Shrinkage Deformation and Shrinkage Stress of Steel Tube Concrete Arch

Literature Overview

This paper by Chen Baochun and Lai Xiuying, published in 2016 in the Journal of the Railway Society of China, addresses a critical but often overlooked aspect of steel tube concrete (CFST) arch bridge design: the shrinkage deformation and resulting shrinkage stresses in the concrete infill. Funded by the National Natural Science Foundation of China (Project U1305245), the research was conducted at Fuzhou University's College of Civil Engineering and provides experimental data and analytical models for predicting shrinkage behavior in CFST members with varying parameters.

Experimental Program and Test Parameters

The experimental program consisted of 11 steel tube concrete specimens and 2 solid concrete sealed specimens, with the following parameters systematically varied: concrete strength grade, fly ash content, steel tube diameter, and steel ratio. The test specimens were monitored over time to capture the time-dependent shrinkage deformation behavior of the CFST members.

Key Experimental Findings

The test results revealed clear trends in the shrinkage deformation behavior of steel tube concrete members. Shrinkage deformation decreases with increasing fly ash content, increases with higher concrete strength grades, and decreases with higher steel ratios. The steel tube diameter has a relatively minor influence on the shrinkage deformation.

These findings have practical significance for material selection in CFST arch bridge design. The use of fly ash as a supplementary cementitious material not only improves the durability and long-term strength of the concrete but also reduces shrinkage deformation, which is beneficial for minimizing shrinkage-induced stresses. However, the trade-off with concrete strength grade must be carefully considered, as higher strength grades produce greater shrinkage.

Shrinkage Prediction Model Evaluation

The researchers evaluated several commonly used shrinkage prediction models against the experimental data. The ACI 209R-92 model demonstrated relatively high prediction accuracy for CFST members with fly ash content not exceeding 20%. When the fly ash content exceeds 20%, the ACI 209R-92 model overestimates the shrinkage, suggesting that model modification is necessary for high-fly-ash concrete applications.

The CEB-FIP MC78 and CEB-FIP MC90 models produced prediction results that were significantly lower than the experimental values, indicating that these models are not suitable for CFST members without significant modification. This finding is important because many international design codes reference these models, and their inaccuracy for CFST applications could lead to underestimation of shrinkage stresses.

Shrinkage Stress Analysis in CFST Arch Bridges

The shrinkage stress analysis for CFST arch bridges revealed that shrinkage self-stress is relatively large and should be considered in design calculations, while shrinkage secondary stress is relatively small and can be neglected in preliminary design stress estimation. This distinction between self-stress and secondary stress is important for practical design methodology.

The commonly used equivalent cooling method for concrete arch shrinkage action is not recommended for CFST arch design. The reasons are twofold: first, the shrinkage deformation range of CFST arches is large and variable, making it difficult to determine a reasonable equivalent cooling value; second, the equivalent cooling method cannot calculate the relatively large shrinkage self-stress. This finding has direct implications for design code provisions and engineering practice.

Engineering Practice Implications

For steel pipe manufacturers supplying tubes for CFST arch bridges, the research highlights the importance of consistent tube diameter and wall thickness, as these dimensions directly affect the steel ratio and consequently the shrinkage behavior. Dimensional tolerances should be maintained within tight specifications to ensure predictable shrinkage performance.

From a concrete technology perspective, the recommendation to limit fly ash content to 20% or less for reliable shrinkage prediction using standard models is a practical guideline for mix design. When higher fly ash content is desired for economic or environmental reasons, the ACI 209R-92 model should be modified to account for the reduced shrinkage, or alternative prediction methods should be developed based on project-specific test data.

The finding that shrinkage self-stress is significant reinforces the need for proper construction sequencing in CFST arch bridges. The timing of concrete pouring relative to steel tube fabrication and erection, as well as the curing conditions, all influence the magnitude of shrinkage stresses that develop in the structure.

Study Insights and Conclusions

This research provides essential experimental data and analytical guidance for the prediction and management of shrinkage effects in steel tube concrete arch bridges. The clear identification of parameter influences on shrinkage deformation, the evaluation of prediction model accuracy, and the recommendation against the equivalent cooling method for CFST applications collectively contribute to more accurate and reliable design practices. Engineers should adopt the ACI 209R-92 model with appropriate modifications for fly ash-containing concretes, account for shrinkage self-stress in detailed design, and maintain tight dimensional controls on steel tubes to ensure consistent steel ratios and predictable shrinkage behavior throughout the service life of CFST arch structures.