STRENGTHENING OF NORMAL AND HIGH STRENGTH CONCRETE CORBELS WITH HORIZONTAL AND INCLINED STRIPES OF CARBON FIBER

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1 International Journal of Civil Engineering and Technology (IJCIET) Volume 7, Issue 3, May June 2016, pp , Article ID: IJCIET_07_03_027 Available online at Journal Impact Factor (2016): (Calculated by GISI) ISSN Print: and ISSN Online: IAEME Publication STRENGTHENING OF NORMAL AND HIGH STRENGTH CONCRETE CORBELS WITH HORIZONTAL AND INCLINED STRIPES OF CARBON FIBER Asst. Prof. Aamer Najim Abbas and Eng. Wahig Abrahim Abd Al-kareem Al-Mustansiriya University, Iraq ABSTRACT In this study, there were two modes of applying the carbon fiber strips on reinforced concrete corbels: the first one is application three horizontal strips and its width is (50 mm) and the other is applying three inclined strips with angle about (45 ), the both modes applied on the two faces of concrete corbel specimens. Two types of concrete were used in this study; normal strength (28 MPa) and high strength concrete (57 MPa). Each types strengthening with two modes of carbon fiber stripes. The ultimate and cracking capacity of tested specimens were improved as a result of strengthening with carbon fiber strips, in addition to development of energy absorption and stiffness characteristics. Key words: Strengthening, Carbon Fiber, High Strength Concrete, Carrying Capacity, Cracking Capacity, Energy Absorption, Deflection. Cite this Article: Aamer Najim Abbas and Wahig Abrahim Abd Al-Kareem, Strengthening of Normal and High Strength Concrete Corbels with Horizontal and Inclined Stripes of Carbon Fiber. International Journal of Civil Engineering and Technology, 7(3), 2016, pp INTRODUCTION Sometimes cracks develop on the concrete surface in a structure because of weakness designing, incorrect placement of bars of reinforcement, temperature variations, change in use of building or due to any other unforeseen reason. In order to improve the strengthening of structures, one of the most common strengthening methods is wrapping by carbon fiber sheets. (1) Carbon fibers could be described as high-performance materials. They could be defined as fibers containing at least 92 % weight of carbon gained by the controlled pyrolysis of convenient fibers. The carbon fibers have been intensively used because editor@iaeme.com

2 Aamer Najim Abbas and Wahig Abrahim Abd Al-kareem they mostly have efficient tensile features, excellent creep resistance, good thermal and electrical conductivities, high thermal and chemical stabilities, low densities and the absence of oxidizing agents. (2) Carbon fibers are used in composites with a lightweight matrix. Where strength, stiffness, lower weight, and outstanding fatigue characteristics are critical requirements, carbon fiber composites are perfectly appropriated to applications. Also carbon fiber composites can be used in the occasion because of the importance of high temperature, high damping and chemical inertness. (3) The main target of using carbon fibers as reinforcement is their resistance to corrosion and rusting. The advantages of using carbon fiber in reinforced concrete depends on an important factors such as length, shape, cross section, bond characteristics of carbon fibers and fiber content. (4) Carbon fiber has been produced with higher flexural strength; higher shear strength and higher modulus of elasticity, hence enhance the deflection of structural members. (4) The main benefits of (CFRP) could be described as increase the durability of strengthening members against alkalis, aggressive materials and corrosion, achieve very high strength with tensile strength greater than (2400MPa) and modulus of elasticity greater than (165 GPa), enhance the fire resistance of structures, saving the cost of maintains, decrease the construction period, it's light weight as a result its easily handling and transportation in the form of rolls, CFRP meets the requirements of using in all wanted lengths, decrease the mechanical fixing and excellent fatigue resistance. But the disadvantages of CFRP could be described as it's susceptible to mechanical impacts and the high cost. (5) CORBELS DIMENSIONS AND REINFORCEMENT DETAILS It had been casted and tested six concrete corbels, three of them is normal strength concrete (NSC), and the other corbels is high strength concrete (HSC). All specimens designed according to shear until failure, see Table (1). The dimensions of column supporting the both opposite sides of corbels were (150 mm depth x 200 mm width x 650 mm height), and the concerned column reinforced at corners with (4ɸ10 mm) deformed longitudinal bars and (8 mm) diameter of closed ties with spacing (150mm) center to center. The dimensions of corbels are shown in Figure (1) below, Corbels were reinforced with (3Ø12mm) of deformed steel bars as a main reinforcement editor@iaeme.com

3 Strengthening of Normal and High Strength Concrete Corbels with Horizontal and Inclined Stripes of Carbon Fiber Specimen Number Figure 1 Description of Tested Corbels Table 1 Details of Concrete Corbels Type of Concrete Mode of Carbon Fiber Arrangement C1 NSC --- C1T NSC Horizontal Strips C1S NSC Inclined Strips C4 HSC --- C4T HSC Horizontal Strips C4S HSC Inclined Strips Concrete Compressive Strength According to (BS1881:Part116) (6), the compressive strength test of concrete ( ) was performed using standard 150 mm3 concrete cubes and 150 mm 300 mm concrete cylinders respectively. Two types of concrete mixes are used (NSC) of (28) MPa and (HSC) of (57) MPa compressive strength. Splitting Tensile Strength (f t) According to (BS1881: Part 117) (7) the splitting tensile test had been carried out in the laboratory by using (150mm*300mm) concrete cylinder, see Table (2) editor@iaeme.com

4 Aamer Najim Abbas and Wahig Abrahim Abd Al-kareem Table 2 Tensile Strength of Concrete Type of Concrete Tensile Strength (MPa) Normal Concrete 1.41 High Strength Concrete 2.92 Flexural Strength (f r ) The flexural strength can be expressed by the modulus of rupture. It is carried out by using (100*100*500) mm prism, loaded with two point loads hydraulic machine of (50 kn ) capacity according to (ASTM C78-02) (8), see Table (3). Table 3 Modulus of Rupture of Concrete Type of Concrete Modulus of Rupture (MPa) Normal Concrete 5.16 High Strength Concrete 7.88 Static Modulus of Elasticity (E c ) The modulus of elasticity is calculated by plotting the stress to strain diagram of loaded axially cylinder. As recommended by (ASTM C469) (9) the chord-modulus method has been used, see Table (4). Type of Concrete Table 4 Static Modulus of Elasticity of Concrete Static Modulus of Elasticity (MPa) Normal Concrete High Strength Concrete Testing of Steel Bars According to ASTM A615 (10), the tensile strength test of steel bars was performed, see Table (5). Nominal Diameter (mm) Table 5 Properties of Steel Bars Bar Type f y f u E s ** Elongation % (MPa) (MPa) (GPa) 8 Deformed Deformed Deformed Cement Tables (6) and (7) illustrate the physical and chemical properties of cement used in this research. The test was performed according to American Specifications ASTM- C150 (11) editor@iaeme.com

5 Strengthening of Normal and High Strength Concrete Corbels with Horizontal and Inclined Stripes of Carbon Fiber Table 6 Physical Properties of Cement Physical Properties Fineness using Blain air permeability apparatus (m 2 /kg) Test Results 392 Initial setting time using Vicat's instruments (hr: min.) 1:48 Final setting time using Vicat's instruments (hr: min.) 4:51 Safety (soundness) using autoclave method (%) 0.01 Compressive strength for cement paste cube (70.7mm) at : (3days) in (N/mm 2 ) or (MPa) Compressive strength for cement paste cube (70.7mm) at : (7days) in (N/mm 2 ) or (MPa) 26.6 Compound Name Table 7 Chemical Composition of Cement Compound Chemical Composition % (weight) Silica SiO Alumina Al2O Iron Oxide Fe2O Lime CaO Magnesia MgO 1.93 Sulfate SO Insoluble Residue I.R Loss On Ignition L.O.I 3.68 Tricalcium aluminates C3A 8.29 (From X.Ray diffraction) Lime Saturation Factor L.S.F 0.83 Fine Aggregate Natural sand with fineness modulus of (2.69) is used. Fine aggregate characterized by rounded particle shape and smooth textures. Only the passing sand from the sieve (4.75mm) is requires achieving the requirement of mixing. The grading is shown in Table (8). No. Sieve size (mm) Table 8 The Grading of Fine Aggregate (Sand) Present work of fine aggregate (% passing) BS882:1992 limit (% passing) (12) editor@iaeme.com

6 Aamer Najim Abbas and Wahig Abrahim Abd Al-kareem Coarse Aggregate The maximum size of crushed gravel is (14mm). The grading is shown in Table (9), which confirms to the BS882:1992 Specification (12). No. Table 9 The Grading of Coarse Aggregate (Gravel) Sieve size (mm) Present work of coarse aggregate (% passing) BS882:1992 limit (% passing) (12) Admixtures In order to produce high strength concrete mixes, super-plasticizer based on poly carboxylic ether must be used. Also, it can be called (high range water reducing agent HRWRA). Glenium51 is one of the new generation of polymer which mainly used in designed super-plasticizer; the normal dosage for Glenium51 is (( ) L/100kg) of cement. Table (10) illustrates the typical properties of super-plasticizer. Table 10 Typical properties of Glenium 51 No. Main action Concrete super plasticizer 1 Color Light brown 2 ph. Value Form Viscous liquid 4 Chlorides Free of chlorides 5 Relative density gm/cm 25 C 6 Viscosity C 7 Transport Not classified as dangerous 8 Labeling No hazard label required Carbon Fiber The technical data information of CFRP sheets and epoxy that used in this work can be clearly seen from Tables (11) and (12) respectively. Table 11 SikaWrap Hex-230C (Carbon Fiber Fabric) Technical Data * Property Results Fiber type High strength carbon fibers Fiber orientation The fabric equipped with special weft fibers which prevent Loosening of the roving (heat set process). Areal weight 225 g/m2 Fabric design thickness 0.13 mm (based on total area of Carbon fibers). Tensile strength of fibers 3500 MPa Tensile E modulus of fibers 230 GPa Elongation at break 1.5 % Fabric length/roll 45.7 m Fabric width 305/610 mm editor@iaeme.com

7 Strengthening of Normal and High Strength Concrete Corbels with Horizontal and Inclined Stripes of Carbon Fiber Property Appearance Table 12 Sikadur-330 (Impregnating Resin) Technical Data* Comp. A: white Comp. B: grey Part A+B mixed: light grey 1.31 kg/l (mixed) Results Density Mixing ratio A : B = 4 : 1 by weight ** Open time 30 min (at + 35C ) Viscosity Pasty, not flowable Application temperature + 15C to + 35 C (ambient and substrate) Tensile strength 30 MPa (cured 7 days at +23 C) Flexural E-modulus 3800 MPa (cured 7 days at +23 C) Concrete Mix Proportions According to ACI committee (13), two concrete mixtures were designed. Anyway, to ensure that the required strengths (28 and 57 MPa) were achieved, many trial mixes were made. The proportions of the suitable mix are as given in Table (13). Table 13 Concrete Mixes (MPa) Cement (Kg/m 3 ) Sand (Kg/m 3 ) Gravel (Kg/m 3 ) W/C Superplastizer Liter/m DISCUSSION AND RESULTS General Behavior and Failure Patterns At the early stages of load application, the specimens appear high stiffness and show high resistance to loads until appearance of the first crack, the vertical displacement is small and no cracks appear. After appearance of the first crack, the stiffness begins to decrease and the vertical displacement begins to increase. At this stage, the flexural cracks begin to appear at the tension face of corbels near the column; the cracks are narrow and increase with load increment. At the advanced stages of loading, diagonal shear cracks start to develop near the supports and propagate quickly towards the column face with an angle about 60. These cracks are wider than the flexural cracks. The failure is sudden and uncontrolled except the strengthened specimens with carbon fiber strips and steel fibers which the failure is more ductile than the un-strengthened specimens. The plates (1) to plate (6) show the failure mode of tested specimens editor@iaeme.com

8 Aamer Najim Abbas and Wahig Abrahim Abd Al-kareem Plate (1) Failure Mode of Corbel (C1) Plate (2) Failure Mode of Corbel (C1T) Plate (3) Failure Mode of Corbel (C1S) Plate (4) Failure Mode of Corbel (C2) Plate (5) Failure Mode of Corbel (C2T) Plate (6) Failure Mode of Corbel (C2S) Load deflection Behavior By studying the load- deflection curve in Figure (2) and Figure (3), it has been found that all the specimens exhibit three stages as indicated below 1. The first linear stage This stage started from the beginning of load application with relatively straight line until the appearance of first crack, this stage represent the elastic behavior of concrete and steel just because the stresses in concrete and steel virtually small as compared with later stages, in other words the specimens returned to the original manner after the load releasing because there is no local slipping between concrete and steel. At this stage of load deflection curves, the specimens have the highest value of flexural rigidity. 2. The second linear stage In this stage there was a change in the slope of load versus deflection curves in comparison with pre cracking phase. This stage started from the appearance of first crack up to the yield of tensile steel bars, this stage characterized by plastic behavior. In other words, specimens did not return to the original manner after the load releasing. At this stage, the specimens have less value of stiffness because extension of the flexural cracks and the loose of bond characteristics between the concrete editor@iaeme.com

9 Load-kN Strengthening of Normal and High Strength Concrete Corbels with Horizontal and Inclined Stripes of Carbon Fiber matrix and steel bars. Thus, the less toughness of structural member means large deformations. 3. The non-linear stage This stage starts at the yield of tensile steel bars until the failure of specimens. At this stage, the specimens have stiffness less than the previous stages because of the increasing the number of cracks, width of cracks and loose of bond between steel and concrete Corbel - C1- Corbel -C1S- Corbel -C1T Figure 2 Load-deflection Curve of Normal Strength Concrete Corbels Corbel -C2- Corbel -C2S- Corbel -C2T Deflection-mm Figure 3 Load-deflection Curve of High Strength Concrete Corbels editor@iaeme.com

10 Aamer Najim Abbas and Wahig Abrahim Abd Al-kareem STRENGTHENING EFFICIENCY OF CARBON FIBER STRIPS Effect of Carbon Fiber Strips on carrying capacity The using of carbon fiber strips in strengthening the concrete corbels effectively enhances the ultimate strength of the specimens. The improvement of strengthening corbels with horizontal strips less than the other specimens with inclined strips because the later are perpendicular on cracks propagation and this configuration play a vital rule in delaying the extension of cracks such as the specimen C1S which have an increasing ratio about 60% over the reference specimen C1, while the improvement of specimen C1T reached to in comparison with same reference specimen. Accordingly, the increasing of the specimen strength of C2T is about % while the specimen C2S is about % in comparison with reference specimen C2, see Table (14). Table 14 Effect of Carbon Fiber Strips on Carrying Capacity Specimen No. Ultimate Loading Percentage of Capacity Improvement (%) C C1T C1S C C2T C2S Effect of Carbon Fiber Strips on Cracks Appearance The strengthening mode does not effect on the appearance of first crack in normal concrete corbels. The amount of improvement is about (63.636%) for specimens C1T and C1S in comparison with reference specimen C1. But, the improvement of high strength concrete corbels is clearly seen in specimens (C2T and C2S) which have the ratio of improvement about 116% and 60% respectively in comparison with reference specimen C4, see Table (15). Table 15 Effect of Carbon Fiber Strips on Cracking Capacity Specimen No. Ultimate Loading Percentage of Capacity Improvement (%) C C1T C1S C C2T C2S Effect of Carbon Fiber Strips on Energy Absorption The energy absorption of tested specimens can be calculated from the area under the load-deflection curve. The using of carbon fiber strips as a strengthening method improved the energy absorption of tested corbels. The strengthened specimens of normal strength concrete C1T and C1S increased about 68% and % respectively as compared with editor@iaeme.com

11 Strengthening of Normal and High Strength Concrete Corbels with Horizontal and Inclined Stripes of Carbon Fiber reference specimen C1. Also, the high strength concrete corbels C2T and C2S have an improvement about % and % respectively over reference specimen C2, see Table (16). The inclined strengthening configuration achieved good energy absorption in comparison with horizontal configuration. The amount of energy absorption gives an indication about its ductility; the greater amount means high ductility. Therefore, the strengthened specimens have a ductility more than the non-strengthened specimens, and the inclined configuration of carbon fiber gives ductility greater than the horizontal one. Conclusions Table 16 Effect of Carbon Fiber Strips on Energy Absorption Specimen No. Ultimate Loading Percentage of Capacity Improvement (%) C C1T C1S C C2T C2S The ultimate carrying capacity of tested corbels is affected positively by using carbon fiber strips as a strengthening method. 2. The cracking capacity of normal strength concrete corbels does not affected by using carbon fiber strips, while the high strength concrete corbels achieved good improvement. 3. The amount of energy absorption increased as a result of using carbon fiber strips. 4. The using of carbon fiber strips does not exchange the failure type of tested corbels. 5. The strengthened corbels by carbon fiber strips have a good stiffness in comparison with un-strengthened corbels. REFERENCES [1] Raghavendra R. H., Atul D. and M. G. Kamath, CARBON FIBERS, April, [2] Norazman M. N., Mohd H. A. B. and Mohammed A. Y., Carbon Fiber Reinforced Polymer (CFRP) as Reinforcement for Concrete Beam, International Journal of Emerging Technology and Advanced Engineering, ISSN , ISO 9001:2008 Certified Journal, 3(2), February 2013, [3] Alaa H. Kadhim AL-Musawi, Experimental Study of Reinforced Concrete Columns Strengthened with CFRP under Eccentric Loading, MS.c. Thesis, University of Al-Mustansiriya, July p. [4] Bayda M. H., Stiffness Enhancement of Reinforced Concrete Beams Strengthened with CFRP Sheeets, MS.c. Thesis, University of Al Mustansiriya, August p. [5] Committee Euro-International du Beton, 1993, Model Code for Concrete Structures, Federation International Precontrainte (CEB FIP) (MC-90), Thomas Teleford, London, UK editor@iaeme.com

12 Aamer Najim Abbas and Wahig Abrahim Abd Al-kareem [6] British Standard Institute, Method for Determination of Compressive Strength of Concrete Cubes, BS 1881: part116: [7] BS , Method for Determination of Tensile Splitting Strength, January [8] ASTM C78 02, Standard Test Method for Flexural Strength of Concrete (Using Simple Beam with Third-Point Loading, Annual Book of American Society for Testing Concrete and Materials, Philadelphia, Pennsylvania, [9] ASTM C 469, Standard Test Method for Static Modulus of Elasticity and [10] Poisson s Ratio of Concrete in Compression, Annual Book of American Society for Testing Concrete and Materials, Philadelphia, Pennsylvania, [11] ASTM A615. Standard Specification for Deformed and Plain Carbon-Steel Bars for Concrete Reinforcement, Annual Book of American Society for Testing Concrete and Materials, Philadelphia, Pennsylvania, [12] ASTM C-150, Standard Specification for Portland Cement, ASTM International, [13] I. Siva Kishore and Ch. Mallika Chowdary, A Study on Waste Utilization of Marble Dust In High Strength Concrete Mix. International Journal of Civil Engineering and Technology, 6(12), 2015, pp.1 7. [14] Thallapaka Vishnu Vardhan Reddy, K. Rajasekhar and Seelanani Janardhana, Study and Performance of High Strength Concrete Using With Nano Silica and Silica Fume. International Journal of Civil Engineering and Technology, 6(11), 2015, pp [15] Aamer Najim Abbas Ali Sabah Ahmed and Saad Khalaf Mohaisen, Rehabilitation of Normal and Reactive Powder Reinforced Concrete Beams Using Epoxy Injection Technique. International Journal of Civil Engineering and Technology, 7(3), 2016, pp [16] BS , Specification for Aggregate from Natural Source for Concrete, December [17] ACI , Standard Practice for Selecting Proportions for Normal, Heavyweight, and Mass Concrete, American Concrete Institute, ACI , Reapproved 2002, 38PP editor@iaeme.com