Effect of Cooling Methods on Axial Compression Performance of Recycled Concrete-Filled Square Steel Tube Columns After High Temperature

Literature Overview

This paper by Ke Xiaojun and colleagues from Guangxi University, published in 2021 in the Journal of Basic Science and Engineering, investigates the residual axial compression performance of concrete-filled square steel tube (CFSST) short columns incorporating recycled aggregate concrete after exposure to high temperatures. The study is particularly relevant given the growing concerns about fire safety in steel-concrete composite structures and the increasing adoption of recycled aggregates in sustainable construction. The authors designed 37 specimens with varying parameters including recycled aggregate replacement ratio, maximum exposure temperature, and cooling method, and conducted axial compression tests to evaluate their post-fire performance.

Core Technical Findings

The experimental results reveal complex interactions between recycled aggregate content, fire exposure conditions, and cooling methods. All specimens exhibited two primary failure modes: steel tube tearing and diagonal compressive failure accompanied by tube bulging. The load-displacement curves showed similar trends across all specimens, with the influence of recycled aggregate replacement ratio being more pronounced than that of cooling method or exposure temperature.

Key Experimental Parameters and Results

The most striking finding is the dramatic reduction in residual load capacity and initial stiffness when water spray cooling is applied compared to natural cooling. The maximum reductions reached 82.3% for residual load capacity and 50.9% for initial stiffness. This is attributed to the thermal shock effect caused by rapid cooling, which induces additional thermal stresses and may exacerbate existing damage in the concrete core and steel tube interface.

Process and Standards Analysis

From a materials science perspective, the high-temperature behavior of CFST columns is governed by several degradation mechanisms. At temperatures above 300°C, concrete begins to lose strength due to dehydration of cement hydrates and microcracking. Between 400°C and 800°C, significant strength loss occurs as free water evaporates and internal steam pressure causes spalling. The steel tube also experiences strength reduction at elevated temperatures, with significant degradation occurring above 550°C according to Eurocode 4.

The use of recycled aggregates introduces additional complexity. Recycled aggregates typically have higher water absorption and lower strength compared to natural aggregates, which can affect the concrete's thermal performance. The interface transition zone between recycled aggregates and the cement paste is often weaker, potentially leading to earlier failure under thermal stress. Relevant standards such as GB/T 50126 for recycled aggregate concrete and GB 50017 for steel structures provide guidelines for the design and assessment of such structures.

The cooling method has profound implications for post-fire structural assessment. Natural cooling allows for more uniform temperature reduction and avoids thermal shock, while water spray cooling, although faster, can cause additional damage due to rapid temperature gradients. This finding is particularly important for fire safety engineering, as the cooling method employed during fire suppression can significantly affect the residual structural capacity.

Integration with Engineering Practice

The findings of this study have direct implications for the design and post-fire assessment of CFST columns incorporating recycled aggregates. Engineers should consider the following practical recommendations:

1. When designing for fire resistance, account for the potential reduction in residual capacity after fire exposure, with particular attention to the cooling method used during fire suppression.
2. For structures using recycled aggregate concrete, conduct additional testing to determine the specific thermal degradation behavior, as recycled aggregates may exhibit different performance compared to natural aggregates.
3. In post-fire structural assessment, consider the cooling method employed during fire suppression when evaluating residual capacity, as water spray cooling may result in significantly lower residual strength than natural cooling.

The proposed residual load capacity calculation formula that accounts for recycled aggregate replacement ratio, exposure temperature, and cooling method provides a practical tool for engineers. The good agreement between calculated and experimental results validates the approach and suggests that it can be incorporated into design codes and assessment procedures.

Key Questions and Reflections

A critical question arising from this study is the long-term durability of recycled concrete-filled steel tube columns after fire exposure and cooling. The thermal cycling experienced during a fire event, followed by rapid cooling, may induce microcracking and damage that progresses over time due to environmental exposure. The immediate residual capacity measured in laboratory tests may not represent the long-term performance of the structure.

Another important consideration is the interface behavior between the steel tube and recycled aggregate concrete after high-temperature exposure. The bond strength between steel and concrete is crucial for the composite action, and this bond may be significantly affected by thermal degradation. The recycled aggregate content may further weaken this interface, potentially leading to debonding and loss of composite action under post-fire loading.

The study also raises questions about the applicability of the proposed calculation formula to different structural configurations and loading conditions. The specimens tested were short columns under axial compression, but in practice, CFST columns are often subjected to combined axial and bending loads, which may result in different failure modes and residual capacity.

Study Insights and Implications

This research provides valuable insights into the post-fire behavior of recycled concrete-filled steel tube columns, addressing an important gap in the knowledge of sustainable construction materials under fire conditions. The finding that cooling method significantly affects residual capacity has important implications for fire safety engineering and post-fire structural assessment.

For welding engineers involved in the fabrication of these columns, the study highlights the importance of maintaining high-quality welds and connections, as these may be critical failure points under post-fire loading. Any weld defects or material inconsistencies can be exacerbated by thermal cycling, leading to premature failure. The quality control procedures specified in relevant welding standards should be rigorously applied to ensure reliable post-fire performance.

In conclusion, this study demonstrates that recycled concrete-filled steel tube columns can maintain significant residual capacity after high-temperature exposure, but the cooling method employed during fire suppression is a critical factor that must be considered in post-fire assessment, and these findings should be integrated into fire safety design and assessment methodologies for sustainable construction.