Zhuojin Pipe Fitting Co., LtdMechanical Properties of Self-Stressing Concrete-Filled Steel Tube Columns Under Axial Compression
Literature Overview
The paper by Chang Xu, Lin Haixiao, and Huang Chengkui (2009), published in the Journal of Henan Polytechnic University, investigates the mechanical behaviour of self-stressing concrete-filled steel tube (SS-CFST) axially compressed short columns. Self-stressing concrete, also known as expansive concrete, develops internal tensile stresses during hydration due to the incorporation of expansive agents (typically calcium sulfoaluminate-based or magnesium oxide-based). When confined within a steel tube, these expansive stresses are converted into beneficial compressive pre-stresses in the concrete core and additional tensile pre-stresses in the steel tube, fundamentally altering the stress state compared to conventional CFST columns.
Core Technical Approach
Self-Stress Mechanism in Confined Conditions
In conventional CFST columns, the steel tube and concrete core are initially stress-free before external loading. The confinement interaction only develops after the concrete begins to expand laterally under axial compression. In SS-CFST columns, the expansive reaction of the concrete generates radial pressure on the steel tube even before any external load is applied. This pre-confinement has several consequences:
| Aspect | Conventional CFST | Self-Stressing CFST |
|---|---|---|
| Initial stress state | Zero in both steel and concrete | Compressive pre-stress in concrete, tensile pre-stress in steel |
| Onset of confinement | After cracking and lateral expansion | Pre-existing confinement from expansive reaction |
| Effective compressive strength of concrete | $f_c$ (unconfined) | $f_c + \Delta f_{conf}$ (pre-confined) |
| Steel tube initial condition | Unstressed | Pre-stressed in tension (potential for early yielding) |
Finite Element Modelling
The FE model employs a three-dimensional formulation with the following constitutive models:
- Steel tube: Bilinear elastic-plastic model with isotropic hardening. The initial tensile pre-stress from self-stress is superimposed on the stress-strain curve, effectively shifting the yield point.
- Concrete core: A modified confinement model (based on the Mander model) that accounts for the pre-existing radial pressure from expansive hydration. The ultimate strain $\varepsilon_{cu}$ is enhanced proportionally to the total confinement pressure (expansive + external load-induced).
- Interface: A bonded contact model with tensile and shear capacity, representing the chemical bond and friction between steel and concrete.
Load-Strain Curve Characteristics
The study confirms that the load-strain curves of SS-CFST columns follow the same three-stage pattern as conventional CFST columns:
- Elastic stage: Linear response with a slope reflecting the composite axial stiffness. The presence of self-stress does not alter the initial stiffness significantly because the pre-stress level is typically much smaller than the elastic limit.
- Elastic-plastic stage: Progressive yielding of concrete and steel, with the confinement effect providing additional load-carrying capacity. The transition from this stage to the next is more gradual in SS-CFST columns due to the pre-existing confinement.
- Post-peak stage: Gradual softening as local buckling of the steel tube progresses.
Parametric Findings and Engineering Implications
Effect of Self-Stress Level
The study demonstrates that increasing the self-stress level (achieved by varying the expansive agent dosage) increases the ultimate bearing capacity of the column. However, the relationship is not linear: beyond a certain threshold, the additional benefit diminishes because the steel tube begins to yield prematurely due to the tensile pre-stress, reducing the effective confinement capacity.
Steel Ratio as the Dominant Parameter
A critical finding is that the steel ratio (the ratio of steel cross-sectional area to total cross-sectional area) is the primary factor governing both the bearing capacity and the shape of the load-displacement curve. This is consistent with the fundamental mechanics of CFST columns, where the steel tube serves as both a load-sharing component and a confinement device. The self-stress modifies the stress state but does not change the fundamental load-sharing mechanism.
| Steel Ratio | Effect on Capacity | Effect on Curve Shape | Effect on Ductility |
|---|---|---|---|
| Low (3–5%) | Moderate increase with self-stress | Slight steepening of elastic-plastic stage | Limited improvement |
| Medium (5–10%) | Significant increase with self-stress | Pronounced plateau in elastic-plastic stage | Noticeable improvement |
| High (10–15%) | Diminishing returns from self-stress | Broad plateau, gradual post-peak decline | Best ductility performance |
Engineering Practice Considerations
Application Scenarios
Self-stressing CFST columns are particularly advantageous in the following scenarios:
- Foundation piles and piers: Where post-tensioning is impractical, self-stressing concrete can provide pre-compression without external tendons.
- Repair and rehabilitation: Expansive concrete can be used to fill damaged CFST columns, restoring and even enhancing the original capacity.
- Low-temperature environments: The expansive reaction can compensate for the volume contraction of concrete during freezing, reducing the risk of internal cracking.
Fabrication and Construction Challenges
- Volume expansion control: The expansive reaction must be carefully controlled to prevent excessive cracking of the concrete core before it gains sufficient strength. This requires precise mixing, controlled placement, and adequate curing.
- Steel tube pre-stress management: The tensile pre-stress in the steel tube can be detrimental if it exceeds the yield strength of the steel. For high-strength steels (e.g., Q460 or higher), the expansive pressure must be limited to ensure the steel remains in the elastic range.
- Quality control: Non-destructive testing (NDT) must account for the pre-stress state. Ultrasonic testing (UT) velocities will be affected by the compressive pre-stress in the concrete, potentially leading to misinterpretation of internal defects if standard calibration curves are used.
Key Questions and Reflections
- The study uses a simplified interface model. In practice, the bond between expansive concrete and steel tube may be different from that of conventional concrete due to the altered pore structure and the presence of expansive hydration products. How does this affect the long-term performance?
- The study focuses on short columns. For slender SS-CFST columns, the pre-stress state may influence the buckling behaviour and the effective length factor. This warrants further investigation.
- The long-term durability of self-stressing CFST columns, particularly in aggressive environments (chloride exposure, carbonation, sulfate attack), remains an open question. The expansive hydration products may be more susceptible to chemical degradation than conventional hydration products.
Study Insights and Implications
This paper provides valuable evidence that self-stressing concrete can be effectively used to enhance the performance of CFST columns. The key insight is that the self-stress acts as an internal pre-confinement mechanism, shifting the stress-strain curve of the concrete core to higher stress levels and improving the composite action. However, engineers must exercise caution in the design of SS-CFST columns, particularly regarding the steel tube pre-stress level and the long-term stability of the expansive reaction. The parametric study confirms that the steel ratio remains the dominant design parameter, and self-stress should be viewed as a supplementary enhancement rather than a primary design strategy.