Theoretical and Experimental analysis of rectangular concrete be ams reinforced internally with Hybrid Fibre Reinforced Polymer (HFRP) reinforcements under pure torsion is carried out in this study. Different parameters like grade of concrete, beam longitudinal reinforcement ratio and transverse stirrups spicing are considered. The basic strength properties of concrete, steel and HFRP reinforcements are determined experimentally. Experiment al torque verses twist relationship is established for variou s values of torque and twist using elastic, plastic theories of torsion. Finally the ultimate torque is determined using experimentaly for different parameters considered in this study. Based on this study, a good agreement is made between theoretical analysis and the experimental behaviour. Keywords: Pure Torsion; Beam ; HFRP Reinforcements ; Steel; Experimental Model Cite this Article: P. Kapildev and G. Kumaran, Theoretical and Experimental on the Behaviour of Rectangular Concrete Beams Reinforced Internal ly with HFRP Reinforcements Under Pure Torsion, International Journal of Advanced Research in Engineering and Technology (IJARET), 11 (3), 2020, pp 29-47. http://www.iaeme.com/IJARET/issues.asp?JType=IJARET&VType=11&IType=3 P. Kapildev and G. Kumaran http://www.iaeme.com/ IJARET/index. asp 30 editor@iaeme.com 1. INTRODUCTION In this new er a, the search of a new material in the construction is a main concern. This is due to the drawbacks in steel reinforcements such as corrosion and high energy wastage. The drawbacks of steel reinforcement can be overcome by using Fibre Reinforced Polymer materials. Since FRP materials occupies an important part in the modern materials. Though FRP materials plays a significant role in construction, its awareness is very less in India compared to developed countries. So there are no recommendations of Indian S tandards for FRP materials but the recommendations for FRP are issued in ACI codes. It is not popular in India because of the less awareness to the manufactures and clients. This creates a lag in the commercial success of this material. Based on its perfor mance in the aspects of strength and durability in worst environmental conditions, it has advantages over conventional reinforcement materials. FRP resources are commercially accessible in the form of cables, sheets, plates etc. The availability of FRP mat erials are possible in the form of reinforced bars in recent days. It is produced by a process called pultrusion for replacing conventional steel reinforcement in concrete members internally. Several imperfections in the surface of FRP bars are made to imp rove the bonding of FRP bars with concrete. In the modern world, constructions of concrete structures are mainly influenced by Fibre reinforcements. The utilization of FRP reinforcements are in concrete members is significantly increasing throughout the world today. Researches in India are also started to concentrate in FRP bars as replacement for conventional reinforcements for concrete members. The application of FRP reinforcements are mainly adopted in the construction of roof decks in i ndustries, equipment in reactors, floor slabs, electrical base pads and seawalls in worst environmental conditions. Hence the FRP bars are recommended by the structural designers. It is also effective in durable properties of concrete members. Most of the researches, all around the world are focused in the implications of FRP bars in structural components of concrete members such as beams and slabs (Nawy et al., 1997; Faza and GangaRao, 1992; Benmokrane, 1995; Sivagamasundari et al ., 2008; Deiveegan et al ., 2010; Saravanan et al ., 2011). The performance of the HFRP concrete beams under pure torsion are studies in the current research. The hybrid bars are reinforced internally in the HFRP concrete beams. Based on the availability of FRP materials in India, th e scope of this research is limited with HFRP reinforcements. This study is mainly divided into two sections such as the theoretical analysis and analytical analysis of HFRP and steel reinforced concrete beams subjected to pure torsion. Space truss formula tion application for traditional reinforced beams was used for HFRB concrete beams. Finite element method was adopted for the analytical analysis of HFRP and steel reinforced concrete beams. Finite element analysis is made by using ANSYS software with vari ous parameters (Sivagamasundari et al ., 2008; Deiveegan et al ., 2010; Saravanan et al ., 2011). The analysis results are validated with the existing theories for beams. 2. MATERIALS 2.1. Concrete Normal Strength Concrete (NSC) of grades M30 and M60 are used in this study. Ordinary Portland cement is used to prepare the concrete. The maximum size of aggregate used is 20 mm and the size of fine aggregate as M -sand ranges between 0 and 4.75 mm. After casting, the specimens are allowed to cure in real environmen tal conditions for about 28 days so as to attain strength. The test specimens are generally tested after a curing period of 28 days. The strength of concrete under uni­axial compression is determined by loading ‘standard test cubes’ (150 mm size) to failur e in a compression testing machine, as per IS 516 ­ 1959. The modulus of elasticity of concrete is determined by loading ‘standard cylinders’ (150 mm Theoretical and Experimental on the Behaviour of Rectangular Concrete Beams Reinforced Internally with HFRP Reinforcements Under Pure Torsion http://www.iaeme.com/ IJARET/index. asp 31 editor@iaeme.com diameter and 200 mm long) to failure in a compression testing machine, as per IS 516: 1959. The mix propor tions of the NSC are carried out as per Indian Standards (IS) 10262­1982 and the average compressive strengths are obtained from laboratory tests (Sivagamasundari et al., 2008; Deiveegan et al., 2010; Saravanan et al., 2011) and are depicted in Table 1(a). Table 1 Properties of Concrete Material/m3 M30 grade of concrete Ratio M60 grade of concrete Ratio Cement 368.42kg 1:1.87:3.50 466.67 kg 1:1.36:2.70 Fine aggregate 690.04 kg 629.16 kg Coarse aggregate 1290.988 kg 1263.55 kg Water/cement ratio 0.38 0.30 Average compressive strength of concrete cubes 42.00 MPa 76.00 MPa 2.2. Reinforcements The mechanical properties of all the types of HFRP reinforcements as per ASTM­D 3916­84 Standards and steel specimens as per Indian standards are obtained from laboratory tests and the results are tabulated in Table 1(b). The tensile strength of steel rein forcements ( S) used in this study, conforming to Indian standards and having an average value of the yield strength of steel is considered for this study. HFRP reinforcements used in this study are manufactured by pultrusion process with the E­glass fibre volume approximately 60% and these fibres are reinforced with epoxy resins. Previous studies were carried out with three different types of GFRP reinforcements (grooved, sand sprinkled & threaded) (ACI 440R­96; Muthuramu , et al., 2005; Sivagamasundari et al., 2008; Deiveegan et al., 2010; Saravanan et al., 2011) with different surface indentations and are designated as Fg, Fs and F t. In this study threaded type HFRP reinforcement is used in place of conventional steel. The diameters of the longitudinal and transverse reinforcements are 10 mm and 8 mm respectively. The standard minimum diameters of the reinforcements as per Indian standards are adopted in this study. The tensile strength properties are ascertained as per ACI standards shown in Table1(b) and are validated by conducting the tensile tests at SERC, Chennai. The compressive modulus of elasticity of GFRP reinforcing bars is smaller than its tensile modulus of elasticity (ACI 440R­96; Lawrence C Bank 2006; Sivagamasundari et al., 2008). It varies be tween 36­ 47 GPa which is approximately 70% of the tensile modulus for GFRP reinforcements. Under compression GFRP reinforcements have shown a premature failures resulting from end brooming and internal fibre micro­buckling. In this study, GFRP stirrups ar e manufactured by Vacuum Assisted Resin Transfer Moulding process, using glass fibres reinforced with epoxy resin (ACI 440R­96; Sivagamasundari et al., 2008; Deiveegan et al., 2010; Saravanan et al., 2011). Based on the experimental study, it is observed t hat the strength of GFRP stirrups at the bend location (bend strength) is as low as 50 % of the strength parallel to the fibres. However, the stirrup strength in straight portion is comparable to the yield strength of conventional steel stirrups. Therefore, in this study, HFRP stirrups strength is taken as 30% of its tensile strength i.e 150 MPa. P. Kapildev and G. Kumaran http://www.iaeme.com/ IJARET/index. asp 32 editor@iaeme.com Table 2 Properties of reinforcements Properties Sand coated Hybrid FRP rod ( Hf) Steel Fe 500 rod ( Fe) Tensile strength (MPa) 1052.04 520 Longitudinal Elastic modulus (GPa) 64.70 210 Strain 0.012 0.002 Poisson’s ratio 0.236 0.3 Figure 1 HFRP reinforcements with end anchorages for tensile tests Figure 2 Failure of HFRP reinforcements tensile test Stress -Strain Curve of Fe 500 steel Sand coated Hybrid FRP Reinforcements ( Hf) Figure 3 Stress­Strain curve for all the reinforcements involved in the present study Theoretical and Experimental on the Behaviour of Rectangular Concrete Beams Reinforced Internally with HFRP Reinforcements Under Pure Torsion http://www.iaeme.com/ IJARET/index. asp 33 editor@iaeme.com 3. THEORETICAL IDEALI SATION OF TORQUE –TWIST (T–𝜽) CURVE 3.1. Theoretical Investigation Theoretical torque verses twist relationship is established for various values of torque and twist using elastic , plastic theories of torsion and also the ultimate torque is determined using space truss analogy (Hsu 1968; MacGrego r et al. , 1995; Rasmussen et al., 1995; Ashar et al., 1996; Khaldown et al ., 1996; Liang et al., 2000; Trahait 2005; Luis et al ., 2008; Chyuan 2010). The theoretical investigation consists different rectangular beams and are designated are as follows; Bp1m1S1Fe; Bp1m1S1Hf; Bp1m2S1Fe; Bp1m2S1Hf ; Bp2m1S1Fe; Bp2m1S1Hf ; Bp2m2S1Fe; Bp2m2S1Hf ; Bp1m1S2Fe; Bp1m1S2Hf ;Bp1m2S2Fe; Bp1m2S2Hf; Bp2m1S2Fe; Bp2m1S2Hf ; Bp2m2S2Fe; Bp2m2S2Hf. These beams are reinforced internally with threaded type Hybrid Fibre Reinforced Polymer Reinforcements and conventional steel reinforcements with different grades of concrete and steel reinforcement ratio under pure torsion is considered in this study. The entire concrete beam is supported on saddle supports which can allow rotation in the direction of application of torsion. Table 3 Various Parameters involved Parameters Description Designation Types of reinforcements Sand coatedHFRP Hf Conventional Fe Concrete grade fcu average cube strength m1&m2 Beam size 160 x 275 mm B Reinforcement ratios 0.56% (2 -Y12 mm bars top & Bottom) 0.85% (3 -Y12 mm bars top & Bottom) p1&p2 Spacing of stirrups 75 mm and 100 mm S1&S2 3.2. Space Truss Analogy The objective of the current research is to improvise the theoretical prediction of the performance of HFRP reinforcements bars under torsion. The performance of concrete beams under torsion is modelled through T (Torque) —  (Twist) curves. The improved theoretical prediction to the actual behaviour is developed from the current theories adopted. The relationship is somewhat linear up to failure, which is sudden and brittle, and occurs immediately after the formation of the f irst torsional crack. Torsion is initiated in a reinforced concrete beams in different ways throughout the progression of load transfer in a structural arrangements. In th e design of reinforced concrete , the terms ‘equilibrium torsion’ and ‘compatibility t orsion’ are frequently utilised to cite to 2 dissimilar torsion -inducing situations. Typically, equilibrium torsion is initiated in beams supporting lateral overhanging protrusion, and is provoked by the eccentricity in the loading. Such torsion is also tr iggered in beams curved in plan and subjected to gravity loads, and in beams where the transverse loads are eccentric with respect to the shear centre of the cross -section (Hsu 1984; Pillai & Menon 2010). The current research is pertained to the utilisation of internal HFRP reinforcements in concrete beams. Already sufficient investigations are conceded for flexural members and it has come in the form of codal standards especially in American Standards ( ACI440XR 2007) . In order to find out the design of conc rete structures reinforced with GFRP reinforcements P. Kapildev and G. Kumaran http://www.iaeme.com/ IJARET/index. asp 34 editor@iaeme.com theoretically, it is desirable to understand fundamental constituent properties and their relations with concrete. Consequently, this section explains the essential properties necessary for theoretical fo rmulations of torque capacity expressions. Le=300 0 mm e P/2 P/2 e Support Saddle support Beam Support Saddle support Strain gauge Figure 4 Beam supported on saddle support -Elevation e=350 mm e=350 mm Le=3000 mm Saddle support Beam Load Point P T= P/2 × e T= P/2 × e Load Point P Saddle support Figure 5 Beam supported on saddle support (plan view) Twisting moment diagram T2 Free body diagram total torque = T T2 T2 T2 Figure 6 Torsional moment diagram Fig. 6 shows the torsional moment diagram. The general theoretical torque twist curve T – 𝜃curves are graphed for three stages and are expressed by their ( 𝜃;T) coordinates. These coordinates are shown in Fig. 7 Theoretical and Experimental on the Behaviour of Rectangular Concrete Beams Reinforced Internally with HFRP Reinforcements Under Pure Torsion http://www.iaeme.com/ IJARET/index. asp 35 editor@iaeme.com Stage 1 constitute the beam’s performance prior to cracking. The slope of the curve constitute the elastic St. Venant stiffness (GC)I. In this step the curve can be implicit as a straight line with starting point in (0;0) and end in ( 𝜃𝑒𝑙;Tel). The theoretical model apprai sed in this research for this phase depends on Theory of Elasticity. Subsequent to cracking, the beams endure a impulsive increase of twist after what it resets the linear behaviour. This phase is identified as Stage 2. It starts at ( 𝜃𝑐𝑟;Tcr) and finished at a definite point of twist ( 𝜃𝑐𝑟). The slope of stage -2 constitutes the torsional stiffness in cracked stage (GC)II. The model observed for stage -2 is based on the space truss analogy with 45° inclined concrete struts and linear behaviour for the materials. The points of the T –𝜃 curve from which the nonlinear behaviour is expressed by through two dissimilar criterions. The first one corresponds to determine the point for which at least one of the torsion reinforcements (longitudinal or transve rsal) reaches the yielding point. The second criterion corresponds to determining the point for which the concrete struts starts to behave nonlinearly, due to high levels of loading (this situation may occur before any reinforcement bar yields). Stage 3 of the curve was plotted by using the Variable Angle Truss -Model, with non linear behaviour of the materials and considering the Softening Effect. The three stages are identified in the T –𝜃 curve of Fig. 7 are characterized separately . Figure 7 Typical T - curve for a reinforced concrete beam under pure torsion where, Tcr = Cracking torque 𝜃𝑐𝑟 = Twist corresponding to T cr for the stage 1 (limit for linear elastic analysis in non cracked and cracked stage) Tly = Torque corresponding to yielding of longitudinal reinforcement 𝜃ly = Twist corresponding to T ly Tty = Torque corresponding to yielding of transversal reinforcement 𝜃ty = Twist corresponding to T ty Tul = Ultimate (maximum) torque 𝜃ul = Maximum twist at beam’s failure. (GC)I =Torsional stif fness of Zone 1 (for linear elastic analysis in non cracked stage); (GC)II = Torsional stiffness of Zone 2 (for linear elastic analysis in cracked stage). P. Kapildev and G. Kumaran http://www.iaeme.com/ IJARET/index. asp 36 editor@iaeme.com 3.2.1. Parameters Considered for A nalyzing the HFRP / Steel R einforce d Concrete Beams are as follows B = 160 mm; D = 275 mm; b1 = 102 mm; d1 = 217 mm; Es = 210000 N/mm2;Ec= 5000√𝑓𝑐𝑘; EHFRP= 64700 N/mm2 ;Cube strengths of concrete m1 = 42.00 MPa; m2 = 76.00 MPa; m = Es/Ec; Al = 1134= 452mm2(2 Nos. of top and bottom); Al = 1136= 678mm2(3 Nos. of top and bottom); At = 2 50.3 = 100.6 mm2; fy =520 MPa; S1 = 75 mmS2 = 100 mm; fHFRP -l =518 MPa ; fHFRP -t =147 MPa. Table 4 Torque verses twist for various Parameters involved Beams Tel el Tcr1 cr1 Teff eff Tyt yt Tul ul Bp1m1FeS1 2.05 0.135 7.35 0.243 8.19 0.541 13.43 5.046 16.27 9.11 Bp1m1HfS1 2.05 0.135 7.35 0.243 8.19 0.541 12.30 14.59 9.91 11.68 Bp1m2FeS1 2.75 0.129 9.89 0.231 11.0 0.515 13.43 5.046 18.27 8.04 Bp1m2HfS1 2.75 0.129 9.89 0.231 11.0 0.515 12.30 14.54 9.91 12.62 Bp2m1FeS1 2.05 0.135 7.35 0.243 8.29 0.547 13.43 3.835 22.93 3.69 Bp2m1HfS1 2.05 0.135 7.35 0.243 8.29 0.547 12.30 10.99 11.26 7.31 Bp2m2FeS1 2.75 0.129 9.89 0.231 11.2 0.521 13.43 3.835 32.93 5.60 Bp2m2HfS1 2.75 0.129 9.89 0.231 11.2 0.521 12.30 10.94 13.26 7.23 Bp1m1FeS2 2.05 0.135 7.36 0.242 8.04 0.531 10.07 4.088 22.09 9.72 Bp1m1HfS2 2.05 0.135 7.36 0.242 8.04 0.531 9.23 11.85 7.48 13.62 Bp1m2FeS2 2.75 0.129 9.89 0.231 11.0 0.505 10.08 4.044 19.10 9.66 Bp1m2HfS2 2.75 0.129 9.89 0.231 10.8 0.505 9.23 11.81 8.91 13.52 Bp2m1FeS2 2.05 0.135 7.35 0.243 8.14 0.537 10.07 3.18 24.46 4.45 Bp2m1HfS2 2.05 0.135 7.35 0.243 8.14 0.537 9.23 9.14 14.81 6.67 Bp2m2FeS2 2.75 0.128 9.89 0.231 10.9 0.511 10.07 3.14 27.26 4.17 Bp2m2HfS2 2.75 0.128 9.89 0.231 10.9 0.511 9.23 9.11 18.81 9.60 4. EXPERIMENTAL OBSERVA TIONS 4.1. Test Setup All the test specimens are instrumented to measure their overall twist and axial deformations using dial gauges and LVDTs. The geometry and reinforcement details of test specimens are shown in Figs. 3.17 to 3.18. Static L oading Torsion testing frame of capacity 10 tonnes is used for testing the rectangular solid beam specimens. The static monotonically increasing loads are applied at the ends with the help of hydraulic jacks manually (100 kN capacity) and are monitored by pressure gauges. This frame is provided with two plungers along with loading wedges at both the ends in orde r to induce uniform torque. Torque is induced by applying eccentric loads along the axis of the beam with an eccentricity of 350mm. The deflections or deformations of the beams are measured by dial gauges and Linear Variable Displacement Transducers (LVDT) . The schematic diagrams of the torsional beam supported on saddle supports are shown in figs. 8 to 9. Saddle Beam Saddle Beam Figure 8 Schematic diagram of saddle support and specimen Theoretical and Experimental on the Behaviour of Rectangular Concrete Beams Reinforced Internally with HFRP Reinforcements Under Pure Torsion http://www.iaeme.com/ IJARET/index. asp 37 editor@iaeme.com Instrumentation All specimens are pasted w ith internal and external surface strain gauges. Internal strain gauges are pasted on the surface of the steel/Hybrid FRP reinforcements at the time of casting the specimens with due precaution. External strain gauges are pasted on the surface of the beam specimens near the support at the middle depth (vertically and horizontally) on both ends of the beam. Strains are measured using strain gauges. The twist induced is then calculated on the basis of the measured deformations. The axial extensions are also m easured with the help of dial gauges with a least count of 0.01 mm mounted on magnetic base stands at the end of beam specimens at both the ends. A Data acquisition system is used with a sampling rate of 50 samples/s to record all LVDT and electrical strai n gauge signals. These electrical signals are converted into strains and are processed with the help of computers. The load is gradually applied with an increment of 1 kN up to the failure of the beams. The experimental set up with instrumentations is show n in fig. 3.20. An arial view of the typical experimental setup with all the instru mentations is shown in fig. 9 Saddle Support These saddle supports are useful for inducing twisting moments along the beam length with an assumption that both the ends are p rovided with uniform torque thereby pure torque is applied. The magnitudes of the twisting moments are entirely determinable from statics alone. The entire saddle support unit consists of top plate, rollers and bottom plate. The top and bottom plates are k ept in between rollers which can permit rotation along the direction of application of loading. can permit rotation along the direction of application of loading. a) Top Plate b) Saddle with rollers Figure 9 Saddle Support All the test specimens with above said parameters are tested in a torsional testing frame and the corresponding observations are made. The test observations are presented in th e form of photographs (Fig. 10 to 13 ) and graphs in the following sectio ns. P. Kapildev and G. Kumaran http://www.iaeme.com/ IJARET/index. asp 38 editor@iaeme.com Figure 10 (a, b) Failure of conventional steel reinforced beams due to yielding of longitudinal and transverse steel ( Bp1m1Fe) (a) (b) Figure 11 (a, b) Failure of conventional steel reinforced beams due to concrete crushing (Bp2m2Fe) Figure 12 Larger diagonal cracks in Hybrid FRP reinforced beams Figure 13 Failure of Hybrid FRP reinforced rectangular beam due to excessive spalling of cover followed by concrete crushing (Bp1m1Hf) Theoretical and Experimental on the Behaviour of Rectangular Concrete Beams Reinforced Internally with HFRP Reinforcements Under Pure Torsion http://www.iaeme.com/ IJARET/index. asp 39 editor@iaeme.com 4.2. Torque and Twist Relationships for various D esignation of Beam Figure 14 Torque verses twist for Bp1m1FeS1 and Bp1m1HfS1 Figure 15 Torque verses twist for Bp1m2FeS1 and Bp1m2HfS1 05101520253035 0 2 4 6 8 10 12 14 16 18Torque in KN.m Twist in degreesBp1m1S1Fe-Sp-Tru Bp1m1S1Hf-Sp-TruBp1m1S1Fe-Exp Bp1m1S1Hf-Exp 05101520253035 0 5 10 15 20Torque in KN.m Twist in degreesBp1m2S1Fe-Sp-Tru Bp1m1S2Hf-Sp-TruBp1m2S1Fe-Exp Bp1m2S1Hf-Exp P. Kapildev and G. Kumaran http://www.iaeme.com/ IJARET/index. asp 40 editor@iaeme.com Figure 16 Torque verses twist for Bp2m1FeS1 and Bp2m1HfS1 Figure 17 Torque verses twist for Bp2m2FeS1 and Bp2m2HfS1 0481216202428 0 2 4 6 8 10Torque in KN.m Twist in degreesBp2m1S1Fe-Sp-Tru Bp2m1S1Hf-Sp-TruBp2m1S1Fe-Exp Bp2m1S1Hf-Exp 048121620242832 0 1 2 3 4 5 6 7 8 9 10Torque in KN.m Twist in degreesBp2m2S1Fe-Sp-Tru Bp2m2S1Hf-Sp-TruBp2m2S1Fe-Exp Bp2m2S1Hf-Exp Theoretical and Experimental on the Behaviour of Rectangular Concrete Beams Reinforced Internally with HFRP Reinforcements Under Pure Torsion http://www.iaeme.com/ IJARET/index. asp 41 editor@iaeme.com Figure 18 Torque verses twist for Bp1m1FeS2 and Bp1m1HfS2 Figure 19 Torque verses Twist for Bp1m2FeS2 and Bp1m2HfS2 05101520253035 0 2 4 6 8 10 12 14 16 18 20Torque in KN.m Twist in degreesBp1m1S2Hf-ExpBp1m1S2Fe-Sp-Tru Bp1m1S2Hf-Sp-TruBp1m2 S2Fe-Exp 05101520253035 0 2 4 6 8 10 12 14 16Torque in KN.m Twist in degreesBp1m2S2Fe-Sp-Tru Bp1m2S2Hf-Sp-TruBp1m2S2Fe-Exp Bp1m2S2Hf-Exp P. Kapildev and G. Kumaran http://www.iaeme.com/ IJARET/index. asp 42 editor@iaeme.com Figure 20 Torque verses twist for Bp2m1FeS2 and Bp2m1HfS2 Figure 21 Torque verses twist for Bp2m2FeS2 and Bp2m2HfS2 051015202530 0 1 2 3 4 5 6 7 8Torque in KN.m Twist in degreesBp2m1Fe S2-Exp Bp2m1Hf S2-Theo Space TrussBp2m1Fe S2-Theo space truss Bp2m1Hf S2-Exp 05101520253035 0 1 2 3 4 5 6 7 8 9 10Torque in KN.m Twist in degreesBp2m2Hf S2-Sp-truBp2m2Hf S2-ExpBp2m2Fe S2-Exp Bp2m2Fe S2-Sp-tru Theoretical and Experimental on the Behaviour of Rectangular Concrete Beams Reinforced Internally with HFRP Reinforcements Under Pure Torsion http://www.iaeme.com/ IJARET/index. asp 43 editor@iaeme.com Table 5 Results obtained from Experimental and Theoretical values Beam (Tcr-EXP/Tcr-Sp Tr) (θcr-EXP/θcr-Sp Tr) (Tul-EXP/Tul-Sp Tr) (θul-EXP/θul-Sp Tr) Bp1m1 FeS1 1.08 1.84 1.08 1.79 Bp1m1HfS1 1.08 1.84 1.21 1.38 Bp1m2 Fe S1 0.86 1.13 1.08 1.79 Bp1m2HfS1 0.86 1.13 1.13 1.01 Bp2m1 Fe S1 1.08 1.84 0.87 1.67 Bp2m1 HfS1 1.08 1.84 0.92 1.44 Bp2m2 Fe S1 0.86 1.13 0.93 1.34 Bp2m2HfS1 0.86 1.13 1.66 1.00 Bp1m1 FeS2 1.08 1.84 0.87 1.07 Bp1m1HfS2 1.08 1. 84 1.27 0.85 Bp1m2 Fe S2 0.86 1.13 0.94 0.68 Bp1m2HfS2 0.86 1.13 1.25 0.50 Bp2m1 Fe S2 1.08 1. 84 1.00 1.05 Bp2m1HfS2 1.08 1. 84 1.16 1.40 Bp2m2 Fe S2 0.86 1.13 0.97 0.85 Bp2m2HfS2 0.86 1.13 1.06 0.62 5. CONCLUSIONS The experimental results in the form of torque verses twist diagrams are shown in Fig. 13 to Fig. 20 and the results are compared with the reference beams. • The torque value of the steel reinforced beams in all the cases are 30 to 45% higher than the Hybri d FRP reinforced beam specimens. The specimens of M 60 grade exhibits lesser value of torque as compared to M 30 grade specimens invariably for all the specimens. • The twist value of the Hybrid FRP reinforced beams in all the cases are 20 to 50% higher tha n the conventionally reinforced beam specimens. The specimens of M 60 grade exhibits higher twist value of torque as compared to M 30 grade specimens invariably for all the specimens. • The specimens having the spacing of stirrups 75mm show identical values of torque and twist value in the ultimate load capacity, both for the steel and Hybrid FRP reinforced concrete beams, inspite of the concrete grade and the reinforcement ratios. • The increased percentages of (0.86%) reinforcements show better performance i n the load carrying capacity, both for the steel and Hybrid FRP reinforced specimens 10% to 15%. The same kind of variation has been noticed for the specimens having the spacing of stirrups 75mm. • The torque -twist behaviour of concrete beams reinforced with HFRP reinforcements are similar to that of concrete beams reinforced with steel reinforcements until the formation of the first torsional crack. The value of first crack ( Tcr) is insensitive to the type of reinforcement used. When cracking occurs, there is an increase in twist under nearly constant torque, due to a drastic loss of torsional stiffness. Beyond this, however, the strength and behaviour depend on the amount and type of longitudinal and torsional reinforcement present in the beam. • From the experimental study it is seen that two types of failures are observed for beams reinforced with steel reinforcements, namely, yielding of longitudinal and transverse reinforcements and concrete crushing. It is also observed from the present study that for beams reinforced with conventional steel reinforcements having a P. Kapildev and G. Kumaran http://www.iaeme.com/ IJARET/index. asp 44 editor@iaeme.com reinforcement ratio lesser than 1% (ie.0.56%) are failed by yielding of longitudinal and transverse reinforcements before crushing of concrete in compression (Fig.3.26) but beams wit h approximately closer to 1% (ie.0.86%) of steel reinforcement ratio are failed by crushing of concrete in compression before yielding of longitudinal or transverse reinforcements (Fig.3.27). • Also it is seen that two types of failures are observed for beam s (Figs.3.25 to 3.30) reinforced with Hybrid FRP reinforcements, namely, concrete crushing and rupture of Hybrid FRP stirrups under tension. Hybrid FRP reinforced concrete beams are failed due to crushing of concrete followed by rupture of Hybrid FRP stirr ups when the reinforcement ratio lesser than 1% (0.56%). It is probably due to the fact that the ultimate tensile strains of Hybrid FRP stirrups (generally HFRP pultruded reinforcements have higher ultimate strains in fact higher than conventional steel, b ut HFRP stirrups are not manufactured by pultrusion process and are indeed, manufactured by vacuum resin bath method which are having lesser ultimate strains but slightly greater than concrete ultimate strains) and are slightly higher than the ultimate com pressive strains of concrete. Such mode of failure is invariably observed for all Hybrid FRP reinforced beams with a reinforcement ratio lesser than 1% and approximately closer to 1%. But none of beams failed due to rupture of Hybrid RFP reinforcements pri or to concrete strain reaches ultimate. It is probably due to the fact that the ultimate tensile strains of Hybrid FRP reinforcements are greater than the ultimate compressive strains of concrete. • Torsional strength and angle of twist increases with the in crease in grade of concrete and percentage of longitudinal and transverse reinforcements. But Hybrid FRP reinforced concrete beams showed higher angle of twist than the conventional reinforcements. T his fact is primarily due to higher tensile strain values for Hybrid FRP reinforcements than the steel reinforcements. • It is also noted that by replacing main and transverse steel reinforcements by an equal percentage of Hybrid FRP reinforcements, reduced their torsional capacities by 30% for lower grade concret e but their increase is reduced by 20% for higher grade concrete and higher percentage of steel . • It is also observed that for steel reinforced concrete beam, the yielding of reinforcement leads to a larger increase in twist with little change in torque, wh ereas Hybrid FRP reinforced beams show no yielding of reinforcements and the twist continues to increase with the increase in torque, there by exhibiting some ductility despite the brittle nature of Hybrid FRP reinforcements. • Higher grade of concrete and h igher percentage of steel, the torque –twist diagrams show a increase in torque together with a rapid increase of beam twist. The failures were characterized by crushing of the concrete at the compressive face, followed by the yielding of steel stirrups and rupture of Hybrid FRP stirrups. This qualitative behaviour is observed in Hybrid FRP/steel beams; consequently the overall behavior of the Hybrid FRP beam is similar to the behavior of conventionally reinforced concrete beams except that the failure of Hy brid FRP stirrups even after concrete crushing. • The post peak values of torsional strength of beams have greater influence on the spacing of stirrups. The minimum spacing of stirrups are arrived based on the Indian Standards. An examination of the curves r eveals that the slope of the curves at the initial stages of loading is mild for Hybrid FRP reinforced specimens whereas for Theoretical and Experimental on the Behaviour of Rectangular Concrete Beams Reinforced Internally with HFRP Reinforcements Under Pure Torsion http://www.iaeme.com/ IJARET/index. asp 45 editor@iaeme.com conventionally reinforced specimen it is steeper. This is primarily due to lower elastic modulus than conventional steel reinforcem ents. • During test all specimens exhibited satisfactory ultimate torsional behaviour, however the post -peak behaviour differs for Hybrid FRP reinforced beam than the conventionally reinforced beams.