The efficacies of the Near Surface Mounted and Externally Bonded Reinforcing techniques for the flexural and shear strengthening of reinforced concrete beams are compared. Both techniques are based on the use of carbon fiber reinforced polymer materials. In the present work the carried out tests are described and the main results are presented and analyzed.Fundação para a Ciência e a Tecnologia (FCT) - POCTI/ECM/59033/2004 - Program “CUTINSHEAR”.Degussa, Portugal.S&P.Unibetão (Braga)

Barros, Joaquim A. O.

Dias, Salvador J. E.

Fortes, Adriano

Universidade do Minho: RepositoriUM

 1 NEAR SURFACE MOUNTED TECHNIQUE FOR THE FLEXURAL AND SHEAR STRENGTHENING OF CONCRETE BEAMS         Joaquim Barros Associate Professor UM Guimarães, Portugal  Salvador Dias PhD Student UM Guimarães, Portugal Adriano Fortes Auxiliary Professor CEFET-BA Salvador, Brazil     ABSTRACT  The efficacies of the Near Surface Mounted and Externally Bonded Reinforcing techniques for the flexural and shear strengthening of reinforced concrete beams are compared. Both techniques are based on the use of carbon fiber reinforced polymer materials. In the present work the carried out tests are described and the main results are presented and analyzed.  1. INTRODUCTION  Near Surface Mounted (NSM) is a strengthening technique based on the use of laminate strips of carbon fiber reinforced polymer (CFRP) materials installed into precut slits opened on the concrete cover of the elements to strengthen [1]. This strengthening technique requires no surface preparation work and, after cutting the slit, requires minimal installation time compared to the Externally Bonded Reinforcing (EBR) technique. A further advantage associated with NSM is its ability to significantly reduce the probability of harm resulting from acts of vandalism, mechanical damages and aging effects. In the present work the efficacies of the NSM and EBR techniques for the flexural and shear strengthening of reinforced concrete beams are compared. For the flexural strengthening, the influence of the longitudinal equivalent reinforcement ratio, ρl,eq, on the strengthening efficiency of both techniques is assessed. For the shear strengthening, the influences of the longitudinal steel reinforcement ratio, ρsl, and the beam depth on the strengthening efficacy of both techniques are evaluated. In this last experimental program, the influence of the inclination of the CFRP laminates in the NSM technique is also investigated. In the present paper the experimental program is described and the results are presented and analyzed.  2 2. STRENGTHENING TECHNIQUES  The NSM technique was made up of the following steps: 1) using a diamond cutter, slits of 4 to 5 mm width and 12 to 15 mm depth were cut on the concrete surface of the elements to strengthen; 2) slits were cleaned by compressed air; 3) CFRP laminates were cleaned by acetone; 4) epoxy adhesive was produced according to supplier recommendations; 5) slits were filled with the epoxy adhesive; 6) epoxy adhesive was applied on the faces of the laminate; and 7) laminates were introduced into the slits and epoxy adhesive in excess was removed. To apply the wet lay-up strips of CFRP sheet by EBR technique, the following procedures were executed: 1) on the zones of the beam’s surfaces where the strips of sheet would be glued, an emery was applied to remove the superficial cement paste (in the shear strengthening experimental program the beam edges were also rounded); 2) the residues were removed by compressed air; 3) a layer of primer was applied to regularize the concrete surface and to enhance the adherence capacity of the concrete substrate; and 4) strips of sheet were glued on the faces of the beam by epoxy resin.   3. MATERIALS  3.1 Concrete and steel bars  Table 1 includes the main properties of the concrete and steel bars used in the experimental program. The average values of the concrete compression strength at 28 days (fcm) and at the date of testing the beams (fcm,j) were evaluated from uniaxial compression tests (3 for each series, at least) with cylinders of 150 mm diameter and 300 mm height. Steel bars were tested and each result is the average of at least five tests.  Table 1 - Properties of the concrete and steel bars Concrete Steel Element type fcm (MPa) fcm,j (MPa) φs (mm) fsym (MPa) fsum (MPa) 5 620 700 Flexural strengthening program 44.2 52.2 (70 days) 6.5 480 570 6 (stirrups) 540 694 6 (long.) 622 1 618 2 702 1 691 2 10 464 581 Shear strengthening program 37.6 1 49.5 2 49.2 1 (227 days)56.2 2 (105 days) 12 574 1 571 2 672 1 673 2 1 A series; 2 B series   3.2 CFRP systems  Two CFRP systems were used on the present work: unidirectional wet lay-up sheets of 80 mm width and precured laminates of 1.4×9.6 mm2 cross-section. These CFRP systems have the properties indicated in Table 2.  3 Table 2 - Properties of the CFRP materials CFRP system Main properties Type Material Tensile strength (MPa) Young's modulus (GPa) Ultimate strain (%) Thickness (mm) Primer 1 12 0.7 3.0 -  Epoxy 1 54 3 2.5 - Flexural strengthening prog. C-Sheet 2401 3700 240 1.5 0.111 S&P C-Sheet (wet lay-up sheet) Sheet Shear strengthening prog. C-Sheet 5301 3000 390 0.8 0.167  Adhesive 1 16-22 5 - - Flexural strengthening prog. 2 2740 158.8 1.7 1.4 CFK 150/2000 (precured laminate) Laminate Shear strengthening prog. 2 2286 166 1.3 1.4 1 According to the supplier; 2 Evaluated from experimental tests  4. FLEXURAL STRENGTHENING  Figure 1 represents the geometry of the beams, the reinforcement arrangement and the number and position of the CFRP. The load configuration and the support conditions are also schematized. The cross-section area of the CFRP systems was evaluated in order to be similar the Af Ef / Asl Es ratio for the tested series, where Af and Ef are the CFRP cross-section area and the Young’s Modulus of the CFRP systems, and Asl and Es (200 GPa) are the cross-section area and the Young’s Modulus of the longitudinal tensile steel bars. In this evaluation the distinct effective depth of the CFRP systems was not considered, but its influence is marginal. Shear reinforcement was selected to assure bending failure prior to shear failure for all beams. The beams were tested at the age of about 70 days [2].  2Ø6.52Ø52Ø6.5Reinforcement arrangement Beam seriesØ6P/2P/2S1 Series S2 Series S3 SeriesR11 and R12NSM11 and NSM12EBR_L11 and EBR_L12Load configuration and support conditions3Ø6.52Ø6.52Ø6.5 2Ø6.52Ø6.5 2Ø6.5 2Ø6.52Ø6.5 2Ø6.5 2Ø6.52Ø6.5 2Ø6.5 2Ø6.52Ø5 2Ø6.5 3Ø6.52Ø5 2Ø6.5 3Ø6.52Ø5 2Ø6.5 3Ø6.510008010 106080 80 80 80 80 80 8080 606060170x(120)100050 300 300 300 50Non-Strengthened1 CFRP laminate (NSM) 2 CFRP laminates (NSM) 3 CFRP laminates (NSM)1 CFRP laminate (EBR) 2 CFRP laminates (EBR) 3 CFRP laminates (EBR)1 layer CFRP sheet (EBR) 2 layers CFRP sheet (EBR) 3 layers CFRP sheet (EBR)170x(120)Non-Strengthened Non-StrengthenedNSM21 and NSM22 NSM31 and NSM32EBR_L21 and EBR_L22 EBR_L31 and EBR_L32EBR_M11 and EBR_M12R21 and R22 R31 and R32EBR_M21 and EBR_M22 EBR_M31 and EBR_M32 Figure 1: Beam series for the flexural strengthening  The force-deflection relationships for the series of tested beams are depicted in Figure 2. The strengthening efficacy is evaluated in terms of the service (Pserv) and the maximum (Pmax) load, see Table 3. Pserv(S) and Pmax(S) are the service and maximum load of a strengthened beam, respectively, while Pserv(R) and Pmax(R) are the service and maximum load of the reference  4 beam. In Table 3 the equivalent reinforcement ratio, ρl,eq = Asl / (b ds) + (Af Ef / Es) / (b df), is also indicated, where b is the beam width and ds and df are the effective depth of the longitudinal steel bars and CFRP systems, respectively. The influence of the ρl,eq on the strengthening efficacy index is graphically represented in Figure 3. In terms of the beam load carrying capacity the NSM technique was the most effective one. The effectiveness, however, decreases with the increasing of ρl,eq. In terms of service load, the EBR based on the use of wet lay-up CFRP sheets was the most efficient, since Ef and df were the largest ones. The beams strengthened by EBR laminates have failed by the premature debonding of the laminates. Two failure modes occurred in the beams strengthened by EBR sheets: in the beams strengthened by one and two layers, the CFRP sheets have ruptured, while in the beams strengthened by three layers, the beam concrete cover has detached, having been bonded to the CFRP sheet. This last failure mode was also occurred in the beams strengthened by the NSM technique, but the detached concrete layer had larger thickness [2].  0204060801000 2 4 6 8 10 12Deflection at mid span (mm)Force (kN)NSMEBR_LEBR_MRBeam’s designationeq,lρ(%)servP(kN)( ) ( )( )RPRPSPserserser −(%) maxP  (kN) ( ) ( )( )RPRPSPmaxmaxmax −  (%) R - 22.1 - 36.6 - NSM 0.28 37.5 70 79.9 118 EBR_L 0.28 31.9 44 38.6 5 S1 series EBR_M 0.28 40.3 82 43.0 17 0204060801000 2 4 6 8 10 12Deflection at mid span (mm)Force (kN)NSMEBR_LEBR_MRBeam’s designationeq,lρ(%)servP(kN)( ) ( )( )RPRPSPserserser −(%) maxP  (kN) ( ) ( )( )RPRPSPmaxmaxmax −  (%) R - 40.5 - 48.5 - NSM 0.49 56.3 39 93.3 92 EBR_L 0.48 57.6 42 83.5 72 S2 series EBR_M 0.48 59.5 47 79.5 64 0204060801000 2 4 6 8 10 12Deflection at mid span (mm)Force (kN)NSMEBR_LEBR_MR Beam’s designationeq,lρ(%)servP(kN)( ) ( )( )RPRPSPserserser −(%) maxP  (kN) ( ) ( )( )RPRPSPmaxmaxmax −  (%) R - 51.5 - 71.8 - NSM 0.73 71.5 39 96.6 35 EBR_L 0.73 74.1 44 86.5 20 S3 series EBR_M 0.73 73.4 43 87.3 22 Figure 2: Force-deflection relationships of the flexural strengthening beams  Table 3 - Main results of the flexural strengthening beam series  5 0204060801001201400.2 0.3 0.4 0.5 0.6 0.7 0.8ρl, eq  (%)[Pmax (S)-P max (R)]/Pmax (R) NSM EBR_L EBR_M Figure 3: Strengthening efficacy index vs longitudinal equivalent reinforcement ratio   5. SHEAR STRENGTHENING  The experimental program was composed by four series of tests. The geometry, reinforcement arrangement and support conditions of the beams of these series are indicated in Figure 4. Each series was constituted by a beam without any shear reinforcement (R) and a beam for each of the following shear reinforcing systems: steel stirrups (S), strips of CFRP sheet (M), laminate strips of CFRP at 90º with the beam axis (VL), and laminate strips of CFRP at 45º with the beam axis (IL). The shear span, a, on the series of beams was two times the depth of the corresponding beams. The concrete clear cover for the top, bottom and lateral faces of the beams was 15 mm. The amount of shear reinforcement applied on the four reinforcing systems was evaluated in order to assure that all beams would fail in shear, at a similar load carrying capacity [3, 4]. Table 4 includes general information of the beams composing the four series. Further information can be found elsewhere [5]. The relationship between the force and the deflection at mid span of the tested beams is represented in Figure 5. Table 5 includes the main results obtained in the four series. Adopting the designation of Fmax,K_R and Fmax,K_S for referring the maximum load of a beam without shear reinforcement and a beam reinforced with steel stirrups, respectively, (K represents the series of tests) the ratios Fmax/Fmax,K_R and Fmax/Fmax,K_S were determined for assessing the efficacy of the shear strengthening techniques, in terms of increasing the beam load carrying capacity. The unreinforced shear R beams have failed by the formation of one shear failure crack without the longitudinal tensile reinforcement has yielded. In the beams reinforced with steel stirrups (S beams) a shear failure crack has occurred. The sudden loss of the load carrying capacity in the S beams corresponds to the moment when a stirrup crossing the shear failure crack has ruptured. In general, beams M have failed by the formation of a shear crack and have pilled off. B12_M beam had a distinct failure mode. This beam has failed by the formation of two “concrete lateral walls” that have separated from the interior concrete volume. A shear crack has formed in this interior concrete volume, and finally the “lateral walls” have ruptured. This complex type of failure has also occurred in B10_VL, B10_IL, B12_VL and B12_IL beams. In A10_VL beam, after the longitudinal tensile reinforcement has yielded, a shear failure crack has formed. A12_VL beam has failed in shear and the shorter bond length of the CFRP laminate strip crossing this crack has slid. Finally, A10_IL and A12_IL beams have ruptured by the formation of a flexural failure crack.  6 A10_ILA10_VL20030050A10_M150200200300200200 200 300300300300A10_SPP300 190 190190300150300190 190 190P300PCross sectionA10_RPP1500 5025PP1504∅10300PPa2∅6A12_VL A12_IL501506x956x1006x100 300 150150150150 300150 150150A12_SP P15015080150 15015080 1503006x95300A12_MPPA12_RP P1500 50Cross section25P P3001504∅12P Pa2∅6 A10 Series A12 Series 150100100 10015050100100 100300B10_VL15080150 80 300B10_IL8060150150P300PB10_S8080 6080P30080 80PB10_MP900PB10_R501504∅1025P150PCross sectiona PP2∅64∅12B12_ILCross section6x50 300B12_VL3x75 406x50 300P P4x75 300B12_SP7x40 3004x75B12_MP P90050B12_RP50 150   3x7540P7x40 P 251502∅6PPa B10 Series B12 Series Figure 4: Beam series for the shear strengthening   Table 4 - Beam series for the shear strengthening Shear strengthening systems Beam’s designation Material Quantity Spacing (mm) Angle (º) A10_R - - - - A10_S Steel stirrups 6φ6 of two branches 300 90 A10_M Strips of S&P C-Sheet 530 8×2 layers of 25 mm (U shape) 190 90 A10_VL S&P laminate strips of CFK 150/2000 16 CFRP laminates 200 90 A10 A10_IL S&P laminate strips of CFK 150/2000 12 CFRP laminates 300 45 A12_R - - - - A12_S Steel stirrups 10φ6 of two branches 150 90 A12_M Strips of S&P C-Sheet 530 14×2 layers of 25 mm (U shape) 95 90 A12_VL S&P laminate strips of CFK 150/2000 28 CFRP laminates 100 90 A series A12 A12_IL S&P laminate strips of CFK 150/2000 24 CFRP laminates 150 45 B10_R - - - - B10_S Steel stirrups 6φ6 of two branches 150 90 B10_M Strips of S&P C-Sheet 530 10×2 layers of 25 mm (U shape) 80 90 B10_VL S&P laminate strips of CFK 150/2000 16 CFRP laminates 100 90 B10 B10_IL S&P laminate strips of CFK 150/2000 12 CFRP laminates 150 45 B12_R - - - - B12_S Steel stirrups 10φ6 of two branches 75 90 B12_M Strips of S&P C-Sheet 530 16×2 layers of 25 mm (U shape) 40 90 B12_VL S&P laminate strips of CFK 150/2000 28 CFRP laminates 50 90 B series B12 B12_IL S&P laminate strips of CFK 150/2000 24 CFRP laminates 75 45  7 0204060801001201401601800 5 10 15 20 25 30 35 40Deflection at mid span (mm)Force (kN) Beams Series A (φ10) Fmax (kN) R_10Amax,maxFF  S_10Amax,maxFF  A10-R 100.40 1.00 0.59 A10-S 169.35 1.69 1.00 A10-M 122.06 1.22 0.72 A10-VL 158.64 1.58 0.94 A10-IL 157.90 1.57 0.93 040801201602002402800 5 10 15 20 25Deflection at mid span (mm)Force (kN) Beams  Series A (φ12) Fmax (kN) R_12Amax,maxFF S_12Amax,maxFF A12-R 116.50 1.00 0.54 A12-S 215.04 1.85 1.00 A12-M 179.54 1.54 0.83 A12-VL 235.11 2.02 1.09 A12-IL 262.38 2.25 1.22 0204060801001201400 2 4 6 8 10 12 14Deflection at mid span (mm)Force (kN) Beams Series B (φ10) Fmax  (kN) R_10Bmax,maxFF S_10Bmax,maxFF B10-R 74.02 1.00 0.61 B10-S 120.64 1.63 1.00 B10-M 111.14 1.50 0.92 B10-VL 131.22 1.77 1.09 B10-IL 120.44 1.63 1.00  0204060801001201401601800 2 4 6 8 10 12 14 16 18 20Deflection at mid span (mm)Force (kN)Beams Series B (φ12) Fmax  (kN) R_12Bmax,maxFF  S_12Bmax,maxFF  B12-R 75.7 1.00 0.48 B12-S 159.1 2.10 1.00 B12-M 143.0 1.89 0.90 B12-VL 139.2 1.84 0.87 B12-IL 148.5 1.96 0.93 Figure 5: Force-deflection relationships of the shear strengthening beams Table 5 - Main results of the shear strengthening beam series  From the results obtained, the following main conclusions can be pointed out: A10-R A10-M A10-ILA10-SA10-VL A12-R A12-MA12-ILA12-S A12-VL B10-R B10-M B10-IL B10-S B10-VL B12-R B12-M B12-VL B12-IL B12-S  8 • The CFRP shear strengthening systems applied in the present work increased significantly the shear resistance of concrete beams; • The NSM shear strengthening technique was the most effective of the CFRP systems. This efficacy was not only in terms of the beam load carrying capacity, but also in terms of deformation capacity at beam failure. Using the load carrying capacity of the unreinforced beams for comparison purposes, the beams strengthened by EBS and NSM techniques showed an average increase of 54% and 83%, respectively; • Increasing the beam depth, laminates at 45º became more effective than vertical laminates; • Fmax of the beams reinforced with steel stirrups and Fmax of the beams strengthened by NSM technique were almost similar; • Failure modes of the beams strengthened by the NSM technique were not so fragile as the ones observed in the beams strengthened by the EBS technique.  6. CONCLUSIONS  The effectiveness of the NSM and EBR techniques for the flexural and shear strengthening of RC beams was compared. For the flexural strengthening, the NSM technique was the most effective, but the difference between the efficacy of NSM and EBR techniques has decreased with the increase of the longitudinal equivalent reinforcement ratio. For the shear strengthening, the NSM was also the most effective technique, and was also the easiest and fastest to apply, and assured the lowest fragile failure modes.  7. ACKNOWLEDGEMENTS  The authors of the present work wish to acknowledge the materials provided by the degussa Portugal, S&P and Unibetão (Braga). The study reported in this paper forms a part of the research program “CUTINSHEAR - Performance assessment of an innovative structural FRP strengthening technique using an integrated system based on optical fiber sensors” supported by FCT, POCTI/ECM/59033/2004.  8. REFERENCES  [1] Barros, J.A.O., Sena-Cruz, J.M., Dias, S.J.E., Ferreira, D.R.S.M. and Fortes, A. S., 2004, "Near surface mounted CFRP-based technique for the strengthening of concrete structures", Workshop on R+D+I in Technology of Concrete Structures - tribute to Dr. Ravindra Gettu, Barcelona, Spain, October, pp. 205-217. [2] Fortes, A. S., 2004, "Estruturas de Concreto submetidas à flexão reforçadas com laminados de CFRP colados em entalhes (NSM for the flexural strengthening of RC structures)", PhD Thesis, Universidade Federal de Santa Catarina, Brasil, May, 213 pp. [3] CEB-FIP Model Code, Comite Euro-International du Beton, Bulletin d’Information nº 213/214, 1993. [4] ACI Committee 440, 2002, “Guide for the design and construction of externally bonded FRP systems for strengthening concrete structures”, American Concrete Institute, 118 pp. [5] Barros, J.A.O. and Dias, S.J.E., 2003, “Shear strengthening of reinforced concrete beams with laminate strips of CFRP”, Proceedings of the International Conference Composites in Constructions - CCC2003, Italia, September, pp. 289-294. 

Near surface mounted thecnique for the flexural and shear strengthening of concrete beams

 1 
NEAR SURFACE MOUNTED TECHNIQUE FOR THE FLEXURAL 
AND SHEAR STRENGTHENING OF CONCRETE BEAMS 
 
 
   
 
 
 Joaquim Barros 
Associate Professor 
UM 
Guimarães, Portugal 
 Salvador Dias 
PhD Student 
UM 
Guimarães, Portugal
 Adriano Fortes 
Auxiliary Professor 
CEFET-BA 
Salvador, Brazil 
 
 
 
 
ABSTRACT 
 
The efficacies of the Near Surface Mounted and Externally Bonded Reinforcing techniques for 
the flexural and shear strengthening of reinforced concrete beams are compared. Both 
techniques are based on the use of carbon fiber reinforced polymer materials. In the present 
work the carried out tests are described and the main results are presented and analyzed. 
 
1. INTRODUCTION 
 
Near Surface Mounted (NSM) is a strengthening technique based on the use of laminate strips 
of carbon fiber reinforced polymer (CFRP) materials installed into precut slits opened on the 
concrete cover of the elements to strengthen [1]. This strengthening technique requires no 
surface preparation work and, after cutting the slit, requires minimal installation time compared 
to the Externally Bonded Reinforcing (EBR) technique. A further advantage associated with 
NSM is its ability to significantly reduce the probability of harm resulting from acts of 
vandalism, mechanical damages and aging effects. 
In the present work the efficacies of the NSM and EBR techniques for the flexural and shear 
strengthening of reinforced concrete beams are compared. For the flexural strengthening, the 
influence of the longitudinal equivalent reinforcement ratio, ρl,eq, on the strengthening 
efficiency of both techniques is assessed. For the shear strengthening, the influences of the 
longitudinal steel reinforcement ratio, ρsl, and the beam depth on the strengthening efficacy of 
both techniques are evaluated. In this last experimental program, the influence of the 
inclination of the CFRP laminates in the NSM technique is also investigated. In the present 
paper the experimental program is described and the results are presented and analyzed. 
 2 
2. STRENGTHENING TECHNIQUES 
 
The NSM technique was made up of the following steps: 1) using a diamond cutter, slits of 4 to 
5 mm width and 12 to 15 mm depth were cut on the concrete surface of the elements to 
strengthen; 2) slits were cleaned by compressed air; 3) CFRP laminates were cleaned by 
acetone; 4) epoxy adhesive was produced according to supplier recommendations; 5) slits were 
filled with the epoxy adhesive; 6) epoxy adhesive was applied on the faces of the laminate; and 
7) laminates were introduced into the slits and epoxy adhesive in excess was removed. 
To apply the wet lay-up strips of CFRP sheet by EBR technique, the following procedures 
were executed: 1) on the zones of the beam’s surfaces where the strips of sheet would be glued, 
an emery was applied to remove the superficial cement paste (in the shear strengthening 
experimental program the beam edges were also rounded); 2) the residues were removed by 
compressed air; 3) a layer of primer was applied to regularize the concrete surface and to 
enhance the adherence capacity of the concrete substrate; and 4) strips of sheet were glued on 
the faces of the beam by epoxy resin. 
 
 
3. MATERIALS 
 
3.1 Concrete and steel bars 
 
Table 1 includes the main properties of the concrete and steel bars used in the experimental 
program. The average values of the concrete compression strength at 28 days (fcm) and at the 
date of testing the beams (fcm,j) were evaluated from uniaxial compression tests (3 for each 
series, at least) with cylinders of 150 mm diameter and 300 mm height. Steel bars were tested 
and each result is the average of at least five tests. 
 
Table 1 - Properties of the concrete and steel bars 
Concrete Steel 
Element type fcm 
(MPa) 
fcm,j 
(MPa) 
φs 
(mm) 
fsym 
(MPa) 
fsum 
(MPa) 
5 620 700 Flexural strengthening 
program 44.2 
52.2 
(70 days) 6.5 480 570 
6 (stirrups) 540 694 
6 (long.) 622 
1 
618 2 
702 1 
691 2 
10 464 581 
Shear strengthening 
program 
37.6 1 
49.5 2 
49.2 1 
(227 days)
56.2 2 
(105 days) 12 574 
1 
571 2 
672 1 
673 2 
1 A series; 2 B series 
 
 
3.2 CFRP systems 
 
Two CFRP systems were used on the present work: unidirectional wet lay-up sheets of 80 mm 
width and precured laminates of 1.4×9.6 mm2 cross-section. These CFRP systems have the 
properties indicated in Table 2. 
 3 
Table 2 - Properties of the CFRP materials 
CFRP system Main properties 
Type Material 
Tensile 
strength 
(MPa) 
Young's 
modulus 
(GPa) 
Ultimate 
strain 
(%) 
Thickness 
(mm) 
Primer 1 12 0.7 3.0 -  Epoxy 1 54 3 2.5 - 
Flexural strengthening prog. C-Sheet 2401 3700 240 1.5 0.111 
S&P C-Sheet 
(wet lay-up sheet) 
Sheet Shear strengthening prog. C-Sheet 5301 3000 390 0.8 0.167 
 Adhesive 1 16-22 5 - - 
Flexural strengthening prog. 2 2740 158.8 1.7 1.4 CFK 150/2000 
(precured laminate) Laminate Shear strengthening prog. 2 2286 166 1.3 1.4 
1 According to the supplier; 2 Evaluated from experimental tests 
 
4. FLEXURAL STRENGTHENING 
 
Figure 1 represents the geometry of the beams, the reinforcement arrangement and the number 
and position of the CFRP. The load configuration and the support conditions are also 
schematized. The cross-section area of the CFRP systems was evaluated in order to be similar 
the Af Ef / Asl Es ratio for the tested series, where Af and Ef are the CFRP cross-section area and 
the Young’s Modulus of the CFRP systems, and Asl and Es (200 GPa) are the cross-section area 
and the Young’s Modulus of the longitudinal tensile steel bars. In this evaluation the distinct 
effective depth of the CFRP systems was not considered, but its influence is marginal. Shear 
reinforcement was selected to assure bending failure prior to shear failure for all beams. The 
beams were tested at the age of about 70 days [2]. 
 
2Ø6.5
2Ø5
2Ø6.5
Reinforcement arrangement Beam series
Ø6
P/2P/2
S1 Series S2 Series S3 Series
R11 and R12
NSM11 and NSM12
EBR_L11 and EBR_L12
Load configuration and support conditions
3Ø6.52Ø6.5
2Ø6.5 2Ø6.5
2Ø6.5 2Ø6.5 2Ø6.5
2Ø6.5 2Ø6.5 2Ø6.5
2Ø6.5 2Ø6.5 2Ø6.5
2Ø5 2Ø6.5 3Ø6.5
2Ø5 2Ø6.5 3Ø6.5
2Ø5 2Ø6.5 3Ø6.5
1000
8010 106080 80 80 80 80 80 8080 606060
17
0x
(1
20
)
1000
50 300 300 300 50
Non-Strengthened
1 CFRP laminate (NSM) 2 CFRP laminates (NSM) 3 CFRP laminates (NSM)
1 CFRP laminate (EBR) 2 CFRP laminates (EBR) 3 CFRP laminates (EBR)
1 layer CFRP sheet (EBR) 2 layers CFRP sheet (EBR) 3 layers CFRP sheet (EBR)
17
0x
(1
20
)
Non-Strengthened Non-Strengthened
NSM21 and NSM22 NSM31 and NSM32
EBR_L21 and EBR_L22 EBR_L31 and EBR_L32
EBR_M11 and EBR_M12
R21 and R22 R31 and R32
EBR_M21 and EBR_M22 EBR_M31 and EBR_M32
 
Figure 1: Beam series for the flexural strengthening 
 
The force-deflection relationships for the series of tested beams are depicted in Figure 2. The 
strengthening efficacy is evaluated in terms of the service (Pserv) and the maximum (Pmax) load, 
see Table 3. Pserv(S) and Pmax(S) are the service and maximum load of a strengthened beam, 
respectively, while Pserv(R) and Pmax(R) are the service and maximum load of the reference 
 4 
beam. In Table 3 the equivalent reinforcement ratio, ρl,eq = Asl / (b ds) + (Af Ef / Es) / (b df), is also 
indicated, where b is the beam width and ds and df are the effective depth of the longitudinal 
steel bars and CFRP systems, respectively. The influence of the ρl,eq on the strengthening 
efficacy index is graphically represented in Figure 3. In terms of the beam load carrying 
capacity the NSM technique was the most effective one. The effectiveness, however, decreases 
with the increasing of ρl,eq. In terms of service load, the EBR based on the use of wet lay-up 
CFRP sheets was the most efficient, since Ef and df were the largest ones. The beams 
strengthened by EBR laminates have failed by the premature debonding of the laminates. Two 
failure modes occurred in the beams strengthened by EBR sheets: in the beams strengthened by 
one and two layers, the CFRP sheets have ruptured, while in the beams strengthened by three 
layers, the beam concrete cover has detached, having been bonded to the CFRP sheet. This last 
failure mode was also occurred in the beams strengthened by the NSM technique, but the 
detached concrete layer had larger thickness [2]. 
 
0
20
40
60
80
100
0 2 4 6 8 10 12
Deflection at mid span (mm)
Fo
rc
e 
(k
N
)
NSM
EBR_LEBR_M
R
Beam’s 
designation
eq,lρ
(%)
servP
(kN)
( ) ( )
( )RP
RPSP
ser
serser −
(%) 
maxP  
(kN) 
( ) ( )
( )RP
RPSP
max
maxmax −  
(%) 
R - 22.1 - 36.6 - 
NSM 0.28 37.5 70 79.9 118 
EBR_L 0.28 31.9 44 38.6 5 S1
 se
rie
s 
EBR_M 0.28 40.3 82 43.0 17 
0
20
40
60
80
100
0 2 4 6 8 10 12
Deflection at mid span (mm)
Fo
rc
e 
(k
N
)
NSM
EBR_L
EBR_M
R
Beam’s 
designation
eq,lρ
(%)
servP
(kN)
( ) ( )
( )RP
RPSP
ser
serser −
(%) 
maxP  
(kN) 
( ) ( )
( )RP
RPSP
max
maxmax −  
(%) 
R - 40.5 - 48.5 - 
NSM 0.49 56.3 39 93.3 92 
EBR_L 0.48 57.6 42 83.5 72 S2
 se
rie
s 
EBR_M 0.48 59.5 47 79.5 64 
0
20
40
60
80
100
0 2 4 6 8 10 12
Deflection at mid span (mm)
Fo
rc
e 
(k
N
)
NSM
EBR_L
EBR_M
R
 
Beam’s 
designation
eq,lρ
(%)
servP
(kN)
( ) ( )
( )RP
RPSP
ser
serser −
(%) 
maxP  
(kN) 
( ) ( )
( )RP
RPSP
max
maxmax −  
(%) 
R - 51.5 - 71.8 - 
NSM 0.73 71.5 39 96.6 35 
EBR_L 0.73 74.1 44 86.5 20 S3
 se
rie
s 
EBR_M 0.73 73.4 43 87.3 22 
Figure 2: Force-deflection relationships 
of the flexural strengthening beams  
Table 3 - Main results of the flexural 
strengthening beam series 
 5 
0
20
40
60
80
100
120
140
0.2 0.3 0.4 0.5 0.6 0.7 0.8
ρl, eq  (%)
[P
m
ax
 (S
)-
P m
ax
 (R
)]
/P
m
ax
 (R
) NSM EBR_L EBR_M
 
Figure 3: Strengthening efficacy index vs longitudinal equivalent reinforcement ratio  
 
5. SHEAR STRENGTHENING 
 
The experimental program was composed by four series of tests. The geometry, reinforcement 
arrangement and support conditions of the beams of these series are indicated in Figure 4. Each 
series was constituted by a beam without any shear reinforcement (R) and a beam for each of 
the following shear reinforcing systems: steel stirrups (S), strips of CFRP sheet (M), laminate 
strips of CFRP at 90º with the beam axis (VL), and laminate strips of CFRP at 45º with the 
beam axis (IL). The shear span, a, on the series of beams was two times the depth of the 
corresponding beams. The concrete clear cover for the top, bottom and lateral faces of the 
beams was 15 mm. The amount of shear reinforcement applied on the four reinforcing systems 
was evaluated in order to assure that all beams would fail in shear, at a similar load carrying 
capacity [3, 4]. Table 4 includes general information of the beams composing the four series. 
Further information can be found elsewhere [5]. The relationship between the force and the 
deflection at mid span of the tested beams is represented in Figure 5. Table 5 includes the main 
results obtained in the four series. Adopting the designation of Fmax,K_R and Fmax,K_S for referring 
the maximum load of a beam without shear reinforcement and a beam reinforced with steel 
stirrups, respectively, (K represents the series of tests) the ratios Fmax/Fmax,K_R and Fmax/Fmax,K_S 
were determined for assessing the efficacy of the shear strengthening techniques, in terms of 
increasing the beam load carrying capacity. 
The unreinforced shear R beams have failed by the formation of one shear failure crack 
without the longitudinal tensile reinforcement has yielded. In the beams reinforced with steel 
stirrups (S beams) a shear failure crack has occurred. The sudden loss of the load carrying 
capacity in the S beams corresponds to the moment when a stirrup crossing the shear failure 
crack has ruptured. In general, beams M have failed by the formation of a shear crack and have 
pilled off. B12_M beam had a distinct failure mode. This beam has failed by the formation of 
two “concrete lateral walls” that have separated from the interior concrete volume. A shear 
crack has formed in this interior concrete volume, and finally the “lateral walls” have ruptured. 
This complex type of failure has also occurred in B10_VL, B10_IL, B12_VL and B12_IL 
beams. In A10_VL beam, after the longitudinal tensile reinforcement has yielded, a shear 
failure crack has formed. A12_VL beam has failed in shear and the shorter bond length of the 
CFRP laminate strip crossing this crack has slid. Finally, A10_IL and A12_IL beams have 
ruptured by the formation of a flexural failure crack. 
 6 
A10_ILA10_VL
200
300
50
A10_M
150200200300200200 200 300
300300300
A10_S
PP
300 190 190190
300150300
190 190 190
P
300
P
Cross sectionA10_R
PP
1500 50
25PP
150
4∅10
300
PPa
2∅6
A12_VL A12_IL
50
150
6x95
6x1006x100 300 150
150150150 300150 150150
A12_S
P P
150150
80150 15015080 150300
6x95300
A12_M
PP
A12_R
P P
1500 50
Cross section
25P P
300
150
4∅12
P Pa
2∅6
 
A10 Series A12 Series 
150
100100 100
150
50
100100 100300
B10_VL
15080150 80 300
B10_IL
8060150150
P
300
P
B10_S
8080 6080
P
30080 80
P
B10_M
P
900
P
B10_R
50
150
4∅10
25P
150
P
Cross section
a PP
2∅6
4∅12
B12_IL
Cross section
6x50 300
B12_VL
3x75 406x50 300
P P
4x75 300
B12_S
P
7x40 3004x75
B12_M
P P
90050
B12_R
P
50 150
   3x7540
P
7x40 
P 25
150
2∅6
PPa
 
B10 Series B12 Series 
Figure 4: Beam series for the shear strengthening  
 
Table 4 - Beam series for the shear strengthening 
Shear strengthening systems Beam’s 
designation Material Quantity Spacing (mm) Angle (º) 
A10_R - - - - 
A10_S Steel stirrups 6φ6 of two branches 300 90 
A10_M Strips of S&P C-Sheet 530 8×2 layers of 25 mm (U shape) 190 90 
A10_VL S&P laminate strips of CFK 150/2000 16 CFRP laminates 200 90 
A
10
 
A10_IL S&P laminate strips of CFK 150/2000 12 CFRP laminates 300 45 
A12_R - - - - 
A12_S Steel stirrups 10φ6 of two branches 150 90 
A12_M Strips of S&P C-Sheet 530 14×2 layers of 25 mm (U shape) 95 90 
A12_VL S&P laminate strips of CFK 150/2000 28 CFRP laminates 100 90 
A
 se
rie
s 
A
12
 
A12_IL S&P laminate strips of CFK 150/2000 24 CFRP laminates 150 45 
B10_R - - - - 
B10_S Steel stirrups 6φ6 of two branches 150 90 
B10_M Strips of S&P C-Sheet 530 10×2 layers of 25 mm (U shape) 80 90 
B10_VL S&P laminate strips of CFK 150/2000 16 CFRP laminates 100 90 
B
10
 
B10_IL S&P laminate strips of CFK 150/2000 12 CFRP laminates 150 45 
B12_R - - - - 
B12_S Steel stirrups 10φ6 of two branches 75 90 
B12_M Strips of S&P C-Sheet 530 16×2 layers of 25 mm (U shape) 40 90 
B12_VL S&P laminate strips of CFK 150/2000 28 CFRP laminates 50 90 
B
 se
rie
s 
B
12
 
B12_IL S&P laminate strips of CFK 150/2000 24 CFRP laminates 75 45 
 7 
0
20
40
60
80
100
120
140
160
180
0 5 10 15 20 25 30 35 40
Deflection at mid span (mm)
Fo
rc
e 
(k
N
)
 
Beams 
Series A (φ10) 
Fmax 
(kN) R_10Amax,
max
F
F
 
S_10Amax,
max
F
F
 
A10-R 100.40 1.00 0.59 
A10-S 169.35 1.69 1.00 
A10-M 122.06 1.22 0.72 
A10-VL 158.64 1.58 0.94 
A10-IL 157.90 1.57 0.93 
0
40
80
120
160
200
240
280
0 5 10 15 20 25
Deflection at mid span (mm)
Fo
rc
e 
(k
N
)
 
Beams  
Series A (φ12) 
Fmax 
(kN) R_12Amax,
max
F
F
 
S_12Amax,
max
F
F
 
A12-R 116.50 1.00 0.54 
A12-S 215.04 1.85 1.00 
A12-M 179.54 1.54 0.83 
A12-VL 235.11 2.02 1.09 
A12-IL 262.38 2.25 1.22 
0
20
40
60
80
100
120
140
0 2 4 6 8 10 12 14
Deflection at mid span (mm)
Fo
rc
e 
(k
N
)
 
Beams 
Series B (φ10) 
Fmax  
(kN) R_10Bmax,
max
F
F
 
S_10Bmax,
max
F
F
 
B10-R 74.02 1.00 0.61 
B10-S 120.64 1.63 1.00 
B10-M 111.14 1.50 0.92 
B10-VL 131.22 1.77 1.09 
B10-IL 120.44 1.63 1.00  
0
20
40
60
80
100
120
140
160
180
0 2 4 6 8 10 12 14 16 18 20
Deflection at mid span (mm)
Fo
rc
e 
(k
N
)
Beams 
Series B (φ12) 
Fmax  
(kN) R_12Bmax,
max
F
F
 
S_12Bmax,
max
F
F
 
B12-R 75.7 1.00 0.48 
B12-S 159.1 2.10 1.00 
B12-M 143.0 1.89 0.90 
B12-VL 139.2 1.84 0.87 
B12-IL 148.5 1.96 0.93 
Figure 5: Force-deflection relationships of the 
shear strengthening beams 
Table 5 - Main results of the shear 
strengthening beam series 
 
From the results obtained, the following main conclusions can be pointed out: 
A10-R 
A10-M 
A10-IL
A10-S
A10-VL 
A12-R 
A12-M
A12-IL
A12-S 
A12-VL 
B10-R 
B10-M 
B10-IL 
B10-S 
B10-VL 
B12-R 
B12-M 
B12-VL 
B12-IL 
B12-S 
 8 
• The CFRP shear strengthening systems applied in the present work increased significantly 
the shear resistance of concrete beams; 
• The NSM shear strengthening technique was the most effective of the CFRP systems. This 
efficacy was not only in terms of the beam load carrying capacity, but also in terms of 
deformation capacity at beam failure. Using the load carrying capacity of the unreinforced 
beams for comparison purposes, the beams strengthened by EBS and NSM techniques 
showed an average increase of 54% and 83%, respectively; 
• Increasing the beam depth, laminates at 45º became more effective than vertical laminates; 
• Fmax of the beams reinforced with steel stirrups and Fmax of the beams strengthened by NSM 
technique were almost similar; 
• Failure modes of the beams strengthened by the NSM technique were not so fragile as the 
ones observed in the beams strengthened by the EBS technique. 
 
6. CONCLUSIONS 
 
The effectiveness of the NSM and EBR techniques for the flexural and shear strengthening of 
RC beams was compared. For the flexural strengthening, the NSM technique was the most 
effective, but the difference between the efficacy of NSM and EBR techniques has decreased 
with the increase of the longitudinal equivalent reinforcement ratio. For the shear 
strengthening, the NSM was also the most effective technique, and was also the easiest and 
fastest to apply, and assured the lowest fragile failure modes. 
 
7. ACKNOWLEDGEMENTS 
 
The authors of the present work wish to acknowledge the materials provided by the degussa 
Portugal, S&P and Unibetão (Braga). The study reported in this paper forms a part of the 
research program “CUTINSHEAR - Performance assessment of an innovative structural FRP 
strengthening technique using an integrated system based on optical fiber sensors” supported 
by FCT, POCTI/ECM/59033/2004. 
 
8. REFERENCES 
 
[1] Barros, J.A.O., Sena-Cruz, J.M., Dias, S.J.E., Ferreira, D.R.S.M. and Fortes, A. S., 2004, 
"Near surface mounted CFRP-based technique for the strengthening of concrete 
structures", Workshop on R+D+I in Technology of Concrete Structures - tribute to 
Dr. Ravindra Gettu, Barcelona, Spain, October, pp. 205-217. 
[2] Fortes, A. S., 2004, "Estruturas de Concreto submetidas à flexão reforçadas com 
laminados de CFRP colados em entalhes (NSM for the flexural strengthening of RC 
structures)", PhD Thesis, Universidade Federal de Santa Catarina, Brasil, May, 213 pp. 
[3] CEB-FIP Model Code, Comite Euro-International du Beton, Bulletin d’Information 
nº 213/214, 1993. 
[4] ACI Committee 440, 2002, “Guide for the design and construction of externally bonded 
FRP systems for strengthening concrete structures”, American Concrete Institute, 118 pp. 
[5] Barros, J.A.O. and Dias, S.J.E., 2003, “Shear strengthening of reinforced concrete beams 
with laminate strips of CFRP”, Proceedings of the International Conference Composites 
in Constructions - CCC2003, Italia, September, pp. 289-294. 


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Near surface mounted thecnique for the flexural and shear strengthening of concrete beams

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