Axial Compression Performance of GFRP Tube-Self-Consolidating Concrete-Steel Tube Composite Medium-Long Columns

Research Significance and Structural Configuration

This study examines the axial compressive behavior of a triple-layer composite column system consisting of a pultruded glass fiber reinforced polymer (GFRP) outer tube, a self-consolidating concrete (SCC) intermediate layer, and a steel inner tube. This hybrid configuration represents an innovative approach to combining the corrosion resistance and high strength-to-weight ratio of GFRP with the ductility and confinement capacity of steel tubes, while utilizing SCC for its excellent workability and reduced need for vibration during placement.

The medium-long column classification indicates that slenderness effects (second-order moments, P-delta effects) are significant and must be considered in the analysis. The interaction between the three components under axial compression creates a complex confinement mechanism where the steel tube provides primary confinement to the concrete, while the GFRP tube provides additional restraint and corrosion protection.

Constitutive Behavior and Load-Transfer Mechanisms

Under axial compression, the load is initially distributed among the three components based on their respective stiffness contributions. As the concrete reaches its peak strength, the steel tube begins to yield and provide lateral confinement, while the GFRP tube mobilizes its hoop strength through Poisson expansion of the concrete. The self-consolidating concrete, with its high flowability, ensures complete filling of the annular space between the steel and GFRP tubes without the risk of voids or honeycombing.

Component	Material Property	Typical Value	Contribution to Axial Capacity
GFRP tube	Longitudinal tensile strength	400-600 MPa	15-25%
GFRP tube	Hoop tensile strength	200-300 MPa	Confinement only
SCC	Compressive strength	40-60 MPa	50-65%
SCC	Flow diameter	650-750 mm	Placement quality
Steel tube	Yield strength	235-345 MPa	20-30%
Steel tube	Wall thickness ratio	0.3-0.8% of diameter	Confinement efficiency

The confinement efficiency of the steel tube is governed by the D/t ratio (diameter-to-wall-thickness ratio), where lower ratios provide higher confinement pressure. The GFRP tube contributes primarily through its hoop strength, which resists the lateral expansion of the concrete and steel tube under compression. The pultruded manufacturing process of the GFRP tube ensures consistent fiber orientation and void content, which is critical for predictable mechanical performance.

Slenderness Effects and Stability Analysis

For medium-long columns, the Euler buckling load and the inelastic buckling behavior become critical design parameters. The composite column system exhibits a higher effective buckling load compared to individual components due to the composite action and the confinement-induced strength enhancement of the concrete. The GFRP tube, being lightweight, reduces the self-weight of the column, which directly improves the slenderness ratio and increases the buckling capacity.

Slenderness Ratio (λ)	Behavior Mode	Governing Mechanism
λ < 30	Short column	Material crushing, confinement failure
30 < λ < 60	Medium column	Inelastic buckling with material yielding
60 < λ < 100	Long column	Elastic buckling, GFRP instability
λ > 100	Very long column	Lateral-torsional buckling

The finite element analysis should account for geometric imperfections, residual stresses from pultrusion, and the nonlinear material behavior of all three components. The GFRP tube may exhibit lateral-torsional buckling at high slenderness ratios due to its relatively low shear modulus compared to its longitudinal modulus.

Manufacturing and Quality Control Considerations

The pultruded GFRP tube must meet strict quality standards for fiber volume fraction (typically 45-55%), void content (<2%), and dimensional tolerances. The steel inner tube should be free of surface defects that could initiate corrosion or stress concentrations. The interface between the GFRP tube and SCC requires careful consideration; a bond-promoting agent or mechanical keying system may be necessary to ensure adequate shear transfer between the polymer matrix and the concrete.

Quality control procedures should include:

Visual inspection and ultrasonic testing of the GFRP tube for delamination and voids
Hydrostatic pressure testing of the steel tube to verify weld integrity and dimensional accuracy
Slump flow and V-funnel testing of the SCC to ensure proper workability before placement
Post-curing verification through rebound hammer testing and core sampling

Study Insights and Engineering Recommendations

This triple-layer composite column system represents a promising solution for corrosive environments where steel-only or concrete-only columns would require extensive maintenance or replacement. The GFRP outer tube provides a corrosion barrier that extends the service life of the steel tube, while the steel tube provides the ductility and confinement that GFRP alone cannot offer. Engineers adopting this system should pay particular attention to the fire resistance of the GFRP component, which requires additional protection (such as intumescent coatings or fire-resistant encasement) to maintain structural integrity during fire exposure. The SCC placement process must be carefully controlled to avoid segregation or bleeding in the confined annular space, and the curing regime should be optimized to achieve full strength development without inducing thermal stresses in the composite system.