Zhuojin Pipe Fitting Co., LtdAxial Compression Performance of Circular Hollow Sandwich Concrete-Filled Steel Tube Members with Different Inner Tube Materials
Literature Overview
This study published in Advances in Steel Construction (2018, Vol. 20, No. 1, pp. 64-72) by Huang Hong, Zhu Yanqi, Guo Xiaoyu, and Chen Mengcheng investigates the mechanical behavior of circular hollow sandwich concrete-filled steel tube (CHS-CFST) members with three different inner tube materials: steel, PVC, and PPR. The research was supported by the National Natural Science Foundation of China (Grants 51378206, 51008122) and the Jiangxi Provincial Youth Science Fund (Grant 20143ACB21020). Five test specimens were designed and subjected to axial compression tests, with full-process finite element simulations performed using ABAQUS. The study addresses a practically relevant question: whether lightweight polymer inner tubes can substitute for steel inner tubes in sandwich CFST members without compromising structural performance.
Core Technical Findings
The experimental results reveal significant differences in failure modes, load-bearing capacity, and deformation characteristics among the three specimen types. The steel inner tube specimens exhibited the highest ultimate load-bearing capacity, combined elastic modulus, and ductility. The PVC and PPR inner tube specimens showed more pronounced diagonal bulging at failure, indicating reduced confinement effectiveness from the inner tube. The lateral strain development rate was fastest for the PVC inner tube specimens, suggesting that the polymer tube provides negligible lateral confinement to the sandwich concrete layer.
Failure Mode Comparison
| Specimen Type | Failure Mode | Ultimate Capacity | Combined Elastic Modulus | Ductility |
|---|---|---|---|---|
| Steel inner tube | Concrete crushing with steel tube yielding | Highest | Highest | Best |
| PVC inner tube | Diagonal bulging, concrete spalling | Lower | Lower | Reduced |
| PPR inner tube | Diagonal bulging, concrete spalling | Lower | Lower | Reduced |
The interaction between the inner tube and the sandwich concrete layer was found to be nearly zero for both PVC and PPR tubes throughout the entire axial compression process. These polymer tubes carried only negligible axial loads, essentially functioning as voids rather than structural contributors. This observation has critical implications for the design philosophy of sandwich CFST members: the inner tube material directly determines whether the sandwich concrete layer receives dual confinement or only outer tube confinement.
Finite Element Modeling Insights
The ABAQUS-based finite element model demonstrated good agreement with experimental results for the initial stiffness of all specimens and the ultimate load-bearing capacity of the steel inner tube specimens. However, the model's accuracy for polymer inner tube specimens was less consistent, particularly in capturing the post-peak behavior and the extent of diagonal bulging. This discrepancy highlights the challenge of accurately modeling the interfacial behavior between polymer tubes and concrete, where the bond mechanism differs fundamentally from steel-concrete interaction.
From a welding and fabrication perspective, the steel inner tube specimens require precise concentricity control between the inner and outer tubes, typically achieved through tack welding of spacer rings at regular intervals. The tolerance for concentricity is critical, as eccentricity in the inner tube reduces the effective sandwich thickness and compromises the dual-confinement mechanism. The sandwich concrete layer must be placed carefully to ensure uniform thickness, typically between 20 and 60 mm, to achieve the intended confinement enhancement.
Engineering Practice Implications
The study concludes that PVC inner tube CHS-CFST members are unsuitable for load-bearing structural applications, while PPR inner tube members can be used in engineering practice. This distinction is significant because PPR (polypropylene random copolymer) offers better mechanical properties than PVC, including higher tensile strength and improved dimensional stability at elevated temperatures. However, the margin of safety for PPR inner tube members remains limited compared to steel inner tube members, and their application should be restricted to non-critical or secondary structural elements.
For practical engineering applications, the following considerations arise from this research:
- The sandwich concrete layer thickness should be optimized based on the inner tube material to ensure adequate confinement effectiveness.
- For polymer inner tube members, the design should not rely on any contribution from the inner tube, treating it as a formwork element only.
- Quality control during fabrication must verify inner tube concentricity, sandwich layer uniformity, and the absence of voids in the concrete placement.
- The connection design between sandwich CFST members and other structural elements must account for the reduced stiffness and ductility of polymer inner tube configurations.
Key Reflections and Study Insights
This research provides valuable guidance for the rational selection of inner tube materials in sandwich CFST members. The finding that polymer inner tubes contribute negligibly to structural performance raises an important design philosophy question: if the inner tube serves only as a void former, should engineers consider alternative lightweight void-forming solutions that are more cost-effective? The answer depends on the specific application requirements, including fire resistance, durability, and long-term serviceability considerations.
From a materials science perspective, the near-zero interaction between polymer tubes and sandwich concrete is expected given the fundamental differences in elastic modulus, thermal expansion coefficient, and surface adhesion characteristics. Steel inner tubes, by contrast, develop a strong bond with concrete through mechanical interlock and chemical adhesion, creating an effective composite action that enhances the confinement of the sandwich concrete layer. This composite action is the primary mechanism by which the sandwich concrete layer achieves higher strength and ductility compared to ordinary concrete.
The study also highlights the importance of experimental validation for novel structural systems. Finite element models, while powerful analytical tools, require careful calibration against experimental data, particularly for systems involving complex material interactions and progressive damage mechanisms. Engineers should exercise caution when extrapolating numerical results to untested configurations, especially when the underlying material behavior has not been thoroughly characterized.
In summary, this research firmly establishes that steel inner tubes are essential for achieving the full structural potential of circular hollow sandwich CFST members, while polymer inner tubes—particularly PPR—offer a viable but limited alternative for non-critical applications. Engineers designing sandwich CFST structures must carefully evaluate the inner tube material selection based on the required structural performance, fabrication constraints, and cost considerations, with full awareness that the inner tube material fundamentally governs the confinement mechanism and ultimate load-bearing behavior of the sandwich concrete layer.