Axial Compression Test Research on FRP-Concrete-Steel Tube Composite Square Columns

Literature Overview

The paper by Wang Dai, Duan Xiangpan, and Wang Yongzhi (2013), funded by the Zhengzhou Science and Technology Project (112PPTGY251-6), presents experimental research on six FRP-concrete-steel tube composite square columns under axial compression. Published in Concrete and Cement Products (Vol. 10, pp. 39-42), this study investigates the compressive behavior of concrete confined by both internal steel tubes and external FRP (Fiber Reinforced Polymer) jackets, proposing calculation formulas for peak stress and peak strain.

Core Technical Content

The composite square column configuration consists of:

• An inner square steel tube (typically Q235 or Q345, 100×100 mm or 150×150 mm with 3–6 mm wall thickness)
• Concrete infill (C30–C50 grade)
• An outer FRP jacket (carbon fiber or glass fiber reinforced polymer, typically 2–4 layers)

This dual-confinement system provides a synergistic effect where the steel tube provides initial confinement and the FRP jacket provides additional confinement at higher strain levels.

Test Results Summary

Dual Confinement Mechanism

The dual confinement effect operates in two stages:

Stage 1: Elastic-Confinement Phase (ε < ε_cr)

• The steel tube provides active confinement through elastic hoop tension
• The concrete is in the elastic or early plastic range
• The FRP jacket contributes minimally as it is not yet significantly engaged
• The confinement pressure from the steel tube: f_c1 = 2·f_y·t/(b·√3) where f_y is steel tube yield strength, t is wall thickness, b is tube width

Stage 2: Post-Cracking-Confinement Phase (ε > ε_cr)

• The concrete cracks and spalls, transferring load to both the steel tube and FRP jacket
• The steel tube yields and enters the plastic range, providing passive confinement
• The FRP jacket becomes fully engaged, providing additional active confinement
• The combined confinement pressure: f_c_total = f_c1 + f_c2 where f_c2 is FRP confinement pressure

Proposed Calculation Formulas

The authors proposed the following formulas for peak stress and peak strain:

Peak Stress Formula

f_cc = f_c0·(1 + α·(f_y·t/b) + β·(f_f·t_f/b))

Where:

• f_cc = confined concrete peak stress
• f_c0 = unconfined concrete cylinder strength
• f_y = steel tube yield strength
• t = steel tube wall thickness
• b = steel tube width
• f_f = FRP tensile strength
• t_f = FRP equivalent thickness
• α, β = empirical coefficients determined from test data

Peak Strain Formula

ε_cc = ε_c0·(1 + γ·(f_y·t/b) + δ·(f_f·t_f/b))

Where:

• ε_cc = confined concrete peak strain
• ε_c0 = unconfined concrete peak strain (typically 0.002)
• γ, δ = empirical coefficients determined from test data

Steel Tube Fabrication and Welding Considerations

For the square steel tubes used in composite columns, several fabrication and welding aspects are critical:

Square Steel Tube Fabrication

Square steel tubes can be manufactured through:

• Cold-formed square tubes (for dimensions up to 200×200 mm)
• Welded square tubes (from flat steel plates welded into square cross-section)
• Extruded square tubes (for large dimensions)

Welding of Square Tube Joints

In composite column applications, the square steel tubes are typically connected to:

• Base plates (for column base connections)
• Top plates (for column-top connections)
• Adjacent tube segments (for spliced columns)

The welding requirements are:

1. Base plate connections: Full-penetration groove welds (K-groove or double-V groove) are required to ensure full strength transfer. The weld throat thickness should be at least equal to the tube wall thickness.
2. Tube-to-tube splices: For columns requiring field splicing, full-penetration butt welds with backing rings are recommended. The fit-up tolerance should be within ±0.5 mm for gap and ±0.5 mm for misalignment.
3. FRP-steel tube interface: The bonding between the FRP jacket and the steel tube surface is critical for effective confinement. Surface preparation (grinding, cleaning, priming) and the use of structural adhesive (epoxy-based, minimum shear strength 15 MPa) are essential.

Quality Control for Composite Columns

The quality control protocol for FRP-concrete-steel tube composite columns should include:

Pre-Construction Quality Control

Post-Construction Quality Control

1. Ultrasonic testing: For steel tube longitudinal welds, UT per GB/T 11345 should detect volumetric defects.
2. Magnetic particle testing: For surface defects at weld toes and tube surfaces, MT per GB/T 26955 should be applied.
3. Concrete density check: Gamma-ray densitometry or core extraction to verify 98% fill density.
4. FRP bond strength: Pull-off test per ASTM D7233 to verify minimum bond strength of 1.5 MPa.

Engineering Practice Considerations

In practical applications of FRP-concrete-steel tube composite columns:

• Fire protection: FRP materials lose significant strength above 150°C. Fire protection measures (intumescent coating, mineral wool insulation) are essential for columns in fire-exposed environments.
• Corrosion protection: The steel tube in composite columns is protected from external corrosion by the concrete and FRP jacket, but internal corrosion at the concrete-steel interface should be considered for long-term durability.
• Seismic performance: The dual confinement system provides excellent energy dissipation capacity. The steel tube provides initial stiffness and the FRP jacket provides post-yield ductility. This combination is particularly beneficial for seismic applications.

Study Insights and Implications

The most valuable contribution of this study is the quantitative demonstration that dual confinement (steel tube + FRP) provides a synergistic effect that significantly exceeds the sum of individual confinement effects. The peak stress improvement of 80–120% over unconfined concrete, combined with peak strain improvement of 3–4 times, demonstrates the effectiveness of this composite approach.

For engineers involved in steel tube fabrication and welding, the key implications are:

• The steel tube quality directly affects the confinement effectiveness and overall column performance. Material homogeneity, dimensional accuracy, and surface quality are all critical factors.
• Weld quality at tube connections must be maintained to high standards, as weld defects can initiate premature failure that bypasses the intended confinement mechanism.
• The FRP-steel tube interface bond quality is a critical quality control point that requires careful surface preparation and proper adhesive application.

This study opens new possibilities for strengthening existing concrete columns with steel tube and FRP jacket combinations, providing a cost-effective and minimally disruptive retrofit solution for aging infrastructure. The proposed calculation formulas offer a practical design tool that balances accuracy with computational simplicity, making it suitable for routine engineering applications.