Evaluation of the Performance of Anchoring Systems for the External Flexural Strengthening of RC Beams

Transcription

1 Evaluation of the Performance of Anchoring Systems for the External Flexural Strengthening of RC Beams T. Ripper ICIST, Instituto Superior Técnico, Lisbon, Portugal S&P Clever Reinforcement Co, Brunnen, Switzerland P. França, A. Costa, & J. Appleton ICIST, Instituto Superior Técnico, Lisbon, Portugal ABSTRACT: Carbon fibres reinforced polymer (CFRP) systems for strengthening concrete structures have emerged as an alternative to traditional strengthening techniques, particularly those regarding steel plate bonding. Both techniques require the appliance of externally bonded plates, glued to the existing concrete surface by resins (epoxy, in most of the cases). The level of strengthening that can be achieved in RC beams under flexion through the use of externally bonded plates both in steel or CFRP is often limited by the early debonding of these plates, due to the minor capacity in transmitting forces of the very cracked concrete layer between the existing reinforcement and the strengthening plates. This investigation aims to analyse the efficiency of external anchoring systems, namely the addition of regularly spaced transverse belts in epoxy impregnated unidirectional carbon fibres fabrics. 1 INTRODUCTION The best exploitation of the glued external strengthening devices applied on reinforced concrete beams strongly depends, among others, of aspects such as follows: Prevention of the occurrence of premature detachment of the strengthening devices; Taking the maximum from the resistant capacity of the material used for strengthening, namely its extension, aspect that will be as so important as noble and expensive is the material (CFRPs, for example) Guarantee of an efficient anchoring system, what implies full suspension of the tensile forces onto the compression zone. Some of the available codes for the design of externally bonded systems (the 44 ACI 2R-2 and the FIB Technical Report 14, for example) make explicit considerations on how to take in account, regarding the particular subjects above referred, the contribution of external anchoring systems for the performance improvement of the flexural strengthened reinforced concrete beams. In the Laboratory of Resistance Materials and Structures of the Instituto Superior Técnico (IST), in Lisbon, Portugal, the authors of this document are developing an experimental programme that aims to analyse the effectiveness of transversal strips made of unidirectional carbon fibres fabrics regularly spaced as external anchoring systems for bonded strengthening devices. 2 THE EXPERIMENTAL PROGRAMME 2.1 Basis The experimental programme that is now taking place at the IST consists in the analysis of a set of reinforced concrete T beams. All these beams have a span large enough (6 m) to take in account most of the scale facts as well as to enable the use of some data related to other tests already carried out at other laboratories. The beams are submitted to two applied loads at the third parts of its span, as shown in figure 1: Figure 1. Load schema. The methodology adopted for the tests consists in analysing the influence of some different factors, but always taking as reference the same beam without any kind of strengthening. The variables to be considered in the whole set of tests are:

2 Amount of original reinforcement; Type of material to be glued: CFRPs or steel plates; Dimensions of the external strips ( U wraps ), as well as its interval and also its Young s modulus; Strengthening device applied on cracked and noncracked beams. The beams were concreted at an industrial plant (SECIL PREBETÃO), where a specific metallic mould has been designed for it (see figures 5). 2.2 The first set of beams The reference beam (V1) taken for the first set is a heavily reinforced one: ρ = A s /A c = 1,6 %, with no staged bars. The aim of this set was the comparison between the reference beam strengthened, in its bottom, by the addition of two CFRP laminates with individual cross section of (1 1,4) mm², glued along 5,6 m centred in the span, thus becoming the beam V2, and (beam V3) with the same laminates plus transversal CFRPs strips, spaced each 5 cm (5 times the interval of the internal stirrups), 1 cm wide, length of the U leg = 15 cm. The schedule for the beams V1, V2 and V3 above referred (longitudinal and cross section) are represented in figure 2, 3 and 4, as follows. Figures 5 The metallic mould 2.3 Relevant characteristics of the materials Figure 2. V1 Reference beam. Figure 3. V2 Beam strengthened just with two longitudinal CFRP laminates. Figure 4. V3 Beam strengthened with two longitudinal CFRP laminates plus CFRP transversal strips each 5 cm. The concrete used for the beams has achieved, in tests made on cubic samples, 33,71 MPa as the average compression resistance (age = 28 days), being 2,2 MPa its standard deviation. The steel used both for the main reinforcement and for the stirrups can be considered as having an yield strength f yk = 58 MPa. The laminates, fabrics as well all the related resins had been supplied by BETTOR-MBT Portugal, being the carbon fibres resistant products manufactured by S&P. The epoxy carbon fibres laminates are labelled as type CFK 2/2, with an average E modulus of about 28 GPa. The adhesive applied (2 mm thick) for gluing the laminate was MBrace Resin 22 and consists in an epoxy paste with the following average main mechanical characteristics: E = 8 GPa; f tk = 7 MPa. The carbon fibre fabrics are labelled as type S&P C Sheet 24. The main characteristics of this product are: E = 242 GPa (just for the carbon fibres); Weight of carbon fibres / m² = 2 g. The resin used for the impregnation of the fabrics is MBrace Resin 55, being used the MBrace Resin 5 as a primer.

3 2.4 Bonding of the laminates and fabrics The gluing of the laminates on the beam V2 and of the laminates and fabrics on beam V3 were made at the same plant where the beams were concreted, thus implying the previous introduction of the internal strain gauges (for the longitudinal steel bars). The preparation of the concrete surfaces (grinding with needles hammer, cleaning by light sandblasting and levelling with an epoxy mortar) as well as the application of laminates and fabrics had been carried out by the specialized technicians of STAP. Figures 6 bellows show some of these procedures. Figures 6 Substrate preparation for laminates and fabrics The beams had been carefully moved from the plant to the IST laboratory on appropriated trucks. 2.5 Instrumentation and tests procedure The single supported beams with an effective span of 6, m had the load applied by means of two EN- ERPAC RH66 hydraulic hollow plunger cylinders with a maximum individual load capacity of 6 kn and a maximum stroke of 15 mm. The load application has been controlled by means of two centre-hole load cells TML CLC-5A, with the nominal capacity of 5 kn each. The system for the application of such heavy load had obliged the use of metallic I profiles and DY- WIDAG bars to guarantee the necessary transmission of the loads into the beams. This system is schematically represented in figure 7 bellow. In any case the load has been applied in two steps up to the cracking situation and from that point on following an interval of 2 15 kn. Electrical resistance strain gauges TML FLA L were attached along the lateral surface of the longitudinal internal bars, prior to concreting the beams. To the surface of the laminates were attached strain gauges TML BFLA-5-8-3L. Both type of TML strain gauges work with a resistance of 12 Ω. The deflection of the beams was monitored throughout the tests with three vertical displacement transducers VISHAY HS 1, output of 5 mv/v. The concrete longitudinal elongation has also been controlled by linear displacement sensors TML CDP 1 and CDP 5 (also with an output of about 5 mv/v), attached to the lateral surface of the beams according the positional of the main internal steel rebar. This procedure has been decided to be used in order to take in consideration the occurrence of large cracks that could disturb the information produced by the electrical strain gauges. The points where the strain-gauges (E), bases for the measurement with non deformable mechanical alongameters (A), vertical displacement transducers (D) and linear displacement sensors (T) were placed are represented on figure 8, as follows. Figure 8 Measurement device system The data acquisition system was an HBM UPM1, which contains an analogue multiplexing module with 1 channels, with an output in correct measurement number format. In this first set of beams up to 4 channels had been used. Figure 7 Load appliance system 3 EXPERIMENTAL RESULTS 3.1 Reference beam V1 The reference beam V1 had shown a very ductile behaviour, reaching a load of 2 2,1 kn that had correspondence with a deflection of about 89 mm (see figure 9, on the next page, taken some minutes before that ultimate stage). At the point of the assumed ultimate load (the test was interrupted do to the extreme deflection of the beam) the main cracks were just reaching the bottom of the slab. The cracking load was about 2 12,2 kn and the load corresponding to the yielding point of the internal steel was 2 181,5 kn.

4 Figures 11 Cracks near to the extremity of the laminates Figure 9 Reference beam near to its ultimate stage 3.2 Beam V2 (just with the laminates) The beam V2 has been strengthened, on its bottom, with two laminates that had a longitudinal length as close to the supports as possible (just 2 cm far), in order to try to anchor them in a zone with reduced tensile tensions. Analysing the behaviour of beam V2 during its loading process, it is possible to identify that the first flexural crack had occurred under a load of about ,8 kn, that means something like 21 % more than V1 at the same situation. The ductility observed was not as evident as for V1, but the ultimate load was 46% higher, reaching 2 291,7 kn with a corresponding maximum deflection of about 57 mm. After reaching a level higher than the service loads shear cracks appeared between the flexural cracks. When the load was about 9% of the ultimate, the main cracks were spaced 3 cm and inbetween minor cracks (with less width and also less vertical length) were spaced 1 cm (see figure 1). The same phenomena (vertical displacement between the two sides of a crack) had occurred also along the main cracks and, at the ultimate stage, the generated deviation forces caused a tensile failure on the layer between the laminates and the internal steel bars, thus developing in debonding the laminates. The concrete prism sent off by these forces came attached to the laminates surface, as is possible to identify in figure 12 bellow. The failure has started from the crack under one of the load application points and continued up to the extremity of the laminates. Figure 12 Laminates after the failure, still holding the concrete prisms sent off by the cracks deviation forces It is important to refer that the maximum elongation measured in surface of the laminates was 7,4, that is higher than the value recommended to be adopted when designing (according to the S&P catalogues, for the laminates with E modulus of about 2 GPa a maximum elongation of 6,5 shall be used). 3.3 Beam V3 (laminates plus fabric strips) Figure 1 Beam V2 under a load of 2 18 kn Close to the ultimate state, heavy cracks occurred near to the extremities of the laminates, inducing a vertical displacement between the two margins of the crack, as it is possible to see in figures 11. The flexural behaviour of beam V3 was much more ductile than of beam V2, showing the relevance of the external anchor system (the 5 cm spaced CFRP fabric strips). The first flexural crack has occurred under a load of about 2 15,2 kn, that is almost the same for V2 and V3. The load corresponding to the yielding of the internal rebar was also similar for V1, V2 and V3.

5 The cracks corresponding to the main tensile forces had also occurred in beam V3, but less spaced (2 to 25 cm) than in beam V2. The ductility observed is much more evident than for V2, as it is possible to understand by observing the comparison graphics in figure 13. The influence of the transversal strips in the control of the laminate peeling is evident in figure 15, where the strips had support the vertical displacement of the laminates. 36, 34, 32, 3, 28, 26, 24, 22, 2, 18, 16, 14, 12, 1, beam CFRPV3e U 8, Figure 15 Strips holding a peeled laminate beam CFRP V2 6, beam V1 Referência 4, 2,,, 1, 2, 3, 4, 5, 6, 7, 8, 9, Figure 13 (P δ) graphics comparing V1, V2 and V3 The ultimate load was 15,5 % higher than for V2, reaching 2 337, kn, with a corresponding maximum deflection of about 82 mm. The peeling phenomena has also occurred, but seeming to be controlled by the transversal strips, that had sustained the induced shear cracks, deviating them at the concrete surface (see figures 14 bellow). The maximum elongation of the laminates (see figure 16), measured at midspan of the beam V3, was 8,9 that is 2 % higher than the achieved for V Figure 16 (P ε) graphic for a laminate on beam V2 The long up and down bend shown in the above graphic means an unload and reload intervention during the tests due to operational reasons of the laboratory. Nevertheless this inconvenience caused no sensible problems for the interpretation of the tests. The contribution of the strips was higher for those closer to the extremities of the laminates than for the others (see graphics on figure 17 bellow) ,25 m,75 m 2,75 m 5 Figures 14 Cracks sustained by the transversal strips Figure 17 (P ε) graphics for some transversal strips 25

6 The failure was caused one more time due to the concentration of the stresses around the shear crack, particularly close to the first and main crack, but the support offered by the strips had conditioned the ultimate load, rising it up to the resistance of the CFRP sheets (see figures 18). The next set of beams will study variables that can be important for the achievement of designing parameters for the transversal strips, such as more fibre weight, higher E modulus, less interval, different geometries and bi-directional fibres. The tests will also analyse the behaviour of these fabrics when controlling steel plates slip. 5 ACKNOWLEDGEMENTS Figures 18 Cracks responsible for the strips failure Observing figure 19 it is possible to see that once again the bonding between the laminates and the concrete cover was considerable, since the concrete prisms sent off by the deviation forces came attached to the laminates surface. The authors express their gratitude to the following companies, do to its great support for the development of this work: ICIST, Instituto Superior Técnico, Lisbon, Portugal; S&P Clever Reinforcement Co, Brunnen, Switzerland; BETTOR MBT S. A., Albarraque, Portugal; STAP S. A., Lisbon, Portugal; SECIL PREBETÃO S. A., Montijo, Portugal. 6 REFERENCES Figure 19 V3 laminates and strips after failure 4 CONCLUSIONS Prevention of the occurrence of premature detachment of the strengthening devices from the concrete substrate can be most accurate if external anchoring systems are applied along the effective span, particularly along the zones where shear crack can be relevant. Further studies must be provided in order to analyse the influence of concentrated loads close to the supports. Extending the strengthening devices as far as possible, looking for its anchoring preferably in a compression zone seems to be a matter to be always taken into consideration. In cases when the strengthening is to be provided with CFRP laminates, the introduction of CFRP fabrics as transversal strips seems also to contribute for higher flexural elongation when designing. CEB-FIP, Fédération Internationale du Béton (21). FIB Technical Report Bulletin 14: Externally Bonded FRP Reinforcement for RC Structures. Concrete Society (2).Technical Report n.º 55: Design Guidance for Strengthening Concrete Structures Using Fibre Composite Materials. ACI, American Concrete Institute (22). ACI 44.2R-2 Guide for the Design and Construction of Externally Bonded FRP Systems for Strengthening Concrete Structures. ACI Committee 44. S&P Clever Reinforcement Company (22). Design Guide FRP. Souza, R. H. F. & Appleton, J. A. & Ripper, T. (1998). Avaliação do Desempenho de Compósitos Armados com Tecido de Carbono como Elemento de Reforço de Vigas de Betão Armado. Jornadas Portuguesas de Engenharia de Estruturas, Lisboa. Dias, S. J. E. & Juvandes, L. & Figueiras, J (22). Comportamento Experimental de Vigas de Betão Armado Reforçadas à Flexão com Sistemas Compósitos de CFRP do Tipo MBrace. Universidade do Porto. Faculdade de Engenharia. Róstasy, F. S. (1998). Expert Opinion n.º 98/322. Braunschweig, Germany. Juvandes, L (1999). Reforço e Reabilitação de Estruturas de Betão Usando Materiais Compósitos de CFRP. PhD Thesis. Universidade do Porto. Faculdade de Engenharia.