Effect of Bar-cutoff and Bent-point Locations on Debonding Loads in RC Beams Strengthened with CFRP Plates

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1 CICE The 5th International Conference on FRP Composites in Civil Engineering September 27-29, 2010 Beijing, China Effect of Bar-cutoff and Bent-point Locations on Debonding Loads in RC Beams Strengthened with CFRP Plates Eftekhar M. R. (mreftekhar@yahoo.com) & Mostofinejad D. Department of Civil Engineering, Isfahan University of Technology, Isfahan, Iran ABSTRACT: In recent years, the use of Fibre Reinforced Polymer (FRP) composites for external ening of concrete structures has emerged as one of the most promising technologies in material and structural engineering. Although bonding of FRP to the soffit and side faces of reinforced concrete beams increases their flexural and shear capacities, debonding of FRP layers from the concrete substrate frequently happens and decreases the expected failure capacity. Over the last two decades, many parameters such as surface preparation of concrete specimens, compressive of concrete and geometrical dimensions of the FRP plate including bonded length, thickness and width, number of plies, and taper end effects have been shown to affect the debonding failure of RC beams ened with FRP laminates. An experimental study was performed to determine the effects of bar-cutoff or bend-point location on design debonding loads of RC beams ened with CFRP sheets. A total of seven 3-m long beams were produced, ened and tested under a 3-point loading. Two specimens served as control, while two had two different types of bar-cutoff locations and two had two different types of bend-point locations. Finally, the last one with four U-shaped strips to prevent debonding of the FRP sheet was used as a bar-cutoff specimen. The analysis of the experimental results was focused mainly on crack distribution and crack widths. The results of the experimental program showed distinct effects of bar-cutoff and bend-point on the total beam behaviour and debonding load, which will be discussed in the fall paper. Keywords: FRP, bar-cutoff, bend point, crack spacing, crack width, debonding load, RC beams. 1 INTRODUCTION A simple method to en RC beams is to use steel or fibre reinforced polymer (FRP) plates, which can be even employed for operating members in constructions. Certain unique properties of composite fibre plates such as their high resistance in moist and corrosive environments or their light weight and ease of application have made them even more attractive than their conventional metal counterparts [1]. The basic concern, however, with bonding composite fibre plates onto concrete members is their premature debonding off the concrete surface, which normally takes place at points prior to the ultimate design load or typically beyond the reinforcement yield point, or as a result of small deflections below expected levels. A number of factors may be involved in this behaviour of CFRP plates that include type of surface conditioning, concrete tensile, plate thickness, number of wrappings, and taper end point. Crack propagation and crack growth are included among the most important causes cited in the literature for this debonding behaviour [2, 3]. The arrangement of reinforcement in ened RC beams can alter the patterns of cracks created in the specimen with respect to crack spacing and debonding [4]. In this study, the effects of bar cut-off and bend point locations on the debonding of ening plates and the pattern of crack propagation have been experimentally investigated in RC beams ened with CFRP plates. 2 SPECIFICATION OF THE EXPERIMENTAL SPECIMENS For the purposes of the present study, a number of bending tests were performed on 7 actual-sized RC beams. The mm beams 3.3 m in length were simply supported at the ends and tested under a three-point loading up to failure. After each load increment, strain gauge, load and LVDT readings were recorded using an electronic data acquisition system. Table (1) summarizes the beam details along with the specifications of both embedded steel bars and external ening. According to the Table,

specimen I500 is not retrofitted with CFRP plates, which is used as control. The other beams listed in Table 1 are retrofitted with two ening plates 0.

2 specimen I500 is not retrofitted with CFRP plates, which is used as control. The other beams listed in Table 1 are retrofitted with two ening plates 0.12 mm thick and 150 mm wide externally on the underside of the beam using the wet lay up method. Table (2) summarizes the specifications of the materials used in the beams. Table 1. Specifications of experimental specimens Specimen No of cut-off L * reinf. or Bent bars (mm) No of plys I500 5Φ I521 5Φ B1I521 5Φ B3I521 5Φ C1I521 5Φ C3I521 5Φ C3UI521 5Φ *) Direct length of cut-off or bend bar at the tensile region (mm) In the specimens B1I521 and B3I521, respectively, 20% and 60% of the tensile reinforcement were bent at an angle of 45 º and extended to the end of the beam in the opposite face at the same level as the compression reinforcement. In C1I521 and C3I521, 20% and 60% of the tensile bars were cut off in the middle of the beam and adjacent to the support point, respectively. The specimen C3UI521 (Fig. 1) is exactly identical to C3I521 in terms of bar arrangement. In addition to the two ening plates, 4 U-shaped FRP strips 50 mm wide have also been used in this specimen to control the middle and taper end debonding of the ening plate. The strips are joined to the two ends of the plate and at beam midspan of the specimen normal to the longitudinal axis of the beam. Fig. (2) shows two samples of the beams tested. FRP adhesive concrete Fig. 2 two samples of the bend point location beams Table 2. Properties of materials Elastic Ultimate Thickness tensile Type per layer s strain (mm) (MPa) (GPa) (%) SikaWrap Hex 230C Flexural Thickness Modulu Type per layer (mm) s s (MPa) (MPa) (MPa) Sikadur Elastic Concrete Cylin- Max. Agg der. Size (mm) s Strength (MPa) (MPa) (GPa) CRACK PROPAGATION AND GENERAL BEHAVIOR OF THE SPECIMENS Fig. 1 C3UI521 specimen In all the specimens ened with two CFRP plates, the general rule of setting back the second plate by 150 mm from the first has been observed (Tapper end) [5]. Table 2 presents the speci- fications of the materials used in the specimens. Upon initial loading, the first flexural crack appears in the specimens at a loading equal to nearly 30 kn. Table (3) presents the details of crack propa- gation in the bar cut-off and bend specimens. Upon the creation of the first flexural crack and as a result of exerting a load on the beam, other flexural and flexural-shear cracks may also appear in the beam, which ultimately leads to the debonding of the ening plate and to beam failure. It is clear from Table (3) that at around the yield load of the tensile reinforcement, which is almost equal to 75% of the ultimate load of the test specimens, the beams experience around 90% of their ultimate number of cracks. In the specimen I521 in which

3 none of the tensile bars is bent or cut off, the numbers of recorded cracks are 21 and 27 at the tensile bar yield point and at the crack stabilization point, respectively. It may be claimed that bar cut-off and bend generally lead to altering the shear and flexural capacities of the specimen adjacent to the cut-off and bend point locations. This leads to an increased number of cracks in the beam compared to the case when no bar cut-off or bend occurs. Additionally, crack width is also affected by the longitudinal bar cut-off and bend. 1.2 Table 3. Crack propagation in specimens First crack Bar yield point Crack stabilization point specimen point Load Load No. of Load No.of cracks (kn) (kn) cracks (kn) B1I B3I C1I C3I C3UI Fig. (3) illustrates the width of the largest flexural crack typically occurring in the middle of the beam. Clearly, under identical load levels, the width of the largest flexural crack is greater in specimens with bar bends than in those with bar cutoffs. Bent bars seem to prevent the widening of cracks at the bar bend point and, consequently, the crack width in specimens with bar bending exceeds that in specimens with bar cut-off in order to attain identical peeling at beam mid-span. Comparison of the crack widths in Fig. (3) reveals that the more bars are cut-off or bent, the lower will be the flexural crack width. In other words, the location of bar bending or cut-off at the point of larger flexural moments leads to greater crack widths. Load (kn) Load (kn) Fig. 3 Effects of bar-cutoff and bent-point location on widening of the largest flexural crack 4 LOAD-DISPLACEMENT BEHAVIOR Fig. 4 depicts the load-displacement curve at the mid-span of the beam for bar cut-off and bar bend specimens. All the specimens exhibited almost similar behaviors prior to the bar yield point with the major differences lying in the debonding load of the ening plate and the corresponding displacement. This Figure also shows a magnified view of the end portion of the load-displacement curve. The rupture in all the ened specimens was due to the debonding of the CFRP plate. B1I521 (Bend bar percent: 20) B3I521 (Bend bar percent: 60) C1I521 (Bar cut-off percent: 20) C3I521 Bar cut-off percent: 60) C3UI521 (Bar cut-off percent: 60 & 4 U strips) Midspan deflection (mm) B1I521 (Bend bar percent: 20) B3I521 (Bend bar percent: 60) C1I521 (Bar cut-off percent: 20) C3I521 Bar cut-off percent: 60) C3UI521 (Bar cut-off percent: 60 & 4 U strips) Midspan deflection (mm) Fig. 4 load-deflection curves for ened RC beams 1 P/Pu B1I521 (Bended Bar Percent: 20) B3I521 (Bebded Bar Percent: 60) C1I521 (Cut-off Bar Percent: 20) C3I521 (Cut-off Bar Percent: 60) C3UI521 (Cut-off Bar Percent: 60 & 4 U Strips) Crack Width (mm) As seen in the Table 4, the debonding of the plate in all the specimens was of the IC type, except for the specimen C3I521 in which initially a shearflexural crack occurred under a load of 137 kn at the longitudinal bar cut-off point (at a distance of 1200 mm from the mid-span). The crack then rapidly propagated up to the neutral axis of the section. The width of this crack at the time of its creation and at its first reading was recorded as 0.2 mm, while the width of flexural cracks at the first reading was 0.02

4 mm. The rapid widening of the crack causes relative displacements on its two sides, which may exert additional stresses on the ening plate on the two sides of the crack. This event at a load of 167 kn in the specimen C3I521 caused part of the taper end to suddenly break away from the beam end (PE debonding). PE debonding is shown in Fig. (5) where the bonding of part of the concrete at the end of the ening plate is clearly seen. Table 4. Specification of debonding type for tested specimens Plate debonding point Deflection Load Type of debondmm) specimen ( (kn) ing B1I IC B3I IC C1I IC C3I PE IC C3UI IC under the debonding of the plate at the middle of the beam. This indicates that application of traverse FRP strips at the bar cut-off point can satisfactorily control the PE debonding of the plate and improve the performance of the specimen. 5 RESULTS This paper investigated the effects of bar-cutoff and bent-point locations in RC beams ened with CFRP plates on the peeling load and their cracking behavior. Based on the experiments, the following results were obtained: The debonding of CFRP plate is affected by bar cut-off percent. Crack growth is the cause of the debonding of the ening plate in specimens with various tensile bar cut-off percentages; however, quite different mechanisms are involved in the formation of cracks that lead to rupture in these specimens. Traverse ening plates cannot prevent their debonding at the middle of the beam. Under identical load levels, the width of the largest flexural crack is larger in bar bend specimens than that in bar cut-off specimens. Bar cut-off or bend increases formability but reduces crack distances. Fig. 5 Plat end debonding in specimen C3I521 It is seen in this figure that the cut-off specimens enjoy a higher load bearing capacity while the bar bend ones exhibit a higher formability. Since crack growth is one of the main causes for the sudden debonding of the ening plate, rupture occurs more rapidly in bar bend specimens due to the wider cracks created. As already mentioned above, the greater load bearing capacity of the specimen can also be attributed to the continuity of the tensile reinforcement and the greater uniformity of bar bend specimens as compared with bar cut-off specimens. The results presented in Table 3 and 4 and comparison of the load-displacement curves reveal the very similar behaviors of the two B3I521 and C3UI521 specimens so that both have ruptured REFERENCES [1] Saadatmanesh, H., and Ehsani, M. R., RC Beams Strengthened with GFRP Plates. I: Experimental Study, J. Struct. Engrg., ASCE, Vol. 117, No. 11, 1991, pp [2] Lu, X. Z., Teng, J. G., Ye, L. P., and Jiang, J. J., Intermediate Crack Debonding in FRP-Strengthened RC Beams: FE Analysis and Strength Model, Journal of Composites for Construction, Vol. 11, No. 2, 2007, pp [3] Pham, H., and Al-Mahaidi, R., Assessment of Available Prediction Models for the Strength of FRP Retrofitted RC Beams, Composite Structures, 66, 2004, pp [4] Eftekhar, M. R., and Mostofinejad, D., Effects of Steel Bar Arrangement on Cracking Pattern and Peeling Load in RC Beams Strengthened with CFRP Plates, FRPRCS-9, Sydney, Australia, 2009.

5 [5] ACI 440.2R-02, Guide for the Design and Construction of Externally Bonded FRP Systems for Strengthening Concrete Strutures, American Concrete Institute, Farmington Hills, Mich., 2002.

STRENGTHENING OF UNBONDED POST-TENSIONED CONCRETE SLABS USING EXTERNAL FRP COMPOSITES

STRENGTHENING OF UNBONDED POST-TENSIONED CONCRETE SLABS USING EXTERNAL FRP COMPOSITES F. El M e s k i 1 ; M. Harajli 2 1 PhD student, Dept. of Civil and Environmental Engineering, American Univ. of Beirut;