Creep Coefficient Prediction Model for Concrete-Filled Steel Tubes

Overview of the Study

This paper by Cao Guohui, Zhang Wang, Hu Jiaxing, Zhang Kai, and Zhang Sheng from Hunan City University, Changsha University of Science and Technology, and Hunan University presents a creep coefficient prediction model for concrete-filled steel tubes (CFST). The research is based on 870-day creep tests conducted on eight cylindrical specimens in a laboratory environment, combined with the hereditary flow theory of concrete creep and creep theory under multiaxial stress states. The model divides CFST creep deformation into three components: recoverable lag elastic deformation, irreversible initial rapid-flow plastic deformation, and irreversible viscous flow. Published in the Journal of Central South University in 2016, Volume 47, Issue 2, pages 628-634, the study was supported by the National Natural Science Foundation of China (Grant 51551801).

Creep Mechanism Analysis

The three-component creep model proposed in this study provides a physically meaningful decomposition of the total creep deformation in CFST members. The recoverable lag elastic deformation corresponds to the delayed elastic response of the concrete core under sustained loading, which partially recovers upon unloading. The irreversible initial rapid-flow plastic deformation represents the early-age creep associated with ongoing hydration and microstructural adjustments in the concrete. The irreversible viscous flow component captures the long-term time-dependent deformation driven by the sustained stress state.

The key finding that CFST creep deformation stabilizes after approximately 100 days, earlier than for plain concrete, is directly related to the confinement effect of the steel tube. The lateral restraint provided by the steel tube limits the development of microcracks in the concrete core, which are the primary mechanism for long-term creep in plain concrete. This earlier stabilization has significant implications for the design of CFST structures, as it means that long-term deformation predictions can be made with greater confidence after the initial 100-day period.

Steel Tube-Concrete Interface and Material Interaction

From a steel pipe manufacturing and material science perspective, the performance of the creep model is fundamentally tied to the quality of the steel tube and the steel-concrete interface. The confinement effectiveness depends on the elastic modulus and yield strength of the steel tube, which in turn depend on the steel grade, manufacturing process, and any residual stresses from forming or welding operations.

The internal surface finish of the steel tube is particularly important for creep performance. A rough or contaminated internal surface promotes better mechanical interlock between the steel tube and concrete, enhancing the composite action and confinement effectiveness. Conversely, a smooth or contaminated surface (e.g., from rust, mill scale, or release agents) may reduce the bond strength and allow relative slip between the steel tube and concrete under sustained loading, potentially increasing the creep deformation.

Creep Model Validation and Application

The prediction model developed in this study is described as concise and effective, reflecting the creep mechanism of CFST members. The model's simplicity is advantageous for practical engineering applications, where complex multi-parameter models may be difficult to calibrate and implement. The validation against 870-day test data on eight cylindrical specimens provides a reasonable basis for the model's credibility, although the limited number of specimens and the laboratory environment conditions warrant cautious extrapolation to field conditions.

The model's applicability is primarily demonstrated for cylindrical CFST specimens under axial compression. Extension to other cross-sectional shapes (square, rectangular, elliptical), other loading conditions (bending, torsion, combined loading), and other environmental conditions (temperature, humidity, chemical exposure) would require additional experimental validation and model calibration.

Engineering Practice Integration

For steel pipe manufacturers supplying CFST members for structural applications, the creep findings have direct implications for product specification and quality control. The earlier stabilization of CFST creep compared to plain concrete means that construction schedules can be optimized, with long-term deflection predictions becoming reliable after approximately 100 days of sustained loading. This is particularly relevant for long-span structures, tall buildings, and bridges where long-term deflection limits are a design criterion.

The manufacturing quality parameters that most affect creep performance include wall thickness uniformity, dimensional accuracy, and internal surface condition. Tight control of wall thickness variation (within ±10% of nominal, as specified in standards such as GB/T 8162 or EN 10216) ensures consistent confinement pressure around the concrete core. Internal surface preparation, including removal of mill scale and rust, is essential for achieving the design bond strength between the steel tube and concrete.

Key Questions and Reflections

A significant question for engineering practice is the effect of steel tube welding residual stresses on the long-term creep behavior of CFST members. Longitudinal welding processes such as SAW and HFW introduce residual stresses in the pipe wall, which may interact with the sustained compressive stress from the concrete core to influence the creep development. While the numerical model may not explicitly account for welding residual stresses, in practice, post-weld stress relief (either thermal or mechanical) should be considered for CFST applications where long-term deformation control is critical.

Another consideration is the effect of concrete filling quality on the creep model's predictions. Incomplete concrete filling, voids, or honeycombing within the steel tube reduce the effective confinement and may significantly alter the creep behavior. Quality control measures during concrete filling, including the use of tremie methods, vibrators, and post-fill inspection by ultrasonic testing, are essential to ensure the assumptions of the creep model are met.

Study Insights and Implications

This study contributes a physically meaningful and practically applicable creep prediction model for CFST members, based on a three-component decomposition of creep deformation and validated against long-term test data. For steel pipe manufacturers and engineers, the key insights are that CFST creep stabilizes earlier than plain concrete due to the confinement effect, the model is concise enough for practical use, and the manufacturing quality of the steel tube directly influences the effectiveness of the confinement mechanism. The findings support the continued adoption of CFST in structural applications where long-term deformation control is a design requirement, provided that manufacturing quality standards are rigorously maintained and the creep model is applied within its validated parameter ranges.