Strengthening of concrete members by means of carbon fiber reinforced polymers (CFRP)

Transcription

1 Strengthening of concrete members by means of carbon fiber reinforced polymers (CFRP) Keywords: carbon fiber reinforced polymers (CFRP), strengthening of concrete, prestressed CFRP-lamellae In this work the method of strengthening of concrete members by means of carbon fiber reinforced polymers (CFRP) is treated, whereas the CFRP-lamellae are glued externally into slots. This innovative technology exhibits some advantages compared to superficially glued lamellae. The results of the experimental studies show that the bond of slotted CFRP-lamellae is significantly higher than the bond of superficially glued lamellae and thereby the tensile strength can be utilized more efficiently. Additionally, slotted lamellae are protected better against mechanical and thermal actions. On the base of the bond tests a method is presented, which enables the application of prestressed slotted CFRP-lamellae and hence the utilization of the mechanical properties of the lamellae may be enhanced. 1. Introduction The innovative combination of materials slotted-in CFRP lamellae and reinforced concrete opens a new and much promising potential for the manufacture of structures and structural components. Through this combination, the opportunity is offered for concrete constructions to be more efficient in terms of fatigue and durability but with a limited consumption of raw material. The retrofitting of structures with carbon-fiber lamellae, in abbreviation CFK-lamellae, stands for a meaningful ecological provision as well, since they contribute to a reduction of the utilized resources in construction. The energy- and material consuming erection of new buildings can often be avoided, as the use of CFK lamellae makes it possible to repair damages or to adapt existing structures for new requirements. Economical benefits arise from the maintenance of the structure, as well as the transportation and the assembly. This paper deals with the behavior of the bondage between the two materials. The results of experimental investigations done in the Institute of Structural Engineering show that the bearing capacity of the bondage of slotted-in CFRP lamellae is considerably higher than that of the superficially bonded ones and thus it is possible to utilize CFRP lamellae more efficiently. Furthermore, slotted-in CFRP lamellae are better protected against mechanical and thermal effects. Consequently, some disadvantages of the superficially bonded CFRP lamellae are overcome by use of this new technique. With a foothold on the experiments for the bondage, a new method was developed that allows CFRP lamellae to be used in a prestressed state as well, utilizing their mechanical parameters at an optimum level. 2. Manufacture and properties of CFRP-lamellae The principal is to mould sheets of any kind of plastic laminates that consist of bonded single layers. In synthetics technology this is called composite or fiber composite materials. They are composed of at least two physically or chemically different components that are firmly bonded with each other through a boundary layer.

2 Carbon fiber lamellae are manufactured with a pultrusion process (pull and extrusion process figure 1). Pultrusion is a discontinuous process for the manufacture of fiber reinforced synthetics profiles. The reinforcement from parallel running carbon fiber rovings or mats is immersed in a reaction resin molding material (Polyester, Vinylester, etc.) and then extruded through a die to the desired profile. The hardening of the profile starts already in the heated extruding die and is completed in the following hardening phase through infrared radiation, high frequency current or convection. The reinforcement consists approximately 60% per weight of the final product. This procedure was developed in 1956 in the USA and is accounted as the composite technology with the highest accession rate. The pultrusion process allows, in that way, to produce endless lamellae, whereas the maximum length is dependent on the roving s bobbin and the transportation potential. Figure 1: Schematic flow of the pultrusion procedure Carbon fiber lamellae, as mentioned above, are manufactured through the pultrusion process and they are provided in unidirectional flat cross sections with a width of 50 to 300 mm, a thickness of 1 to 3 mm and can be supplied with a length of up to 500 m. The mechanical properties in the longitudinal direction can mainly be determined according to the type of the fibers and their volume content (approximately 70 %). In the direction of the fibers they exhibit a very high tensile strength (up to 3000 N/mm 2 ) and stiffness (up to N/mm 2 ), while their performance against fatigue is excellent as well. Their strength in the lateral direction and consequently their shear resistance are low and they are primarily influenced by the properties of the matrix. Their maximum strain ranges between 1,5 and 2,0 % and considering a partial safety factor of 2,0 referring to the failure strain, the design value for strain can be assumed between 0,6 and 0,8 %. Carbon fiber lamellae can be fastened to the repaired structural component either untensioned or prestressed. They are delivered at the construction site in rolls (250 m rolls of approx. 25 kg weight), so they can be also employed in cases of accessibility space shortage [1], [2], [3]. 3. Application of slotted-in CFRP lamellae in concrete construction By reason of the problems in the bondage between concrete surfaces and superficially fastened CFRP lamellae a new method of application was developed in the late nineties, the so-called slotted-in CFRP lamellae method (figure 2). By use of that, carbon fiber lamellae can be fixed in slots vertically to the surface of the concrete element and thus to essentially improve the ductile behavior and eliminate the danger of a brittle failure, in contrast to the superficially fastened lamellae. For this purpose, slots are incised normal to the concrete element, to a depth smaller than the concrete cover. Since the concrete cover has often a small and varying value, for the implementation of this method it is of utter importance to know the values and therefore the exact measurements should be included already in the design stage. In order to avoid damaging the stirrups or the lateral reinforcement by incising the slots, the available concrete cover should be of at least 25 mm. In case of bi-axial layers of reinforcement the application is hard to be realized. Depending on the geometry of the lamellae (thickness up to 2 mm), these slots are usually 15 to

3 30 mm deep and up to 3 mm wide. After careful cleaning of the slots, the CFRP lamellae are finally fixed inside by use of epoxy resin. beam stirrup detail plate reinforcement h reinforcement h c b CFK-lamellae b b concrete cover CFK-lamella epoxy resin Figure 2: Strengthening with slotted-in lamellae This method in comparison to the superficially fastened CFRP lamellae features the following advantages: [5], [6] - Better behavior of the bondage between lamellae and concrete surface, so more efficient usage of the lamellae and selection of smaller lamellae cross sections - Unevenness of the concrete surfaces can be easily adapted to, by a proper slot depth - Incising of the slots is often less expensive than evening and roughening of the concrete surfaces in case of superficially fastened lamellae - Slotted-in lamellae are protected from mechanical damage, while they exhibit a more favourable performance in case of fire Ideal fields of application of slotted-in lamellae are: - Increase of resistance in negative moments (joint moments) - Slots in compression members (increase of tension components of moments) At this point, it should also be mentioned that CFRP lamellae as strengthening components can be applied not only in concrete structures but also in wood and especially in steel and composite constructions. 4. Prestressing of CFRP lamellae With a method developed in the Institute of Structural Engineering (BOKU), a prestressing before the fastening of the CFRP lamellae can be realized and thus a better utilization of their material properties. Aim of the study is to set the foothold for the practical application of this method. Reinforced concrete products with prestressed carbon fibers are superior to comparable structural elements from many points of view: - Significantly reduced displacements at serviceability state - Improvement of the crack pattern and reduction of the crack width

4 - Remarkably higher strengthening function - Improved ductility - Secure anchorage with flat elements - Minimum intervention to the available structural material - The low weight of CFRP products leads to reduction of transportation costs 4.1 Description of the system There is a distinction between the temporary anchorages for the procedure process and the permanent anchorages (figure 3) for a long lasting anchorage of the prestressing forces in the element. In the following, the principals of the prestressing procedure will be presented, with main focus on the permanent anchorages. The other end of the prestressed system, that is the temporary anchorage, is treated theoretically. The body of the permanent anchorage consists of a two-part discoidal anchoring hull with wedge-shaped cuts. The two corresponding wedges are fastened to the end of the CFRP lamellae through an insert of 1,5 mm thick aluminum plates. On loading they develop expansion forces that are transferred in concrete through the anchoring hull. The adhesion of the wedges with the lamella is particularly important, since the expansion forces in the beginning are not enough to hold the lamella in the anchorage, so the slip would be too big or even the lamella would be detached. The temporary anchorage consists of two parts: the anchoring hull with wedge shaped cuts on the one hand and the adaptor for the prestressing force introduction on the other. The adaptor, set in motion by a high-pressure pump, slides as a cart on previously assembled steel plates. The transfer of the prestressing forces from the adaptor to the anchoring hull takes place through mechanical form closure. The advantage of this decoupled system lies in the fact that the anchor hull of the temporary anchorage can stay as a permanent anchor, while the pump and the adaptor can be removed and used in the next prestressing location. An additional mortaring on the face of the anchorage can provide a smoother load transfer in concrete. Until the complete hardening of the mortar the anchor hull should be fixed in place with a bolt.

5 wedges aluminium - platelets lamella 1. Drilling the hole 2. Lamella with wedges epoxy resin anchor 3. Placing the anchor anchor wedges 4. Threading the lamella 5. Prestressing the lamella Figure 3: Permanent anchorage

6 4.2 Prestressing procedure The procedure of the prestressing (figure 4) is, as mentioned above, a two-step procedure with implementation of two different types of anchorages. The prestressing force is introduced through a hydraulic system which functions as a small dual piston pump. When the lamella comes in tension, the movable anchorage is secured with a bolt, the pump is removed and the face of the anchorage is mortared. After hardening of the mortar and the two-component adhesive, with the lamella fastened in the slot, the bold can be removed as well. The whole of the prestressing forces are transferred in the structural element through face pressure of the anchor hull. The steel plates are dismantled and can be used in further experimental setups. The basic steps of the procedure are presented (figure 4). 1. Drilling the hole + cutting the slots + placing the permanent anchor ceiling wall permanent anchor slot 2. Placing the first steel plate and the temporary anchor + threading the lamella wall lamella 3. Placing the second steel plate + prestressing the lamella 1. steel plate temporary anchor wall lamella 2. steel plate press adaptor Figure 4: Prestressing procedure (basic outline)

7 4.3 Results of the investigation and prospects To investigate the operability of the anchorage system at the end of the lamellae, experiments with and without influence of the free edge of the compound, for concrete classes C 20/25 and C 50/60 were performed. Mechanical properties of the used CFRP lamellae: E Modulus: MPa Thickness: 1,4 mm Tensile Strength: MPa Width: 20 mm Failure strain: 1,4 % The failure mode for experiments of both concrete classes without influence of a free edge was lamella failure, where following failure loads was reached: Concrete C 20/25: 66,9 kn Concrete C 50/60: 76,9 kn Up to an edge distance of 200 mm, a failure of the edge of the concrete element can be observed. The geometry of the breakout body is similar to the one of shear loaded dowels close to the edge. As the load increases cracks initiate from the side of the anchor hull, both in direction of the loading and the opposite direction. The cracks and the free edge form an angle of about 30. For smaller distances from the edge (up to 100 mm) this angle is not noticed here cracks propagate steeper. By excess of the concrete s tensile strength, an abrupt failure of the element s edge takes place. On the other hand, with an edge distance of 250 mm only capillary cracks develop in concrete, with a propagation angle of about 30 as well. In that case, the edge of the plate can resist the loading conditions and the lamella failure is decisive. Figure 5: Experiments with influence of a free edge

8 Influence of the free edge Failure loads [%] Edge distance [mm] Figure 6: Failure loads compared to the results without influence of the free edge As every new development, this concept is afflicted with a lot of teething troubles. In the future, it s essential to improve the system and to make it more applicable for the practice with regard to the temporary anchorage. A definitive evaluation of the prestressed slotted-in CFRP lamellae is not yet possible with the available experiments. However, it is important to collect experience from the practice in order to help this very promising technique to get widely accepted. 5. Literature [1] Luggin W., Die Applikation vorgespannter CFK-Lamellen auf Brettschichtholzträger (The applixation of CFRP-lamellae on glued-laminated timber structures in German), Dissertation, BOKU Vienna2000 [2] Bergmeister K., Vorlesungsskript zu Konstruktion I, 6. Auflage (Lecture notes for Construction I, 6 th edition in German), Oktober 2002 [3] Bergmeister K., Kohlenstofffasern im Konstruktiven Ingenieurbau (Carbon fibers in structural engineering in German), Ernst & Sohn Verlag, Berlin 2003, 300 pg. [4] [1.September 2003] [5] Blaschko A., Zum Tragverhalten von Betonbauteilen mit in Schlitze eingeklebten CFKLamellen (On the bearing beahvior of concrete structural compounds with slotted-in CFRP lamellae in German), Dissertation, TU Munich, Juni 2001 [6] [8. September 2003]