Axial Compression Behaviour of Medium-Long CFST Columns Filled with Self-Stressing Self-Consolidating High-Strength Concrete

Literature Overview

The paper by Xu Lihua, Xu Fangzhou, Zhou Penghua, Gu Yushan, Wu Min, and Xu Mingyao (2016, China Civil Engineering Journal, Vol. 49, No. 11) presents an experimental investigation into the axial compression performance of medium-long concrete-filled steel tube (CFST) columns filled with self-stressing self-consolidating high-strength concrete (SSS-HSC). The study is motivated by a practical engineering need: conventional concrete in CFST columns often suffers from poor compaction in dense reinforcement zones and significant shrinkage, which can lead to gaps between the concrete core and the steel tube, thereby degrading the composite action that is the very foundation of CFST column performance.

Sixteen column specimens were designed and tested, varying initial self-stress level, slenderness ratio, and concrete compressive strength. The test programme was comprehensive, capturing failure modes, load-longitudinal deformation curves, load-deflection curves, and ultimate bearing capacity for each specimen.

Experimental Programme and Key Parameters

Specimen Design Parameters

The self-stressing property of the concrete is achieved through expansive agents (typically calcium hydroxide or magnesium oxide-based) that generate internal expansive pressure during hydration. The self-consolidating property ensures full densification without vibration, which is particularly important for CFST columns where access for vibration equipment is extremely limited.

Key Findings and Technical Analysis

Effect of Initial Self-Stress

The most striking finding is that increasing the initial self-stress by 5 MPa results in a 19% increase in ultimate bearing capacity, along with an improvement in ductility. This is a substantial improvement and directly addresses the gap problem between concrete and steel tube. The self-stress effectively pre-compresses the concrete core against the steel tube wall, establishing intimate contact from the outset and maintaining it throughout the loading history. From a welding and fabrication perspective, this is particularly important because any gap between concrete and steel tube in a CFST column can lead to local buckling of the tube wall under compression, which would be exacerbated by the stress concentrations at welded joints in the tube fabrication.

The mechanism behind this improvement is straightforward but elegant: the self-stress creates a radial confining pressure on the concrete core even before external loading is applied. This pre-confinement enhances the triaxial compressive strength of the concrete and delays the onset of concrete crushing. Additionally, the self-stress helps to close any micro-cracks that may develop during the early hydration period, thereby improving the long-term durability of the column.

Effect of Slenderness Ratio

As expected from classical column theory, increasing the slenderness ratio from 5 to 12 reduces the ultimate bearing capacity by 16.9%. The failure mode for most specimens was bending instability (flexural buckling), which is consistent with the Euler buckling theory for slender columns. However, the reduction in capacity was less than what would be predicted by pure Euler theory, indicating that the concrete core provides additional stability against lateral deflection. This is an important practical finding: CFST columns with self-stressing concrete retain more of their theoretical capacity at moderate slenderness ratios than conventional CFST columns.

Effect of Concrete Strength

Increasing the concrete compressive strength by 36.3% (from 60 MPa to 80 MPa) resulted in a 16.3% increase in ultimate bearing capacity. The non-proportional increase (16.3% vs. 36.3%) is attributed to the fact that higher-strength concrete is more brittle and provides less confinement benefit to the steel tube. This is consistent with the well-established behaviour of CFST columns and underscores the importance of the steel tube in providing ductility and confinement, rather than relying solely on concrete strength.

Proposed Design Formula and Engineering Applicability

Based on the experimental results and with reference to existing Chinese design codes (GB 51229-2017 for CFST structures), the authors propose an axial compression stability bearing capacity calculation formula for SSS-HSC CFST medium-long columns. The formula incorporates the self-stress effect through an additional confining pressure term, the slenderness effect through a stability coefficient, and the concrete strength effect through the conventional composite action term.

From an engineering practice standpoint, the proposed formula is a valuable addition to the designer's toolkit, but its application requires careful consideration. The self-stress level must be reliably achieved and maintained throughout the concrete curing period, which depends on the quality of the expansive agent, the water-binder ratio, and the curing conditions. In field conditions, achieving and verifying the specified self-stress level is challenging and may require non-destructive testing methods such as ultrasonic pulse velocity measurements or pressure cell monitoring during curing.

Quality Control Considerations

For engineers involved in the fabrication and erection of CFST columns with self-stressing concrete, several quality control measures are essential:

1. Concrete mix design verification: The expansive agent dosage must be precisely controlled, and trial mixes should be conducted to verify both the self-consolidating flow properties and the self-stress development rate.
2. Filling process control: The self-consolidating concrete should be placed in layers if the column height exceeds a certain limit (typically 3–5 m per lift) to prevent excessive internal pressure buildup and potential bursting of the steel tube.
3. Curing monitoring: Temperature and pressure monitoring during curing is recommended to ensure that the self-stress develops as expected and that no thermal cracking occurs.
4. Steel tube fabrication quality: The steel tube must be manufactured to tight dimensional tolerances, and any welding defects (such as incomplete fusion, porosity, or undercut) must be repaired before concrete filling, as these defects can compromise the tube's ability to contain the self-stressing concrete.

Study Insights and Reflections

The concept of self-stressing self-consolidating concrete for CFST columns is a significant innovation that addresses a long-standing practical problem. In my experience, the gap between concrete and steel tube in CFST columns is one of the most common and consequential defects encountered in construction quality inspections. The self-stress approach offers a proactive solution rather than a reactive one, which is philosophically superior from a quality management perspective.

However, I would caution that the 19% capacity improvement reported for a 5 MPa increase in self-stress should be validated through large-scale tests and long-term monitoring before being adopted in critical structural applications. The long-term stability of the self-stress — whether it is maintained over decades of service or gradually relaxes due to concrete creep — remains an open question that requires further investigation.

Conclusion

This study makes a meaningful contribution to the field of CFST column technology by demonstrating that self-stressing self-consolidating high-strength concrete can significantly enhance the axial compression performance of medium-long CFST columns. The proposed design formula provides a practical tool for engineers, and the quality control considerations outlined above should be incorporated into any specification for such columns. The technology holds promise for applications in high-rise buildings, bridge piers, and industrial structures where CFST columns are subjected to high axial loads and where traditional concrete compaction methods are impractical.