Self-Stress Test of Large-Diameter Steel Pipe Expansive Concrete

Literature Overview

This paper by Zhou Mingru and colleagues from Lanzhou University of Technology (2014) presents experimental research on the self-stress characteristics of expansive concrete in large-diameter steel pipes (φ1500mm). The work was conducted at the Gansu Provincial Key Laboratory of Civil Engineering Disaster Prevention and Mitigation. The study addresses a significant gap in existing research, as most expansive concrete studies focus on small-scale specimens, while large-diameter applications in engineering practice require different considerations.

Background and Research Significance

Expansive concrete is widely used in steel pipe-concrete composite structures to compensate for concrete shrinkage and to create compressive prestress in the steel pipe. The expansive agent (typically calcium sulfoaluminate-based or magnesium oxide-based) causes the concrete to expand during hydration, which is partially restrained by the steel pipe, resulting in beneficial self-stress.

The significance of this study lies in the large diameter of the test specimens (φ1500mm), which is representative of actual engineering applications such as large-diameter piles, bridge piers, and storage tank foundations. Small-scale tests may not accurately represent the expansive behavior in large-diameter applications due to differences in heat generation, temperature gradients, and restraint conditions.

Test Program and Specimen Details

The test groups varied the expansive agent dosage to investigate its effect on self-stress development. Strain gauges were installed on both the steel pipe surface and within the core concrete to measure the strain development over time.

Self-Stress Calculation Methodology

The self-stress was calculated considering the simultaneous effects of concrete expansion and steel pipe restraint. The calculation methodology follows the principle of strain compatibility:

1. The free expansion strain of the concrete (ε_exp) is measured from the core concrete strain gauges.
2. The actual strain in the concrete (ε_conc) is measured from the core concrete strain gauges after accounting for restraint.
3. The steel pipe strain (ε_steel) is measured from the steel pipe surface strain gauges.
4. The self-stress in the steel pipe is calculated as: σ_steel = E_steel × ε_steel
5. The self-stress in the concrete is calculated as: σ_conc = E_conc × (ε_exp - ε_conc)

The study found that the self-stress value is approximately 3 MPa, which is consistent with the theoretical predictions for the given expansive agent dosage and restraint conditions.

Key Technical Findings

Effect of Expansive Agent Dosage:

• Both 10% and 12% expansive agent dosages produce measurable self-stress, with the 12% dosage producing slightly higher values. However, the difference is not proportional to the dosage increase, indicating diminishing returns at higher dosages.

Time-Dependent Behavior:

• The self-stress develops over time as the expansive agent reacts with the cement hydration products. The peak self-stress is typically reached within 7-14 days of concrete placement.
• Temperature effects play a significant role, as the heat of hydration accelerates the expansive reaction. In large-diameter pipes, the core temperature can be significantly higher than the surface temperature, leading to non-uniform self-stress distribution.

Large-Diameter Effects:

• The large diameter (φ1500mm) results in significant heat generation during hydration, which affects both the expansive reaction rate and the concrete strength development.
• The restraint condition in large-diameter pipes is more complex than in small-scale specimens, as the steel pipe itself can deform elastically under the expansive pressure.

Engineering Practice Implications

Steel Pipe Manufacturing and Installation:

• The self-stress of approximately 3 MPa is beneficial for the structural performance of steel pipe-concrete composite members, as it creates a compressive prestress in the steel pipe that counteracts tensile stresses from service loads.
• The large wall thickness (50mm) used in this study is representative of heavy-duty applications. For thinner-walled pipes, the self-stress may be higher due to less restraint deformation, but the overall structural benefit may be similar.
• The welding joints in the steel pipe must be designed to accommodate the self-stress without developing cracks. The weld quality is critical, as any defect at the weld could become a crack initiation point under the expansive pressure.

Construction Considerations:

• The timing of concrete placement is critical. The expansive agent reacts most vigorously in the first 7 days, so the concrete must be placed in a manner that allows uniform expansion without creating differential stresses.
• The curing regime must be carefully controlled to ensure that the expansive reaction proceeds at an optimal rate. Too rapid curing (such as steam curing) can accelerate the expansive reaction and potentially cause excessive self-stress.
• For large-diameter applications, thermal management is essential. The heat of hydration can reach 70-80°C in the core of a φ1500mm pipe, which affects both the expansive reaction and the concrete strength development.

Study Insights

This research fills an important gap in the understanding of expansive concrete behavior in large-diameter steel pipes. The self-stress of approximately 3 MPa is a valuable design parameter for engineers working on large-diameter steel pipe-concrete composite structures. The study confirms that the simultaneous consideration of expansion and restraint is essential for accurate self-stress prediction, and that small-scale tests may not be representative of large-diameter applications.

For steel pipe manufacturers, the key takeaway is that the self-stress from expansive concrete is a beneficial design feature that should be incorporated into the structural analysis. The quality of the steel pipe fabrication, including weld quality and dimensional accuracy, directly affects the self-stress distribution and the overall structural performance.