Unidirectional Tensile Testing of Grouted Sleeves with Seamless Steel Pipes After High Temperature Exposure

Literature Overview

This 2024 paper published in "Journal of Henan Polytechnic University" by Zhao Jun, Wu Mingxun, Yao Dunmin, and Ma Bin investigates the mechanical performance of seamless steel pipe grouted sleeves after exposure to elevated temperatures. The research addresses a critical concern in prefabricated construction: the fire resistance and post-fire structural integrity of mechanical connections using seamless steel pipe grouted sleeves. The study was supported by the National Natural Science Foundation (Project 12162010) and Guangxi Science and Technology Plan (Projects AD20159085 and AA20302006).

Test Parameters and Experimental Design

The researchers conducted unidirectional tensile tests on grouted sleeve specimens with seamless steel pipes, varying four key parameters: exposure temperature, cooling method, protective layer thickness, and rebar diameter. The test matrix covers a comprehensive range of practical conditions encountered in fire scenarios.

Key Experimental Results

The test results reveal several important trends regarding the degradation of grouted sleeve performance after high temperature exposure:

The flexural strength of the grouting material shows greater sensitivity to temperature than compressive strength, with reductions of 59% and 81% at 800°C and 1000°C respectively, compared to compressive strength reductions of 44% and 72% at the same temperatures. This differential degradation pattern indicates that the grouting material's ability to resist bending moments is more critically affected by high temperatures than its ability to resist compression.

Two distinct failure modes were observed: rebar fracture and rebar pull-out. Water-cooled specimens and specimens without protective layers exhibited pull-out failure, with the critical temperature for pull-out failure dropping to 800°C compared to room temperature conditions. The presence of a protective layer effectively improved both ductility and load-bearing capacity.

The ultimate bearing capacity reduction increases with rebar diameter, indicating that larger diameter rebars experience more severe performance degradation after high temperature exposure. This trend may be attributed to the greater thermal gradient within larger diameter rebars and the increased interface area between rebar and grouting material.

Engineering Practice Implications

For steel pipe manufacturers and prefabricated construction practitioners, this research provides critical design and fabrication guidance:

• Steel pipe material selection: The seamless steel pipe used in grouted sleeves must maintain dimensional stability and mechanical integrity after fire exposure. Pipes conforming to GB/T 8162 or GB/T 8163 should be selected with appropriate steel grades that maintain properties at elevated temperatures.
• Protective layer design: The study confirms that protective layers significantly improve fire resistance. Engineers should specify appropriate fire protection materials and thicknesses based on the expected fire scenario.
• Grouting material specification: The greater sensitivity of flexural strength to temperature suggests that grouting material formulations should be optimized for flexural performance retention at elevated temperatures.
• Rebar diameter considerations: Larger diameter rebars experience greater capacity reduction after fire exposure. Design should account for this differential degradation when selecting rebar sizes for connections exposed to potential fire scenarios.

FMEA Analysis of Failure Modes

Applying Failure Mode and Effects Analysis (FMEA) to the observed failure modes:

The grouting material degradation presents the highest risk priority number, emphasizing the importance of grouting material quality and fire protection measures.

Study Insights and Conclusions

The research provides a comprehensive understanding of the post-fire behavior of seamless steel pipe grouted sleeves. The derived bearing capacity calculation formulas show good agreement with experimental results, offering a practical predictive tool for post-fire structural assessment.

From a steel pipe manufacturing perspective, the study highlights the importance of pipe material quality and dimensional accuracy in fire-exposed applications. The seamless steel pipe serves as the outer casing for the grouted sleeve, and its integrity directly affects the overall connection performance. Pipes with consistent wall thickness, proper surface finish, and appropriate steel grade are essential for reliable performance.

The finding that water cooling increases sensitivity to fire damage is particularly noteworthy. This thermal shock effect suggests that fire suppression strategies should consider the potential for secondary damage to structural connections. Engineers should design connections with adequate reserve capacity to withstand both the thermal effects of fire and the thermal shock of subsequent cooling.

The protective layer's effectiveness in improving ductility and capacity demonstrates the value of fire protection measures in prefabricated construction. Future research should explore advanced fire protection materials and application methods specifically designed for steel pipe grouted sleeve connections.

Summary

This study provides essential data on the post-fire mechanical performance of seamless steel pipe grouted sleeves used in prefabricated construction. The findings on temperature-dependent strength degradation, failure mode transitions, and the effectiveness of protective layers offer practical guidance for fire-resistant design of mechanical connections. Engineers should incorporate these results into connection design specifications and fire protection planning, ensuring that prefabricated building connections maintain adequate structural integrity after fire exposure.