Eccentric Compression Performance of Circular Steel Tube Recycled Concrete Short Columns with Built-In CFRP Tube

Literature Overview

This study investigates the eccentric compression behavior of circular steel tube columns filled with recycled concrete (RC) and incorporating an internal carbon fiber reinforced polymer (CFRP) tube. The research addresses two sustainability objectives simultaneously: the use of recycled aggregate concrete to reduce natural resource consumption and the incorporation of CFRP to enhance structural performance. The eccentric loading condition simulates realistic boundary conditions encountered in practical structures where perfect axial alignment is rarely achieved.

Core Technical Content and Key Findings

Recycled concrete, produced by replacing natural aggregate with crushed concrete waste, typically exhibits lower compressive strength (10-25% reduction) and higher porosity compared to natural aggregate concrete. However, its environmental benefits are significant, reducing landfill waste and conserving natural resources. The incorporation of a CFRP tube within the steel tube creates a sandwich composite system where the CFRP tube provides additional confinement to the recycled concrete core while also acting as a tensile reinforcement that resists the tensile stresses induced by eccentric loading.

The eccentric compression creates a non-uniform stress distribution across the column cross-section, with compressive stresses concentrated on the side near the load application point and tensile or reduced compressive stresses on the opposite side. The CFRP tube, with its high tensile strength (3000-5000 MPa) and elastic modulus (120-230 GPa), effectively resists the tensile stresses that would otherwise cause concrete cracking and spalling on the tension side.

Interpretation of Technical Points

The sandwich configuration (steel tube - CFRP tube - recycled concrete) creates a hierarchical confinement system. The outer steel tube provides the primary confinement through its plastic deformation under high loads, while the inner CFRP tube provides secondary confinement through its high elastic stiffness. The CFRP tube also acts as a tensile hoop reinforcement, preventing radial cracking of the recycled concrete core that would otherwise propagate from the tension side under eccentric loading.

The recycled concrete, due to its higher porosity and lower interfacial transition zone (ITZ) strength, is more susceptible to cracking under tensile stresses. The CFRP tube effectively bridges these cracks, maintaining load transfer continuity across the cracked sections. This crack-bridging mechanism is particularly important for eccentrically loaded columns where the tension zone is critical for load redistribution.

From a fabrication perspective, the assembly of the sandwich system requires careful attention to concentricity between the steel tube and CFRP tube. Misalignment creates eccentric loading of the CFRP tube itself, reducing its effectiveness. The gap between the CFRP tube and steel tube must be uniform to ensure even concrete placement and avoid localized voids. Typical concentricity tolerances of ±2 mm are recommended for the CFRP tube positioning.

The CFRP tube is typically manufactured through filament winding or pultrusion processes, following standards such as ASTM D5229 (pultruded glass or carbon fiber reinforced polymer composite profiles) or ASTM D7512 (wound carbon fiber reinforced polymer composite profiles). The tube must be verified for fiber volume fraction, fiber orientation, and interlaminar strength through destructive testing per ASTM D2344 or ASTM D3518.

Process and Standards Analysis

The design and fabrication of CFRP-reinforced recycled concrete steel tube columns involves multiple material standards and fabrication specifications:

The eccentric compression testing follows standard protocols per GB/T 50152 or EN 1992-1-1, applying a controlled eccentric load to the column top while measuring mid-height deflection and strain at multiple locations. The test data provides load-deflection curves, strain distributions, and failure mode observations that validate analytical models and inform design equations.

Integration with Engineering Practice

The combination of recycled concrete and CFRP reinforcement addresses multiple sustainability and performance objectives simultaneously. Applications include:

• Building columns in regions with abundant construction waste but limited natural aggregate
• Seismic retrofits of existing structures where additional confinement is needed without adding significant weight
• Bridge piers in environmentally sensitive areas where material sourcing must be sustainable
• Industrial structures requiring high eccentric load capacity with limited space

The fabrication process requires specialized equipment for CFRP tube manufacturing and assembly. The CFRP tube must be cut to length with precision to avoid delamination at the cut ends, and the cut ends should be sealed with epoxy resin to prevent moisture ingress. The steel tube and CFRP tube must be concentrically aligned using positioning fixtures during concrete placement.

Quality control should include: recycled aggregate quality verification (water absorption, ITZ strength); CFRP tube non-destructive testing (ultrasonic C-scan for delamination); concrete strength verification; dimensional inspection of the assembly; and eccentric compression load testing on representative specimens.

Key Questions and Reflections

The long-term durability of the CFRP tube in the concrete environment raises questions about interfacial bond degradation over time. The alkaline environment of concrete (pH 13-14) can potentially degrade the epoxy matrix of the CFRP tube, reducing its mechanical properties over decades of service. Accelerated aging tests (alkali immersion, freeze-thaw cycling, sustained loading) should be conducted to assess long-term performance.

The interaction between the steel tube and CFRP tube under fire conditions is another concern. CFRP materials begin to degrade at temperatures above 200°C, losing significant strength above 300°C. In fire scenarios, the CFRP tube may lose its effectiveness before the steel tube, creating a sudden reduction in confinement. Fire-resistant coatings or protective layers may be necessary for fire-critical applications.

Study Insights and Implications

The research demonstrates that the combination of recycled concrete and CFRP reinforcement creates a viable composite system for eccentrically loaded columns. The CFRP tube effectively compensates for the reduced strength of recycled concrete by providing superior tensile resistance and additional confinement. This approach offers a practical pathway toward sustainable construction without sacrificing structural performance.

For pipeline and structural engineers, the sandwich confinement concept has broader applications. The principles of hierarchical confinement and material synergy can be applied to the design of pipe systems where multiple layers of protection or reinforcement are required. The emphasis on recycled materials and high-performance composites reflects the industry's growing commitment to sustainability and resource efficiency.