Residual Mechanical Properties of Square Steel Tube Recycled Aggregate Concrete After High Temperature Exposure

Literature Overview

This study, published in the Journal of Shenyang University of Technology in 2015 by Wang Bing and colleagues from Shenyang University and Shenyang Hengji Industrial Co., Ltd., investigates the residual mechanical properties of square steel tube recycled aggregate concrete (RAC) columns subjected to eccentric compression after exposure to elevated temperatures. The research is supported by the National Natural Science Foundation of China (Grant No. 51308347) and the Shenyang Science and Technology Project (F13171900). The study uses ABAQUS finite element simulation to analyze the effects of temperature, recycled aggregate replacement rate, and eccentricity ratio on the structural behavior of these composite columns.

Core Technical Findings

The authors validated their finite element model by comparing simulation results with experimental data, confirming the accuracy of the numerical approach. The key findings are as follows:

• As the exposure temperature increases, the ultimate load-bearing capacity of the square steel tube RAC columns gradually decreases.
• When the exposure temperature is held constant, increasing the recycled aggregate replacement rate leads to a slow decrease in ultimate load-bearing capacity, but the reduction is not significant.
• As the eccentricity ratio increases, the residual load-bearing capacity decreases significantly, with a pronounced decline.

Interpretation of Technical Points

The finding that recycled aggregate replacement rate has only a minor effect on residual capacity after high temperature exposure is particularly interesting from a materials science perspective. Recycled aggregate typically contains adherent old mortar, which has lower strength and higher porosity than natural aggregate. One might expect a more significant degradation in performance. However, the presence of the steel tube confinement provides a confining pressure that compensates for the reduced aggregate strength, especially at elevated temperatures where the concrete cover may have already spalled or cracked.

The significant effect of eccentricity ratio is consistent with classical column theory. Under eccentric loading, the section experiences both axial compression and bending, and the neutral axis shifts toward the loaded side. At elevated temperatures, the concrete in the compression zone may have already degraded, reducing the effective compression area and amplifying the effect of eccentricity. The steel tube, while maintaining its structural integrity better than concrete at high temperatures, cannot fully compensate for the loss of concrete contribution in the tension zone.

Integration with Engineering Practice

For fire protection design of steel tube concrete structures, this study provides important guidance:

1. The residual capacity of square steel tube RAC columns should be evaluated based on the expected fire temperature and duration, not just the room temperature capacity.
2. The use of recycled aggregate in steel tube concrete columns is feasible even for structures that may be exposed to elevated temperatures, provided that the steel tube provides adequate confinement.
3. Eccentric loading conditions, which are common in frame structures under lateral loads, should be considered in fire safety design, as they significantly reduce residual capacity.
4. Fire protection measures, such as fire-resistant coatings or encasement, should be designed to limit the temperature rise in the steel tube to below critical thresholds.

From a welding perspective, the integrity of the square steel tube longitudinal and transverse welds is critical. During a fire event, differential thermal expansion between the steel tube and the concrete core can induce additional stresses at weld joints. If the welds have residual stresses from fabrication or if there are weld defects, they may become initiation sites for cracking at elevated temperatures. Therefore, welding procedures for square steel tube concrete columns should include post-weld stress relief or controlled cooling to minimize residual stresses.

Key Questions and Reflections

The study raises the question of whether the finite element model accurately captures the progressive spalling of concrete at high temperatures. Concrete spalling is a complex phenomenon influenced by moisture content, aggregate type, and confinement level. If the model does not account for spalling, the predicted residual capacity may be optimistic. Additionally, the study does not address the effect of fire duration, which is critical in real fire scenarios. A column that is exposed to high temperature for a short duration may retain more capacity than one exposed for a prolonged period.

Another consideration is the long-term durability of recycled aggregate concrete in structural applications. While the mechanical properties at room temperature and after fire exposure may be acceptable, the permeability of recycled aggregate concrete may be higher, leading to increased susceptibility to carbonation and chloride ingress over the service life of the structure.

Study Insights and Implications

This study demonstrates that recycled aggregate concrete can be used in square steel tube concrete columns without significant loss of residual fire resistance, provided that the steel tube confinement is adequate. The key design parameter is the eccentricity ratio, which should be minimized in fire-critical structural elements. For engineers designing fire-resistant steel tube concrete structures, the recommended approach is to conduct finite element analysis that includes temperature-dependent material properties, to limit eccentricity ratios in critical columns, and to ensure high-quality welding of steel tube components to maintain structural integrity during fire exposure.