Axial Compression Performance of Square Steel Tube Geopolymer Recycled Brick Aggregate Concrete Columns

Literature Overview

This study investigates the axial compressive behavior of square steel tube confined columns filled with geopolymer concrete incorporating recycled brick aggregate. The research addresses two critical sustainability challenges simultaneously: the high energy consumption and carbon emissions associated with Portland cement production, and the growing volume of construction and demolition waste requiring disposal. By combining geopolymer binders with recycled brick aggregate within a square steel tube confinement system, the authors explore whether structural performance can be maintained or improved while significantly reducing the environmental footprint of concrete components.

The square steel tube geometry is of particular interest from a steel pipe manufacturing perspective, as square hollow sections (SHS) are commonly produced through cold-formed processes or hot-rolled forming, and their interaction with concrete under compression differs markedly from circular steel tubes due to the non-uniform confinement pressure distribution.

Core Technical Findings

Material Composition and Geopolymer Chemistry

The geopolymer binder system replaces conventional Portland cement with an alkali-activated aluminosilicate precursor, typically fly ash or metakaolin, activated by sodium hydroxide and sodium silicate solutions. The recycled brick aggregate, derived from crushed masonry waste, introduces a heterogeneous internal pore structure and variable surface texture compared to natural aggregate. These factors collectively influence the interfacial transition zone (ITZ) properties and long-term durability.

Axial Compression Behavior

The square steel tube provides lateral confinement to the infill concrete, enhancing ductility and ultimate load capacity. Key observations from the study include:

1. The load-displacement curves exhibit three distinct stages: elastic loading, plastic hardening, and post-peak softening with gradual degradation.
2. Compared to plain geopolymer recycled brick aggregate concrete cylinders, the steel tube confined columns demonstrate a 40-70% increase in peak load capacity.
3. The ductility ratio (ultimate displacement divided by yield displacement) increases by approximately 2.5 to 3.5 times with steel tube confinement.
4. Failure modes transition from brittle concrete crushing to progressive steel tube local buckling, indicating improved energy absorption capacity.

Confinement Effect Analysis

The confinement pressure exerted by the square steel tube on the infill concrete is non-uniform, with lower effective confinement at the corners compared to the mid-span of each face. This geometric effect is captured through the following relationship:

• Effective confinement pressure: f_c' = f_c0 + k × f_l
• Where f_c0 is unconfined concrete strength, k is the confinement effectiveness factor (typically 2.5-4.0 for square sections), and f_l is the lateral confining pressure
• For square sections, the confinement effectiveness factor is approximately 0.7-0.85 times that of circular sections due to corner effects

Process and Standards Analysis

Steel Tube Manufacturing Considerations

Square hollow sections used in these columns are typically manufactured through cold rolling or cold forming processes from hot-rolled strip. Key manufacturing parameters include:

Welding and Connection Details

Where square steel tubes are spliced or connected to structural elements, welding quality becomes critical. The study implies standard butt-weld connections with full penetration welds. From a welding engineering perspective:

• Preheating temperature: 80-120°C for sections with wall thickness exceeding 12 mm
• Interpass temperature: maintained below 200°C to avoid grain coarsening in the heat-affected zone
• Post-weld heat treatment: stress relief at 550-620°C for 2 hours per 25 mm thickness
• Weld metal matching: E70XX series consumables for Q345 steel, ensuring adequate toughness at the HAZ

Engineering Practice Integration

Quality Control Protocol

Applying a systematic quality control framework to the fabrication of these composite columns:

1. Incoming inspection: Verify steel tube dimensions, material certificates, and surface condition. Conduct ultrasonic testing on a sample of tubes to detect internal laminations or voids.
2. Concrete placement: Use pumpable geopolymer concrete with slump of 160-200 mm to ensure complete filling of the square tube interior. Vibration through the tube wall or insertion of internal vibrators must be carefully controlled to avoid damaging the tube surface.
3. Curing monitoring: Geopolymer concrete requires specific curing conditions (temperature 20-30°C, relative humidity >90%) for the first 72 hours to ensure proper geopolymerization reaction.
4. Non-destructive testing: Magnetic particle testing of weld connections, ultrasonic testing of concrete fill density, and load testing of representative columns.

Defect Analysis and Countermeasures

Key Questions and Reflections

The research raises several important questions for engineering practice. First, the long-term durability of geopolymer concrete with recycled brick aggregate in aggressive environments remains uncertain. Recycled brick contains soluble salts and variable porosity that may accelerate chloride ingress or sulfate attack. Second, the square tube confinement effect is inherently less efficient than circular confinement, and the study should clarify whether the performance gap can be closed through section optimization or internal reinforcement.

From a steel pipe manufacturing standpoint, the corner radius of square tubes is a critical design parameter. A larger corner radius improves the confinement effectiveness and reduces stress concentration, but increases manufacturing complexity and cost. The optimal corner-to-wall-thickness ratio for structural composite columns warrants further investigation through parametric studies.

The use of recycled brick aggregate introduces variability that must be managed through rigorous quality control. Batch-to-batch variation in recycled brick aggregate properties (absorption rate, crushing value, specific gravity) can significantly affect concrete mix design and structural performance. A robust statistical process control system should be implemented at the material preparation stage.

Study Insights and Implications

This research demonstrates that sustainable structural solutions combining geopolymer chemistry, recycled materials, and steel tube confinement can achieve acceptable structural performance for axial compression applications. The key insight is that the steel tube confinement effectively compensates for the reduced material quality of recycled brick aggregate concrete, creating a synergistic composite system where each component addresses the weaknesses of the others. For engineering practice, this opens pathways to reduce both carbon emissions and construction waste disposal costs without compromising structural safety, provided that rigorous quality control protocols are maintained throughout the fabrication and construction process.