Many researchers worldwide have extensively used fibre-reinforced polymer (FRP) strengthening materials and near-surface mounted (NSM) to enhance the shear and flexural strengths of reinforced concrete (RC) beams. However, studies on torsional strengthening are limited. Although a few studies have focused on torsional strengthening, none of them simultaneously investigated torsion with shear and/or bending moment. This study aims at demonstrating the behaviour of RC beams strengthened with FRP sheets (strips) with different configurations and NSM steel bars with different spacing that was subjected to combined actions of torsion and bending moment and making a comparison between them. Seven beams with a dimension of 15 × 25 × 200 cm were casted. One of the beams was not strengthened; three of them were strengthened with carbon FRP, and the others were strengthened with NSM steel bar. The angle of twist at torque intervals, first cracking torque, ultimate torque, and ultimate twist angle of the conventional and strengthened beams during the testing process are compared. Results show a significant improvement in the torsional performance of RC beams using carbon FRP and NSM steel bar. The test beams that were strengthened with CFRP wrapping showed better enhancement in the ultimate torsional moment as opposed to the beams that were strengthened with NSM steel bar.
A considerable amount of torque can accumulate in many concrete members, including curved bridge elements, spandrel beams, horizontally curved members, and eccentrically loaded beams. The torsional capacity of these members needs to be maximized due to several factors, including structural damage, deterioration, and increased loading.
Strengthening or upgrading of structural elements must be performed after a certain period to increase the service life of these elements [
Several externally bonded reinforcement (EBR) methods of increasing the service life of RC structural elements are available; for example, fibre-reinforced polymer (FRP) strengthening sheets are used to wrap RC structural elements. This technique is inexpensive and can be applied to completely degraded structures. Compared with traditional techniques, repairing concrete structures by using FRP sheets has many advantages. FRP sheets have high tensile strength, extremely low weight, high corrosion resistance, and fast installation. Moreover, changing the geometry of the structure is unnecessary when these sheets are used. This strengthening technique has already been proven to be effective for the shear and flexural strengthening of RC beams [
Considered a suitable alternative to the EBR method, the NSM strengthening method involves cutting grooves in the concrete cover of a beam specimen and then inserting and embedding reinforcement into these grooves by using an adhesive. The NSM method presents numerous advantages over the EBR method, including its higher bonding productivity and better protection. The NSM method can also address the limitation of the EBR method in its maximum strain, which is below the ultimate strain due to premature debonding. The greater confinement granted by the adhesive and the surrounding concrete is considered the best advantage offered by the NSM method [
The literature review above indicates that FRP strengthening materials have been extensively used to improve the flexural and shear strengths of RC beams. NSM strengthening steel bars have also been used to improve the flexural and shear strengths of RC beams. However, studies on torsional strengthening by both techniques are limited, and none of them compared these methods for choosing the best one. Therefore, this study aims to investigate the characteristics of RC beams strengthened with both methods (FRP sheet and NSM steel bar) having different configurations under the combined actions of torsion and bending moment and making a comparison between them.
To avoid beam failure at torsional and flexural cracking loads, each beam was designed to have a steel reinforcement ratio (
The reinforcement percentage provided in a beam was higher than the minimum requirement [
Dimensions and reinforcement details of the test beams.
The reinforcement for the specimens included 2Φ16 and 2Φ12 mm diameter steel bars at the bottom and top, respectively. End zones that were 0.4 m long on each end of the beam were reinforced with Φ10 mm stirrups spaced at 75 mm from centre to centre to force failure in the halfway zone of the tested beam. The test region of (1.0 m) was selected in such a way that at least one complete spiral crack formed along the length of the region; therefore, Φ10 mm (
All of the RC beams used the same concrete mix with compressive strength (
Seven rectangular RC beams with 250 mm depth, 150 mm width, and 2000 mm length were casted by using ready-mixed concrete. The central part of these beams was specifically designed to display torsion failure. The length of the central part was set to 1.0 m to allow the formation of at least one spiral crack at the 45° angle in the longitudinal axis of these beams. One specimen was stored without strengthening and referred to as the control specimen. The strengthened beams were then divided into two groups. The first group consists of three beams and strengthened with CFRP, whilst the second group also consists of three beams and strengthened with near-surface mounted (NSM) steel bar.
CFRP composite wrap spacing was the main parameter investigated for this group. A full-wrap CFRP composite was employed to strengthen specimen 100C100. By contrast, a 100 mm width CFRP composite was used to strengthen two other beam specimens, and all-around wraps were placed at a spacing of 150 and 200 mm c/c using one layer of the CFRP composites. All beam specimens were tested under combined torsion and bending. Figure
Strengthening of the test beams.
Design of the concrete beams.
Beams | Strengthening technique | CFRP transverse strip | NSM steel bar spacing (mm) | ||
---|---|---|---|---|---|
No. of layers | Width (mm) | Spacing (mm) | |||
Control | Unstrengthened | ||||
100C100 (full wrap) | CFRP sheet | 1 | Full | — | |
100C150 | 1 | 100 | 150 | — | |
100C200 | 1 | 100 | 200 | — | |
NSM100 | NSM steel bar | — | — | — | 100 |
NSM150 | — | — | — | 150 | |
NSM200 | — | — | — | 200 |
NSM groove spacing was the main parameter investigated for this group. Figure
The experiment used carbon fibre fabric SikaWrap®-300C and epoxy-based saturated resin Sikadur-330. Unidirectional CFRP fabrics (SikaWrap-300C) with a thickness of 0.166 mm per ply were also utilised. The elastic modulus, ultimate tensile strength, and elongation at failure of the fibre were 230 GPa, 3900 MPa, and 15 mm/m, respectively, according to the manufacturer. The FRP sheets were bonded to concrete by using two pieces of “rubber-toughened cold-curing-construction epoxy adhesive (Sikadur-330)” with a density, elastic modulus, and tensile strength of 1310 kg/m3, 3800 MPa, and 30 MPa, respectively. Tables
SikaWrap®-300C (woven carbon fibre fabric for structural strengthening).
Characteristics | Note |
---|---|
Fibre type | Midstrength carbon fibres |
Fibre orientation | 0° (unidirectional) |
Areal weight | 300 ± 15 g/m2 |
Fabric design thickness | 0.166 mm (based on fibre content) |
Tensile strength of fibres | 3,900 N/mm2 (nominal) |
Thickness | 1.3 mm per layer (impregnated with Sikadur®-330) |
1.0 mm per layer (impregnated with Sikadur®-300) | |
Tensile E-modulus | 230,000 N/mm2 |
Elongation at break | 1.5% (nominal) |
Sikadur-330 (two-part epoxy impregnation resin).
Appearance and colours | Resin part A: white (paste); hardener part B: grey (paste) |
---|---|
Density | 1.31 kg/l (mixed) |
Mixing ratio | A : B = 4 : 1 by weight |
Open time | 30 min (at + 35°C) |
Viscosity | Pasty, not flowable |
Service temperature | −40°C to +50°C |
Tensile strength | 30 MPa (cured seven days at +23°C) |
Flexural E-modulus | 3800 MPa (cured seven days at +23°C) |
The bond between the RC beam and CFRP was given appropriate attention during the strengthening process. When fixing the CFRP to the concrete surface, a handheld grinder was utilised to level the concrete surface disclosing the aggregate. The grinder was used again to arciform the concrete corners to a minimum radius of 13 mm [
The beams were cleaned by washing with pressurized water and allowed to dry prior to CFRP sheet application. Loose particles and defilements from the specimen’s surface were removed by this procedure. The beams were also wire brushed and vacuumed prior to the CFRP sheet application. Depending on substrate roughness, the resin (Sikadur-330) was mixed and applied to the prepared concrete surface by using a brush at the amount of approximately 0.75 kg/m2 to 1.25 kg/m2. The SikaWrap®-300C sheet was cut into strips with 100 mm width by scissors for the required length for all the specimens (estimated overlap greater than 150 mm). The (SikaWrap®-300C) strip was applied to the resin with a special plastic roller until the resin was squeezed out between the roving. Then, the SikaWrap®-300C fabric was applied onto the resin coating in the appropriate direction.
Figure
CFRP sheet reinforcement ratios for the strengthened beams.
Ready-mixed concrete also was used for all three beams in this group. The compressive strength (
Three bars with similar diameters and three welded bars (Ø10 mm) were subjected to uniaxial tensile tests to determine their yield and ultimate strength (
Table
Mechanical properties of the concrete and NSM steel bars.
Material | Compressive strength (MPa) | Yielding tensile strength (MPa) |
---|---|---|
Concrete | 48 | — |
Steel bars Ø10 mm | — | 541 |
Welding steel bars Ø10 mm | — | 298 |
Sikadur®-30 LP (two-part epoxy impregnation resin).
Appearance and colours | 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 : |
Layer thickness | 30 mm max |
Open time | 90 minutes (at +25°C) |
Viscosity | Pasty, not flowable |
Service temperature | –40°C to +45°C (when cured at >+23°C) |
Tensile strength | 15 MPa to 18 MPa (when cured for seven days at +23°C) |
Shear strength | 17 MPa to 21 MPa [+40°C to +55°C (7 days)] |
The NSM steel reinforcement ratios for the strengthened beams in this group are shown in Figure
NSM steel reinforcement ratios for the strengthened beams.
CFRP sheet and NSM steel reinforcement ratios for the strengthened beams.
The installation of the strengthening steel bars began by cutting grooves into the concrete cover of the specimens after 28 days of curing. These grooves were cut in the transversal direction around the beam cross section, whilst maintaining dimensions greater than 1.5 db × 1.5 db (where db denotes the diameter of the NSM steel reinforcement). A special concrete saw (handle grinder) with a diamond cutting saw blade was used for the cutting. A hammer drill and chisel were used to remove any remaining concrete lugs and to roughen the lower surface of the grooves. These grooves were then smoothened and cleaned with a wire brush and a high-pressure air jet. The strengthening steel bars were bent into a U-shape, and two U-shaped steel bars were used for each closed groove (one at the top and the other at the bottom). In this way, these steel bars were overlapping for approximately 100 mm and welded together to form a closed stirrup at each round groove. These grooves were then filled with an epoxy adhesive groove filler (Sikadur-30 LP) around the steel bar and the surface was levelled as shown in Figure
Grooving, NSN steel bar installation, welding, and groove filling with epoxy.
Figure
Schematic of the test setup for applying combined torsion and bending.
Load application on the beam specimens to be tested under the combined action of torsion and bending utilised a fabricated loading frame from the civil engineering laboratory. Rotation around the longitudinal beam axis was arranged via a special support condition, and lever arms were attached to the specimen to provide a torsional moment, as shown in Figure Three dial gauges were used. Two of them for measuring displacements were positioned under the lever arm, and one was placed at the centre to measure central displacement. A distance of 400 mm was maintained between the centre of the support and lever arm to achieve bending and torsion. The load of the hydraulic jack was transferred to the specimen through a spreader beam resting on the end of a lever arm attached to the specimen. Thus, half of the applied load acted at the end of each lever arm. The length of the specimen between supports was 1.8 m, with 0.1 m projection outside the support. The central 1.0 m length of the specimen was subjected to combined bending and torsion, whereas 0.4 m length of the beam near each support was subjected to bending moment and shear force. The torque in the middle part of the specimen was the product of the load at the end of each lever arm (half the total of applied load) multiplied by the length of the lever arm from the centre of the specimen. The twist angle at each lever arm was achieved from a vertical displacement of a lever arm end point and length of the lever arm. The overall twist angle in the middle part of the specimen was equal to the sum of twist angles at the couple of the lever arm.
Figure
Test setup with the loading frame.
The circular rotation of the supports and the transmission of applied load from the centre of the machine to the two points that express the moment arm must be facilitated by the experimental conditions. Figures
As shown in Figure
Angle of twist measurements.
Table
Experimental results of the tested beams.
Beam code | Str. Tech. |
|
|
%Incr. |
|
%Incr. |
|
%Inc. |
---|---|---|---|---|---|---|---|---|
Control | Un-str. | 48 | 4.50 | — | 10.75 | 0 | 4.77 | 0 |
100C100 | CFRP sheet | N.A | — | 25.15 | 134 | 10.49 | 120 | |
100C150 | 7.75 | 72 | 21.85 | 103 | 10.39 | 118 | ||
100C200 | 7.00 | 56 | 19.88 | 85 | 9.17 | 92 | ||
NSM100 | NSM steel bar | 8.50 | 89 | 15.50 | 44 | 3.97 | −17 | |
NSM150 | 8.00 | 78 | 14.75 | 37 | 3.64 | −24 | ||
NSM200 | 7.00 | 56 | 14.35 | 33 | 3.47 | −27 |
Str. Tech.: strengthening technique; %Incr.
The ultimate torsional moment carrying capacity of the control and strengthened beams are shown in Figure
Ultimate torsional moment carrying capacity.
The ultimate torsional moment carrying capacity of the control and strengthened beams with respect to the strengthening material (CFRP sheet and NSM steel bar) and reinforcement ratio are shown in Figure
FRP and NSM steel reinforcement ratios and increase in ultimate torque in percentage.
The percentage of enhancement in the ultimate torsional moment (
The percentage of enhancement in the ultimate torsional moment (
The ultimate twist angle carrying capacity of the control and strengthened beams are shown in Figure
Enhancement percentage of the ultimate twist angle carrying capacity.
Figure
Influence of CFRP and NSM steel bar spacing on the torsional strength.
The enhancement percentage of the ultimate torsional moment (
Figure
Torque versus angle of twist at each concrete beam.
Figure
Mode of failure for specimens tested under combined torsion and bending. (a) Control specimen, (b) full transverse wrapping (100C100), (c) 100C150, (d) 100C200, (e) NSM100, (f) NSM150, (g) NSM200.
The full torsional strength of CFRP-strengthened RC beams can be analysed by the design codes using the principle of superposition from both the CFRP and steel reinforcement.
The ultimate torsional strength for the FRP-strengthened tested beams,
The design equation to calculate the ultimate torsional strength of a reinforced concrete beam,
FIB Bulletin-14, 2001 design model [
Equation (
The full torsional strength of the NSM-strengthened RC beams can be analysed by the design codes using the superposition principle of both the NSM steel bar and internal steel stirrups.
The
Equation (
Table
Comparison of the experimental and analytical ultimate torsional moments.
Beam code | Ultimate torsional moment |
|
|
---|---|---|---|
Experimental | Analytical | ||
Control | 10.75 | 13.32 | 0.81 |
100C100 (full wrap) | 25.15 | 26.23 | 0.96 |
100C150 | 21.85 | 24.83 | 0.88 |
100C200 | 19.88 | 20.13 | 0.99 |
NSM100 | 15.50 | 25.95 | 0.60 |
NSM150 | 14.75 | 21.13 | 0.70 |
NSM200 | 14.35 | 18.08 | 0.79 |
Experimental versus analytical ultimate torsional moments at each concrete beam.
For strengthened RC beams 100C100, 100C150, and 100C200, the predicted values are in good agreement with the experimental one. However, for the strengthened beam NSM100, NSM150, and NSM200 the predicted values are obviously higher than the experimental values.
Apart from the shear and flexural strengths of RC beams, this study also focused on the torsional behaviour of RC beams strengthened with various CFRP wrapping configurations and NSM steel bar under the combined effect of torsion and bending. The following conclusions were obtained from the experimental work: Despite the CFRP wrapping configurations and NSM steel bar spacing, higher torsional resistance than that of the control beam was observed for all strengthened beams. The test beams with CFRP wrapping showed better enhancement in the ultimate torsional moment as opposed to the beams that were strengthened with NSM steel bar. The percentage of enhancement in the ultimate torsional moment ( The ductility of all the CFRP-strengthened beams increased; while it decreased for NSM steel bar-strengthened beams. The percentage of enhancement in ultimate twist angle ( The enhancement percentage of the ultimate torsional moment ( Cracks in the strengthened beams spread more extensively along their length compared with the singular cracks that formed in the control beam. Failure in the concrete beams was delayed when CFRP strip and NSM steel bar were used to strengthen the beams. However, this delay unavoidably occurred in the unwrapped spaces between strips (for CFRP-strengthened beams) and spaces between NSM steel bar grooves (for NSM steel bar-strengthened beams). The predicted analytical values for control beam and CFRP-strengthened beams are in good agreement with the experimental one. However, for the NSM steel bar-strengthened beams the predicted values are obviously higher than the experimental one.
Fibre-reinforced polymer
Carbon fibre-reinforced polymer
Glass fibre-reinforced polymer
Reinforced concrete
Near-surface mounted
Gross area of the concrete cross section, mm2
Area of the NSM steel stirrup reinforcement, mm2
Cross-sectional area bounded by the centre line of the shear flow according to ACI 318-14, mm2
Cross-sectional area bounded by the centre line of the shear flow (NSM-welded steel bar stirrup), mm2
Total area of steel longitudinal bars, mm2
Area of the transversal steel reinforcement (stirrups), mm2
Area of the NSM-welded steel reinforcement (stirrups), mm2
Width of the CFRP strips, mm
Cross section dimensions of beam
Modulus of elasticity of FRP at ultimate
Concrete compressive strength, MPa
Stresses in the longitudinal and transverse steel reinforcements, MPa
Yield stress of transversal steel reinforcement, MPa
Yield stress of the NSM-welded steel reinforcement, MPa
Number of plies of CFRP sheets
Perimeter of the strengthened beam cross section using CFRP, mm
Perimeter of the NSM steel stirrup, mm
Perimeter of the centerline of the shear flow in space truss analysis, mm
Perimeter of the steel stirrup, mm
Spacing of steel stirrups, mm
Centre-to-centre spacing of FRP strips, mm
Centre-to-centre spacing of the NSM steel stirrups, mm
Horizontal spacing of the NSM-welded stirrups, mm
Cracking torque, kN·m
Torsional contribution of longitudinal steel reinforcement, kN·m
Torsional contribution of transverse steel reinforcement, kN·m
Thickness of fibre laminate, mm
Fabric design thickness, mm
Ultimate torsional capacity of the strengthened beam, kN·m
Analytical ultimate torsional moment, kN·m
Experimental ultimate torsional moment, kN·m
Ultimate torsional capacity from FRP reinforcement, kN·m
Ultimate torsional capacity from NSM reinforcement, kN·m
Ultimate torsional capacity from steel reinforcement, KN·m
Effective FRP strain, mm/mm
Characteristic value of effective FRP strain, mm/mm
Ultimate FRP strain, mm/mm
Angle of diagonal crack with respect to the member axis, deg
Ultimate twist angle, deg
Total steel reinforcement ratio for each of the longitudinal and transverse reinforcement, mm2/mm2
Area of the NSM steel stirrup reinforcement ratio, mm2/mm2
Ratio of the longitudinal steel bar, mm2/mm2
Ratio of transverse steel stirrups, mm2/mm2.
The data used to support the findings of this study are included within the article.
The sponsors had no role in the design of the study, in the collection, analyses, or interpretation of data, in the writing of the manuscript, and in the decision to publish the results.
The authors declare no conflicts of interest.
Both authors conceived and designed the experiments. Nasih Askandar analysed the data and wrote the paper, and Abdulkareem Mahmood made necessary revisions.
The authors acknowledge the support provided by Salahaddin University in Erbil and Sulaimani Polytechnic University in Sulaymaniyah.