Radial Expansion Compressive Stress Test on Steel Tubes by Static Breaking Agent

Literature Overview

Published in the Journal of Harbin Institute of Technology in 2020, this study by Li Ruisen and colleagues from the Key Laboratory of Structural Engineering Disaster and Control investigates the radial expansion compressive stress generated by static breaking agents within steel tubes. The research is part of a National Key R&D Program (2017YFC0806100) focused on intelligent disaster prevention and mitigation in civil engineering. The work provides fundamental data essential for understanding the mechanics of static demolition and breaking operations.

Experimental Methodology

The study employs a sophisticated experimental approach using seamless steel tubes to simulate drilled holes in concrete or rock. Twenty-one test specimens were instrumented with resistance strain gauges to measure the radial expansion compressive stress during the hardening and expansion process of static breaking agents.

Key experimental parameters varied across specimens:

The "confinement degree" is defined as the ratio of steel tube cross-sectional area to static breaking agent cross-sectional area, analogous to the "hoop constraint theory" used in steel tube concrete mechanics. This definition provides a quantitative measure of how much the surrounding material restricts the expansion of the breaking agent.

Key Experimental Findings

Stress Distribution Along Hole Depth

The radial expansion compressive stress is not uniformly distributed along the hole depth. The distribution follows a characteristic pattern:

• Mid-section of hole: Generally exhibits the highest expansion stress
• Bottom of hole: Also shows elevated stress levels, often comparable to or slightly lower than mid-section
• Top of hole (near opening): Significantly lower stress due to material overflow and reduced confinement

This non-uniform distribution has critical implications for the effectiveness of static breaking operations. The stress concentration at mid-depth and bottom suggests that the breaking action is most intense away from the free surface.

Effect of Hole Diameter

Larger hole diameters produce several distinct effects:

• Greater differential between bottom and top expansion stress
• Higher maximum expansion stress achievable at mid-section and bottom within the same time period
• Increased overflow of breaking agent material from the hole opening

The relationship between hole diameter and stress development rate indicates that larger holes allow more rapid stress buildup at the critical mid-section and bottom locations, which is beneficial for demolition efficiency.

Effect of Hole Depth and Confinement Degree

• Hole depth has minimal influence on the development rate of expansion stress but significantly increases the absolute magnitude of stress at mid-section and bottom locations
• Confinement degree similarly does not affect development rate but increases the ultimate stress levels achievable
• Both parameters demonstrate that greater restriction leads to higher internal stresses, consistent with the fundamental mechanics of constrained expansion

Practical Applications and Design Guidelines

The experimental data directly informs the design of static breaking operations in demolition and construction:

1. Hole design optimization: Larger hole diameters are preferred to maximize expansion stress and improve breaking efficiency. However, practical constraints such as equipment availability, structural integrity of the remaining material, and safety considerations must be balanced.
2. Hole depth selection: Adequate hole depth is essential to achieve sufficient confinement and develop the necessary stress levels. Shallow holes will result in premature overflow and reduced effectiveness.
3. Confinement management: The surrounding material's stiffness and continuity directly affect the stress buildup. In applications where the surrounding material is weak or discontinuous, the confinement degree may be insufficient to develop effective breaking stresses.
4. Agent placement strategy: Given the non-uniform stress distribution, strategic placement of breaking agent within drilled holes should consider that the most effective breaking action occurs at mid-depth and bottom locations.

Quality Control and Safety Considerations

From a quality assurance perspective, several factors must be controlled during static breaking operations:

• Agent mixing ratio: The water-to-powder ratio must be precisely controlled to ensure proper expansion characteristics and timing
• Hole cleanliness: Debris and moisture in drilled holes can affect agent performance and stress development
• Temperature control: Ambient and material temperature significantly affect the expansion rate and magnitude
• Safety monitoring: The non-uniform stress distribution means that unexpected failure modes are possible, particularly at locations of stress concentration

Study Insights and Reflections

This research provides valuable quantitative data that bridges the gap between the empirical practice of static breaking and fundamental mechanical understanding. The concept of confinement degree, borrowed from steel tube concrete mechanics, proves to be an effective analytical framework for understanding breaking agent performance. The finding that hole diameter significantly affects stress distribution suggests that current industry practices, which often use relatively small drill diameters, may be suboptimal from a mechanical efficiency standpoint. However, the study is limited to steel tube confinement, and the behavior within actual concrete or rock matrices may differ due to material heterogeneity, pre-existing cracks, and non-linear material response. Future research should extend these findings to in-situ conditions and develop predictive models that account for the complex interaction between breaking agent expansion and the surrounding material's fracture mechanics.