Shrinkage Compensation of Recycled Concrete in Steel Tube Concrete Axial Compression Short Columns

Literature Overview

This paper by Xiang Jun, Zhang Weidong, and Hua Zhujun, published in Advances in Steel Structure Construction in 2013 (Volume 15, Issue 6, pp. 52–57), investigates the effect of shrinkage compensation on the mechanical properties of recycled concrete used in steel tube concrete axial compression short columns. The research is funded by the Jiangsu Provincial Department of Education Natural Science Project. The study addresses a critical issue in the use of recycled concrete in structural applications: the excessive drying shrinkage of recycled concrete, which can lead to cracking, reduced bond strength, and compromised structural performance.

Background and Motivation

Recycled concrete, produced by incorporating recycled coarse and fine aggregates derived from demolished concrete, is an important material for sustainable construction. However, recycled concrete typically exhibits higher drying shrinkage than conventional concrete due to the higher porosity and water absorption of recycled aggregates. This excessive shrinkage can cause several problems in steel tube concrete structures, including the formation of gaps between the concrete core and the steel tube, reduced composite action, and potential cracking of the concrete surface.

The use of expansive agents to compensate for the shrinkage of recycled concrete is a well-established approach in conventional concrete technology. However, the effectiveness of this approach in the context of steel tube concrete structures, where the concrete is confined by a steel tube, has not been extensively investigated. This study fills this gap by systematically examining the effect of different expansive agent dosages on the shrinkage compensation and mechanical performance of steel tube recycled concrete short columns.

Experimental Program

The experimental program involved the preparation of recycled concrete specimens with varying dosages of expansive agent. The specimens were tested for drying shrinkage deformation and axial compression strength. Steel tube recycled concrete short columns were also fabricated and tested under axial compression to evaluate the effect of shrinkage compensation on the composite behavior of the steel tube and recycled concrete.

The test parameters included the expansive agent dosage, which was varied across a range of values to identify the optimal dosage for shrinkage compensation. The specimens were cured under standard conditions and subjected to drying shrinkage tests to measure the shrinkage deformation over time. The axial compression tests were conducted on short column specimens to measure the ultimate load, peak stress, and deformation characteristics.

Key Experimental Findings

The test results reveal several important findings regarding the effect of shrinkage compensation on recycled concrete in steel tube concrete structures. The following table summarizes the key observations:

The results demonstrate that the addition of an appropriate amount of expansive agent can improve both the strength of the recycled concrete and the ultimate load capacity of the steel tube recycled concrete short columns. However, excessive expansive agent dosage leads to a reduction in both the concrete strength and the column ultimate load. This finding highlights the importance of optimizing the expansive agent dosage to achieve the best balance between shrinkage compensation and strength performance.

The study also found that the interaction between the steel tube and the recycled concrete in shrinkage-compensated specimens develops earlier than in uncompensated specimens. This earlier interaction is attributed to the reduced shrinkage deformation, which maintains a tighter bond between the concrete and the steel tube from an earlier stage of loading. The deformation characteristics of the shrinkage-compensated and uncompensated specimens are relatively similar, indicating that the shrinkage compensation does not significantly alter the overall deformation behavior of the column.

Technical Analysis of Shrinkage Compensation

The mechanism of shrinkage compensation through expansive agents involves the controlled expansion of the concrete matrix during hydration, which counteracts the drying shrinkage that would otherwise occur. In the context of steel tube concrete, this expansion is particularly beneficial because it maintains the contact pressure between the concrete core and the steel tube, ensuring continuous composite action throughout the service life of the structure.

The optimal expansive agent dosage identified in this study is critical for engineering practice. Too little expansive agent results in inadequate shrinkage compensation, while too much expansive agent can cause excessive expansion that may lead to internal stresses, cracking, or even bursting of the steel tube. The optimal dosage represents a balance between these competing effects and should be determined through testing for each specific application.

The earlier onset of steel tube-concrete interaction in shrinkage-compensated specimens has important implications for the design of steel tube recycled concrete columns. The earlier interaction means that the confinement effect is developed at lower load levels, which can improve the overall ductility and energy dissipation capacity of the column. This finding is particularly relevant for seismic applications, where the ability to undergo large deformations without catastrophic failure is essential.

Comparison with Conventional Steel Tube Concrete

The study provides a useful comparison between shrinkage-compensated steel tube recycled concrete and conventional steel tube concrete. The deformation characteristics of the two types of specimens are found to be relatively similar, which suggests that shrinkage-compensated recycled concrete can be used as a viable substitute for conventional concrete in steel tube concrete applications. This finding is significant for sustainable construction, as it demonstrates that recycled materials can be used without compromising structural performance.

However, the study also reveals some differences between the two types of specimens. The shrinkage-compensated recycled concrete specimens exhibit earlier interaction onset, which may indicate a more efficient utilization of the composite action. This difference should be considered in the design of steel tube recycled concrete structures, as it may require adjustments to the design assumptions and safety factors.

Engineering Practice Considerations

From a fabrication and quality control perspective, the use of expansive agents in recycled concrete for steel tube concrete applications requires careful attention to several factors. The expansive agent must be compatible with the cement and other admixtures used in the concrete mix. The dosage must be accurately controlled to ensure consistent shrinkage compensation across all specimens. The mixing and placing process must be controlled to ensure uniform distribution of the expansive agent throughout the concrete matrix.

The quality of the recycled aggregates used in the concrete mix is also critical. Recycled aggregates with high porosity and water absorption may require higher dosages of expansive agent to achieve adequate shrinkage compensation. The properties of the recycled aggregates should be characterized through laboratory testing, and the expansive agent dosage should be adjusted accordingly.

The concrete filling process in steel tube concrete columns must be carefully controlled to ensure complete filling without voids. The use of expansive agents may affect the workability of the concrete, and adjustments to the mix design may be necessary to maintain adequate pumpability and compaction. The quality of the concrete-steel tube bond should be verified through non-destructive testing methods during quality control.

Study Insights and Outlook

This research provides valuable insights into the use of shrinkage compensation to improve the performance of recycled concrete in steel tube concrete structures. The findings demonstrate that the addition of an appropriate amount of expansive agent can enhance both the strength and the composite behavior of steel tube recycled concrete columns. The optimal expansive agent dosage identified in this study provides a practical guideline for engineers designing steel tube recycled concrete structures.

The study also contributes to the broader goal of sustainable construction by demonstrating that recycled materials can be used in structural applications without compromising performance. The use of recycled concrete in steel tube concrete columns can reduce the demand for virgin aggregates and contribute to waste reduction in the construction industry. The shrinkage compensation approach provides a practical solution to the durability challenges associated with recycled concrete, making it a viable option for structural applications.

Future research should focus on the long-term durability of shrinkage-compensated steel tube recycled concrete under various environmental conditions, including exposure to moisture, temperature cycling, and chemical attack. The effect of different types of expansive agents on the shrinkage compensation and mechanical performance of recycled concrete also warrants investigation. Overall, this study provides a solid foundation for the practical application of shrinkage-compensated recycled concrete in steel tube concrete structures.