Load-Bearing Capacity Analysis of Steel Tube Coal Gangue Concrete Flexural Members

Literature Overview

The research by Li Guochang, Wang Zhaoqiang, and Shao Yumei, published in the Journal of Shenyang Jianzhu University (Natural Science Edition) in 2005, investigates the flexural behavior and load-bearing capacity of steel tube coal gangue concrete (STCGC) members. Funded by the Ministry of Construction Science and Technology Project (303-2-027), this study addresses the structural performance of a composite member that combines steel tubes with coal gangue concrete, an innovative material system that promotes the utilization of coal gangue, a major industrial waste product in coal mining regions.

Coal gangue, the byproduct of coal washing and mining, contains a significant volume of waste material that poses environmental challenges. Incorporating coal gangue into concrete as a partial replacement for natural aggregates offers a pathway for waste utilization while producing a structural material with acceptable engineering properties. The steel tube confinement further enhances the concrete's compressive strength and ductility, making the composite member suitable for flexural applications in industrial and civil structures.

Experimental Design and Test Parameters

Five specimens with different steel ratios (the ratio of steel tube cross-sectional area to total cross-sectional area) were tested under flexural loading. The test program included:

The steel ratio is a critical design parameter that governs the contribution of the steel tube to the overall flexural capacity. Higher steel ratios provide more steel reinforcement but reduce the concrete volume and may affect the composite action between the steel and concrete.

Load-Deformation Behavior and Characteristic Points

The study identifies three characteristic points on the load-deflection curve, dividing the flexural response into three stages:

1. Elastic stage: The member behaves linearly, with stresses proportional to strains. The steel tube and concrete act together in a fully composite manner, with no cracking in the concrete.
2. Elastic-plastic stage: The outer fiber of the steel tube begins to yield, and microcracks develop in the concrete. The load continues to increase but at a reduced rate. The composite action remains effective, but the stress distribution becomes nonlinear.
3. Hardening stage: The steel tube undergoes significant plastic deformation and strain hardening. The concrete in the compression zone may be crushed. The load reaches a maximum and then may decrease or remain constant.

The study defines the ultimate limit state point as the point at which the plastic deformation of the section is fully developed and the outermost fiber of the tension-side steel tube is about to enter the strain hardening region. This definition provides a rational basis for determining the design flexural capacity.

Load-Bearing Capacity Formula

Based on the experimental data and the identified failure mechanism, the study derives a load-bearing capacity calculation formula for STCGC flexural members. The formula accounts for:

• The contribution of the steel tube in both tension and compression zones.
• The contribution of the coal gangue concrete in the compression zone.
• The composite action between the steel tube and the concrete fill.
• The effect of steel ratio on the overall capacity.

The calculated results showed good agreement with the experimental results, validating the proposed formula for design applications.

Engineering Practice Implications

For engineers considering the use of STCGC members in structural applications, this study provides the following guidance:

• Steel ratio is the primary design variable. The flexural capacity increases with steel ratio, but the rate of increase diminishes at higher ratios due to the reduced concrete contribution. An optimal steel ratio should balance capacity, economy, and constructability.
• Coal gangue concrete properties must be verified. The mechanical properties of coal gangue concrete vary depending on the gangue source, grading, water-cement ratio, and curing conditions. Standard concrete property tests should be conducted for each batch used in structural applications.
• The composite action between steel and concrete is essential. Full composite action requires adequate bond between the steel tube interior surface and the concrete. Surface preparation, mechanical interlock (such as internal ribs or perforations), and proper concrete placement are critical for achieving the designed capacity.
• The proposed capacity formula should be validated for different member sizes and loading conditions. The formula was derived from specific test specimens and should be verified through additional testing before being applied to different geometries or loading scenarios.

Reflections and Broader Significance

This study contributes to the broader goal of sustainable construction by demonstrating that coal gangue, an abundant industrial waste, can be effectively utilized in structural composite members. The environmental benefits of coal gangue utilization are substantial, as it reduces landfill volume, conserves natural aggregates, and sequesters waste material that would otherwise contribute to environmental degradation. However, the structural performance of coal gangue concrete must be rigorously verified, as the gangue particles may have different mechanical properties (lower strength, higher porosity, different thermal expansion) compared to natural aggregates. The long-term durability of STCGC members, particularly their resistance to corrosion of the steel tube and degradation of the concrete, should be evaluated through accelerated aging tests. The study provides a solid foundation for the further development of STCGC structural systems, but additional research is needed to address durability, fire resistance, and seismic performance before widespread engineering adoption.