Axial Compression Performance of Square and Circular Steel Tube Recycled Concrete Columns After High-Temperature Water Spray Cooling

Literature Overview

This paper by Chen Zongping and Zhou Ji, published in Industrial Construction in 2020, compares the axial compression performance of square and circular steel tube recycled concrete (RAC) short columns after exposure to high temperatures followed by water spray cooling. The study investigates the effects of section shape, peak temperature, recycled coarse aggregate replacement rate, and cooling method on the residual load capacity, initial axial stiffness, ductility, and energy dissipation capacity of the columns. A total of 54 specimens (27 square and 27 circular) were tested. The research was supported by multiple funding sources including the National Natural Science Foundation of China (Grant No. 51578163).

Core Technical Points

The study addresses a critical engineering scenario: the residual structural performance of STC columns after fire exposure and subsequent cooling. Fire exposure is one of the most severe accidental loading scenarios that structures may encounter, and understanding the residual capacity of STC columns after fire is essential for post-fire structural assessment and potential repair or reuse decisions.

The key findings reveal significant differences between square and circular sections in their response to fire and cooling. Square section specimens exhibit more significant degradation in load capacity, ductility, and energy dissipation capacity as temperature increases, compared to circular section specimens. The cooling method has a significant effect on the initial axial stiffness and energy dissipation capacity of square section specimens, while it significantly affects the load capacity and ductility of circular section specimens.

Performance Degradation Summary

Interpretation of Technical Points

The differential behavior of square and circular sections after fire exposure can be attributed to several factors. First, the confinement effectiveness of circular steel tubes is inherently superior to that of square steel tubes because the circular cross-section provides uniform radial confinement without stress concentration at corners. Second, the thermal expansion mismatch between the steel tube and the concrete fill creates additional stresses, and these stresses are more severe in square sections due to the non-uniform thermal expansion distribution.

The water spray cooling method introduces rapid temperature gradients that can cause thermal shock cracking in the concrete and potentially lead to spalling of the concrete cover near the steel tube inner surface. The differential effects of cooling on square versus circular sections suggest that the stress state within the composite column after cooling is highly dependent on the section geometry.

The counterintuitive finding that increasing recycled aggregate replacement rate increases the ductility and energy dissipation of square sections but decreases them for circular sections is particularly noteworthy. This may be related to the interaction between the recycled aggregate's weaker interfacial transition zone and the different confinement mechanisms of square versus circular sections.

Engineering Practice Integration

From a steel pipe manufacturing perspective, the choice between square and circular sections for STC columns in fire-prone environments has significant implications. Circular sections are generally preferred for their superior confinement effectiveness and more predictable post-fire behavior. However, square sections offer practical advantages in terms of alignment with architectural grids and ease of connection to steel beams.

The manufacturing quality of the steel tube is particularly critical in fire-exposed applications. Any manufacturing defects such as weld inclusions, wall thickness variations, or surface irregularities can act as stress concentrators during thermal loading, potentially initiating cracks that propagate during cooling. For circular tubes, the longitudinal seam weld is a critical feature, and its quality directly affects the post-fire performance of the column.

In terms of welding, the connections between the steel tube and any external reinforcement or connection plates must be designed to accommodate the thermal expansion and contraction that occurs during fire exposure and cooling. Welding residual stresses can interact with thermal stresses, potentially leading to premature failure of the connection. This is particularly important for square sections where the corner regions experience higher stresses.

Key Questions and Reflections

The study provides valuable comparative data on the post-fire performance of square and circular STC columns with recycled concrete. However, several practical questions remain. First, what is the maximum temperature at which a square STC column can retain acceptable residual load capacity for continued use after fire? Second, how does the recycled aggregate replacement rate affect the thermal conductivity of the concrete fill, and does this influence the temperature distribution within the column during fire exposure?

Third, from a manufacturing standpoint, are there specific steel tube manufacturing requirements that should be specified for fire-prone applications? For example, should the steel tube material be selected for its high-temperature strength retention rather than room temperature yield strength? Should the welding procedure be optimized to minimize residual stresses that could interact with thermal stresses?

The study also raises the question of whether the use of recycled aggregate in fire-prone applications is advisable. While the environmental benefits of using recycled aggregate are clear, the reduced thermal performance and the complex interaction between recycled aggregate and fire exposure warrant careful consideration in design.

Study Insights and Implications

This research provides important comparative data on the post-fire behavior of square and circular STC columns with recycled concrete. The findings suggest that circular sections generally offer more reliable post-fire performance, particularly in terms of load capacity and ductility retention. For steel pipe engineers, the key implication is that the selection of section shape must consider not only the structural efficiency under normal loading but also the residual performance after fire exposure. The study also highlights the need for further research on the interaction between recycled aggregate properties and fire exposure, as well as the development of manufacturing specifications that address the specific requirements of fire-prone STC applications.