Bond-Slip Performance of Steel Tube Alkali-Activated Concrete Composite Columns

Research Background and Problem Statement

The study by Ren Zhigang, Xu Tian'en, Li Qi, and Feng Yixuan from the School of Civil Engineering and Architecture at Wuhan University of Technology addresses a critical interface issue in composite column design: the bond-slip behavior between steel tubes and alkali-activated concrete (AAC) in composite (overlaid) columns. Funded by the Sanya Science and Education Innovation Park Open Fund (2022KF001), this research was published in the Journal of Building Science and Engineering (2025, Vol. 42, Issue 6, pp. 77-86) under classification TU398. The motivation is clear: as the construction industry seeks lower-carbon alternatives to Portland cement concrete, AAC offers significant advantages in reduced CO2 emissions and enhanced durability. However, the bond performance between steel tubes and AAC in composite column configurations has not been sufficiently characterized, creating uncertainty in design and quality control.

Experimental Program and Key Parameters

The authors designed and tested 10 push-out specimens representing steel tube AAC composite columns. The experimental variables included steel fiber volume fraction, tube ratio (含管率), and volumetric stirrup ratio (体积配箍率). The push-out test methodology is the standard approach for evaluating interface bond performance in composite members, as recognized by Eurocode 4 and relevant Chinese standards.

The load-slip curves obtained from the tests were classified into three typical forms, reflecting different failure modes and bond degradation mechanisms. The observation that specimens remained largely intact after testing, with no visible cracks on the external concrete surface and only localized damage at the loading end, indicates that AAC provides superior ductility and crack resistance compared to conventional concrete under similar loading conditions.

Key Findings and Technical Analysis

The principal findings can be summarized as follows:

1. Characteristic bond strength increases with both steel fiber volume fraction and volumetric stirrup ratio, indicating that mechanical interlock and confinement are the dominant bond enhancement mechanisms.
2. Residual bond strength decreases with increasing tube ratio, suggesting that higher steel tube proportions reduce the effective concrete contribution to interface shear resistance.
3. An optimal cross-sectional dimension range exists that maximizes ultimate bond strength, beyond which the tube ratio becomes counterproductive.
4. Grey relational analysis was employed to quantify the sensitivity of characteristic bond strengths to each parameter. The results showed that steel fiber volume fraction has the strongest correlation with ultimate bond strength, while tube ratio most strongly influences residual bond strength.

From a metallurgical and materials engineering perspective, the role of steel fibers in enhancing bond performance is well understood. Steel fibers bridge microcracks at the steel-concrete interface, redistribute stress concentrations, and provide additional mechanical interlock through their three-dimensional reinforcement network. The optimal fiber volume fraction typically falls between 1.5% and 3.0% for AAC matrices, beyond which workability issues arise during placement.

The grey relational analysis approach is particularly valuable for engineering practice because it provides quantitative sensitivity rankings without requiring complex regression models. For design engineers, this means that when optimizing composite column specifications for bond performance, steel fiber content should be prioritized as the primary design variable, followed by stirrup configuration, and then tube ratio selection.

Engineering Practice Implications

For steel pipe manufacturers and pipe fitting producers, this research has several direct implications. First, AAC is increasingly specified for composite construction applications, and understanding its bond behavior with steel tubes is essential for ensuring structural safety. Second, the finding that an optimal tube ratio exists means that simply increasing the steel tube proportion does not monotonically improve structural performance — this has cost implications for material procurement and fabrication. Third, the emphasis on steel fiber volume fraction suggests that AAC mix designs for composite columns should specify fiber content as a critical quality control parameter, with incoming material testing protocols accordingly.

The push-out test methodology itself should be adopted as a routine quality verification procedure for composite columns using AAC, with acceptance criteria derived from the characteristic bond strength values established in this study. Engineers should note that the three typical load-slip curve forms provide a practical basis for categorizing test results and identifying anomalous specimens that may indicate workmanship defects such as incomplete AAC placement, inadequate compaction, or poor interface preparation.

In conclusion, this research fills an important knowledge gap in the bond mechanics of steel tube AAC composite columns, providing quantitative design guidance through grey relational analysis and establishing that steel fiber volume fraction is the most influential parameter for ultimate bond strength. The practical takeaway for the steel pipe industry is that AAC composite columns represent a viable low-carbon structural solution, provided that interface bond performance is properly designed and verified through standardized push-out testing.