Hydration Heat Stress Analysis of CFST Under Low-Temperature Pouring Conditions

Literature Overview

This research by Zhou Dawei, Deng Nianchun, and Guo Xiao from Guangxi University, published in the Journal of Railways Science and Engineering in 2020 (Vol. 17, No. 11, pp. 2807-2815), investigates the hydration heat stress behavior of concrete-filled steel tube (CFST) arch ribs under low-temperature pouring conditions. Funded by multiple sources including the National Natural Science Foundation of China (51868006, 51878186, 51738004), the China Railway Corporation Science and Technology Program (2017G006-B), and the Guangxi Natural Science Foundation (2018GXNSFAA138067), the study is based on the engineering background of the Yarlung Zangbo River Grand Bridge on the Lhasa-Nyingchi Railway in the Qinghai-Tibet Plateau region. The research employed a constant-temperature experimental chamber to simulate low-temperature conditions and conducted long-term continuous temperature field monitoring on a full-scale 1.6 m diameter CFST specimen.

Core Technical Content

Experimental Setup and Conditions

The experimental program was designed to replicate the challenging construction conditions of the Qinghai-Tibet Plateau, where ambient temperatures can be significantly below zero during the construction season. Key experimental parameters included:

Key Research Findings

1. Hydration Induction Period: The concrete with admixtures exhibited an induction period of approximately 4 hours. During this period, the hydration reaction is minimal, and the temperature and thermal stress changes are relatively small. This induction period has a significant impact on the overall thermal-mechanical behavior of the CFST member.
2. Residual Strain After Hydration: After the hydration process is complete, significant residual strains remain in both the longitudinal and radial directions. These residual strains have a substantial impact on the closing temperature (合龙温度), which is a critical parameter for the structural integrity of arch ribs.
3. Hydration Heat Model: The introduction of an equivalent age concept into a hyperbolic hydration heat model provides a good representation of the hydration temperature distribution in CFST members. The equivalent age approach accounts for the temperature-dependent rate of hydration, which is particularly important under low-temperature conditions where the hydration rate is significantly reduced.
4. Thermal Stress Development: The accelerated development of concrete mechanical properties due to hydration temperature rise leads to significant radial and hoop stresses in the confined concrete. The calculation of hydration temperature effects must account for the influence of temperature on concrete mechanical properties.
5. Critical Control Period: The control of hydration temperature stresses should focus on the initial cooling stage, as this is when the thermal gradients are most severe and the concrete has not yet developed sufficient strength to resist thermal cracking.

Equivalent Age Model

The paper introduces an equivalent age concept to account for the temperature-dependent hydration rate:

The equivalent age is defined as a function of the actual age and the temperature history, allowing the use of standard hydration heat models developed under standard curing conditions to predict behavior under non-standard temperature conditions. The hyperbolic model form captures the characteristic hydration heat release curve, with the equivalent age scaling the time axis to reflect the actual temperature conditions.

Engineering Practice Implications

Low-Temperature Construction Challenges

The construction of CFST arch ribs in cold regions presents several unique challenges:

• Delayed hydration: Low ambient temperatures slow down the hydration process, extending the time required to achieve design strength.
• Thermal gradients: The difference between the internal hydration temperature and the external cold environment creates significant thermal gradients, leading to thermal stresses.
• Steel tube cooling: The steel tube acts as a heat sink, cooling the concrete core and potentially causing thermal cracking at the steel-concrete interface.
• Freeze risk: If the concrete temperature drops below freezing during the early age, ice crystal formation can cause permanent damage to the concrete microstructure.

Mitigation Measures

Based on the research findings, the following mitigation measures are recommended for low-temperature CFST construction:

1. Admixture optimization: Use of appropriate low-temperature admixtures to control the induction period and hydration rate, ensuring adequate early strength development.
2. Insulation: Application of thermal insulation to the exterior of the steel tube to reduce heat loss and minimize thermal gradients.
3. Temperature monitoring: Continuous monitoring of internal and external temperatures during the hydration period to detect and respond to adverse thermal conditions.
4. Pouring timing: Scheduling concrete pouring during the warmest part of the day to maximize the initial concrete temperature.
5. Heating measures: Use of external heating or heated water for concrete mixing and placement in extremely cold conditions.

Implications for Steel Tube Manufacturing

For CFST arch ribs constructed in low-temperature environments, the following steel tube manufacturing considerations are important:

• Thermal conductivity: The steel tube's thermal conductivity affects the rate of heat transfer from the concrete core to the environment, influencing the thermal gradient development.
• Surface treatment: Surface treatments that reduce heat transfer (such as thermal insulation coatings) can be applied to the exterior of the steel tube to minimize heat loss.
• Dimensional accuracy: Precise dimensional control of the steel tube is essential to ensure uniform concrete cover and consistent thermal-mechanical behavior.
• Material selection: Steel grades with appropriate thermal expansion coefficients should be selected to minimize thermal mismatch between the steel tube and concrete core.

Key Technical Insights

The finding that the initial cooling stage is the critical period for thermal stress control is particularly important for construction planning. During this stage, the concrete has not yet developed significant strength, making it vulnerable to thermal cracking. Construction teams should implement enhanced monitoring and protective measures during this period.

The equivalent age concept provides a practical tool for predicting hydration behavior under non-standard temperature conditions. This approach allows engineers to use standard hydration models while accounting for the actual temperature history, simplifying the analysis of complex thermal-mechanical problems in CFST construction.

The observation that significant residual strains remain after hydration completion has important implications for the design of arch ribs. These residual strains affect the closing temperature and must be accounted for in the structural analysis to ensure proper fit-up and load distribution in the completed arch structure.

Summary

This paper provides valuable experimental data and analytical tools for understanding and predicting the hydration heat stress behavior of CFST arch ribs under low-temperature pouring conditions. The introduction of the equivalent age concept into the hyperbolic hydration heat model offers a practical approach for predicting thermal behavior under non-standard conditions. The identification of the initial cooling stage as the critical period for thermal stress control provides clear guidance for construction planning and quality control. Engineers working on CFST structures in cold regions should incorporate these findings into their design and construction practices, implementing appropriate monitoring, insulation, and temperature control measures to ensure the structural integrity and long-term performance of CFST arch ribs in challenging environmental conditions.