Axial Compressive Bearing Capacity Calculation Method for Steel Fiber Reinforced Geopolymer Concrete Filled Square Steel Tubes

Literature Overview and Research Background

This study develops a calculation method for the axial compressive bearing capacity of square steel tube short columns filled with steel fiber reinforced geopolymer concrete (SFRC-GPC). The research addresses an emerging composite construction system that combines the structural advantages of steel tubes with the unique properties of geopolymer concrete and steel fiber reinforcement. Geopolymer concrete, an alkali-activated cementitious material that utilizes industrial by-products such as fly ash and slag as binders, offers significant environmental benefits over conventional Portland cement concrete while achieving comparable or superior mechanical properties. The incorporation of steel fibers further enhances ductility, crack resistance, and energy absorption capacity, making this composite system particularly attractive for seismic applications and heavy-load structural members.

The focus on square steel tubes, as opposed to circular sections, reflects the practical advantages of square sections in construction: easier connection details, better space utilization, and more uniform stress distribution under biaxial loading conditions. However, square sections exhibit different buckling behavior and concrete confinement mechanisms compared to circular tubes, necessitating specialized calculation methods.

Core Technical Methodology

The research methodology combines experimental investigation with theoretical modeling to develop a reliable calculation method. The experimental program includes axial compression tests on square steel tube columns filled with SFRC-GPC, varying key parameters such as steel fiber volume fraction, geopolymer concrete strength grade, steel tube dimensions, and slenderness ratio. The theoretical development involves analyzing the stress-strain behavior of the confined SFRC-GPC, the interaction mechanism between the steel tube and the concrete core, and the overall column failure mode.

The calculation method development follows a systematic approach:

1. Characterization of SFRC-GPC unconfined compressive strength and strain behavior through cylinder tests.
2. Determination of confined SFRC-GPC stress-strain relationships considering the lateral confinement provided by the square steel tube.
3. Analysis of steel tube stress state under axial compression with consideration of concrete dilation effects.
4. Formulation of bearing capacity equations based on equilibrium, compatibility, and material constitutive laws.
5. Validation and calibration against experimental results.

Key Technical Parameters and Constitutive Relationships

The confined SFRC-GPC stress-strain model is a critical component of the calculation method. The model accounts for the enhanced compressive strength and significantly improved post-peak ductility resulting from the steel tube confinement. The confinement pressure in square tubes differs from circular tubes due to the non-uniform lateral pressure distribution; the corners of the square section experience lower lateral pressure than the mid-span of the flat faces. This non-uniformity is typically addressed through an effective confinement pressure model that applies a reduction factor to the nominal confinement pressure.

The steel fiber contribution to the bearing capacity is primarily through enhanced post-peak behavior and improved strain capacity rather than significant strength increase. At fiber volume fractions of 1.0 to 1.5 percent, the ductility improvement is most pronounced, with energy absorption capacity increasing by 30 to 50 percent compared to plain geopolymer concrete. The calculation method incorporates a fiber enhancement factor that modifies the stress-strain curve in the post-peak region.

Bearing Capacity Calculation Framework

The proposed calculation method establishes the axial bearing capacity as the sum of contributions from the steel tube and the confined concrete core, with interaction effects accounted for through compatibility conditions. The general framework follows:

1. Steel tube contribution: Calculated based on the stress-strain behavior of the steel under combined axial and hoop stress states, considering the lateral pressure exerted by the expanding concrete core.
2. Confined concrete core contribution: Based on the confined SFRC-GPC stress-strain relationship, integrated over the cross-sectional area.
3. Interaction factor: A correction factor accounting for the non-uniform confinement in square sections, typically ranging from 0.85 to 0.95 depending on the section geometry and steel tube slenderness.

The method demonstrates good agreement with experimental results, with prediction accuracy within ±10 percent for most test specimens. The accuracy improves significantly when the steel fiber volume fraction and geopolymer concrete strength are within the tested parameter ranges.

Process and Material Interaction Analysis

The failure mode of SFRC-GPC filled square steel tube short columns under axial compression is characterized by a sequence of events that reflects the progressive interaction between the steel tube and the concrete core. Initially, both materials deform elastically under increasing axial load. As the concrete reaches its unconfined compressive strength, it begins to dilate laterally, exerting increasing pressure on the steel tube walls. The steel tube responds by developing hoop stresses that provide confinement, preventing premature concrete failure and enabling the concrete to sustain higher axial stresses.

The steel fibers play a crucial role in the post-peak behavior by bridging cracks and maintaining load-carrying capacity after initial concrete cracking. The fibers prevent sudden loss of confinement pressure, ensuring a more gradual transition from the peak load to the descending branch of the load-displacement curve. This behavior is particularly beneficial in seismic applications where energy dissipation and ductility are paramount.

The square section geometry introduces a unique challenge in the form of corner effects. At the corners of the square tube, the concrete core experiences less lateral confinement because the steel tube walls diverge at 90-degree angles. This results in earlier concrete crushing at the corners compared to the mid-span of the flat faces. The calculation method accounts for this through an effective confinement area concept that reduces the nominal concrete area based on the degree of corner influence.

Engineering Practice Integration

The SFRC-GPC filled steel tube system offers compelling advantages for practical applications in heavy-load columns, bridge piers, and seismic structural members. The environmental benefit of geopolymer concrete, which reduces CO2 emissions by approximately 60 to 80 percent compared to Portland cement concrete, aligns with sustainable construction goals. The enhanced ductility from steel fiber reinforcement makes the system suitable for regions with high seismic activity, where ductile behavior under cyclic loading is essential.

However, several practical considerations must be addressed for widespread implementation. The workability of geopolymer concrete, particularly with steel fiber additions, can be challenging during placement in confined steel tube spaces. Proper vibration and compaction techniques are essential to avoid voids and ensure uniform fiber distribution. The corrosion protection of the steel tube remains a critical concern, as geopolymer concrete, while offering good chemical resistance, may not provide the same alkaline environment as Portland cement concrete for passive corrosion protection of embedded steel.

Key Questions and Reflections

The research raises important questions regarding the long-term durability and time-dependent behavior of the SFRC-GPC composite system. Geopolymer concrete exhibits different aging characteristics compared to Portland cement concrete, with continued strength development over extended periods but potentially different shrinkage and creep behavior. The interaction between these time-dependent effects and the steel tube confinement requires further investigation for long-term serviceability predictions.

A significant reflection concerns the standardization challenges. The calculation method developed in this study is specific to the tested material combinations and geometric configurations. Extending the method to different geopolymer compositions, fiber types, and section geometries requires additional experimental validation. The lack of established design codes for geopolymer concrete applications represents a barrier to practical adoption, and the research contributes to building the technical foundation for future code development.

Study Insights and Implications

This research makes a significant contribution to the structural engineering community by providing a validated calculation method for an innovative composite system that combines environmental sustainability with enhanced structural performance. The SFRC-GPC filled square steel tube system represents a promising solution for heavy-load applications where both strength and ductility are required. For practicing engineers, the findings underscore the importance of understanding material interaction mechanisms in composite systems and the value of rigorous experimental and analytical approaches in developing reliable design methods. The environmental credentials of geopolymer concrete, combined with the proven structural reliability of steel tube confinement, position this system as a viable alternative to conventional reinforced concrete columns in demanding structural applications.