Practice-oriented anchoring and overlapping solutions for FRP reinforcement
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Report in the annual report 2022
Anchoring, but properly!
Bending test on a plate with overlapping FRP and steel reinforcement
Thanks to their outstanding material properties, fiber-reinforced plastics (FRP) enable particularly efficient, lightweight and economical structures. Due to their high tensile strength, corrosion resistance and very low thermal conductivity, FRP reinforcements are particularly well suited for thin, highly stressed concrete structures and in aggressive environments. To fully exploit the potential of these innovative materials, experimentally verified knowledge of bond behavior is required.
To this end, pull-out tests were first carried out on various FRP reinforcements and steel reinforcement in this project. As a result of the investigations, the force-slip relationships for the investigated variants were evaluated and compared. In order to further investigate the bond behavior of the FRP bars, nonlinear numerical investigations were also carried out using the FE software ANSYS. The calculations focused on the local deformation behavior of the concrete as well as the internal stress states and internal cracking.
In addition, the load-bearing behavior on slabs with overlapping FRP reinforcement and conventional reinforcing steel was investigated. In this context, different types of tests were realized: tensile tests on pure FRP bars and on steel bars to determine the material properties such as tensile strength and Young’s modulus, and tests with overlapping FRP and steel bars to investigate the load transfer in the overlapping area. Quasi-continuous fiber optical strain measurements were used in all tests to quantify the strain development in the bars. As accompanying investigation, pull-out tests with a long bond length were performed to assess the suitability of different fiber optical sensors and different application variants for the lap tests. From the lap tests, it was found that the combined lap of FRP bars and reinforcing steel does not negatively affect the load-carrying capacity, but leads to higher load-carrying capacities than the test specimens with overlapping conventional reinforcing steel.
Report from the annual report 2021
Anchoring of FRP reinforcement
Bending test of FRP bars with FOS
Fibre-reinforced plastics (FRP) make it possible to produce particularly efficient, lightweight and economical components in the construction industry. Due to their high tensile strength, insensitivity to corrosion and very low thermal conductivity, FRP reinforcements can be usefully employed in particular for slender, highly stressed concrete components as well as in aggressive environments. In order to fully exploit the potential of these innovative materials in civil engineering, experimentally verified knowledge of the bond behaviour is required. For the anchorage of FRP tendons, a technology is also needed that allows rapid anchorage on the construction site. This research project is therefore focused on developing new, practical anchoring options for reinforcement and tendons made of FRP. Experimental and numerical investigations form the basis of the project.
In order to evaluate the bond behaviour of FRP reinforcement and concrete within the framework of the experimental investigations, the strain development in the rebar is considered in addition to the bond stress-slip relationship. Fibre optic sensors (FOS) based on Rayleigh scattering are used for strain measurements. FOS have already been widely used on steel and concrete surfaces. For the application on FRP bars, however, there are only a few investigations. To formulate material and handling recommendations for a reliable application of FOS on FRP bars, tensile and bending tests on pure FRP bars as well as pull-out tests with FOS were carried out. In this context, two types of fibre coatings (with acrylate and polyimide coating) and two types of adhesives (epoxy and cyanoacrylate + silicone) were used for the application, so that a total of four fibre coating-adhesive combinations were investigated. In addition, the type of application (bonding in a groove and on the bar surface) was varied. The investigations were used to test the plausibility and reliability of the different combinations, to measure the bond stresses between the bars and the concrete in the pull-out tests and to determine the bending and tensile strength of the FRP bars. The results of these investigations serve as a basis for the further four-point bending tests on concrete components with FRP reinforcement or tendons.
Report from the annual report 2020
Well anchored
Numerical investigation on FRP anchorages
Fibre composite plastics (FRP) are used extremely successfully in many areas of technology and also enable particularly efficient, lightweight and economical components in the construction industry. FRP reinforcements are characterised by high tensile strength, corrosion resistance and very low thermal conductivity. Thus, FRP reinforcements can be used especially for slender, highly stressed concrete components and in aggressive environments.
To be able to use the potential of these innovative materials, experimentally verified knowledge of the composite load-bearing behaviour is required to design anchorage and overlap lengths safely and economically. For the anchoring of FRP tendons, technology is also required which allows for quick anchoring on the construction site. Safe and efficient anchoring of (pre-stressed) FRP reinforcements opens up further areas of application for this corrosion-resistant and extremely high tensile strength type of reinforcement.
In this research project new, applicable anchorage possibilities for FRP reinforcement and prestressing bars are therefore being developed. For this purpose, experimental and numerical investigations are carried out on reinforcement made of glass fibre reinforced plastics (GFRP) and on prestressing bars made of carbon fibre reinforced plastics (CFRP).
In the case of GFRP reinforcement, the focus is on the investigation and optimisation of the composite behaviour to be able to reliably realise comparatively short anchorage and lap lengths. Compared to anchoring and lapping of steel bar reinforcement, these are often oversized, which makes the economic use of FRP reinforcement and the structural design more difficult.
For CFRP tendons, a new type of local anchoring using a polymer material is being developed and investigated. The use of conventional steel wedges, as used for anchoring steel tendons, is not efficiently possible due to the transverse pressure sensitivity of anisotropic CFRP tendons. Against this background, a “softer” polymer material is used to wedge the bars and to reduce the transverse pressure at the anchoring point to such an extent that a failure of the bars can be excluded. At the same time, a high anchoring effect can be achieved and high practicability of the anchoring technology on the construction site can be guaranteed.