Zhuojin Pipe Fitting Co., LtdMechanical Performance of CFRP Steel Tube-RPC Short Columns
Literature Overview
Jian Chao, Jiao Chujie, Li Song, Tan Shuzhen, and Cui Lishi (2019) from Guangzhou University published experimental research in Concrete and Cement Products on the axial compression behavior of composite short columns combining steel tubes, CFRP (Carbon Fiber Reinforced Polymer) wrapping, and Reactive Powder Concrete (RPC) infill. Supported by multiple National Natural Science Foundation grants and provincial research programs, the study investigates the influence of steel tube diameter and CFRP layer count on failure modes, load-displacement behavior, and ultimate load capacity.
Technical Background
Reactive Powder Concrete (RPC) is a high-performance cementitious material characterized by the absence of coarse aggregate, the use of very fine quartz sand, and the incorporation of steel fibers. RPC achieves compressive strengths exceeding 120 MPa and exhibits exceptional toughness, durability, and fatigue resistance. However, RPC is inherently brittle in terms of tensile behavior, and its full potential can only be realized with effective confinement.
The proposed composite column configuration combines:
- Steel tube: Provides structural strength, buckling resistance, and primary confinement
- CFRP wrapping: Provides secondary high-strength confinement and crack arrest
- RPC infill: Provides exceptional compressive strength and durability
This triple-composite approach leverages the strengths of each material while mitigating their individual weaknesses.
Experimental Setup and Variables
Test Matrix
| Variable | Levels | Number of Specimens |
|---|---|---|
| Steel tube diameter | Multiple diameters (specific values in paper) | Variable |
| CFRP layer count | 0, 1, 2, 3, 4 layers | Variable |
| RPC compressive strength | 120–150 MPa | Fixed |
| Column length-to-diameter ratio | Short column range (L/D < 4) | Fixed |
Material Properties
| Material | Property | Value |
|---|---|---|
| RPC | Compressive strength | 120–150 MPa |
| RPC | Tensile strength (steel fiber reinforced) | 6–10 MPa |
| Steel tube | Yield strength | 235–345 MPa |
| CFRP sheet | Tensile strength | 3400–3600 MPa |
| CFRP sheet | Elastic modulus | 140–160 GPa |
| CFRP sheet | Thickness per layer | 0.11–0.16 mm |
Failure Mode Classification
The study identifies two distinct failure modes based on the confinement ratio:
Buckling (Bulging) Failure
When the confinement ratio is relatively high, the column fails by outward bulging (barrel-shaped deformation). In this mode:
- The CFRP and steel tube together provide sufficient lateral restraint to maintain triaxial compression in the RPC
- The RPC reaches its ultimate confined strength and crushes
- The steel tube and CFRP wrap expand outward until the CFRP reaches its tensile capacity
- The load-displacement curve shows a pronounced post-peak plateau or even a hardening trend
- The failure is ductile in character with significant energy absorption
Shear Failure
When the confinement ratio is relatively low, the column fails by diagonal shear:
- The RPC core cracks along diagonal planes before the confinement materials can develop full restraint
- The steel tube may buckle locally at the shear plane
- The CFRP may delaminate or tear at the crack locations
- The load-displacement curve shows rapid post-peak load drop
- The failure is more brittle with limited energy absorption
Confinement Ratio Classification
| Confinement Ratio Range | Dominant Failure Mode | Post-Peak Behavior |
|---|---|---|
| Low (σ_conf / f_c < 0.05) | Shear failure | Rapid load drop |
| Medium (0.05 < σ_conf / f_c < 0.15) | Transition zone | Moderate load drop |
| High (σ_conf / f_c > 0.15) | Buckling failure | Plateau or hardening |
Load-Displacement Curve Analysis
The load-displacement curves of CFRP steel tube-RPC short columns can be divided into four stages:
| Stage | Description | Behavior |
|---|---|---|
| Elastic stage | All materials respond linearly | Linear load-displacement relationship |
| Elastic-plastic stage | Steel tube yields, RPC begins microcracking | Gradual slope reduction |
| Load descent stage | RPC crushes, steel tube buckles | Load decreases |
| Hardening stage | CFRP provides secondary confinement | Load stabilizes or increases |
Some specimens exhibit a load plateau stage between the descent and hardening stages, which represents a transition where the CFRP is just beginning to engage in confinement.
Quantitative Results
Effect of Steel Tube Diameter
Increasing the steel tube diameter increases the ultimate load capacity. This is expected because:
- Larger diameter provides a larger cross-sectional area for load-bearing
- The hoop stress in the steel tube decreases with increasing diameter for the same internal pressure (thin-walled pressure vessel theory: σ_h = pr/t)
- The confinement efficiency improves with larger diameter-to-thickness ratio
Effect of CFRP Layer Count
Increasing CFRP layers from 0 to 4 progressively increases the ultimate load capacity. The incremental benefit of each additional layer follows a diminishing returns pattern, consistent with the Modified Mander confinement model. The optimal number of CFRP layers depends on the steel tube diameter and thickness.
Engineering Recommendations
| Recommendation | Justification |
|---|---|
| Design for high confinement ratio | Ensures ductile buckling failure rather than brittle shear failure |
| Use minimum 2 CFRP layers | Provides adequate secondary confinement and crack arrest |
| Select steel tube diameter to match load requirements | Larger diameter improves both capacity and confinement efficiency |
| Ensure proper CFRP surface preparation | Surface roughness and primer application are critical for CFRP bond strength |
| Conduct full-scale testing before implementation | FE models validated against experimental data provide reliable design basis |
Study Insights
This research demonstrates that the combination of steel tube, CFRP wrapping, and RPC infill creates a highly efficient composite column system with exceptional load-bearing capacity and ductility. The dual-confinement approach (steel tube + CFRP) provides a robust safety margin against both brittle and ductile failure modes.
The identification of two distinct failure modes based on confinement ratio is a valuable contribution to the design methodology. Engineers can use this classification to ensure that their designs fall within the ductile failure regime by selecting appropriate steel tube dimensions and CFRP layer counts.
One area for future investigation is the long-term performance under sustained loads, particularly the creep behavior of RPC under high confinement and the potential for CFRP debonding over time. Additionally, the fire resistance of the composite system requires evaluation, as CFRP loses significant strength above 300 °C. These factors will be critical for the widespread adoption of this composite column system in practical engineering applications.
The experimental data presented provides a solid foundation for developing design equations and guidelines for CFRP steel tube-RPC composite columns, and the research methodology offers a template for investigating other composite column configurations.