Axial Compression Performance of Steel Tube Columns Filled with Coarse Aggregate Ultra-High Performance Concrete

Literature Overview

The study by Zeng Yanqin, Xu Lihua, Wu Fanghong, Yu Min, and Chi Yin, published in Engineering Mechanics in 2022, investigates the axial compression performance of circular steel tube columns filled with coarse aggregate ultra-high performance concrete (CA-UHPC). This research was supported by the National Natural Science Foundation of China Key Project. Fourteen short column specimens were designed and tested, considering three parameters: steel fiber content, coarse aggregate replacement ratio, and steel tube thickness. The study provides experimental data and a load-bearing capacity calculation formula for CA-UHPCFST short columns under axial compression.

Core Technical Points

Ultra-high performance concrete (UHPC) is known for its exceptional compressive strength, which can exceed 150 MPa, and its excellent durability properties. However, conventional UHPC is expensive due to the use of fine aggregates only and the high cement content. The introduction of coarse aggregate into UHPC, known as CA-UHPC, aims to reduce costs while maintaining high performance. The study investigates how the coarse aggregate replacement ratio affects the axial compression performance of steel tube concrete columns, which is a critical question for the practical application of CA-UHPC in structural engineering.

The test results reveal that the confining coefficient (ξ), defined as the ratio of the steel tube cross-sectional area to the concrete cross-sectional area multiplied by the steel yield strength divided by the concrete compressive strength, is the key parameter governing the failure mode. When ξ is between 0.217 and 0.883, the specimens primarily exhibit shear failure, characterized by diagonal cracking and localized crushing. When ξ is between 0.883 and 1.431, the specimens primarily exhibit barrel-shaped failure, where the steel tube bulges outward due to the lateral pressure from the confined concrete.

Failure Mode and Load-Deformation Behavior

The shear failure mode occurs at lower confining coefficients, where the steel tube provides insufficient lateral confinement to prevent diagonal cracking in the concrete core. The concrete fails by shear sliding along the diagonal planes, and the steel tube may experience localized yielding at the failure region. The barrel-shaped failure mode occurs at higher confining coefficients, where the steel tube provides adequate lateral confinement, and the concrete fails in a more ductile manner with the steel tube expanding outward.

The load-deformation curves of the CA-UHPCFST columns exhibit a distinct three-stage behavior: an initial linear elastic stage, a nonlinear hardening stage, and a post-peak softening stage. The peak load is significantly higher than that of conventional steel tube concrete columns, due to the high compressive strength of the CA-UHPC. The post-peak behavior is characterized by a gradual strength degradation, which indicates good ductility and energy absorption capacity. The steel tube provides effective lateral confinement to the CA-UHPC core, preventing sudden failure and allowing the column to sustain loads well beyond the peak.

Influence of Design Parameters

The steel fiber content has a positive effect on the load-bearing capacity and ductility of the CA-UHPCFST columns. Steel fibers bridge cracks in the concrete, improving the tensile strength and toughness of the composite material. The fibers also enhance the bond between the concrete and the steel tube, which is critical for the effective transfer of lateral pressure from the concrete to the steel tube. However, excessive steel fiber content may affect the workability of the concrete, leading to poor compaction and reduced interface bond quality.

The coarse aggregate replacement ratio has a complex influence on the column performance. At the same steel tube thickness, the introduction of coarse aggregate effectively increases the load-bearing capacity of the specimens. This is attributed to the improved aggregate interlock and the increased volume of the concrete core. However, the coarse aggregate may also create weak interfaces within the concrete matrix, which could affect the long-term durability and fatigue performance of the column.

The steel tube thickness directly affects the confining pressure provided to the concrete core. Thicker steel tubes provide greater lateral confinement, which increases the load-bearing capacity and ductility of the column. The confining pressure is calculated based on the steel tube thickness, yield strength, and the geometric configuration of the column. The study demonstrates that the confining pressure is the primary mechanism by which the steel tube enhances the performance of the CA-UHPC core.

Load-Bearing Capacity Calculation Formula

Based on the test results and reference to existing steel tube concrete codes, the study proposes a load-bearing capacity calculation formula for CA-UHPCFST short columns under axial compression. The formula accounts for the contributions of the steel tube, the CA-UHPC core, and the interaction effects between the two materials. The formula is expressed as a function of the steel tube yield strength, the CA-UHPC compressive strength, the steel tube cross-sectional area, and the concrete cross-sectional area, with appropriate confinement coefficients that reflect the enhanced performance of the CA-UHPC.

The proposed formula provides a practical tool for the design of CA-UHPCFST columns. It should be noted that the formula is based on the test data from this study and may require calibration for different concrete mix designs, steel grades, and column geometries. The formula should be validated with additional experimental data before being adopted in design codes.

Welding and Fabrication Considerations

The fabrication of CA-UHPCFST columns requires careful attention to the welding of the steel tube and the concrete pouring process. The steel tube should be a seamless or welded pipe with adequate wall thickness to provide effective lateral confinement. The welds in the steel tube, if any, should be full-penetration butt welds inspected by ultrasonic testing (UT) to ensure full fusion and absence of defects. The weld heat-affected zone (HAZ) should be examined for hardness and microstructural changes, as the welding process may affect the mechanical properties of the steel tube near the weld.

The concrete pouring process is critical for ensuring the quality of the CA-UHPC core. The CA-UHPC should be poured in layers to ensure proper compaction and avoid voids or honeycombing. The pouring rate should be controlled to prevent excessive lateral pressure on the steel tube, which could cause distortion or bulging. The interface between the CA-UHPC and the steel tube should be free of laitance or other weak layers, which could reduce the bond strength and the effectiveness of the composite action.

Integration with Engineering Practice

The CA-UHPCFST column concept offers several advantages for structural engineering applications. The high compressive strength of the CA-UHPC allows for smaller column cross-sections, which reduces the overall structural weight and increases the available floor area. The excellent durability of the CA-UHPC improves the service life of the structure, reducing maintenance costs over the design life. The steel tube provides effective lateral confinement, which enhances the ductility and energy absorption capacity of the column, making it suitable for seismic-resistant design.

In practice, the CA-UHPCFST column concept is particularly suitable for high-rise buildings, long-span structures, and infrastructure projects where high strength and durability are required. The use of coarse aggregate in the UHPC mix reduces the material cost compared to conventional UHPC, making the concept more economically viable. However, the workability of the CA-UHPC must be carefully controlled during the mixing and pouring process to ensure proper compaction and interface quality.

Key Questions and Reflections

Several important questions arise from this study that warrant further investigation. First, the long-term durability of the CA-UHPCFST columns under cyclic loading and environmental exposure is a critical concern. The coarse aggregate may create additional interfaces within the concrete matrix, which could be susceptible to crack propagation and corrosion. Second, the fatigue performance of the CA-UHPCFST columns requires investigation, as the steel fiber content and the coarse aggregate may affect the fatigue strength of the composite material. Third, the seismic performance of the CA-UHPCFST columns should be evaluated through cyclic loading tests, as the ductility and energy absorption capacity are critical for seismic-resistant design.

From a quality control standpoint, the CA-UHPCFST columns require rigorous inspection protocols. The steel tube should be inspected for dimensional accuracy, weld quality, and surface defects. The CA-UHPC should be inspected for compressive strength, workability, and density. The interface between the steel tube and the CA-UHPC should be inspected by non-destructive testing methods such as ultrasonic testing (UT) to detect delamination or voids.

Study Insights and Implications

This study provides valuable experimental data and a practical calculation formula for the design of CA-UHPCFST short columns under axial compression. The identification of the confining coefficient as the key parameter governing the failure mode is a significant finding that simplifies the design process. The proposed load-bearing capacity calculation formula offers a practical tool for engineers to design CA-UHPCFST columns with adequate safety and economy.

For practicing engineers, the key takeaway is that the CA-UHPCFST column concept is a promising solution for high-strength, durable structural columns. The design of these columns requires careful consideration of the confining coefficient, the steel fiber content, the coarse aggregate replacement ratio, and the steel tube thickness. Future research should focus on the long-term durability, fatigue, and seismic performance of CA-UHPCFST columns, as well as the development of design code provisions that specifically address this composite structural system.