USE OF CFRP LAMINATES FOR STRENGTHENING OF REINFORCED CONCRETE CORBELS

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1 International Journal of Civil Engineering and Technology (IJCIET) Volume 6, Issue 11, Nov 2015, pp , Article ID: IJCIET_06_11_002 Available online at ISSN Print: and ISSN Online: IAEME Publication USE OF CFRP LAMINATES FOR STRENGTHENING OF REINFORCED CONCRETE CORBELS Khalid K. Shadhan and Mohsen M. Mohammad Kadhim University of Babylon College of Engineering ABSTRACT This work consists of fabrication and testing of sixteen reinforced concrete corbels. Fourteen of these corbels have been strengthened or repaired by various plies of Carbon Fiber Reinforced Polymer (CFRP) laminates. The experimental variables considered in the test program include, the CFRP pattern scheme (i.e. location, orientation, and number of layers), bond type (partial warping, full warping, and using CFRP anchors), and damaged ratio. Two modes of failure were observed, debonding and rupture of the CFRP laminates. Significant increase in ultimate capacity ranging from 17% to 71% was registered in the strengthening specimens and from 13% to 65% in repaired specimens. Key words: CFRP, Reinforced Concrete and Corbels Cite this Article: Shadhan, K. K. and Mohammad Kadhim, M. M. Use of CFRP Laminates for Strengthening of Reinforced Concrete Corbels. International Journal of Civil Engineering and Technology, 6(11), 2015, pp INTRODUCTION Corbels are structural members commonly used to support the reinforced concrete beam element, especially in precast structures. Their primal function is to transfer the horizontal and vertical forces to principal structural members. Corbels are cantilevers having shear span-to-depth ratios not greater than unity, which tend to act as simple trusses or deep beams, rather than flexural members (ACI , 2011), thus they need special provisions and act differently from the normal cantilever beams, this difference in behavior is due to the steep inclination of tension cracks and influence of compression strut. While a significant number of researches had been made on ordinary reinforced concrete corbels, a much fewer number were made for strengthening or repairing 11 editor@iaeme.com

2 Khalid K. Shadhan and Mohsen M. Mohammad Kadhim corbels using FRP composite materials (Elgwady et el.,1999 ;Corry and Dolan, 2001; Erfan et el.,2010; Abed Attiah, 2010;Ahmed et. el.,2013). This experimental research is focused on the behavior of reinforced concrete corbels strengthened and repaired with CFRP laminates. 2. Experimental program 2.1. Specimen description The test specimen consisted of double corbels and a short column. Each side had a total depth of 190 mm, 150 mm breadth and 200 mm length. The column supporting the two corbels has a section of 200 mm 150 mm and length of 440 mm as shown in Figure (1). Two 10 mm bars were used as primary reinforcement, placed at the bottom of the beam with an effective cover of 25 mm, and one 8 mm bars used as secondary reinforcement. Column part is reinforced with six deformed bars having a 12 mm diameter and supported with ties having a 10 mm diameter placed every 85 mm. The usage of this high amount of column reinforcement is to assure the corbel failure in the testing phase. The yield strength for the steel bar are 467, 589 and 536 MPa for 8, 10, and 12 mm, respectively. Figure 1 Geometry and reinforcement details of the tested specimen 2.2. Concrete materials and mix design During the design phase of the experimental program, 30 to 35 MPa concrete compressive strength was chosen to mimic relatively real strength used in practice for similar members. The mix was proportioned by weight at 1:1.7:2.2:0.45 for cement, sand, gravel and water respectively. Locally available materials were used throughout and showed conformity with the specifications required CFRP material description and application Sika Wrap Hex-230C type was used as CFRP strengthening sheets. While, the epoxy resin is of type Sikadur-330. The technical properties of the CFRP sheets and 12 editor@iaeme.com

3 Use of CFRP Laminates for Strengthening of Reinforced Concrete Corbels impregnation resin used in this study as given by the manufacturer are presented in Table (1) and (2), respectively. A two-part impregnation resin (Sikadur -330) is mixed in required proportion, until the color is a uniform grey, and then applied with a special tool to the concrete surface to a thickness of ~1.5 mm. The adhesive is also applied to the CFRP sheet to the same thickness. The CFRP sheet is then placed on the concrete,epoxy to epoxy, and a rubber roller is used to properly seat the CFRP sheet by exerting enough pressure so the epoxy is forced out on both sides of the CFRP sheet and the adhesive line did not exceed 2mm in thickness. After the specimens are confined, they are left in laboratory conditions for 7 days after gluing to be completely cured. Table 1 Technical properties of CFRP Laminates Type SikaWarp Hex-230C Tensile strength (MPa) Elongation at Break (%) Tensile modulus (GPa) Thickness (mm) Weight (g/m 2 ) >3500 > Table 2 Technical properties of impregnation resin Type Tensile strength (MPa) Elongation at Break (%) Flexural modulus (MPa) Sikadur Pot live (minute) at 15C o :90min at 35C o :30min Density (kg/l) Specimens identification and strengthening schemes The CFRP strengthening patterns were chosen to modeling the possibilities in the field, i.e. some of the CFRP patterns that cannot apply in the field had been neglected. Two control specimens of CON1 and CON2 with no strengthening had been applied. Table (3) in conjunction with Figure (2) illustrates the specimen identification and CFRP strengthening schemes. For investigation of influence of number of layers and the bond type six different specimens had been made, three of which H1P,H2P and H3P with CFRP bond only on the face of the corbels and the other three H1F, H2F, and H3F with full bonded CFRP over all the faces of specimens. All of the strips in the six specimens were horizontally applied. The specimen H2A had two horizontal strips partially bonded on the shear face of the corbels. This specimen had upgraded bond strength with the usage of four CFRP anchor (two for each of strips) applied near the edge of corbel. The applying of anchors was after the casting and curing of the specimens. Specimens 2H2F and 3H2F had two and three layers of pair horizontal CFRP strips respectively, the two specimens V6F and IF had fully warped CFRP strips with orientation of 90 and 45 degrees, respectively. The last three specimens were for measuring the efficiency or repair technique. Specimens H3V6F-0%, H3V6F-35%, and H3V6F-70% are tested with damage ratio of 0%, 35%, and 70% respectively. All of these three specimens were repaired with three full warped horizontal strips, and had additional of six fully warped vertical strips editor@iaeme.com

4 Khalid K. Shadhan and Mohsen M. Mohammad Kadhim Table 3 Characteristics of Corbel specimens Specimen identification Compressive strength, f cu No. of CFRP strips No. of CFRP layers CFRP Bond type Damaged ratio, (%) CON Nil Nil Nil Nil CON Nil Nil Nil Nil H1P Partial warp 0 H2P Partial warp 0 H3P Partial warp 0 H1F Full warp 0 H2F Full warp 0 H3F Full warp 0 H2A CFRP anchors 0 2H2F Full warp 0 3H2F Full warp 0 V6F Full warp 0 IF Full warp 0 H3V6F Full warp 0 H3V6F Full warp 35 H3V6F Full warp EXPRIMENTAL RESULTS The main objective of this research is to investigate the efficiency of CFRP when used as strengthening and/or repairing material for reinforced concrete corbels, and its influence on the load carrying capacity and behaviors of the corbels. Test results were analyzed based on ultimate load carrying capacity, load-deflection curve, crack patterns and failure mode. Table (4) in conjunction with Figure (3) and (4) shows test results for all corbels specimens. The results show that the CFRP strips can delay the formation of first crack.this delay is mainly depending on the CFRP existence near the weak points (i.e. corbelcolumn junction) and the amount of CFRP in the specimens. This is due the high moment at this point. After this crack a shear crack began to develop at the loading area from the end of the bearing plate continuing toward the corbel face with an angle varied from 40 to 60 degrees. This was the typical case in all of the tested specimens except the specimen with inclined CFRP strips, specimen IF, which the typical first crack did not took place at column-corbel junction point due to existence of CFRP strips exactly at the very point of junction. In this specimen the first crack was a shear crack at the loading area. While all control and strengthened specimens failed suddenly and exhibited no ductility. Generally, the deflections of the specimens strengthened with CFRP were lower than the deflections of the corresponding control beam, Figure (3). The application of CFRP strips proved to be an effective method of strengthening reinforced concrete corbels. The test results show that the increase varied from 16% in specimen H1P to 71% in specimen V6F in load carrying capacity of un-strengthened 14 editor@iaeme.com

5 Use of CFRP Laminates for Strengthening of Reinforced Concrete Corbels corbels. These increases are depending on many factors such as number of CFRP strips, Types of bonding, Angle of bonding, damage ratio, and CFRP pattern. Figure 2 Specimens identification and strengthening schemes 15 editor@iaeme.com

6 Khalid K. Shadhan and Mohsen M. Mohammad Kadhim Figure 2 Continued The crack patterns after the failure of the specimens were different from specimen to another. While some specimens had several miner crack, others had few major ones. It has been found that the application of CFRP strips restrained the cracking propagation; however the quantity and location of the CFRP strips had been found to have a great influence on the crack pattern. Figure (4) gives illustrated figures of 16 editor@iaeme.com

7 Use of CFRP Laminates for Strengthening of Reinforced Concrete Corbels Specimen cracking pattern in the tested specimens. The black lines in the figure represent the cracks while the gray areas represent the crushing of the concrete The CFRP pattern showed a great influence on the failure mechanism of the tested specimens. While the un-strengthened control specimens had elastic failure with widening of shear cracks, all of the strengthened specimens, except one specimen, had sudden failure. This specimen was H3P that failed in semi-ductile way. That is because this specimen failed by debonding of CFRP accompany with concrete cover separation, and due to elastic characteristics of concrete that influence the failure mechanism this specimen failed in this unique way. The specimens H1P, H2P, H3P, H1F, H2F, 2H2F, 3H3F, and H2A failed by debonding of one (or more) CFRP strip from concrete surface, although the position of debonding differ in the specimens. For the specimens that had increased bond strength i.e. H1F, H2F, 2H2F, 3H2F, and H2A, the debonding was at the mid- span of the specimen, while for other specimens that had no bonding strengthening, i.e. H1P, H2P, and H3P the debonding was at the sides of the specimen. For the specimens IF and V6F the failure was due to rupture in one of CFRP strips. For specimens H3F, H3V6F0%, H3V6F35%, and H3V6F70% the failure was by combined factor of CFRP strips deboning and rupture. Cracking load, (kn) Table 4 Ultimate load capacity and failure mode for tested beams *Increase in cracking load, (%) Ultimate load,(kn) *Increase in ultimate load, (%) Failure mode CON1 90 Nil 211 Nil Concrete crushing CON2 83 Nil 242 Nil Concrete crushing H1P CFRP debonding H2P CFRP debonding H3P CFRP debonding H1F CFRP debonding H2F CFRP debonding H3F CFRP rupture H2A CFRP debonding 2H2F CFRP debonding 3H2F CFRP debonding V6F CFRP rupture IF CFRP rupture H3V6F CFRP rupture H3V6F CFRP rupture H3V6F CFRP rupture *compared with the average of the control specimens 17 editor@iaeme.com

8 Khalid K. Shadhan and Mohsen M. Mohammad Kadhim (a): H1P, H2P and H3P (b): H1F, H2F and H3F (c): H2F, 2H2F and 3H3F (d): H3F, IF and V6F (e): H2P, H2B and H2F (f): H3V6F (0), H3V6 (35) and H3V6F (70) Figure 3 Load-Deflection curve for tested specimens 18 editor@iaeme.com

9 Use of CFRP Laminates for Strengthening of Reinforced Concrete Corbels Figure 4 Crack patterns after failure for corbels specimens 19 editor@iaeme.com

10 Khalid K. Shadhan and Mohsen M. Mohammad Kadhim 4. CONCLUSIONS 1. Using carbon fiber reinforced polymer (CFRP) strips is an effective technique for strengthening of reinforced concrete corbel. The ultimate carrying capacity is increased by 16% to 71%. 2. The orientation of CFRP strips found to have the largest effect on increased load carrying capacity, the percentage of increase differ from 35% for horizontal strips to 71% for vertical strips. 3. The presence of CFRP laminates can increase the cracking load of R.C corbels by acceptable amount. This increase is found to vary from 12% in specimen with horizontal CFRP strips to 46% for specimen with vertical CFRP strips. 4. The damage ratio of the specimen has reverse effect on the effectiveness of the repair. Thus for corbels with high damage ratio (greater than or equal 70%), the application of CFRP laminates are not an effective technique to improve the characteristics of the corbels. 5. The debonding in the CFRP laminates (which is the main mode of failure) is sudden and have a brittle nature. Thus for more effective strengthening, using of CFRP anchors to improve the bond strength is recommended. REFERENCES [1] Abed Attiah M., (2010), Behavior of reinforced concrete corbels strengthened with carbon fiber reinforced polymer strips, PhD thesis, Basrah University. [2] Ahmad, S., Elahi, A., Kundi, S., and Haq, W. (2013), Investigation of shear behavior of Corbel beams strengthened with CFRP, Life Science Journal [3] American Concrete Institute, ACI Committee 318. (2011) Building code requirements for structural concrete (ACI ) and commentary, American Concrete Institute, Farmington Hills, MI [4] Corry, R. W. and Dolan, C. W. Strengthening and repair of a column bracket using a carbon fibre reinforced polymer (CFRP) fabric. Journal of PCI Journal, January-February, 2001, pp [5] Elgwady, M. A., Rabie, M., and Mosatafa, M. T. Strengthening of corbles using CFRP, an experimental program, Cairo university, Giza, Egypt, [6] Erfan, A. M., Abdel-Rahman, G. T., Nassifand, M. K. and Hammad, Y. H. Behavior of reinforced concrete corbels strengthened with CFRP fabrics, Benha University, [7] Ivanova, I., Assih, J., Li, A. and Delmas, Y. Influence of Fabrics Layers on Strengthened Reinforced Concrete Short Corbels. International Journal of Civil Engineering and Technology, 5(11), 2014, pp editor@iaeme.com