Creep Effects During Construction of Steel Tube-Expanded and Recycled Concrete Structures

Overview and Research Context

The paper by Wang Haiyang and Zha Xiaoxiong (Harbin Institute of Technology, Shenzhen Graduate School) together with Huang Haochun and Wang Xiaodong (China Construction Fifth Engineering Bureau Co., Ltd.), published in "Industrial Construction" (Vol. 41, No. 6, 2011, pp. 43-46, ISSN 1000-8993), investigates the influence of creep on CFST structures during staged construction, with specific attention to the effects of expansive agents and recycled concrete. This research addresses a practical construction challenge that is often overlooked in design: the time-dependent deformation that occurs during the sequential pouring and curing of concrete within steel tubes.

The classification code TU398 places this work within the domain of CFST structural behavior. The research is particularly relevant for large-diameter CFST columns where concrete is placed in multiple layers, and for sustainable construction applications using recycled aggregate concrete.

Construction Methodology and Creep Mechanism

In practice, large-diameter CFST columns are constructed through staged concrete placement:

1. Initial placement - The first layer of concrete is poured and allowed to cure.
2. Subsequent layers - Additional layers are placed at intervals, each of which imposes a sustained load on the previously placed layers.
3. Expansive agent contribution - When expansive agents are used, the concrete undergoes volume increase during hydration, generating internal stresses and additional creep deformation.

The creep mechanism in this context is distinct from the conventional creep of a uniformly loaded member because:

• The load history is non-uniform, with different concrete layers experiencing different durations of sustained loading.
• The expansive agent introduces time-dependent internal stresses that superimpose on the external loads.
• Recycled aggregate concrete exhibits different creep characteristics compared to normal concrete due to the weaker interfacial transition zone between recycled aggregates and cement paste.

Key Experimental Parameters

Experimental Findings

Effect of Expansive Agent Dosage

The experimental results reveal that:

• Ultimate load capacity - The use of expansive agents has a positive effect on the ultimate load-bearing capacity of normal CFST members. The improvement is attributed to the enhanced composite action resulting from the initial contact pressure generated by concrete expansion.
• Creep deformation - As the expansive agent dosage increases, the creep deformation also increases. This is because the expansive agent introduces additional time-dependent strains that contribute to the overall deformation.
• Interaction effect - The beneficial effect on ultimate capacity and the detrimental effect on creep deformation create a design trade-off that must be carefully managed.

Effect of Loading Interval

The time interval between successive loading stages significantly influences the creep behavior:

• Longer intervals - When the loading interval is larger, the creep deformation amplitude is significantly reduced. This is because the concrete has more time to gain strength and stiffness before the next load increment is applied.
• Ultimate capacity - The effect of loading interval on ultimate load capacity is relatively minor compared to its effect on creep deformation. This suggests that the long-term creep deformation is the primary concern, not the ultimate strength.

Recycled Concrete Behavior

The behavior of steel tube-recycled concrete structures differs notably from normal concrete:

• Creep deformation - Expansive agents have a more pronounced effect on the creep deformation of recycled concrete. The deformation increases proportionally with expansive agent dosage, and the absolute values are significantly higher than for normal concrete.
• Ultimate capacity - The effect of expansive agents on the ultimate load-bearing capacity of recycled concrete CFST members is relatively small compared to normal concrete. This suggests that the composite action enhancement is less effective in recycled concrete, possibly due to the weaker interface properties.
• Design implications - Engineers using recycled aggregate concrete in CFST applications must exercise greater caution regarding creep deformation, particularly when expansive agents are employed.

Engineering Practice Considerations

For practical construction of CFST structures, the following recommendations emerge:

• Optimized loading schedule - Extending the time interval between concrete placement stages can significantly reduce creep deformation without substantially affecting ultimate capacity.
• Expansive agent selection - The dosage of expansive agent should be carefully selected to balance the benefits of enhanced composite action against the increased creep deformation.
• Recycled concrete monitoring - Structures using recycled aggregate concrete require more rigorous monitoring of long-term deformation, as the creep behavior is more sensitive to expansive agent dosage.
• Construction quality control - The uniformity of concrete placement and curing is critical, as non-uniform expansion can lead to localized stresses and potential cracking.

Key Questions and Reflections

The study provides valuable insights into the time-dependent behavior of CFST members during construction, but several questions remain open. The long-term creep behavior beyond the test duration (90-180 days) is not fully characterized, and extrapolation to the service life of the structure requires additional data. Additionally, the interaction between creep and the expansive agent's delayed expansion is not fully resolved, as the expansive reaction may continue for extended periods depending on the specific product used.

The study also does not address the effect of temperature on creep behavior, which is relevant for structures in extreme climates or those subjected to thermal loading during service.

Summary

This experimental study provides essential guidance for the construction of CFST structures using expansive agents and recycled concrete. The findings demonstrate that loading intervals have a significant influence on creep deformation but limited effect on ultimate capacity, while expansive agent dosage affects both creep deformation and ultimate capacity in opposite directions. For recycled concrete, the creep sensitivity to expansive agents is notably higher, requiring careful design and construction practices. Engineers should incorporate these findings into their construction planning and quality control procedures to ensure the long-term performance of CFST structures.