Shrinkage Strain Calculation Model for Steel Tube Expansive Concrete

Literature Overview

This paper by Cao Guohui, Zhang Kai, Hu Jiaxing, and He Min from Hunan City University and Hunan University was published in the Journal of Central South University (Science and Technology) in 2015 (Volume 46, Issue 3, pp. 1094-1099). Funded by the Hunan Provincial Natural Science Foundation (14JJ4062) and the Hunan Provincial Higher Education Research Project (Key Project) (12A027), the study investigates the shrinkage behavior of expansive concrete within steel tubes and proposes a calculation model for the shrinkage strain of steel tube expansive concrete.

Core Technical Findings

The research conducted long-term shrinkage deformation tests on three concrete cylindrical specimens under indoor natural conditions. The test data were analyzed using least-squares nonlinear fitting to study the shrinkage mechanism of steel tube concrete. The key findings are summarized as follows:

The expansive agent effectively suppresses the early shrinkage strain of ordinary concrete (within 100 days), but its effect on late-stage shrinkage is minimal. For steel tube concrete, the expansive agent has a significant influence on the early shrinkage strain (within 300 days), with the early shrinkage strain remaining nearly constant. The shrinkage strain of steel tube concrete is primarily caused by the self-shrinkage of the core concrete, with the self-shrinkage strain being approximately 1×10⁻⁴.

Calculation Model Development

The proposed calculation model accounts for four key factors influencing the core concrete shrinkage: the expansive agent content, the concrete strength grade, the cement type, and the curing method. The model was developed based on the nonlinear fitting of experimental data and provides a means to predict the shrinkage deformation behavior of steel tube expansive concrete over time.

The self-shrinkage mechanism in steel tube concrete is fundamentally different from drying shrinkage. Self-shrinkage occurs due to the internal chemical shrinkage of the cement paste as hydration proceeds, leading to a reduction in the volume of the gel pores. In a steel tube environment, the absence of external moisture loss means that drying shrinkage is minimal, and the dominant shrinkage mechanism is self-shrinkage. The expansive agent partially compensates for this self-shrinkage by generating internal expansion through the hydration of expansive compounds such as calcium sulfoaluminate (CSA) or calcium hydroxide (CH).

Engineering Practice Implications

For engineers designing steel tube concrete structures, particularly in bridge and tunnel applications where the steel tube creates a sealed environment, the shrinkage behavior is a critical design consideration. The self-shrinkage of approximately 1×10⁻⁴, while seemingly small, can lead to significant internal stresses in the steel tube and the concrete interface over time. The proposed calculation model allows for the prediction of these stresses during the design phase, enabling appropriate countermeasures such as the selection of expansive agents with suitable expansion rates, the optimization of curing protocols, and the design of the steel tube-to-concrete interface to accommodate differential deformations.

From a steel tube fabrication perspective, the shrinkage of the core concrete can induce compressive stresses in the steel tube, which may affect the long-term structural performance. The welding joints at the steel tube ends and any internal stiffeners must be designed to accommodate these stresses without cracking or fatigue failure. The use of expansive concrete to pre-stress the steel tube through internal expansion is a beneficial approach, but the timing and magnitude of the expansion must be carefully controlled to avoid excessive stresses during the early hardening stage.

Study Insights and Reflections

This research addresses a practical challenge in steel tube concrete engineering that is often overlooked in design practice: the long-term shrinkage behavior of the core concrete in a sealed steel tube environment. The distinction between drying shrinkage and self-shrinkage is crucial, as the steel tube prevents moisture loss to the environment, making self-shrinkage the dominant mechanism. The proposed calculation model, which incorporates multiple factors including expansive agent content, concrete strength grade, cement type, and curing method, represents a comprehensive approach to predicting shrinkage behavior. I find the finding that the expansive agent's effect on early shrinkage is significant but its effect on late shrinkage is minimal to be particularly important for long-term durability assessments. The model can be integrated into finite element analyses of steel tube concrete structures to predict time-dependent stresses and deformations, enhancing the accuracy of long-term serviceability predictions.