Dynamic Mechanical Properties of Reactive Powder Concrete-Filled Steel Tubes After High Temperature Exposure

Literature Overview

This paper by Jiang Meng and colleagues from the National Key Laboratory of Explosion Shock Disaster Prevention and Mitigation at the PLA University of Science and Technology investigates the dynamic compressive behavior of reactive powder concrete-filled steel tubes (RPC-FST) subjected to elevated temperatures prior to impact loading. The study employs the Split Hopkinson Pressure Bar (SHPB) apparatus to characterize the strain-rate sensitivity and residual mechanical capacity of these composite members after exposure to fire-like conditions at 200 °C and 300 °C. The research is funded by the National Natural Science Foundation of China (Grants 51378498 and 51578541) and the Jiangsu Provincial Natural Science Foundation (Grant BK20141066), and was published in the journal Explosion and Shock Waves in 2017 (Volume 37, Issue 3, pages 405-414).

From a steel pipe engineering perspective, this work is significant because it bridges the gap between static fire-resistance design and the dynamic load scenarios that may occur during or after a fire event — such as secondary explosions, debris impact, or structural collapse loading. Understanding how the steel tube component and the RPC infill interact under combined thermal and dynamic stress is critical for the safety design of fire-exposed pipeline systems and structural enclosures.

Core Technical Findings

The experimental results reveal several key phenomena that warrant careful engineering interpretation:

• Residual capacity retention: After exposure to 200 °C and 300 °C, RPC-FST specimens retained relatively good impact resistance, ductility, and structural integrity. This indicates that the confinement effect provided by the steel tube continues to function effectively even after moderate thermal degradation of the concrete core.
• Reduced strain-rate effect: Under impact loading, the strain-rate sensitivity of RPC-FST is noticeably weaker than that of plain RPC (reactive powder concrete without steel tube confinement). This is a counterintuitive but physically explainable observation — the steel tube constrains lateral expansion of the concrete, thereby suppressing the dynamic hardening mechanisms that typically arise from microcrack propagation at high strain rates.
• Post-fire strength increase: With increasing fire exposure temperature, the peak stress of RPC-FST gradually increases, deformation capacity enhances, and impact resistance improves. The dynamic enhancement factor (DEF) also increases with fire temperature, indicating that the strain-rate effect becomes more pronounced in specimens that have undergone higher thermal exposure.

Technical Analysis and Interpretation

The apparent increase in peak stress and dynamic enhancement factor after fire exposure can be attributed to several mechanisms operating within the composite system. During heating, the steel tube undergoes thermal expansion, which pre-compresses the RPC core. Upon cooling, differential contraction between the steel and concrete phases introduces residual compressive stresses in the concrete, effectively pre-stressing the infill. This pre-stress state suppresses initial microcrack formation during subsequent impact loading, leading to higher apparent strength and a more pronounced strain-rate response.

The reduction in strain-rate sensitivity relative to plain RPC is a critical finding for engineering design. In practice, this means that the steel tube confinement effectively decouples the dynamic response of the composite from the extreme strain-rate hardening of the concrete alone, leading to more predictable and designable behavior under impact.

Engineering Practice Implications

For pipeline and structural engineering applications where fire exposure followed by dynamic loading is a credible scenario — such as in petrochemical facilities, offshore platforms, or military infrastructure — this research provides valuable data for performance-based design. The following practical considerations emerge:

1. Fire-resistance design philosophy: Traditional fire-resistance design focuses on maintaining structural integrity for a specified duration (e.g., 60, 90, or 120 minutes). This study suggests that post-fire dynamic performance should also be considered, particularly for structures exposed to potential blast or impact events.
2. Steel tube specification: The confinement effectiveness depends on the steel grade, wall thickness, and geometric proportions. For applications where post-fire dynamic capacity is critical, higher-strength steel tubes with adequate ductility (e.g., Q345 or Q420 grade per GB/T 1591) may be specified to ensure continued confinement after thermal cycling.
3. RPC mix design: Reactive powder concrete, with its fine aggregate and high cementitious content, is inherently susceptible to thermal degradation. The steel tube effectively mitigates spalling and loss of concrete cover, but the RPC mix design should still account for expected service temperatures.
4. Welding considerations: The steel tube fabrication process, including any welded joints (e.g., longitudinal or circumferential welds), must maintain its integrity through the thermal exposure and subsequent impact. Weld HAZ properties, residual stresses, and potential weld defects become critical factors in the overall post-fire dynamic performance.

Key Questions and Reflections

Several questions arise from this study that merit further investigation. First, the study examines temperatures up to 300 °C, which represents moderate fire exposure. What happens at higher temperatures (500 °C, 800 °C, or 1000 °C) where the steel tube itself begins to lose significant strength? Second, the study focuses on axial compression; the behavior under bending, shear, or combined loading after fire exposure remains to be explored. Third, the long-term effects of thermal cycling (repeated heating and cooling) on the steel-concrete bond and the composite dynamic response are not addressed.

From a quality control standpoint, the fabrication quality of RPC-FST members is paramount. The steel tube must be manufactured with tight dimensional tolerances to ensure uniform concrete confinement. Welded joints in the steel tube, if present, must be inspected by appropriate NDT methods (RT, UT, or MT) to ensure no defects that could propagate under combined thermal and dynamic loading. The concrete placement process must ensure complete filling without voids, which can be verified by ultrasonic testing or radiographic inspection.

Study Insights and Implications

This research contributes meaningfully to the understanding of composite structural behavior under coupled thermal and dynamic loading. The finding that steel tube confinement reduces strain-rate sensitivity while maintaining or enhancing post-fire capacity is particularly valuable for engineers designing structures that must survive fire events and subsequent dynamic scenarios. The SHPB methodology employed provides reliable dynamic property data that can be incorporated into numerical models for performance-based design.

For the steel pipe industry, this work underscores the importance of developing and specifying steel tubes that maintain their mechanical properties under elevated temperatures. Pipeline steels designed for high-temperature service (e.g., API 5CT grades with enhanced creep resistance) may offer advantages in composite applications where fire exposure is a design consideration. Furthermore, the welding processes used to fabricate these tubes must be carefully controlled to ensure that the HAZ does not become a weak link under thermal cycling conditions.

Reference Value and Outlook

The study provides a solid experimental foundation for the dynamic design of RPC-FST members after fire exposure. Future research should extend the temperature range to include severe fire scenarios, investigate the effects of different steel grades and tube geometries, and develop design equations that account for the coupled thermal-dynamic behavior. Integration of these findings into building codes and pipeline design standards would be a natural progression, enabling engineers to specify composite members with quantified post-fire dynamic performance.