C3-V4.2-VI: Development of compact anchorages for unbonded tendons

Table of contents

1. 1. 1. Project data
      2. Report in the annual report 2017
      3. Report in the annual report 2017
      4. Report in the annual report 2016

Project data

Titel | Title TP C3-4.2-VI: Entwicklung kompakter Verankerungselemente für Spannverfahren ohne Verbund im Verbundvorhaben C3-V4.2 Vorgespannter Carbonbeton für Straßenbrücken und Flächentragwerke | TP C3-4.2-VI: Development of compact anchorages for unbonded tendons as part of the joint research project C3-V4.2 Pre-stressed carbon concrete for road bridges and shell structures Förderer | Funding Bundesministerium für Bildung und Forschung (BMBF); Projektträger: PT Jülich / C³ – Carbon Concrete Composite Zeitraum | Period 05.2016 – 04.2019 Teilprojektleiter | Subproject manager Prof. Dr.-Ing. Dr.-Ing. E.h. Manfred Curbach Bearbeiter | Contributor Dipl.-Ing. Oliver Steinbock Projektpartner | Project partners FG Entwerfen und Konstruieren – Massivbau TU Berlin (project coordinator) | Hentschke Bau GmbH, Bautzen | Dywidag-Systems International, Unterschleissheim| SGL Carbon GmbH, Meitingen | sbp, Stuttgart | GINKGO Projektentwicklung GmbH, Dresden | solidian GmbH, Albstadt | Ingenieurbüro KORDES+ZIEGENHORN Partner, Dresden (associated partner) | Ingenieurbüro Prof. Dr.-Ing. Roland Fink, Radebeul (associated partner)

Report in the annual report 2017

A PRESTRESSING SYSTEM WITH CFRP LAMELLAE

Recently, especially in the field of bridge construction, the trend has moved away from prestressing with full bond towards non-bond structures. The main reason for that is the high flexibility. In general, non-bond prestressing systems offer the possibility of replacement, post-tensioning and post-strengthening. However, the major prerequisite for doing so is that the anchorage elements, significant due to their missing bond, also meet those requirements. Against this background, a compact prestressing system has been developed as part of the joint project C3-V4.2. It is geared to existing non-bond systems for prestressing strands so it can be applied for both building construction and the transverse prestressing in bridge construction.

Finally, a modular anchor system, following the principle of friction clamp anchoring, has been developed. The anchor system is composed of tension plates with different planed surfaces, which grip the CFRP lamella by a specific prestressing of the screws. The combination of graded prestressing and surface treatment at the anchor zone allows an almost consistent load application while the bond length is comparatively small.

In collaboration with the C³-project C3-V1.5 “Demolition, removal and recycling of C³-structural members”, the usability of the prestressing systems was not only able to be investigated in small-sized tests, but also with a whole floor slab. The floor slab consists of a pressure arch and a tension member and picks up the idea of the historical Möller girder. Compared to a conventional flat slab the cross section without any transverse reinforcement is severely thin and it just meets the common limitation of deformation for building construction in conjunction with a prestressing. The principle of friction-clamp anchoring was applied both to the temporary prestressing process and to the permanent anchoring in the component itself.

After a successful initial load test until failure of the slats at free length, another lamella was re-installed and then subjected to permanent loading and a further load until failure. No negative effects for the reuse of the anchor bodies could be detected nor a slippage of the slats under continuous load so that the desired goals were achieved.

Report in the annual report 2017

COMPACT ANCHORS FOR CARBON TENDONS

The triumph of prestressed concrete began at the end of the 20th century, several years after the establishment of reinforced concrete took place. Indeed, there had been prior attempts to prestress concrete, but they failed because of insufficient steel strengths, or because the required amount of prestressed could not be maintained, due to creep and shrinkage losses of the concrete. Today, there are CRP fibres elements available with a high tensile strength that is suitable for prestressing. The development of compact anchoring elements, as part of the project C3-V4.2 – Pre-stressed Carbon Concrete for Road Bridges and Surface Structures, focus on the analysis of pre-stressed carbon concrete.

The specific challenge of anchoring carbon tension elements is the sensitivity of lateral pressure of the tension elements. Concerning the anchoring possibilities, the sensitivity of the structural components requires special considerations; even more, if the retightening, replacement and reusability requirements of the anchoring system are taken into consideration.

As a result of these requirements, a combined anchorage was developed on the basis of friction and clamping mechanisms. As opposed to systems that are available on the market, the pre-stress load is introduced in the elements by directly tightening a pair of screws that, in conjunction with variable surface roughness, enables an even load application. Thereby, a failure in the anchorage area can be avoided. Numerical investigations provided results with good prospects and are supposed to be further verified by experimental means during the course of the project. CFRP lamellas were chosen as tension elements since they proved to be beneficial to prevent local stress concentrations. Moreover, CFRP lamellas are easier to install than round cross sections, and they can be easily replaced.

The system is being designed as un-bonded pre-stress, and it is primarily supposed to be employed for structural applications. In this context, an adequate form-optimized slab system was developed, which will be experimentally tested soon.

Report in the annual report 2016

Compact Anchorage Elements for Carbon Tendons

Tendons made of carbon are already in use, as can be seen in several pilot projects around the world. They have mainly been used in bridges or repairs. Due to the high forces in the clamping members and the cross-pressure sensitivity of the material, the space requirement for the anchoring elements is large. This is where the research project for the development of compact anchorages for carbon tendons with small cross sections starts. The development of non-bonded carbon composite members offers advantages regarding sustainability (interchangeability) as well as in application. The compact design is intended to make the clamping system applicable in future, practical application such as the transverse pre-tensioning of bridge superstructures, or as prestressing in building construction. While the goal of prestressing bridge constructions is to limit the crack widths, prestressing is also done in buildings to limit the deformation of the structure.

In 2016, performing the basic evaluation and determining the resulting constraints laid in the foreground. Within a material-independent evaluation of the tensioning systems which were available on the market, it was found that in conventional unbonded steel tendons and with a small cross section, monostrands are almost always, which can be stacked in ducts. The monostrands are frequently anchored with wedges. Anchorages via deviation, thickened heads or threads and sleeves are uncommon. It is an advantage of steel that the material forgives concentrated load inlets or stress peaks, as occurs with these conventional systems.

In the case of prestressing elements made of carbon, a greater variety of anchorages and construction types for the tensile members was found. In addition to carbon lamellas and strands, there are bars, couplings and strand loops available on the market. However, due to their differing material properties, the solutions of steel anchorages cannot be transferred directly to carbon. Therefore, a new type of clamping member and its associated anchoring is necessary, one which can meet the requirements of the construction industry. The development of compact anchoring elements is part of the composite project C3-V4.2 – pre-stressed carbon concrete for road bridges, plates and shell structures – which examine the entire range of pre-stressed carbon concrete.