Static Behaviour of Steel Tube Polypropylene Fibre Ultra-High-Strength Stone Slag Concrete Short Columns

Literature Overview

This paper by Chen Guocan, published in the Journal of Wuhan University (Engineering Edition) in 2010 (Vol. 43, No. 5, pp. 617–622), presents an experimental investigation into the axial compressive behaviour of steel tube columns filled with polypropylene fibre-reinforced ultra-high-strength stone slag concrete. Nineteen specimens were tested with varying diameter, diameter-to-thickness ratio, and concrete strength grade. The research was funded by the Fujian Provincial Natural Science Foundation (No. 2007J0163) and the Putian Science and Technology Plan Project (No. 2007G26). The study addresses a critical practical problem in steel tube reinforced concrete (SRC) construction: the debonding phenomenon caused by excessive self-shrinkage of high-strength concrete within steel tubes.

Core Technical Findings

Material and Specimen Parameters

Four-Stage Load–Strain Behaviour

The load–average strain curves identified four distinct stages:

1. Elastic stage – linear relationship between load and strain; both steel tube and core concrete act elastically.
2. Elastic-plastic stage – microcracking initiates in the core concrete; the curve deviates from linearity as plastic deformation develops.
3. Load-decreasing stage – macroscopic cracking propagates; the steel tube begins to buckle locally.
4. Load-recovery stage – the steel tube confinement effect fully mobilises, arresting crack propagation and recovering load capacity.

Key Performance Indicators

Technical Interpretation

The incorporation of polypropylene fibres into ultra-high-strength stone slag concrete addresses the self-shrinkage debonding problem that plagues conventional SRC columns with high-strength concrete fills. When ultra-high-strength concrete is placed inside a steel tube, the significant self-shrinkage during hardening creates tensile stresses at the steel-concrete interface, leading to radial gaps that compromise the composite action. Polypropylene fibres, with their high aspect ratio and bond strength, bridge microcracks during the shrinkage phase and maintain the intimate contact between the concrete core and the steel tube wall.

The confinement index, defined as the ratio of the circumferential confinement stress provided by the steel tube to the unconfined compressive strength of the core concrete, emerges as the dominant parameter. This is consistent with the well-established mechanical model of SRC columns where the steel tube provides lateral confinement proportional to its yield strength and thickness. The diameter-to-thickness ratio D/t controls the hoop stress capacity of the steel tube—lower D/t ratios yield higher confinement efficiency but may be impractical from a fabrication and welding standpoint.

The four-stage load–strain response, particularly the load-recovery stage, is of considerable engineering significance. It indicates that even after significant damage to the concrete core, the steel tube maintains structural integrity through plastic deformation and provides continued load-bearing capacity. This behaviour is directly analogous to the strain-hardening behaviour observed in heavily confined concrete columns and is critical for seismic design where energy dissipation through ductile deformation is essential.

Connection with Steel Pipe Manufacturing and Welding Practice

From a steel pipe manufacturing perspective, the confinement performance of the steel tube is directly dependent on the quality of the pipe material and fabrication process. For SRC columns, the steel tubes are typically fabricated from structural steel conforming to GB/T 3077 or Q345B/Q355B grades, with wall thicknesses ranging from 4 mm to 12 mm. The manufacturing route—whether seamless (GB/T 8162), ERW (GB/T 3091), or HFW—must ensure uniform wall thickness and absence of defects at the weld seam that could become stress concentrators under compressive loading.

The welding of end caps, stiffener rings, and connection plates to the steel tube is a critical quality control point. According to GB 50017 and SY/T 0269, the following welding considerations are essential:

The residual strain range of 8.67%–24.9% implies that the steel tube undergoes significant plastic deformation. This demands that the pipe material possess adequate elongation (typically ≥ 20% for Q345B per GB/T 699) and that the welding process does not introduce excessive hardness in the heat-affected zone that could limit local ductility. For GTAW or SAW welding of stiffener rings, a controlled heat input of 1.0–2.5 kJ/mm is recommended to avoid over-tempering of the base metal or excessive grain growth in the HAZ.

Study Insights and Implications

The research demonstrates that the combination of ultra-high-strength stone slag concrete and polypropylene fibres within a steel tube creates a composite column system with exceptional ductility and energy dissipation capacity. The recommended regression-based formula for ultimate bearing capacity provides a practical design tool, though its application should be validated against specific project conditions. For engineering practice, the key takeaway is that the confinement index should be optimised during the design phase by selecting appropriate D/t ratios and steel grades, while ensuring that the pipe fabrication and welding quality meets the stringent requirements of the composite action mechanism. The shear-type failure mode with high residual strength is particularly attractive for seismic applications where columns must sustain large inelastic deformations without catastrophic collapse.