Effect of Void Defects on Hysteresis Performance of Circular CFST Members

Literature Overview

This paper, published in Industrial Construction (Vol. 47, Issue 9, 2017, pp. 146-151) by Han Hao, Liao Feiyu, and Li Yongjin from Fujian Agriculture and Forestry University, investigates the influence of spherical-cap-shaped void defects between the steel tube and concrete core on the hysteresis behavior of circular CFST compression-bending members under cyclic loading. Funded by the National Natural Science Foundation of China (Grants 51578154 and 51308121), the research employs finite element modeling with advanced material constitutive models to analyze the interaction mechanism between steel and concrete in the presence of voids.

Finite Element Modeling Approach

Material Models and Constitutive Relationships

The research incorporates three critical material behaviors that are essential for accurate simulation of CFST members under cyclic loading:

Material Behavior	Implementation	Significance
Bauschinger effect	Kinematic hardening model for steel	Captures yield surface translation during load reversal
Concrete damage accumulation	Damage plasticity model	Represents progressive micro-cracking under cyclic stress
Stiffness degradation	Strain-softening law	Accounts for loss of load-carrying capacity

Contact Model and Element Selection

The finite element model employs appropriate element types for each component and defines the steel-concrete contact interface to accurately capture the interaction behavior. The void region is modeled as a spherical-cap-shaped separation zone where the contact constraint is intentionally removed, simulating the loss of composite action in that region.

Results and Mechanism Analysis

Model Validation

The finite element model was validated against experimental results, showing good agreement in:

Failure mode prediction (local buckling pattern of the steel tube)
Lateral load (P) versus lateral displacement (Δ) hysteresis curves
Peak load and residual strength values

Influence of Void Defects on Hysteresis Performance

Performance Indicator	Effect of Void Defect	Mechanism
Peak load	Reduced	Loss of confinement in void region
Initial stiffness	Reduced	Discontinuity in load transfer path
Hysteresis loop area	Reduced	Decreased energy dissipation capacity
Ductility	Reduced	Accelerated local buckling initiation
Residual strength	Significantly reduced	Compromised composite action
Cyclic degradation rate	Accelerated	Stress concentration at void edges

Steel-Concrete Interaction Mechanism

The research reveals that the interaction between the steel tube and concrete core operates through three primary mechanisms:

Frictional shear transfer: Tangential stress transfer at the steel-concrete interface due to relative slip resistance
Confinement pressure: Radial confining stress exerted by the steel tube on the concrete core, which is lost in the void region
Dilatation interaction: The tendency of concrete to expand laterally under compression engages the steel tube in tension, creating the composite strengthening effect

When a void defect exists, all three mechanisms are disrupted in the affected region. The void edge creates a stress concentration zone where the remaining intact interface experiences elevated shear stresses, potentially accelerating interface degradation in subsequent load cycles.

Engineering Practice Implications

Quality Control for CFST Fabrication

Control Item	Risk of Void Formation	Prevention Measure
Concrete pumping pressure	Insufficient pressure creates voids	Maintain pressure > 0.5 MPa above pipe elevation
Concrete slump	Too low slump causes segregation	Use high-flow concrete (slump ≥ 200 mm)
Pipe internal cleanliness	Debris blocks concrete flow	Clean pipe interior before concreting
Concreting sequence	Layered filling creates interfaces	Fill from bottom in one continuous pour
Vibration (if used)	Insufficient vibration traps air	Use internal vibrator with proper spacing

Defect Detection Methods

From a non-destructive testing perspective, void defects in CFST members can be detected using:

Ultrasonic testing (UT): Impulse-echo method to detect steel-concrete interface discontinuities
Thermal infrared imaging: Temperature differences between filled and void regions during heating/cooling cycles
Vibration analysis: Modal frequency changes indicate mass distribution anomalies
Electromagnetic methods: Eddy current or magnetic flux leakage for surface-near voids

Study Insights and Reflections

This research is significant because it quantifies the structural consequences of what is often considered a minor construction defect—the void between steel and concrete. In practice, voids are frequently overlooked because they are internal and invisible during construction. However, this study demonstrates that even moderate void areas can significantly degrade the hysteretic performance, which is the primary design criterion for seismic-resistant structures.

The use of the Bauschinger effect in the material model is particularly noteworthy. Many simplified analyses of CFST members neglect this effect, leading to overestimation of energy dissipation capacity. The inclusion of this effect, combined with concrete damage accumulation, provides a more realistic representation of cyclic behavior and explains why void defects have a disproportionate impact on hysteresis performance compared to monotonic strength.

The practical implication for engineers is clear: void prevention and detection must be elevated to critical quality control activities in CFST construction. The financial cost of a void defect is not merely the material replacement but the potential degradation of seismic performance that may not be discovered until a major earthquake event.