For strengthening of reinforced concrete structures (FRP strips) carbon fibre lamellae are available for several years. Bonded non-tensioned CFRP strips have no corrosion, no ageing and no fatigue.
CFRP strips, besides many advantages such as corrosion resistance, ease of handling and application or insensitivity to material fatigue, have also some disadvantages.
As a result of the rigid bond of the CFRP strip onto the cracked concrete, high tensile stresses arise in the strip in the cracked concrete zone and very high bond shear stresses occur in the vicinity of cracks. Because of the limited tensile strength of the concrete close to the surface, failure of the strip strengthening occurs in most cases in the form of bond failure in the concrete. From the linear-elastic material behaviour of CFRP strips up to failure, results moreover a limited deformation capacity of the strengthened reinforced concrete section.
If the CFRP strip is tensioned and anchored at the ends, the material strength can be utilized to a higher extent and there is less uncertainty regarding ultimate resistance of the strengthened structural part. In zones where no bond is established between concrete and CFRP strip, plastic deformations of the strengthened reinforced concrete section can be allowed.
The aim of the project is the development of a stressing anchorage for CFRP strips.
The development is divided into 3 phases:
Phase 1: Steel tensioning system
As a first step a biasing system for CFRP strips is developed with a steel anchor head.
With pilot applications and long-term tests the practicality and long-term stability of the system are detected at the end of Phase 1.
Phase 2: Plastic tensioning system
Next step will be replacement of the steel biasing system by a developed system made of plastic components. Thus corrosion can be eliminated. This system is in pilot applications and long-term tests shall be checked. It is specifically observed the aging behaviour of plastic components.
Phase 3: Final development work
Project will be completed and sent to further stage if necessary.
The results of the research have already been implemented and applied in different projects. The CFRP prestressing system has been implemented in practice. However, it is to be further optimized.
Information on design of structural strengthening with as without bond allows to weigh the advantages and disadvantages of both application methods and to decide which type of strengthening is best suited for a specific structural part.
The description of a number of different kinds of structures strengthened with tensioned CFRP strips completes final report, showing the possibilities and limits of this strengthening method.