Zhuojin Pipe Fitting Co., LtdCreep Coefficient Prediction Model for Concrete-Filled Steel Tubes
Literature Overview
The paper by Cao Guohui, Zhang Wang, Hu Jiaxing, Zhang Kai, and Zhang Sheng, published in the Journal of Central South University (Science and Technology) in 2016, presents a creep coefficient prediction model for concrete-filled steel tubes (CFST) based on 870 days of experimental testing on cylindrical specimens. This research addresses a fundamental but often overlooked aspect of CFST structural design: the time-dependent deformation behavior that significantly affects long-term serviceability and ultimate limit state design.
Experimental Foundation
The experimental program consists of 8 cylindrical specimens subjected to sustained loading for 870 days under controlled indoor environmental conditions. This duration is particularly significant because it captures the transition from early-age rapid creep to long-term stabilized creep behavior, providing data that extends beyond the typical 180-365 day testing periods found in most literature.
Key Experimental Observations
| Time Period | Creep Behavior Characteristic | Engineering Significance |
|---|---|---|
| 0-30 days | Rapid initial creep development | Critical for early-age formwork and support removal decisions |
| 30-100 days | Transitional phase with decreasing creep rate | Primary period for construction phase deformations |
| After 100 days | Creep deformation stabilizes | Long-term deformation prediction becomes more reliable |
| 100-870 days | Minimal additional creep | Confirms earlier stabilization trend |
Theoretical Framework
The proposed model is built upon two theoretical foundations:
- Hereditary flow theory of concrete creep: This approach considers the time-varying nature of concrete's viscoelastic response, accounting for aging effects and the superposition of creep from different loading histories.
- Multi-axial stress creep theory: Recognizes that the concrete within a steel tube is subjected to a triaxial stress state, which fundamentally alters creep behavior compared to uniaxially loaded conventional concrete.
Creep Decomposition Model
The model decomposes total CFST creep deformation into three components:
| Component | Nature | Time Dependency | Physical Mechanism |
|---|---|---|---|
| Recoverable delayed elastic deformation | Elastic | Recovers upon unloading | Viscoelastic response of cement paste |
| Irrecoverable initial rapid plastic deformation | Plastic | Immediate and early-age | Microcrack initiation and expansion |
| Irrecoverable viscous flow deformation | Viscous | Long-term progressive | Sliding between hydration products |
Technical Analysis and Interpretation
Comparison with Conventional Concrete
The most striking finding is that CFST creep deformation stabilizes significantly earlier than conventional concrete. This acceleration is attributed to the confining pressure exerted by the steel tube, which:
- Reduces the development of microcracks in the concrete core
- Increases the effective stress state, promoting more uniform load distribution
- Limits lateral expansion, thereby reducing the driving force for time-dependent deformation
This earlier stabilization has direct implications for construction scheduling. In CFST structures, formwork and temporary supports can potentially be removed earlier than in conventional RC structures, leading to cost savings in temporary works.
Model Validation and Applicability
The model's simplicity and computational efficiency make it suitable for practical engineering applications. The formula structure allows direct integration into structural analysis software through user-defined material subroutines or equivalent stiffness reduction factors.
Engineering Practice Applications
Welding and Fabrication Considerations
For CFST members in practical construction, the creep behavior interacts with fabrication processes in several important ways:
- Post-welding creep: Welding-induced residual stresses in the steel tube create additional stress concentrations that influence the creep response of the confined concrete. The heat-affected zone (HAZ) of welded joints may exhibit different creep characteristics due to microstructural changes.
- Longitudinal weld effects: In LSAW or UOE manufactured CFST members, the longitudinal weld introduces a material discontinuity that can affect the uniformity of confining pressure, potentially creating localized creep variations.
- Connection details: Welded joints between CFST columns and steel beams introduce rigid connections that redistribute stresses over time as creep develops, potentially altering the intended load path.
Design Code Implications
Current design codes for CFST structures, including GB 51248 (Chinese code for concrete-filled steel tube structures) and ASCE 36 provisions for composite construction, incorporate simplified creep reduction factors. The proposed model offers a more refined approach that could potentially be adopted in future code revisions.
The practical design implications include:
- More accurate prediction of long-term deflections in CFST beams and columns
- Improved assessment of prestress losses in prestressed CFST members
- Better evaluation of time-dependent stress redistribution in indeterminate CFST frames
- Enhanced understanding of fatigue life prediction where creep contributes to cumulative damage
Key Technical Points for Practitioners
The stabilization of creep at approximately 100 days is a critical threshold for construction planning. Engineers should consider this timeframe when:
- Determining the minimum age before applying full service loads to CFST structures
- Planning the sequence of construction activities in multi-story CFST buildings
- Evaluating the long-term performance of CFST structures in applications sensitive to time-dependent deformation, such as precision manufacturing facilities or sensitive equipment foundations
The three-component creep decomposition provides a physically meaningful framework that can be extended to account for environmental factors such as temperature variations and humidity changes, which are particularly relevant for CFST structures in aggressive environments.
Study Insights and Outlook
This research provides a solid experimental and theoretical foundation for understanding CFST creep behavior. The 870-day testing duration is commendable and provides confidence in the long-term predictions. The proposed model's simplicity and physical basis make it practical for engineering application while maintaining acceptable accuracy.
A limitation worth noting is the indoor environmental conditions of the testing. Real-world CFST structures are exposed to temperature cycling, humidity variations, and potentially aggressive chemical environments that could accelerate or modify creep behavior. Future research should extend these studies to outdoor exposure conditions and consider the combined effects of creep and corrosion in CFST members.
The earlier stabilization of CFST creep compared to conventional concrete represents a significant advantage for construction efficiency and long-term serviceability. This finding should be communicated to designers and specifiers to encourage the adoption of CFST solutions in applications where time-dependent deformation is a critical concern.