Axial Compression Mechanical Properties of Steel Tube Ultra-High Strength Concrete Short Columns with High-Titanium Slag Sand

Literature Overview

The paper by Zhou Xiaojun, Zhan Yulin, Mu Tingmin, Xu Li, and Pang Shuai, published in Science Technology and Engineering (2022), presents an experimental study on the axial compression mechanical properties of steel tube ultra-high strength concrete (UHSC) short columns using high-titanium slag sand as a fine aggregate. The research was supported by the National Natural Science Foundation of China (52008340) and the Ministry of Education Chunhui Program (191643). The study investigates the influence of high-titanium slag sand replacement rate on the axial compression behavior of steel tube concrete columns.

Material Properties and Experimental Design

The high-titanium slag is identified as a suitable internal curing material, which can be used as a fine aggregate to prepare ultra-high strength concrete. The internal curing effect of high-titanium slag helps to reduce the shrinkage of the core concrete, thereby minimizing the risk of debonding and void formation between the concrete and the steel tube. This is a critical advantage for steel tube concrete structures, where the bond between the steel tube and the core concrete is essential for the effective transfer of loads and the development of the confinement effect.

The experimental program involves 10 steel tube concrete short columns subjected to axial compression testing. The following table summarizes the key parameters:

The replacement rates are varied to investigate the influence of the high-titanium slag sand content on the mechanical properties of the steel tube concrete columns.

Key Findings and Technical Interpretation

The experimental results reveal several important findings regarding the axial compression behavior of steel tube UHSC columns with high-titanium slag sand:

1. Failure mode consistency: The high-titanium slag sand replacement rate has no significant influence on the failure mode, load-displacement curve, stress-strain curve, or stress-lateral deformation coefficient curve. All specimens exhibit a shear failure mode, which is typical for steel tube concrete columns under axial compression.
2. Load capacity and stiffness: As the high-titanium slag sand replacement rate increases, the load capacity and initial stiffness of the specimens increase. The maximum values are achieved at a replacement rate of 60%, after which the properties may decrease or plateau.
3. Confinement effect limitation: Due to the excessively high strength of the core concrete and the relatively low steel ratio of the specimens, the confinement effect of the steel tube on the core concrete is limited. As a result, the measured load capacity is lower than the calculated load capacity predicted by the relevant design codes.

Engineering Practice Implications

The findings of this study have several important implications for the design and construction of steel tube UHSC columns:

1. High-titanium slag sand optimization: The optimal replacement rate of 60% for high-titanium slag sand should be considered in the design of steel tube UHSC columns. This replacement rate provides the best balance between load capacity, stiffness, and the internal curing effect.
2. Internal curing benefit: The internal curing effect of high-titanium slag sand helps to reduce the shrinkage of the core concrete, which is beneficial for maintaining the bond between the concrete and the steel tube. This is particularly important for UHSC, which typically has high shrinkage due to its low water-to-binder ratio.
3. Confinement effect consideration: The limited confinement effect of the steel tube on UHSC is a critical design consideration. The high strength of the core concrete may exceed the confinement capacity of the steel tube, leading to premature concrete failure. Designers should account for this limitation when using UHSC in steel tube concrete columns.
4. Steel ratio optimization: The relatively low steel ratio of the specimens may have contributed to the limited confinement effect. Increasing the steel ratio, either by using thicker steel tubes or by adding internal reinforcement, may improve the confinement effect and the overall load capacity.
5. Design code revision: The discrepancy between the measured and calculated load capacities suggests that the existing design codes may not adequately account for the behavior of steel tube UHSC columns. The design codes should be revised to incorporate the specific behavior of UHSC in steel tube concrete structures.

Key Questions and Reflections

Several questions arise from this study that warrant further investigation. First, the study focuses on short columns under axial compression, but the behavior of long columns under combined axial and bending loads may differ. The slenderness ratio and the loading eccentricity can significantly influence the load capacity and failure mode of steel tube concrete columns.

Second, the study does not address the long-term behavior of the columns, including the effects of creep, shrinkage, and sustained loading. The internal curing effect of high-titanium slag sand may influence the long-term shrinkage behavior, but this aspect is not investigated in the study.

Third, the study does not discuss the durability of the high-titanium slag sand concrete. The long-term durability, including resistance to carbonation, chloride ingress, and freeze-thaw cycles, is an important consideration for structural applications.

Summary

This study provides valuable experimental data on the axial compression mechanical properties of steel tube UHSC short columns with high-titanium slag sand, with clear findings on the optimal replacement rate of 60% and the limited confinement effect of the steel tube on UHSC. The results are directly applicable to the design of steel tube UHSC columns, with practical recommendations for high-titanium slag sand optimization and steel ratio consideration. Engineers should carefully consider the limitations of the confinement effect and the need for design code revision when using UHSC in steel tube concrete structures.