Impact Resistance of CFST and Recycled Aggregate CFST Members A Review

Literature Overview

This comprehensive review by Li Wengui, Luo Zhiyu, Long Chu, and Long Binghuang from Hunan University and Wuhan University of Technology synthesizes research on the impact resistance of circular steel tube concrete (CFST) members and recycled aggregate CFST (RACFST) members. The review covers the effects of strain rate, temperature, confinement ratio, material strength, impact energy, and external confinement on the impact performance of CFST members. Published in the Journal of Architecture and Civil Engineering (2016, Vol. 33, No. 4, pp. 25-35), the work was supported by the National Natural Science Foundation (Project No. 51408210) and the Central University Basic Scientific Research Business Special Fund (531107040800).

Core Technical Findings

The review concludes that recycled aggregate CFST members possess good impact resistance, providing a theoretical basis for the further development of RACFST applications. The review systematically examines the dynamic mechanical behavior of CFST members under various influencing factors and extrapolates these findings to RACFST members based on the known characteristics of recycled aggregate concrete.

Influencing Factor	Effect on CFST Impact Resistance	Mechanism
Strain rate	Increases strength and stiffness	Dynamic strengthening of concrete and steel
Temperature	Generally reduces capacity	Thermal degradation of materials
Confinement ratio (D/t)	Higher ratio reduces impact resistance	Reduced confinement effectiveness
Concrete strength	Higher strength may reduce ductility	Trade-off between strength and deformation capacity
Impact energy	Higher energy increases damage	Progressive damage accumulation
External confinement	Improves impact resistance	Additional restraint on lateral expansion

Interpretation of Technical Points

The impact resistance of CFST members is fundamentally governed by the dynamic interaction between the steel tube and the concrete core under high strain rate loading. Unlike static or quasi-static loading, impact loading introduces strain rate effects that significantly alter the material behavior of both the steel tube and the concrete. Concrete exhibits a well-documented dynamic strength increase with increasing strain rate, described by the Dynamic Increase Factor (DIF) in various constitutive models. Steel, while less rate-sensitive than concrete, also shows some dynamic strengthening, particularly at elevated strain rates.

The confinement ratio (D/t, where D is the outer diameter and t is the steel tube wall thickness) plays a critical role in impact resistance. A lower confinement ratio provides stronger confinement of the concrete core, which enhances the concrete's compressive strength and ductility under impact loading. However, a very low confinement ratio may be structurally impractical or uneconomical. The optimal confinement ratio for impact resistance represents a balance between structural efficiency and impact performance.

For recycled aggregate concrete (RAC), the presence of recycled coarse and fine aggregates introduces additional complexity to the impact behavior. Recycled aggregates typically have higher porosity and lower density than natural aggregates, which can affect the dynamic wave propagation characteristics within the concrete core. The interfacial transition zone (ITZ) between the recycled aggregate and the cement paste is generally weaker than that of natural aggregate, potentially creating preferential damage paths under impact loading. However, the steel tube confinement may mitigate some of these weaknesses by providing lateral restraint that compensates for the reduced aggregate quality.

Standards and Testing Methodology

The impact testing of CFST members typically employs several methods, each with distinct characteristics:

Testing Method	Typical Energy Range	Strain Rate Range	Application
Drop weight hammer	50-5000 J	1-1000 s⁻¹	Laboratory specimen testing
Split Hopkinson Pressure Bar	10-500 J	100-10000 s⁻¹	Material-level dynamic characterization
Pendulum impact	50-2000 J	1-50 s⁻¹	Structural element testing
Projectile impact	Variable	Variable	Full-scale structural testing
Blast loading	High energy	Very high rates	Protection and defense applications

The review notes that most CFST impact research has been conducted using drop weight hammer testing, which provides a practical balance between experimental complexity and the ability to generate meaningful impact conditions. The strain rate range achievable with drop weight testing (typically 1-1000 s⁻¹) covers the range relevant to most engineering impact scenarios, including vehicular impacts, falling objects, and explosive blasts.

Integration with Engineering Practice

The practical significance of this review extends to several engineering domains:

Bridge engineering: CFST columns in bridges are potential targets for vehicular impacts, and understanding their impact resistance is essential for protective design.
Building protection: CFST structural members in buildings may be subjected to impacts from construction equipment, falling objects, or security threats.
Marine and offshore structures: CFST members used in marine applications may experience impact from vessels, ice, or wave loading.
Protective structures: CFST members in blast-resistant or impact-resistant buildings require validated impact performance data.

For engineers designing CFST members for impact resistance, the review provides guidance on the key parameters that should be considered in design: appropriate confinement ratio selection, material strength optimization, and the potential benefits of external confinement systems. The extrapolation to RACFST members suggests that recycled aggregate concrete can be used in CFST applications where impact resistance is a design consideration, provided that appropriate design adjustments are made to account for the modified material properties.

Key Questions and Reflections

The review identifies several areas where further research is needed. The dynamic behavior of RACFST members under impact loading has not been extensively studied experimentally, and the current understanding relies largely on extrapolation from CFST and RAC behavior separately. This gap in knowledge represents both a research opportunity and a design challenge. Additionally, the interaction between the recycled aggregate particles and the steel tube under impact loading—particularly the potential for aggregate fragmentation and its effect on the confinement mechanism—deserves more detailed investigation.

From a materials science perspective, the microstructure of recycled aggregates plays a crucial role in dynamic performance. The ITZ quality, the degree of cement paste removal from recycled aggregates, and the surface texture of the recycled particles all influence the dynamic response of RACFST members. Future research should incorporate microstructural characterization techniques, including scanning electron microscopy (SEM) and X-ray computed tomography (XCT), to establish clear correlations between microstructure and macroscopic impact performance.

Study Insights and Implications

This review serves as an essential reference for engineers and researchers working with CFST and RACFST members in impact-prone applications. The systematic organization of research findings by influencing factor provides a clear framework for understanding the complex behavior of these composite members under dynamic loading. The conclusion that RACFST members possess good impact resistance is encouraging for the sustainable construction community, as it suggests that recycled materials can be used in structural applications without significant compromise in impact performance. For practitioners, the key message is that impact resistance should be considered as a design parameter for CFST members in applications where impact loading is credible, and that the selection of confinement ratio, material strengths, and external protection systems can be optimized to achieve the required impact performance level.