Axial Compression Performance of Square Steel Tube Confined Reactive Powder Concrete Short Columns

Literature Overview

This paper by Hao Wenxiu, Yu Wanli, Zhang Siyuan, Xu Xiao, and Song Shanshan (2021), published in "Industrial Construction," presents experimental and numerical research on the axial compression mechanical properties of square steel tube confined reactive powder concrete (RPC) short columns. Funded by Hebei Provincial High School Science and Technology Research Project (ZD2018209) and Hebei Provincial Key Research and Development Project (18227209D), the study investigates the effects of steel fiber content and spiral reinforcement spacing on failure modes, bearing capacity, and ductility through nine physical tests and ABAQUS finite element analysis.

Core Technical Content

The research addresses a critical challenge in ultra-high strength concrete (UHSC) applications: the inherent brittleness of RPC when confined within steel tubes. The authors propose a dual-confinement approach combining steel fibers and internal spiral reinforcement to enhance the deformation capacity of the composite system.

Key experimental findings include:

• The combination of steel fibers and spiral reinforcement significantly improves deformation capacity
• Ultimate bearing capacity increases by 10% to 25% compared to conventional steel tube RPC columns
• Steel fiber content and spiral reinforcement spacing are the primary parameters affecting performance
• The ABAQUS finite element model accurately captures the mechanical behavior and provides additional parametric insights regarding wall thickness ratio and spiral spacing

Key Technical Parameters and Analysis

Interpretation of Technical Points

Reactive powder concrete is an ultra-high strength concrete (typically exceeding 150 MPa compressive strength) characterized by its fine aggregate composition, high cement content, and steel fiber reinforcement. While RPC offers exceptional strength, its brittleness limits its application in structures requiring ductility, particularly under seismic loading or impact conditions.

The dual-confinement approach addresses this limitation through two mechanisms:

1. Steel fibers bridge microcracks and provide post-cracking tensile resistance, delaying concrete failure and enabling more uniform stress distribution within the RPC matrix.
2. Spiral reinforcement provides macroscopic confinement, restricting lateral expansion of the RPC core and maintaining the compressive strength enhancement beyond the initial crack formation.

The square steel tube geometry introduces additional complexity compared to circular sections. In square sections, the corners experience different confinement effects than the mid-span of each face. The corners, being closer together, provide more effective confinement, while the mid-span regions have greater freedom for lateral expansion. This geometric asymmetry must be considered in design.

The 10-25% bearing capacity increase is significant in the context of RPC applications where material costs are already elevated. This improvement allows engineers to either reduce cross-sectional dimensions for weight savings or maintain dimensions for increased safety margins.

Standards and Engineering Practice Integration

The application of steel tube RPC columns falls within the realm of special structural applications where ultra-high strength is required. Relevant standards include:

• GB/T 51219-2016: Technical specification for reactive powder concrete
• GB 50993-2014: Specification for design of steel tube concrete structures
• JGJ/T 110-2017: Technical specification for steel tube concrete structures

From a fabrication perspective, square steel tube RPC columns present specific challenges:

1. Concrete placement in square sections requires careful attention to corner filling, as corners are prone to honeycombing
2. Spiral reinforcement installation within square sections requires precise fabrication and positioning
3. Steel fiber mixing must be carefully controlled to ensure uniform distribution without segregation

The finite element model developed in this study provides a valuable tool for design optimization. Engineers can use the model to evaluate different parameter combinations without extensive physical testing, reducing development time and costs. However, the model must be validated against additional experimental data for different section sizes and loading conditions before widespread use in design.

Key Questions and Reflections

Several important considerations emerge from this research. First, while the study demonstrates significant improvements in ductility, the absolute ductility values of the composite system should be evaluated against seismic design requirements. The improvement relative to plain RPC may not be sufficient for high-seismic zones.

Second, the study focuses on short columns under pure axial compression. In practical applications, columns are subjected to combined axial and bending loads, and the interaction between the confinement mechanisms and bending behavior needs investigation. The P-M interaction curves for square steel tube RPC columns with dual confinement are essential for practical design.

Third, the long-term behavior of RPC under sustained loading is a concern. RPC's low permeability and high strength may result in significant creep effects, particularly when confined within steel tubes. The time-dependent behavior could affect the long-term performance of the composite system.

From a cost-benefit perspective, the addition of spiral reinforcement and increased steel fiber content adds to material costs. The 10-25% bearing capacity increase must be weighed against the additional costs, particularly in applications where material costs are a primary concern.

Study Insights and Implications

This research demonstrates that the dual-confinement approach effectively addresses the brittleness limitation of RPC in steel tube applications. The combination of steel fibers and spiral reinforcement provides complementary mechanisms that synergistically enhance both strength and ductility. For engineering practice, the key takeaway is that RPC columns should not be used in their plain form for applications requiring ductility; instead, the dual-confinement approach offers a practical solution. The finite element model provides a design tool that should be further developed and validated for broader application. Future research should extend to eccentric loading, cyclic loading, and long-term behavior to provide comprehensive design guidance for seismic and long-duration applications.