Effect of Interface Defects on Bending Performance of CFST Members

Literature Overview

This paper by Liu Minghui, Han Bing, and Duo Juntao from Beijing Jiaotong University investigates the effect of interface defects between the steel tube and core concrete on the bending performance of circular CFST members. The study combines experimental testing with acoustic emission (AE) monitoring and finite element analysis. Funded by the China Railway Corporation Research and Development Program (2017G010-C), the paper was published in the China Civil Engineering Journal in 2019.

Interface Defect Characterization

Interface defects between the steel tube and core concrete are common in in-service CFST structures and arise from poor concrete placement, inadequate compaction, corrosion of the tube inner surface, or construction joints. These defects reduce the bond strength between the steel tube and concrete, leading to premature slippage, reduced composite action, and degraded structural performance.

Defect Range and Performance Degradation

Defect Range	Load Capacity Reduction	Mid-Span Deflection Increase	Stiffness Reduction Factor
0% (No defect)	Baseline	Baseline	1.0
25%	3–5%	8–12%	0.92–0.95
50%	8–12%	18–25%	0.82–0.88
75%	15–20%	30–40%	0.70–0.78
100%	25–35%	50–65%	0.55–0.65

Acoustic Emission Monitoring Insights

The AE monitoring technique captures the initiation and propagation of internal damage in the concrete core during loading. The study reveals that interface defects significantly alter the AE signal characteristics: the number of AE events increases dramatically in the early loading stage when defects are present, indicating premature cracking and debonding. The AE energy release rate is higher for specimens with larger defect ranges, confirming that the damage is more extensive and localized.

Finite Element Model and Parametric Study

The finite element model is calibrated against experimental results and then used to study the influence of section steel ratio, core concrete strength, and interface defect range on the bending stiffness of CFST members. The model employs a cohesive zone model to simulate the interface behavior, with damage initiation and evolution criteria based on the bilinear traction-separation law.

Key Parametric Findings

Parameter Pair	Interaction Effect	Engineering Implication
Steel ratio vs. defect range	Opposite effects on stiffness	Higher steel ratio mitigates defect damage
Concrete strength vs. defect range	Same direction effects	Higher concrete strength amplifies defect damage
Steel ratio vs. concrete strength	Complementary	Both contribute to overall stiffness

Welding Quality and Interface Integrity

From a welding and fabrication perspective, the interface defects studied in this paper are closely related to the quality of the steel tube inner surface. Welding defects such as spatter, oxide inclusion, or weld bead protrusion on the inner surface of longitudinally welded pipes can act as stress concentrators and initiate interface debonding. The weld quality of the steel tube itself, particularly the integrity of the longitudinal weld seam, affects the local buckling behavior of the tube under bending, which in turn influences the interface stress distribution.

Defect Prevention Measures

Source of Defect	Prevention Measure	Inspection Method
Poor concrete compaction	Use vibration compaction, low-slump concrete	Core sampling, UT
Inner surface corrosion	Apply anti-corrosion coating	Visual inspection, thickness measurement
Weld spatter on inner surface	Use internal shielding, clean after welding	Visual inspection, MT
Construction joints	Use bond-breaking agent, roughen surface	Visual inspection, pull-off test

Study Insights and Implications

The empirical expressions for stiffness reduction factors derived from the numerical analysis provide practical tools for engineers to assess the structural capacity of in-service CFST members with known interface defects. The finding that higher steel ratio mitigates the adverse effect of defects is particularly significant for rehabilitation engineering: increasing the steel ratio through adding internal steel reinforcement or external steel plates can effectively compensate for interface degradation. However, the finding that higher concrete strength amplifies the defect effect suggests that using high-strength concrete without addressing interface quality may be counterproductive. For welding engineers, the key message is that maintaining a clean, smooth, and defect-free inner surface of the steel tube is not merely a cosmetic requirement but a structural necessity that directly affects the long-term performance of CFST members under bending loads.