Experimental investigation of the contributions of CFRP and externally collar strengthening to the seismic performance of RC columns with different cross-sections

Structures 24 (2020) 266–281

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Experimental investigation of the contributions of CFRP and externally collar strengthening to the seismic performance of RC columns with different cross-sections

T

Tamer Dirikgil Department of Civil Engineering, Erciyes University, 38280 Kayseri, Turkey

A R T I C LE I N FO

A B S T R A C T

Keywords: RC column Hysteretic behavior CFRP wrap Externally collar Cross-section size

In this study, the performance of nine reinforced concrete (RC) short columns under cyclic lateral loading effect was investigated experimentally. The columns are divided into three series with cross-sectional dimensions of 300 × 400, 300 × 550 and 300 × 700 mm2. Each series includes three columns that are strengthened with one Reference column, two CFRP Wrap and Externally Collar columns. Aspect ratios (αs) of all columns in the series are 1.5. Therefore, the columns were tested under the same behavior demand according to the cross-section dimension. Reference columns in each series are perfect columns with sufficient stirrup spacing as required by regulations. The performance of RC columns was evaluated in terms of crack development, hysteretic character, stiffness, ductility, and energy consumption. The results showed that CFRP wrap and collar strengthening significantly improved the performance of the columns.

1. Introduction Columns are the most important reinforced concrete (RC) carrier system elements in order to ensure the expected performance under the influence of seismic loads of the carrier system. The performance of the RC columns therefore directly affects the carrier system. A defect that will adversely affect the seismic performance of the columns may cause severe damage to the structure or it may completely collapse. For this reason, RC columns must have sufficient rigidity as well as sufficient ductility within the framework of earthquake-resistant structural design [1–3]. One of the most important reasons for the failure to achieve the expected performance in RC columns is that the column cannot meet the displacement demand under seismic load. This is mostly seen in short columns. Effective lengths are shorter than other floor columns, so the stiffness of short columns is quite high. Therefore, they cannot show the desired ductile behavior by trying to compensate for the lateral load effect caused by seismic motion with their stiffness. In this case, the element which cannot exhibit flexural behavior by displacement shows the shear dominant behavior. If the lateral load effect exceeds the shear strength of the column, sudden shear failure occurs in the column. It is recommended to increase the shear reinforcement in order to increase ductility and shear strength for such columns which exhibit a shear dominant behavior. In many earthquake regulations, for these columns that exhibit shear dominant behavior, the requirement of the column's confinement zone conditions to be maintained throughout the entire

column is defined [4–6]. However, FEMA 274 [7] reported that shortcolumn damage was reported, although it was very well-detailed in the Japan-Kobe earthquake. In this case, the requirements of the regulation are not sufficient to obtain the desired performance from a column with shear dominant behavior. In addition, due to design errors or defects in the construction phase, the need for strengthening the columns that do not comply with the requirements of the regulation is a problem faced by civil engineers. For columns with aspect ratio (αs) less than 2.5, the effect of shear deformations increases in the lateral load [1,8]. Experimental and numerical studies are carried out by the researchers in order to direct the shear dominant behavior to flexure or to reduce the defects of the behavior. Shear behavior is very dangerous for the safety of people‘s life and property as it causes a brittle failure. Therefore, this situation increases the importance of this subject. In this context, some experimental studies have been made for the design recommendations for RC short columns [9,10], fiber reinforcement for concrete mixture [11,12] or different strengthening applications to increase ductility [13–18]. Studies on the design details of short columns have been carried out to investigate the effect of shear reinforcement ratio and/or longitudinal reinforcement ratio on behavior or to present new design proposals. Dirikgil and Atas have proved that the diagonal reinforcement design which is placed between the stirrups significantly increases the shear strength of the columns and reduces the damage caused by the shear cracks in the core concrete [3]. In addition to this, strengthening

E-mail address: [email protected]. https://doi.org/10.1016/j.istruc.2020.01.014 Received 10 October 2019; Received in revised form 12 December 2019; Accepted 14 January 2020 2352-0124/ © 2020 Institution of Structural Engineers. Published by Elsevier Ltd. All rights reserved.

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Nomenclature Ag As As,col Ash b E Ec Ee Eel Ef Ep Es Es,col Et fy,col fywk h I l M N nf

s sc sf tf V Vc VCFRP Vcollar Vmax Vr Vs Vu Vy wc wf αs γ

Gross cross-sectional area Stirrup cross-section area Collar cross-section area Effective shear area Perpendicular dimension to the lateral load Modulus of elasticity Consumed energy Recoverable elastic energy Area up to yield point under the strength envelope of the column Elasticity modulus of CFRP Unrecoverable plastic energy Stored energy Elasticity modulus of Collar Area under the strength envelope of the column tensile strength of collar Tensile strength of stirrup steel Parallel dimension to the lateral load Moment of inertia height of the lateral load from foundation upper-level & effective length of the column Bending moment Axial load level Number of CFRP wrap layers on one face

Δ Δu Δy εf εs,col μd μE ρt

Stirrup spacing Center-to-center spacing of Collars Center-to-center spacing of CFRP strips Effective thickness for a layer of CFRP Shear force Shear strength of concrete Contribution of the CFRP sheets to shear strength Contribution of the Collars to shear strength Strength of the column Shear strength of column cross-section Contribution of the stirrups to shear strength Strength of the column at the ultimate point Yield strength of the column Collar width Width of the CFRP strip Aspect ratio Coefficient of variation of concrete shear contribution depending on ductility Lateral displacement Ultimate displacement Yield displacement Effective strain limit of CFRP Collar strain Displacement ductility index Energy ductility index Longitudinal reinforcement ratio

reinforcement detail. Reinforcement conditions of confinement zone in the reference (Ref) columns were maintained throughout the entire column. Other columns were strengthened with CFRP wrap and externally collar. Stirrup spacing of strengthened columns is higher than Ref column. This makes the strengthened columns defective in terms of being short columns. Therefore, in terms of short column design, Ref columns are perfect columns according to the requirements of current regulation, while strengthened columns are weaker than Ref columns in terms of stirrup spacing. The nine columns tested were divided into three series with different cross-section dimensions. Each series included three columns, one for reference and two for CFRP wrapped and Collared columns. The performances of the strengthened columns were demonstrated by comparison with the Ref column in the series and the columns in the other series. Strengthening applications were carried out considering the damage mechanisms of the columns. The objective of this study is to demonstrate the applicability of the proposed applications in different cross-section dimensions and the effectiveness of the strengthening applications. Besides, it was aimed to investigate the contribution of the proposed applications to the performance of RC short columns with different cross-sectional dimensions at the same aspect ratio. Therefore, except for cross-section dimensions and strengthening applications, parameters are the same for all columns such as longitudinal reinforcement ratio, material quality, aspect ratio, etc.

studies are often carried out to increase the strength and ductility of the RC short columns as well as to reduce the damage mechanisms. In recent years, one of the most widely used materials in these strengthening studies is FRP and its derivative wraps. The contribution of FRP wraps under the monotonic or cyclic axial load to the confinement pressure was investigated and stress–strain models were created by the studies on the non-reinforced concrete samples [19–24]. In many studies, the application of FRP and its derivative wraps on columns are investigated and recommendations are given regarding the application method, number of layers and effectiveness of FRP [25–30]. However, it is also important to investigate the change in the efficiency of FRP wraps according to the behavior of RC short columns. Moreover, in most of the studies where FRP wrapped columns were tested low concrete strength or insufficient shear reinforcement was used. Another important subject is to investigate the contribution of FRP wraps to performance in elements with sufficient concrete quality and/or reinforcement detail. External jacketing applications are also made to increase the strength of the columns. In this regard, the most common strengthening method is steel jacketing. The studies show that steel jacketing increases the strength of the column [31–35]. The application of steel jacketing can significantly increase the shear strength of columns. However, low flexural stiffness or local buckling in the jacket weakens the effectiveness of the jacket due to increased displacement demand [17]. From this point of view, the effectiveness of the application has been started to be investigated by making the application of externally collar [16–18]. It has been shown that the externally collars are very successful in improving the performance of short columns with different aspect ratios that exhibit shear dominant behavior [2]. Investigation of the efficiency of externally collar by selecting cross-section size, cross-section shape, concrete strength, and aspect ratios as variables is another important issue that will contribute to the literature. In this study, performances of nine RC short columns under constant axial load and reversible repetitive lateral load were investigated. The performance expression describes the seismic performance of RC columns in this study. Tested columns are full scale. These columns comply with short column design rules in terms of both material quality and

2. Experimental program Nine RC columns were tested within the scope of the study. The performances of the columns were investigated under the effect of constant axial load and cyclic lateral load. The aim of this study is to compare the results of the Ref columns with the strengthened columns and to show the effectiveness of the strengthening applications. In order to investigate the applicability of strengthening methods in columns of different cross-section sizes, experimental studies were conducted on three different cross-section dimensions. The performances of the test columns were evaluated in terms of hysteretic behavior, shear behavior, ductility, stiffness, and energy consumption capacity. The properties of 267

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experimental studies, the aspect ratios of the columns were kept constant in order to see the effectiveness of strengthening applications at different cross-section dimensions. In all columns, the aspect ratio is αs = 1.5 with shear dominant behavior. The aspect ratio for single curvature columns is expressed as αs = M/Vh = l/h. Since the crosssection dimensions of the columns in the direction of lateral load are different, the lateral loading levels are determined according to the cross-section dimensions in order to maintain a constant aspect ratio. Therefore, the lateral load was applied to 300 × 400 mm2, 300 × 550 mm2 and 300 × 700 mm2 cross-sectioned columns from the height of 600 mm, 825 mm and 1050 mm, respectively. Lateral load level (h) is the height from the column foundation to the center of the hydraulic jack. Similarly, since the effect of cross-section dimension on behavior was evaluated, the axial load was kept constant. The columns were therefore tested under the effect of the same axial load. All columns were subjected to a constant axial load of 700kN. In this case, the axial load indexes are 0.25, 0.19 and 0.15 for the 300 × 400 mm2, 300 × 550 mm2 and 300 × 700 mm2 columns, respectively. These load indexes are greater than 0.1, which is the minimum axial load index required for columns to be designed and dimensioned as columns [4]. Reversible repetitive cyclic lateral load was applied to the RC short columns under the influence of constant axial load. The lateral loading conditions of the RC short columns during the experiment were carried out following the same procedure, including Reference columns. Testing the columns by following the same load procedure helps to minimize errors in the energy consumption data. The lateral loading procedure was applied by following the application method in FEMA461 [34] which is specified as the application of quasi-static load for the displacement controlled of structural components. Due to the brittle behavioral characteristics of short columns, it is more appropriate to continue the experiment with small increases in the displacement, because it is possible that major damages may occur at small displacement values. In this way, both behavior is observed more clearly and sudden failures that occur after a few cycles are avoided. A cycle is that lateral load follows the zero-push–pull-zero path. Each cycle is repeated two times according to the procedure. Therefore, repeated two cycles in a row constitute one step (Fig. 8a). Since at least 10 steps of cyclic loading should be applied as required by the procedure, the load steps have been determined to reach the target displacements in the 10th step. The target displacement is 3% relative drift with respect to the effective length of the column. In this situation, the target displacements of the SC300 × 400, SC300 × 550 and SC300 × 700 series of columns are 18.6 mm, 25.6 mm, and 32.5 mm, respectively. The size of the load steps was increased 1.4 times until the 10th step, and 1.3 times after the 10th step if the column didn’t fail. In Fig. 8b, the loading steps that applied to the each series columns are given graphically with step displacements of all series.

the materials used for production and strengthening of the columns were obtained by material tests. Some of the characteristic properties obtained from material experiments are given in Table 1. 2.1. Test columns The test columns are divided into three series and cross-section dimensions of these series columns are 300 × 400 mm2, 300 × 550 mm2 and 300 × 700 mm2. Each series consists of three columns, one of which is a Reference (Ref) column, one of which is strengthened with CFRP wrap and the other is strengthened with externally collar. The damage at the plastic hinge areas of the columns directly affects the performance. It is important that the applied strengthening methods are effective against shear and flexure effects. Besides, strengthening methods should be applicable to existing carrier systems in accordance with the type of the used material. By considering these criteria, the most suitable configurations for the strengthening methods were determined. Thus, the most suitable configurations of the strengthening methods in terms of both applicability and efficiency were evaluated. Therefore, CFRP wrapped columns were fully wrapped with CFRP wraps up to half of the height of cross-section dimension in the load direction (200, 275, 350 mm), then wrapped up to loading level in sf = 100 mm spacing and wf = 50 mm thick strips. Collared columns were wrapped with 2 external collars just above the foundation and then collared with strips placed sc = 100 mm spacing. External collars were produced in accordance with the cross-section dimensions of the columns. The detailed visuals of the produced external collars are given in Fig. 1. The inner dimensions of the collars were produced to be 1 mm smaller than the cross-section of the column in both directions. In this way, the collar confining was ensured strongly from the outside of the column. Ø10/100 mm spacing stirrups were used in Ref columns and Ø10/150 stirrups were used in CFRP wrapped and externally collared columns. Cross-section and length-details of the test columns can be seen in Figs. 2–6. Notations of the columns were made as SC-b × h-X. The symbols in this notation system represent as follow; SC: Short Column b × h: Cross-section dimensions (b: perpendicular dimension to the lateral load, h: parallel dimension to the lateral load) X: Suggested method or Reference Ref: Reference C: External Collar CFRP: Carbon Fiber Reinforced Polymer Notations, reinforcement and strengthening details of the test columns are given in Table 2. 2.2. Test setup The test setup consists of two lateral loading units and one axial loading unit. These units include load cells, hinges, and other fasteners. A single point cyclic loading at large lateral load levels causes overloading of the fasteners and endangers occupational health. For this reason, cyclic loading was applied to the test columns as mutual-lateral loading. When one of the hydraulic jacks is loading and the oil pressure of the other is released, there is no reaction observed against it. A view of the test-setup is given in Fig. 7.

Table 1 Some characteristic properties of the materials. Concrete

Mean Compr. Strength (MPa) 22.13

1.86

24351.51

Steel Rebar

Mean Yield Strength (MPa)

Mean Tensile Strength (MPa)

Modulus of Elasticity (MPa)

520.02 453.28

627.53 561.08

202868.76 200017.92

Thickness (mm)

Mean Tensile Strength (MPa)

Modulus of Elasticity (MPa)

0.331 10

4900 451.83

230000 161857.36

2.3. Experimental procedure

Ø10 Ø16

In this study, the effect of cross-section dimension on the behavior of columns with shear dominant behavior and the effectiveness of proposed strengthening methods were investigated. Therefore, the parameters that are effective in evaluating the performance of RC short columns are strengthening applications and cross-section dimension. In

Strengthening Materials CFRP Collar

268

Tensile Strength (MPa)

Modulus of Elasticity (MPa)

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Fig. 1. Details of externally collar.

the first stage. However, in the following steps, these cracks are widening and this causes the decrease of flexural stiffness of the column. These cracks extend as inclined to the foundation after reaching the sides. In the next steps, shear cracks occur on the sides of the column. Shear cracks occur at the first stage at the capillary level. In subsequent steps, new capillary shear cracks appear while the first occurring shear cracks are enlarged. In the meantime, the transverse cracks that occurred in the first steps on the column faces in the direction perpendicular to the loading are also enlarged. After this, vertical cracks occur due to the compressive crushes on the column faces perpendicular to the load and yield of longitudinal reinforcement. After the yield of longitudinal reinforcement, the strength envelope of the column approaches the horizontal. This is the point where the flexural stiffness of the column decreases and inelastic behavior begins. In the meantime, the cracks expanding in each cycle cause the decrease of flexural stiffness of the column. Therefore, the hysteresis curve in each cycle proceeds closer to the horizontal than the previous cycle. When the peak load of the column is reached, the width of the shear cracks increases considerably. The width of the shear cracks continues to increase in subsequent cycles. In this case, the shear strength of the column decreases due to crack development. Ultimately, the column fails with sudden expansion in the shear crack. At this stage, the lateral load response of the column decreases dramatically. Although the subsequent cycles were not important for performance evaluation,

3. Results and discussion In this section, performances of nine RC short columns under the effect of constant axial load and cyclic lateral load are given depending on cross-section dimension and efficiency of strengthening methods. Obtained results are presented as crack development, hysteretic behavior, stiffness, ductility, energy consumption capacity, and shear behavior. 3.1. Crack development Crack development is an important parameter for monitoring and understanding the behavior of RC short columns. The crack development under the influence of lateral loads and the damage that happens as a result of it allows a visual assessment of the performance history of the columns. In this section, the general crack developments of nine tested RC short columns are explained and then the damages are revealed within the scope of the study. The first crack development in the tested columns occurs in the column-foundation junction zone where the side perpendicular to the lateral load. This first crack development is also observed just above the column-foundation junction zone in the first steps of the lateral loading. These cracks, which occur horizontally at the capillary level, do not significantly change the behavior of the column in the elastic region in

Fig. 2. Cross-section details of test column series. 269

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Fig. 3. Longitudinal-section details of columns.

Fig. 4. View of Ref columns.

Fig. 5. View of CFRP wrapped columns.

Fig. 6. View of Collared columns.

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Table 2 Reinforcement details and strengthening applications of the test columns. Series

Column

Long. Reinf.

Stirrup

Strengthening

300 × 400

SC300 × 400-Ref SC300 × 400-CFRP SC300 × 400-C

8Ø16 8Ø16 8Ø16

Ø10/100 Ø10/150 Ø10/150

– 200 mm CFRP full wrap + 50 mm wide 50 mm spaced strip Two combined Collar + 50 mm wide 50 mm spaced collar

300 × 550

SC300 × 550-Ref SC300 × 550-CFRP SC300 × 550-C

10Ø16 10Ø16 10Ø16

Ø10/100 Ø10/150 Ø10/150

– 200 mm CFRP full wrap + 50 mm wide 50 mm spaced strip Two combined Collar + 50 mm wide 50 mm spaced collar

300 × 700

SC300 × 700-Ref SC300 × 700-CFRP SC300 × 700-C

12Ø16 12Ø16 12Ø16

Ø10/100 Ø10/150 Ø10/150

– 200 mm CFRP full wrap + 50 mm wide 50 mm spaced strip Two combined Collar + 50 mm wide 50 mm spaced collar

Fig. 7. View of the test setup.

Fig. 8. (a) Load steps (b) Loading procedure.

respond to more cycles in the plastic phase and thus to exhibit more ductile behavior. As the demand for displacement increases, the damage level increases. Increased displacement demand causes ruptures in the CFRP wraps at peak load level. CFRP wraps have lost its effectiveness with the expansion of these ruptures. External collars continued to be effective even at further damage level. Damages in collared columns began to concentrate in areas between the collars. As the shear cracks could not be enlarged, it was observed that a slip occurred parallel to the load direction in the cross-section of the column. Deformation in the form of outward curvature has occurred on the externally collar. The post-test damage images of the test columns are given in Fig. 10. CFRP wraps and externally collars have significantly limited the increase of damage to the column. This is clearly seen in the

cyclic loading continued in the experiments to observe further damage levels. Buckling occurs in the longitudinal reinforcement at further damage level. Buckling of the reinforcement causes spalling of the cover-concrete. Moreover, the buckling of the longitudinal reinforcement causes shortening of the column. The spalling also occurs in concrete covers on the sides of the column. By continuing cyclic loading for several more cycles, the ruptures occur particularly in the longitudinal reinforcements placed at the corners. In Fig. 9, the crack development stages of the columns are given as a visual. CFRP wraps and externally collars significantly limited the expansion of shear cracks. Restriction of the crack development has enabled both the strength of the column to be increased and the peak load to occur in further steps than the Ref columns. This allowed the column to 271

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ultimate points have not been taken into account in performance evaluations. The strength and displacement values of the yield, peak and ultimate points of the columns are given in Table 3. The strength envelopes of the series can be seen in Fig. 12. When hysteretic behavior is examined, it is seen that CFRP wraps and externally collar significantly increase the performance of RC short columns. Hysteretic characteristics of strengthened columns are stable. Reduction in strength of Ref columns is more immediate after the ultimate point. Both peak and ultimate points of CFRP wrapped and collared columns in each series are one step ahead of Ref columns. For example, in the SC300 × 400 series, the peak point of the Ref column was in step 8 and the ultimate point was in step 10, whereas the peak and ultimate points of the CFRP wrapped and collared columns occurred in steps 9 and 11, respectively. Thus, the hysteretic behavior of CFRP wrapped and collared columns maintained its stability to greater displacement levels. In the SC300 × 400 series, the Ref column performs a drift ratio of 0.61% compared to the normal storey height, while the drift ratios of the CFRP wrapped and collared columns are 0.81% and 0.79%, respectively. This difference is more obvious in the SC300 × 550 and SC300 × 700 series. While the drift ratios of Ref, CFRP wrapped and collared columns were 0.84%, 1.09% and 1.1%, respectively in the SC300 × 550 series, drift ratios were 1.07%, 1.40%, and 1.82% in the SC300 × 700 series. The drift ratio can be expected to decrease as the cross-section dimension increases, but it should be noted that the aspect ratios of the columns are the same at this point. CFRP and collar strengthening significantly increased the strength of the columns. The strengths of the SC300 × 400-CFRP and SC300 × 400-C columns are 1.15 and 1.26 times higher in the SC300 × 400 series, 1.21 and 1.23 times higher in the SC300 × 550 series, and 1.21 and 1.26 times higher in the SC300 × 700 series than the SC300 × 400-Ref column, respectively. Considering that the average strengths of the SC300 × 400-Ref, SC300 × 550-Ref, and SC300 × 700-Ref columns are as 300.19kN, 414.46kN, and 501.85kN

Fig. 9. Crack development stages of test columns.

photographs of further step and collapse of the columns. Since CFRP wrap and externally collar applications are performed meticulously, the unwinding in both of CFRP wraps and collars did not occur. 3.2. Hysteretic behavior and strength Hysteresis curves reveal the behavioral characteristics of the elements under cyclic loading. The hysteresis curves and strength envelopes of 9 tested RC short columns are given in Fig. 11. The yield points ( ), peak loads ( ) and ultimate loads ( ) of the columns are marked on the strength envelopes. The ultimate loads of the columns were evaluated up to 80%Vmax after the peak load, where Vmax is the peak load of the column. The ultimate point is also the point where the performance levels of the columns are evaluated. Behaviors beyond the

Fig. 10. Damage mechanisms of test columns. 272

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Fig. 10. (continued)

series (see Section 2.3). Thus, the effect of cross-section dimension on behavior independent of aspect ratio was investigated. In columns with the same aspect ratio, the strength increased significantly as the crosssection dimension increased. Compared to the SC300 × 400 series, the average push-pull strength of columns in the SC300 × 550series increased by 1.34 ~ 1.44 times, while it increased by 1.67 ~ 1.75 times in the SC300 × 700 series. Average strength of the columns in the

respectively, it can be seen that these increases in the strength of abovementioned columns are quite large. It is evident that the strength and stiffness will increase substantially in columns where the lateral load is applied at the same height from the foundation. This situation occurs due to the increase in moment of inertia as the cross-section dimension increases in columns. In this study, lateral load is applied to RC columns with the same aspect ratio of each 273

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Fig. 10. (continued)

On the other hand, the ultimate displacement (Δu) is the point corresponding to 80% Vmax after the peak load on the descending branch. So, displacement ductility (μd) was evaluated as μd = Δu/Δy. One of the graphical representations used to calculate the secant stiffness and ductility values of the columns is given as a visual in Fig. 13. The displacement ductility directly depends on the yield displacement value. Thus, even if the large yield displacement of a column has also resulted in large ultimate displacement, the displacement ductility may be numerically small. Although the ductility of displacement is an important evaluation criterion in performance assessments, it should not be a single assessment criterion for this reason. In addition, the energy consumption data must be taken into consideration. Therefore, in this section, energy ductility (μE) values have been considered in addition to displacement ductility values. Energy ductility is the ratio of the total energy consumed up to the ultimate point (Et) to the energy consumed in the elastic stage limited by the yield point (Eel) [36]. Structures or structural elements do not exhibit ideal elasto-plastic behavior. The most important part of the behavioral characteristics of the systems occurs in the nonlinear region after the yield point. Therefore, some researchers have used energy ductility as well as displacement ductility

SC300 × 700 series is increased by 1.21 ~ 1.25 times compared to the SC300 × 550 series. This shows that cross-section dimension significantly affects the behavior, even if the aspect ratio is the same (see Fig. 11). 3.3. Stiffness and ductility Stiffness and ductility are important parameters for determining the elastic and plastic deformation capacity of the structural elements under seismic loadings. Structural elements must have sufficient stiffness and sufficient ductility in terms of earthquake-resistant structure design. To determine the initial stiffness and displacement ductility, the Ky line and Kpl line plotted on the hysteresis curves of the columns were used. Secant stiffness is the slope of the Ky line passing 75% of the peak load. Kpl line is the line which combines the coordinates of the peak point cycle (Vmax, Δmax) and the coordinates of the second previous cycle from the peak point cycle (Vmax-2, Δmax-2). The displacement corresponding to the intersection of the Kpl line and the Ky line is determined as the yield displacement (Δy). Yield displacement points were confirmed by strain-gauge data placed on longitudinal reinforcement. 274

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Fig. 11. Hysteresis curves and strength envelopes of the test columns. Table 3 Displacement and strength values of the test columns. Test Column

Yield Point Displacement (Δy, mm)

Peak Load (Vmax, kN) Strength (Vy, kN)

Ultimate Point Displacement (Δu, mm)

Strength (Vu, kN)

Push

Pull

Push

Pull

Push

Pull

Push

Pull

Push

Pull

SC(300 × 400)-Ref SC(300 × 400)-CFRP SC(300 × 400)-C

2.80 3.29 3.37

−4.42 −3.39 −3.25

217.92 280.57 281.99

−242.95 −264.27 −270.25

288.65 354.71 386.64

−311.74 −338.31 −371.34

18.71 24.26 24.26

−17.82 −24.11 −23.29

264.45 288.30 364.51

−289.71 −267.98 −313.51

SC(300 × 550)-Ref SC(300 × 550)-CFRP SC(300 × 550)-C

4.66 4.46 4.48

−3.84 −4.48 −4.53

330.89 390.53 366.90

−309.06 −348.59 −339.58

416.44 509.42 516.34

−412.48 −489.62 −500.32

25.81 33.25 33.30

−24.35 −32.39 −32.50

399.85 494.99 495.06

−364.73 −451.06 −451.62

SC(300 × 700)-Ref SC(300 × 700)-CFRP SC(300 × 700)-C

6.56 6.24 6.99

−6.11 −6.52 −7.03

384.39 416.93 430.69

−403.78 −465.92 −450.80

518.30 599.16 625.41

−485.39 −610.96 −641.87

31.84 42.06 54.56

−32.44 −42.18 −54.92

518.30 599.16 625.41

−485.39 −595.73 −627.96

behavior [2,3], but in here which aspect ratio is not a parameter. The flexural stiffness of the columns is defined by k = EI/l3 (E; modulus of elasticity, I; moment of inertia and l; effective length). E = bh3/12 for rectangular cross-section. Therefore, the flexural stiffness of the columns is directly proportional to the cube of cross-section dimensions in the loading direction, and inversely proportional to the cube of effective length. For columns with the same aspect ratio, as the cross-section dimension increases, moment of inertia increases and also effective length increases. In this case, the stiffness of the columns increases by h3 and decreases by l3. This explains why there is no significant difference between the secant stiffness of the test columns. The displacement ductility (μd) of the test columns was calculated

[36–39]. Energy ductility values are greater than displacement ductility values because they represent the proportion of areas. Energy ductility is calculated by considering the area under the strength envelopes of the test columns (μE = Et/Eel) (Fig. 14). The stiffness and ductility values of the test columns are given in Table 4. Ductility comparisons can be seen in the histograms given in Figs. 15 and 16. There is no significant difference between the initial (secant) stiffness of the columns. Since the columns were tested with the same aspect ratio, the increase in cross-section dimensions did not change the stiffness. At this point, it is seen that the aspect ratio is as important parameter as the cross-section dimension for shear dominant behavior. The author has studied the effects of aspect ratio on shear dominant 275

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by considering the average displacement obtained during the push and pull steps. Thus, the evaluated ductility values are the average of the ductility obtained in the push and pull zones. When the displacement ductility is examined, it is seen that the ductility of SC300 × 550 series columns is higher than SC300 × 400 series columns. The yield displacement (Δy) of the columns in the SC300 × 550 series are slightly higher than the SC300 × 400 series, but the difference between the ultimate displacement (Δu) is greater between these two series (Table 3, Fig. 12). Although the aspect ratios were the same, the SC300 × 400 series columns failed in smaller displacement demands as their shear strength is smaller. The increase in cross-section dimension makes the displacement ductility of the SC300 × 550 series columns greater than the SC300 × 400 series. However, this situation is reversed in SC300 × 700 series columns. The lateral loading levels of columns in the SC300 × 700 series are high and because of this their effective lengths are larger, the desired displacement demands are higher. The ultimate displacements of the SC300 × 700 series columns are larger than the columns in the other series. However, the displacement ductility values are small due to the large yield displacements. At this point, it should be emphasized that the collar application performed very successfully even in large displacement demands. SC300 × 700-C column is the most ductile column in all columns. Although the CFRP wrap was successful in the SC300 × 550 series, it failed to meet the increasing displacement demands of the SC300 × 700 series. This was due to the rupture of the CFRP wraps for large displacement demands. In addition, CFRP wrapped and collared columns are more ductile than Ref columns in each series. This shows that the applied strengthening techniques are very successful. Displacement ductility of the test columns are given graphically and numerically in Fig. 15. When energy ductility (μE) is evaluated, the effect of increasing cross-section dimension on both energy consumption and ductile behavior is more clearly seen in columns with the same aspect ratio. The ductility increases with the increasing cross-section dimension in CFRP wrapped and collared columns with the same aspect ratio. However, similar to displacement ductility, the increase in cross-section dimension decreases the ductility of the SC300 × 700-Ref column. This shows that in columns exposed to greater lateral load effects with the increase in cross-section dimension, the prevention of damage development contributes significantly to ductility. Collared columns performed very well in each series. Energy ductility of the test columns is given graphically and numerically in Fig. 16. CFRP wrapped and collared columns are far more successful than Ref columns in terms of ductile behavior for both displacement and energy consumption. At this point, it should be emphasized that Ref columns are perfect columns produced in accordance with the reinforcement details of current regulations. 3.4. Energy consumption The energy consumption capacity is an important parameter in assessing the performance of columns that are very important structural system components in RC frames and/or mixed systems. The energy consumption of the columns depends on axial load level, yield displacement, number of cycles, support conditions, cross-sectional details, material properties and reached peak load level. The identification of energy consumption zones is given visually at Fig. 17. Energy consumption values of the test columns are given visually in Fig. 18 and numerically in Table 5. When the energy consumption of the test columns is examined, it is clearly seen that the energy consumption capacity increases as the cross-section size increases. Energy consumption of CFRP wrapped and collared columns are higher than Ref columns in each series. This difference is more apparent in the SC300 × 550 and SC300 × 700 series. In the SC300 × 400 series, the energy consumptions of the SC300 × 400-CFRP and SC300 × 400-C columns are 1.79 and 1.85 times higher than the SC300 × 400-Ref column, respectively. In the SC300 × 550 series, the energy consumptions of the SC300 × 550-CFRP and SC300 × 550-C columns are

Fig. 12. Strength envelopes of the series. 276

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Fig. 13. Visualization of the stiffness and displacement ductility evaluation diagram.

consumed energy. This is due to the increase in both the inelastic energy area and the elastic energy area as a result of the increase in lateral displacement demand and the load. Inelastic energy consumption (Ep) values of SC300 × 400-Ref and SC300 × 550-Ref columns are close to elastic energy consumption (Ee), whereas in SC300 × 700-Ref column, inelastic energy consumption is smaller than elastic energy consumption. The inelastic energy consumption of CFRP wrapped and collared columns are greater than the elastic energy consumption and differ significantly from the Ref columns. This clearly shows that CFRP wrapped and collared columns consume more inelastic energy than Ref columns. CFRP and Collar strengthening significantly improved the behavior of the columns. The SC300 × 700-Ref column has a larger cross-section dimension than the SC300 × 550-CFRP and SC300 × 550-C columns. Although the energy consumption capacities increase significantly with increasing cross-section dimension, the energy consumption of the SC300 × 550-CFRP and SC300 × 550-C columns is higher than the SC300 × 700-Ref column. This clearly demonstrates that CFRP and Collar applications improve the behavior of the columns very effectively in the inelastic stage.

Fig. 14. Definition of hysteretic energy ductility response.

1.82 and 1.82 times higher than the SC300 × 550-Ref column, respectively. In the SC300 × 700 series, the energy consumptions of the SC300 × 700-CFRP and SC300 × 700-C columns are 2.04 and 3.10 times higher than the SC300 × 700-Ref column, respectively. The stored energy in SC300 × 700 series columns is more than the

3.5. Shear strength envelope Many researchers are studying the evaluation of shear strength of

Table 4 Stiffness and ductility values of test columns. Test Column

Mean Secant Stiffness (kN/ mm)

Disp. Ductility (μd) Push *(C7/C1)

Pull *(C8/C2)

Mean

Consumed Elastic Energy (kNmm)

Consumed Plastic Energy (kNmm)

Energy Ductility (μE)

SC(300 × 400)-Ref SC(300 × 400)-CFRP SC(300 × 400)-C

73.50 94.72 96.19

6.67 7.38 7.21

4.03 7.11 7.17

5.35 7.24 7.19

1231.08 1274.67 1113.83

30665.65 55884.61 57785.37

24.91 43.84 51.88

SC(300 × 550)-Ref SC(300 × 550)-CFRP SC(300 × 550)-C

84.30 97.00 87.84

5.53 7.45 7.44

6.35 7.24 7.17

5.94 7.35 7.31

1893.09 2265.79 1686.15

60565.74 111126.35 111857.03

31.99 49.05 66.34

SC(300 × 700)-Ref SC(300 × 700)-CFRP SC(300 × 700)-C

72.19 77.17 75.52

4.85 6.74 7.80

5.31 6.47 7.81

5.08 6.61 7.81

2404.64 2414.41 2525.71

95046.19 144843.83 220110.94

39.53 59.99 87.15

*At the Table 3, Column7(Δu)/Column 1 (Δy) and Column8(Δu)/Column2(Δy). 277

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In most of the models, the effective shear area is expressed as 80% of the gross cross-sectional area (Ag) of the column. In some models, it is expressed as the product of the section width (b) and effective depth (d). (Ash = 0,80Ag or Ash = bd). Priestley et al. [40] evaluated the shear strengths of reinforced concrete columns in part of concrete contribution, the contribution of the shear reinforcement and axial load. They have defined the decrease in the shear strength of concrete according to increasing ductility with the following Eq. (1).

Vc = γ fc′ (0.8Ag )

(1)

Here γ is 0.29 if the displacement ductility (μd) is less than 2, 0.10 if it is greater than 4. The γ factor is linearly decreasing when the ductility is between 2 and 4. A few years later, Xiao and Martirossyan [41] proposed that the contribution of concrete to shear decreases more dramatically when they investigate the seismic performances of high strength concrete columns. Accordingly, the γ factor is defined by the following equations.

Fig. 15. Displacement ductility of test columns.

γ = 0.29

for μd ⩽ 2

γ = 0.29 − 0.12(μd − 2)

for 2 ⩽ μd ⩽ 4

γ = 0.05 − 0.025(μd − 4) for 4 ⩽ μd ⩽ 6 γ=0 for 6 ⩽ μd

(2)

Recently, Howser et al. [43] have conducted a numerical parameter study. They evaluated the change of ductility depending on the longitudinal reinforcement ratio and concrete strength (Eq. (3)). Therefore, the determination of the γ factor and the contribution to the shear strength of the concrete has been revised by taking into consideration both the strength and the longitudinal reinforcement of the section.

γ = 0.29

or μd ⩽ 2

γ = 0.29 − 0.12(μd − 2) for 2 ⩽ μd ⩽ r γ = 0.53 − 0.095r − 0.025μd for r ⩽ μd ⩽ q γ = 0.53 − 0.095r − 0.025q for q ⩽ μd r = 35ρt − 0.011fc′ + 3.8

Fig. 16. Energy ductility of test columns.

q = −144ρt − 0.03fc′ + 4.3

for r ⩽ q

q=r

for q < r

(3)

It is important that the columns are directed to flexural behavior without reaching the shear strength in order to avoid brittle behavior. Shear strength envelopes define the shear strengths of the elements which taken into consideration due to ductility [44,46]. The combination of these shear strength envelopes, which are given depending on the ductility, and strength envelopes, which are reduced to ductility axis, provide a great convenience for the evaluation of the behavior. Shear strength envelopes of test columns and strength envelopes reduced to ductility axis are given together in the Fig. 19. The shear strengths of the test columns were determined using the equations (4)(6) and (7)-(10).

Fig. 17. Energy consumption zones.

reinforced concrete columns [40–46]. Most of these studies are mainly based on the testing of columns. The contribution of the shear strength envelope, depending on some parameters, to the shear strength of the concrete is expressed by evaluating the obtained results. These shear strength envelopes are formed in the reinforced concrete column. The most important design parameters considered for assessing the contribution to the shear strength of concrete are; displacement ductility (μ), axial load level (N), longitudinal reinforcement ratio (ρt), size effect and effective shear area (Ash).

Ref Columns − Vr = Vc + Vs

(4)

Collared Columns − Vr = Vc + Vs + Vcollar

(5)

CFRP wrapped Columns − Vr = Vc + Vs + VCFRP

(6)

N ⎞ ' Vc = 0.166 ⎜⎛1 + ⎟ bd f c (SI unit) 13.8Ag ⎠ ⎝

(7)

Vs =

As f ywk d s

β

(8)

ACI318-14 [4] and FEMA273 [47] take β = 1, while Priestley et al. [35] takes β = cot30. In this study, β taken as 1. 278

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Fig. 18. Energy consumptions of test columns.

4. Conclusions

Table 5 Energy consumption values of test columns. Test Column

In this study, the applicability and efficiency of two different strengthening methods to RC short columns with different cross-section dimensions were investigated. The Collar strengthening proposed and applied using a different configuration was compared to one of the most efficient applications of CFRP wrapping. At this point, it should be emphasized that Ref columns are perfect columns. The conclusions of this study are as follows: CFRP Wrap and Collar strengthening significantly limited the shear cracks in columns. Hysteretic character in CFRP Wrapped and Collared columns is highly improved compared to Ref columns. The strength reduction in these columns is even more stable even after the ultimate point. CFRP Wrap and Collar applications significantly increased the shear strength of columns and limited the damage development. CFRP wrapped and Collared columns with varying cross-section dimension in each series are more ductile than the perfect Ref columns. As a result of this, the energies consumed by the CFRP wrapped and Collared columns are significantly higher than the Ref columns. In each series, the energies consumed by the columns in the elastic stage are close to each other. Therefore, CFRP wrapped and Collared columns performed significantly better in inelastic stage compared to Ref columns. This is an important finding in directing the columns, which are forced to shear-dominant behavior, to the flexural behavior. Both Collar and CFRP wrapping strengthening applications significantly improved the seismic performance of RC short columns in each series (SC300 × 400, SC300 × 550, SC300 × 700). The increase in cross-section dimension increased the strength of the columns even if aspect ratio was the same. The increase in the crosssection dimension did not reduce the effectiveness of strengthening applications. Especially, Collar strengthening was successful in columns with large cross-section dimensions.

Energy (kNmm = Nm) Ec*

Es*

Ee*

Ep*

SC(300 × 400)-Ref SC(300 × 400)-CFRP SC(300 × 400)-C

31896.73 57159.28 58899.20

31980.56 50213.10 56887.37

14005.16 18974.80 21967.02

17975.40 31238.30 34920.35

SC(300 × 550)-Ref SC(300 × 550)-CFRP SC(300 × 550)-C

62458.84 113392.14 113543.18

61043.08 104302.38 104226.65

26244.09 42242.10 38653.57

34798.99 62060.28 65573.08

SC(300 × 700)-Ref SC(300 × 700)-CFRP SC(300 × 700)-C

71850.01 146434.45 222636.65

86764.92 155604.59 226095.53

45860.88 68008.47 92408.98

40904.04 87596.12 133686.55

*Ec: Consumed; Es: Stored; Ee: Recoverable Elastic; Ep: Unrecoverable Plastic Energy.

Vcollar =

As, col εs, col Es, col h β s

(9)

Strain data of the collars were obtained with strain-gauges during the experiments. Yield in collars did not occur before the shear strength of the section is reached. The collar yielding appeared at the stage where the longitudinal reinforcement buckled and the ultimate point of the column. Therefore, the contribution of the collars to shear strength is within elastic limits and complies with Hooke’s law. In this case, since the contribution of the collars to shear strength will be as much as the yield strength, it is considered as εs, col Es, col = f y, col

VCFRP =

2nf t f wf Ef εf d sf

(10)

When the shear strength envelopes are examined, it is seen that the strength envelopes of the Ref columns are very close to the shear strength envelopes. Immediately following these steps, a sudden collapse of the Ref columns occurred. This shows that Ref columns tend to shear dominant behavior. The strength envelopes of CFRP wrapped and Collared columns are pretty below the shear strength envelopes. Therefore, applied strengthening methods have been successful in increasing the shear strength of the column as well as directing the behavior to the flexure. In CFRP wrapped and Collared columns, the strength in the plastic phase was maintained over a longer range and the reduction in strength was more stable than the Ref columns. Strength losses suddenly occurred in Ref columns.

Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This research was supported by TUBITAK (The Scientific and Technological Research Council of Turkey) under grant number 279

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Fig. 19. Shear strength envelopes of the test columns.

115M264. I would like to thank Oguzhan Atas and Oguz Dugenci from Erciyes University for their supports in the laboratory, and Naziye Dirikgil from Aberystwyth University for language editing.

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