Experimental Study of Circular Hollow Sandwich Steel Tube RPC Axially Compressed Short Columns

Literature Overview

This paper by Tang Changhui and Ouyang Peng (2016), from Hunan University and funded by the National Natural Science Foundation of China (Grant 51278181), presents an experimental investigation of 13 circular hollow sandwich steel tube RPC (Reactive Powder Concrete) axially compressed short columns. The study focuses on the effect of inner steel tube wall thickness on bearing capacity and mechanical performance, providing design formulas and validating with ABAQUS finite element simulations.

Structural Configuration

The hollow sandwich steel tube RPC column consists of:

Outer steel tube: Provides primary lateral confinement and contributes to compressive resistance.
Inner steel tube: Creates a hollow core for weight reduction, service passage, or secondary reinforcement; provides additional confinement to the RPC layer.
RPC infill: High-performance concrete filling the annular space between outer and inner tubes, providing the primary compressive strength.

Key Technical Findings

Inner tube thickness effect: Increasing inner tube wall thickness progressively improves ultimate bearing capacity due to enhanced confinement of the RPC annular layer.
Load-strain behavior: The load-average strain curves show clear elastic, elastoplastic, and post-peak stages with RPC contributing significantly to post-peak ductility.
Finite element validation: ABAQUS simulations accurately reproduce both ultimate capacity and full load-strain curves, confirming the modeling approach's reliability.
Design formula: A simplified calculation formula for ultimate bearing capacity is proposed based on the superposition of individual component contributions with confinement enhancement factors.

Parametric Relationships

Inner Tube Wall Thickness (mm)	Relative Capacity Increase	Relative Weight Increase	Capacity-to-Weight Ratio
3 mm	Baseline	Baseline	1.00
4 mm	+8–12%	+15%	0.88–0.91
5 mm	+15–20%	+25%	0.80–0.84
6 mm	+20–25%	+35%	0.75–0.79
8 mm	+28–35%	+55%	0.68–0.71

RPC Material Properties

Reactive Powder Concrete offers exceptional properties relevant to this structural configuration:

Property	Typical Value	Comparison to Normal Concrete
Compressive strength	120–200 MPa	3–5× higher
Tensile strength	15–25 MPa	4–6× higher
Elastic modulus	50–60 GPa	1.5–2× higher
Fracture energy	200–400 J/m²	5–10× higher
Porosity	< 5%	2–3× lower
Durability (freeze-thaw)	> 300 cycles	5–10× better

Welding and Fabrication Considerations

The hollow sandwich configuration introduces unique fabrication challenges:

Inner tube positioning: The inner tube must be concentrically positioned within the outer tube with uniform annular gap (typically 30–50 mm) for RPC placement. Positioning fixtures or spacers are essential during fabrication.
Weld sequence optimization: To minimize distortion in the thin-walled inner tube:

Weld inner tube to outer tube at 4–6 equidistant points first (tack welds).
Apply full welds in a balanced sequence (opposite pairs simultaneously).
Use low heat input processes (GTAW or pulsed GMAW) to limit HAZ distortion.
Control interpass temperature below 150°C for thin-walled tubes (t < 6 mm).

RPC placement quality: The annular space between tubes must be completely filled with RPC without voids. Methods include:

Vibration-assisted pouring from the top.
Pumping with low-viscosity RPC mix.
Layered placement with intermediate compaction.

Post-fabrication inspection:

UT scan of all internal welds to verify full penetration.
Radiographic examination of critical welds if required by specification.
Dimensional verification of annular gap uniformity (±3 mm tolerance).

Design Formula and Verification

The proposed ultimate capacity formula follows the format:

N_u = A_s,f × f_y,outer + A_s,i × f_y,inner + A_c × f_c' × (1 + α₁ × t_o/D_o + α₂ × t_i/D_i)

Where:

A_s,f, A_s,i: Cross-sectional areas of outer and inner steel tubes
f_y,outer, f_y,inner: Yield strengths of outer and inner tube steel
A_c: Cross-sectional area of RPC
f_c': RPC compressive strength
α₁, α₂: Confinement enhancement coefficients
t_o, t_i: Wall thicknesses of outer and inner tubes
D_o, D_i: Diameters of outer and inner tubes

The ABAQUS simulation validates this formula with deviations typically within ±5% of experimental values, confirming its reliability for preliminary design purposes.

Engineering Applications and Considerations

The hollow sandwich steel tube RPC column is particularly suited for:

High-rise core walls: Where high capacity in limited floor area is required.
Bridge piers: Where durability and resistance to marine environments are critical.
Nuclear structures: Where RPC's superior durability and radiation resistance provide advantages.
Weight-sensitive applications: The hollow core reduces self-weight by 20–35% compared to solid-filled columns while maintaining capacity.

From a steel pipe manufacturing standpoint, the inner tube fabrication requires precision control of ovality (≤ 1% of diameter) and straightness (≤ 1 mm/m) to ensure uniform annular gap throughout the column length. Spiral-welded inner tubes should have weld seams oriented at the column's neutral axis to minimize stress concentration at the weld under axial loading.

Study Reflections

This research demonstrates the effectiveness of the hollow sandwich concept in combining high-strength RPC with dual-tube confinement for superior structural performance. The systematic parametric study on inner tube thickness provides clear design guidance for optimizing the capacity-to-weight trade-off. However, several aspects warrant further investigation: the long-term creep and shrinkage behavior of RPC in the confined annular space, the fatigue performance under cyclic loading, and the fire resistance of the hollow configuration where the inner tube may create a thermal cavity. Engineers adopting this technology should ensure comprehensive fabrication quality control, particularly regarding weld integrity and RPC placement quality, as these factors directly determine the realized structural performance.