Prestressed FRP Strengthening of Steel Pipe Anchorage Systems

Literature Overview and Technical Challenge

The paper by Lu Yi, Zhang Yu, Jia Bin, Zhang Chuntao, and Li Zuzhan (2015), published in Industrial Construction, addresses a critical engineering challenge in fiber-reinforced polymer (FRP) strengthening applications for steel pipe structures: the development of a practical prestressing and anchorage system. While FRP wrapping has been widely adopted for structural strengthening of concrete and steel members, the application of prestress to FRP wraps on steel pipes presents unique technical difficulties related to tensioning, anchorage, and long-term bond performance.

The significance of this research lies in its practical focus on developing a dedicated anchorage device that integrates both tensioning and anchoring functions, enabling the effective application of prestress to FRP wraps on steel pipe surfaces. This capability is essential for achieving the full potential of FRP strengthening, as prestress significantly enhances the load-bearing capacity and serviceability performance of strengthened members compared to passive (unprestressed) FRP wrapping.

Anchorage Device Design and Testing

The research developed a novel integrated FRP cloth prestressing and anchoring device consisting of two primary functional components:

1. Rotary anchor bolt mechanism: A rotating bolt assembly that grips the FRP cloth and applies tension through rotational advancement, similar in principle to a turnbuckle but adapted for the low-ductility FRP material.
2. Bolt-fixed anchorage: A bolt-clamped anchorage system that secures the FRP cloth in its prestressed state, preventing relaxation and maintaining the applied prestress over the service life of the strengthening system.

Anchorage Device Components and Functions

The tensioning mechanism uses the rotary anchor bolt to incrementally advance and grip the FRP cloth, gradually building up the desired prestress level. Once the target prestress is achieved, the bolt-fixed anchorage clamps the FRP cloth securely, transferring the prestress through the adhesive bond to the steel pipe substrate.

Prestress Performance and Strain Distribution Analysis

The research conducted prestress application and anchorage performance tests on FRP-strengthened steel pipe specimens. The test results provided critical data on:

1. Prestress application efficiency: The ratio of measured FRP strain to applied bolt tension, which indicates the effectiveness of the tensioning mechanism in transferring prestress to the FRP wrap.
2. Strain distribution along the steel pipe: The spatial distribution of FRP strain along the pipe length, which reveals the stress transfer characteristics and identifies regions of potential stress concentration.
3. Anchorage integrity: The ability of the bolt-fixed anchorage to maintain prestress under sustained loading conditions without significant relaxation.

The strain distribution analysis revealed that the FRP prestress is not uniformly distributed along the steel pipe length. Near the anchorage points, the strain is lower due to the local constraint imposed by the anchorage hardware, while the strain increases toward the mid-span of the strengthened region. This non-uniform distribution is an important consideration for design, as it affects the effective prestress level and the load-bearing capacity enhancement.

Engineering Application and Design Considerations

For practical engineering application of prestressed FRP strengthening on steel pipes, several critical considerations must be addressed:

1. Surface preparation: The steel pipe surface must be thoroughly prepared to ensure adequate adhesive bond strength. This includes removal of coatings, rust, and contaminants, followed by roughening to enhance mechanical interlock. The surface preparation quality directly affects the prestress transfer efficiency and long-term bond durability.
2. Adhesive selection and application: The adhesive used to bond the FRP cloth to the steel pipe must have adequate shear strength, fatigue resistance, and environmental durability. The adhesive layer thickness and application quality are critical parameters that affect the prestress transfer and bond performance.
3. Prestress level control: The applied prestress must be controlled within a safe range that provides beneficial strengthening without causing premature debonding or steel pipe yielding. The maximum allowable prestress level depends on the FRP material properties, adhesive strength, and steel pipe yield strength.
4. Environmental durability: The prestressed FRP strengthening system must be designed for the specific environmental conditions of the application, including temperature extremes, UV exposure, chemical exposure, and moisture ingress. These factors affect the long-term bond performance and FRP material degradation.

Design Parameters and Performance Indicators

Study Insights and Reflections

This research makes a practical contribution to the field of FRP structural strengthening by developing a dedicated prestressing and anchorage system for steel pipe applications. The integrated design of the rotary anchor bolt and bolt-fixed anchorage provides a practical solution to the tensioning and anchoring challenge that has limited the adoption of prestressed FRP strengthening.

From a materials engineering perspective, I emphasize the importance of long-term durability testing for prestressed FRP systems. The prestress introduces a sustained tensile stress in the FRP material, which may accelerate matrix cracking and fiber-matrix debonding under environmental exposure. Accelerated aging tests and long-term monitoring data are essential for establishing reliable design life predictions.

The research also highlights the need for standardized design procedures for prestressed FRP strengthening of steel pipes. Current design codes for FRP strengthening primarily address passive (unprestressed) applications, and the design methodology for prestressed systems requires further development and codification. The strain distribution data obtained from this research provides a valuable foundation for developing such design procedures.

The development of practical prestressing and anchorage devices for FRP strengthening of steel pipes represents an important advancement in structural rehabilitation technology, enabling engineers to achieve superior strengthening performance compared to passive FRP wrapping systems.