Zhuojin Pipe Fitting Co., LtdShrinkage Compensation of Recycled Concrete and Axial Compression Behavior of Steel Tube Recycled Concrete Short Columns
Literature Overview
This paper by Xiang Jun, Zhang Weidong, and Hua Zhujun (2013), published in Progress in Steel Building Structures (Vol. 15, No. 6, pp. 52-57), investigates the use of expansive agents to compensate for the excessive drying shrinkage of recycled concrete and examines the resulting mechanical performance of steel tube recycled concrete (STRC) short columns. The research was supported by the Natural Science Project of the Jiangsu Provincial Department of Education.
Background and Motivation
Recycled concrete, produced by incorporating crushed concrete aggregates from demolished structures, offers significant environmental benefits by reducing natural resource consumption and construction waste. However, recycled concrete exhibits several disadvantages compared to virgin concrete:
- Higher drying shrinkage due to increased porosity of recycled aggregates
- Reduced compressive strength owing to residual mortar on aggregate surfaces
- Greater permeability and lower long-term durability
- Potential for premature cracking and steel tube-concrete debonding in composite members
The excessive shrinkage of recycled concrete is particularly problematic in steel tube concrete applications, where differential shrinkage between the concrete core and steel tube can lead to debonding, loss of composite action, and reduced load-bearing capacity.
Experimental Approach
The researchers investigated the effect of different expansive agent dosages on:
- Drying shrinkage compensation of recycled concrete
- Compressive strength of recycled concrete
- Axial compression behavior of STRC short columns
The experimental program included varying the expansive agent content to identify an optimal dosage that maximizes shrinkage compensation without compromising strength.
Key Findings
The experimental results reveal several important relationships:
| Parameter | Effect of Expansive Agent | Optimal Range |
|---|---|---|
| Drying shrinkage | Reduced with increasing dosage | Moderate dosage |
| Concrete compressive strength | Increases with moderate dosage, decreases with excessive dosage | Moderate dosage |
| STRC column ultimate load | Increases with moderate dosage, decreases with excessive dosage | Moderate dosage |
| Interaction onset timing | Earlier in compensated specimens | All compensated specimens |
| Deformation characteristics | Similar to uncompensated STRC | — |
Interaction Mechanism Enhancement
A particularly significant finding is that the interaction between the steel tube and recycled concrete core develops earlier in shrinkage-compensated specimens compared to uncompensated ones. This occurs because the expansive agent creates internal expansive pressure in the concrete core, which pushes against the steel tube and establishes contact and composite action at lower external loads. In uncompensated specimens, the concrete may shrink away from the tube wall, creating a gap that delays interaction onset.
Dosage Sensitivity
The non-monotonic relationship between expansive agent dosage and mechanical performance indicates an optimal dosage window. Excessive expansive agent leads to:
- Over-expansion causing internal micro-cracking
- Disruption of the aggregate-paste interface
- Reduced effective confinement pressure due to concrete over-expansion beyond steel tube elastic capacity
Engineering Practice Implications
For the implementation of STRC members in practice, several considerations emerge:
- Expansive agent selection and dosing: The optimal dosage must be determined through preliminary testing for each specific recycled concrete mix. Standard expansive agents (such as calcium sulfoaluminate-based or magnesium oxide-based types) should be evaluated for compatibility with recycled aggregates.
- Curing conditions: The expansive properties of the agent require adequate moisture for hydration. In steel tube concrete members, the sealed environment may actually favor expansive agent performance, as moisture loss is limited.
- Quality control: The variability of recycled aggregates introduces additional uncertainty. Batch-to-batch variation in aggregate properties may shift the optimal expansive agent dosage, necessitating quality control measures.
- Design considerations: The earlier onset of interaction in compensated STRC members is beneficial for design, as it means the composite action can be relied upon at lower load levels. However, the expansive pressure must be accounted for in the steel tube stress calculation, as it introduces an initial compressive hoop stress.
Study Insights and Reflections
This research addresses a practical barrier to the widespread adoption of recycled concrete in structural steel tube applications. The shrinkage compensation approach is particularly elegant because it simultaneously solves two problems: reducing the detrimental effects of shrinkage and enhancing the composite action between steel and concrete. The finding that deformation characteristics remain similar between compensated and uncompensated specimens suggests that the fundamental mechanical behavior is preserved, with the primary benefit being earlier interaction development and slightly improved strength. From a sustainability perspective, this work supports the circular economy approach to construction materials, demonstrating that recycled aggregates can be effectively used in composite structural systems with appropriate mix design modifications. The research also highlights the importance of understanding the time-dependent behavior of composite members, as the expansive pressure from the agent continues to develop over time, potentially altering the stress state in the steel tube.