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Behavior of concrete beams reinforced with hybrid steel and FRP composites

Behavior of concrete beams reinforced with hybrid steel and FRP composites HBRC Journal (2018) 14, 300–308 Housing and Building National Research Center HBRC Journal http://ees.elsevier.com/hbrcj Behavior of concrete beams reinforced with hybrid steel and FRP composites Suzan A.A. Mustafa , Hilal A. Hassan Structural Engineering Department, Faculty of Engineering, Zagazig University, Egypt Received 8 August 2016; revised 31 December 2016; accepted 19 January 2017 KEYWORDS Abstract This paper presents a nonlinear finite element model to investigate the behavior of hybrid Fiber Reinforced Polymers and steel reinforcement. Different types of Fiber Reinforced Polymers; Concrete beam; CFRP and GFRP; were used along with steel rebars in the studied concrete beams. The study was Hybrid; conducted using the nonlinear finite element program ‘‘ANSYS”. Nonlinear material models for the Steel reinforcement; components of the concrete beam were used in the three dimensional finite element models. The FRP reinforcement; Flexure; outcomes got from finite element analysis were confirmed against experimental results. A broad Finite element analysis parametric study was conducted to explore the effect of replacing steel reinforcement by different types of FRP bars. The study showed that the contribution of steel rebars to FRP rebars in concrete beams improved beam ductility and eliminated the unfavorable brittle failure of the concrete beam. In addition, it is better to use steel rebars as top reinforcement in concrete beams with hybrid rein- forcement. In hybrid GFRP/steel reinforced concrete beams, a significant reduction in stiffness and a noticeable increase in the beams’ deflection after the initiation of first crack and yielding of steel reinforcement were observed. On the other hand, in hybrid CFRP/steel reinforced concrete beams showed a better performance during cracking initiation and propagation. 2017 Housing and Building National Research Center. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Introduction few decades ago. The well-known high cost of FRP material might be a preventive to its use. However, FRP is a quite competitive material to use. It has a noticeable high strength The use of fiber-reinforced polymer (FRP) composites for and stiffness ratios to density, In addition to their resistance reinforcing or rehabilitation of structural members started to corrosion. The light weight of FRP provides significant Corresponding author. savings in labor cost. Furthermore, fatigue durability, and E-mail addresses: samustafa@eng.zu.edu.eg (S.A.A. Mustafa), the transparency of FRP bars to magnetic and electrical fields hilalcivil@yahoo.com (H.A. Hassan). makes them a applicable alternative to steel reinforcement Peer review under responsibility of Housing and Building National in applications sensitive to electromagnetic fields such as Research Center. magnetic resonance imaging; Hollaway [1]. In reinforced concrete structures, steel plays the main rule of ductility which is the ratio of post yield deformation to yield Production and hosting by Elsevier deformation. This traditional meaning of ductility cannot be http://dx.doi.org/10.1016/j.hbrcj.2017.01.001 1687-4048  2017 Housing and Building National Research Center. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Behavior of concrete beams reinforced with hybrid steel and FRP composites 301 applied to structures reinforced with FRP reinforcement due analytical finite element modeling and/or experimental studies to the linear elastic behavior of FRP bars. A lot of exertion to investigate the behavior of reinforced concrete beams rein- has been made to enhance and characterize the ductility of forced or strengthened with FRP. beams reinforced with FRP bars. The ductility index for These studies focused on simply supported and continuous FRP reinforced structures could be calculated using the energy FRP-reinforced concrete beams [7–9]. El-Mogy et al. [10,11] based method and the deformation based method; Jaeger et al. presented an experimental and a finite element study on the [2]. flexural behavior of continuous FRP-reinforced concrete Harris et al. [3] tried to mimic the elastic plastic behavior of beams. El-Mogy et al. [12] concluded that deflection can be the steel by using hybrid FRP rebars. They utilized pseudo- decreased when increasing the transverse GFRP reinforcement ductile materials at which two or more different FRP reinforc- in continuous concrete beams inspite of keeping the longitudi- ing materials are combined. They noticed that the ductility nal reinforcement the same. Chitsazan et al. [13] studied exper- index of the examined beams is almost the same as that of imentally the enhancement of the flexural behavior of concrete the traditional steel- reinforced beams. The complicated man- beams with GFRP bars by using high strength concrete and ufacturing process in addition to the high cost of the hybrid increasing the effective depth over the breadth of the beam. bars restricted its practical applications. Another methodology Deifalla et al. [14] found that the bonded GFRP stirrups were to enhance the property of the beam is to make the ductility of more effective as transversal reinforcement compared with the the system strongly dependent on the concrete properties. bent steel stirrups and the bent GFRP stirrups. It improved the Alsayed and Alhozaimy [4] examined 18 steel and FRP rein- ultimate torsional strength and increased the corresponding forced concrete beams and found that with the addition of unit angle of twist and the maximum stirrup strain as well as 1% steel fibers, the ductility index can be increased by as much the major concrete crack width in L-shaped beam subjected as 100%. ACI 440 [5] recommends that the FRP reinforced to torsion. Rana et al. [15] investigated the effect of carbon structures should be over reinforced and concrete beams have fiber/glass fiber weight ratio on both strain sensitivity and ten- to be designed so that they fail by concrete crushing rather sile properties of concrete beams. They observed that the stud- than by rupture FRP reinforcement. Li and Wang [6] demon- ied composites with lowest amount of carbon fibers led to best strated that the GFRP rebars reinforcing engineered cementi- strain sensitivity and good mechanical properties. tious composite material improved flexural practices and The principle goal of this study is to build up a nonlinear ductility of the concrete element. Many studies utilized finite element model to investigate the behavior and strength (a) Beam Tested by Rafi et al. (2008) (b) Beam Tested by El-Mogy et al. (2010) Figure 1 Dimensions and details of verified experimental beams. 302 S.A.A. Mustafa, H.A. Hassan of concrete beams reinforced by hybrid steel and different verify the developed finite element model. The two specimens types of FRP bars. Both GFRP and CFRP bars were used tested by El-Mogy et al. [10] were two-spans continuous con- as negative and positive reinforcement of concrete beams. crete beams tested in flexure. One beam was reinforced with The finite element commercial program ANSYS [16] was uti- GFRP bars (GS1), and the other was reinforced with steel bars lized in the analysis. Nonlinear material properties of the beam (SS1). The reinforcement of the two beams is detailed in Fig. 1 components were used. The results obtained from the model (b). The length of the beams was 6000 mm with a rectangular were confirmed against the test results conducted by previous cross section with 200 mm and 300 mm width and depth experimental tests of Rafi et al. [17] and El-Mogy et al. [10]. respectively. The beams were continuously supported over An intensive parametric study was conducted to investigate two equal clear spans of 2800 mm each. The overall length the effect of the ratio of steel contribution, amount of FRP of beam (BRC1) tested by Rafi et al. [17] was 2000 mm. The reinforcement and finally FRP type. beam was simply supported with a rectangular cross-section of 120 mm width and 200 mm depth. The concrete beam had two CFRP longitudinal bars on the tension side in addition Finite element modeling to two steel longitudinal bars on the compression side. A four-point static loading technique was utilized to examine General the simply supported beam with a span of 1750 mm, as detailed in Fig. 1(a). The dimensions and material properties In order to accurately simulate the actual behavior of the con- of the verified specimens are summarized in Table 1. cerned beam, all its components; concrete beam, steel bars, FRP bars and stirrups; have to be modeled properly. Mean- Finite element type and mesh while, choosing the element types and mesh size are important as well in building the model to provide accurate results with To obtain an accurate simulation of the actual behavior of the reasonable computational time. concrete beam reinforced with steel and different types of FRP Recent experimental tests on concrete beam reinforced with bars, the elements composing the finite element model had to steel and FRP conducted by Rafi et al. [17] (specimen BRC1) be chosen properly. The mesh size was carefully selected to and El-Mogy et al. [10] (specimens SS1 and GS1) were used to Table 1 Details of the verified specimens. Rafi et al. [17] Beam Bottom Reinforcement Top Reinforcement f Mpa cu BRC1 Type U (mm) f (Mpa) E(Gpa) type U (mm) f (Mpa) E(Gpa) u u CFRP 9.5 1676 135.9 Steel 8 566 194 42.5 El-Mogy et al. [12] Beam Bottom Reinforcement Top Reinforcement f Mpa cu SS1 Type U (mm) f (Mpa) E (Gpa) Type U (mm) f (Mpa) E (Gpa) y y Steel 15.9 485 200 Steel 15.9 485 200 35 GS1 Type U (mm) f (Mpa) E (Gpa) Type U (mm) f (Mpa) E (Gpa) u u GFRP 15.9 740 46 GFRP 15.9 740 46 Specimen (BRC1): Stirrups u6 mm @ 100 mm. Specimens (SS1) & (GS1): Stirrups u8 mm @ 120 mm. Figure 2 A typical figure of the 3-D FE mesh (selected concrete elements were removed to illustrate reinforcement). Behavior of concrete beams reinforced with hybrid steel and FRP composites 303 obtain high accuracy of results with reasonable computational time. The aspect ratio of the used solid elements was kept as possible within the recommended range; between 1 and 3. The analysis was performed using the ANSYS [16] program. Both material and geometric non-linearity were considered in the analysis. The three-dimensional eight-node solid element SOLID65 was used to model concrete. Each node has three translational degrees of freedom. This element uses linear interpolation functions for displacements. This element is capable of simu- lating the cracking and crushing of brittle materials. Before cracking or crushing, the concrete is assumed to be an isotropic elastic material. After crushing, concrete loses its stiffness in all directions. After cracking, the concrete is assumed to be ortho- tropic having stiffness based on a bilinear softening stress- strain response in the crack normal direction. The 3-D spar element (LINK180) was used to simulate steel, CFRP, GFRP rebars and the stirrups. Link180 is a two-nodes uniaxial tension-compression element with three translational DOF at each node. The elements were embedded in concrete sharing its nodal points. Nonlinearity and plastic deformations are simulated in this element. In order to pre- clude early warnings and premature failure messages due to concrete crushing at the positions of loading and supports, eight-node solid element; solid 185; was used to model the loading plates. A typical figure of the three dimensional finite element mesh of the studied beams is shown in Fig. 2. Material modeling The material properties of the components of the pre-tested specimens were considered as detailed in Table 1. In all cases, the ultimate strain of the concrete at failure was taken as 0.0035 and the Poisson’s ratio of concrete was taken 0.2. A multi-linear isotropic stress-strain relation was used for model- ing concrete material in compression. This relationship con- sists of two portions. The first portion is an ascending curve represented by the numerical expressions; Eqs. (1) and (2), [18] along with Eq. (3) [19]. The curve starts at zero stress and zero strain toward a value of 0.3f , calculated from Eq. (3). The rest points of the ascending curve are obtained from Eq. (1). The strain at ultimate stress of concrete is calculated via Eq. (2). The descending branch which represents strain softening of the ideal stress-strain curve of concrete was ignored as recommended in previous studies [20,21] in order Figure 3 (a) Typical stress-strain curve for concrete; (b) Typical to avoid convergence problems. A bilinear relationship was stress-strain curve for FRP; (c) Typical stress-strain curve for steel. used to represent the stress-strain curve of the steel reinforce- ment while a linear elastic behavior was used for the FRP movement in the loading Y- direction was allowed. Other rebars. The Poisson’s ratio was assumed to be 0.3 for steel rein- nodes were free to displace in any direction except the nodes forcement and 0.2 for FRP. For stirrups and loading plates, representing the supports. The load was applied in small incre- the stress-strain relation was considered linear. Fig. 3 shows ments as usually recommended in testing concrete structures to the stress-strain relations of the different materials used. avoid non-convergence problems. This was achieved with the aid of the load steps and sub-steps. Cracking and crushing of Boundary conditions and load application concrete elements were monitored during the loading steps. The load was applied until failure in all beams. Following the testing procedures conducted by Rafi et al. [17] and El-Mogy et al. [10], simply supported boundary conditions Verification of finite element model were applied at the position of edge support. Due to symmetry of all the pre-tested beams, only half of each beam was mod- eled, as shown in Fig. 2. The nodes in the middle symmetry To validate the finite element model, a comparison was held surface were prevented to displace in Z- direction, while their with available pre-tested beams. The ultimate loads and the 304 S.A.A. Mustafa, H.A. Hassan corresponding maximum deflection of the tested specimens Failure of the beam with steel reinforcement was due to steel (P ,D ) and the finite element analysis (P ,D ) as well as yielding. The steel yielded at mid-span and at the middle T T FE FE the load–deflection curves, and deformed shapes after failure support. Tensile cracks in concrete were generated. The first have been investigated for all types of reinforced concrete crack was at about 23% of the ultimate load at the middle beams. In addition, crushing and cracking pattern of the con- support of the beam, as shown in Fig. 4(b)-a. At the crete beams of the compared specimens is shown. Good agree- next load step, cracks began at mid-span as shown in ment has been achieved between both results. The mean values Fig. 4(b)-b, then all cracks increased and propagated until of P /P and D /D ratios were 0.98 and 1.07 respectively. failure. In concrete beam reinforced with GFRP rebars, the T FE T FE The corresponding coefficients of variation (COV) were about failure was in concrete. The GFRP rebars did not reach their 0.04 and 0.053 respectively. Figs. 4–6 plot the load–deflection tensile stress or strain at failure of the beam neither at mid- curves of the modeled beams. In addition, cracking and crush- span nor at the middle support. The first cracks were noticed ing patterns for the reinforced concrete beams are illustrated. at the middle support and at mid-span simultaneously at The results of the proposed 3-D nonlinear finite element model about 22% of the ultimate load, as shown in Fig. 5(b)-a. matched the experimental results fairly well and the finite ele- Afterwards, the cracks increased and propagated until failure ment model successfully predicted the behavior of the beams. happened in concrete. Concrete cracks and crushing are ,"# shown in Fig. 5(b). In the simply supported beam simulating that tested by f ¼ E e 1 þ ð1Þ 0 Rafi et al. [17] with CFRP rebars, concrete failure was noticed. The CFRP bars did not reach its tensile stress or 2f strain at the failure of the beam. The first crack was at about e ¼ ð2Þ 22% of the beam ultimate load at the middle of the span. The produced cracks increased and propagated until failure. Compressive cracks and failure was noticed in the concrete, E ¼ ð3Þ e as shown in Fig. 6. (a) Load-Deflection Curve (b) Cracking and Crushing Pattern Figure 4 Beam with steel reinforcement. (a) Load-Deflection Curve (b) Cracking and Crushing Pattern Figure 5 Beam with GFRP reinforcement. Behavior of concrete beams reinforced with hybrid steel and FRP composites 305 (b) Cracking and Crushing Pattern (a) Load-Deflection Curve Figure 6 Beam with CFRP reinforcement. Table 2 Studied parameters. Group Model Bottom Reinf. Top Reinf. Notes P/P Failure mode Ref Bottom Reinf. Top Reinf. SM1 4/16 S 3/16 S Steel Steel 1 Steel yielding & concrete crushing GI - S M2 3/16 S + 1/16 G 3/16 S Steel + GFRP Steel 0.94 Steel yielding & concrete crushing M3 2/16 S + 2/16 G 3/16 S Steel + GFRP Steel 0.88 Steel yielding & concrete crushing M4 1/16 S + 3/16 G 3/16 S Steel + GFRP Steel 0.74 Steel yielding & concrete crushing M5 4/16 G 3/16 S GFRP Steel 0.71 Steel yielding & concrete crushing GI - G M6 3/16 S + 1/16 G 3/16 G Steel + GFRP GFRP 0.81 Steel yielding & concrete crushing M7 2/16 S + 2/16 G 3/16 G Steel + GFRP GFRP 0.72 Steel yielding & concrete crushing M8 1/16 S + 3/16 G 3/16 G Steel + GFRP GFRP 0.68 Steel yielding & concrete crushing M9 4/16 G 3/16 G GFRP GFRP 0.65 Concrete crushing CI - S M10 3/16 S + 1/16 C 3/16 S Steel + CFRP Steel 1.006 Steel yielding & concrete crushing M11 2/16 S + 2/16 C 3/16 S Steel + CFRP Steel 0.995 Steel yielding & concrete crushing M12 1/16 S + 3/16 C 3/16 S Steel + CFRP Steel 0.98 Steel yielding & concrete crushing M13 4/16 C 3/16 S CFRP Steel 0.95 Steel yielding & concrete crushing CI - C M14 3/16 S + 1/16 C 3/16 C Steel + CFRP CFRP 1.02 Steel yielding & concrete crushing M15 2/16 S + 2/16 C 3/16 C Steel + CFRP CFRP 1.02 Steel yielding & concrete crushing M16 1/16 S + 3/16 C 3/16 C Steel + CFRP CFRP 1.02 Steel yielding & concrete crushing M17 4/16 C 3/16 C CFRP CFRP 1.02 Concrete crushing GII - S M18 4/18 G 3/16 S GFRP Steel 0.81 Steel yielding & concrete crushing M19 4/20 G 3/16 S GFRP Steel 0.86 Steel yielding & concrete crushing M20 4/22 G 3/16 S GFRP Steel 0.94 Steel yielding & concrete crushing GII - G M21 4/18 G 3/16 G GFRP GFRP 0.71 Concrete crushing M22 4/20 G 3/16 G GFRP GFRP 0.71 Concrete crushing M23 4/22 G 3/16 G GFRP GFRP 0.71 Concrete crushing CII - S M24 4/18 C 3/16 S CFRP Steel 1.04 Steel yielding & concrete crushing M25 4/20 C 3/16 S CFRP Steel 1.13 Steel yielding & concrete crushing M26 4/22 C 3/16 S CFRP Steel 1.19 Steel yielding & concrete crushing CII - C M27 4/18 C 3/16 C CFRP CFRP 1.22 Concrete crushing M28 4/20 C 3/16 C CFRP CFRP 1.27 Concrete Crushing M29 4/22 C 3/16 C CFRP CFRP 1.32 Concrete crushing S:Steel, G: Glass FRP (GFRP), C: Carbon FRP (CFRP), P : Ultimate load of reference beam (M1). Ref Dimensions, loading pattern and material properties followed Parametric study exactly the experimental work. The studied beams were divided into 8 groups with different hybrid reinforcement, as A total of 29 concrete beams were analyzed in the current detailed in Table 2. parametric study. The study was performed on continuous All the analyzed beams behaved similarly until first crack beam similar to the pre-examined by El-Mogy et al. [10]. (the initial linear part of the load-deflection curves) Figs. 7 306 S.A.A. Mustafa, H.A. Hassan and 8. The behavior of hybrid FRP/steel reinforced concrete age of its ultimate value. This demonstrates that this reinforc- beams was compared to the behavior of the reference steel ing arrangement is suitable for flexural failure control. On the reinforced concrete beam (M1). The stiffness of the hybrid other hand, beams with GFRP bars as top reinforcement and reinforced concrete beams with various percentages of FRP hybrid GFRP/steel bars as lower reinforcement, compressive and steel rebars lied between those of their counterpart FRP stresses reached almost half of the GFRP ultimate value. Since and steel reinforced beams, as shown in Figs. 7 and 8(a and the compressive strength of the GFRP bars reduces by up to b). Increasing the steel percentage to the GFRP rebars in the 45% with respect to the value in tension [21], it is better to studied concrete beams increases the stiffness of the beam after use steel bars as top reinforcement in concrete beams with the stage of first cracks. While in beams with hybrid CFRP/ hybrid GFRP/steel bars as lower reinforcement. steel rebars, increasing the ratio of steel to CFRP rebars Beams with hybrid GFRP/steel reinforcement cracked over slightly increased the stiffness of the concrete beams however the middle support and at mid-span simultaneously and no increase in the ultimate load capacity of the beams was showed an increase in deflection at this loading level due to noticed. stiffness reduction of the beam. After this phase, a reduction In the studied beams, it was noticed that steel yielded at first in the beams’ stiffness was noticed from the decreased slope then concrete crushing is followed in most of the beams. No of the load-deflection curve. The higher the percentage of the rupture in GFRP or CFRP was noticed at first. This is attrib- GFRP bars with respect to the steel bars, the lower beam uted to the high value of the FRP rupture strain considered in strength was observed, as shown in Fig. 7. this study (e = 0.016 and e = 0.012) compared to As shown in Fig. 7(a and b), the biggest stiffness reduction GFRP CFRP the steel yield strain considered (e = 0.0024). For speci- was found in beams with GFRP reinforcement. Adding steel steel mens with FRP rebars as upper and lower reinforcement, the rebars to the bottom reinforcement of the beam decreased failure was crushing in concrete which is the unfavorable brit- the stiffness reduction. This is attributed to the small value tle failure in spite of increasing the area of reinforcement. of GFRP elastic modulus. It was noticed that in concrete Therefore, providing steel rebars to the FRP rebas in the top beams reinforced with hybrid GFRP/steel rebars, the frist and bottom reinforcement ensures the desirable ductile flexural cracks were initiated over the middle support and at mid- behavior. span simultaneously; as shown in Fig. 9(a); regardless the type The observed failure modes of the studied beams are pre- of the upper reinforcement; FRP or steel bars. However, in sented in Table 2. All the hybrid GFRP/steel reinforced con- concrete beams with hybrid CFRP/steel reinforcement, cracks crete beams failed due to yielding in steel rebars then were recorded over the middle support at first then after a crushing in concrete occurred. Using steel rebars as upper rein- number of sub-steps (about 4–7 kN), cracks were noticed at forcement with hybrid GFRP/steel rebars as bottom reinforce- mid-span, as shown in Fig. 9(c and d). Fig. 9(b and e) show ment increased the ultimate capacity of the concrete beams by the cracking and crushing pattern of the GFRP/steel and the 5% up to 16%; according to the ratio of the steel bars to the CFRP/steel reinforced concrete beams respectively. No sudden GFRP bars. Stress of the GFRP bars reached a small percent- increase in deflection was observed at the first cracks loading (a) Group (GI-S) (b) Group (GI-G) (c) Group (GII-S) (d) Group (GII-G) Figure 7 Load-deflection curves of beams with hybrid GFRP/Steel reinforcement. Behavior of concrete beams reinforced with hybrid steel and FRP composites 307 (a) Group (CI-S) (b) Group (CI-C) (c) Group (CII-S) (d) Group (CII-C) Figure 8 Load-deflection curves of beams with hybrid CFRP/steel reinforcement. Figure 9 Typical cracking/crushing pattern in FRP/steel reinforced concrete beams. level, as shown in Fig. 8. Replacing steel rebars by CFRP Conclusions rebars maintained the ultimate capacity of the beams. Comparing results of hybrid GFRP/steel reinforced and A nonlinear finite element analysis of the flexural behavior of hybrid CFRP/steel reinforced concrete beams with the same hybrid GFRP/steel and CFRP/steel-reinforced concrete beams diameter of reinforcing bars and the same ratio of FRP rebars has been investigated in this paper. The study considered the to steel rebars, the comparison revealed that the former type ultimate load carrying capacity, deflection and cracking pat- exhibited a noticeable reduction in stiffness than the hybrid tern of the beams. The material as well as geometric nonlinear- CFRP/steel reinforced concrete beams. This is because of the ities has been considered in the finite element model. A reduced effective moment of inertia due to the lower elastic parametric study of hybrid reinforced concrete beams was per- modulus of GFRP bars compared to that of CFRP bars. formed. The following conclusions are outlined from this The rate of the increase in the ultimate capacity of the concrete analysis. beams decreased with the increase of the amount of reinforce- ment. In beams with GFRP rebars as top and bottom rein- In hybrid GFRP/steel reinforced concrete beams, the steel forcement, no noticeable increase in the ultimate capacity of reinforcement improved the beam stiffness, ductility and the beam when the area of reinforcement exceeded 0.42%. load resistance after cracking. The higher the amount of FRP reinforcement, the less the rate The higher the amount of GFRP reinforcement, the less the of increase of the ultimate capacity of the concrete beam. So, it rate of increase of the ultimate capacity of the concrete is recommended that the FRP reinforcement ratio should be beam selected to optimum use of FRP reinforcement. 308 S.A.A. Mustafa, H.A. Hassan The higher the percentage of the GFRP bars with respect to [7] M.A. Aiello, L. Ombres, Structural performances of concrete beams with hybrid Fiber- reinforced polymer-steel the steel bars, the lower beam strength reinforcements, J. Compos. Constr. 6 (2002) 133–140. It is better to use steel bars as top reinforcement in concrete [8] B. Saikia, J. Thomas, A. Ramaswamy, R.K. Nanjunda, beams with hybrid GFRP/steel bars as lower reinforcement. Performance of hybrid rebars as lon-gitudinal reinforcement in The contribution of steel rebars to GFRP/CFRP rebars in normal strength concrete, Mater. Struct. 38 (2005) 857–864. the top and bottom reinforcement of concrete beams pro- [9] R.J. Gravina, S.T. Smith, Flexural behaviour of indeterminate vides ductility and stiffness improvement of beams and concrete beams reinforced with FRP bars, J. Eng. Struct. 30 (9) ensures the desirable ductile flexural behavior and avoids (2008) 2370–2380. the unfavorable brittle failure. [10] M. El-Mogy, A. El-Ragaby, E. El-Salakawy, Flexural behavior In hybrid GFRP/steel reinforced concrete beams, a signifi- of continuous FRP-reinforced concrete beams, J. Compos. cant decrease in stiffness and a noticeable increase in the Constr. 14 (6) (2010) 669–680. [11] M. El-Mogy, A. El-Ragaby, E. El-Salakawy, Experimental beams’ deflection after the initiation of first crack and yield- testing and finite element modeling on continuous concrete ing of steel reinforcement is noticed. On the contrary, beams reinforced with fibre reinforced polymer bars and hybrid CFRP/steel reinforced concrete beams showed a stirrups, Can. J. Civ. Eng. 40 (2013) 1091–1102. better performance during cracking initiation and [12] M. El-Mogy, A. El-Ragaby, E. El-Salakawy, Effect of propagation. transverse reinforcement on the flexural behavior of continuous concrete beams reinforced with FRP, J. Compos. Constr. 14 (6) (2011) 672–681. Conflict of interest [13] I. Chitsazan, M. Kobraei, M.Z. Jumaat, P. Shafigh, An experimental study on the flexural behavior of FRP RC beams The authors report that they have no conflict of interests to and a comparison of the ultimate moment capacity with ACI, J. declare. Civ. Eng. Constr. Technol. 1 (2) (2010) 27–42. [14] A. Deifalla, M. Hamed, A. Saleh, T. Ali, Exploring GFRP bars as reinforcement for rectangular and L-shaped beams subjected References to significant torsion: an experimental study, Eng. Struct. 59 (2014) 776–786. [1] L.C. Hollaway, Key Issues in the Use of Fibre Reinforced [15] S. Rana, E. Zdraveva, C. Pereira, Fangueiro R and Correia A. Polymer (FRP) Composites in the Rehabilitation and Development of Hybrid Braided Composite Rods for Retrofitting of Concrete Structures, Woodhead Publishing Reinforcement and Health Monitoring of Structures. The Limited, University of Surrey, UK, 2011. Scientific World Journal; Volume 2014, Article ID 170187, 9 [2] G.L. Jaeger, G. Tadros, A.A. Mufti, Balanced Section, Ductility pages. and Deformability in Concrete with FRP Reinforcement. [16] ANSYS Release 15.0 Finite Element Analysis System, SAS IP Research Report No. 2-1995, Industry Center for Computer- Inc. Aided Engineering, Technical University of Nova Scotia, [17] M.M. Rafi, A. Nadjai, F. Ali, D. Talamona, Aspects of Halifax, Nova Scotia, Canada. behaviour of CFRP reinforced concrete beams in bending, [3] H.G. Harris, W. Somboonsong, F.K. Ko, New ductile hybrid Constr. Build. Mater. 22 (2008) 277–285. frp reinforcing bar for concrete structures, J. Compos. Constr. 2 [18] P. Desayi, S. Krishnan, Equation for the stress-strain curve of (1) (1998) 28–37. concrete, ACI 61 (3) (1964) 345–350. [4] S.H. Alsayed, A.M. Alhozaimy, Ductility of concrete beams [19] J.M. Gere, Mechanics of materials, sixth ed., Thomson Learning reinforced with FRP bars and steel fibers, J. Compos. Mater. 33 Inc., USA, 2004. (19) (1999) 1792–1806. [20] A. Buyukkaragoz, Finite element analysis of the beam ¨ ¨ ¨ [5] 2001, ACI Committee 440, Guide for the Design and strengthened with prefabricated reinforced concrete plate, Sci. Construction of Concrete Reinforced with FRP Bars (ACI Res. Essays 5 (6) (2010) 533–544. 440.1R-01), Am. Concrete Institute, Farmington Hills, Mich. [21] A. Nanni, A. De Luca, H.J. Zadeh, Reinforced Concrete with [6] V.C. Li, S. Wang, Flexural behaviors of glass fiber-reinforced FRP Bars Mechanics and Design, Taylor & Francis Group, polymer (GFRP) reinforced engineered cementitious composite USA, 2014. beams, ACI Mater. J. 99 (1) (1999) 11–21. http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png HBRC Journal Taylor & Francis

Behavior of concrete beams reinforced with hybrid steel and FRP composites

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Taylor & Francis
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© 2017 Housing and Building National Research Center. Production and hosting by Elsevier B.V.
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1687-4048
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10.1016/j.hbrcj.2017.01.001
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Abstract

HBRC Journal (2018) 14, 300–308 Housing and Building National Research Center HBRC Journal http://ees.elsevier.com/hbrcj Behavior of concrete beams reinforced with hybrid steel and FRP composites Suzan A.A. Mustafa , Hilal A. Hassan Structural Engineering Department, Faculty of Engineering, Zagazig University, Egypt Received 8 August 2016; revised 31 December 2016; accepted 19 January 2017 KEYWORDS Abstract This paper presents a nonlinear finite element model to investigate the behavior of hybrid Fiber Reinforced Polymers and steel reinforcement. Different types of Fiber Reinforced Polymers; Concrete beam; CFRP and GFRP; were used along with steel rebars in the studied concrete beams. The study was Hybrid; conducted using the nonlinear finite element program ‘‘ANSYS”. Nonlinear material models for the Steel reinforcement; components of the concrete beam were used in the three dimensional finite element models. The FRP reinforcement; Flexure; outcomes got from finite element analysis were confirmed against experimental results. A broad Finite element analysis parametric study was conducted to explore the effect of replacing steel reinforcement by different types of FRP bars. The study showed that the contribution of steel rebars to FRP rebars in concrete beams improved beam ductility and eliminated the unfavorable brittle failure of the concrete beam. In addition, it is better to use steel rebars as top reinforcement in concrete beams with hybrid rein- forcement. In hybrid GFRP/steel reinforced concrete beams, a significant reduction in stiffness and a noticeable increase in the beams’ deflection after the initiation of first crack and yielding of steel reinforcement were observed. On the other hand, in hybrid CFRP/steel reinforced concrete beams showed a better performance during cracking initiation and propagation. 2017 Housing and Building National Research Center. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Introduction few decades ago. The well-known high cost of FRP material might be a preventive to its use. However, FRP is a quite competitive material to use. It has a noticeable high strength The use of fiber-reinforced polymer (FRP) composites for and stiffness ratios to density, In addition to their resistance reinforcing or rehabilitation of structural members started to corrosion. The light weight of FRP provides significant Corresponding author. savings in labor cost. Furthermore, fatigue durability, and E-mail addresses: samustafa@eng.zu.edu.eg (S.A.A. Mustafa), the transparency of FRP bars to magnetic and electrical fields hilalcivil@yahoo.com (H.A. Hassan). makes them a applicable alternative to steel reinforcement Peer review under responsibility of Housing and Building National in applications sensitive to electromagnetic fields such as Research Center. magnetic resonance imaging; Hollaway [1]. In reinforced concrete structures, steel plays the main rule of ductility which is the ratio of post yield deformation to yield Production and hosting by Elsevier deformation. This traditional meaning of ductility cannot be http://dx.doi.org/10.1016/j.hbrcj.2017.01.001 1687-4048  2017 Housing and Building National Research Center. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Behavior of concrete beams reinforced with hybrid steel and FRP composites 301 applied to structures reinforced with FRP reinforcement due analytical finite element modeling and/or experimental studies to the linear elastic behavior of FRP bars. A lot of exertion to investigate the behavior of reinforced concrete beams rein- has been made to enhance and characterize the ductility of forced or strengthened with FRP. beams reinforced with FRP bars. The ductility index for These studies focused on simply supported and continuous FRP reinforced structures could be calculated using the energy FRP-reinforced concrete beams [7–9]. El-Mogy et al. [10,11] based method and the deformation based method; Jaeger et al. presented an experimental and a finite element study on the [2]. flexural behavior of continuous FRP-reinforced concrete Harris et al. [3] tried to mimic the elastic plastic behavior of beams. El-Mogy et al. [12] concluded that deflection can be the steel by using hybrid FRP rebars. They utilized pseudo- decreased when increasing the transverse GFRP reinforcement ductile materials at which two or more different FRP reinforc- in continuous concrete beams inspite of keeping the longitudi- ing materials are combined. They noticed that the ductility nal reinforcement the same. Chitsazan et al. [13] studied exper- index of the examined beams is almost the same as that of imentally the enhancement of the flexural behavior of concrete the traditional steel- reinforced beams. The complicated man- beams with GFRP bars by using high strength concrete and ufacturing process in addition to the high cost of the hybrid increasing the effective depth over the breadth of the beam. bars restricted its practical applications. Another methodology Deifalla et al. [14] found that the bonded GFRP stirrups were to enhance the property of the beam is to make the ductility of more effective as transversal reinforcement compared with the the system strongly dependent on the concrete properties. bent steel stirrups and the bent GFRP stirrups. It improved the Alsayed and Alhozaimy [4] examined 18 steel and FRP rein- ultimate torsional strength and increased the corresponding forced concrete beams and found that with the addition of unit angle of twist and the maximum stirrup strain as well as 1% steel fibers, the ductility index can be increased by as much the major concrete crack width in L-shaped beam subjected as 100%. ACI 440 [5] recommends that the FRP reinforced to torsion. Rana et al. [15] investigated the effect of carbon structures should be over reinforced and concrete beams have fiber/glass fiber weight ratio on both strain sensitivity and ten- to be designed so that they fail by concrete crushing rather sile properties of concrete beams. They observed that the stud- than by rupture FRP reinforcement. Li and Wang [6] demon- ied composites with lowest amount of carbon fibers led to best strated that the GFRP rebars reinforcing engineered cementi- strain sensitivity and good mechanical properties. tious composite material improved flexural practices and The principle goal of this study is to build up a nonlinear ductility of the concrete element. Many studies utilized finite element model to investigate the behavior and strength (a) Beam Tested by Rafi et al. (2008) (b) Beam Tested by El-Mogy et al. (2010) Figure 1 Dimensions and details of verified experimental beams. 302 S.A.A. Mustafa, H.A. Hassan of concrete beams reinforced by hybrid steel and different verify the developed finite element model. The two specimens types of FRP bars. Both GFRP and CFRP bars were used tested by El-Mogy et al. [10] were two-spans continuous con- as negative and positive reinforcement of concrete beams. crete beams tested in flexure. One beam was reinforced with The finite element commercial program ANSYS [16] was uti- GFRP bars (GS1), and the other was reinforced with steel bars lized in the analysis. Nonlinear material properties of the beam (SS1). The reinforcement of the two beams is detailed in Fig. 1 components were used. The results obtained from the model (b). The length of the beams was 6000 mm with a rectangular were confirmed against the test results conducted by previous cross section with 200 mm and 300 mm width and depth experimental tests of Rafi et al. [17] and El-Mogy et al. [10]. respectively. The beams were continuously supported over An intensive parametric study was conducted to investigate two equal clear spans of 2800 mm each. The overall length the effect of the ratio of steel contribution, amount of FRP of beam (BRC1) tested by Rafi et al. [17] was 2000 mm. The reinforcement and finally FRP type. beam was simply supported with a rectangular cross-section of 120 mm width and 200 mm depth. The concrete beam had two CFRP longitudinal bars on the tension side in addition Finite element modeling to two steel longitudinal bars on the compression side. A four-point static loading technique was utilized to examine General the simply supported beam with a span of 1750 mm, as detailed in Fig. 1(a). The dimensions and material properties In order to accurately simulate the actual behavior of the con- of the verified specimens are summarized in Table 1. cerned beam, all its components; concrete beam, steel bars, FRP bars and stirrups; have to be modeled properly. Mean- Finite element type and mesh while, choosing the element types and mesh size are important as well in building the model to provide accurate results with To obtain an accurate simulation of the actual behavior of the reasonable computational time. concrete beam reinforced with steel and different types of FRP Recent experimental tests on concrete beam reinforced with bars, the elements composing the finite element model had to steel and FRP conducted by Rafi et al. [17] (specimen BRC1) be chosen properly. The mesh size was carefully selected to and El-Mogy et al. [10] (specimens SS1 and GS1) were used to Table 1 Details of the verified specimens. Rafi et al. [17] Beam Bottom Reinforcement Top Reinforcement f Mpa cu BRC1 Type U (mm) f (Mpa) E(Gpa) type U (mm) f (Mpa) E(Gpa) u u CFRP 9.5 1676 135.9 Steel 8 566 194 42.5 El-Mogy et al. [12] Beam Bottom Reinforcement Top Reinforcement f Mpa cu SS1 Type U (mm) f (Mpa) E (Gpa) Type U (mm) f (Mpa) E (Gpa) y y Steel 15.9 485 200 Steel 15.9 485 200 35 GS1 Type U (mm) f (Mpa) E (Gpa) Type U (mm) f (Mpa) E (Gpa) u u GFRP 15.9 740 46 GFRP 15.9 740 46 Specimen (BRC1): Stirrups u6 mm @ 100 mm. Specimens (SS1) & (GS1): Stirrups u8 mm @ 120 mm. Figure 2 A typical figure of the 3-D FE mesh (selected concrete elements were removed to illustrate reinforcement). Behavior of concrete beams reinforced with hybrid steel and FRP composites 303 obtain high accuracy of results with reasonable computational time. The aspect ratio of the used solid elements was kept as possible within the recommended range; between 1 and 3. The analysis was performed using the ANSYS [16] program. Both material and geometric non-linearity were considered in the analysis. The three-dimensional eight-node solid element SOLID65 was used to model concrete. Each node has three translational degrees of freedom. This element uses linear interpolation functions for displacements. This element is capable of simu- lating the cracking and crushing of brittle materials. Before cracking or crushing, the concrete is assumed to be an isotropic elastic material. After crushing, concrete loses its stiffness in all directions. After cracking, the concrete is assumed to be ortho- tropic having stiffness based on a bilinear softening stress- strain response in the crack normal direction. The 3-D spar element (LINK180) was used to simulate steel, CFRP, GFRP rebars and the stirrups. Link180 is a two-nodes uniaxial tension-compression element with three translational DOF at each node. The elements were embedded in concrete sharing its nodal points. Nonlinearity and plastic deformations are simulated in this element. In order to pre- clude early warnings and premature failure messages due to concrete crushing at the positions of loading and supports, eight-node solid element; solid 185; was used to model the loading plates. A typical figure of the three dimensional finite element mesh of the studied beams is shown in Fig. 2. Material modeling The material properties of the components of the pre-tested specimens were considered as detailed in Table 1. In all cases, the ultimate strain of the concrete at failure was taken as 0.0035 and the Poisson’s ratio of concrete was taken 0.2. A multi-linear isotropic stress-strain relation was used for model- ing concrete material in compression. This relationship con- sists of two portions. The first portion is an ascending curve represented by the numerical expressions; Eqs. (1) and (2), [18] along with Eq. (3) [19]. The curve starts at zero stress and zero strain toward a value of 0.3f , calculated from Eq. (3). The rest points of the ascending curve are obtained from Eq. (1). The strain at ultimate stress of concrete is calculated via Eq. (2). The descending branch which represents strain softening of the ideal stress-strain curve of concrete was ignored as recommended in previous studies [20,21] in order Figure 3 (a) Typical stress-strain curve for concrete; (b) Typical to avoid convergence problems. A bilinear relationship was stress-strain curve for FRP; (c) Typical stress-strain curve for steel. used to represent the stress-strain curve of the steel reinforce- ment while a linear elastic behavior was used for the FRP movement in the loading Y- direction was allowed. Other rebars. The Poisson’s ratio was assumed to be 0.3 for steel rein- nodes were free to displace in any direction except the nodes forcement and 0.2 for FRP. For stirrups and loading plates, representing the supports. The load was applied in small incre- the stress-strain relation was considered linear. Fig. 3 shows ments as usually recommended in testing concrete structures to the stress-strain relations of the different materials used. avoid non-convergence problems. This was achieved with the aid of the load steps and sub-steps. Cracking and crushing of Boundary conditions and load application concrete elements were monitored during the loading steps. The load was applied until failure in all beams. Following the testing procedures conducted by Rafi et al. [17] and El-Mogy et al. [10], simply supported boundary conditions Verification of finite element model were applied at the position of edge support. Due to symmetry of all the pre-tested beams, only half of each beam was mod- eled, as shown in Fig. 2. The nodes in the middle symmetry To validate the finite element model, a comparison was held surface were prevented to displace in Z- direction, while their with available pre-tested beams. The ultimate loads and the 304 S.A.A. Mustafa, H.A. Hassan corresponding maximum deflection of the tested specimens Failure of the beam with steel reinforcement was due to steel (P ,D ) and the finite element analysis (P ,D ) as well as yielding. The steel yielded at mid-span and at the middle T T FE FE the load–deflection curves, and deformed shapes after failure support. Tensile cracks in concrete were generated. The first have been investigated for all types of reinforced concrete crack was at about 23% of the ultimate load at the middle beams. In addition, crushing and cracking pattern of the con- support of the beam, as shown in Fig. 4(b)-a. At the crete beams of the compared specimens is shown. Good agree- next load step, cracks began at mid-span as shown in ment has been achieved between both results. The mean values Fig. 4(b)-b, then all cracks increased and propagated until of P /P and D /D ratios were 0.98 and 1.07 respectively. failure. In concrete beam reinforced with GFRP rebars, the T FE T FE The corresponding coefficients of variation (COV) were about failure was in concrete. The GFRP rebars did not reach their 0.04 and 0.053 respectively. Figs. 4–6 plot the load–deflection tensile stress or strain at failure of the beam neither at mid- curves of the modeled beams. In addition, cracking and crush- span nor at the middle support. The first cracks were noticed ing patterns for the reinforced concrete beams are illustrated. at the middle support and at mid-span simultaneously at The results of the proposed 3-D nonlinear finite element model about 22% of the ultimate load, as shown in Fig. 5(b)-a. matched the experimental results fairly well and the finite ele- Afterwards, the cracks increased and propagated until failure ment model successfully predicted the behavior of the beams. happened in concrete. Concrete cracks and crushing are ,"# shown in Fig. 5(b). In the simply supported beam simulating that tested by f ¼ E e 1 þ ð1Þ 0 Rafi et al. [17] with CFRP rebars, concrete failure was noticed. The CFRP bars did not reach its tensile stress or 2f strain at the failure of the beam. The first crack was at about e ¼ ð2Þ 22% of the beam ultimate load at the middle of the span. The produced cracks increased and propagated until failure. Compressive cracks and failure was noticed in the concrete, E ¼ ð3Þ e as shown in Fig. 6. (a) Load-Deflection Curve (b) Cracking and Crushing Pattern Figure 4 Beam with steel reinforcement. (a) Load-Deflection Curve (b) Cracking and Crushing Pattern Figure 5 Beam with GFRP reinforcement. Behavior of concrete beams reinforced with hybrid steel and FRP composites 305 (b) Cracking and Crushing Pattern (a) Load-Deflection Curve Figure 6 Beam with CFRP reinforcement. Table 2 Studied parameters. Group Model Bottom Reinf. Top Reinf. Notes P/P Failure mode Ref Bottom Reinf. Top Reinf. SM1 4/16 S 3/16 S Steel Steel 1 Steel yielding & concrete crushing GI - S M2 3/16 S + 1/16 G 3/16 S Steel + GFRP Steel 0.94 Steel yielding & concrete crushing M3 2/16 S + 2/16 G 3/16 S Steel + GFRP Steel 0.88 Steel yielding & concrete crushing M4 1/16 S + 3/16 G 3/16 S Steel + GFRP Steel 0.74 Steel yielding & concrete crushing M5 4/16 G 3/16 S GFRP Steel 0.71 Steel yielding & concrete crushing GI - G M6 3/16 S + 1/16 G 3/16 G Steel + GFRP GFRP 0.81 Steel yielding & concrete crushing M7 2/16 S + 2/16 G 3/16 G Steel + GFRP GFRP 0.72 Steel yielding & concrete crushing M8 1/16 S + 3/16 G 3/16 G Steel + GFRP GFRP 0.68 Steel yielding & concrete crushing M9 4/16 G 3/16 G GFRP GFRP 0.65 Concrete crushing CI - S M10 3/16 S + 1/16 C 3/16 S Steel + CFRP Steel 1.006 Steel yielding & concrete crushing M11 2/16 S + 2/16 C 3/16 S Steel + CFRP Steel 0.995 Steel yielding & concrete crushing M12 1/16 S + 3/16 C 3/16 S Steel + CFRP Steel 0.98 Steel yielding & concrete crushing M13 4/16 C 3/16 S CFRP Steel 0.95 Steel yielding & concrete crushing CI - C M14 3/16 S + 1/16 C 3/16 C Steel + CFRP CFRP 1.02 Steel yielding & concrete crushing M15 2/16 S + 2/16 C 3/16 C Steel + CFRP CFRP 1.02 Steel yielding & concrete crushing M16 1/16 S + 3/16 C 3/16 C Steel + CFRP CFRP 1.02 Steel yielding & concrete crushing M17 4/16 C 3/16 C CFRP CFRP 1.02 Concrete crushing GII - S M18 4/18 G 3/16 S GFRP Steel 0.81 Steel yielding & concrete crushing M19 4/20 G 3/16 S GFRP Steel 0.86 Steel yielding & concrete crushing M20 4/22 G 3/16 S GFRP Steel 0.94 Steel yielding & concrete crushing GII - G M21 4/18 G 3/16 G GFRP GFRP 0.71 Concrete crushing M22 4/20 G 3/16 G GFRP GFRP 0.71 Concrete crushing M23 4/22 G 3/16 G GFRP GFRP 0.71 Concrete crushing CII - S M24 4/18 C 3/16 S CFRP Steel 1.04 Steel yielding & concrete crushing M25 4/20 C 3/16 S CFRP Steel 1.13 Steel yielding & concrete crushing M26 4/22 C 3/16 S CFRP Steel 1.19 Steel yielding & concrete crushing CII - C M27 4/18 C 3/16 C CFRP CFRP 1.22 Concrete crushing M28 4/20 C 3/16 C CFRP CFRP 1.27 Concrete Crushing M29 4/22 C 3/16 C CFRP CFRP 1.32 Concrete crushing S:Steel, G: Glass FRP (GFRP), C: Carbon FRP (CFRP), P : Ultimate load of reference beam (M1). Ref Dimensions, loading pattern and material properties followed Parametric study exactly the experimental work. The studied beams were divided into 8 groups with different hybrid reinforcement, as A total of 29 concrete beams were analyzed in the current detailed in Table 2. parametric study. The study was performed on continuous All the analyzed beams behaved similarly until first crack beam similar to the pre-examined by El-Mogy et al. [10]. (the initial linear part of the load-deflection curves) Figs. 7 306 S.A.A. Mustafa, H.A. Hassan and 8. The behavior of hybrid FRP/steel reinforced concrete age of its ultimate value. This demonstrates that this reinforc- beams was compared to the behavior of the reference steel ing arrangement is suitable for flexural failure control. On the reinforced concrete beam (M1). The stiffness of the hybrid other hand, beams with GFRP bars as top reinforcement and reinforced concrete beams with various percentages of FRP hybrid GFRP/steel bars as lower reinforcement, compressive and steel rebars lied between those of their counterpart FRP stresses reached almost half of the GFRP ultimate value. Since and steel reinforced beams, as shown in Figs. 7 and 8(a and the compressive strength of the GFRP bars reduces by up to b). Increasing the steel percentage to the GFRP rebars in the 45% with respect to the value in tension [21], it is better to studied concrete beams increases the stiffness of the beam after use steel bars as top reinforcement in concrete beams with the stage of first cracks. While in beams with hybrid CFRP/ hybrid GFRP/steel bars as lower reinforcement. steel rebars, increasing the ratio of steel to CFRP rebars Beams with hybrid GFRP/steel reinforcement cracked over slightly increased the stiffness of the concrete beams however the middle support and at mid-span simultaneously and no increase in the ultimate load capacity of the beams was showed an increase in deflection at this loading level due to noticed. stiffness reduction of the beam. After this phase, a reduction In the studied beams, it was noticed that steel yielded at first in the beams’ stiffness was noticed from the decreased slope then concrete crushing is followed in most of the beams. No of the load-deflection curve. The higher the percentage of the rupture in GFRP or CFRP was noticed at first. This is attrib- GFRP bars with respect to the steel bars, the lower beam uted to the high value of the FRP rupture strain considered in strength was observed, as shown in Fig. 7. this study (e = 0.016 and e = 0.012) compared to As shown in Fig. 7(a and b), the biggest stiffness reduction GFRP CFRP the steel yield strain considered (e = 0.0024). For speci- was found in beams with GFRP reinforcement. Adding steel steel mens with FRP rebars as upper and lower reinforcement, the rebars to the bottom reinforcement of the beam decreased failure was crushing in concrete which is the unfavorable brit- the stiffness reduction. This is attributed to the small value tle failure in spite of increasing the area of reinforcement. of GFRP elastic modulus. It was noticed that in concrete Therefore, providing steel rebars to the FRP rebas in the top beams reinforced with hybrid GFRP/steel rebars, the frist and bottom reinforcement ensures the desirable ductile flexural cracks were initiated over the middle support and at mid- behavior. span simultaneously; as shown in Fig. 9(a); regardless the type The observed failure modes of the studied beams are pre- of the upper reinforcement; FRP or steel bars. However, in sented in Table 2. All the hybrid GFRP/steel reinforced con- concrete beams with hybrid CFRP/steel reinforcement, cracks crete beams failed due to yielding in steel rebars then were recorded over the middle support at first then after a crushing in concrete occurred. Using steel rebars as upper rein- number of sub-steps (about 4–7 kN), cracks were noticed at forcement with hybrid GFRP/steel rebars as bottom reinforce- mid-span, as shown in Fig. 9(c and d). Fig. 9(b and e) show ment increased the ultimate capacity of the concrete beams by the cracking and crushing pattern of the GFRP/steel and the 5% up to 16%; according to the ratio of the steel bars to the CFRP/steel reinforced concrete beams respectively. No sudden GFRP bars. Stress of the GFRP bars reached a small percent- increase in deflection was observed at the first cracks loading (a) Group (GI-S) (b) Group (GI-G) (c) Group (GII-S) (d) Group (GII-G) Figure 7 Load-deflection curves of beams with hybrid GFRP/Steel reinforcement. Behavior of concrete beams reinforced with hybrid steel and FRP composites 307 (a) Group (CI-S) (b) Group (CI-C) (c) Group (CII-S) (d) Group (CII-C) Figure 8 Load-deflection curves of beams with hybrid CFRP/steel reinforcement. Figure 9 Typical cracking/crushing pattern in FRP/steel reinforced concrete beams. level, as shown in Fig. 8. Replacing steel rebars by CFRP Conclusions rebars maintained the ultimate capacity of the beams. Comparing results of hybrid GFRP/steel reinforced and A nonlinear finite element analysis of the flexural behavior of hybrid CFRP/steel reinforced concrete beams with the same hybrid GFRP/steel and CFRP/steel-reinforced concrete beams diameter of reinforcing bars and the same ratio of FRP rebars has been investigated in this paper. The study considered the to steel rebars, the comparison revealed that the former type ultimate load carrying capacity, deflection and cracking pat- exhibited a noticeable reduction in stiffness than the hybrid tern of the beams. The material as well as geometric nonlinear- CFRP/steel reinforced concrete beams. This is because of the ities has been considered in the finite element model. A reduced effective moment of inertia due to the lower elastic parametric study of hybrid reinforced concrete beams was per- modulus of GFRP bars compared to that of CFRP bars. formed. The following conclusions are outlined from this The rate of the increase in the ultimate capacity of the concrete analysis. beams decreased with the increase of the amount of reinforce- ment. In beams with GFRP rebars as top and bottom rein- In hybrid GFRP/steel reinforced concrete beams, the steel forcement, no noticeable increase in the ultimate capacity of reinforcement improved the beam stiffness, ductility and the beam when the area of reinforcement exceeded 0.42%. load resistance after cracking. The higher the amount of FRP reinforcement, the less the rate The higher the amount of GFRP reinforcement, the less the of increase of the ultimate capacity of the concrete beam. So, it rate of increase of the ultimate capacity of the concrete is recommended that the FRP reinforcement ratio should be beam selected to optimum use of FRP reinforcement. 308 S.A.A. Mustafa, H.A. Hassan The higher the percentage of the GFRP bars with respect to [7] M.A. Aiello, L. 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Journal

HBRC JournalTaylor & Francis

Published: Dec 1, 2018

Keywords: Concrete beam; Hybrid; Steel reinforcement; FRP reinforcement; Flexure; Finite element analysis

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