BEHAVIOR OF SHORT CONCRETE COLUMNS REINFORCED BY CFRP BARS AND SUBJECTED TO ECCENTRIC LOAD

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1 International Journal of Civil Engineering and Technology (IJCIET) Volume 6, Issue 10, Oct 2015, pp Article ID: IJCIET_06_10_002 Available online at ISSN Print: and ISSN Online: IAEME Publication BEHAVIOR OF SHORT CONCRETE COLUMNS REINFORCED BY CFRP BARS AND SUBJECTED TO ECCENTRIC LOAD Prof. Dr. Nameer A. Alwash and Ahmed Hamid Jasim Civil Engineering, College of Engineering, Babylon University-Iraq ABSTRACT This paper focused to study the behavior of short concrete column reinforced by CFRP bars and subjected to eccentric axial load. It was based on ten concrete columns with normal concrete. All columns were identical in size and the nominal dimensions. The model dimensions selected in the present investigation was a square section of 140*140mm and a total length of 820mm. The length between corbels (middle portion) was 400mm, for eccentric loaded columns, variable values of eccentricity (e) were studied herein. Three of columns were cast as control specimens or for the purpose of comparison, one without reinforcement and two with longitudinal steel reinforcement, the other specimens were as longitudinal reinforced by CFRP bars. The behavior of these columns will be quantified by conducting axial load moment (P-M) and load deflection (P-Δ) response analysis, the latter is used to study the secondary column effect. Additionally, the failure mechanisms of these columns will also be identified and quantified. Ultimately, The experimental results were studied and compared with data published by a nonlinear three dimensional finite element (F.L.) analyses have been used to analyze the tested columns, This has been done by using the available computer program (ABAQUS/Standard 6.13.). The study concludes that the CFRP bars contribute about 14.51% of column capacity under axial load. For Column reinforced with CFRP bars gave some decrease in ultimate load 3.78% under axial load with respect to the column reinforced with steel bars, while it gave clear increase in ultimate load 38.21% under load eccentricity e/h values (0.857). It can be concluded that using CFRP reinforcement has a significant effective on ultimate load capacity of columns with high eccentricity. Key words: Short Concrete Column, CFRP Bars, eccentricity editor@iaeme.com

2 Prof. Dr. Nameer A. Alwash and Ahmed Hamid Jasim Cite this Article: Prof. Dr. Nameer A. Alwash and Ahmed Hamid Jasim. Behavior of Short Concrete Columns Reinforced by CFRP Bars and Subjected To Eccentric Load. International Journal of Civil Engineering and Technology, 6(10), 2015, pp INTRODUCTION Corrosion of the reinforcement is one of the major reasons for deterioration of reinforced concrete structures with conventional steel. Fiber reinforced polymer (FRP) composites can be a solution to overcome the corrosion problem of reinforced concrete structures which is exposed to harsh environmental conditions. The common types of FRP composites for concrete construction include carbon, glass, and aramid ;CFRP, GFRP, and AFRP respectively. In addition to be corrosion resist, FRP composites possess light weight, nonmagnetic and strength comparable or greater to that of steel strength depending on the types (Choo, C.C.2005) (1). 2. LITERATURE REVIEW Paramanantham (1993) (2) tested seventeen 8 x 8 x 72 in. (200 x 200 x 1800 mm) concrete beam-columns reinforced with GFRP bars. From the results of experiments he reported that GFRP bars would be stressed up to 70 percent of its ultimate strength in pure flexure, and up to 20 to 30 percent in compression. 3. PROGRAM OF THE WORK The experimental program was intended to focus the behavior of concrete columns with a square section reinforced with CFRP longitudinal bars and subjected to eccentric axial load. It was based on ten concrete columns with normal concrete cast in laboratory of Civil Engineering Department of Babylon University. All columns were identical in size and the nominal dimensions. The model dimensions selected in the present investigation was a square section of 140*140mm and a total length of 820mm. The length between corbels (middle portion) was 400mm, see figure (1). For eccentric loaded columns, variable values of eccentricity e were studied herein. The test specimen details are summarized in table (1), each column is identified by symbols where the first two symbols set describe the kind of longitudinal reinforcement; (CF) refers to carbon fiber reinforced polymers bars, (PC) denotes plain concrete without reinforcement while (ST) denotes main steel reinforcement. The third symbol is a number that refers to the value of load eccentricity in mm. The forth symbol describes the average compressive strength (fc`) according to experimental compressive strength result of concrete specimens of columns; (A) refers to fc`=29.5 MPa editor@iaeme.com

3 Behavior of Short Concrete Columns Reinforced by CFRP Bars and Subjected To Eccentric Load Column designation Table 1 Details of the tested columns specimens. Longitudinal reinforcement Longitudinal reinforcement ratio ρ % Eccentricity e (mm) e/h (mm) CF0A CF1A CF2A CF4A CF6A CF8A CF12A ST0A ST12A PC0A Figure 1 CFRP reinforced column details (All dimensions in mm). The columns (except PC0A) contained the same transverse reinforcement of deformed bars with 4.5mm and spaced at 92mm as well as the corbels reinforced well to prevent premature failure at this portion of the specimens during the tests and to concentrate the failure in the middle portion. 4. MATERIAL PROPERTIES Steel Reinforcement: two types of steel reinforcing bars were used in the tested columns of this study: first, deformed steel bar nominal diameter Ø10mm were used as corbel reinforcement. Second, deformed steel bars nominal diameter Ø5mm were used as transverse ties and longitudinal reinforcement. The results of testing steel reinforcement are summarized in table (2) editor@iaeme.com

4 Prof. Dr. Nameer A. Alwash and Ahmed Hamid Jasim Nominal diameter (mm) Table 2 Specification and test results of steel reinforcing bars Measured diameter (mm) Yield stress (MPa) Ultimate strength (MPa) CFRP Reinforcement: Aslan 201 series were used in this study, as standard product. Features of CFRP bars are sand coating and helical wrap surface. The nominal diameter is 6 mm which was used to reinforce the CFRP RC column specimens in the longitudinal directions. Table (3) represents the results of tests as which provided from the manufacturer of the CFR Pbars. Nominal Diameter (mm) Nomina l Area (mm 2 ) Table 3 Aslan 200 mechanical properties. Guaranteed Tensile Strength (f*fu ) (MPa) Ultimate Tensile Load (kn) Tensile Modulus of Elasticity(GPa) Ultimate Strain (%) % 5. THE TEST RIG AND EQUIPMENT Columns were tested in a vertical position and under compressive eccentric loading with hinged-hinged end conditions up to failure. The applied load are measured by a hydraulic machine with capacity of 670 kn, the load was applied with a loading increment rate of about 10% of estimated ultimate load of column. During the test of the columns, the main characteristics of their structural behavior were measured at every stage of loading. The load at first crack as well as the ultimate failure load. The measurements, which were recorded during the tests, are the following: strain in concrete and displacements. The linear variable differential transducers (LVDT) with mm accuracy were used to measure the lateral deflection at the tension side of column in mid-height. Figure (2) shows testing column. Figure 2 shows testing column editor@iaeme.com

5 Behavior of Short Concrete Columns Reinforced by CFRP Bars and Subjected To Eccentric Load 6. RESULT AND DISCUSSIONS 6.1 First Cracking Load and Cracks Pattern The visible first cracking load (Pcr) and ultimate load(pu) which are obtained from experiments are presented in table (4).From this table, one can see the visible first crack load of all the specimens varied from (9.4%) to (80.1%) ultimate loads. This big change in the proportions is due to the compressive strength, and eccentricity of load. The columns with concentric load or small eccentricity were under compression stress and therefore the cracks need big load to appear. Because the stress is concentrated near the supports so cracks appear firstly in the supports (corbel area). Whereas the first cracks appear in the tension face at middle of column or near of it when the column under high eccentricity of loading due to the concrete tensile stress reaches the ultimate tensile strength. The experimental results of columns indicate that the CFRP bars contributed 14.51% of column capacity for CF0A under axial load comparison with column PC0A. On the other hand, when making a comparison between the columns that reinforced with steel and CFRP bars show that the column reinforced with CFRP bars gave some decrease in ultimate load 3.78% under axial load with respect to the column reinforced with steel bars, while it gave clear increase in ultimate load 38.21% under load eccentricity 12cm that agreed to e/h values (0.857). Table 4 Test results of the ultimate load and the first crack load and location of first crack. Column's Symbol Ultimate load Pu (kn) First crack load Pcr (kn) (Pcr/Pu) % Location of first crack load PC corbel ST corbel CF0A corbel CF1A corbel CF2A corbel under loading CF4A mid-height CF6A cm above mid-height CF8A mid-height CF12A cm above mid-height ST12A mid-height 6.2. Deflection Lateral displacements measurements at mid-height were taken until the ultimate load. The recorded loads, lateral displacement at mid-height of Columns CF1A, CF2A, CF4A, CF6A, CF8A, CF12A and ST12Aare presented in figure (3) editor@iaeme.com

6 Load kn Prof. Dr. Nameer A. Alwash and Ahmed Hamid Jasim CF1A CF2A CF4A CF6A CF8A CF12A ST12A Lateral Displacement mm Figure 3 Load - Lateral Displacement at mid-height of the columns. Based on figure (3), the following observations can be seen for tested columns: In general the experimental load versus mid-height lateral deflection behavior was noticed to have three distinguished stages; The first is being an initial straight portion of the load-deflection curve representing the elastic stage, the second is a nonlinear portion with distinct change in slope with increasing deflections (elastic-plastic stage), and the third is also a nonlinear portion but has characteristics in which a slight increase in load results in a larger deflection (represent the plastic stage). Lateral deflection at mid-height of column increases whenever load eccentricity increases Failure Modes of Test Specimens The manners of failure of all test columns specimens are listed in table (5).Failure mode of the specimens CF0A, CF1A and CF2A occurred suddenly due to cleavage of the concrete and simultaneous rupturing some of the longitudinal CFRP bars. Also, for the specimen CF4A failure occurred suddenly due to spalling of concrete cover. In addition, the failure of specimen CF6A occurred gradual due to spalling of concrete cover, all above specimens failure can be classified as compressive failure mode. Whereas failure manner of the remaining specimens CF8A, CF12A, and ST12A were gradual expanding in the tension zone and reduction in compression zone even the remaining outermost concrete crumbles. Therefore CF8A can be classified as combined failure mode and CF12A and ST12A as concrete tension failure mode editor@iaeme.com

7 Behavior of Short Concrete Columns Reinforced by CFRP Bars and Subjected To Eccentric Load Table 5 Manners of failure of test columns specimens. Column's Symbol Failure Mode at Ultimate Failure State PC0A Cleavage of the concrete Sudden ST0A CF0A, CF1A, CF2A Cleavage of the concrete and simultaneous buckling of the longitudinal steel Cleavage of the concrete and simultaneous rupturing of CFRP bar Sudden Sudden CF4A Progressive concrete crushing Sudden CF6A Progressive concrete crushing Gradual CF8A Combined concrete failure Gradual CF12A, ST12A concrete tension failure Gradual 7. ULTIMATE LOAD-MOMENT OF COLUMNS REINFORCED BY CFRP BARS A comparison between the ultimate load-moment of the tested columns and finite element analysis using the available computer program (ABAQUS /Standard 6.13.) are shown in table (6) and figure (4). Table 6 Ultimate load-moment comparison between experimental and finite element of columns (ρ=0.646%, fc`=29.5mpa) Column's Symbol e (mm) Experimental F.E. ABAQUS PukN Mu kn.m PukN Mu kn.m Percentage difference% CF0A CF1A CF2A CF4A CF6A CF8A CF12A Pure moment as a beam editor@iaeme.com

8 Pu kn Prof. Dr. Nameer A. Alwash and Ahmed Hamid Jasim Experimental F.E Mu kn.m Figure 4 Ultimate load-moment interaction diagram of experimental and finite element of columns (ρ=0.646%, fc`=29.5mpa) The table (6) shows the ultimate load - moment results of experimental and F.E. ABAQUS of columns that reinforced with CFRP bar, as well as the table displays the percentage increase or decrease in the ultimate load-moment of finite element analysis with respect to tested columns. One can see that the maximum percent of difference is 13.01% and minimum percent of difference is -3.68%. These results show an acceptable agreement between the finite element and experimental results of tested columns, with underestimated ultimate load from finite element analysis which is preferable. 8. PARAMETRIC STUDY The main objective of this section is to investigate the effect of several important parameters on the behavior of concrete column. These parameters include percent of CFRP reinforcement of column (ρ) and compressive strength of concrete. Where (ρ = Acf/ Ag), Ag and Acfare the gross column cross sectional area and area of CFRP reinforcement, respectively CFRP Reinforcement Ratio ρ In order to investigate the effect of ρ on the ultimate load-moment behavior of column, three different CFRP bar ratios namely ρ = %, % and % respectively were studied and the other properties are the same for the considered column. Figure (5) shows the effect of ρ on ultimate load-moment of column editor@iaeme.com

9 Pu kn Pu kn Behavior of Short Concrete Columns Reinforced by CFRP Bars and Subjected To Eccentric Load F.E. ρ=0.6463% F.E. ρ=1.4543% F.E. ρ=2.5857% Mu kn.m Figure 5 Effect of (ρ) on ultimate load-moment interaction diagram of column (fc`=29.5 MPa). As shown in figure (5), increasing the CFRP reinforcement ratio ρ enhances the behavior of concrete column reinforced by CFRP bar clearly, and this may reaches (52%) for the considered cases. 8.2 Compressive Strength In order to investigate the effect of (fc`) on the ultimate load-moment behavior of column three different fc` (25, 29.5 and MPa) were adopted. Figure (6) show the effect of fc` on ultimate load-moment of column F.E. fc`= MPa F.E. fc`= 29.5 MPa F.E. fc`= 25 MPa Mu kn.m Figure 6 Effect of fc` on ultimate load-moment interaction diagram of column editor@iaeme.com

10 Prof. Dr. Nameer A. Alwash and Ahmed Hamid Jasim As shown in figure (6), increasing the concrete compressive strength enhances the behavior of concrete column reinforced by CFRP bar clearly. However, such enhanced is not significant in case of high eccentricities. 9. CONCLUSIONS Based on the results obtained from the experimental work, finite element analysis, the following conclusions are drawn; CFRP bars contribute about 14.51% of column capacity under axial load. Column reinforced with CFRP bars gave some decrease in ultimate load 3.78% under axial load with respect to the column reinforced with steel bars, while it gave clear increase in ultimate load 38.21% under load eccentricity e/h values (0.857). It can be concluded that using CFRP reinforcement has a significant effective on ultimate load capacity of columns with high eccentricity. The general behavior of the finite element models which were analyzed by (ABAQUS/Standard 6.13) shows good or acceptable agreement with the experimental results. Maximum difference in ultimate load was (13.01%). Failure mode of the columns under pure compression condition occurre suddenly due to cleavage of the concrete and simultaneous rupturing some of the longitudinal CFRP bars, with increasing in eccentricity of load tension zone will appear and expand at the expense of compression area and the occurred failure changes from suddenly due to spalling of concrete cover to gradual due to spalling of concrete cover, (all these failure can be classified as compressive failure mode), with high eccentricity gradual expanding in the tension zone and reduction in compression zone even the remaining outermost concrete crumbles. REFERENCES [1] Choo, C. C., "Investigation of Rectangular Concrete Columns Reinforced or Prestressed with Fiber Reinforced Polymer (FRP) Bars or Tendons " Doctoral Dissertation, College of Engineering at the University of Kentucky, 2005,1 pp. [2] Paramanantham, N. S., Investigation of the Behavior of Concrete Columns Reinforced with Fiber Reinforced Plastic Re-bars, MS thesis, Lamar University, Beaumont, TX, 1993, Cited in 1. [3] Samir A.Al Mashhadi and Ahmed Hassan Hadi. Strengthening Reinforced Concrete One Way Slabs by Overlay Technique & Using CFRP. International Journal of Civil Engineering and Technology, 5(2), 2014, pp [4] Adnan Ibrahim Abdullah, Dr. Muyasser M. Jomaa'h and Dr. Alya'a Abbas Al- Attar. Flexural Behavior of Fiber Reinforced Concrete I- Beams Strengthened with (CFRP). International Journal of Civil Engineering and Technology, 5(1), 2014, pp [5] Javaid Ahmad and Dr. Javed Ahmad Bhat. Ductility of Timber Beams Strengthened Using CFRP Plates. International Journal of Civil Engineering and Technology, 4(5), 2013, pp [6] Yaman S.S. Al-Kamaki, Riadh Al-Mahaidi and Azad A. Mohammed. Behavior of Concrete Damaged By High Temperature Exposure and Confined with CFRP Fabrics. International Journal of Civil Engineering and Technology, 5(8), 2014, pp editor@iaeme.com