Zhuojin Pipe Fitting Co., LtdExperimental Study on Axially Compressed Circular Hollow Sandwich Steel Tube RPC Short Columns
Literature Overview
This paper by Tang Changhui and Ouyang Peng from Hunan University investigates the axial compressive behavior of circular hollow sandwich steel tube confined reactive powder concrete (RPC) short columns. Funded by the National Natural Science Foundation of China (51278181), the study tests 13 specimens with varying inner steel tube wall thickness and provides a recommended formula for ultimate bearing capacity calculation. ABAQUS finite element simulations were conducted to validate the analytical model against experimental results.
Core Technical Findings
The research focuses on the effect of inner steel tube wall thickness on the bearing capacity and mechanical behavior of the sandwich column. The load-strain curves obtained from both experiments and simulations show good agreement, validating the finite element model. The key findings include:
| Parameter | Observation | Engineering Implication |
|---|---|---|
| Inner tube wall thickness increase | Ultimate capacity increases | But with diminishing returns at high thickness ratios |
| Load-strain curve shape | Elastic → hardening → plateau → descending | Characteristic of confined RPC behavior |
| FE simulation accuracy | Good agreement with experimental curves | Model is reliable for parametric studies |
| Recommended formula | Provided in the paper | Can be used for preliminary design |
Reactive powder concrete (RPC), also known as engineered cementitious composite (ECC) in some contexts, is characterized by ultra-high compressive strength (typically 120-200 MPa or higher), high ductility, and excellent durability. When confined within a double-walled steel tube configuration, the RPC benefits from lateral confinement that prevents premature splitting and enhances its strain-hardening behavior.
Sandwich Column Design and Fabrication Challenges
The circular hollow sandwich steel tube RPC column is a sophisticated composite member that combines the benefits of multiple materials in an optimized configuration. The fabrication process involves several critical steps:
- Inner tube fabrication: The inner steel tube must be manufactured with tight dimensional tolerances (typically ±0.5 mm for diameter and ±0.2 mm for wall thickness) to ensure uniform concrete cover thickness.
- Centering and alignment: The inner tube must be precisely centered within the outer tube using spacers or centering devices. Any eccentricity introduces unintended bending moments under axial loading.
- RPC placement: RPC has a very low workability (slump flow typically 200-280 mm) and requires specialized placement techniques, including high-frequency vibration or pumping with appropriate admixtures. The narrow annular gap between inner and outer tubes (typically 30-80 mm) makes complete consolidation challenging.
- Welded joint integrity: Any field welds connecting column segments must maintain the concentricity of both tubes. Angular distortion at the weld can shift the inner tube relative to the outer tube, creating an eccentric concrete layer.
Material and Welding Specifications
| Component | Recommended Material | Key Properties | Standard Reference |
|---|---|---|---|
| Outer steel tube | Q345B or Q390 | fy ≥ 345 MPa; δ ≥ 21% | GB/T 1591, GB/T 8163 |
| Inner steel tube | Q345B or Q390 | fy ≥ 345 MPa; δ ≥ 21% | GB/T 1591, GB/T 8163 |
| RPC | Custom mix design | fcu ≥ 120 MPa; fct ≥ 6 MPa | GB/T 50082, EN 206 |
| Butt welds | Matched electrode/wire | Tensile strength ≥ parent metal | GB 50661, ISO 5817 |
| Fillet welds (if applicable) | Full penetration required | No slag inclusion > 1 mm | ISO 5817 Level B |
The welding of thin-walled inner tubes (typically t = 3-8 mm) requires careful heat input control to avoid burn-through and excessive distortion. Gas tungsten arc welding (GTAW) is preferred for the root pass due to its precise heat control, followed by gas metal arc welding (GMAW) or flux-cored arc welding (FCAW) for subsequent passes. Back-gassing with argon is essential to prevent root oxidation on thin-walled tubes.
Engineering Practice and Quality Control
The quality of sandwich steel tube RPC columns depends heavily on the consistency of fabrication. A comprehensive quality control plan should include:
- Pre-weld inspection: Verification of tube dimensions, surface cleanliness (Sa 2.5 per ISO 8501-1), and fit-up gap (typically 2-4 mm for butt joints).
- Welding procedure qualification (WPQR): Each welding process, electrode type, and parameter combination must be qualified per GB/T 19866 or ISO 15614.
- In-process monitoring: Real-time monitoring of welding current, voltage, travel speed, and gas flow rate.
- Post-weld NDT: 100% visual inspection (VT) and magnetic particle testing (MT) on all welds; radiographic testing (RT) or phased array ultrasonic testing (PAUT) on a statistically determined sample (typically 10-20% of weld length).
- Dimensional verification: Post-weld measurement of ovality, angularity, and straightness to ensure compliance with fabrication tolerances.
- RPC placement verification: Density test, slump flow test, and post-placement ultrasonic testing to detect voids or honeycombing in the annular zone.
Study Insights and Reflections
The sandwich steel tube RPC column represents an advanced composite structural element that leverages the complementary properties of steel, RPC, and the geometric configuration of a double-walled tube. The experimental and numerical results demonstrate that this configuration achieves high ultimate capacity with favorable ductility characteristics. However, the practical adoption of such columns is constrained by fabrication complexity and cost. The narrow annular gap for concrete placement, the need for precise centering, and the specialized requirements for RPC mixing and placement all add to the construction cost. Engineers considering this solution should carefully evaluate the cost-benefit ratio compared to conventional single-walled STC columns with higher-strength concrete. The recommended capacity formula provided in the paper should be validated against a broader database of specimens before being incorporated into design codes. From a welding quality perspective, the thin-walled inner tube welds are the most critical quality control point, and investment in qualified welders, calibrated equipment, and comprehensive NDT is essential to ensure the structural integrity of these advanced composite members.