Zhuojin Pipe Fitting Co., LtdInterface Bonding Performance of Steel Tube Concrete Under Constant High Temperature
Literature Overview
The paper by Chen Jun, Xue Yuanyuan, Tan Qinghua, and Wu Jinliang (2018), published in the China Civil Engineering Journal, investigates the interface bonding behavior of steel tube concrete (STC) members under constant high-temperature exposure. The interface bond between the steel tube and concrete core is fundamental to the composite action of STC members, governing the load transfer between the two materials and the overall structural performance. The study was supported by the National Natural Science Foundation of China (Grant No. 51308539) and the Xiangtan University Doctoral Startup Fund.
Experimental Program and Key Findings
The experimental program employed push-out slip tests on STC specimens subjected to constant temperatures ranging from 20°C to 900°C. The primary variables were constant temperature, length-to-diameter ratio, and diameter-to-thickness ratio. The tests produced temperature-time curves and load-slip relationship curves that characterize the interface bonding behavior under thermal exposure.
| Parameter | Effect on Average Bond Strength | Maximum Reduction |
|---|---|---|
| Constant Temperature (20–900°C) | Non-monotonic: decreases, then increases, then decreases | Up to 90% |
| Length-to-Diameter Ratio | Decreasing trend | Up to 50% |
| Diameter-to-Thickness Ratio | Minor effect | Within 10% |
The most striking finding is the non-monotonic behavior of average bond strength with respect to temperature. The bond strength decreases initially with increasing temperature, then increases in a moderate temperature range, and finally decreases again at higher temperatures. This three-stage behavior is attributed to the complex interplay between thermal expansion mismatch, moisture evaporation, chemical changes in the cement paste, and the formation of a thermal oxide layer on the steel tube surface.
Mechanism Analysis
The initial decrease in bond strength (approximately 20°C to 400°C) is primarily due to the differential thermal expansion between steel and concrete, which generates radial stresses at the interface that partially overcome the mechanical interlock and chemical adhesion. The subsequent increase in bond strength (approximately 400°C to 600°C) may be attributed to the densification of the interfacial transition zone as free water evaporates, and the formation of a thin iron oxide layer that can enhance mechanical interlock. The final decrease (above 600°C) results from the decomposition of cement hydrates, loss of chemical adhesion, and the degradation of the aggregate-cement paste bond.
The length-to-diameter ratio effect is explained by the stress distribution along the interface. Longer specimens develop a more uniform stress distribution, but the effective bonded length is limited by the frictional resistance, leading to a reduction in average bond strength as the ratio increases. The diameter-to-thickness ratio has a minor influence because the confinement effect of the steel tube on the concrete is relatively insensitive to wall thickness within the range studied.
Fire Engineering Implications
The results of this study have direct implications for the fire design of STC structures. The significant reduction in interface bond strength at high temperatures means that the composite action between the steel tube and concrete may be compromised during a fire event. Engineers must account for the potential separation of the steel tube from the concrete core when assessing the fire resistance of STC members. The push-out test data can be used to calibrate numerical models that predict the interface behavior under fire conditions, enabling more accurate assessment of the structural performance of STC members in fire scenarios.
Study Insights and Reflections
This research provides critical data for the fire safety design of STC structures, which are increasingly used in bridges, industrial plants, and nuclear facilities where fire resistance is a primary design concern. The non-monotonic bond strength behavior with temperature is a particularly important finding, as it challenges the common assumption that bond strength monotonically decreases with temperature. Design codes and fire rating procedures should be updated to incorporate this more nuanced understanding of interface behavior. From a practical standpoint, engineers should consider the use of high-temperature resistant bond agents or mechanical interlock devices (such as shear studs or corrugated steel tubes) to enhance the interface bond strength at elevated temperatures. The relatively minor effect of diameter-to-thickness ratio suggests that wall thickness selection can be optimized for other design criteria without significantly compromising the fire performance of the interface bond.