality and durability of CFST bridge structures.