International Concrete Abstracts Portal

A new design concept, experimental assessment and numerical simulation of a wedge anchor system for prestressing CFRP rods are presented. This compact and reusable anchor required no presetting load and was capable to carry the full design strength of the rod. It consists of an outer cylinder (barrel), a number of wedges, and a soft metal sleeve. The contacting surfaces of the wedges and barrel have a circular profile along the length of the anchor. Monotonic tensile testing was carried out. The relationship of the tensile load and displacement of the rod was established. A three dimensional finite element model was developed to simulate the anchor components. Contact pressure on the rod surface for different presetting distances was low at the loading end of the rod and increased at the free end. Tensile load and displacement relationships was found and compared to that of the experimental investigation.

The need for sustainable structures is the driving force for the growing international interest in using advanced materials such as fiber reinforced polymer (FRP) materials for civil infrastructure applications. To date, FRP materials have not realized their full potential within the construction industry. One of the contributing factors is the limited information regarding their long-term performance. In particular, since the service life of a civil engineering structure is typically 50 to 100 years, knowledge of the long-term durability of FRPs is of prime importance. The research work presented in this paper is the result of a research collaboration between North Carolina State University and the University of Cambridge. The research investigates the durability of concrete beams prestressed with CFRP bars and compares the results with those of companion beams prestressed with steel wires. A total of 15 beams have been constructed and tested under different mechanical and environmental conditions. The parameters included in the program were the level of sustained stress in the bars and wires (55 and 70 percent of the ultimate bar or wire strength), the environmental exposure condition (air exposure and continuous exposure to 15 percent by mass salt water spray at 54 °C temperature), the length of time under sustained load (9 and 18 months) and the method of testing (with or without application of cyclical loading prior to static testing to failure). The experimental program illustrates that CFRP prestressed concrete beams exhibit comparable ultimate strength and fatigue strength properties in comparison to equivalent steel prestressed concrete beams. Furthermore, test results show that the beams prestressed with steel wires did not survive the environmental exposure over 12 months whereas the beams prestressed with CFRP bars survived up to the end of the 18 month long extreme environmental exposure, indicating the excellent durability of CFRP in the marine environment. Provided that provision is made for the lack of CFRP ductility in comparison to steel, the research undertaken has indicated that CFRP prestressed concrete is a durable, appropriate option when designing structures for use in offshore environments.

Fiber reinforced polymer (FRP) composites have been widely used as externally bonded reinforcements to strengthen or rehabilitate deteriorated concrete structures. However, premature debonding failure due to the limitation of bond at the FRP-concrete interface is often encountered which limits FRP material potential for flexural strength increase. To minimize debonding failures and mechanical damage such as vehicular traffic and impact, etc., near surface mounted (NSM) FRP technology has emerged as another structural rehabilitation method. This study aims at evaluating a new strengthening using prestressed NSM carbon fiber reinforced polymer (CFRP) tendons. Although NSM FRP tendons are less susceptible to debonding than externally bonded FRP laminates, the load-carrying capacity of RC structures with NSM reinforcements is still likely to be governed by "premature" debonding failure. An experimental program with design variables such as prestress level and bonding agents was carried out to study the strengthening performance in terms of stiffness, cracking behavior and load-carrying capacity. The experimental results are presented and discussed in this paper. Experimentally validated bonding method is also proposed in this study.

The monotonic flexural performance of reinforced concrete beams strengthened with external post-tensioned CFRP tendons was investigated. Variables include straight and double draped tendon profiles, and two post-tensioning conditions: at service load (50% of the yielding capacity - in-service beam) or after overloading to twice the steel reinforcement yield strain (overloaded beam). The beams were tested to failure in four-point bending. Experimental results indicated that the tendon profiles investigated did not affect the beam behavior. Overloading of the beam prior to post-tensioning had a significant effect on the deflection capacity of the beam, with a slight effect on the yield and ultimate capacities. A strain reduction approach, developed previously for unbonded steel tendons, was extended to model the beam behavior. The loading history of the beam prior to strengthening was explicitly accounted for in the model. The analytical predictions showed good agreement with the experimental values.

Strengthening of concrete structures with fibre reinforced polymer materials has today grown to be a widely used method over most parts of the world. As a way of higher utilization of the FRP (Fibre Reinforced Polymers) prestressing has proved to be beneficial. Most of the research done with prestressing Carbon Fibre Reinforced Polymers (CFRP) for strengthening has been with surface bonded plates. However, in this paper a presentation is given where CFRP quadratic rods are bonded in the concrete cover in sawed grooves and then immediately prestressed. Testing has, proven this to be an advantageous way of bonding CFRP to the concrete. There is also a tendency that the shear forces between the CFRP and the concrete are transferred more efficiently compared to surface bonded plates and sheets. In the tests performed, no mechanical device has been used to keep the prestress during testing, which then means that the adhesive has to transfer all shear stresses to the concrete. These tests have then been compared with concrete beams strengthened with prestressed external steel and CFRP tendons and similar load carrying capacity has been obtained.