Axial Compression Behavior of Circular Hollow Sandwich Steel-Concrete Members with Different Inner Tube Materials

Literature Overview

This paper, published in Progress in Steel Building Structures (2018, Vol. 20, No. 1, pp. 64-72) by Huang Hong and colleagues from East China Jiaotong University, Nanchang Institute of Technology, and Tianjin University, investigates the influence of inner tube material selection on the mechanical performance of circular hollow sandwich steel-concrete (CHSSC) members under axial compression. The study is supported by the National Natural Science Foundation of China (Grants 51378206 and 51008122) and the Jiangxi Provincial Youth Science Fund (20143ACB21020). Five specimens were designed and tested, incorporating three types of inner tubes: steel, PVC, and PPR. Both experimental testing and finite element (FE) simulation using ABAQUS were conducted to evaluate failure modes, load-strain relationships, average stress-strain behavior, and transverse strain evolution.

Core Technical Content and Key Findings

The authors designed a systematic experimental matrix to isolate the effect of inner tube material while maintaining geometric and material consistency of the outer steel tube and infill concrete. The test specimens represent a practical engineering scenario where cost reduction or functional separation (e.g., fluid transport, corrosion protection) is achieved through a hollow sandwich configuration.

Experimental Configuration

Failure Mode Analysis

The paper reveals distinct failure behaviors across the three material configurations. The steel inner tube specimens exhibited conventional buckling patterns with relatively uniform circumferential deformation. In contrast, PVC and PPR inner tube specimens displayed pronounced diagonal bulging at failure, indicating that the absence of structural interaction between the inner tube and the sandwich concrete layer significantly alters the failure mechanism. This observation has direct implications for steel pipe design: when the inner tube does not participate in load transfer, the concrete ring behaves more like a standalone confined concrete cylinder, with reduced lateral restraint effectiveness.

Load-Bearing Capacity and Deformation Characteristics

The experimental results demonstrate that PVC and PPR inner tube specimens have lower ultimate load-bearing capacity, combined elastic modulus, and ductility compared to steel inner tube specimens. The PVC inner tube specimens exhibited the fastest transverse strain development, suggesting minimal restraint from the inner tube on the sandwich concrete layer. This is a critical finding from a pipe engineering perspective: the sandwich concrete layer, when not structurally bonded to the inner tube, cannot develop effective confinement, leading to premature concrete crushing and reduced overall member performance.

Finite Element Model Validation

The FE model developed using ABAQUS was able to reasonably simulate the initial stiffness of all specimens and the ultimate load-bearing capacity of steel inner tube specimens. However, the model showed limitations in capturing the full nonlinear behavior of PVC and PPR inner tube specimens, particularly in the post-peak regime. This discrepancy likely stems from the difficulty in modeling the near-zero interaction between polymer inner tubes and concrete, which introduces contact nonlinearity and potential delamination effects.

Engineering Practice Integration

From a steel pipe and pipe fitting engineering standpoint, this research carries several practical implications:

• Pipe-in-pipe configurations: In offshore and pipeline engineering, double-wall pipe systems (pipe-in-pipe) are used for thermal insulation and structural redundancy. The findings here reinforce that only when both walls are structurally coupled (through bonding, grouting, or mechanical interlock) does the sandwich layer contribute meaningfully to load capacity.
• Material compatibility in composite pipes: When considering non-metallic inner liners in steel pipes (e.g., FBE-lined pipes or polymer-lined pipes for corrosion protection), the research confirms that these liners provide essentially zero structural contribution under axial loading. Engineers must not credit such liners in structural design calculations.
• Quality control implications: For sandwich steel-concrete pipe columns used in bridge piers or building columns, the bond quality between inner tube and concrete is critical. Non-destructive testing (NDT) methods such as ultrasonic testing (UT) and pull-off tests should be employed to verify interface integrity.

Key Questions and Reflections

Several questions arise from this study that deserve further investigation:

1. What is the minimum structural contribution threshold for an inner tube to be considered a composite element rather than a mere liner?
2. How does the manufacturing process of the inner tube (extrusion for PVC/PPR vs. rolling/welding for steel) affect surface roughness and consequently the concrete-tube interface bond?
3. For welded steel inner tubes, would the weld seam location (longitudinal or spiral) influence the failure mode of the sandwich concrete layer?

The authors' conclusion that PVC inner tube CHSSC members are unsuitable for load-bearing structures while PPR inner tube members may be used in engineering applications is noteworthy. The distinction between PVC and PPR, despite both being non-structural polymers, suggests that PPR's slightly superior mechanical properties (higher modulus and elongation at break) provide marginal but meaningful confinement improvement.

Study Insights and Implications

This research provides valuable guidance for engineers designing hollow steel-concrete composite columns and pipe-based structural systems. The fundamental principle is clear: structural compositing requires mechanical interlock or chemical bonding between constituent materials. In the context of steel pipe fabrication, this translates to the importance of surface preparation, concrete mixture design (with appropriate aggregate grading and steel fiber reinforcement), and curing practices to ensure adequate bond development. For quality assurance in composite pipe column production, I would recommend implementing a PDCA cycle: Plan the bond verification protocol, Do the interface testing at critical locations, Check the results against acceptance criteria, and Act on any deficiencies through additional grouting or mechanical anchorage. The research ultimately reinforces the metallurgical principle that composite performance is governed by the weakest interface, not the strongest constituent.