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International Journal of Civil Engineering and Technology (IJCIET) Volume 10, Issue 03, March 2019, pp. 336–348, Article ID: IJCIET_10_03_035 Available online at http://www.iaeme.com/ijmet/issues.asp?JType=IJCIET&VType=10&IType=3 ISSN Print: 0976-6308 and ISSN Online: 0976-6316 © IAEME Publication Scopus Indexed NUMERICAL INVESTIGATION ON THE PUNCHING BEHAVIOR OF RC FLAT SLABS STRENGTHENING BY TRM AND FRP Majid H. Abdulhussein, Dr. Muhammad J. Kadhim Department of Civil Engineering, Babylon University, College of Engineering, Babylon, Iraq ABSTRACT In this paper, the effectiveness of textile-reinforced mortar (TRM) and fiberreinforced polymer (FRP), as a means of improving the punching behavior of reinforced concrete flat slabs were numerically investigated. Finite element (FE) model using ABAQUS computer program was developed to analyze eight half-scaled slabs, in terms of load-carrying capacity, ductility, stiffness, and crack patterns. These eight specimens were divided into two groups (G1 and G2) with four specimens for each of them. Specimens of G1 was similar to that of G2 in all details but differ in the eccentricity of the applied load. Specimens of G1 were tested with concentric load, while these of G2 were tested with 150 mm eccentricity. For each group, one specimen was built as control (unstrengthened), one was strengthened by FRP-sheet, and the other two was strengthened by TRM-jacket with two different mesh opening (10 and 20 mm). The results obtained from FE analysis showed that the efficiency of TRM in increasing the punching shear capacity of strengthened slabs was less than that of FRP. In addition, the slabs strengthened by TRM showed stiffer behavior than that strengthened by FRP, but lesser ductile. TRM effectiveness was sensitive to the mesh size of the textile. When the mesh size decreased, stiffness was increased and ductility was decreased. Key words: flat slab, punching sheer, stiffness, ductility, TRM, FRP. Cite this Article: Majid H. Abdulhussein, Dr. Muhammad J. Kadhim, Numerical Investigation on the Punching Behavior of RC Flat Slabs Strengthening by TRM and FRP, International Journal of Civil Engineering and Technology 10(3), 2019, pp. 336–348. http://www.iaeme.com/IJCIET/issues.asp?JType=IJCIET&VType=10&IType=3 1. INTRODUCTION The flat plate is a two-way framing systems composed of uniform slabs supported directly by columns without drop panels or capitals. This type of system has many applications in the construction industry due to many advantages that it offers, which include the simplified formwork, fast construction, reduction of story heights and architectural flexibility. Park and Gamble (2000) [1] indicated that for each ten stores in a structure, an additional store may be added automatically for the same overall height in the flat plate systems, as compared to the http://www.iaeme.com/IJCIET/index.asp 336 [email protected] Numerical Investigation on the Punching Behavior of RC Flat Slabs Strengthening by TRM and FRP other systems having the same height. Despite of these benefits, there are many drawbacks with using of this system. The important one is punching shear failure at the slab columnconnection that is caused by shear transferring and the supporting column moments. In nature, punching shear failure is brittle and maybe leads to building progressive collapse. In general, two types of punching can be distinguished: symmetrical punching and non-symmetrical. It can be said that the punching is symmetrical if the geometry, the load, the bearing conditions, and the composition of the structural element (concrete and reinforcement) can be considered symmetrical with respect to the two axes of symmetry. If one of these conditions is missing, it will be possible to enter the term of non-symmetrical punching. In this case, it is still possible to distinguish between two different types: non-symmetrical punching without eccentricity and non-symmetrical punching with eccentricity. The difference between these two types is that, in case of eccentric punching, the conditions of non-symmetry lead to generate a bending moment transfers from the slab to the column, and that what called by the unbalanced moment [2]. The phenomenon of transmission of the moment between the slab and the column is one of the main problems of the study of eccentric punching. Existing reinforced concrete (RC) slabs have usually been designed without shear reinforcement. Previous design codes have made possible to assume that the shear capacity of regular reinforced concrete was sufficient. Punching strength in slabs can become insufficient due to several reasons such as changes of building usage and loading, need of installing new services that requires openings in the slabs, and in the relevant updated design codes [3]. Over the past decade, a number of research has dealt with various strengthening techniques for RC flat slabs in order to increase the punching shear capacity. These include enlarging the supporting area by adding concrete capitals or steel collars at the connection zone [4], [5], introducing post-installed shear reinforcement around columns [6], using of post-installed prestressed members [7], and more recently applying FRP sheets to the tension face [8], [9]. Some of these methods provides an enough additional strength to the slabs; however, they are elaborate, difficult to install, expensive and aesthetically not pleasing. Strengthening slabs with FRPs is simple, does not require excessive labor or equipment, and does not change the appearance of the slab. However, the FRP strengthening technique has a few disadvantages mainly associated with the use of epoxy resins, namely high cost, poor performance in high temperatures, inability to apply on wet surfaces, as discussed by Triantafillou et al. (2018) [10]. One possible solution to the above problems would be the replacement of organic with inorganic binders, e.g. cement-based mortars, leading to the replacement of FRP with textilereinforced mortar (TRM) [11]. A TRM is a composite comprises high-strength fibers made of carbon, basalt or glass in form of textiles embedded into inorganic materials such as cementbased mortars. The textiles typically consist of fiber rovings woven or stitched at least in two orthogonal directions, thus creating an open-mesh geometry. TRM is a relatively low cost strengthening material, friendly for manual workers and compatible to concrete or masonry substrates material, whereas can be applied on wet surfaces or at low temperatures. The same material can also be found in the literature as FRCM. Significant research effort has been put in the last decade to use TRM system as strengthening materials for reinforced concrete (RC) members [12]–[14]. Few research studies have been reported in the technical literature on using TRM to strengthen a flat slab against punching shear failure [15], [16]. To the author knowledge, the effect of applying TRM jackets on the punching shear behavior of the RC flat slabs subjected to combined action of shear force and unbalanced moment were not investigated in the literature. In this paper, a nonlinear finite element analysis using ABAQUS program (2016) was conducted to investigate the effect of using TRM composite system on the punching behavior of flat slabs in terms of load carrying capacity, stiffness, ductility, and crack patterns thereby compared that with the effect of using CFRP system. The parameter http://www.iaeme.com/IJCIET/index.asp 337 [email protected] Majid H. Abdulhussein, Dr. Muhammad J. Kadhim investigated in the numerical analysis are the eccentricity of the load (zero and 150 mm), type of external strengthening (CFRP and TRM), and mesh opening of the carbon fiber textile (10 and 20 mm). 2. CONSTRUCTION AND VERIFICATION OF FINITE ELEMENT MODEL 2.1. Experimental specimens and investigated parameters Three slab specimens test by Abdulhussein (2018) [17] were selected to simulate the finite element model. The slab’s dimensions of 1200 × 1200 × 100 mm represent models on the scale of 1/2 with respect to a real multi-story building, taking into consideration the supporting clearness, Fig. 1. The tested specimen represented the zone of negative moments enclosed by the lines of contra-flexure around an interior column. In addition, a 150 × 150 mm central column stub was extended from the compression faces of the slab and a rectangular bracket was connected to the top of the column in order to simulate the unbalanced moment. The perimeter of the slab was simply-supported on 25 mm diameter steel rods fixed on the upper rectangular steel base of a rigid steel frame. All test specimens, which have same dimensions, were reinforced with a flexural reinforcement ratio 2.24% in one orthogonal gathered set on the tension side. one of the three slabs (S1-e75-CFRP) was strengthened by adding a (1000×1000 mm) unidirectional carbon fiber sheet (CFRP) to the tension face of the slab as explain in Fig. 2 a. whereas the other two slabs S3-e75-TRM1 and S4-e75-TRM2 were strengthened by using textile-reinforced mortar (TRM) with two different mesh openings 10 and 20 mm, respectively, as shown in Fig. 2 b, c. All the three slabs were tested under an eccentricity equal to half-length of the column side (75 mm). A comparison between the simulation and the testing results was made and Based on the validity of the numerical model, the numerical investigation was extended to analyzed the three specimens under concentric and high-eccentric loading (i.e., under an eccentricity equal to zero and to the column dimension, 150 mm). The analyzed specimens were also divided into two groups, namely G1 and G2 based on eccentricity amount 0 and 150 mm, respectively. Each group compares three strengthened specimens and one control specimen (unstrengthened) as presented in Table 1. (b) Section A-A (a) Top view Figure 1 Geometry of specimen and typecal reinforement(dimension in mm). http://www.iaeme.com/IJCIET/index.asp 338 [email protected] Numerical Investigation on the Punching Behavior of RC Flat Slabs Strengthening by TRM and FRP (a) Specimen (S1-e75-CFRP) (b) Specimen (S3-e75-TRM1) (c) specimen (S4-e75-TRM2) Figure 2 Details and configrations of CFRP and TRM strengthening systems (dimensions in mm). Table 1 Details of invesitgated specimens. Eccentricity Specimen Strengthening systems Descriptiona mm S2-e00-XX Control specimen (unstrengthened) S1-e00-CFRP CFRP-sheet + epoxy risen. G1 0 b S3-e00-TRM1 Carbon fiber textile + two layers of cement mortar. c S4-e00-TRM2 Carbon fiber textile + two layers of cement mortar. S5-e150-XX Control specimen (unstrengthened) S1-e150-CFRP CFRP-sheet + epoxy risen. G2 S3-e150-TRM1 150 Carbon fiber textile + two layers of cement mortar. S4-e150-TRM2 Carbon fiber textile + two layers of cement mortar. a) Specimen description composed of three parts: the first represent slab number, the second is a number stands for the load eccentricity, and the third is a character symbolizes for the strengthening type, characters “XX” was used when there were no strengthening provided. b and c) the numbers 1 and 2 referred to the mesh size of the textile, which equal to 10 mm and 20 mm, respectively. Group No. 2.2. Finite element modeling In this paper, a three-dimensional nonlinear finite element analysis has been carried out to simulate the punching behavior of RC flat slabs by using a powerful nonlinear finite element package ABAQUS/Standard 2016.The geometry, applied load, and boundary condition were simulated to be similar to that in experimental work. The modeling including element types, mesh details, and interactions will be presented in this section. 2.2.1. Mesh details Selection of mesh size is an important step in finite element modeling. Before the analysis is started, an adequate pre-analysis of different mesh densities to determine the best density that giving the required accuracy according to the level of analysis complexity. Therefore, a convergence analysis was made on the FE model to get the appropriate mesh size. It can be observed that the change in the results can be ignored when the mesh size decreased from 30 http://www.iaeme.com/IJCIET/index.asp 339 [email protected] Majid H. Abdulhussein, Dr. Muhammad J. Kadhim to 20 mm and the FE results become more accurate with experimental ones, therefore, 25 mm mesh size was selected for the analysis. 2.2.2. Material modeling, element types, and contraction In general, five parts were involved in the modeling of the specimens. These five parts were concrete slab, concrete column, flexural reinforcement of the slabs, reinforcing bars of the column, and bearing plate. These parts were drawn separately and then assembled and merged to form the modeling specimens. Fig. 3 explains the assembly of these five parts. Flexural Concrete column Column Bearing plate reinforcemen reinforcemen Concrete slab Figure 3 The assembled parts of FE model. Concrete material ABAQUS offers different constitutive models to analyze concrete structures. In this paper, the concrete damaged plasticity (CDP) is chosen for the punching shear simulations. The concrete damaged plasticity model is based on the scalar isotropic damage assumption considering the stiffness degradation in both compression and tension [18], [19]. Tensile cracking and compressive crushing of concrete are assumed as two main failure mechanisms in this model. The nonlinear behavior of the concrete material is represented by an equivalent uniaxial stress-strain response. Concrete in tension can be characterized by stress-crack displacement response instead of a stress-strain relationship due to its brittle behavior, Fig.4 a. In this study, bilinear stiffening response is used and calculated according to Genikomsou (2016) [18]. While the uniaxial stress-strain response of concrete in compression is elastic until the initial yield is reached, Fig.4 b. To define the stress-strain curve of concrete, the model of Collins and Mitchell (1997) [20] were used after converting the nominal strain to inelastic strain. The concrete of the slab and the column was meshed into solid brick elements to achieve suitable stress distribution in the 3D Finite element analysis. There are several forms of solid brick elements available in ABAQUS. In this study, Linear Hexahedral elements with reduced integration (C3D8R) have been selected. Steel materials The required input parameters for material definition of steel materials, includes density, elastic and plastic behavior. Elastic behavior of steel material is defined by specifying Young’s modulus (Es) and Poisson’s ratio () of which typical values are 200 GPa and 0.3, respectively. Plastic behavior is defined in a tabular form, included yield stress and corresponding plastic strain. According to Hibbit et al. (2011) [21], true stress and logarithmic strain should be defined, so input values of stress in each point for an isotropic material are calculated according to that. yielding strength of the flexural reinforced is listed in Table 2. In this simulation, the steel reinforcement for the slab and column was divided into Linear Truss element (T3D2). While the bearing plate was divided into Linear Hexahedral elements http://www.iaeme.com/IJCIET/index.asp 340 [email protected] Numerical Investigation on the Punching Behavior of RC Flat Slabs Strengthening by TRM and FRP with reduced integration (C3D8R). Steel reinforcement was linked by embedded region constrain to the surrounding concrete. TRM-jacket TRM was represented by a linear truss element (T3D2), for the textile, embedded into a solid brick element for mortar (i.e. Linear Hexahedral elements with reduced integration (C3D8R) have been selected to simulate the mortar matrix). To model the textile, an equivalent diameter for the textile was calculated and a circular profile was assumed. A perfect bond was assumed not only between the textile and the surrounding mortar but also between TRM composite system and concrete substrate. The properties of the textile and the mortar, based on the manufacturer’s product data sheet, were presented in Table 2. The constitutive model used to simulate mortar was adopted from Awani (2015) [22]. CFRP-sheet A unidirectional CFRP lamina can usually be treated as an orthotropic material whose mechanical properties in the ﬁber direction are different from those in the other two orthogonal directions. That is, the elastic modulus, shear modulus and Poisson’s ratios are different in different directions. the FRP lamina is modeled as plane stress element, and the mechanical properties of the FRP lamina can be obtained from the two constituents (i.e., ﬁbers and epoxy) and their volume fractions based on the mechanics of materials approach [23] are calculated and listed in Table 3. A bilinear cohesive model available in ABAQUS is a best select for modelling the interface behavior between CFRP lamina and concrete surface, as shown in Fig. 4 b. The cohesive model defines surfaces of separation and prescribes their interaction by describing a proportional displacement at each contact point. The definition of the model is characterized by the parameters, initial stiffness, shear strength, fracture energy and curve shape of the bond slip model. These parameters as a function of the adhesive and concrete properties are determined according to Obaidat (2011) [24]. . (a) Bilinear cohesive model [24] (a) Bilinear tension softening [18] Figure 4 Bilnear tension softening and cohesive model used in this study. http://www.iaeme.com/IJCIET/index.asp 341 [email protected] Majid H. Abdulhussein, Dr. Muhammad J. Kadhim Table 2 Properties of Materials used in this study Parameter Concrete Average compressive strength (MPa) Average modulus of rapture (MPa) Mortar* Compressive strength (MPa) Tensile strength (MPa) Property Parameter Carbon fiber textile* 42.38 Tensile strength of fibers (GPa) 7.38 Elastic modulus of fibers (GPa) Equivalent dry fiber thickness (mm) 70 Width per one textile (mm) 6.0 Fracture strain (%) Steel Yield strength of flexural reinforcement (MPa) * As provided by manufacture. Property 4.9 230 0.4 4.0 2.0 461 Table 3 Properties of CFRP laminate used in this study. Composite CFRP Longitudinal Young’s modulus E1 (MPa) 77,560 Transverse Young’s modulus E2 (MPa) 6,600 Major Poisson's Ratio V12 --0.267 Tensile strength Shear modulus G12 Thickness of lamina (MPa) 3,500 (MPa) 2,540 (mm) 1 3. RESULTS AND DISCUSSION As previously mentioned, three half-scale specimens were used for calibrating a 3D nonlinear finite element model. These specimens were strengthened with two different system (CFRP and TRM) and tested experimentally under an eccentricity equal to half of the column side (75 mm). A comparison between the simulation and the testing results showed good validity of the numerical analysis where the punching load of the analyzed models are more than the experimental values by a difference less than 2.91%. Fig. 5 depict the experimental and numerical curve for the three slabs. Based on that, the constructed FE model used to conduct a numerical study on the punching behavior of these strengthened specimens with concentric and high-eccentric load (i.e. with 0 and 150 mm eccentricity). The concentrically loaded specimens named as G1 and eccentrically loaded specimens named as G2, as explained in Table 1. In addition, one unstrengthened specimen was built as control for each group. The output data that have been extracted from the analysis are the load-carrying capacity and central deflection, which are directly obtained from the ABAQUS simulation. The summary of the results are given in Table 4. Table 4 Numerical results. Group No. G1 G2 Specimens Symbol S1-e00-CFRP S3-e00-TRM1 S4-e00-TRM2 S2-e00-XX S1-e150-CFRP S3-e150-TRM1 S4-e150-TRM2 S5-e150-XX Ultimate Load Ultimate Deflection KN 362.14 335.24 306.56 259.29 216.45 206.63 186.88 155.84 mm 12.82 8.11 8.49 10.08 11.92 5.62 5.78 8.72 http://www.iaeme.com/IJCIET/index.asp 342 Energy Absorption Index --3.85 2.95 2.71 3.27 3.15 1.84 1.52 2.84 Uncracked Stiffness Cracked Stiffness Loss in Stiffness KN/mm 62.33 89.88 67.52 45.74 35.02 47.39 41.07 31.13 KN/mm 36.37 66.59 52.97 28.03 21.86 36.90 32.00 20.23 % 58 74 78 61 62 78 78 65 [email protected] Numerical Investigation on the Punching Behavior of RC Flat Slabs Strengthening by TRM and FRP S1-e75-CFRP 250 Experimental Numerical 200 200 150 150 100 100 50 50 0 0 2 4 6 8 10 12 0 14 0 250 Experimental Numerical Punching Load (KN) Punching Load (KN) 250 S4-e75-TRM2 S3-e75-TRM1 Experimental Numerical 200 150 100 50 0 2 Central Deflection (mm) 4 6 8 10 Central Deflection (mm) 12 0 2 4 6 8 10 12 Central Deflection (mm) Figure 5 Comparison between the experimental and numerical load-deflection curves. 3.1. Load-deflection curves Concentrically loaded specimens Evaluate the efficiency of using CFRP and TRM strengthening techniques on punching shear behavior under concentric loading. In comparison with the control specimen (S2-e00-XX), the analytical results showed that strengthening of slabs by installing CFRP-sheet on the tension face of the slab reduced the deflection at the failure load of control specimen by 24.70%. Whereas the load carrying capacity and the ultimate deflection increased by 39.67% and 27.18%, respectively. The slab strengthening with TRM1 showed a reduction in the ultimate deflection and deflection at the failure load of control specimen by 19.54 and 58.83%, respectively. However, the ultimate punching capacity improved by 29.29% in compare with the control specimen. Appling TRM2 as an external strengthening system decreased the ultimate deflection and deflection at the failure load of control specimen by 15.77 and 47.32%, respectively. Whereas the ultimate punching strength improved by 18.28% in compare to the control slab. Fig. 6 shows the variety of the load-deflection curves for the three specimens and the reference one. From the above mention, the efficiency of using TRM strengthening system in increasing the punching shear capacity of strengthened slabs was less than that of CFRP system. However, TRM efficiency was sensitive to the mesh size of the carbon fiber textile. It was found that using of carbon fiber textile with mesh size 20 and 10 mm increased the punching capacity by 18.28 and 29.29%, respectively as compared with control specimen. Eccentrically loaded specimens Examining the efficiency of using CFRP and TRM strengthening techniques on punching shear behavior of flat slabs under high-eccentric loading (i.e. under eccentricity equal to the column side, 150 mm). It was noted from the analytical results that adding CFRP-sheet on the tension face of the slab increased the load carrying capacity and the ultimate deflection by about 38.90% and 36.70%, respectively, as compared with the control specimen (S5-e150XX). On the other hand, installing TRM-jackets on the tension face of the slabs was slightly affected by the mesh size of the textile. It was observed that using textile with 10 and 20 mm mesh size resulted in increasing the punching capacity of the specimens by around 38.90 and 32.60%, respectively, and decreasing the ultimate central deflection by about 35.55 and 33.72%, respectively. Fig. 6 presents the load-deflection curves for eccentrically loaded specimens. 3.2. Stiffness of analyzed slabs In general, the stiffness can be expressed as the slope of the load-displacement relationship. Marzouk and Hussein (1991) [25] stated that "For most slabs failing in punching shear, the http://www.iaeme.com/IJCIET/index.asp 343 [email protected] Majid H. Abdulhussein, Dr. Muhammad J. Kadhim 350 350 300 300 Punching Load (KN) Punching Load (KN) load deflection curves can be represented by two straight lines with different slopes". initial stiffness (Uncracked stiffness, Ki) described by the slope of the Load-displacement curve reaching up to the first change in the slope (first cracking load), while secant stiffness (cracking stiffness, Ks) defined by the slope of the load-displacement curve extending up to the first yielding of the flexural reinforcement [26], see Fig 7a. Table 5 elucidates the results of the initial and secant stiffness. In addition, the percentage of reduction in the stiffness after cracks initiated also calculated and summarized in this Table. It can be concluded that using CFRP and TRM as external flexural reinforcement increased the initial and secant stiffness. Compared to the control specimens, the maximum increase in the uncracked and cracking stiffness was about 96.49 and 137.56%, respectively, for concentrically loaded specimens, and about 52.24 and 82.41%, respectively, for eccentrically loaded specimens. Specimens strengthening with TRM jacket showed a better improvement in stiffness and that was sensitive to the mesh size of the textile (i.e. stiffness increased, when the mesh size decreased). The effects of applying external strengthening on initial and secant stiffness are explained in Fig. 8. It was observed from Fig.8 that increasing the stiffness in by applying external strengthening was highly affected by eccentricity amount. For concentrically loaded specimens, the average increase in the initial and secant stiffness was around 60.13 and 85.42%, respectively. While for eccentrically loaded specimens (i.e., in case of eccentricity 150 mm), the average increase in the initial and secant stiffness in was around 32.23 and 49.56%, respectively. It was shown that the best improvement in the stiffness was by applying TRM of 10 mm mesh size on the tension face of the slab. 250 200 150 Control: S2-e00-XX Specimens of G1: S1-e00-CFRP S4-e00-TRM2 S3-e00-TRM1 100 50 250 200 150 Control: S5-e150-XX Specimens of G2: S4-e150-TRM2 S1-e150-CFRP S3-e150-TRM1 100 50 0 0 0 2 4 6 8 10 12 14 16 0 Central Deflection (mm) 2 4 6 8 10 12 14 16 Central Deflection (mm) Figure 6 load-deflection curves (left: concentric, right: eccentric) (a) Evaluation of initial and secant (b) Evaluation of energy absorption Figure 7 Determination of stiffness and energy absorption index. http://www.iaeme.com/IJCIET/index.asp 344 [email protected] Numerical Investigation on the Punching Behavior of RC Flat Slabs Strengthening by TRM and FRP +96.49% 110 100 40 +12.51% 31.13 50 45.74 60 +31.94% 70 +52.24% +47.63% +36.27% 80 S4-e150-TRM2 S3-e150-TRM1 S1-e150-CFRP S5-e150-XX S4-e00-TRM2 10 S3-e00-TRM1 20 S1-e00-CFRP 30 S2-e00-XX Initial Stiffness (KN/mm) 90 +137.56% 0 80 +82.41% S4-e150-TRM2 S3-e150-TRM1 S1-e150-CFRP 20.23 S5-e150-XX S4-e00-TRM2 10 S3-e00-TRM1 20 S1-e00-CFRP 30 28.02 40 +8.07% +29.74% 50 +58.20% +88.97% 60 S2-e00-XX Secant Stiffness (KN/mm) 70 0 eccentricity = 150 mm eccentricity = 0 Eccentricity Effect Figure 8 Effect of eccentricity and external strengthening on the initial and secant stiffness. 3.3. Ductility of analyzed slabs Abdulraheem and Mohammed (2018) [27] stated that calculating the energy absorption index (EAI) provides a better approach for determining the ductility of RC members. Energy absorption index (EAI) defined by Husain et al. (2017) [26] as the ratio of the total area under load-deflection curve to that under the ascending portion only as explained in Fig 7 b. According to that, the effect of concentric and high-eccentric loading on the ductility of externally strengthened specimens was determined by calculating and comparing the energy absorption index of them with the control specimens. Table 4 summarizes the results of the energy absorption index of the analyzed specimens. Fig. 9 Show the effect of applying external strengthening on the energy absorption index with different cases of eccentricity, (i.e., zero and 150 mm eccentricities). It can be clarified from the results that using CFRP-strengthening technique increased the ductility of the tested specimen and that was also affected by the eccentricity as shown in Fig 9. In the other hand, TRM-strengthened slabs showed a different behavior in terms of ductility than CFRPstrengthened slabs. Using TRM-strengthening techniques showed a reduction in the ductility of the specimens and that was sensitive to the mesh size of the textile used. It was also noted that increased the eccentricity of the load decreased the calculated ductility. 3.4. Crack patterns The cracks in Finite Element Analysis spreads within the slab near the column. It starts tangentially close to the column and so extends radially as the load increases. At the failure load, the punching cone is obvious due to the sudden cracks opening. Concrete damaged plasticity model considers that the cracking initiates when the maximum principal plastic strain (PE) is positive [18]. The direction of the cracks is taken into account to be perpendicular to the maximum principal plastic strains (PE) and so, the orientation of the http://www.iaeme.com/IJCIET/index.asp 345 [email protected] Majid H. Abdulhussein, Dr. Muhammad J. Kadhim cracks are visualized through the maximum principal plastic strains [28]. The tensile principal stresses can be utilized in Finite Element Analysis in order to indicate crack patterns but the maximum principal plastic strains provide a higher illustration of the cracks [18]. For that reason, the strains will be used for viewing the crack patterns for all tested slabs. Fig. 10 showed the crack patterns (on the tension face of the slabs) of the all analyzed specimens. It can be seen from Fig. 10 that the crack propagation of specimens strengthened by CFRP were within the direction of the fiber distribution. While the crack patterns for slabs strengthened by TRM-jackets were similar to that of control ones, but these cracks were spread in different directions towards the edges. In addition, presence of eccentricity of applied load resulted in excessive damage to the half tension surface of the slab (right portion of the photograph shown in Fig. 10 as compared with the other half). Whereas the crack patterns of specimen tested under concentric load was approximately symmetric about the two axes. -35.21% +46.46% S3-e150-TRM1 S4-e150-TRM2 2.84 +11.07% -9.79% 3.27 3 S1-e150-CFRP S5-e150-XX S4-e00-TRM2 S3-e00-TRM1 1 S1-e00-CFRP 2 S2-e00-XX Energy apsorbtion index 4 -17.21% +17.69% 5 0 eccentricity = 150 mm eccentricity = 0 Eccentricity Effect Figure 9 Effect of eccentricity and external strengthening on energy absorption index. S2 S5- S1- S3- S1- S3- S4- S4- Figure 10 Crack patterns of analyzed specimens. http://www.iaeme.com/IJCIET/index.asp 346 [email protected] Numerical Investigation on the Punching Behavior of RC Flat Slabs Strengthening by TRM and FRP 4. CONCLUSIONS Based on the results obtained in this paper using numerical analysis for the RC fat slabs strengthened by CFRP and TRM and tested with two different eccentricities (0 and 150 mm), the conclusions can be drawn as the following: For concentrically loaded specimens, using CFRP increases the punching load and the central deflection of the slabs by about 39.67 and 27.18%, respectively. While using TRM with 10 and 20 mm mesh opening increases the punching load by around 29.29 and 18.28%, respectively, but decreases the central deflection by 58.83 and 47.32, respectively as Compared with control specimens. For eccentrically loaded specimens, using CFRP increases the punching load and the central deflection of the slabs by about 38.9 and 36.7%, respectively. While using TRM with 10 and 20 mm mesh opening increases the punching capacity by around 38.90 and 32.60%, respectively, and decreases the central deflection by about 35.55 and 33.72%, respectively, as compared with control specimens. It was observed that increasing the stiffness by applying external strengthening was highly affected by eccentricity amount. For concentrically loaded specimens, the average increase in the initial and secant stiffness was around 60.13 and 85.42%, respectively. While for eccentrically loaded, the average increase in the initial and secant stiffness was around 32.23 and 49.56%, respectively. It was shown that the best improvement in the stiffness was by applying TRM and that was sensitive to the mesh opening of the textile (i.e. stiffness increased, when the mesh size decreased). 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