Axial Compression Performance of Square Steel Tube Confined Reactive Powder Concrete Short Columns

Literature Overview

This research by Hao Wenxiu, Yu Wanli, Zhang Siyuan, Xu Xiao, and Song Shanshan (2021, Industrial Construction, Vol. 51, No. 6, pp. 117-122) investigates the axial compressive behavior of square steel tube confined reactive powder concrete (RPC) short columns. The study was supported by the Hebei Provincial Higher Education Science and Technology Research Project (ZD2018209) and the Hebei Provincial Key Research and Development Project (18227209D). Nine specimens were tested, varying the steel fiber content and spiral stirrup spacing to evaluate their effects on failure patterns, load-bearing capacity, and ductility.

Material Properties and Design Parameters

Reactive powder concrete (RPC) is an ultra-high-performance concrete characterized by very high compressive strength, typically exceeding 120 MPa, and excellent durability. When confined within a steel tube, the composite action between the steel tube and the RPC core can further enhance the structural performance. The addition of steel fibers and spiral stirrups provides additional reinforcement mechanisms.

Experimental Parameters

The steel fiber content was varied to study its contribution to post-peak ductility and crack resistance. The spiral stirrup spacing was varied to examine its confinement effect on the RPC core. Both parameters were selected based on practical construction considerations and typical design values used in engineering practice.

Experimental Results and Analysis

Failure Patterns

The failure patterns of the specimens reveal the combined effect of the steel tube confinement and the internal reinforcement. The steel fibers bridge cracks in the RPC, delaying the propagation of major cracks and maintaining load-carrying capacity beyond the initial cracking. The spiral stirrups provide additional confinement to the RPC core, particularly in the regions away from the steel tube.

Load-Bearing Capacity Enhancement

The ultimate load-bearing capacity was enhanced by 10% to 25% compared to specimens without steel fibers and spiral stirrups. This enhancement is attributed to the combined effects of steel fiber reinforcement, spiral stirrup confinement, and the steel tube constraint. The synergistic effect of these three reinforcement mechanisms results in a more significant improvement than the sum of individual contributions.

Ductility Improvement

The deformation capacity of the specimens was significantly improved through the combined use of steel fibers and spiral stirrups. The steel fibers provide crack-bridging capacity and improve the post-peak energy absorption. The spiral stirrups confine the RPC core, preventing lateral expansion and maintaining the integrity of the composite column under large deformations.

Finite Element Analysis

An ABAQUS finite element model was established to further analyze the effects of edge-to-thickness ratio and stirrup spacing on the column's load-bearing capacity and ductility. The FEA model complements the experimental results by enabling parametric studies that would be impractical to conduct experimentally. The model validation against experimental data confirmed the accuracy of the numerical approach.

The edge-to-thickness ratio is a critical parameter for steel tube design. A lower ratio (thicker walls relative to the tube width) provides greater confinement but increases material cost. The FEA results help identify the optimal ratio for different design objectives.

Engineering Practice Implications

For engineers designing composite columns using RPC, this study provides valuable guidance on the optimal use of steel fibers and spiral stirrups. The 10%-25% capacity enhancement justifies the additional cost of steel fibers and stirrups in applications where ductility and post-peak performance are critical, such as seismic zones and impact-resistant structures.

From a welding perspective, the fabrication of square steel tubes for RPC columns requires attention to the quality of the longitudinal and transverse welds. Square tubes are typically fabricated from flat plates welded at the corners, and the weld quality at these corners directly affects the confinement effectiveness. The residual stress at the corner welds can influence the stress distribution in the composite column under load.

Study Reflections and Outlook

This study contributes to the growing body of knowledge on ultra-high-performance concrete composite columns. The combination of RPC, steel fibers, spiral stirrups, and steel tube confinement represents a multi-level reinforcement strategy that can achieve exceptional structural performance. However, the study is limited to short columns under axial compression. Future research should extend to slender columns, eccentric loading, and cyclic loading to fully characterize the structural behavior under realistic loading conditions.

The economic analysis of using RPC in composite columns should also be considered. While RPC offers superior mechanical properties, its material cost is significantly higher than conventional concrete. The 10%-25% capacity enhancement must be weighed against the cost increase to determine the overall economic viability for different applications.