Mechanical Properties of Eccentrically Loaded Steel Tube Recycled Concrete Columns After High Temperature Exposure

Literature Overview

This 2018 study by Chen Zongping, Liang Houren, and Wang Cheng from Guangxi University, published in Fire Science and Technology, investigates the residual mechanical performance of steel tube recycled concrete (STRC) eccentrically loaded columns after exposure to elevated temperatures. The research addresses two important sustainability and safety concerns simultaneously: the use of recycled coarse aggregate in structural concrete and the fire resistance performance of steel tube concrete columns.

Core Technical Findings

Experimental Program and Test Matrix

The researchers tested 18 steel tube recycled concrete column specimens after exposure to various elevated temperatures. The experimental program was designed to systematically investigate the effects of two key parameters: the fire exposure temperature and the recycled coarse aggregate replacement rate.

Performance Degradation Patterns

The mechanical performance indicators examined included ultimate load, peak displacement, displacement ductility coefficient, and energy dissipation coefficient. The key findings regarding degradation patterns are:

Effect of temperature:

• As exposure temperature increases, all mechanical performance indicators degrade monotonically for both circular and square steel tube specimens.
• The temperature effect is more pronounced for circular steel tube specimens than for square steel tube specimens. This counterintuitive result can be attributed to the different confinement mechanisms: circular tubes provide uniform confinement pressure, so when the concrete degrades at high temperature, the confinement effectiveness is uniformly reduced. Square tubes, on the other hand, provide non-uniform confinement with higher confinement at corners and lower confinement at mid-span of walls. At high temperatures, the degraded concrete in the low-confinement regions may still be partially supported by the corner regions, resulting in a more gradual overall degradation.

Effect of recycled aggregate replacement rate:

• As the recycled aggregate replacement rate increases, all mechanical performance indicators degrade for both tube shapes.
• The replacement rate effect is more pronounced for square steel tube specimens than for circular steel tube specimens. This is because the non-uniform confinement in square tubes amplifies the negative effects of recycled aggregate on concrete properties. The recycled aggregate, with its higher water absorption and lower density, creates additional non-uniformity in the concrete core properties, which interacts adversely with the non-uniform confinement of the square tube.

Interaction Effects

An important finding is the interaction between temperature and replacement rate effects. The combined effect is not simply additive; rather, the two factors interact in a complex manner that depends on the tube shape. For circular tubes, the temperature effect dominates, while for square tubes, the replacement rate effect becomes more significant at elevated temperatures.

Engineering Practice Implications

Fire Design of Steel Tube Recycled Concrete Columns

The research provides important guidance for the fire design of SRC columns incorporating recycled aggregates:

• Residual strength estimation: Engineers should use the degradation curves provided in the research to estimate the residual load capacity of STRC columns after fire exposure, considering both the exposure temperature and the recycled aggregate replacement rate.
• Square tube preference for fire resistance: The finding that square tubes exhibit more gradual degradation with temperature suggests that square steel tubes may offer better fire performance for recycled concrete columns, particularly when high replacement rates are used. However, this must be balanced against the generally lower overall confinement effectiveness of square tubes compared to circular tubes.
• Replacement rate limits: For fire-exposed structures, the recycled aggregate replacement rate should be limited to ensure adequate residual strength. The research suggests that replacement rates above 70% may result in unacceptable residual strength degradation at temperatures above 600°C.
• Fire protection measures: Additional fire protection measures, such as intumescent coatings or fire-resistant concrete, may be necessary for STRC columns with high recycled aggregate replacement rates to ensure adequate fire resistance.

Welding and Fabrication Considerations

The use of recycled aggregates in steel tube concrete columns has implications for the steel tube fabrication and welding process:

• Tube geometry tolerance: Recycled concrete has different rheological properties compared to conventional concrete, which may affect the concrete filling process and the resulting concrete-steel interface. The steel tube geometry, including out-of-straightness and out-of-roundness tolerances, becomes more critical when using recycled concrete, as the non-uniform concrete properties can interact with geometric imperfections to create stress concentrations.
• Weld quality for fire-exposed structures: The welding quality of steel tubes in fire-exposed structures is critical because welding defects can serve as crack initiation sites under thermal cycling. The heat-affected zone (HAZ) of longitudinal welds may experience additional degradation at elevated temperatures, particularly if the steel grade has a low carbon equivalent. Post-weld heat treatment (PWHT) may be necessary to ensure adequate toughness retention at fire exposure temperatures.
• Recycled aggregate and steel tube compatibility: The recycled aggregate content in the concrete affects the thermal expansion coefficient and thermal conductivity of the concrete core. These properties influence the thermal stress distribution in the steel tube during fire exposure. The steel tube material selection should consider the thermal compatibility with the recycled concrete core.

Key Questions and Reflections

The research provides valuable data on the fire performance of STRC columns, but several important questions remain. First, the long-term durability of STRC columns after fire exposure is not addressed. The recycled aggregate, with its higher porosity and water absorption, may be more susceptible to carbonation and chloride ingress after fire damage, leading to accelerated corrosion of the steel tube.

Second, the research focuses on static loading after fire exposure. The dynamic response of STRC columns under post-fire seismic loading, a realistic hazard scenario, requires additional investigation. The interaction between fire-damaged concrete, thermally degraded steel tube properties, and cyclic seismic loading is complex and not fully understood.

Third, the research does not address the environmental and economic aspects of using recycled aggregates in fire-exposed structures. The cost-benefit analysis of using recycled aggregates must consider not only the initial material savings but also the potential increased maintenance costs due to reduced fire resistance and durability.

Study Insights and Implications

This research makes an important contribution to the sustainable design of steel tube concrete structures by quantifying the fire performance trade-offs associated with recycled aggregate use. The finding that tube shape significantly moderates the interaction between temperature and replacement rate effects provides a practical design lever: engineers can select tube shapes to optimize the fire performance of recycled concrete columns. For steel pipe manufacturers and welding engineers, the research highlights the importance of considering the entire structural system, including the concrete core composition, when specifying steel tube materials and welding procedures for fire-exposed applications. The welding quality requirements for fire-exposed STRC columns should be elevated to account for the additional thermal degradation of the HAZ and the potential for fire-induced cracking at weld locations. Ultimately, the research supports the continued development of sustainable construction practices while emphasizing the need for rigorous fire performance assessment of recycled material systems.