Effect of Interface Debonding Defects on Axial Compression Performance of Square Steel Tubular Concrete Columns

Research Background and Motivation

Steel tubular concrete (STC) columns represent one of the most efficient composite structural members, combining the compressive strength of concrete with the confining action and tensile capacity of a steel tube. However, during construction, imperfect concrete placement, inadequate compaction, or differential shrinkage between steel and concrete can lead to interface debonding — a condition where the steel tube separates from the infill concrete, creating a void or gap at the steel-concrete interface. This study investigates how such debonding defects influence the axial compression behavior of square STC columns, providing critical data for quality control and structural assessment.

Core Technical Findings

The research conducts both experimental testing and numerical modeling to quantify the impact of interface debonding on column capacity, stiffness, and failure mode. The experimental program includes columns with controlled debonding areas at various locations — top, bottom, middle, and distributed — to simulate realistic construction defects. The following table presents representative results:

The study reveals that interface debonding has a nonlinear effect on axial capacity. Small debonding areas (below 10 percent of the interface) produce minimal reduction in ultimate load, as the remaining bonded area can still provide adequate confinement. However, as the debonding area increases beyond 20 percent, the load reduction accelerates dramatically, and the ductility of the column degrades significantly. This is because the confining pressure exerted by the steel tube on the concrete is directly proportional to the bonded interface area; loss of bond means loss of confinement, which means reduced concrete strength and premature failure.

Mechanism Analysis and Metallographic Insights

From a materials science perspective, the interface between steel and concrete is a critical zone where stress transfer occurs. In a properly constructed STC column, the steel tube exerts a radial confining pressure on the concrete through friction and mechanical interlock at the interface. When debonding occurs, this confinement mechanism is disrupted, and the concrete core behaves more like an unconfined concrete column, losing the strength enhancement that is the primary advantage of the composite system.

The research identifies three primary mechanisms of debonding initiation:

1. Differential shrinkage: Concrete shrinkage during curing can pull the concrete away from the steel tube, particularly when the steel tube has a low surface roughness or when the concrete mix has high water-cement ratio.
2. Inadequate compaction: In tall columns, concrete may not be fully compacted near the top, leaving voids that evolve into debonding zones under load.
3. Thermal effects: During fire exposure or in high-temperature service environments, differential thermal expansion between steel and concrete can cause interface separation.

For welding engineers, the relevance extends to the steel tube fabrication process. The surface condition of the steel tube — including mill scale, rust, and surface roughness — directly affects the bond strength at the steel-concrete interface. According to GB/T 50010-2010 and ACI 318-19, the bond strength between steel and concrete depends on surface preparation. For STC columns, the steel tube interior should be free of loose mill scale, rust, and oil contamination. Shot blasting to achieve a surface profile of Sa 2.5 (near-white metal cleaning) is recommended for critical applications.

Quality Control and Engineering Recommendations

The study provides valuable guidance for quality control procedures in STC column fabrication and construction. The following table summarizes recommended inspection and testing protocols:

From a welding perspective, the fabrication of square steel tubes for STC columns involves longitudinal and transverse welding of plate sections. The quality of these welds is directly related to the structural integrity of the column. Any porosity, lack of fusion, or slag inclusion in the weld can serve as a stress concentration point that initiates debonding or cracking under axial compression. Therefore, radiographic testing (RT) or ultrasonic testing (UT) of all welds is essential, with acceptance criteria per GB/T 3323 or ASME Section V, Article 4.

Study Insights and Reflections

This research underscores a fundamental principle that I have emphasized throughout my career: the performance of composite steel-concrete members is only as good as the weakest link in the load-transfer chain. In the case of STC columns, that weakest link is often the steel-concrete interface, which is invisible to the naked eye and difficult to inspect during construction. The finding that even moderate debonding (15–20 percent of the interface area) can reduce column capacity by 8–12 percent and significantly degrade ductility should alarm all engineers involved in STC design and construction. It calls for more rigorous quality assurance procedures, including mandatory interface bond testing and advanced NDT methods for detecting internal debonding. For those of us who fabricate and weld steel pipe sections for such applications, the message is clear: surface preparation, welding quality, and geometric accuracy are not merely fabrication concerns — they are structural safety concerns.