Transfer Project T3
Strengthening of beams and T-beams with textile reinforced concrete
Director
Prof. Dr.-Ing. Manfred Curbach
Institute of Concrete Structures
Mitarbeiter
M.Sc. Frank Schladitz
Dipl.-Ing. Annett Brückner
Aims
In the subproject T3 the acquired knowledge of the basic research (Subproject D1) were transferred onto building components with practically relevant dimensions. The production process of the textile reinforced strengthening layers was to be examined under practical building conditions concerning producibility and profitability.
Sample structures being adjusted optimally as possible to the high demands of shear force strengthening were to be developed for the textile reinforcement. Oppositional requirements such as high tensile strength at short anchorage lengths had to be weighted and aligned to the textile production technology in cooperation with the subproject A1.
Decisive for the carrying capacity of a shear force strengthening is its anchorage at the reinforced concrete component. Theoretical models and experimental research are used to comprehend the carrying mechanism of the anchorage. Additional constructive solutions of the anchorage had to be developed depending on the load-bearing capacity of the bonding joint between web and strengthening layer.
The derived design approaches from the basic research were to be checked regarding their validity for large scale components. Experimental research should show whether or not size effect influence the shear force bearing capacity of the components.
Results
Sample fabric for shear force strengthening
Practical applications have shown, that fabrics with 45° inclined rovings need additional warp threads in 0°-direction in order to ensure a sufficient form stability. The displacement resistance of the fabric is too low only by stitching and coating of the rovings. The warp threads avoid the laminar contortion of the structure as an essential precondition for the dimensionally stable cut of the fabric.
Fabrics with a mesh size of 7.2 mm and a roving fineness of 1200 tex are too tight as reinforcement of strengthening. The fine-grained concrete can penetrate the fabric only with difficulty due to the low openings between the fibre bundles so that the time expenditure is very high for inserting the reinforcement. Fabrics with equal roving fineness but larger interspaces of 10.8 mm could be processed in considerably shorter times.
The coating of the fabric has a decisive influence onto the flexural behaviour, the anchorage length and and the tensile strength of a structure. In terms of shear force strengthening fabrics with a low flexural rigidity should be used so as to enable a realisation of bending ups with small transition radiuses at cross section edges. Contradictory to that are the simultaneously wanted high tensile strengths allowing economic reinforcements with low layer numbers.
15% coating rate having been found in cooperation with the (Subproject A1) is an acceptable compromise for the conflictive practical requirements. The developed sample fabric is displayed in figure 1.
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Influence of the reinforcement onto the main state of stress
The principal stress condition within the web of a component depends on the relation of the extensional stiffness in main compression and tensile stress direction. According to the load the stiffnesses change due to the nonlinear material behaviour of the reinforced concrete. Especially in direction of the tensile stress an increasing loading reduces the stiffness owing to crack formation and yielding of the transverse reinforcement. The result is a more and more inclined compressive stress inclination.
During tests the changes of the compressive stress inclination can be verified measuring the deformation at the surface of the component (e.g. through photogrammetry). Though the deformation state is not identical with the stress state due to the nonlinear material behaviour and the anisotropism of the composite material, but it depends on it. Thus all changes in the deformation state are to be expected similarly also in the stress state.
The tested specimens show all the described lowering angle of inclination until the state of failure (Fig. 2). The steeper inclination at the strengthened beams in comparison with the un-strengthened reference both at the same load level and in the state of failure is decisive for the design.
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Anchoring of the reinforcement by bond
Textile reinforced shear strengthening can be anchored onto the reinforced steel component by the bond of the old and the new concrete alone. That was verified by the ultimate load increase in the tests. The load carrying capacity of the bonding joint limits the possible reinforcement effect though. The failure of the bonding joint is being characterized by the successively spreading debonding of the reinforcement layer.
The limit load carrying capacity of the bonding joint should be determined through measuring the relative displacements between the reinforced concrete component and the strengthening layer. At the upper border of the strengthening displacement transducers were aligned evenly so as to measure the displacement in vertical and horizontal direction.
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B
All tests show a disproportionate increase of the relative displacements firstly at measuring point 15 which is to be seen directly next to the single load (Fig. 3). After that follows measuring points 13 and 11 with slightly higher load. The lowest relative displacements showed always the outer measuring points 9 and 7. That leads to the conclusion that the debonding of the strengthening layer starts at the centre of the beam proceeding to the supports. Occurring shear cracks disrupt a further continuing of the debonding.
The debonding of the strengthening layer starts at highly differing load levels as the comparison of the individual tests points. A limit of the load carrying capacity could not be determined until now. Scattering adhesive tensile strengths as well as qualitative differences within the completion of the bonding joint are suspected as a reason for the different beginning of the bonding failure.
Constructive elements for anchoring of the strengthening layer
Anchorages of the strengthening layer by constructive elements were tested at small, separate shear test specimens with regard to their practicability and load-bearing capacity. The test set-up as well as the used specimen were being developped in (Subproject C1) (Curbach) (Fig. 4).
The function of the constructive means of anchoring is the transfer of the vertically acting adhesion tensile load onto the web of the reinforced concrete beam. The best results within the shear tests were being achieved with continuous, prestressed anchores. The lateral pressure resulting from the pre-stress was able to prevent a bond failure in the old concrete as in the case of the reference tests without anchorage means.
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With augmenting pre-stress the load-carrying capacity of the anchorage increases, still staying below the reference test one. Reason for that is the reduced reinforcement cross section of the strengthening within the area of the anchores due to the needed drilling. A compensation through an additionally reinforcement layer within the anchorage length was not tested experimentally until now.
Anchorage solutions with pneumatically driven nails or drilled-in anchor bolts turned out to be unqualified. Both possibilities did not result in sufficient lateral pressure so as to increase the load-bearing capacity of the bonding joint. Additionally the nails have the disadvantage that the driving nail into the old concrete enduringly damages the old concrete and thus the compound.
Calculation approaches
According to the experimental research the shear resistance of a component does not only result from the load-carrying capacity of the web reinforcement. After the truss model the tested beams and T-beams have the same load-bearing capacity of the steel stirrups but differing test loads (the un-strengthened T-beams had a considerably higher ultimate loading than the comparable beams). The load-carrying capacity difference clarifies that, in spite of the shear slenderness of 3.3 an important part of the loading was carried through an arched girder.
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The distribution of the loading among web reinforcement and arched girder is not clearly determinable with experimental research. Nevertheless it can be assumed, that the shear slenderness and strain condition of the web reinforcements affect the distribution.
The load-bearing capacity of the web reinforcement consists both of the steel stirrups and the textile strengthening. These two portions can not be determined clearly in experiments since the measured strains of the concrete surface do not allow any conclusions on the strains of the reinforcement in the concrete.
The classic strut-and-tie-model of the steel stirrups is unsuitable for the dimensioning of the textile reinforcement since the tension struts of the textile reinforcements often end outside the bending compression zone. For that reason a new strut-and-tie-model for shear force strengthening was being developed.
Strut-and-tie-model of the textile reinforcement
According to the generally accepted strut-and-tie-model (z. B. DIN 1045-1) all tension struts have to be anchored within the bending compression zone of a component. As the experimental investigations prove, considerable increases of the load-bearing capacity load are also possible, if the reinforcements end outside the bend pressure zone.
The anchoring outside the compression zone can be interpreted with a strut-and-tie-model with double inclined compression struts (Fig. 5). Unlike the traditional model there is no horizontal compression chord since it is assumed that the reinforcement is situated completely within the area of the tensile-bending zone. All compression struts are oriented towards the single load and idealise the load transfer as arched girder with tie rod. Due to the existing tension struts various arched girders can be linked. These are being steeper inclined than the simple arched girder so that the shear force bearing capacity of the linked arched girders is higher at the ultimate limit state of the compression struts.
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The inclination change within the compression strut enables the attachment of the diagonal tension strut. In the cross section of component the tension struts of the textile reinforcement are being anchored by an arched girder with tie rod at the web (Fig. 5b). The tie rod in the cross section is either realised through the adhesion tensile strength of the old and new fine-grained concrete or through anchorage elements, which brace the two lateral surfaces of the strengthening against each other through the web. An anchorage of the reinforcement within the pressure zone is not required according to this model.
RCalculative stress of the bonding joint
NLateron strengthened reinforced concrete components act as composite section of two composed sections. In their contact joint the sections are linked rigidly. The loads within the bonding joint are of decisive importance for the dimensioning of a shear force strengthening. Fails the bond the whole strengthening layer is not capable of bearing without constructive anchorage elements.
Calculatively both shear and adhesive tensile stresses have to be transferred in the bonding joint. The shear stresses result from the restraint relative displacements between the sections (Fig. 6). The bonding enforces the strain distribution at the whole composite section. Under bending loads the zero crossing of the strains is in the centre of gravity of the whole cross section, not in the centre of gravity of the single sections. That means, that the two sections are being loaded by opposed normal forces being in a balance through the shear stresses in the bonding joint.
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The adhesive tensile stresses in the bonding joint are being caused by the anchorage of the textile reinforcement. Analog to the traditional reinforced concrete the tensile forces of textile reinforcement have to be transferred to the surrounding concrete in the end anchorage areas and the surroundings of bending and shear cracks. The resulting tensile forces act vertically to the bonding joint and load the adhesive bonding between the reinforced concrete component and the strengthening layer (Fig. 5b).
Publications
2011
- Curbach, M.; Ortlepp, R.: Leichtes Bauen mit ultrahochfesten und Textilbetonen. In: Bauforschung und Baupraxis, Heft 10: “Wie wollen wir in Zukunft bauen?” – Festschrift zum 60. Geburtstag von Prof. Dr.-Ing. Wolfram Jäger, Dresden, 19.04.2011. S.17-22 - ISBN 978-3-86780-216-1
2010
- Ortlepp, R.; Brückner, A.; Curbach, M.: Influence of Textile Reinforcement on the Principle Stress Condition of Strengthened RC Beams. 3rd International fib Congress, Washington, D.C., May 29 - June 2 2010. Paper 147 – DVD-Rom
2009
- Ortlepp, R.; Brückner, A.; Lorenz, E.: Verbundversagensmechanismen im Verankerungsbereich von textilbewehrten Feinbetonverstärkungsschichten. In: Curbach, M. (Hrsg.), Jesse, F. (Hrsg.): Textile Reinforced Structures : Proceedings of the 4th Colloquium on Textile Reinforced Structures (CTRS4) und zur 1. Anwendertagung, Dresden, 3.-5.6.2009. SFB 528, Technische Universität Dresden, D–01062 Dresden : Eigenverlag, 2009, S. 433-446 – ISBN 978-3-86780-122-5 URN: urn:nbn:de:bsz:14-ds-1244049139702-05245
- Jesse, F.; Curbach, M.: Verstärken mit Textilbeton. In: Bergmeister, K.; Fingerloos, F.; Wörner, J.-D. (Hrsg.): Beton-Kalender 2010. Teil I, Berlin : Ernst & Sohn, 2009, S. 457-565
- Curbach, M.; Jesse, F.: Eigenschaften und Anwendung von Textilbeton. In: Beton- und Stahlbetonbau 104 (2009) 1, S. 9-16 – doi:10.1002/best.200800653
- Curbach, M.; Jesse, F.; Ortlepp, R.; Brückner, A.; Weiland, S.: Textile Bewehrung im Betonbau. In: Baradiy, S. et al.: Tagungsband zum 8. Bautechnikforum am 20. März 2009. TU Chemnitz : Selbstverlag, 2009
- Schladitz, F.; Strobelt, J.: Verstärkung eines Tonnendaches – Ein Erfahrungsbericht. In: Curbach, M. (Hrsg.), Jesse, F. (Hrsg.): Textile Reinforced Structures : Proceedings of the 4th Colloquium on Textile Reinforced Structures (CTRS4) und zur 1. Anwendertagung, Dresden, 3.-5.6.2009. SFB 528, Technische Universität Dresden, D–01062 Dresden : Eigenverlag, 2009, S. 586-587 – ISBN 978-3-86780-122-5
- Strobelt, J.; Schladitz, F.; Lorenz, E.: Textilbeton in der Praxis: Verstärkung eines Tonnendaches. Beratende Ingenieure 39 (2009) 9/10, S. 41-45
- Curbach, M.; Hauptenbuchner, B.; Ortlepp, R.; Weiland, S.: Sanierung von Stahlbetonschalen mit textiler Bewehrung. In: Beton im Denkmal. Ästhetik, Funktion, Technologie. Tagungsband 28. Mitteldeutsches Bau-Reko-Kolloquium am 29. April 2009 in Freiberg. Weimar : Verlag der Bauhaus Universität, 2009 – ISBN 978-3-86068-381-1
- Weiland, S.; Curbach, M.: Interaktion gemischter Bewehrungen bei der Verstärkung von Stahlbeton mit textilbewehrtem Beton. In: Curbach, M. (Hrsg.), Jesse, F. (Hrsg.): Textile Reinforced Structures : Proceedings of the 4th Colloquium on Textile Reinforced Structures (CTRS4) und zur 1. Anwendertagung, Dresden, 3.-5.6.2009. SFB 528, Technische Universität Dresden, D–01062 Dresden : Eigenverlag, 2009, S. 553-564 – ISBN 978-3-86780-122-5 RN: urn:nbn:de:bsz:14-ds-1244051366655-25294
2008
- Curbach, M.; Michler, H.; Weiland, S.; Jesse, D. Textilbewehrter Beton – Innovativ! Leicht! Formbar! In: BetonWerk International 11 (2008) 5, S. 62-72
- Schladitz, F.: Torsion strengthening of reinforced concrete components using Textile Reinforced Concrete (TRC). In: 7th International PhD Symposium in Civil Engineering, Stuttgart, 11.-13.09.2008. – Book of Abstracts, pp. 171-172 and CD-ROM
- Schladitz, F.; Curbach, M.: Increase in the torsional resistance of reinforced concrete members using Textile Reinforced Concrete (TRC). In: 2nd International Conference on Concrete Repair, Rehabilitation and Retrofitting (ICCRRR), Cape Town, 24.-26.11.2008. – Book of Abstracts pp. 391-392and CD-ROM – ISBN 978-0-415-46850-3
2007
- Köckritz, U.; Weiland, S.; Curbach, M.; Cherif, Ch.: Gut bedacht mit beschichteten Bewehrungs-textilien Beschichtete gitterartige Bewehrungstextilien und deren Einsatz zur Verstärkung einer Hyparschale in Schweinfurt. Kettenwirkpraxis 41 (2007) 2, S. 20-23
- Brückner, A.; Ortlepp, R.; Curbach, M. Anchoring of TRC Shear Strengthening for T-Beams. In: Proceedings of the 8th International Symposium on Fiber Reinforced Polymer Reinforcement for Concrete Structures (FRPRCS-8), Patras, 16.–18.07.2007. Book of Abstracts and CD-ROM, S. 640–641 – Paper-ID 17-8
- Curbach, M.; Brückner, A.; Ortlepp, R.; Weiland, S.: Textilbewehrter Beton – Anwendung für Verstärkungen. In: DAFSTB (Hrsg.): Neue Entwicklungen im Betonbau: Tagungsband der Fachtagung 2007 des Deutschen Ausschusses für Stahlbeton in Zusammenarbeit mit der Bundesanstalt für Materialforschung und -prüfung, Berlin, 8.–9.3.2007.
- Diederichs, U.; Hauptenbuchner, B.; Müther, U.; Weiland, S.: Obnova hyperparabolické skořepi-nové střechy nad auditorium maximum university aplikovaných věd ve Schweinfurtu, německo (Restauration of the hyper parabolic shell roof over the auditorium maximum of the university of applied science in Schweinfurt, Germany). In: SANACE 2007, 17th International Symposium REPAIR 2007, Brno, 16.-18. May 2007
- Rustler, G.; Schmidt, F.; Weiland, S.: Stahlbeton-Schalentragwerk der Fachhochschule Schweinfurt – Neuartige Verstärkung mit textilbewehrtem Beton. In: bau intern. Zeitschrift der Bayrischen Staatsbauverwaltung für Hochbau, Städtebau, Woh-nungsbau, Straßen- und Brückenbau (2007), September/Oktober, Nr. 5, S. 16-18
2006
- Curbach, M.; Ortlepp, R.; Triantafillou, T. C.: TRC for rehabilitation. In: Brameshuber, W. (Hrsg.): Textile Reinforced Concrete : State-of-the-Art Report of RILEM Technical Committee 201 – TRC: Textile Reinforced Concrete. Bagneux : RILEM, Report 36, 2006, S. 221–236 – ISBN 2-912143-99-3
- Brückner, A.; Ortlepp, R.; Curbach, M.: Textile Reinforced Concrete for Strengthening in Bending and Shear. In: Materials and Structures. 39 (2006) 8, S. 741–748 – doi: 10.1617/s11527-005-9027-2
2005
- Brückner, A.; Ortlepp, R.; Curbach, M.: Neueste Forschungsergebnisse zur Querkraftverstärkung mit textilbewehrtem Beton. (Newest Research Findings of Shear Strengthening with Textile Reinforced Concrete). In: Techtextil Symposium, Frankfurt, 07.–09.06.2005. – CD-ROM
- Brückner, A.; Ortlepp, R.; Weiland S.; Curbach, M.: Shear Strengthening with Textile Reinforced Concrete. In: 3rd International Conference Composites in Construction, Lyon, 11.-13.07.2005. Lyon : Université Lyon / Laboratoire Mécanique Matériaux et Structures, 2005, S. 1307-1314
- Brückner, A.; Ortlepp, R.; Weiland, S.; Curbach, M.: Textile Structures for Shear Strengthening. In: Alexander, M. (Edt.); Beushausen, H.-D. (Edt.); Dehn, F. (Edt.); Moyo, P. (Edt.): International Conference on Concrete Repair, Rehabilitation and Retro- fitting, Cape Town, 21.-23.11.2005. London : Taylor & Francis Group, 2005, p. 456-457 – Book of abstracts and CD-Rom
2004
- Möller, B.; Brückner, A.; Ortlepp, R.; Steinigen, F.: Verstärken mit textilbewehrtem Beton - Experimente und numerische Simulation. In: Beton- und Stahlbetonbau 99 (2004) 6, S. 466-471 – doi:10.1002/best.200490118