Comparative Analysis of Bonding Performance between FRP Tube Concrete Piles and Steel Tube Concrete Piles

Overview of the Study

This paper by Wang Zhengzhen, Gong Weiming, Dai Guoliang, and Liu Lingfeng from Southeast University, published in the Journal of Southeast University in 2019, presents a systematic comparative study of the bonding performance between fiber-reinforced polymer (FRP) tube concrete piles and steel tube concrete piles. The research was conducted through push-out tests on four groups of specimens, examining the effects of bonding method, concrete type, specimen slenderness ratio, and outer tube wall thickness on the bonding strength. The work was supported by the National Key R&D Program of China and the National Natural Science Foundation of China.

Test Matrix and Methodology

The push-out test is the standard method for evaluating the bond strength between the outer tube and the inner concrete core in composite concrete-filled tube piles. In this test, the concrete core is pushed out of the tube under axial loading, and the maximum load and the corresponding slip at the tube-concrete interface are recorded. The four groups of specimens varied in bonding method, concrete type, slenderness ratio, and wall thickness to systematically investigate the influence of each parameter.

Key Findings

The most striking finding is that under identical test conditions, the bonding strength of steel tube concrete specimens is significantly greater than that of FRP tube concrete specimens. This is attributed to the higher elastic modulus and yield strength of steel compared to FRP, which provides greater confinement pressure on the concrete core and creates a stronger mechanical interlock at the interface. The difference in bonding strength between the two materials is substantial and has direct implications for the design of composite piles in terms of load capacity and serviceability.

The use of expansive concrete was found to be the most effective method for improving bonding strength, increasing it by nearly ten times compared to normal concrete. Expansive concrete develops internal expansion upon hydration, which creates radial pressure on the outer tube and enhances the frictional resistance at the tube-concrete interface. This finding is particularly significant for FRP tube concrete piles, where the relatively low stiffness of the FRP tube may limit the effectiveness of mechanical interlock.

The slenderness ratio and wall thickness also showed positive effects on bonding strength, but to a lesser degree than the concrete type. A higher slenderness ratio provides a larger contact area between the tube and the concrete, while a thicker wall provides greater confinement stiffness. The bonding method also influenced the strain distribution: for directly bonded specimens, the stress near the load application point developed first and propagated along the length of the specimen; for threaded specimens, the strain developed simultaneously along the entire height of the specimen due to the mechanical interlock provided by the threads.

Engineering Design Implications

The bonding performance of composite piles directly affects their axial load capacity, lateral load capacity, and service life. For steel tube concrete piles, the high bonding strength ensures effective composite action between the steel tube and the concrete core, allowing the full capacity of both materials to be utilized. For FRP tube concrete piles, the lower bonding strength may result in partial composite action, where the FRP tube and the concrete core act more independently, reducing the overall structural efficiency.

The finding that expansive concrete can dramatically improve the bonding strength of FRP tube concrete piles opens up a practical pathway for enhancing the structural performance of FRP composite piles without modifying the tube material or geometry. Engineers designing FRP tube concrete piles should consider specifying expansive concrete mixtures with carefully controlled expansion rates to optimize the radial pressure at the tube-concrete interface without causing cracking or excessive internal stresses.

Quality Control and Testing

For quality control of composite piles, the push-out test provides a direct measure of the bonding performance, but it is a destructive test that can only be performed on test specimens. Non-destructive evaluation methods such as ultrasonic testing can be used to detect voids or delamination at the tube-concrete interface, while magnetic flux leakage testing can identify defects in the steel tube. For FRP tube concrete piles, the lack of magnetic properties limits the applicability of magnetic testing methods, and alternative techniques such as acoustic emission or thermography may be required.

The strain distribution patterns observed in the tests provide valuable information for the design of strain monitoring systems in instrumented piles. For directly bonded piles, the strain near the load application point is the critical parameter to monitor, while for threaded piles, the strain distribution is more uniform and can be assessed at multiple locations along the pile length.

Study Insights

This comparative study provides essential baseline data for engineers evaluating the structural performance of FRP tube concrete piles relative to their steel tube counterparts. The finding that expansive concrete can enhance bonding strength by an order of magnitude is particularly significant, as it suggests that the performance gap between FRP and steel tube concrete piles can be substantially narrowed through appropriate material selection. The systematic investigation of multiple parameters provides a comprehensive understanding of the factors governing bonding performance, enabling engineers to make informed design decisions based on the specific requirements of each project.