Axial Compression Performance of Hollow GFRP Tube-Concrete-Steel Tube Composite Columns

Literature Overview

The paper by Li Wen, Zhang Xuan, and Na Yu, published in 2017 in the Journal of Xuzhou Institute of Technology (Natural Science Edition) (Volume 32, Issue 2, pages 18–25), investigates the axial compression behavior of a novel composite column consisting of a hollow glass fiber reinforced polymer (GFRP) tube, concrete infill, and an inner steel tube. The research is supported by the National Natural Science Foundation of China (51308028) and the Heilongjiang Provincial Department of Education Science and Technology Research Project (12543023). The authors conducted axial compression tests on two physical specimens and validated the use of ANSYS finite element software for modeling, then extended the numerical analysis to eight additional specimens with varying GFRP tube wall thicknesses and member dimensions.

Composite Column Configuration and Design Rationale

The proposed composite column configuration consists of three concentric layers: an outer hollow GFRP tube, a middle concrete layer, and an inner steel tube. This triple-layer design combines the advantages of each material: the GFRP tube provides corrosion resistance and lightweight properties for the outer shell, the concrete provides compressive strength and fire resistance, and the inner steel tube provides ductility and tensile capacity. The hollow configuration, with the steel tube positioned internally rather than as the outermost layer, is a departure from conventional steel tube concrete (SRC) columns and introduces unique structural behavior.

The design rationale addresses several practical concerns in modern construction: the corrosion resistance of GFRP eliminates the need for protective coatings or cathodic protection on the outer surface; the lightweight nature of GFRP reduces the overall weight of the column; and the internal steel tube ensures that the column retains ductile failure characteristics, which are essential for seismic resistance.

Experimental and Numerical Results

The experimental program included two physical specimens subjected to axial compression loading until failure. The specimens were instrumented with displacement transducers, load cells, and strain gauges to capture the complete load-deformation response, including the elastic stage, cracking stage, yielding stage, and post-peak softening stage. The experimental results were used to validate the ANSYS finite element model, with particular attention to the convergence of load-displacement curves, failure modes, and stress distributions.

The numerical analysis extended to eight additional specimens with varying parameters:

The key findings from the study are summarized as follows:

• When specimen dimensions are held constant, increasing the GFRP tube wall thickness improves both the ductility and the ultimate load-bearing capacity of the composite column.
• When specimen dimensions are increased, the improvement in load-bearing capacity from increasing GFRP tube wall thickness diminishes, indicating a size effect on the contribution of the GFRP layer.
• The load-displacement curves exhibit a clear bilinear or trilinear characteristic, with an initial elastic stage, a transition stage associated with concrete cracking, and a post-peak stage where the steel tube provides confinement and ductility.
• The failure mode involves progressive cracking and crushing of the concrete, followed by yielding and bulging of the inner steel tube, with the GFRP tube providing restraint against excessive outward deformation.

Technical Interpretation and Engineering Relevance

The diminishing effect of GFRP tube wall thickness on load capacity as specimen size increases is an important observation that has implications for the economic optimization of composite column design. For small-diameter columns, the GFRP tube contributes significantly to the overall load capacity through direct load-bearing and confinement of the concrete. However, for large-diameter columns, the concrete and steel tube dominate the load-bearing capacity, and the GFRP tube primarily serves as a protective and corrosion-resistant outer shell. This suggests that for large-diameter columns, a thinner GFRP tube may be economically optimal, with the wall thickness selected based on corrosion protection requirements rather than structural contribution.

From a steel pipe and welding perspective, the inner steel tube in this composite column configuration presents specific fabrication and welding challenges. The steel tube must be precisely manufactured to fit within the concrete layer with adequate clearance for placement and compaction. The longitudinal and circumferential welds of the steel tube must be of high quality, as they are embedded within the concrete and cannot be visually inspected after placement. Non-destructive testing (NDT) of all welds must be completed before concrete placement, with particular emphasis on phased array ultrasonic testing (PAUT) for detecting volumetric defects and time-of-flight diffraction (TOFD) for detecting planar defects such as lack of fusion and cracks.

The interaction between the GFRP tube and the concrete is another critical aspect. The bond between GFRP and concrete is typically weaker than the bond between steel and concrete, due to the lower surface energy of the polymer matrix. This can lead to debonding under compressive loads, reducing the effectiveness of the GFRP tube as a confinement element. Surface treatments such as sandblasting, mechanical roughening, or chemical priming of the GFRP tube inner surface can improve the bond strength, and these treatments must be specified in the fabrication and construction procedures.

Study Insights and Reflections

This research contributes to the growing body of knowledge on hybrid composite columns that combine fiber-reinforced polymers with conventional structural materials. The triple-layer configuration of GFRP-concrete-steel tube represents an innovative approach to achieving corrosion resistance, structural strength, and ductility in a single column element. However, several aspects require further investigation:

• The long-term behavior of the composite column under sustained loads, including creep and stress relaxation of the GFRP tube and concrete, has not been fully addressed.
• The fire resistance of the composite column, particularly the performance of the GFRP tube at elevated temperatures, needs to be evaluated, as GFRP materials can experience significant strength loss at temperatures above 200 degrees Celsius.
• The seismic performance of the composite column, including hysteretic behavior and energy dissipation capacity, should be investigated through cyclic loading tests.
• The fabrication tolerances and assembly procedures for the triple-layer column, including the alignment of the steel tube within the concrete layer and the bonding of the GFRP tube to the concrete surface, need to be standardized.

For welding engineers, the key consideration is that the steel tube welds in this composite column are permanently embedded and cannot be reworked after concrete placement. This means that the welding quality assurance program must be rigorous, with full inspection of all welds using multiple NDT methods before concrete placement. The welding procedure specifications (WPS) must be qualified for the specific steel grade and wall thickness, and welder performance qualifications must be current. Any weld defects detected during NDT must be repaired and re-inspected before the column is sealed with concrete.

In conclusion, this research demonstrates that the hollow GFRP tube-concrete-steel tube composite column is a viable structural solution that combines the advantages of all three materials. The parametric study reveals that the GFRP tube wall thickness has a significant effect on the load capacity and ductility of small-diameter columns but a diminishing effect on large-diameter columns. Engineers designing such columns must carefully balance structural performance, corrosion resistance, and economic considerations, and must ensure that the embedded steel tube welds meet the highest quality standards since they cannot be inspected or repaired after concrete placement.