The New Surface Mounted (NSM) technique was applied to increase the shear resistance of
Reinforced Concrete (RC) beams. For this purpose, laminates of Carbon Fiber Reinforced Polymer (CFRP)
were introduced into thin slits made on the concrete cover of the lateral faces of the beams to be strengthened.
In the present paper the influence of concrete strength on the effectiveness of the NSM technique is assessed
by an experimental program. From the obtained results it can be concluded that the NSM shear strengthening
technique with CFRP laminates is still effective in RC beams with low concrete strength. However, as minimum
is the concrete strength as less effective is the NSM technique.Fundação para a Ciência e a Tecnologia (FCT) - PTDC/ECM/73099/200

Barros, Joaquim A. O.

Dias, Salvador J. E.

Universidade do Minho: RepositoriUM

1 EXPERIMENTAL PROGRAM  The influence of the concrete mechanical properties on the performance of the NSM technique using CFRP laminates for the shear strengthening of RC beams (Fig. 1) is assessed by experimental research, which main features are presented in this publica-tion.    Figure 1. NSM technique with CFRP laminates for the shear strengthening of RC beams.  1.1 Test series and materials properties Figure 2 presents the T cross section beam prototype used in the experimental program. The reinforce-ment systems were designed to assure that all the tested beams failed in shear. To localize shear fail-ure in only one of the beam shear spans, a three point load configuration of a distinct length of the beam shear spans was selected. To avoid shear fail-ure in the Lr beam span, steel stirrups φ6@75mm were applied in this span. In terms of the shear rein-forcement systems the differences between the tested beams were localized in the Li beam span. Five arrangements of NSM CFRP laminates were used in RC beams that were manufactured with a concrete of a compressive strength at the age of beam tests of 39.7 MPa (fcm = 39.7 MPa) and 18.6 MPa (fcm = 18.6 MPa). The differences between the beams with fcm = 39.7 MPa and the beams with fcm = 18.6 MPa are restricted to the mode of preventing brittle spalling of the concrete cover at the supports (Fig. 2). To overcome the difficulties to bend φ32 mm longitudinal tensile bars, their ends were welded to steel plates in the beams with the lower concrete strength (Fig. 2).    ∅6//150 in L (lateral concrete cover = 22 mm)2∅32+1∅163 layers of U strips of wet lay-up CFRP sheet with the fibers direction at 0º100 = 900iLA160150B45ºF2222Lr = 1350 1004506∅12180ri and ∅6//75 in L d = 360300100The CFRP shear strengthening was distributed along the AB line∅6//75 in L r ∅12//50 (vertical stirrups)Steel plate∅6//65 (horizontal stirrups)∅6//65 (horizontal stirrups)Steel plate∅12//50 (vertical stirrups)FLi = 900 = 1350rL45ºBA The CFRP shear strengthening was distributed along the AB line200 200  Figure 2. Geometry and steel reinforcements applied in all tested beams (dimensions in mm): a) beams with fcm = 39.7 MPa and b) beams with fcm = 18.6 MPa.  For each type of concrete five arrangements of NSM CFRP laminates were applied in five RC beams with a percentage of steel stirrups of 0.10% (ρsw = 0.10%), and in five RC beams with a percent-age of steel stirrups of 0.17% (ρsw = 0.17%). Three distinct inclinations of CFRP laminates (45º, 60º and 90º) were studied and, for inclined laminates, two levels of CFRP percentage (ρf) were analyzed (Table 1 and Fig. 3). For both percentages of CFRP, the spacing of laminates for each inclination was ob-CICE 2010 - The 5th International Conference on FRP Composites in Civil EngineeringSeptember 27-29, 2010 Beijing, China Influence of the concrete properties in the effectiveness of the NSM CFRP laminates for the shear strengthening of RC beams Salvador Dias (sdias@civil.uminho.pt) & Joaquim Barros ISISE, University of Minho, Guimarães, Portugal     ABSTRACT: The New Surface Mounted (NSM) technique was applied to increase the shear resistance of Reinforced Concrete (RC) beams. For this purpose, laminates of Carbon Fiber Reinforced Polymer (CFRP) were introduced into thin slits made on the concrete cover of the lateral faces of the beams to be strengthened. In the present paper the influence of concrete strength on the effectiveness of the NSM technique is assessed by an experimental program. From the obtained results it can be concluded that the NSM shear strengthening technique with CFRP laminates is still effective in RC beams with low concrete strength. However, as mini-mum is the concrete strength as less effective is the NSM technique. Beam lateral surfaceBeam bottom tensile surface a) b) tained with the purpose that the shear strengthening contribution of the CFRP would be similar (Dias and Barros 2009). The three point beam bending tests (Fig. 2) were carried out using a servo closed-loop control equip-ment, taking the signal read in the displacement transducer, placed at the loaded section, to control the test at a deflection rate of 0.01 mm/second. The concrete compressive strength was evaluated at 28 days and at the age of the beam tests, carrying out direct compression tests with cylinders of 150 mm diameter and 300 mm height, according to EN 206-1 (2000). In the tested beams, high bond steel bars of 6, 12, 16 and 32 mm diameter were used. The values of their main tensile properties were obtained from uniaxial tensile tests performed according to the recommendations of EN 10002-1 (1990). The tensile properties of the CFK 150/2000 S&P laminates were characterized by uniaxial ten-sile tests carried out according to ISO 527-5 (1997). Table 2 includes the average values obtained from these experimental programs. According to this ta-ble, the properties of the steel and CFRP are similar in the two types of beams (beams with the higher and lower concrete strength). The MBrace Resin 220 (Degussa 2003) epoxy adhesive was used to bond the laminates to the concrete.  Table 1. Shear reinforcement configurations of the tested beams. Number of laminates  CFRP angle  [θf] a (º) CFRP spacing [sf] (mm) CFRP percentage [ρf] b (%) Percentage of steel stirrups ρsw = 0.10% c ρsw = 0.17% d2×4 45 275 0.08 2S-4LI45 4S-4LI45 2×4 60 243 0.07 2S-4LI60 4S-4LI60 2×7 90 114 0.13 2S-7LV 4S-7LV 2×7 45 157 0.13 2S-7LI45 4S-7LI45 2×6 60 162 0.11 2S-6LI60 4S-6LI60 a Angle between the CFRP fiber direction and the beam axis; b The CFRP percentage was obtained from ( ) ( )ffwfff θsinsbbaρ 2= being af = 1.4 mm and bf = 9.5 mm the di-mensions of the laminate cross section and bw = 180 mm is the beam web width; c 2S-R is the reference beam without CFRP (Fig. 3); d 4S-R is the reference beam without CFRP (Fig. 3).  Table 2. Values of the properties of intervening materials. Materials Beams manufactured with  the higher concrete strengthBeams manufactured with  the lower concrete strengthConcretea fcm = 31.7 MPa  (at 28 days) fcm = 15.9 MPa  (at 28 days) fcm = 39.7 MPa  (age of beam tests) fcm = 18.6 MPa  (age of beam tests) Steel Tensile strength (MPa) φ6 φ12 φ16 φ32 φ6 φ12 φ16 φ32 fsym (yield stress) 542 453 447 759 539 453 429 734 fsum (maximumstress) 594 591 566 902 595 581 563 885 CFRP Laminatesb ffum (MPa) Efm    (GPa) εfum     (‰) ffum (MPa) Efm   (GPa) εfum    (‰) 2741.7 170.9 16.0 2847.9 174.3 16.3 a fcm is the average value of the concrete compressive strength; b ffum, Efm and εfum are the maximum tensile strength, Young’s modulus and maximum strain of the CFRP laminates. 18x75 1001350243243 4913503618x75FF18x751001005x1805x1806x1141004S-4LI454S-7LV581004S-R18x751350F135015818x75F100100FF100 3x3006x114 15858100135018x75100 3x300F10018x75100 3x300 18x75 100100F3x300F10018x75100 10018x753x300F275138 212275 135079 365x157 1350122 243 243243 13504981 5x162 135092S-7LV2S-4LI452S-7LI452S-4LI602S-6LI602S-R100 18x753x300F100275138 2122755x1625x1575x1805x1805x1805x1801004S-6LI60811004S-4LI604S-7LI45122 2437910010010018x7513509F Figure 3. Localization of the steel stirrups (continuous line) and CFRP laminates (dashed line) (dimensions in mm). 1.2 Discussion of tests results The recorded force-displacement diagrams (F-u) in the loaded section obtained for the tested beams are reported in Figure 4.   0501001502002503003504004505000 2 4 6 8 10Force (kN)Deflection at loaded-section (mm)2S-R2S-4LI452S-4LI600501001502002503003504004505000 2 4 6 8 10Force (kN)Deflection at loaded-section (mm)2S-R2S-4LI452S-4LI600501001502002503003504004505000 2 4 6 8 10Force (kN)Deflection at loaded-section (mm)2S-R2S-7LV2S-7LI452S-6LI600501001502002503003504004505000 2 4 6 8 10Force (kN)Deflection at loaded-section (mm)2S-R2S-7LV2S-7LI452S-6LI600501001502002503003504004505000 2 4 6 8 10Force (kN)Deflection at loaded-section (mm)4S-R4S-4LI454S-4LI600501001502002503003504004505000 2 4 6 8 10Force (kN)Deflection at loaded-section (mm)4S-R4S-4LI454S-4LI600501001502002503003504004505000 2 4 6 8 10Force (kN)Deflection at loaded-section (mm)4S-R4S-7LV4S-7LI454S-6LI600501001502002503003504004505000 2 4 6 8 10Force (kN)Deflection at loaded-section (mm)4S-R4S-7LV4S-7LI454S-6LI60 Figure 4. Force vs deflection at the loaded-section for the tested beams (left part: beams with fcm = 39.7 MPa; right part: beams with fcm = 18.6 MPa).  Up to critical diagonal crack (CDC) initiation the strengthened and its corresponding reference beam had similar F-u, regardless of the percentage of steel stirrups and the percentage and orientation of the CFRP. At CDC initiation the load decay observed in the reference beams did not occur in the CFRP shear strengthened beams, revealing that the presence of the CFRP delayed the propagation of the shear crack. This results in an increase in the beam’s load carrying capacity. The maximum load (Fmax) and the corresponding deflection at loaded-section (uFmax) for all the tested beams are included in Table 3. The values of the refmaxmax FFΔ ratio (gain due to CFRP where refmaxmaxmax FFFΔ -=  and refmaxF is the maximum force of reference beam) for the beams with fcm = 39.7 MPa and fcm = 18.6 MPa are included in Table 3 and rep-resented in Figure 5, being visible the increase of the NSM effectiveness with the increase of the concrete strength. According to the values into Table 3, the average value of the refmaxmax FFΔ  ratio for NSM ar-rangements adopted in beams with higher and lower concrete compressive strength was 26.5% and 22.4%, respectively (the values regarding the 4S-7LV shear strengthening configuration was excluded for this analysis). Independently of the concrete strength and the percentage of existing steel stirrups, the inclined laminates were more effective than ver-tical laminates. An increase of the percentage of CFRP produced an increase of the shear strengthen-ing contribution. In general the failure modes of the tested beams with CFRP have been influenced by the concrete strength, since at failure a certain concrete volume was attached to the laminates (concrete fracture). This justifies the better shear strengthening contribu-tion of the CFRP in the beams with fcm = 39.7 MPa. The failure modes of the beams with NSM laminates are also influenced by the percentage of the CFRP. In some beams with the higher percentage of CFRP a detrimental group effect (Fig. 6) between neighboring laminates occurred that originated a separation of parts of the concrete cover, which had already been observed in previous experimental pro-grams (Barros and Dias 2006; Rizzo and De Loren-zis 2009; Dias and Barros 2009) and analytical re-search (Bianco et al. 2009).  Table 3. Relevant results of the load capacity up to beam’s failure.  Beams (ρsw = 0.10%)Fmax (kN) Gain due to CFRP (%) uFmax (mm) Beams (ρsw = 0.17%)Fmax (kN) Gain due to CFRP (%) uFmax(mm)f cm = 39.7 MPa 2S-R 303.8 - 5.88 4S-R 371.4 - 6.252S-4LI45 392.8 29.3 6.45 4S-4LI45 442.5 19.1 7.932S-4LI60 386.4 27.2 6.90 4S-4LI60 443.8 19.5 6.912S-7LV 374.1 23.1 7.17 4S-7LV 427.4 15.1 9.752S-7LI45 421.7 38.8 7.93 4S-7LI45 478.1 28.7 8.262S-6LI60 394.4 29.8 7.87 4S-6LI60 457.6 23.2 7.31f cm = 18.6 MPa 2S-R 226.5 - 5.29 4S-R 303.8 - 7.202S-4LI45 283.0 24.9 5.79 4S-4LI45 347.2 14.3 9.282S-4LI60 281.6 24.3 5.57 4S-4LI60 345.6 13.8 7.672S-7LV 273.7 20.8 4.55 4S-7LV 315.2 3.8 5.982S-7LI45 306.5 35.3 4.79 4S-7LI45 356.4 17.3 7.832S-6LI60 297.7 31.4 5.84 4S-6LI60 362.3 19.3 8.36 Figure 5 also shows that the amount of existing steel stirrups plays a very important role on the ef-fectiveness of the NSM shear strengthening tech-nique with CFRP laminates. In fact, the effective-ness of the CFRP was higher in the beams with the lower percentage of steel stirrups analysed (ρsw = 0.10%). According to Figure 5, for an increase from 0.10% to 0.17% in the percentage of steel stirrups in the Li beam span (about 70%), the NSM strengthen-ing effectiveness decreased in about 70% and 55% (the value regarding the solution with vertical lami-nates was excluded for this evaluation) for the beams with fcm = 39.7 MPa and fcm = 18.6 MPa, re-spectively. It emerges that a formulation for the pre-diction of the NSM shear strengthening contribution cannot neglect the percentage of existing steel stir-rups.  29.3 27.223.138.829.824.9 24.320.835.331.4010203040502S-4LI45 2S-4LI60 2S-7LV 2S-7LI45 2S-6LI60ΔFmax /(F max, ref)  (%)Beams shear strengthened with NSM CFRP laminatesLower percentage of  CFRP Higher percentage of  CFRP19.1 19.515.128.723.214.3 13.83.817.3 19.3010203040504S-4LI45 4S-4LI60 4S-7LV 4S-7LI45 4S-6LI60ΔFmax /(F max, ref)  (%)Beams shear strengthened with NSM CFRP laminatesLower percentage of  CFRP Higher percentage of  CFRP Figure 5. Concrete strength influence in the effectiveness of the NSM shear strengthening technique with CFRP laminates (left column: fcm = 39.7 MPa; right column: fcm = 18.6 MPa): a) beams with ρsw = 0.10%; b) beams with ρsw = 0.17%.    Figure 6. Influence of CFRP percentage on failure modes (beams with fcm = 18.6 MPa). 2 CONCLUSIONS The following conclusions can be obtained from the experimental results: • The NSM CFRP laminates shear strengthening technique is still effective in beams of concrete with an average compressive strength of 18.6 MPa at the age of the tests, which can be considered as the lowest concrete strength class for structural purposes. The CFRP shear strengthening configu-rations provided an increase not only in terms of maximum load, but also in terms of load carrying capacity after shear crack formation. The concrete strength has an important role on the effectiveness of the NSM shear strengthening technique, since this effectiveness decreases with the decrease of the concrete strength. In fact, when the same NSM CFRP laminates arrangements were applied in a group of beams of concrete compressive strength (fcm) equal to 39.7 MPa and in another group of beams of fcm=18.6 MPa, the CFRP laminates were more effective in the former beams. • Inclined laminates were more effective than verti-cal laminates and an increase of the percentage of laminates led to an increase of the shear capacity of the beams. A detrimental effect of the increase of the percentage of the existing steel stirrups exists in terms of the effectiveness of the NSM technique for the shear resistance of RC beams. • An analytical formulation for the prediction of the NSM shear strengthening contribution should take into account the concrete mechanical properties, the percentage and orientation of the CFRP and the percentage of the existing steel stirrups. 3 ACKNOWLEDGEMENTS The authors wish to acknowledge the support pro-vided by the “Empreiteiros Casais”, Degussa, S&P® and Secil (Unibetão, Braga). The study reported in this paper forms a part of the research program sup-ported by FCT, PTDC/ECM/73099/2006.  4 REFERENCES Barros, J.A.O. & Dias, S.J.E. 2006. Near surface mounted CFRP laminates for shear strengthening of concrete beams. Journal Cement and Concrete Composites 28(3): 276-292. Bianco, V.; Barros, J.A.O.; Monti, G. 2009. Three dimensional mechanical model for simulating the NSM FRP strips shear strength contribution to RC beams. Engineering and Struc-tures Journal 31(4): 815-826. Degussa Construction Chemicals Portugal. 2003. Technical Report MBrace Resin 220. Dias, S.J.E. & Barros, J.A.O. 2009. Performance of reinforced concrete T beams strengthened in shear with NSM CFRP laminates. Engineering Structures doi:10.1016/j.engstruct.2009.10.001. EN 206-1. 2000. Concrete - Part 1: Specification, performance, production and conformity. European standard, CEN: 69pp. EN 10002-1. 1990. Metallic materials - Tensile testing. Part 1: Method of test (at ambient temperature). European Stan-dard, CEN: 35pp. ISO 527-5. 1997. Plastics - Determination of tensile properties - Part 5: Test conditions for unidirectional fibre-reinforced plastic composites. International Organization for Stan-dardization (ISO): 9 pp. Rizzo, A. & De Lorenzis, L. 2009. Behaviour and capacity of RC beams strengthened in shear with NSM FRP reinforce-ment. Construction and Building Materials 23: 1555-1567. 2S-4LI45 2S-7LI45 a) b) 

Influence of the concrete properties in the effectiveness of the NSM CFRP laminates for the shear strengthening of RC beams

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Influence of the concrete properties in the effectiveness of the NSM CFRP laminates for the shear strengthening of RC beams

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