Flexural Mechanical Behavior of Steel Tube Ultra-High Strength Concrete

Literature Overview

This paper by Zhou Xiaojun, Zhan Yulin, and Mu Tingmin, published in Building Structure (2024, Vol. 54, No. 6, pp. 32-37), presents experimental research on the flexural mechanical properties of steel tube ultra-high strength concrete (UHSC) members. The study is conducted in the context of the Guansheng Qijiang Bridge project and involves three groups of six flexural specimens with concrete compressive strengths ranging from 80.3 MPa to 115.2 MPa. The research investigates the effects of concrete strength on flexural bearing capacity, deformation characteristics, and failure modes of CFST members. This work was supported by the Ministry of Education Chunhui Program (191643) and the Sichuan Provincial Department of Education Science and Technology Project (15ZA0141). The study is significant because ultra-high strength concrete in steel tubes represents an emerging structural material combination with potential for high-performance bridge applications.

Core Technical Analysis

Experimental Program and Specimen Configuration

The experimental program consists of three groups of flexural specimens, with each group containing two specimens, for a total of six specimens. The specimens represent steel tube ultra-high strength concrete (CFST) members subjected to four-point bending. The key variables in the experimental program are the concrete compressive strength and the section steel ratio (ρ_s).

The use of a constant section steel ratio across the three groups allows the effect of concrete strength to be isolated as the primary variable. The section steel ratio, defined as the ratio of the steel tube cross-sectional area to the total cross-sectional area, is a critical parameter in CFST design because it governs the relative contributions of the steel tube and the concrete core to the overall structural response.

Flexural Failure Mode

One of the most important findings of this study is that the flexural failure mode of steel tube ultra-high strength concrete members is essentially the same as that of conventional CFST members. The primary failure mechanism is excessive deflection leading to overall flexural failure, characterized by the following features:

• Overall bending failure: The member fails as a whole rather than through a localized mechanism such as web crippling or flange local buckling. This indicates that the steel tube provides adequate lateral confinement to the concrete core, preventing premature local buckling of the compression flange.
• Local bulging in the compression zone: Local outward bulging of the steel tube is observed in the compression zone, indicating that the concrete core exerts lateral pressure on the steel tube that exceeds the local buckling resistance of the tube wall. However, this bulging does not lead to immediate failure but rather contributes to the post-peak deformation capacity of the member.
• Post-yield ductility: After the member enters the yielding stage, the bearing capacity decreases only gradually, indicating good flexural ductility. This is attributed to the confinement effect of the concrete on the steel tube, which prevents premature buckling, and the ductile behavior of the steel tube itself.

Effect of Concrete Strength on Flexural Performance

The most striking finding of this study is that, at a constant section steel ratio, the increase in concrete strength from 80.3 MPa to 115.2 MPa has a relatively small effect on the flexural failure mode, bearing capacity, and ductility of the CFST member. This finding has important implications for the design of CFST members using ultra-high strength concrete.

The reasons for this observation are multifaceted:

• Steel tube dominance in flexure: In flexure, the steel tube contributes significantly to the tensile capacity of the section through the tension flange, while the concrete core primarily provides compressive resistance and lateral confinement. The steel tube's contribution to the flexural capacity is largely independent of the concrete strength, as long as the concrete provides adequate confinement to prevent premature buckling.
• Confinement saturation: At concrete strengths above 80 MPa, the confinement pressure exerted by the concrete on the steel tube is sufficient to prevent local buckling of the compression flange. Further increases in concrete strength provide additional confinement, but this additional confinement is not fully utilized because the steel tube wall thickness and geometry already provide adequate buckling resistance.
• Brittle concrete behavior: Ultra-high strength concrete is inherently more brittle than conventional concrete, with reduced strain capacity and more sudden failure. This brittleness limits the benefit of higher concrete strength in flexure, where ductile behavior is essential for achieving the full plastic moment capacity of the section.

Implications for Design

The experimental findings suggest that for CFST flexural members, the concrete strength is not the primary driver of flexural performance when the section steel ratio is held constant. This has several design implications:

• Cost optimization: Engineers may consider using moderate-strength concrete (80-90 MPa) rather than ultra-high strength concrete (100-115 MPa) for CFST flexural members, as the additional cost of higher-strength concrete may not be justified by a significant improvement in flexural capacity or ductility.
• Section steel ratio optimization: Since the concrete strength has limited effect on flexural performance, the section steel ratio becomes the more important design parameter. Increasing the section steel ratio, through thicker steel tube walls or larger tube diameters, is likely to provide more significant improvements in flexural capacity than increasing the concrete strength.
• Mixed design approach: For bridge applications where ultra-high strength concrete is used for other reasons (such as reducing section dimensions or increasing service life), the CFST members can be designed with the knowledge that the concrete strength contribution to flexural capacity is secondary to the steel tube contribution.

Engineering Practice and Quality Control

The application of steel tube ultra-high strength concrete in bridge engineering requires careful attention to several quality control aspects. The concrete mix design must ensure adequate workability for pumping and placement inside the steel tube, as ultra-high strength concrete is typically more viscous and less workable than conventional concrete. The use of high-range water reducers and fine aggregate grading optimization are essential for achieving the required workability without compromising strength.

The concrete placement process inside the steel tube must ensure complete filling without voids or segregation. For large-diameter tubes, the use of internal vibrators or external vibration of the tube itself is necessary. The placement sequence should be planned to minimize the risk of concrete segregation and to ensure uniform compaction. Post-placement inspection, including ultrasonic testing of the concrete-steel interface, is recommended to verify the quality of the concrete infill.

The welding and fabrication of the steel tube members must meet the requirements of applicable standards such as GB 50017 or AWS D1.1. The steel tube walls must be free of defects that could initiate stress concentrations under flexural loading. Surface treatment of the steel tube inner surface, such as shot blasting or chemical cleaning, is recommended to improve the bond between the steel tube and the concrete core.

The research by Zhou Xiaojun and colleagues provides valuable experimental data on the flexural behavior of steel tube ultra-high strength concrete members. The findings challenge the conventional assumption that higher concrete strength always leads to better structural performance in CFST members. Engineers designing CFST bridge members should carefully evaluate the cost-benefit of using ultra-high strength concrete versus optimizing the steel tube geometry and section steel ratio. The Guansheng Qijiang Bridge project provides a practical context for applying these findings in real bridge engineering, and the lessons learned should inform future design codes and standards for CFST structures.