Effect of Longitudinal Reinforcement Strength on Axial Compression of Reinforced Steel Tube Recycled Concrete Columns

Literature Overview

This paper investigates how the yield strength grade of longitudinal reinforcement steel bars influences the axial compressive behavior of steel tube confined recycled concrete columns. While previous research has extensively studied the effects of concrete strength, steel tube dimensions, and recycled aggregate replacement ratio, the role of reinforcement steel grade has received relatively less attention despite being a critical design parameter that directly impacts both the structural capacity and the economic efficiency of composite columns. The study systematically examines the influence of reinforcement strength ranging from HRB400 to HRB600 grades on the load-displacement response, failure mode, and load-sharing mechanism among the constituent materials.

Test Matrix and Specimen Configuration

The experimental program included 36 square steel tube recycled concrete columns with varying longitudinal reinforcement grades, organized according to a systematic design of experiments approach.

Each reinforcement grade was tested with at least three specimens per parameter combination to ensure statistical reliability. The specimens were loaded under monotonic axial compression at a displacement rate of 0.5 mm/min using a 2000 kN universal testing machine.

Influence of Reinforcement Strength on Load-Displacement Response

The results demonstrate that increasing the reinforcement yield strength from HRB400 to HRB600 improves the ultimate bearing capacity by approximately 6–12%, depending on the reinforcement ratio and concrete strength. However, this improvement is not linear; the marginal benefit of upgrading from HRB500 to HRB600 is smaller than from HRB400 to HRB500, indicating a diminishing return effect. The peak load displacement remains relatively constant across reinforcement grades, suggesting that the ductility of the column is primarily governed by the steel tube and concrete properties rather than the reinforcement strength.

The load-sharing ratio analysis reveals that the reinforcement contribution to the total bearing capacity increases from approximately 12% for HRB400 bars to 16–18% for HRB600 bars at a reinforcement ratio of 2.5%. The steel tube contribution remains dominant at 40–50%, while the confined recycled concrete contributes 35–45% of the total capacity.

Failure Mode Analysis and Strain Distribution

Strain measurement using rosette gauges on the steel tube surface and individual reinforcement bars reveals important insights into the load-transfer mechanism. For HRB400 reinforcement, the bars reach yield strain (approximately 0.002) well before the peak load, indicating that the reinforcement contributes primarily through plastic deformation at failure. For HRB600 reinforcement, the bars reach yield strain closer to the peak load, suggesting a more efficient utilization of the reinforcement strength but also a more abrupt transition to failure.

The failure mode remains consistent across reinforcement grades: the steel tube undergoes local inward buckling near the mid-height, accompanied by concrete crushing at the corners and splitting along the diagonals. The longitudinal bars experience outward bowing in the post-peak stage, with the buckling amplitude being larger for higher-strength bars due to their reduced plastic strain capacity at yield.

Study Insights and Design Recommendations

From a practical design perspective, the study suggests that HRB500 reinforcement offers the best balance between bearing capacity improvement and ductility retention. The upgrade to HRB600 provides only marginal capacity gains while potentially reducing the overall ductility of the column, which is undesirable for seismic applications. The reinforcement strength effect is more pronounced in columns with higher reinforcement ratios and lower concrete replacement ratios, indicating that the interaction between reinforcement and concrete is optimized when the concrete quality is relatively high. Engineers should also consider the cost-effectiveness of higher-strength reinforcement, as the price premium for HRB600 over HRB500 may not be justified by the modest capacity improvement. This study provides valuable data for the rational selection of reinforcement grades in steel tube recycled concrete column design.