Strength and Lateral Deformation Coefficient of Coal Gangue Concrete Under Steel Tube Confinement

Literature Overview

This research by Li Guochang and Zhong Shantong (published in 2002, Volume 35, Issue 3, pp. 20-23 in the journal "Harbin Institute of Architecture and Engineering") investigates the mechanical behavior of coal gangue concrete (CGC) confined within circular steel tubes under axial compression. Funded by the Liaoning Provincial Natural Science Foundation Major Project (Grant 9910300201), this study represents an early contribution to the field of recycled material concrete-steel tube composite systems. The work combines theoretical analysis with experimental data to derive design formulas for the ultimate compressive strength and lateral deformation coefficient of the confined coal gangue concrete core.

Core Technical Findings

Fundamental Confinement Mechanism

The study establishes that steel tube confinement significantly enhances the compressive strength of coal gangue concrete through the following mechanism:

1. Under axial compression, the concrete core tends to expand laterally due to Poisson's effect.
2. The steel tube restrains this lateral expansion, imposing a confining pressure on the concrete.
3. The confining pressure transforms the concrete stress state from uniaxial to triaxial, significantly increasing its compressive strength and ductility.
4. The lateral deformation coefficient characterizes the relationship between the steel tube's lateral expansion and the concrete's lateral strain demand.

Influence of Steel Ratio and Concrete Grade

The key finding that lower-grade coal gangue concrete is more suitable for combination with steel tubes reflects the fundamental principle that confinement provides greater relative benefit to weaker materials. This is analogous to the well-established behavior of normal concrete, where low-strength concretes exhibit more pronounced confinement benefits.

Theoretical Framework

The derived formulas for the confined coal gangue concrete incorporate:

1. Ultimate compressive strength: Expresses the enhanced strength as a function of the unconfined concrete strength, steel ratio, steel yield strength, and the confinement efficiency factor.
2. Lateral deformation coefficient: Characterizes the geometric relationship between the steel tube's elastic-plastic lateral deformation and the concrete's lateral strain, serving as a key parameter in the confinement model.

The theoretical approach likely follows the established framework of Mander et al. (1988) and Piao Yulian's work on steel tube confinement, adapted for the specific material properties of coal gangue concrete.

Material Characteristics of Coal Gangue Concrete

Coal gangue concrete presents unique challenges compared to conventional normal concrete:

• Aggregate properties: Coal gangue aggregates typically exhibit lower compressive strength, higher water absorption, and greater variability than natural aggregates.
• Bond characteristics: The paste-aggregate interface may be weaker, affecting the overall concrete behavior under confinement.
• Durability concerns: Coal gangue may contain reactive minerals (such as pyrite) that could cause long-term deterioration, though this is secondary to the short-term structural performance studied here.
• Environmental benefits: Using coal gangue as aggregate addresses waste disposal challenges while reducing demand for natural aggregates.

Engineering Practice and Quality Control

For the fabrication of steel tube coal gangue concrete columns, the following quality control measures are essential:

1. Coal gangue aggregate preparation: Proper crushing, washing, and grading of coal gangue aggregates is critical. The aggregate should be tested for compressive strength (per GB/T 14685), water absorption (per GB/T 14684), and soundness (per GB/T 14685 acid-soluble sulfate method).
2. Concrete mixing and placement: The pumpability and workability of CGC must be sufficient for placement within the steel tube. Superplasticizers should be used to achieve adequate workability without excessive water addition. Placement should be continuous to avoid cold joints within the tube.
3. Steel tube preparation: The interior surface of the steel tube should be cleaned and dry to ensure proper bond between the concrete and tube wall. Any rust scale should be removed by shot blasting or grinding.
4. Curing requirements: After placement, adequate curing (minimum 7 days of moisture retention) is essential for strength development, particularly for coal gangue concrete which may have slower hydration kinetics.

Study Insights and Reflections

The finding that lower-grade coal gangue concrete benefits more from steel tube confinement has profound implications for sustainable construction. It suggests that rather than investing in high-quality aggregates and cement to achieve high concrete strength, engineers can utilize lower-grade recycled materials and compensate through structural confinement design. This philosophy aligns with the broader trend toward resource-efficient construction and circular economy principles in the building industry.

The lateral deformation coefficient is a particularly important parameter for practical design, as it governs the interaction between the steel tube's elastic deformation and the concrete's plastic lateral expansion. Accurate prediction of this coefficient requires knowledge of both the concrete's Poisson's ratio and the steel tube's elastic-plastic behavior. In practice, the coefficient may vary with strain level, being higher at early stages (elastic behavior) and lower at later stages (plastic flow of concrete).

The research from 2002 represents pioneering work in an area that has gained renewed importance with the growing availability of industrial waste materials and the increasing demand for sustainable construction materials. The fundamental principles established in this study remain valid and applicable to modern recycled material concrete-steel tube composite systems.