COMPARISON BETWEEN CARBON AND STEEL FIBER REINFORCED POLYMERS WITH OR WITHOUT ANCHORAGE

Transcription

1 COMPARISON BETWEEN CARBON AND STEEL FIBER REINFORCED POLYMERS WITH OR WITHOUT ANCHORAGE Konstantinos KATAKALOS MSc, MSc, PhD Candidate Laboratory of Strength of Materials and Structures, Aristotle University of Thessaloniki (AUTh) University Campus, Thessaloniki, Greece, George C. MANOS Professor, Director of Laboratory of Strength of Materials and Structures Aristotle University of Thessaloniki (AUTh) University Campus, Thessaloniki Christos G. PAPAKONSTANTINOU MSc, PhD, Research Associate Laboratory of Strength of Materials and Structures, Aristotle University of Thessaloniki (AUTh) University Campus, Thessaloniki, Greece, Abstract The aim of this study is to investigate the shear behavior of reinforced concrete beams strengthened with either Carbon (CFRP) or Steel (SFRP) fiber Reinforced Polyme rs. The investigation also includes the use of an anchoring system. The strengthened reinforced concrete beams along with a non-strengthened control beam were tested monotonically under four point bending loading conditions. Moreover, an analytical model is introduced, that can be used to analyze the shear performance of the strengthened beams. Totally nine 1:1 scale beams with a span of 3000mm where fabricated and tested. The experimental results indicate that the failure of the strengthened beams was based on the debonding of the strengthening sheet when no anchoring system was used. At the other cases the failure occurred due to fracture of the FRP sheet or failure of the anchoring system. Furthermore, the use of an anchoring system increases the overall capacity of the beam. In conclusion a strengthening system with SFRP can provide an effective alternative to commercially available systems. Keywords: Anchorage, Carbon fiber reinforced polymers (CFRP), Reinforced concrete Beams, Shear Upgrade, Steel fiber reinforced polymers (SFRP) 1. Introduction A wide range of fiber reinforced composite materials have been successfully used for the repair and strengthening of existing reinforced concrete (R/C) structures [1]. Strengthening with externally bonded FRPs is particularly common, due to the numerous advantages related with their use. These advantages include the speed and ease of installation, the low weight and their high tensile strength [4]. The most commonly used strengthening systems are glass or carbon fiber reinforced polymer (FRP) composite materials [1-5]. Lately, a new strengthening system based on high strength steel fibers known as steel reinforced polymers (SRP) [ 3] was introduced as an alternative having high strength and ability of inelastic deformations. The shear capacity of reinforced concrete (R/C) structural elements, with low value of the steel transverse reinforcement ratio (stirrups), can be enhanced by applying as transverse external reinforcement such materials in the form of strips [1, 3, 4]. However, the exploitation of the high tensile strength of such FRP s or SRP s is usually rather Page 1 of 8

2 low due to the debonding type of failure. In the case that these strips can be fully wrapped around the structural member, the degree of exploitation of these materials increases and the mode of failure can then result due to fiber fracture of the external reinforcement [2,6]. However, there are many practical applications that full wrapping cannot be practically materialized. This paper presents results from an experimental investigation that focuses on the upgrade of the shear capacity of 3m span R/C beams strengthened with either CFRP or SRP open hoop strips, placed with an anchoring device in order to avoid the debonding mode of failure and thus increase the exploitation of the CFRP/SRP materials. This specific anchoring device is developed at the Laboratory of Strength of Materials and Structures, in Thessaloniki, Greece under the patent number WO [6]. Finally an analytical approach is applied for predicting the shear strength of such R/C beams which is validated by the measured results. 2. Experimental Setup 2.1 Materials Totally ten reinforced concrete beams were fabricated and tested at the Laboratory of Experimental Strength of Materials and Structures of Aristotle University of Thessaloniki. The compressive strength of concrete was measured equal to 20MPa. The internal reinforcement yielded at f y =527MPa and failed at a stress equal to f u =645MPa. The strengthening scheme consisted of either uniaxial CFRP sheets or uniaxial high strength steel fiber sheets (SFRP) and a commonly used organic resin (Sikadur 330). SFRP sheets were provided by Bekaert Industries specifically for this project. The width of sheets was kept constant and equal to 100mm and resulted to an equivalent width equal to 0,1184mm. Both CFRP sheets and organic resin were provided by SIKA Hellas which is gratefully acknowledged. The type of CFRP is SikaWrap 230C with nominal thickness equal to mm and a modulus of elasticity equal to 234 GPa. The maximum elongation was recorded equal to for either CFRP or SFRP coupons. 2.2 Geometry and loading of specimens The present research program includes the investigation of ten rectangular R/C beams under monotonic loading conditions. The cross section of the beams was 120X360 (mm). All specimens were longitudinally reinforced with 3Ø20mm bars at the top and 3Ø20mm at the bottom of the beam. Table 1. Experimental variables of specimens BEAM NAME SHEAR REINFORCEMENT ANCHORAGE FRP WIDTH (mm) FRP axial DISTANCE (mm) RB NO RBs Ø8/ 250 (internal) RB200C CFRP NO RB200Ca CFRP YES RB200S SFRP NO RB200Sa SFRP YES RB150C CFRP NO RB150Ca CFRP YES RB150S SFRP NO RB150Sa SFRP YES Two control beams were tested. The first one, named as RB, had no internal shear reinforcement whereas the second, named as RBs, had Ø8/250mm as transverse reinforcement. The remaining eight beams did not have any internal transverse reinforcement, Page 2 of 8

3 thus were strengthened with externally bonded open hoop CFRP or SFRP sheets, with or without an anchorage device. The strengthened beams were designed in such way that would be able to upgrade their shear capacity at least to the point of the RBs strength. Table 1 that follows depicts the variables that were taken under consideration for each specimen. All beams were subjected under four point monotonic loading conditions. The overall beam span was 2700mm whereas the shear span was equal to the 1/3 of the total length, thus 900mm. Figure 1 represents the experimental setup. The loads as well as the vertical displacements at two points, under the application of the load, were recorded. Strain gauges were placed to every FRP sheet in order to record the developed strains on them. The strain gages were placed along the direction of carbon or steel fibers and no both sides of the beam. They were also placed at the middle distance of both width and height of the sheet, due to the fact that that the shear cracks are difficult to be predicted. Figure 1 also shows a photographic demonstration of the experimental setup. Fig. 1. Experimental Setup (sketch and photo) The present investigation also focuses on the application of a novel anchoring device which was developed and patented at the Laboratory of Experimental Strength of Materials and Structures of Aristotle University of Thessaloniki. Rod Steel plate Bolts C or S FRP Zoom Anchoring Device Steel plate Rod Bolts C or S FRP Rod Bolts FRP Steel plate Fig. 2. Patented Anchoring Device (WO ) Page 3 of 8

4 The developed anchoring device is transforming the axial tensile forces, applied on the FRP sheet, to shear forces imposed through the bolts into the concrete mass of the beam. Figure 2 shows a front face and a section of the novel anchoring device. 3. Experimental results and model of prediction 3.1 Experimental Results - Discussion Totally ten R/C rectangular beams were investigated. For all beams not only load and vertical deflection were recorded but strains at each FRP sheet as well. Table 2 depicts the obtained data from the experimental investigation. The experimental measured shear force is depicted at fourth column of Table 2 whereas the measured strains of the FRP sheets that the shear crack is intersecting are mentioned at fifth and sixth columns of the same table. The mentioned strain gages were placed above the shear crack, thus making most of the measurements more reliable. BEAM NAME SHEAR REINFORCE- MENT ANCHORAGE Table 2. Experimental results STRAIN STRAIN EXPERIMENTAL of 1 st of 2 nd SHEAR FORCE SHEET SHEET (kn) (μstrain) (μstrain) CALCULATED SHEAR FORCE (kn) CRB NO MODE OF FAILURE Brittle shear crack Ø8/ 250 Yield of (internal) stirrups RB200C CFRP NO Debonding RB200Ca CFRP YES CFRP fracture RB200S SFRP NO Debonding RB200Sa SFRP YES Anchorage failure RB150C CFRP NO Debonding RB150Ca CFRP YES CFRP fracture RB150S SFRP NO Debonding RB150Sa SFRP YES Anchorage failure The shear strength of the control beam (CRB) without any stirrups was recorder equal to 39.4kN, whereas the specimen with Ø8/250mm as transverse reinforcement was increased to 90.9kN. The strengthening scheme that took place to the remaining of the beams had an objective to increase the shear strength at a lower limit equal to. From the results shown in Table 2, it is evident that the maximum recorded shear load for all strengthened beams was significantly greater than that of the CRB. In Table 3 the percentage of the increase of the shear load is presented. % INCREASE compared to CRB % INCREASE compared to Table 3. Percentage increase of the shear load compared to CRB and CRB RB200C RB200Ca RB200S RB200Sa RB150C RB150Ca RB150S RB150Sa The shear load of the strengthened beams, compared to CRB, met an increase of more than 200%, but not less than 130%. The largest percentages of that increase were observed, as it was expected, for the strengthened beams that the novel anchoring device was used in Page 4 of 8

5 combination with FRP sheet. The biggest increase of the shear load was observed for specimen RB150Ca and was equal to 212,2%. For this strengthened beam the mode of failure was located at the fracture of the CFRP sheet which demonstrates that the novel anchoring device anchored the CFRP sheet properly on the concrete beam. Specimen RB150Sa exhibit a maximum shear load equal to 119kN that resulted to a 202% increase compared to CRB. For this specimen the anchoring devised posed an upper limitation to the shear load of the beam due to the fact that the device failed it self, before the failure of SFRP sheet. Nevertheless the strains that were recorded reached 7000μstrain which is close to the maximum strain at failure of SFRP sheet (9000μstrain). This specimen reached the maximum recorded average displacement for the maximum load, fact that results to an increase of an inelastic behavior. By comparing the beams that were strengthened with CFRP vs. SFRP it could be noted that the overall behavior was similar. SFRP sheets could be utilized as an alternative material for strengthening applications. More specifically, focusing on strengthened beams without the use of the anchoring device, RB150S exhibit the biggest shear load (103kN) which is very close to RB150C s (101kN). Figure 3 presents the shear load vs. the average displacement of all beams in two graphs. The first one shows the behavior of the strengthened beams without an anchoring device, compared with the control beams CRB and, whereas the second graph deals with those that the anchoring device was utilized for the proper attachment of FRP sheets Shear Load (kn) RB200S RB150C Shear Force vs Displacement w /o anchoring device RB150S RB200C CRB RB150C RB200C RB150S 20 RB200S CRB Displacement (mm) Shear Force vs Displacement with anchoring device 20 RB200Sa CRB Displacement (mm) Fig. 3. Shear Force vs. Displacement graphs w/o and with the novel anchoring device Specimens, where the novel anchoring device was utilized, exhibit a clear increase of the shear load as well as of the developed average displacement. The premature mode of failure, the debonding of FRP sheet, changed to either fracture of FRPs or failure of the anchoring device, when the sheets were properly anchored. The prevention of the premature delamination resulted to an increase of the specimen s strength. However, the initial stiffness remains the same for all specimens. On the other hand after the first cracking the stiffness of the strengthened beams is greater than the stiffness of the control beams. This phenomenon is stronger for the specimens that FRP sheets were anchored. The behavior of the control beam with stirrups () was sufficiently reached by the strengthened beams without the anchoring device, despite the fact that the premature debonding of the sheets, either CFRP or SFRP, posed an upper limitation. The rest of the strengthened beams, where the FRP sheets were properly anchored, presented a better behavior than, concerning not only the shear force but the average displacement as well. Finally, as far as the measured strains are concerned it could be said that the existence of the anchoring device is allowing the development of greater strains on FRP sheets compared to the cases that the anchoring device was not utilized, where a limitation was posed due to the debonding Shear Load (kn) RB150Ca RB200Sa RB150Sa RB200Ca CRB RB150Ca RB200Ca RB150Sa Page 5 of 8

6 3.2 Model of prediction The prediction is based on the observed during the experimental sequence dominant shear crack pattern that leads to the shear mode of failure for each specimen that employs external CFRP or SFRP transverse reinforcement. It takes into account the following contributions: 1. The compressive zone of the beam V c = 0.3*f c 2/3 *h c *b, where f c is the concrete compressive strength, h c the height of the compressive zone (as estimated from the observed behavior), b the width of the beam. For the specific research program V c was experimentally measured equal to 39.4kN (shear load of control beam CRB) 2. The contribution of the external transverse reinforcement in the form of CFRP or SFRP sheets that are mobilized by the observed mode of failure (V FRP ). This contribution was based on strain measurements that were taken from strain gages attached externally on the surface of the CFRP or SFRP sheets. More specifically, V FRP is calculated using the experimentally recorded strains from both sides of the beam (ε 1 +ε 2 ), the thickness of FRP (t f ), the modulus of elasticity (E f ) and the width (b f ) following the equation: V FRP =[(ε 1 +ε 2 )* E f ]*(b f *t f ). 3. The contribution of the steel stirrups, if they exist (V w ). 4. The contribution of the bottom side longitudinal reinforcement from dowel action (V d ). This contribution is based on the geometry of the observed shear failure mechanism which was measured in detail at the laboratory. For this purpose, a simple numerical simulation of this dowel action was formed, which is depicted in Fig. 4. All the supports offered to the longitudinal reinforcement by the transverse reinforcement are simulated by springs with the corresponding elastic / post-elastic and cross-sectional properties of the actual CFRP or SFRP sheets and steel stirrups, if they exist. Due to the volume of concrete, the rotations at the two ends of this numerical simulation for the dowel action were restrained; moreover, the employed numerical model simulates the possibility for the longitudinal reinforcement to develop plastic hinges with parameters based on the cross-sectional area and yield stress of the actual longitudinal reinforcement; this yield stress value was measured at the laboratory. The limit state contribution of the dowel action of the longitudinal reinforcement, as resulted from this numerical simulation, was found equal to V d =3.41kN for the specimens that the FRP sheets had an axial distance equal to 200mm (Fig. 4a). Furthermore for specimens where FRP sheets had a distance of 150mm V d was found equal to 4.07kN 50mm 100mm 50mm 25mm 100mm 25mm Figure 4. Numerical simulation for the calculation of dowel action a) 200mm, b) 150mm On table 3 the predicted values for the shear strength, for each of the eight tested specimens, is demonstrated as they resulted from the sum of all four of the above mentioned contributions; the same table also lists the corresponding experimental measured shear load values. Finally a ratio of the experimental value devided by the calculated value is depicted as well (V exp / V cal ). As can be seen, a reasonably good agreement is reached between the predicted and the corresponding measured shear strength values. It must be pointed out that the predicted values were based on the exact observation of the failure mechanism as well as on corresponding Page 6 of 8

7 measurements of the FRP strains. Table 4. Predicted and observed shear load values for the tested R/C beam specimens CALCULATED EXPERIMENTAL V exp BEAM V c V FRP V D SHEAR SHEAR FORCE / NAME (kn) (kn) (kn) FORCE V cal V exp (kn) V cal (kn) RB200C RB200Ca RB200S RB200Sa RB150C RB150Ca RB150S RB150Sa Conclusions The results of the present investigation resulted to the following conclusions a) Steel Fiber Reinforced Polymers (SFRP) that were specially fabricated for this spe cific research program could be succesfully used as an alternative strengthening material. b) Reinforced Concrete beams (R/C) that were s trengthened without the use of the anchoring device exhibit a limitation to the maximum shear load and displacement due to the debonding of the FRP sheet. Despite that fact the limited developed shear load was greater than the control s beam with stirrups (). c) When the novel anchoring device is utilized for the proper attachment of the FRP sheets, either carbon or steel, to the R/C beams, a further increase of the shear capacity of the beams is observed. At some case this increase is greater that 210% compared to the control beam without stirrups and 35% greater compared with. d) The vertical displacements also increased especially when the anchoring device was utilized. e) An increase of the post cracking stiffness was observed for all strengthened specimens. This was more pronounced for specimens that an anchoring system was used. f) The developed analytical model, taking into consideration the observed measured values of the investigation, and introducing the mechanism of dowel action, can predict the experimental values. Acknowledgements Carbon fibers and epoxy resins were provided by Sika Hellas Steel fibers are not commercially available and were provided for the present study by Bekaert Industries Partial financial support for this investigation was provided by the Hellenic Earthquake Planning and Protection Organization (EPPO) The employed, in this study, anchoring device is patented under the no. WO Page 7 of 8

8 5. References [1] MANOS, G..C., KATAKALOS, K., KOURTIDES, V, The influence of concrete surface preparation when fiber reinforced polymers with different anchoring devices are being applied for strengthening R/C structural members, Applied Mechanics and Materials, Vol, 82, 2011, Pages [2] PAPAKONSTANTINOU, C.G., KATAKALOS, K., Flexural behavior of reinforced concrete beams strengthened with a hybrid inorganic matrix - Steel fiber retrofit system, Structural Engineering and Mechanics, Vol. 31, Issue 5, March 2009, Pages [3] CASADEI, P., NANNI, A., ALKHRDAJI, T., Steel-reinforced polymer: An innovative and promising material for strengthening infrastructures, Concrete Engineering International, Vol. 9, No. 1, 2005, pp [4] PLEVRIS, N., TRIANTAFILLOU, T. C., Time-dependent behavior of RC members strengthened with FRP laminates, Journal of Structural Engineering, Vol. 120, No. 3, 1994, pp [5] KATAKALOS, K., PAPAKONSTANTINOU, C.G., Fatigue of reinforced concrete beams strengthened with steel-reinforced inorganic polymers, Journal of Composites for Construction, Vol. 13, Issue 2, 2009, Pages [6] MANOS G.. C., KATAKALOS, K., KOURTIDES, V, Construction structure with strengthening device and method, European Patent Office, Patent Number WO (A1) Page 8 of 8