ComparativeInvestigationonTorsionalBehaviourofRCBeam ...inforcement ratios for the strengthened beams in this group are shown in Figure 4. CFRP sheet and NSM steel re-inforcement ratios

Research ArticleComparative Investigation on Torsional Behaviour of RC BeamStrengthened with CFRP Fabric Wrapping and Near-SurfaceMounted (NSM) Steel Bar

Nasih Askandar 1,2 and Abdulkareem Mahmood3

1Department of Civil Engineering, Salahaddin University-Erbil, Erbil, Iraq2Sulaimani Polytechnic University, Sulaymaniyah, Iraq3Department of Civil Engineering, Salahaddin University, Erbil, Iraq

Correspondence should be addressed to Nasih Askandar; [email protected]

Received 24 June 2019; Accepted 7 September 2019; Published 23 September 2019

Academic Editor: Arnaud Perrot

Copyright © 2019 Nasih Askandar and Abdulkareem Mahmood. *is is an open access article distributed under the CreativeCommons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided theoriginal work is properly cited.

Many researchers worldwide have extensively used fibre-reinforced polymer (FRP) strengthening materials and near-surfacemounted (NSM) to enhance the shear and flexural strengths of reinforced concrete (RC) beams. However, studies on torsionalstrengthening are limited. Although a few studies have focused on torsional strengthening, none of them simultaneously in-vestigated torsion with shear and/or bending moment.*is study aims at demonstrating the behaviour of RC beams strengthenedwith FRP sheets (strips) with different configurations and NSM steel bars with different spacing that was subjected to combinedactions of torsion and bending moment and making a comparison between them. Seven beams with a dimension of15× 25× 200 cm were casted. One of the beams was not strengthened; three of them were strengthened with carbon FRP, and theothers were strengthened with NSM steel bar. *e angle of twist at torque intervals, first cracking torque, ultimate torque, andultimate twist angle of the conventional and strengthened beams during the testing process are compared. Results show asignificant improvement in the torsional performance of RC beams using carbon FRP and NSM steel bar.*e test beams that werestrengthened with CFRP wrapping showed better enhancement in the ultimate torsional moment as opposed to the beams thatwere strengthened with NSM steel bar.

1. Introduction

A considerable amount of torque can accumulate in manyconcrete members, including curved bridge elements,spandrel beams, horizontally curved members, and eccen-trically loaded beams. *e torsional capacity of thesemembers needs to be maximized due to several factors,including structural damage, deterioration, and increasedloading.

Strengthening or upgrading of structural elements mustbe performed after a certain period to increase the service lifeof these elements [1, 2]. Strengthening materials can beapplied to RC structures in two ways, namely, (1) the ex-ternally bonded reinforcement (EBR) method, where thestrengthening materials are externally applied to the

concrete surface, and (2) the near-surface mounted (NSM)method, where the strengthening materials are insertedwithin grooves that are pre-cut into the concrete cover [3].

Several externally bonded reinforcement (EBR) methodsof increasing the service life of RC structural elements areavailable; for example, fibre-reinforced polymer (FRP)strengthening sheets are used to wrap RC structural ele-ments. *is technique is inexpensive and can be applied tocompletely degraded structures. Compared with traditionaltechniques, repairing concrete structures by using FRPsheets has many advantages. FRP sheets have high tensilestrength, extremely low weight, high corrosion resistance,and fast installation. Moreover, changing the geometry ofthe structure is unnecessary when these sheets are used. *isstrengthening technique has already been proven to be

HindawiAdvances in Civil EngineeringVolume 2019, Article ID 9061703, 15 pageshttps://doi.org/10.1155/2019/9061703

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effective for the shear and flexural strengthening of RCbeams [4–31]. A few studies focused on torsion strength-ening [1, 2, 32–40].

Considered a suitable alternative to the EBR method, theNSM strengthening method involves cutting grooves in theconcrete cover of a beam specimen and then inserting andembedding reinforcement into these grooves by using anadhesive. *e NSM method presents numerous advantagesover the EBR method, including its higher bonding pro-ductivity and better protection. *e NSM method can alsoaddress the limitation of the EBR method in its maximumstrain, which is below the ultimate strain due to prematuredebonding.*e greater confinement granted by the adhesiveand the surrounding concrete is considered the best ad-vantage offered by the NSMmethod [41].*is strengtheningtechnique has already been proven to be effective for theshear and flexural strengthening of RC beams [42–58]. Manystudies have also used conventional steel bars instead of FRPmaterials for flexural and shear strengthening by employingthe NSM method [59–64]. However, only few of thesestudies have focused on torsion strengthening [65, 66], andno previous work has investigated the application of con-ventional steel bars for the torsion strengthening of RCbeams.

*e literature review above indicates that FRPstrengthening materials have been extensively used to im-prove the flexural and shear strengths of RC beams. NSMstrengthening steel bars have also been used to improve theflexural and shear strengths of RC beams. However, studieson torsional strengthening by both techniques are limited,and none of them compared these methods for choosing thebest one. *erefore, this study aims to investigate thecharacteristics of RC beams strengthened with bothmethods(FRP sheet and NSM steel bar) having different configu-rations under the combined actions of torsion and bendingmoment and making a comparison between them.

2. Materials and Testing Methods

2.1. Reinforcement and Formwork. To avoid beam failure attorsional and flexural cracking loads, each beam wasdesigned to have a steel reinforcement ratio (ρ) of 2.3%(equation (1)) for each of the longitudinal and transversereinforcement to the volume of concrete [33, 36, 67]:

ρ � ρsl + ρst �Asl

Ac+

At

Ac·pt

s. (1)

*e reinforcement percentage provided in a beam washigher than the minimum requirement [1] to control theintegrity of the beam beyond cracking; moreover, this willreveal the case of a defective beam in terms of reinforcement.*e steel reinforcement details and cross-sectional di-mensions of each beam are presented in Figure 1. All beamswere under-reinforced according to the American ConcreteInstitute code ACI 318-14 [67] to simulate torsion-deficientbeams under some future loading condition. *e minimumspacing of the transverse reinforcement was also purposefullyexceeded to facilitate the observation of torsional failure andto hinder the stirrup from restricting the torsional cracks.

*e reinforcement for the specimens included 2Φ16 and2Φ12mm diameter steel bars at the bottom and top, re-spectively. End zones that were 0.4m long on each end of thebeam were reinforced with Φ10mm stirrups spaced at75mm from centre to centre to force failure in the halfwayzone of the tested beam. *e test region of (1.0m) wasselected in such a way that at least one complete spiral crackformed along the length of the region; therefore, Φ10mm(fy � 541MPa) stirrups spaced at 200mm on centres wereutilised as reinforcement. Furthermore, plywood sheets(1.22m× 2.44m× 18mm) were used to cast the concretebeams.

All of the RC beams used the same concrete mix withcompressive strength (fc) of 48MPa. Lafarge ConcreteCompany (local supplier) provided the ready-mix concretefor casting the concrete beams.

2.2. Torsion Strengthening Configurations. Seven rectangularRC beams with 250mm depth, 150mmwidth, and 2000mmlength were casted by using ready-mixed concrete. *ecentral part of these beams was specifically designed todisplay torsion failure. *e length of the central part was setto 1.0m to allow the formation of at least one spiral crack atthe 45° angle in the longitudinal axis of these beams. Onespecimen was stored without strengthening and referred toas the control specimen. *e strengthened beams were thendivided into two groups. *e first group consists of threebeams and strengthened with CFRP, whilst the second groupalso consists of three beams and strengthened with near-surface mounted (NSM) steel bar.

2.2.1. CFRP Torsion Strengthening Configurations. CFRPcomposite wrap spacing was the main parameter in-vestigated for this group. A full-wrap CFRP composite wasemployed to strengthen specimen 100C100. By contrast, a100mm width CFRP composite was used to strengthen twoother beam specimens, and all-around wraps were placed ata spacing of 150 and 200mm c/c using one layer of theCFRP composites. All beam specimens were tested undercombined torsion and bending. Figure 2 shows thewrapping arrangement, and Table 1 presents the testspecimen design.

2.2.2. NSM Steel Bar Torsion Strengthening Configurations.NSM groove spacing was the main parameter investigatedfor this group. Figure 2 presents the groove details for allthree strengthened beams. Grooves 20mm wide and 20mmdeep were cut in the 25mm cover zone of the concretebeams, and two U-shaped conventional steel bars wereembedded in these grooves. One of these steel bars had aU-shaped stirrup at the top whilst the other bar had aU-shaped stirrup at its bottom. To create an enclosed circuitstirrup, these U-shaped steel bars were welded together withat least a 100mm overlap between their legs. *e grooveswere vertical (having angle of inclination of 90° with respectto the longitudinal axis of the beam). *e groove spacing forthese beams was set to 100, 150, and 200mm. Sikadur-30P

2 Advances in Civil Engineering

epoxy, a two-part epoxy that produces a clear liquid whenmixed, was used as an adhesive to ll the grooves of thestrengthened beams. Table 1 lists the information related tothis system.

2.3. FRP Material

2.3.1. CFRP Material Properties. �e experiment used car-bon bre fabric SikaWrap®-300C and epoxy-based saturated

1000mm

2000mm

400mm

150mm

250mm

2Ø12mm

2Ø16mm

Ø10mm

Ø10mm @75mm c/c Ø10mm @200mm c/c Ø10mm @75mm c/c100mm 100mm

25mm

25mm

25m

m

25m

m

400mm

Figure 1: Dimensions and reinforcement details of the test beams.

Strengtheningtype

Beamname Strengthening detail

Control

100C100

100C150

100C200

NSM100

NSM150

NSM200

Unstrengthened

Strengthened beamwith CFRP strip

Strengthened beamwith NSM steel bar

2000mm

1000mm 100mm100mm

100mm

150mm

200mm

100m

m10

0mm

150m

m20

0mm

400mm400mm

C

Figure 2: Strengthening of the test beams.

Advances in Civil Engineering 3

resin Sikadur-330. Unidirectional CFRP fabrics (SikaWrap-300C) with a thickness of 0.166mm per ply were also utilised.*e elastic modulus, ultimate tensile strength, and elongationat failure of the fibre were 230GPa, 3900MPa, and 15mm/m,respectively, according to the manufacturer. *e FRP sheetswere bonded to concrete by using two pieces of “rubber-toughened cold-curing-construction epoxy adhesive (Sika-dur-330)” with a density, elastic modulus, and tensile strengthof 1310 kg/m3, 3800MPa, and 30MPa, respectively. Tables 2and 3 list the information related to this system.

2.3.2. Preparing and Installing CFRP. *e bond between theRC beam and CFRP was given appropriate attention duringthe strengthening process. When fixing the CFRP to theconcrete surface, a handheld grinder was utilised to level theconcrete surface disclosing the aggregate. *e grinder wasused again to arciform the concrete corners to a minimumradius of 13mm [3] and reduce the stress concentration in thefibres at the edges. *is stress concentration will lead torupture failure of CFRP sheets at corner edges before reachingtheir ultimate strength. Water and compressed air were usedto clean the concrete surface from released particles and dirt.

2.3.3. Applying CFRP to the Concrete Substrate Surface WasRegulated in Steps. *e beams were cleaned by washing withpressurized water and allowed to dry prior to CFRP sheetapplication. Loose particles and defilements from thespecimen’s surface were removed by this procedure. *ebeams were also wire brushed and vacuumed prior to theCFRP sheet application. Depending on substrate roughness,the resin (Sikadur-330) was mixed and applied to the pre-pared concrete surface by using a brush at the amount ofapproximately 0.75 kg/m2 to 1.25 kg/m2. *e SikaWrap®-300C sheet was cut into strips with 100mmwidth by scissorsfor the required length for all the specimens (estimatedoverlap greater than 150mm). *e (SikaWrap®-300C) stripwas applied to the resin with a special plastic roller until theresin was squeezed out between the roving. *en, theSikaWrap®-300C fabric was applied onto the resin coatingin the appropriate direction.

2.3.4. FRP Ratio as Transverse Reinforcement. Figure 3presents the FRP ratios for the strengthened beams(group 1). Equation (2) was used to calculate the volumetricratios of CFRP reinforcement, ρf [34, 35]:

ρf �nf tfdbfPf

Acsf. (2)

2.4. NSM Steel Bar Strengthening Technique

2.4.1. NSM Steel Bar Materials. Ready-mixed concrete alsowas used for all three beams in this group. *e compressivestrength (fc′) of the supplied concrete was evaluated after28 days. *e average results of three concrete cylinders(100mm in diameter and 200mm in height) on the day ofthe testing were considered for computing the compressivestrength. Sikadur-30P epoxy, a two-part epoxy that producesa clear liquid when mixed, was used as an adhesive to fill thegrooves of the strengthened beams.

*ree bars with similar diameters and three weldedbars (Ø10mm) were subjected to uniaxial tensile tests todetermine their yield and ultimate strength (fy, fu)

Table 2: SikaWrap®-300C (woven carbon fibre fabric for structuralstrengthening).Characteristics NoteFibre type Midstrength carbon fibresFibre orientation 0° (unidirectional)Areal weight 300± 15 g/m2Fabric designthickness

0.166mm(based on fibre content)

Tensile strength offibres 3,900N/mm

2 (nominal)

*ickness

1.3mm per layer(impregnated with Sikadur®-330)1.0mm per layer(impregnated with Sikadur®-300)Tensile E-modulus 230,000N/mm2

Elongation at break 1.5% (nominal)

Table 1: Design of the concrete beams.

Beams Strengthening techniqueCFRP transverse strip

NSM steel bar spacing (mm)No. of layers Width (mm) Spacing (mm)

Control Unstrengthened100C100 (full wrap)

CFRP sheet1 Full —

100C150 1 100 150 —100C200 1 100 200 —NSM100

NSM steel bar— — — 100

NSM150 — — — 150NSM200 — — — 200

Table 3: Sikadur-330 (two-part epoxy impregnation resin).

Appearance andcolours

Resin part A: white (paste); hardener part B:grey (paste)

Density 1.31 kg/l (mixed)Mixing ratio A : B� 4 :1 by weightOpen time 30min (at + 35°C)Viscosity Pasty, not flowableService temperature − 40°C to +50°CTensile strength 30MPa (cured seven days at +23°C)Flexural E-modulus 3800MPa (cured seven days at +23°C)


following the recommendations of ASTM A370-10 2010[28].

Table 4 summarizes the mechanical properties of theconcrete and NSM steel bars, the average compressivestrength of the concrete (fc′), and the average test results forthe steel bars, whilst Table 5 presents the properties of theepoxy based on the specications supplied by themanufacturer.

2.4.2. NSM Steel Reinforcement Ratio. �e NSM steel re-inforcement ratios for the strengthened beams in this groupare shown in Figure 4. CFRP sheet and NSM steel re-inforcement ratios for the strengthened beams for bothgroups are shown in Figure 5. �e volumetric ratios of NSMsteel bar reinforcement, ρnst, were calculated by using thefollowing equation:

ρnst �vlume of steel stirrupVolume of concrete

�AnstpnstAcS

. (3)

2.4.3. Specimen Preparation. �e installation of thestrengthening steel bars began by cutting grooves into theconcrete cover of the specimens after 28 days of curing.�ese grooves were cut in the transversal direction aroundthe beam cross section, whilst maintaining dimensionsgreater than 1.5 db× 1.5 db (where db denotes the diameterof the NSM steel reinforcement). A special concrete saw(handle grinder) with a diamond cutting saw blade was usedfor the cutting. A hammer drill and chisel were used toremove any remaining concrete lugs and to roughen thelower surface of the grooves. �ese grooves were thensmoothened and cleaned with a wire brush and a high-pressure air jet. �e strengthening steel bars were bent into aU-shape, and two U-shaped steel bars were used for eachclosed groove (one at the top and the other at the bottom). Inthis way, these steel bars were overlapping for approximately100mm and welded together to form a closed stirrup at eachround groove. �ese grooves were then lled with an epoxyadhesive groove ller (Sikadur-30 LP) around the steel barand the surface was levelled as shown in Figure 6. To ensurethat the epoxy reached its full strength, the beamwas kept forat least two weeks of curing time.

2.5. Test Setup and Instrumentation. Figure 7 shows thedetails of the test setup. �e load at the active support wasapplied using a 200 ton hydraulic jack. �e jack had a500mm lever arm from the centroidal beam axis. �eperiodical applied load was measured using a compression

0.00

0.35 0.24 0.18

0.0

0.5

1.0

1.5

2.0

Control 100C100 100C150 100C200

ρ � %

Beam name

Figure 3: CFRP sheet reinforcement ratios for the strengthenedbeams.

Table 4: Mechanical properties of the concrete and NSM steel bars.

Material Compressivestrength (MPa)Yielding tensilestrength (MPa)

Concrete 48 —Steel bars Ø10mm — 541Welding steel barsØ10mm — 298

0.00

1.47

0.98

0.73

0.0

0.5

1.0

1.5

2.0

Control NSM100 NSM150 NSM200

ρ nst

%

Beam name

Figure 4: NSM steel reinforcement ratios for the strengthenedbeams.

Table 5: Sikadur®-30 LP (two-part epoxy impregnation resin).Appearance andcolours

Part A: white; Part B: black; parts A +B:light grey

Density (at 23°C) ∼1.65 kg/lt (parts A +B)Mixing ratio Part A :B� 3 :1 by weight or volumeLayer thickness 30mm maxOpen time 90minutes (at +25°C)Viscosity Pasty, not §owableService temperature –40°C to +45°C (when cured at >+23°C)

Tensile strength 15MPa to 18MPa (when cured for sevendays at +23°C)Shear strength 17MPa to 21MPa [+40°C to +55°C (7 days)]

0.00

0.350.24 0.18

1.47

0.980.73

0.0

0.5

1.0

1.5

2.0

Control 100C100 100C150 100C200 NSM100 NSM150 NSM200

(ρf o

r ρns

t) %

Beam name

Figure 5: CFRP sheet and NSM steel reinforcement ratios for thestrengthened beams.


load cell with a 100 ton capacity. *e hydraulic jack had amovable length of 250mm, thus providing a 57.3° twistcapacity for the beam. *e reaction arm had 500mmeccentricity from the centroidal axis of the beam. *ebeam elongated longitudinally after cracking and wasallowed to slide and elongate freely to avoid any

longitudinal restriction and consequent compression.*is feature was obtained by supporting the beam ends onrollers at the unresisting support. *e twist angle of thefree end (the point of applying torque) was measured by adial gauge with the aid of the downward distance of thelever arm at that point.

Figure 6: Grooving, NSN steel bar installation, welding, and groove filling with epoxy.

Concrete beam

Lever arm

Applied load

Dial gauge

Supports

Spreader beam

Figure 7: Schematic of the test setup for applying combined torsion and bending.


Load application on the beam specimens to be testedunder the combined action of torsion and bending utilised afabricated loading frame from the civil engineering labo-ratory. Rotation around the longitudinal beam axis wasarranged via a special support condition, and lever armswere attached to the specimen to provide a torsional mo-ment, as shown in Figure 7. *e specimen underwent puretorsion when the location of a lever arm coincided with thesupport. *e lever arm was maintained beyond the twosupports to apply combined bending and torsion.

(i) *ree dial gauges were used. Two of them formeasuring displacements were positioned under thelever arm, and one was placed at the centre tomeasure central displacement.

(ii) A distance of 400mm was maintained between thecentre of the support and lever arm to achievebending and torsion.

(iii) *e load of the hydraulic jack was transferred to thespecimen through a spreader beam resting on theend of a lever arm attached to the specimen. *us,half of the applied load acted at the end of each leverarm.

(iv) *e length of the specimen between supports was1.8m, with 0.1m projection outside the support. *ecentral 1.0m length of the specimen was subjected tocombined bending and torsion, whereas 0.4m lengthof the beam near each support was subjected tobending moment and shear force. *e torque in themiddle part of the specimen was the product of theload at the end of each lever arm (half the total ofapplied load) multiplied by the length of the leverarm from the centre of the specimen. *e twist angleat each lever arm was achieved from a vertical dis-placement of a lever arm end point and length of thelever arm. *e overall twist angle in the middle partof the specimen was equal to the sum of twist anglesat the couple of the lever arm.

2.6. Test Procedure. Figure 8 shows the hydraulic testingmachine in the civil engineering laboratory that was utilisedto test the beam specimens.

*e circular rotation of the supports and the trans-mission of applied load from the centre of themachine to thetwo points that express the moment arm must be facilitatedby the experimental conditions. Figures 7 and 8 show theparticular clamping loading frame used in this study. *isframe consisted of an I-section (200mm× 80mm× 8mm)attached to two steel channels (100mm× 50mm× 8mm) bywelding. After insertion around the beam cross section bylarge bolts, these steel channels connect from the bottom,and each arm utilises two bolts. Attached steel clamps workas lever arms for applied torque with separated faces toconnect them over the sample. *e final shape is similar to abracket. *ese lever arms are suitable for providing therequired eccentricity of 500mm with respect to the longi-tudinal axis. As shown in Figures 7 and 8 the transmission ofthe loads from the centre of the machine to the two lever

arms was achieved using an I-section steel spreader beamwith 200mm depth and 2m length. Increment readings ateach load were acquired through recorded camera videos,strains from the data logger, and cracks were recorded inaccordance with their occurrence.

2.7. Angle of Twist Measurements. As shown in Figure 9, adial gauge connected to the bottom of a lever arm at a point(500mm) from the centre of the longitudinal axis of thebeam was used to evaluate the angle of twist. *e downwardvalue of the lever arm was recorded by the dial gauge todetermine the twist angle in radians.

3. Results and Discussion

Table 6 shows a summary of the cracking torque (Tcr), ul-timate torque (Tu), and ultimate twist angle (θu) of theconcrete beams. Overall, the results demonstrate the sig-nificant improvement of Tcr, Tu, and θu of the concretebeams with the use of FRP sheets and NSM steel bar.

3.1. Ultimate Torsional Moment Carrying Capacity. *e ul-timate torsional moment carrying capacity of the controland strengthened beams are shown in Figure 10. Nonlinearimprovement of the ultimate torsional moment carryingcapacity of the strengthened beams in relation to the controlbeam, especially for beams that were strengthened with NSMmethod, was also observed. *e strengthening method andmaterials used were mainly affecting the improvementrather than FRP strip and NSM steel bar spacing. *emaximum ultimate torsional moment of 25.15 kN·m wasdemonstrated by beam 100C100, and beamNSM200 showedthe minimum torsional moment of 14.35 kN·m. *e valuesfor the other strengthened beams were between those of thetwo beams. Ultimate torsional moment for beams 100C150,100C200, NSM150, and NSM200 were 21.85, 19.88, 15.50,and 14.75 kN·m, respectively.

3.2. Influence of Material and Techniques Used for Strength-ening on Torsional Strength. *e ultimate torsional momentcarrying capacity of the control and strengthened beamswith respect to the strengthening material (CFRP sheetand NSM steel bar) and reinforcement ratio are shown inFigure 11. Nonlinear improvement of the ultimate torsionalmoment carrying capacity of the strengthened beams inrelation to the control beam was also observed. *estrengthening material and techniques used mainly influ-enced the improvement.

*e percentage of enhancement in the ultimate torsionalmoment (Tu) for beams 100C100 and NSM100, with thesame spacing but with different strengthening technique andmaterial and with strengthening material reinforcementratio of 0.35% and 1.47%, is 134% and 44%, respectively.

*e percentage of enhancement in the ultimate torsionalmoment (Tu) with respect to the control specimen for beams100C150 and NSM150, with 150mm spacing andstrengthening material reinforcement ratio of 0.24% and


0.98%, is 103% and 37%, respectively. While this percentagefor beams 100C200 and NSM200, with 200mm spacing andstrengthening material reinforcement ratio of 0.18% and0.73%, is 85% and 33%, respectively.

3.3. Inuence of Strengthening Method on Beam Ductility.�e ultimate twist angle carrying capacity of the control andstrengthened beams are shown in Figure 12. �e

strengthening method and materials used were mainlyin§uencing the ductility of the strengthened beams. Beamsthat were strengthened with CFRP sheet represented moreductility than the control beam, and the others werestrengthened with NSM steel bar. Beams 100C100, 100C150,and 100C200 that were strengthened with CFRP sheet, the

Loading frame Load cell

Spreaderbeam

Lever arm

Dial gauge

Concretebeam

Hydraulic jack(200 kN capacity)

Camera

Supports

Figure 8: Test setup with the loading frame.

500mm

θ

Down reading

Figure 9: Angle of twist measurements.

10.75

25.1521.85

19.88

15.50 14.75 14.35

0

5

10

15

20

25

30


Ulti

mat

e tor

que (

kN·m

)

Beam name

Figure 10: Ultimate torsional moment carrying capacity.

00

134

10385

44 37 333524 18

147

98

73

0

40

80

120

160


% in

crea

se o

f ulti

mat

e tor

que

Beams

% increase of ultimate torque[(ρft and ρnst)%]/100

Figure 11: FRP and NSM steel reinforcement ratios and increase inultimate torque in percentage.

Table 6: Experimental results of the tested beams.

Beam code Str. Tech. fc′MPa Tcr kN·m %Incr.Tcr Tu kN·m %Incr.Tu θu deg./m %Inc. θuControl Un-str.

48

4.50 — 10.75 0 4.77 0100C100

CFRP sheetN.A — 25.15 134 10.49 120

100C150 7.75 72 21.85 103 10.39 118100C200 7.00 56 19.88 85 9.17 92NSM100

NSM steel bar8.50 89 15.50 44 3.97 − 17

NSM150 8.00 78 14.75 37 3.64 − 24NSM200 7.00 56 14.35 33 3.47 − 27Str. Tech.: strengthening technique; %Incr.Tcr: increase of cracking torque in percentage; %Incr.Tu: increase of ultimate torque in percentage; %Incr. θu:increase of ultimate twist angle in percentage; Un-str.: Unstrengthened.


enhancement percentage of their ultimate twist angle (θu)with respect to the control beam was 120%, 118%, and 92%,respectively, whilst beams NSM100, NSM150, and NSM200that were strengthened with NSM steel bar, the diminutionpercentage of their ultimate twist angle (θu) with respect tothe control beam was 17%, 24%, and 27%, respectively.

3.4. Inuence of CFRP and NSM Steel Bar Spacing on theTorsional Strength. Figure 13 shows the in§uence of CFRPand NSM steel bar spacing on the enhancement percentageof ultimate torsional strength of the strengthened beamswith respect to the control beam.�e spacing of CFRP sheetsfor beams that were strengthened with CFRP sheet had agreat eªect on enhancement percentage of ultimate torsionalstrength of the strengthened beams than that beamsstrengthened with NSM steel bar.

�e enhancement percentage of the ultimate torsionalmoment (Tu) of beams 100C100, 100C150, and 100C200 thatwere strengthened with CFRP sheet and spacing of 100, 150,and 200mm is 134%, 103%, and 85%, respectively, whilst itsvalue for beams NSM100, NSM150, and NSM200 with thesame spacing but with NSM steel bar strengthening methodis 44%, 37%, and 33%, respectively.

3.5. Torque-Twist Comparison. Figure 14 shows the torque-twist behaviour of the control and strengthened beams.Utilising CFRP sheets for the same load, the control beam hada smaller torque carrying capacity and higher twist anglevalues compared with the strengthened beams. Amongst all ofthe strengthened beams, 100C100 and 100C150 exhibited thebest ductility. Furthermore, the tendency of the torque-twistangle of all the beams did not signicantly change beforecracking occurred. Owing to the stirrup, external CFRP stripsor NSM steel bar that exhibited torque resistance in the post-cracking stage, all curves showed a reliable slope to reach theultimate torque of beams. �erefore, an increase in torsionalrigidity of the beams was observed, and the loading stoppedafter reaching the ultimate torque.

3.6. Crack Pattern and Failure Modes. Figure 15 shows thefailure modes of the unstrengthened (control) and CFRP-strengthened RC beams under combined bending andtorsion. �e torsional moment resulted in the failure of all

the tested beams. �e number of cracks in the strength-ened beams was larger than that in the control beam; thus,the strengthened beams had higher tensile stress. �eemergence of §exural cracks was observed in the controlbeam at the midlength of one or both of the beam verticalfaces. Tension cracks generated and propagated in a spiralform. �e cracks gradually widened as the load increased,

0

120 118

92

–17 –24 –27–40–20

020406080

100120


% in

crea

se in

ulti

mat

e tw

ist an

gle

Beam name

Figure 12: Enhancement percentage of the ultimate twist anglecarrying capacity.

134

103

85

4437 33

0

20

40

60

80

100

120

140

100 150 200

% in

crea

se o

f Tu

Spacing (mm)

Poly. (CFRP)Poly. (NSM )

CFRPNSM

Figure 13: In§uence of CFRP and NSM steel bar spacing on thetorsional strength.

0

5

10

15

20

25

30

0 2 4 6 8 10 12

Torq

ue (k

N·m

)

Control100C100100C150100C200

NSM100NSM150NSM200

Angle of twist (degree/m)

Figure 14: Torque versus angle of twist at each concrete beam.


with the two lever arms rotating relative to one anotheraround the RC beam centroidal axis along with bending.Most of the concrete cracks in the strengthened beamswere dispersed through the concrete surfaces between theFRP strips and NSM steel bar grooves. In addition, fibrefailure in specimen 100C100 was observed from the edgeof the central portion of the beam specimen, and suddenfailure of the RC beam occurred after the emergence of thefirst fibre crack. *e FRP sheets ruptured after concretecracking in specimens 100C150 and 100C200. In NSM100,the first crack appeared at the edge of the test region inboth sides near the lever arm. *is crack propagated veryslowly and dispersed throughout the test region in a spiralformation immediately before the failure caused by themain crack near the lever arm was widened to approxi-mately 6mm. For NSM150, the first crack appeared at thecentre of the test region and in both sides of the specimen;these cracks propagated and dispersed throughout thecentral portion of the test region in a spiral formation, andthe widening of the main crack at the centre of the testregion resulted in a failure. For NSM200, the first crackappeared at a quarter length in both sides of the test regionand resulted in a failure.

4. Analytical Predictions

4.1.CFRP-StrengthenedBeams. *e full torsional strength ofCFRP-strengthened RC beams can be analysed by the design

codes using the principle of superposition from both theCFRP and steel reinforcement.

*e ultimate torsional strength for the FRP-strengthenedtested beams, Tu, can be achieved by adding the contributiondue to fibres and due to reinforced concrete beam, as follows:

Tu � Tu,RC + Tu,FRP. (4)

*e design equation to calculate the ultimate torsionalstrength of a reinforced concrete beam, Tu,RC, recommendedby [67, 68] is as follows:

Tu,RC � TT + TL, (5)

TT �2(0.85) · Ao · At · fyv

S, (5a)

TL �2Ao · Asl · fsl

Potan θ, (5b)

tan θ �

��At · fst · Po

Asl · fsl · S

. (5c)

FIB Bulletin-14, 2001 design model [69] states that anexternally bonded FRP laminate will grant contribution tothe torsional capacity only if full wrapping around thebeam’s cross section is applied, so that the tensile forcescarried by the FRP on each side of the cross section maycreate a continuous loop. *e technical document alsoprovides for the possibility of using inclined FRP strips as astrengthening solution. Based on the assumption of thevalidity of the truss mechanism, the following equations(6)–(8) were provided to predict the FRP contribution tostrength Tu,FRP:

Tu,FRP � 2εfk,eEfutfbf

Sfbh · cot θ, (6)

εfk,e � 0.8εfe, (7)

εfe � 0.17fc′

2/3

Efuρf⎛⎝ ⎞⎠

0.3

εfu. (8)

Equation (2) was used to calculate the volumetric ratiosof CFRP reinforcement, ρf [34, 35].

4.2. NSM Steel Bar-Strengthened Beams. *e full torsionalstrength of the NSM-strengthened RC beams can be ana-lysed by the design codes using the superposition principleof both the NSM steel bar and internal steel stirrups.

*e Tu of the NSM-strengthened tested beams can becalculated as follows by adding the contributions of the NSMsteel bar and the reinforced concrete beam:

Tu � Tu,RC + Tu,NSM. (9)

Tu,RC is calculated as follows following the recommen-dations of [67, 68]:

(a)

(b)

(c)

(d)

(e)

(f)

(g)

Figure 15: Mode of failure for specimens tested under combinedtorsion and bending. (a) Control specimen, (b) full transversewrapping (100C100), (c) 100C150, (d) 100C200, (e) NSM100,(f ) NSM150, (g) NSM200.


Tu,RC � TT + TL, (10)

TT �2(0.85) · Ao · At · fyv

S, (10a)

TL �2Ao · Asl · fsl

Potan θ, (10b)

tan θ �

��At · fst · Po

Asl · fsl · S

. (10c)

Equation (10) can also be used to calculate Tu,NSM asfollows:

Tu,NSM �2(0.85) · A°,NSM · At,NSM · fyv,NSM

SNSM. (11)

Table 7 and Figure 16 present the comparison of theexperimental and the predicted analytical results of the totaltorsional capacity using ACI 318-14M [67] and FIB Bulletin-14, 2001 [69] design model equations. It was observed thatthe total torsional capacity was over-estimated by 19% forthe control beam. *is was due to taking into considerationthe effect of longitudinal steel reinforcement (dowel action)

contribution to the ultimate torsional moment of reinforcedconcrete beams.

For strengthened RC beams 100C100, 100C150, and100C200, the predicted values are in good agreement withthe experimental one. However, for the strengthened beamNSM100, NSM150, and NSM200 the predicted values areobviously higher than the experimental values.

5. Conclusions

Apart from the shear and flexural strengths of RC beams,this study also focused on the torsional behaviour of RCbeams strengthened with various CFRP wrapping config-urations and NSM steel bar under the combined effect oftorsion and bending. *e following conclusions were ob-tained from the experimental work:

(i) Despite the CFRP wrapping configurations andNSM steel bar spacing, higher torsional resistancethan that of the control beam was observed for allstrengthened beams.

(ii) *e test beams with CFRP wrapping showed betterenhancement in the ultimate torsional moment asopposed to the beams that were strengthened withNSM steel bar. *e percentage of enhancement inthe ultimate torsional moment (Tu), with respect tocontrol specimen for beams 100C100, 100C150,and 100C200, were 134%, 103%, and 85%, re-spectively. While this percentage for beamsNSM100, NSM150, and NSM200 were 44%, 37%,and 33%, respectively.

(iii) *e ductility of all the CFRP-strengthened beamsincreased; while it decreased for NSM steel bar-strengthened beams. *e percentage of en-hancement in ultimate twist angle (θu), with re-spect to the control specimen for beams 100C100,100C150, and 100C200, were 120%, 118%, and92%, respectively. While the diminution per-centage of their ultimate twist angle (θu), withrespect to the control specimen for beamsNSM100, NSM150, and NSM200, were 17%, 24%,and 27%, respectively.

(iv) *e enhancement percentage of the ultimate tor-sional moment (Tu) proportionally increased withincreasing of CFRP and NSM steel bar ratio (de-creasing the CFRP strip and NSM steel barspacing).

(v) Cracks in the strengthened beams spread moreextensively along their length compared with thesingular cracks that formed in the control beam.

(vi) Failure in the concrete beams was delayed whenCFRP strip and NSM steel bar were used tostrengthen the beams. However, this delay un-avoidably occurred in the unwrapped spaces be-tween strips (for CFRP-strengthened beams) andspaces between NSM steel bar grooves (for NSMsteel bar-strengthened beams).

10.7

5

25.1

5

21.8

5

19.8

8

15.5

0

14.7

5

14.3

5

13.3

2

26.2

3

24.8

3

20.1

3

25.9

5

21.1

3

18.0

8

0

5

10

15

20

25

30

Control 100C100(full wrap)

100C150 100C200 NSM200 NSM150 NSM200

Ulti

mat

e tor

siona

l mom

ent (

kN·m

)

Beam code

Tu,Exp.Tu,Predict

Figure 16: Experimental versus analytical ultimate torsionalmoments at each concrete beam.

Table 7: Comparison of the experimental and analytical ultimatetorsional moments.

Beam codeUltimate torsionalmoment Tu (kN·m) Tu,Exp./Tu,An

Experimental AnalyticalControl 10.75 13.32 0.81100C100 (full wrap) 25.15 26.23 0.96100C150 21.85 24.83 0.88100C200 19.88 20.13 0.99NSM100 15.50 25.95 0.60NSM150 14.75 21.13 0.70NSM200 14.35 18.08 0.79Tu,Exp.: experimental ultimate torsional moment; Tu,An.: analytical ultimatetorsional moment.


(vii) *e predicted analytical values for control beamand CFRP-strengthened beams are in goodagreement with the experimental one. However, forthe NSM steel bar-strengthened beams the pre-dicted values are obviously higher than the ex-perimental one.

Nomenclature

FRP: Fibre-reinforced polymerCFRP: Carbon fibre-reinforced polymerGFRP: Glass fibre-reinforced polymerRC: Reinforced concreteNSM: Near-surface mountedAc: Gross area of the concrete cross section, mm2Anst: Area of the NSM steel stirrup reinforcement, mm2

Ao: Cross-sectional area bounded by the centre line ofthe shear flow according to ACI 318-14, mm2

Ao,NSM: Cross-sectional area bounded by the centre line ofthe shear flow (NSM-welded steel bar stirrup),mm2

Asl: Total area of steel longitudinal bars, mm2At: Area of the transversal steel reinforcement

(stirrups), mm2At,NSM: Area of the NSM-welded steel reinforcement

(stirrups), mm2bf: Width of the CFRP strips, mmBxh: Cross section dimensions of beamEfu: Modulus of elasticity of FRP at ultimatef′c: Concrete compressive strength, MPafst, fsl: Stresses in the longitudinal and transverse steel

reinforcements, MPafyv: Yield stress of transversal steel reinforcement, MPafyv,NSM: Yield stress of the NSM-welded steel

reinforcement, MPanf: Number of plies of CFRP sheetsPf: Perimeter of the strengthened beam cross section

using CFRP, mmPnst: Perimeter of the NSM steel stirrup, mmPo: Perimeter of the centerline of the shear flow in

space truss analysis, mmpt: Perimeter of the steel stirrup, mmS: Spacing of steel stirrups, mmSf: Centre-to-centre spacing of FRP strips, mmSn: Centre-to-centre spacing of the NSM steel stirrups,

mmSNSM: Horizontal spacing of the NSM-welded stirrups,

mmTcr: Cracking torque, kN·mTL: Torsional contribution of longitudinal steel

reinforcement, kN·mTT: Torsional contribution of transverse steel

reinforcement, kN·mtf: *ickness of fibre laminate, mmtfd: Fabric design thickness, mmTu: Ultimate torsional capacity of the strengthened

beam, kN·mTu,An: Analytical ultimate torsional moment, kN·mTu,Exp: Experimental ultimate torsional moment, kN·m

Tu,FRP: Ultimate torsional capacity from FRPreinforcement, kN·m

Tu,NSM: Ultimate torsional capacity from NSMreinforcement, kN·m

Tu,RC: Ultimate torsional capacity from steelreinforcement, KN·m

εfe: Effective FRP strain, mm/mmεfk,e: Characteristic value of effective FRP strain, mm/mmεfu: Ultimate FRP strain, mm/mmΘ: Angle of diagonal crack with respect to the member

axis, degθu: Ultimate twist angle, degρ: Total steel reinforcement ratio for each of the

longitudinal and transverse reinforcement,mm2/mm2ρnst: Area of the NSM steel stirrup reinforcement ratio,

mm2/mm2ρsl: Ratio of the longitudinal steel bar, mm2/mm2ρst: Ratio of transverse steel stirrups, mm2/mm2.

Data Availability

*e data used to support the findings of this study are in-cluded within the article.

Disclosure

*e sponsors had no role in the design of the study, in thecollection, analyses, or interpretation of data, in the writingof the manuscript, and in the decision to publish the results.

Conflicts of Interest

*e authors declare no conflicts of interest.

Authors’ Contributions

Both authors conceived and designed the experiments.Nasih Askandar analysed the data and wrote the paper, andAbdulkareem Mahmood made necessary revisions.

Acknowledgments

*e authors acknowledge the support provided by Sala-haddin University in Erbil and Sulaimani PolytechnicUniversity in Sulaymaniyah.

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ComparativeInvestigationonTorsionalBehaviourofRCBeam ...inforcement ratios for the strengthened beams in this group are shown in Figure 4. CFRP sheet and NSM steel re-inforcement ratios