Introduction to Concrete-Filled Steel Tubes (CFST)
In modern civil engineering, the use of Concrete-Filled Steel Tube (CFST) structures has become a common practice, combining the high strength of steel with the robustness of concrete. Steel pipes are used as permanent formworks for concrete, providing multiple benefits like improved compressive strength, strain capacity, ductility, and energy dissipation. These composite structures are commonly used in bridges, high-rise buildings, and seismic-resistant structures due to their enhanced mechanical properties and ability to resist fire and corrosion.
The interface between the steel pipe and concrete plays a crucial role in the overall performance of CFST structures. Debonding and void formation at this interface can reduce the load-bearing capacity and stability of the structure. Therefore, strengthening the bond between the steel and concrete is critical for improving the overall durability and service life of CFST components.
The Need for Reinforcement in CFST Structures
To enhance the bond strength and mitigate potential issues like debonding or emptying, researchers have proposed various structural modifications to the inner surface of steel tubes. Among these strategies, internally welded steel bars have gained attention as an effective reinforcement method. By welding circular steel bars on the inner surface of the steel pipe, the mechanical interlock between the concrete and steel is enhanced, improving the overall bond strength and the performance of CFST components under different loading conditions.
Despite the promising benefits, there is limited research on the optimal application, parameters, and effects of welded steel bars in CFST structures. This study aims to bridge this gap by analyzing the impact of internally welded steel bars on the bond strength, dynamic response, and overall performance of CFST interfaces.
Experimental Setup
For the study, a Q420qD steel pipe and C60 self-compacting compensating concrete were used in the experiments. The steel pipe had a thickness of 32 mm and an outer diameter of 1,400 mm. The concrete was pumped into the steel tube using a Zoomlion ZLJ5180THBE truck-mounted pump (with a maximum pressure of 28 MPa) and vacuum-assisted perfusion equipment. The concrete mixture was carefully designed to ensure uniform filling and minimal void formation, crucial for the compactness of the final structure.
Push-Out Tests and Strain Measurements
The experiments involved push-out tests to assess the bond strength at the steel-concrete interface. Strain gauges were placed in three sections of the steel tube to monitor the strain distribution and response to applied loads. The stress distribution was also observed at multiple measurement points along the tube. The strain measurements indicated good pressure-holding capacity and minimal deformation under loading.
Ultrasonic Testing
To assess the bond strength and compactness of the concrete, ultrasonic testing was performed. The results showed that the concrete had excellent compactness, with consistent readings for ultrasonic wave velocity at different intervals (2nd, 5th, and 7th days). The ultrasonic testing confirmed that the concrete-filled steel tube exhibited strong cohesion between the steel and concrete components, with no significant debonding detected.
Results and Observations
1. Bond Strength Improvement:
The introduction of internally welded steel bars significantly enhanced the bond strength between the steel pipe and the concrete. The increase in bond strength reduced the likelihood of debonding and improved the overall performance of the CFST structure under various loading conditions. The experimental results showed that with an increased number of welded steel bars, the push-out length was reduced, indicating stronger adhesion between the concrete and steel.
2. Impact of Welded Steel Bars:
Internally welded steel bars not only improved the bond strength but also contributed to better dynamic stability and vibration resistance of the structure. The steel bars provided additional restraint, which helped maintain the structural integrity of the CFST under dynamic and seismic loading conditions.
3. Finite Element Simulation:
The finite element simulations aligned with the experimental findings, providing insights into the bond-slip characteristics of CFST components. These simulations confirmed that the internal welded steel bars enhanced the overall stability and performance of the structure by improving the interaction between the steel tube and concrete.
4. Concrete Compactness:
The vacuum-assisted perfusion method used in the experiments ensured a high degree of compactness in the concrete, contributing to its overall stability. This method also effectively removed air bubbles, which is crucial for achieving a uniform distribution of concrete and preventing void formation that could lead to debonding.
Key Takeaways:
• Internally welded steel bars significantly enhance the bond strength between steel pipes and concrete in CFST structures.
• The push-out test results indicate that increasing the number of welded steel rings reduces the push-out length, improving the bonding performance.
• Ultrasonic testing confirmed that the concrete had good compactness, ensuring the integrity of the steel-concrete interface.
• Finite element simulations supported experimental data, offering insights into the bond-slip characteristics of CFST components.
• The vacuum-assisted perfusion method used in the study ensured uniform concrete filling and minimized void formation.
• Internally welded steel bars also improve the dynamic stability and vibration resistance of CFST structures, making them more resilient under seismic and dynamic loads.
• This reinforcement method can significantly contribute to long-term durability and the overall performance of concrete-filled steel tube structures, particularly in applications like bridge engineering.
This study highlights the effectiveness of internally welded steel bars as a reinforcement strategy to improve the bond strength and dynamic performance of concrete-filled steel tube structures, providing valuable insights for future design and construction in civil engineering.
