Fourth Asia-Pacific Conference on FRP in Structures (APFIS 2013) 11-13 December 2013, Melbourne, Australia

© 2013 International Institute for FRP in Construction

TORSIONAL STRENGTHENING OF CONCRETE MEMBERS USING CFRP COMPOSITES:

A STATE-OF-THE-ART REVIEW

Ghaidak Al-Bayati1,2 * and Riadh Al-Mahaidi1 1 Swinburne University of Technology, Melbourne, Australia

2 University of Baghdad, Baghdad, Iraq *corresponding author: galbayati@swin.edu.au

ABSTRACT The use of carbon fibre reinforced polymer (CFRP) composites in reinforced concrete members has emerged as one of the most promising technologies to address the rehabilitation requirements of buildings and bridges. Both externally-bonded reinforcement (EBR) and near-surface mounted (NSM) techniques have been investigated by many researchers, both experimentally and theoretically. The aim of this paper is to present a review of the literature concerning the torsional behaviour of reinforced concrete beams strengthened with CFRP. KEYWORDS FRP, RC beams, strengthening, EBR, NSM, torsion, shear. INTRODUCTION In order to avoid the risk of collapse of damaged buildings, it is often impossible to demolish all such buildings and replace them with new constructions. Recently, the most critical issue in the area of structural engineering around the world has been the need for the repair and rehabilitation of existing infrastructure. The most significant reasons for the deficiency of structures are increased service loading, design errors, diminished strength through senility and environmental degradation, more rigid updates in design codes, and the need for seismic retrofitting. The use of fibre-reinforced polymers (FRPs) with their notable mechanical properties has shown promise in this area and has been in the forefront of strengthening technology (Gosbell 2002). The use of carbon fibre-reinforced polymer (CFRP) materials is growing, especially to increase the resistance of reinforced concrete (RC) beams. The high strength-to-weight ratio, high durability, electromagnetic neutrality, ease of handling, rapid execution with less work, and virtually unlimited size, geometry and dimensions are advantages of the use of CFRP (ACI 2002; Bakis et al. 2003). CFRP materials can be used as externally-bonded reinforcement (EBR), where CFRP sheets or laminates are applied as bonding by wet lay-up to the external faces of the elements (Salom et al. 2004; Ameli et al. 2007; Chalioris 2008). CFRP can also be used in the near-surface mounted (NSM) technique, where CFRP bars (circular, square or rectangular cross-section) are installed into pre-cut slots on the concrete cover of the structural elements to be strengthened (De Lorenzis and Nanni 2001; Nanni et al. 2004). It has been shown that the flexural capacity and shear capacity of existing reinforced concrete members can be increased by using CFRP reinforcement (Pham and Al-Mahaidi 2004; Borros and Dias 2003). Many structural elements are subjected to important torsional moments and shear forces, such as girders supporting eccentric columns and spandrel beams. Strengthening for increased torsional and shear capacity may be required in beams as well as in bridge box girders (fib-bulletin 14 2001). CFRP has been used successfully for this purpose (Hii and Al-Mahaidi 2006; Mohammadizadeh et al. 2009). In order to study the performance of strengthened members with respect to pure torsion or combined shear and tension, our current research investigates and evaluates the performance and capacity of strengthened concrete beams. This paper presents a review of the available information concerning the torsional and shear-torsional behaviour of RC members strengthened using EBR and NSM techniques. FIBRE-REINFORCED POLYMER (FRP) Fiber-reinforced polymer (FRP) materials have been used extensively as an alternative reinforcement for new construction, as well as for strengthening and repairing existing structures, and their use has increased in recent

years (Bakis et al. 2002). For the torsional-shear strengthening of concrete beams, FRP materials can be applied by the following two major techniques: externally-bonded FRP sheets and strips applied to the external faces of the elements, and near-surface mounted (NSM), which involves the installation of CFRP into slits on the concrete cover of the beams’ lateral faces. Externally-bonded reinforcing (EBR) Many studies have been carried out to examine the use of externally-bonded FRP and its effectiveness in strengthening concrete structures. Table 1 summarises, to the best of the authors’ knowledge, the research that has been reported using the EBR technique.

Table 1. Research that has used EBR FRP Authors and year Number of

specimens Shape,

Specimen length (mm)

Strengthening types Anchorage system

Number of layers

Others

Zhang et al. 2001 Ten One Rec 1150 Control No 1, 2, 3 Pre-crack Six 90 strips*^ Two Longitudinal strips One 90 strips* &

Longitudinal strips Ghobarah et al.

2002 Eleven Three Rec 2440 Control No 1 CFRP

Three 90 strips^ One Entire beam One 45 strips one side One 45 strips spiral around

the beam One Entire beam GFRP One 90 strips

Salom et al. 2004 Six Two Spandrel 2438

Control Steel anchor 1,2 Cyclic loading Two 0/90 U entire beam* One ±45 U entire beam* One U entire beam

Hii and Al-Mahaidi 2006

Six Two Rec, box 2500

Control No 1,2 Photogrammetry, cracking twist and

strength Four 90 strips*

Ameli et al. 2007 Twelve Two Rec 1900 Control No 1,2 CFRP & GFRP, numerical modeling

by ANASYS computer program

Four Entire beam* Two 90 strips Two U strips Two U entire beam

Jing et al. 2007 Four One Box 3400 Control No 1,2 Cyclic loading, combined loads One U anchorage strips

Two U anchorage strips* & longitudinal strips

Chalioris 2008 Fourteen Six Rec, rec, T 1600

Control No

1,2 3 control beams with different ratios

of steel stirrups Three Entire beam* Two U entire beam*

Three 90 strips^ Mohammadizadeh

et al. 2009 Sixteen Six Rec, rec, rec

2000 Control Steel anchor 1,2 Three groups with

the same stirrups and different

longitudinal reins.

Two 90 strips Two U strips Six Entire beam*

Deifalla and Ghobarah 2010a

Six Two T 3400 Control Steel anchor 1,2 Combined loads One U entire beam One Extended U entire beam Two Entire beam*

*= multi layers were used with this type, ^= different spacing strips were used with this type The FRP materials are typically applied to the concrete surface using a bonding agent after adequate surface preparation of the concrete, typically involving sandblasting, water jetting, and the application of a suitable primer. Once applied, depending on the bond, several days of curing are typically required to achieve the full

bond strength of the system (Hag-Elsafi et al. 2002). Many FRP configurations have been used to evaluate the contributions of external transverse or longitudinal FRP reinforcements applied to beams with rectangular, box, spandrel and flanged cross-sections. Most studies in this review have focused on pure torsion strengthening, and very few have considered torsion combined with shear. To increase the resistance of beams to these loads, the configurations shown in Figure 1 have been adopted.

d cb a

e g h f

k l i j

Configuration sample

Description Studies

a Entire beam for rectangular beam Ghobarah et al. 2002, Ameli et al. 2007, Chalioris 2008, Mohammadizadeh et al. 2009

b U entire beam for rectangular beam

Ameli et al.2007

c 90 strips for rectangular beam Zhang et al.2001, Ghobarah et al.2002, Hii and Al-Mahaidi 2006, Ameli et al. 2007, Chalioris 2008, Mohammadizadeh et al. 2009

d U strips for rectangular beam Ameli et al. 2007, Mohammadizadeh et al. 2009 e Longitudinal strips for rectangular

beam Zhang et al. 2001

f 90 strips & longitudinal strips for rectangular beam

Zhang et al. 2001

g 45 strips one side for rectangular beam

Ghobarah et al.2002

h 45 strips spiral around beam for rectangular beam

Ghobarah et al.2002

i 90 strips for box beam Hii and Al-Mahaidi 2006 j U anchorage strips & longitudinal

strips for box beam Jing et al. 2007

k U anchorage strips for box beam Jing et al. 2007 l Extended U entire beam for

spandrel beam (with multiple FRP orientations)

Salom et al. 2004

m U entire beam for T beam Deifalla and Ghobarah 2010a, Chalioris 2008 n Extended U entire beam for T

beam Deifalla and Ghobarah 2010a

o Entire beam for T beam Deifalla and Ghobarah 2010a

om n

Figure 1. FRP configurations for the current review

The configurations can be summarised as follows:

(1) Full wrapping (all faces wrapped) with vertical strips or along the entire length as in Figure 1 (a), (c), (f), (i), (l) and (o): Continuous sheets or vertical strips (90° strips) with various dimensions and spacings have been completely wrapped around the cross-section of beams excluding spandrel beams.

(2) U-jacketed (three faces wrapped normally) with vertical strips or along the entire length as in Figure 1 (b), (d),( j), (k), (m), and( n): Continuous sheets or vertical strips (U strips) with various dimensions and spacings have been used by bonding them on the bottom and both vertical sides of the rectangular, box, and spandrel beams, and the web of T beams, with or without extended FRP anchorages.

(3) Inclined FRP strips as in Figure 1 (g) and (h): 45° FRP strips have been used by bonding them on one side of the beam or in a spiral around the beam with rectangular cross-section beams.

Full-wrap FRP strengthening technique The full-wrap FRP strengthening technique includes continuous wrapping and strips. To the best of the authors’ knowledge, Zhang et al. (2001) were the first to investigate externally-bonded FRP, and they used CFRP sheets bonded with either or both transverse and longitudinal strips. Ten reinforced concrete beams were tested, one control specimen, seven strengthened with CFRP hoop strips, two with longitudinal strips, and one with both. The number of layers and the loading history prior to strengthening were the main factors in their study. It was found that the carbon fibres at larger loads within the single ply strips improved the tensile stress level because of the efficient utilisation of material. Zhang et al. (2001) concluded that pre-cracking made little difference to the torsional capacities, and the widths and distribution of cracks were relatively irregular. Further, CFRP longitudinal strips on the short or long side surfaces were not as effective as transverse strips in increasing ultimate strength, as they were debonded by cracking and twisting deformation. Many researchers have used continuous wrapping and strips of FRP to strengthen beams with rectangular cross-sections, and the results have been compared with the results of different configuration techniques to quantify the advantages of the techniques (Ghobarah et al. 2002; Ameli et al. 2007; Chalioris 2008; Mohammadizadeh et al. 2009). In general, the studies have shown that continuous wrapping is much more efficient than strips. For example, Ghobarah et al. (2002) tested eleven rectangular beams 150mm wide by 350mm deep. Complete wrapping and vertical strips were used with GRFP- and CFRP-strengthened materials. The results of these patterns were compared with other strengthening patterns. A direct comparison was made between CFRP and GFRP, and it showed very little difference in terms of elastic responses and post-cracking behaviour. Complete wrapping was the most effective of the strengthening layouts in increasing stiffness and torsional capacity. They found that the torsional capacities corresponded to the ratio of the beam covered with CFRP, whereas the greater confining pressure was provided by the larger ratio of covered area. Ameli et al. (2007) presented a variety of configurations of FRP applied to twelve rectangular reinforced concrete beams 150mm wide by 350mm deep. Two beams were references and ten were wrapped and tested with different FRP configurations. Complete wrapping with one and two FRP layers and vertical strips were employed, using GRFP and CFRP materials to compare the results with other types of configurations used in this study such as U-jacket strips and a U-jacket along the entire beam length. Obtaining the ductility factor was achieved by finding the ratio of the twist angle of ultimate torque to twist angle at a yield to demonstrate the effect of FRP strengthening on ductility. Numerical modelling of FRP-strengthened RC beams was accomplished using the ANSYS computer program. It was reported that CFRP materials increased the strength more than GFRP, and the percentage increases in the ultimate torque for the full-wrap strengthening technique were from 45 to 143% for CFRP and 34 to 110% for GFRP, depending on the configuration. The results also showed a boost in the twist angle and ductility for various FRP configurations. Although CFRP-strengthened beams failed almost directly after reaching the peak, the GFRP post-peak behaviour took some time to occur. Hence, GFRP may be a better choice for earthquake strengthening methods. According to the experimental observations, EBR FRP limited crack width propagation and widened the spread along the length, which increased the concrete’s contribution to torsion. The cracks were distributed more uniformly in fully-wrapped beams than other configurations. Chalioris (2008) tested two groups of rectangular beams with different cross-sections and without stirrups using carbon FRP sheets as complete wrapping and strips as external transverse reinforcement. The dimensions of the cross-section of the six specimens were 100 mm wide by 200 mm wide. The width and depth of the cross-section of five beams were 150 mm and 300 mm. The contribution of the FRP fabrics and the effective strain were obtained for all beams. This study showed that full wrapping with continuous sheets is more effective for torsional upgrading than the use of the same volume of separate strips, and torsional failure of fully-wrapped beams happened at high levels of strength along with tensile rupture of the sheets. Mohammadizadeh et al. (2009) evaluated the effect of different steel reinforcement ratios on the response of strengthening beams with the same volumetric ratios of complete wrapping and strips of CFRP reinforcement. Three groups of beams with several amounts of torsional steel reinforcement were tested under pure torsion. The dimensions of the

cross-sections of all beams were 150 mm by 350 mm for width and depth respectively. The total steel ratios, including longitudinal and transverse reinforcements, were 1.56%, 2.13%, and 3.03% in the first, second, and third groups, respectively. It was concluded that the beams strengthened with complete wrapping with either one ply or two plies were not affected by the use of various steel stirrup ratios. However, the various steel reinforcement ratios were more obvious for the beams strengthened with strips. Hii and Al-Mahaidi (2006) carried out an experimental study to investigate the torsional behaviour of solid and box cross-section RC beams with vertical strips of FRP using the EBR technique. Six beams 350 mm wide and 500 mm deep were used for this test. Photogrammetry was used to obtain accurate measurements of the torsional deformation (Armer 2001). The results showed that the rupture of the CFRP strips happened before the cracks significantly widened. The increases in both cracking and ultimate strength were up to 40% and 78% respectively, compared with the base specimens. Although the cracking strengths were larger for the solid beams, the ultimate strengths were similar to those of the box-section specimens. In addition, the strip spacing and the number of CFRP layers were closely related to the increased capacity of the beams. The EBR FRP efficiently limited crack width development due to an important aggregate interlocking action. T-beams strengthened with EBR FRP complete wrapping and subjected to combined shear and torsion were evaluated by Deifalla and Ghobarah (2010a). Four beams were strengthened. Two beams were strengthened with full wrap, whereas the flange of the beams was covered by one FRP layer, and the web of the beams was covered with one U-jacket layer and two U-jacket layers respectively. The results were compared with the results of the control beams and other strengthened beams, as will be described in detail in the following section. It was observed that the shear and torsion carrying capacities were increased up to 63% and 71% more than the control beam for the fully wrapped beams strengthened with one and two U-jacket layers on the web, respectively. According to these previous studies, the ultimate strength of beams strengthened with full-wrap EBR FRP sheets along the entire beam is significantly higher than that of beams strengthened by other strengthening techniques including full-wrap strips. The premature debonding and peeling of FRP may cause early failure in the beams before the attainment of the maximum FRP capacity. The usage of anchors obviously increases the beam strength capacity (Deifalla and Ghobarah 2010a). The EBR FRP limits crack width propagation and broadens the spacing between cracks, which increases the resistance of concrete (Hii and Al-Mahaidi 2006; Ameli et al. 2007). However, the use of full wrapping sheets or strips to strengthen existing structures faces practical limitations. U-jacketed FRP strengthening technique The U-jacket FRP strengthening technique is quite practical for most existed beams, disregarding the increase in strength and ductility. On the other hand, the FRP full-wrap strengthening technique is not likely to be successfully used in most strengthening conditions due to the lack of access to all sides of the beam. Salom et al (2004) studied the effect of strengthening on the torsional behaviour of spandrel beams using composite laminates. Six RC beams were used with an L-shaped cross-section, as shown in Figure 2. Figure 2. Spandrel beam detail (Salom et al. 2004) Two beams were control beams, and the rest were retrofitted with a U-wrap along the entire beam using CFRP composite laminate around the cross-section, except for the lateral face of the 102 mm flange. Different orientations were applied to the beams, including 0°/90°, ±45°, and 90°. A steel anchorage was used with the plate and angle connected to each other by steel bolts. A half-cyclic loading was applied at predetermined increments. The main conclusion was that the anchored (±45) laminate was the most effective retrofit of the

spandrel beams. The torsional strength of the retrofitted beams was more than the control beams by 77%. Although it is not always practical to use oriented laminates, the orientation allowed the composite material to be stressed in the fibre direction. Jing et al. (2007) investigated the effect of the –U- jacket strengthening technique to the response of RC box beams under the combined actions of bending moment, shear and cyclic torque. Three box beams strengthened with EBR fibre-reinforced polymer sheets and one control box beam were tested. One beam was strengthened with vertical strips wrapped in the transverse direction with an anchorage length on the top surface. The other two beams were retrofitted with vertical (one or two layers) and horizontal strips, and two FRP longitudinal strips were pasted on the bottom surface. Skeleton curves were presented in this study as an envelope developed from the torque versus twist angle cycle curve by gathering together the peak value of each cycle in the same loading direction. It was concluded that the torsional and deformation capacities increased with the number of FRP sheets in the vertical direction. However, FRP in the longitudinal direction had a poorer retrofit effect than FRP in the vertical direction. A number of studies have used U-jacket FRP to strengthen beams with rectangular cross-sections (Ameli et al. 2007; Mohammadizadeh et al. 2009) or T cross-sections (Chalioris 2008), and the results have been compared with the results of different configuration techniques, as mentioned previously. Ameli et al. (2007) tested four rectangular cross-section beams strengthened with U-jackets along the entire beam and U-jacket strips using GRFP and CFRP materials. The results were compared with full wrapping along the entire beam and strips. They found that CFRP material was more effective than GFRP, and the percentage increases in ultimate torque for the U-jacket strengthening technique were from 16% and 33% for CFRP, and 14% and 32% for GFRP. The ultimate strengths for the full-wrap technique were much higher than for the U-jacket technique. Mohammadizadeh et al. (2009) tested two rectangular cross-section strengthened beams with U-jacket strips using CFRP with different steel reinforcement ratios (1.56%, and 2.13%). It was claimed that the use of various steel stirrup ratios had a small effect on the CFRP contribution. However, Chalioris (2008) tested two T cross-section beams strengthened with U-jackets along the entire length using one and two plies of FRP sheets. He concluded that the beams exhibited premature debonding failure at the interface between the concrete and the FRP sheet adhesive. Finally, a comparison of U-jacket and complete wrapping strengthening techniques for T-beams subjected to combined shear and torsion has been achieved by Deifalla and Ghobarah (2010a). Six beams (two controls and four strengthened beams) were tested, and a diagram of the loading and support system for the test set-up is presented in Figure 3. The beam behaves as a continuous beam over three supports (points G, H, and F) and the controlled loads L1, L2, and L3 are applied at points A, D, and E, respectively.

Figure 3. Graphic of the forces and moments on the tested beams (Deifalla and Ghobarah 2010a)

Four T-beams were strengthened with different strengthening techniques, and two beams were controls tested with 0.5 and 0.1 torque-to-shear ratios respectively. The first strengthing technique was the anchored U-jacket, and the second was the extended U-jacket. The third strengthening technique was the fully-wrapped section. Finally, the fourth strengthening technique was the fully-wrapped section with two FRP layers around the web. It was found that the shear and torsion carrying capacities were improved up to 71% more than those of the control beam. The ratio between the torque and the angle of twist, which is known as the torsional stiffness, increased with decreased torque-to-shear ratio, and decreasing the torque-to-shear ratio delayed concrete cracking. Using a steel anchorage system at the location observed by Chalioris (2008) delayed premature end anchorage failure. According to the above studies, the ultimate strength of a beam with a strengthened U-jacket wrap is lower than that of a beam strengthened by full wrapping, and the usage of anchors heightens beam strength (Salom et al. 2004). Inclined FRP strips strengthening technique To the best of the authors' knowledge, the study by Ghobarah et al. (2002) is the only torsional test to study external FRP strengthening at a 45° angle to the longitudinal axis, where the inclined strips are almost orthogonal to the torsional cracks. As mentioned previously, in the first EBR technique these researchers tested eleven beams with several application patterns of CFRP and GFRP material. Complete wrap, vertical strips, 45° full spiral strip, and 45° strips on one side only, were the CFRP application patterns tested in this study. They concluded that spiral 45° wrapping was much more efficient than vertical strips for torsional strengthening. However, strips placed on one side only at a 45° orientation provided little improvement in torsional capacity, and the cut-out in the fibre wrap was the reason. As mentioned previously, complete wrapping was the most effective strengthening layout used in this program. NEAR-SURFACE MOUNTED (NSM) Early de-bonding has become the major issue in the EBR FRP strengthening technique. To reduce these problems, the near-surface mounted (NSM) technique (Nanni et al. 2004) provides greater resistance to de-bonding. It is used to strengthen beams by installing CFRP bars into grooves cut in the concrete cover of the beam lateral faces using epoxy adhesive as a bonding material (De Lorenzis and Nanni 2001). Many studies have indicated that the NSM technique is feasible and beam strength capacity could be increased significantly compared with EBR application. A square cross-section groove opening is required for the bars, which is a relatively time-consuming procedure, and the bond thickness around the bar is non-uniform. As a result, CFRP strips with small cross-sections are used by installing them into narrow slits, which are easily made using conventional cutting equipment (Barros and Dias 2003). The ratio between the bond perimeter and the area of the cross-section in these strips is higher than the values that could have been obtained if bars had been used, which has effects in terms of strengthening effectiveness because of the nearly constant thickness of strips. (El-Hacha and Rizkalla 2004). NSM is currently a promising strengthening technique for RC structures because of the installation requirements, where only cutting the groove without removing the finishing is needed. In addition, it has minimal installation time compared with the EBR technique. The other advantages are the ability to reduce the possibility of harm resulting from mechanical damage, fire, and vandalism, and the ability to develop higher strain in FRP (De Lorenzis and Teng, 2007). Further, the higher confinement provided by the surrounding concrete and adhesive is the most effective benefit of the NSM technique (El-Hacha and Rizkalla 2004). To date, many studies have been conducted on the flexural strengthening of concrete beams using this technique, but shear as an important issue in reinforcement capacity has attracted little attention No studies focusing on the torsional strengthening of concrete beams have been carried out, as mentioned in the previous section. This section will focus on recent studies that have been reported on RC beams strengthened in shear using the NSM technique (Table 2). The experimental studies reported to date are listed according to the shape of the beam's cross-section.

Table 2. Experimental studies of NSM FRP strengthened beams Authors and

year Number of specimens Shape,

Specimen length (mm)

Strengthening types Others

Barros and Dias (2006)

Twenty Four Rec, Rec, 1600

Control: no stirrups Con. compressive strength for two series

(A,B)-age of beam test = 49.2 & 56.2 MPa

respectively

Four Steel stirrups, 6Ø6 & 10Ø6 Four EBR U strips, 8 & 14 strips 2 layers Four NSM 90 strips, 2 sides, 16 & 28 strips Four NSM 45 strips, 2 sides, 12 & 24 strips

Dias and Barros (2008)

Twelve Three T 2450 Control: no stirrups, 0.1%,0.24% Con. compressive strength-age of beam

test = 31.1 MPa Three NSM 90 strips, 2 sides, 0.06%,0.1%,0.16% Three NSM 45 strips, 2 sides, 0.06%,0.1%,0.16% Three NSM 60 strips, 2 sides, 0.06%,0.09%,0.13%

Dias and Barros (2009)

Fourteen Three T 2450 Control: no stirrups, 0.1%,0.17% Con. compressive strength-age of beam

test = 39.7 MPa Four NSM 90 strips, 2 sides, 0.08%,0.13% applied for

0.1%,0.17% steel stirrups Four NSM 45 strips, 2 sides, 0.08%,0.13% applied for

0.1%,0.17% steel stirrups Four NSM 60 strips, 2 sides, 0.07%,0.11% applied for

0.1%,0.17% steel stirrups El-Hacha and Wagner (2009)

Seven Two Rec 2000 Control: no stirrups, 120 mm s* Con. compressive strength-age of beam test =24.8-38.2 MPa

Five NSM 90 strips, 2 sides, 60, 120, 240, 360, 180-360 mm s*

Dias and Barros (2010)

Fifteen Three T 2450 Control: no stirrups, 0.1%,0.28% Con. compressive strength-age of beam

test = 39.7 MPa Three NSM 90 strips, 2 sides, 0.08%,0.13%,0.18% Three NSM 45 strips, 2 sides, 0.08%,0.13%,0.19% Three NSM 60 strips, 2 sides, 0.07%,0.11%,0.16% Three EBR U strips, 0.07%,0.10%,0.21%

Lim (2010) Nine One Rec 3000 Control: no stirrups ^ means that there were different steel stirrups

spacing for the different beams, Con.

compressive strength at 28 days = 40 MPa

Three EBR 2 sides strips, 125, 150 ,250 mm s* Two EBR 2 sides strips, 250 mm s*, steel stirrups^ One NSM 90 strips, 2 sides, 150 mm s* One EBR & NSM strips, 2 sides, 150 mm s* One EBR & NSM strips, 2 sides, 250 mm s*, steel stirrups

Dias and Barros (2011)

Thirteen Three T 2650 Control: no stirrups, 0.1%,0.17% Con. compressive strength-age of beam

test = 18.7 MPa Two NSM 90 strips, 2 sides, 0.13% applied for 0.1%,0.17% steel

stirrups Four NSM 45 strips, 2 sides, 0.08%,0.13% applied for

0.1%,0.17% steel stirrups Four NSM 60 strips, 2 sides, 0.07%,0.11% applied for

0.1%,0.17% steel stirrups Dias and

Barros (2012a) Twenty-

two Four T 2450 Control: no stirrups, 0.1%,0.17%, 0.28% Con. compressive

strength-age of beam test = 39.7 MPa

Five NSM 90 strips, 2 sides, 0.08%, 0.13% applied for 0.1%,0.17% steel stirrups, and 0.18% applied for 0.1% only

Five NSM 45 strips, 2 sides, 0.08%, 0.13% applied for 0.1%,0.17% steel stirrups, and 0.19% applied for 0.1% only

Five NSM 60 strips, 2 sides, 0.07%, 0.11% applied for 0.1%,0.17% steel stirrups, and 0.16% applied for 0.1% only

Three EBR U strips, 0.07%,0.10%,0.21% Dias and

Barros (2012b) Seventeen Three T 2800 Control: no stirrups, 0.1%,0.16% Con. compressive

strength-age of beam test = 59.4 MPa, # two beams have the same

CFRP% and Steel stirrups% (0.08, 0.1 respectively) but the

number of CFRP crossing by shear crack

is different

Two NSM 90 strips, 2 sides, 0.08%, 0.13% applied for 0.1% steel stirrups

Four NSM 45 strips, 2 sides, 0.08%, 0.13% applied for 0.1%,0.16% steel stirrups

Three# NSM 45 strips, 2 sides, 0.08% applied for 0.1%,0.16% steel stirrups, with pre-cracking

Four NSM 60 strips, 2 sides, 0.07%, 0.11% applied for 0.1%,0.16% steel stirrups

One NSM 60 strips, 2 sides, 0.07% applied for 0.16% steel stirrups, with pre-cracking

NSM CFRP percentage depends on the CFRP cross-section, the beam web width and spacing of the CFRP, and inclination of CFRP. * s= spacing.

Rectangular section Barros and Dias (2006) studied the efficiency of the NSM technique by testing four series of beams. Each group comprised five beams. The percentage and inclination of the NSM CFRP laminates were assessed by testing two

strengthened beams in each series, one with laminates positioned at 90º and another with laminates positioned at 45º in relation to the beam axis. One beam from each group was tested to examine the effect of beam depth without shear reinforcement, and the results were obtained from testing one beam reinforced with steel stirrups in each group. One beam was strengthened with CFRP sheet strips applied according to the EBR technique to compare the efficacy of the EBR and NSM techniques. The researchers demonstrated that the NSM shear strengthening technique was the easiest to apply and the most effective of the CFRP techniques for both beam load-carrying capacity and deformation capacity. For the load-carrying capacity, the beams strengthened by EBR and NSM techniques displayed an average boost of 54% and 83%, respectively. Likewise, for the deflection at 0.95 Fmax after peak load, the increases were 77% and 307%, respectively. However, the NSM strengthening technique provided almost similar Fmax to that of the beam reinforced with steel stirrups, and an increase of 9% in the deflection. Furthermore, the failure mode of the NSM technique was not as brittle as those observed in the EBR technique, and the NSM technique was faster to apply than the EBR technique. Seven reinforced concrete beams with a rectangular cross-section were cast and tested by El-Hacha and Wagner (2009) to investigate the effectiveness of vertical NSM CFRP strips. The NSM CFRP strips were applied for shear strengthening at different spacings on RC beams weak in shear. The researchers studied the relationship between the mode failure of the beams and the spacing between the strips, and found that all beams strengthened with strips with spacings up to 240 mm failed in flexure. However, the beams with spacings of 180-360 mm and 360 mm failed by main shear cracking between the strips. It was reasoned that the effective depth of the beams should be greater than the spacing between the strips to avoid brittle failure in shear before yielding of the longitudinal steel. The effectiveness of the NSM technique to increase the shear capacity of beams was shown. Finally, Lim (2010) examined the shear strengthening effectiveness of nine beams strengthened with NSM CFRP strips and EBR CFRP strips. The length of the beams was 3000 mm, and the cross-section was 200 × 300 mm and they were tested under monotonically increasing concentrated load. One beam was not strengthened and tested as a control, and five beams were tested as beams strengthened with EBR FRP strips with different spacings and steel stirrups. Three beams were strengthened using NSM FRP strips. One was strengthened with 25 mm NSM with a spacing of 150 mm, whereas the rest were strengthened with the combined use of NSM and EBR CFRP strips. Combined T-shaped strips were inserted into the slits with epoxy. It was concluded that shear stiffness and strength were considerably developed for the beams strengthened by combined NSM and EBR compared with the control beam. The shear cracking of the beam strengthened with NSM and EBR strips was propagated diagonally to the close epoxy grooves without crossing the epoxy. It was also found that the combined strips of NSM and EBR can redistribute the total stress. Although the result shows that the NSM and EBR strips are a perfect combination to resist load, the technique is difficult and time-consuming to execute and has the same undesirable disadvantages as the EBR FRP technique, including premature de-bonding. For all three research studies presented in this section, the gains due to the NSM CFRP technique are shown in Table 3, and the contribution ratio of the CFRP is defined as

∆FF

% F FF

100

∆FF

(1) It is note-worthy that the maximum load capacity provided by the NSM CFRP strengthening technique was achieved without steel stirrups, with a maximum percentage of CFRP, and with 45º CFRP configuration. However, it was achieved with 90º CFRP configuration for the studies which included only vertical strips.

  Max.  %  Strengthened beam type 

Barros and Dias (2006)  125 no steel stirrups, CFRP laminates at 150 mm spacing, 45º CFRP orientation,  49.3 MPa concrete compressive 

strength  

El‐Hacha and Wagner (2009) 

101 no steel stirrups, CFRP laminates at 120 mm spacing, 

90º CFRP orientation,  31.3  MPa concrete compressive strength 

Lim (2010) 

NSM  133 no steel stirrups, CFRP laminates at 150 mm spacing, 90º CFRP orientation,  40  MPa concrete compressive 

strength at 28 days 

NSM+EBR  155 no steel stirrups, CFRP laminates at 250 mm spacing, 90º CFRP orientation,  40  MPa concrete compressive 

strength at 28 days 

Table 3. Maximum CFRP contribution in terms of the load-carrying capacity (maximum force).

T Section To the authors' best knowledge, to date Barros and Dias are the only authors who have studied the shear behaviour of T-section reinforced concrete beams using the NSM technique. The papers of Dias and Barros (2008; 2010; 2011; 2012a; 2012b) and Dias and Barros (2009) investigated the influence of the NSM shear strengthening technique with CFRP laminates in experimental studies of T cross-section RC beams (Figure 4). The design of the reinforcement system was intended to ensure that the shear failure mode occurred in only one shear span of the beams. Strain modifications along the FRP laminates and one steel stirrup were detected using four and three strain gauges respectively. For all studies mentioned above, slits were opened in the concrete cover 5 mm wide and 12–15 mm deep. Adhesive was then applied to the slit and on the faces of the laminates. Thereafter, the laminates were slotted into the slits and excess adhesive was removed.

)c

)b(

)aFigure 4. The geometry and steel reinforcement of the tested beams (all dimensions in mm), (a) Dias and Barros 2008; 2010; 2012a and Dias and Barros 2009, (b) Dias and

Barros 2011 , (c) Dias and Barros 2012b. Many variables have been considered in these studies, the first being the percentage and inclination of the CFRP. Three percentages of CFRP were applied for each of three laminate inclinations (45º, 60º and 90º) for the studies by Dias and Barros (2008; 2010; 2012a). The highest percentage of CFRP laminates was designed to supply a maximum load similar to that of the control RC beam, which was reinforced with a ratio of steel stirrups of 0.28% and 24% for Dias and Barros (2010; 2012a), and Dias and Barros (2008), respectively. However, Dias and Barros (2009) and Dias and Barros (2012b) used two percentages of CFRP for each of three laminate inclinations (45º, 60º and 90º) to obtain the same CFRP contribution. Also, Dias and Barros (2011) adopted the same configuration used in Barros and Dias (2009; 2012b), except that they used one per cent of CFRP for 90º orientation. They concluded that an increase of the percentage of CFRP led to an increase of the beam’s shear resistance, and inclined laminates were more effective than vertical laminates. The 45° orientation was more effective than the 60° orientation, and the 60° orientation was more effective than the vertical arrangement. On the other hand, only Dias and Barros (2008) reported that the configuration at 60° was the most effective of the arrangements adopted. They explained that as being because the shear cracks were almost perpendicular to the 60° orientation. According to Dias and Barros (2012b), the maximum load of NSM shear strengthening configurations adopted increased between 35% and 62%, compared with the maximum load of the reference

beams. Regardless of the orientation of the CFRP, an increase of the beam shear capacity was produced by an increase of the CFRP percentage for all strengthened beams. In addition, control beams reinforced with steel stirrups similar to the highest CFRP percentage were tested in three studies (Dias and Barros (2008; 2010; 2012a). The load capacity of the highest CFRP percentage was larger than the load capacity of the control beam, according to Dias and Barros (2008). In contrast, the load capacity of the control beam was the larger according to Dias and Barros (2010; 2012a). Furthermore, to compare the effectiveness of the NSM and EBR CFRP-strengthening techniques, Dias and Barros (2010; 2012a) applied unidirectional U-shaped CFRP wet lay-up sheets to a corresponding RC beam strengthened according to the EBR technique for each percentage of NSM FRP laminate applied to the beams. They concluded that the NSM technique was more effective than EBR because NSM provided a higher increase, not only in terms of load-carrying capacity, but also in terms of stiffness after shear crack formation, and it also provided higher values of maximum strains measured in the CFRP. Dias and Barros (2010) found that the NSM shear-strengthened beams showed a higher stiffness, while the EBR shear-strengthened beams presented similar initial stiffness up to the de-bonding of the wet lay-up CFRP strips. Also, the maximum strain of the NSM was the higher of the maximum strains measured in the CFRP. Dias and Barros (2012a) demonstrated that the NSM presented 90% of the maximum load of the reference beam, whereas the EBR shear-strengthened beam had a maximum load that was 79% of the maximum load of the reference beam. The percentage of existing steel stirrups was also one of the main parameters assessed in these experimental studies. Dias and Barros (2009) and Dias and Barros (2011; 2012a) examined two percentages of the steel stirrups (01% and 0.17%) by using two different spacings between the stirrups. Dias and Barros (2009) and Dias and Barros (2011) used the same number of beams in the two groups, Dias and Barros (2012a) used some beams with 0.17%. On the other hand, Dias and Barros (2012b) included 0.1% and 0.16% of existing steel stirrups in their study with different numbers of beams in each group. Furthermore, the aforementioned CFRP configurations with different inclinations and percentages were applied in beams with 0.1% and 0.17%. It was found that increasing the percentage of steel stirrups in the NSM CFRP technique plays a substantial role in the shear behaviour of RC beams, and has a negative effect on the beam’s resistance to shear in terms of the gain due to CFRP. The existence of cracks was an important parameter in the shear behaviour of the strengthened beams investigated by Dias and Barros (2012b). Four beams were loaded until the appearance of shear cracks prior to the CFRP application. The load was then removed, and NSM CFRP laminate was applied to the beams in the unloaded state in order to simulate practical conditions. Subsequently, the beams were examined up to failure. The results showed that an expected loss of initial stiffness was the only difference between NSM CFRP beam behaviour with and without pre-cracks. However, the efficiency of the NSM shear strengthening technique was not affected by the pre-cracking in respect of the load-carrying capacity and ultimate deflection. In the non-pre-cracked beams, the mobilization of the CFRP laminates began when the shear crack formed. However, in the pre-cracked beams, the mobilization of the CFRP laminates initiated after the opening of the cracks. Finally, the effect on concrete strength has been studied. This parameter is considered important for RC structures with low concrete strength. In Dias and Barros (2011), the concrete compressive strength at the age of tested beams (18.7MPa) was used to assess the shear behaviour of NSM-strengthened beams with low strength concrete. It was concluded that the NRM CFRP technique increases the shear resistance of the beams with low concrete strength. They compared their results with available experimental results under the same test conditions and high compressive strength (39.7 MPa) that were obtained by Dias and Barros (2009), Dias and Barros (2010) and Dias and Barros (2012b), where the concrete compressive strength at the age of tests (59.4 MPa) was used to assess the shear behaviour of NSM-strengthened beams with high strength concrete. The results indicate that concrete strength plays a role in the efficacy of the NSM technique. In higher strength concrete, the fracture failure of concrete surrounding a laminate was not as obvious as in beams of lower strength concrete. In fact, the NSM technique is more efficient when applied to RC beams of high-strength concrete. The contribution of the NSM CFRP to shear resistance and strain in the laminates was greater. For all studies presented in this section, the gains due to the NSM CFRP technique are shown in Table 4, and the contribution ratio of the CFRP is calculated using Equation 1. It can be noticed that the maximum load capacity provided by the NSM CFRP strengthening technique is achieved with the minimum percentage of steel stirrups, the maximum percentage of CFRP, and in most cases, with 45º CFRP configuration. The CFRP is highly functional with high compressive strength concrete, and the maximum upper contribution of CFRP is achieved.

Max. ∆ %

Table 4. Maximum CFRP contribution in terms of load-carrying capacity (maximum force).

Strengthened Beam type

Dias and Barros (2008) 28.9 0.1% steel stirrups, 0.13% CFRP, 60º CFRP orientation, 31.1 MPa concrete compressive strength

Dias and Barros (2009) 38.8 0.1% steel stirrups, 0.13% CFRP, 45º CFRP orientation, 39.7 MPa concrete compressive strength

Dias and Barros (2010) 47 0.1% steel stirrups, 0.19% CFRP, 45º CFRP orientation, 39.7 MPa concrete compressive strength

Barros and Dias (2011) 35.3 0.1% steel stirrups, 0.13% CFRP, 45º CFRP orientation, 18.7 MPa concrete compressive strength

Dias and Barros (2012a) 47 0.1% steel stirrups, 0.19% CFRP, 45º CFRP orientation, 39.7 MPa concrete compressive strength

Dias and Barros (2012b) 56.6 0.1% steel stirrups, 0.13% CFRP, 45º CFRP orientation, 59.4 MPa concrete compressive strength

CONCLUSIONS A new class of structural material, fibre-reinforced polymer (FRP), has been used to rehabilitate and strengthen RC members subjected to torsion or combined shear and torsion. Externally-bonded reinforcement (EBR) and near-surface mounted (NSM) are two different techniques of applying FRP to beams that have been used with epoxy as a bonding agent. The literature on FRP application on beams has been reviewed for torsional behaviour and shear and torsional behaviour, and the following conclusions can be drawn: (1) The use of the EBR FRP technique has been reported in different strengthening layouts. Full-wrap along the

entire beam has been shown to be the most effective layout to improve the resistance and ductility of the beam. Elevated levels of FRP strength have been achieved with an increase in inhibition of crack propagation. The spacing and number of layers of FRP strips have a considerable effect on the capacity of the beam. Although U-jackets along the entire beam and strips are practical to apply, they experience premature debonding, which limits development in the strength of FRP. In general, the use of anchors produces a longer response and delays failure. However, it may cause concrete crushing on the unstrengthened area, and it may have effects on the strength of the beam. Although the use of inclined 45° spiral strips is considered the most effective layout, it has limitations in practice. Finally, applying 45° strips to one side of the beam is not an effective layout.

(2) There are no data on the torsional behaviour of RC beams strengthened with the NSM technique using epoxy or cement-based adhesives. The use of the NSM FRP technique has been reported for beams strengthened with shear only. Comparison between the NSM and EBR CFRP-strengthening techniques has been achieved, and the effects of inclination of the FRP strips, FRP percentage, steel stirrup percentage, pre-cracking, and the concrete strength have been investigated. The NSM technique is more effective than the EBR technique in increasing beam capacity. In general, 45° strips display higher resistance to shear compared with 60° strips, which are more efficient than vertical strips. Disregarding the FRP strips’ inclination, increasing the percentage of FRP results in an increase in resistance to shear. The percentage of steel stirrups has a detrimental effect on the FRP contribution to the shear resistance, and the FRP contribution increases with decreased percentage of steel stirrups. The pre-cracking test has no effect on the load capacity and ultimate deflection of the strengthened beam. The increase of the concrete strength has a positive effect on the activity of the NSM strengthening technique, which is more effective when applied to beams with high concrete strength. Since debonding or fracture is not observed in FRP strips in the NSM technique, the failure mode is not brittle, as observed in the EBR technique.

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