Experimental Study on Mechanical Properties of Plastic-Steel-Concrete Composite Short Columns

Literature Overview

This paper by Feng Wenxian, Tan Jianchao, Guo Yongchang, and Liu Feng, published in "Engineering Earthquake Resistance and Reinforcement" in 2009 (Vol. 31, No. 6, pp. 28-32), presents an experimental investigation of a novel composite column type: the solid plastic-steel-concrete short column. The work was supported by the National Natural Science Foundation of China (10872052), the Guangdong Provincial Natural Science Foundation Key Project (07117419), and multiple Guangdong Provincial Science and Technology Program projects. The authors propose replacing the outer steel tube of a double-tube steel-concrete column with a plastic tube to create a new composite member, and they investigate its mechanical properties through systematic testing.

Novel Composite Column Configuration

The proposed plastic-steel-concrete composite column consists of three concentric layers:

1. Inner layer: Solid concrete core providing compressive strength.
2. Middle layer: Steel tube providing lateral confinement to the concrete and contributing directly to load-bearing capacity.
3. Outer layer: Plastic tube (polymer-based) replacing the traditional outer steel tube, providing corrosion protection and additional confinement.

This configuration addresses several limitations of conventional steel-concrete composite columns:

• Corrosion resistance: The plastic outer tube eliminates the corrosion problem that plagues steel tubes in aggressive environments.
• Weight reduction: The plastic tube is significantly lighter than a steel tube of equivalent thickness, reducing the overall weight of the column.
• Cost reduction: Plastic materials are generally less expensive than steel, reducing material costs.
• Maintained confinement: The plastic tube provides sufficient lateral confinement to the steel tube, preventing outward buckling of the steel tube walls.

Test Matrix and Key Variables

The experimental program varied three primary parameters to assess their influence on column performance:

Additional tests examined the effect of FRP (Fiber-Reinforced Polymer) reinforcement on the bearing capacity of the composite columns.

Mechanical Performance and Deformation Characteristics

The experimental results revealed several important findings regarding the mechanical behavior of the plastic-steel-concrete composite columns:

Load-Displacement Behavior:

• The columns exhibited a clear three-stage response: elastic stage, yielding stage, and post-peak descending stage.
• The elastic stage extended to approximately 60-70% of the peak load, indicating good initial stiffness.
• The yielding stage was characterized by gradual load increase with increasing displacement, reflecting the progressive yielding of the steel tube.
• The post-peak descending stage showed gradual load reduction, indicating ductile failure behavior.

Strain Distribution:

• The concrete core experienced compressive strains significantly higher than those in plain concrete, demonstrating the effectiveness of the steel tube confinement.
• The steel tube experienced compressive strains that increased with load, with local buckling occurring at higher loads for higher D/t ratios.
• The plastic tube experienced compressive strains that remained within the elastic range for most of the loading, indicating that the plastic tube primarily provides confinement rather than direct load-bearing.

Failure Modes:

• Columns with low D/t ratios failed by concrete crushing with limited steel tube buckling.
• Columns with high D/t ratios failed by local buckling of the steel tube, followed by concrete crushing.
• The plastic tube did not fail in any test specimen, confirming its role as a protective and confining layer.

Effect of FRP Reinforcement

The FRP reinforcement tests demonstrated that wrapping the composite columns with FRP sheets can further enhance their bearing capacity. The FRP sheets provide additional lateral confinement, which:

• Increases the peak load by 10-25% depending on the FRP configuration.
• Improves the ductility by delaying the onset of concrete crushing.
• Provides a redundant load path in the event of steel tube buckling.

The effectiveness of FRP reinforcement depends on the bond quality between the FRP and the plastic tube surface, which requires proper surface preparation and adhesive application.

Engineering Practice Implications

For engineers considering the use of plastic-steel-concrete composite columns in practice, the following considerations emerge:

• Applicability: The composite columns are particularly suitable for corrosive environments such as marine structures, chemical plants, and underground facilities where steel corrosion is a major concern.
• Design methodology: The design should account for the interaction between the concrete core, steel tube, and plastic tube, with appropriate safety factors for each component.
• Quality control: The bond quality between the steel tube and concrete, and between the plastic tube and steel tube, must be verified through destructive or non-destructive testing.
• Fire resistance: The plastic tube may degrade at elevated temperatures, requiring fire protection measures for structural applications in fire-prone environments.

Study Insights and Reflections

This paper presents a creative approach to solving the corrosion problem in steel-concrete composite columns by replacing the outer steel tube with a plastic tube. The experimental results demonstrate that the proposed configuration maintains the mechanical performance of conventional steel-concrete columns while offering significant advantages in corrosion resistance, weight, and cost.

The systematic variation of D/t ratio, plastic tube thickness, and concrete strength provides a comprehensive understanding of the parameter sensitivity of the composite column behavior. The findings suggest that the inner steel tube's D/t ratio is the most critical parameter, as it directly governs the local buckling behavior and confinement effectiveness.

The limitation of this study lies in the focus on short columns under axial compression, without addressing the behavior under combined axial and bending loads, which is more representative of real structural conditions. Additionally, the long-term behavior under sustained loads, including creep and shrinkage effects, deserves further investigation. Nevertheless, this work provides a valuable foundation for the development of corrosion-resistant composite columns, and it opens up new possibilities for the application of steel-concrete composite members in aggressive environments.