Post-High-Temperature Mechanical Properties of Square Steel Tube Recycled Concrete Axially Compressed Short Columns

Literature Overview

This paper by Chen Zongping, Ye Peihuan, Xue Jianyang, and Li Ling (published in Industrial Construction, Vol. 44, No. 11, 2014, pp. 13-18) investigates the axial compression behavior of square steel tube recycled concrete (RC) short columns after exposure to elevated temperatures. The research was funded by the National Natural Science Foundation of China (Grant 51268004), the "Bague Scholar" Construction Engineering Special Fund, Guangxi Science and Technology Project (Grant 12118023-3), and Guangxi Natural Science Foundation (Grant 2013GXNFSFDA019025). Twenty-four specimens were tested with recycled coarse aggregate replacement rate and temperature as the two primary variables.

Experimental Design and Parameters

The test program was systematically designed with two main variables:

Each specimen was first exposed to the designated elevated temperature, then cooled to ambient conditions, and subsequently loaded under axial compression to failure. This approach simulates the post-fire condition of structural columns in steel tube recycled concrete construction.

Key Experimental Results

Effect of Temperature

As temperature increases, the following trends were observed:

The non-monotonic behavior of ductility and energy dissipation is particularly interesting and warrants detailed discussion. At moderate temperatures, the degradation of concrete strength and steel properties reduces ductility. At higher temperatures, the increased plasticity of the steel tube and the softened concrete may contribute to greater deformation capacity, resulting in increased ductility despite lower strength.

Effect of Recycled Aggregate Replacement Rate

As the recycled aggregate replacement rate increases:

The initial increase in elastic stiffness with recycled aggregate replacement is counterintuitive and may be attributed to the rougher surface texture of recycled aggregates providing better interfacial bonding with the mortar matrix, leading to improved stress transfer at low strain levels. However, at higher replacement rates, the inherent porosity and weaker interfacial transition zone of recycled aggregates lead to stiffness reduction.

Bearing Capacity Calculation

The paper proposes a calculation method for the ultimate bearing capacity of post-high-temperature square steel tube recycled concrete short columns, considering the temperature-dependent degradation of both steel and concrete properties, as well as the confinement effect of the square steel tube on the recycled concrete core.

Technical Analysis from Steel Pipe and Welding Perspective

Square Steel Tube Fabrication and Welding

The square steel tubes used in these columns are typically fabricated from steel plates using submerged arc welding (SAW) or electric resistance welding (ERW) processes. Key fabrication considerations include:

1. Plate selection: The steel grade must meet requirements for both ambient temperature structural performance and post-fire residual strength. Common grades include Q235B, Q345B, and Q345C per GB/T 1591 and GB/T 700.
2. Welding process: SAW is preferred for thicker plates (above 6 mm) due to its high deposition rate and deep penetration. The welding parameters must be optimized to minimize heat-affected zone (HAZ) softening and residual stresses.
3. Weld quality: For post-fire structural applications, weld integrity is critical. The HAZ of SAW welds may have different thermal properties compared to the base metal, potentially affecting the column's behavior during fire exposure. UT inspection of all longitudinal welds is essential.
4. Post-weld treatment: Stress relief annealing may be considered to reduce residual stresses that could be exacerbated by thermal exposure during fire events.

Recycled Concrete and Steel Tube Interaction

The recycled concrete contains recycled coarse aggregates that have been previously used in concrete construction and subsequently demolished. These aggregates have:

• Higher porosity and water absorption compared to natural aggregates
• Weaker interfacial transition zone (ITZ) between aggregate and mortar
• Residual mortar attached to aggregate surfaces
• Potentially lower crushing strength

The square steel tube provides confinement to the recycled concrete, which is particularly important because the weaker ITZ of recycled aggregates would otherwise lead to premature concrete failure under compression. The confinement pressure from the steel tube effectively limits lateral expansion of the concrete, increasing its compressive strength and ductility.

Post-Fire Steel Properties

The steel tube properties after elevated temperature exposure are governed by the temperature- dependent degradation of yield strength, elastic modulus, and ultimate strength. The Eurocode 3 (EN 1993-1-2) and relevant Chinese codes (GB 51249) provide temperature-dependent reduction factors for structural steel. The welding HAZ may exhibit different degradation behavior compared to the base metal, as the microstructure in the HAZ is affected by the original welding thermal cycle.

Engineering Practice Implications

Design Considerations

1. Temperature-dependent design: Columns using recycled concrete in steel tubes must be designed considering post-fire residual capacity, not just ambient temperature performance.
2. Steel tube wall thickness: Adequate wall thickness is essential to maintain confinement after fire exposure, as the steel tube may lose significant strength at elevated temperatures.
3. Recycled aggregate quality control: The quality of recycled aggregates directly affects the column's post-fire performance. Strict quality control of recycled aggregates, including screening, washing, and strength testing, is essential.
4. Weld inspection after fire: Following a fire event, the integrity of steel tube welds should be inspected using MT and UT to detect any thermal damage or cracking.

Quality Assurance for Recycled Concrete Steel Tube Columns

Key Questions and Reflections

The research raises important questions about the applicability of recycled concrete in steel tube columns for structures in regions with significant fire risk. While the post-fire performance is characterized, the question of whether the column can be safely reused after a fire event remains. The non-monotonic behavior of ductility with temperature suggests that there may be an optimal temperature range beyond which structural performance deteriorates rapidly. Additionally, the interaction between recycled aggregate properties and steel tube welding quality under fire conditions is complex and warrants further investigation.

Study Insights and Implications

This research provides valuable data on the post-high-temperature behavior of square steel tube recycled concrete columns, contributing to the sustainable construction agenda by demonstrating that recycled aggregates can be used in steel tube concrete columns with predictable post-fire performance. The proposed bearing capacity calculation method offers a practical design tool. For steel pipe fabrication and welding engineers, the key insight is that the welding quality of square steel tubes is critical for post-fire structural performance, and the HAZ properties must be considered in the overall assessment of column integrity after fire exposure. The combination of recycled concrete and steel tube confinement represents a viable sustainable construction approach, provided that appropriate quality control and design considerations are implemented.