Pre-stressed FRP for the in-situ strengthening of timber structures

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1 Pre-stressed FRP for the in-situ strengthening of timber structures Lehmann Martin 1*, Milena Properzi 1, Frédéric Pichelin 1, Pascal Triboulot 3 1 University of Applied Sciences Bern HSB, Biel. Switzerland 3 ENSTIB University Nancy1. France * Corresponding author University of Applied Science HSB, Solothurnstrasse 12, CH-254 Biel, Switzerland, lam5@bfh.ch 1 Summary Since 194s, composite materials, formed by the combination of two or more distinct materials in a microscopic scale, have gained more popularity in the engineering field. Fibre Reinforced Polymers (FRP) are a relatively new class of composite materials manufactured from artificial fibres and resins, with demonstrated efficiency in the field of reinforcement and rehabilitation of structures. However, while timber has been successfully reinforced over the past few decades using various materials and reinforcing techniques, only a few of those methods have reached the commercial market. Moreover, compared to the steel, the FRP composites show a different structural behaviour and their utilisation for the in-situ external bonding has not yet been fully explored. In order to gain quantitative and qualitative knowledge on structural adhesive and fibre-reinforced polymer (FRP), suitable for in-situ poststrengthening of timber structures, the authors investigated an application method using ed carbon fibre strips. In this method the timber beam is cambered prior to the installation of the FRP lamella. This cambering is done with a in the height adjustable prop placed in the middle of the beam. The force in the FRP is not constant. It peaks in the middle of the beam were it is mostly needed and is zero towards the ends were high stress would causes delamination. A calculation model was developed and verified. Thus, this paper reports on the latest progress made on the development of the in-situ strengthening of timber beams with ed carbon fibres. Keywords: Timber structure, on-site reinforcement, carbon fibres, pretension, strain measurements 2 Introduction The refurbishment of old buildings often comes hand in hand with an increase of the dead- and live load. This and the higher safety factors often make a reinforcement of the old beams necessary. The use of steel is often not regarded as an adequate solution in a timber construction. This because of the increase of the structures self weight and the appearance. The use of FRP allows methods, which are nearly invisible. In order to gain the needed contribution of the reinforcement to the service limit state a solution involving ing is requested otherwise the amount of carbon fibre would be very high and therefore the method not cost efficient. Pre-stressed carbon fibres are commercially used to strengthen concrete and steel structures (especially old structures in cast iron). However until now the methods were not successfully applied to timber this mostly because of the delaminating of the carbon fibre strip due to the concentrated shear stress at the beams end. The idea to overcome this obstacle with resins with higher bonding strength resulted mostly in timber failure due to tension perpendicular to the grain. This failure was mostly two to three centimetre above the carbon strip. In the year 21 a method involving decreased pretension force towards the ends was experimented at the HSB in cooperation with the EMPA [3]. The results were quite promising; however the applications method involves rather large equipment and is therefore not preferable for on-site applications. Acting in the

2 framework of the COST action E34 Bonding of Timber the authors initiated a research project. In the investigated method the timber beam is cambered prior to the installation of the FRP lamella. This cambering is done with a in the height adjustable prop placed in the middle of the beam. 3 Objectives The objectives of the study were to verify the strengthening ration with slag and prestressed CFK and to determine the improvements in deflections. Furthermore a suitable resin for in-situ application was determined and a calculation model was developed. The aim of the study was to develop a method for on-site reinforcement of structural timber beams using CFK-strips. 4 Material and Methods The key of the method is that the timber beam is cambered prior to the installation of the FRP lamella. This cambering is done with a in the height adjustable prop placed in the middle of the beam (Figure 1). This allows the ing of a beam quite easily on site and also solves the delamination because the moment introduced in the beam has a triangular shape. Therefore the shear stress in the glue line is constant and quite low. The stress introduced in the FRP lamella is not constant over its length. It is also related to the force present in the prop, which is itself limited by the strength of the timber beam and the possibilities to hold the beam in place on its end. No literature was found concerning this method of ing FRP reinforcement, but it is assumed that this method is used on building sites without considering the prestress effect. This leads to a very conservative approach. Therefore in order to use the full capacity of this system a calculations model was developed and verified with testing of structural sized specimens. Figure 1: The black drawing shows the system prior to intervention. In red is the chambered system after installation of the prop. The red arrow symbolises the in the height adjustable prop. In the first series the feasibility was tested and the influence of the level of cambering prior to the gluing was tested. In this series three different resins were used. In the second series beams in structural size were tested. Series 1 was done with relatively clear specimen of Picea abies. For the second series glue laminated timber (GL 24 Picea abies / Abies alba [4]) was used. The five-percentage fractile is at 24 MPa and this class is commonly used for constructions in Switzerland. The Sikadur -3 and Sikadur 33 are solvent-free epoxy resins, which are already used for gluing FRP strips to concrete and steel. The Sikadur -3 was already used in combination with timber. The Sikadur 331 W is a water based epoxy resin and currently not used in combination with timber or CFK. The advantages of this resin are the relative low costs and no odours emissions during the application. The carbon fibre strips used for this research were Sika Carbodur lamellas (5 x 1.2 mm) with a MOE of 165 GPa.

2 x 15 B 5 SD 33 2 28% 4 x 53 1.2 x 15 C 5 SD -3 2 28% 4 x 53 1.2 x 15 D 5 SD -3 1 14% 4 x 53 1.2 x 15 E 5 SD -3 3 43% 4 x 53 1.2 x 15 F 5 SD -3 4 x 53 1.2 x 15 Series 2 G 1 SD 33 2 5% 12 x 16 1.

3 Table 1: Series 1 Samples tested during the research. sample number of specimens resin level 1 [MPa] 1 in % of estimated MOR of timber cross section timber [mm] cross section CFK [mm] Control x 53 - A 5 SD 331W 2 28% 4 x x 15 B 5 SD % 4 x x 15 C 5 SD % 4 x x 15 D 5 SD % 4 x x 15 E 5 SD % 4 x x 15 F 5 SD -3 4 x x 15 Series 2 G 1 SD % 12 x x 5 1 Bending stress in timber during the gluing of the carbon lamellas. The production of the small specimen was done upside down. The curing of the resin was done at 5 C this allowed a curing overnight. In order to bend the beam during the production weight was hung in the middle of the specimen (Figure 2). The controls were also heated to 5 C overnight in order to check influence of the heating on the timber properties. Figure 2: Production of a small specimen. (Sample E weight 225 kg) Figure 3: Production of two large specimens. In the foreground are the cables of the Sika Carboheater system visible. The Production of the large specimens was done upside down. The desired of the timber beam during the production was achieved with the testing machine. The curing of the resin was done at elevated temperatures. The heading was done with the Sika Carboheater system (Figure 3), this system sends a current through the carbon lamellas and due to their resistance they will heat up. The temperature is controlled with a k-wire in the glue line. The temperature of the glue line was risen over 3 minutes to 9 C and remained at this level for another 6 minutes. The clamps are to prevent the CFK lamella from lifting due to the vapour pressure. This vapour pressure results from the heating of the timber (MC 12%) near the glue line. The clamps and the load were released after the glue line was cooled down to 4 C, which is lower then the allowed service temperature. All the specimens were tested in four point according to EN 48 [2]. In order to measure the strain in the CFK strain gauges were used. The strain distribution over the length of the CFK lamella was measured. These measurements allowed the verification of the stress distribution in the CFK

4 lamella. On the specimens in sample G strain gauges were also used on the timber. This was done in order to help analyse the load-bearing behaviour up to failure / = strain gauge = dial gauge Figure 4: Four point setup as used for sample A to F / = strain gauge = dial gauge Figure 5: Four point set up as used for sample G. The strain Gauges on the timber were glued on the position Calculation model In order to calculate the stress present in the composite section, after the installation of the FRP lamella, the equilibrium of the forces after the removal of the prop needs to be calculated. In order to gain the desired information three stages are introduced. Stage 1 represents the System after the prop is installed but before installation of the CFK lamella. Stage two represents the effect of the removal of the prop on the system. In order to model the removal of the prop the force introduced by the prop is applied from above. Then the stress is in the cross-section of the timber-frp composite is calculated. This allows determining the stress, which is present in the FRP after the removal of the prop. The adding of stage one and stage two results in stage. This stage represents the system s equilibrium. The stresses present in the system are due to the ing and the structures own weight. The forces due to the ing are internal forces only. The force in the FRP (P ) is not constant over its length (Figure 6). This is due to the distribution of the moment, which is introduced by the prop. The negative Moment in the timber (M ) is due to the prestress force present in the FRP, which is not applied on the neutral axis. The normal forces in the timber (N ) are equal to the tension in the FRP. The positive moment (M g ) yields from the systems own weight.

5 m g tension in FRP (P ) compression in timber (N ) moment in timber (M ) internal forces yielding from ing m g (M g ) Figure 6: Stage in the calculation model. This is the result of the addition of stage 1 and stage 2 which represents the system after installed reinforcement and removal of the prop without additional load. The knowledge of the internal forces present in the system helps in the calculation of the load bearing capacities. This calculation can be done after the rules of the design standards. 5 Results 5.1 Small specimens The measurements of the force introduced in the small specimen verified the feasibility of the system (Table 2). Not one of the specimens failed due to delaminating. However the force was mostly over estimated with the calculation model only in sample D with a low level of prestressing and in sample B with Sikadur 33 resin the measurements were in the same range as the calculation. The results of the testing of the small specimens showed that the resin type has no obvious influence on the load bearing capacity. Table 2: + Overview of the results of the small specimens. The values presented are the average of the five specimen in each sample. Max. moment Increase of moment improvement stiffness (EI) Crosshead movement at failure Camber due to the prestressing stress measured stress calculated MOE timber only [knm] [%] [%] [mm] [mm] [MPa] [MPa] [MPa] Control Sample A Sample B Sample C Sample D Sample E Sample F The specimens of the sample B showed different failure behaviour as the other samples (Figure 7 and Figure 8). In sample D a trend towards a longer non-linear part of the graph was clear visible.

6 Specimen 12 Specimen Force [N] 6 Force [N] Crosshead movement [mm] Crosshead movement [mm] Figure 7: Force - deformation graph of an specimen in sample B. Figure 8: Force- deformations graph of an specimen of sample C The increase in the stiffness (EI) was over all samples in the same range. The water-based resin had quite a large shrinkage during the curing. Some of the specimens in this sample failed during testing in the glue line between FRP and timber. This happened without clear visible influence on the maximal load bearing capacity. But it was assumed that with three times wider CFK lamellas this shrinkage of the resin could cause a high amount of internal stress in the glue line and therefore be an obstacle in structural sized beams. Therefore this resin was not chosen for the large specimens. Due to the fact that the specimens produced using Sikadur 33 reached the highest level of ing and had an interesting failure behaviour this resin was chosen for the production of the large specimens. The intermediate level of ing was chosen for the large specimens. This mainly due to the fact that the forces involved in the production could be easily handled on a building site. Furthermore there was no obvious difference in the load bearing capacity between the high and the intermediate level. However some specimens of the higher level failed in shear during testing. 5.2 Structural sized specimens The analyses of the distribution of the force in the CFK lamella showed a triangular shape (Figure 9). This was expected and allows the assumption that the shear stress in the glue line is constant over the entire length of the specimen. Therefore no delamination needs to be expected in beams prestressed as described above. The non-linearity of the graph in Figure 9 is due to the variation of the MOE over the length of the timber beam. The calculated stress maximum in the CFK lamella due to ing corresponded well with the stress measured (Table 3) Table 3: Overview of the results of the structural sized specimens. (The values shown are the average values of the whole sample) Max. moment Estimated increase of moment improvement stiffness (EI) Crosshead movement at failure Camber due to the prestressing stress measured stress calculated MOE timber [knm] [%] [%] [mm] [mm] [MPa] [MPa] [MPa] Sample G 26.8 ~

Distribution of the stress in the CFK lamella 3 25 Stress [MPa] 2 15 1 5 5 1 15 2 25 3 35 4 Position of the strain gauges [mm] Figure 9: Prestress distribution in the CFK lamella.

7 Distribution of the stress in the CFK lamella 3 25 Stress [MPa] Position of the strain gauges [mm] Figure 9: Prestress distribution in the CFK lamella. Figure 1: Four point test setup as used for the structural sized specimens. The strain measurements during the testing showed that towards the maximum capacity the timber starts to fail in tension and the CFK lamella keeps the beam up (Figure 11 and Figure 12). This works only as long as there is no large fracture on the tension side of the timber. A large fracture in the timber causes a significant amount of tension perpendicular to the grain on the interface timber - CFK and the lamella peeled of. This failure occurred in the timber or the CFK lamella but never in the glue line. Some specimen had quite a abrupt failure without any significant non-linear part in the force-deflection and force-strain graphs. The specimens with a higher MOE of the timber beam had only some visible crushing of the timber on the compression side. Strain meassured on timber Strain messured on FRP Gauge 1 Force [N] Bodem Top Force [N] Gauge 2 Gauge 3 Gauge 4 Gauge 5 Gauge 6 Gauge Gauge Strain [µm/m] Strain [µm/m] Figure 11: Strain measurements on the timber during testing. The strain at zero force is due to the ing. Figure 12: Strain measurements on the CFK lamella during testing. The strain at zero force is due to the ing. The specimen with a higher MOE in timber tended to have a higher capacity than all of the others. The increase of the capacity due to the CFK lamella was estimated with the assumption that the timber by itself would have failed at the same tension stress as it failed in the composite. This calculation resulted in a resistance of about 13% for the reinforced beam compared to a timber beam of the same size. This value is in the same range as the value presented in [1] for prestressed beams. The increase of the stiffness (EI) however, is with 14 % significant lower than the value presented in the paper mentioned above. The cambering due to the ing has a

8 significant contribution in the service limit state. With four millimetres this chamber is about 3% of the allowable deflection. 6 Conclusions The estimated increase of the load bearing capacity in of 3% on the structural sized beams is a significant contribution to the ultimate limit state. However the expected increase of load bearing capacity in with higher forces could not be verified. The cambering due to prestressing has a significant contribution in the service limit state and justifies the ing, especially because the described method is very easy and fast to apply. The total contribution (including the chamber) of the reinforcement to the service limit state is around 4%. The strain measurements showed that this method for ing results in an equal distribution of the shear stress and is therefore a possible solution for the delamination of ed FRP lamellas. The calculated stress distribution and the measured strain corresponded well. Furthermore it showed that the use of strain gauges is possible in timber. The only disadvantage of the strain gauges used on timber is that they need to be calibrated in order to determine the local MOE under the strain gauges. But after the calibration the measurements can be used to do quantitative stress calculations with an accuracy, which can be accepted considering the variability of timber. To insure that the beam is visually pleasing the carbon lamella could be slotted in the beam. The longterm behaviour of the described system would however need to be researched. 7 Literature 1. BRUNNER Maurice and SCHNÜRIGER Marco, 25; Strengthening timber beams with prestressed artificial fibres: the delamination problem; in: COST C12 Final Conference Proceedings January 25: Improvement of Buildings Structural Quality by New Technologies (London) 2. EN 48:23 Timber structures Structural and glued laminated timber Determination of some physical and mechanical properties (Zürich) 3. SCHNÜRIGER Marco and BRUNNER Maurice, 24; Timber beams strengthened with prestressed fibres: Delamination; in: Proceedings of the 8 th World Conference on timber engineering wcte 24 Volume 1; Lathi, Finland 4. SIA 265:23 Timber Structures (Zürich)

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