Axial Compression Performance of Aluminum Powder Aerated Steel Pipe Concrete Short Columns

Literature Overview

This paper by Guo Jiangpei, Tian Anguo, Gong Nina, Zhang Feng, Cai Xiaoning, and Gu Xinxin from China University of Mining and Technology and Huaihai Institute of Technology, published in Concrete and Cement Products in 2019 (No. 4, pp. 42-45), presents experimental research on the axial compression performance of short columns made with aluminum powder aerated self-stressing steel pipe concrete. The study was supported by the National Natural Science Foundation of China (Grant No. 51508220), Jiangsu Provincial University Natural Science Foundation (Grants No. 16KJB570001 and 17KJB570001), and Jiangsu Provincial Construction System Science and Technology Project (Grant No. 2017ZD098).

Core Viewpoints and Technical Approach

The research designed six short column specimens: four aluminum powder aerated self-stressing steel pipe concrete columns and two conventional steel pipe concrete columns for comparative testing. The columns were subjected to axial compression tests to evaluate their mechanical behavior under loading.

The key findings are summarized as follows:

Interpretation of Technical Points

Mechanism of Performance Enhancement

The aluminum powder aerated self-stressing concrete achieves its enhanced performance through a two-stage mechanism. First, the aluminum powder reacts with calcium hydroxide in the concrete matrix to produce hydrogen gas, which creates a uniform pore structure and induces internal expansion. Second, this internal expansion generates self-stress (autogenous prestress) within the concrete, which is transferred to the surrounding steel pipe, creating a beneficial compressive stress state in the steel and an additional confinement effect on the concrete.

The extension of the elastic stage to above 87% of the ultimate bearing capacity is particularly significant from a serviceability design perspective. In conventional steel pipe concrete columns, the transition from elastic to inelastic behavior occurs at a relatively low load level, meaning that the column enters the plastic range well before reaching its ultimate capacity. This can lead to excessive deformations under service loads, which is undesirable for structural components where dimensional stability is important. The self-stressing effect effectively preloads the column, shifting the entire stress-strain curve and allowing the column to remain elastic up to a much higher fraction of its ultimate capacity.

Effect of Prestress Level

The study confirms that higher prestress levels lead to more pronounced improvements in both elastic range extension and ultimate bearing capacity. This is expected because higher prestress provides greater initial confinement to the concrete, delaying the onset of microcracking and plastic deformation. However, in practical applications, there are practical limits to the achievable prestress level, constrained by the aluminum powder content, the concrete mix design, and the risk of excessive expansion that could lead to formwork damage or surface cracking.

Comparison with Other Prestressing Methods

The self-stressing approach offers several advantages over traditional prestressing methods for steel pipe concrete columns. Unlike external prestressing, which requires additional hardware and installation complexity, self-stressing is achieved entirely through the concrete mix design. Unlike internal prestressing using tendons, which is impractical for large-diameter steel pipe concrete columns, self-stressing provides uniform prestress throughout the concrete cross-section. The prestress loss is also lower than in conventional prestressed concrete because the self-stress is generated in situ and is less susceptible to relaxation mechanisms.

Integration with Engineering Practice

The findings of this study have direct applications in the design of steel pipe concrete columns for buildings, bridges, and industrial structures. The 15% improvement in ultimate bearing capacity can translate into significant material savings, as smaller diameter steel pipes or lower grade concrete can be used to achieve the same design capacity. The extension of the elastic stage to above 87% of ultimate capacity improves the serviceability behavior of the columns, reducing deflections and cracking under normal operating loads.

In terms of construction practice, the aluminum powder aerated concrete requires careful control of the aluminum powder content and mixing procedure to ensure uniform aeration and consistent self-stress development. The aluminum powder should be added gradually during mixing to prevent premature reaction and localized over-expansion. The concrete should be placed promptly after mixing to ensure that the aeration process occurs within the formwork before the concrete sets.

For quality control purposes, the self-stress level of the aerated concrete can be monitored using embedded pressure gauges or strain gauges during the curing period. The achieved self-stress should be compared with the design target to verify that the mix design is producing the intended prestress level. Non-destructive testing methods such as rebound hammer testing and ultrasonic pulse velocity testing can be used to assess the quality of the aerated concrete in place.

Key Questions and Reflections

One important question is the long-term durability of the self-stressing steel pipe concrete columns. The aluminum powder reaction is essentially complete within a few days of concrete placement, but the long-term behavior of the aerated concrete under sustained loading, environmental exposure, and cyclic loading is not fully characterized by short-term compression tests. The uniform pore structure created by the aeration process may affect the permeability and durability of the concrete, and this should be evaluated through permeability and freeze-thaw testing.

Another consideration is the cost-effectiveness of the aluminum powder aerated concrete approach. While the 15% improvement in bearing capacity is attractive, the cost of aluminum powder and the additional quality control requirements must be weighed against the savings from reduced steel pipe diameter or lower concrete grade. A life-cycle cost analysis would be needed to determine whether the self-stressing approach is economically advantageous for a given application.

Study Insights and Implications

This paper demonstrates that aluminum powder aerated self-stressing concrete is a viable and effective approach to enhancing the performance of steel pipe concrete columns. The combination of improved elastic range, increased ultimate capacity, and reduced prestress loss makes this approach particularly attractive for applications where serviceability and long-term performance are critical. The study provides valuable experimental data that can be used to develop design guidelines and code provisions for self-stressing steel pipe concrete columns. For practicing engineers, the key takeaway is that the self-stressing approach can significantly improve the structural efficiency of steel pipe concrete columns, but careful attention must be paid to mix design, construction quality control, and long-term durability assessment.