A promising strengthening strategy, using carbon fiber reinforced polymer (CFRP) materials, consists in applying CFRP laminate strips into pre-cut slits opened in the concrete cover of the elements to strengthen. This strengthening technique is designated by Near Surface Mounted (NSM) and has been successfully used to increase the flexural and the shear resistance of concrete and masonry structures. The present work describes an efficient strategy, using steel fiber reinforced concrete (SFRC) and NSM CFRP laminates, for the strengthening of existing reinforced concrete (RC) slabs. The use of a SFRC compression overlay can provide the necessary ductility for attaining high level of tensile stress in the CFRP strengthening system and therefore preventing the concrete crushing failure mode. In the present work, the effectiveness of this technique to increase the service and ultimate load carrying capacity of RC slabs is assessed by an experimental program. A numerical strategy was developed to predict the load-deflection relationship of this type of elements. The results are presented and analyzed, and the performance of the numerical model is appraised.The authors acknowledge the Portuguese Science and Technology Foundation (FCT) for the PhD grant number SFRH / BD / 11232 / 2002. Thanks also for the companies “Companhia Geral de Cal e Cimento S.A. (SECIL)”, Sika S.A., “Central do Pego”, “Pedreiras Bezerras”, Bekaert NV, “Degussa Construction Chemicals Portugal S.A.”, S&P® Reinforcement, which have supplied cement; overlay bond product; fly ash; aggregates; steel fibres; superplasticizer and CFRP adhesive; and CFRP laminate, respectively

Barros, Joaquim A. O.

Bonaldo, Everaldo

Lourenço, Paulo B.

Universidade do Minho: RepositoriUM

EFFICIENT STRENGTHENING TECHNIQUE FOR REINFORCED CONCRETE SLABS SFRC and CFRP laminate strips   Everaldo Bonaldo1, Joaquim A. O. de Barros2 and Paulo B. Lourenço2 1PhD Student; 2Associate Professor University of Minho, Department of Civil Engineering, P-4800-058 Guimarães, PORTUGAL   Abstract: A promising strengthening strategy, using carbon fiber reinforced polymer (CFRP) materials, consists in applying CFRP laminate strips into pre-cut slits opened in the concrete cover of the elements to strengthen. This strengthening technique is designated by Near Surface Mounted (NSM) and has been successfully used to increase the flexural and the shear resistance of concrete and masonry structures. The present work describes an efficient strategy, using steel fiber reinforced concrete (SFRC) and NSM CFRP laminates, for the strengthening of existing reinforced concrete (RC) slabs. The use of a SFRC compression overlay can provide the necessary ductility for attaining high level of tensile stress in the CFRP strengthening system and therefore preventing the concrete crushing failure mode. In the present work, the effectiveness of this technique to increase the service and ultimate load carrying capacity of RC slabs is assessed by an experimental program. A numerical strategy was developed to predict the load-deflection relationship of this type of elements. The results are presented and analyzed, and the performance of the numerical model is appraised.  Key words: flexural strengthening; reinforced concrete slabs; CFRP laminate; thin bonded overlay; steel fibre reinforced concrete; epoxy adhesive.   1. INTRODUCTION  The Near-Surface Mounted (NSM) strengthening technique has been used in the recent years, with remarkable efficiency, to increase the flexural strength1-4 and the shear resistance1,5 of reinforced concrete elements. The NSM technique involves the embedment of CFRP bars - of circular, square or rectangular cross-section - into grooves opened on the concrete surface. When compared to the Externally Bonded Reinforcing (EBR) technique, the Everaldo Bonaldo, Joaquim A. O. Barros and Paulo B. Lourenço 2 NSM technique assures a higher anchoring capacity to the FRP. As a consequence, a high tensile stress can be mobilized in the CFRP, as long as the member load carrying capacity is not limited by a premature failure mode. For RC slabs of low or medium concrete strength, the increment of the flexural resistance that NSM can provide might be limited by the maximum allowable compression strain in the most compressed concrete fibre. This drawback can be overcome by adding a concrete layer in the compression zone of the existent slab6. To attain the desired structural performance (e.g. full composite action), the new concrete overlay and the existent concrete slab should behave monolithically. A sound bond between the new layer and the existing concrete slab can be guaranteed if a proper epoxy compound is used7,8.   2. EXPERIMENTAL WORK  2.1 Slab specimens, test set-up and materials  To assess the efficiency of the hybrid strengthening technique for the increase of flexural load carrying capacity of RC slabs, the slab strip specimens represented in Figure 1 were used. The cross section dimensions and the test set up of the tested slab strip specimens are also illustrated in Figure 1. Two unstrengthened RC slabs formed a control set (SL01 and SL06), three slabs were strengthened with CFRP laminates according to NSM technique (SL03S, SL04S and SL08S), and three were strengthened with NSM laminates and a compression SFRC overlay (SL02SO1, SL05SO1 and SL07SO2). The number of CFRP laminate strips applied in each RC slab was evaluated in order to obtain an increase of 50% in the service load, which was assumed as the load producing a mid-span displacement of l/250 = 1800 mm/250 = 7.2 mm. Each slab specimen was tested in simply supported conditions, with a clear span of 1.8 m, and under line loads at 0.6 m from the supports, see Figure 1(b). The monotonic loading was controlled by the LVDT placed at slab mid span (see Figure 1(b)), using a displacement velocity of 20 µm/s up to failure of the slab. Figure 2 outlines the arrangement of the strain gauges (SG) applied to measure the strains in the CFRP laminates, steel bars and concrete. Tables 1 and 2 include the main mechanical properties of the materials used in the present work. In Table 1 fcm is the compressive strength and Ec the elastic modulus, fctm,fl and fctm,ax the flexural and axial tensile strength, respectively, of plain concrete. In Table 2, E is the elastic modulus, σu and εu the ultimate strength and strain, respectively; σsy and εsy the steel yield stress and strain in tension; fcm the compressive strength, feqm,2 and feqm,3 the equivalent flexural tensile strength parameters of SFRC overlays; tf and Wf the thickness and width of the CFRP. Strengthening of RC slabs with CFRP laminate and SFRC overlay 33080507523150 7550epoxy adhesive300507550SFRC overlay2323801502φ6mm75502φ6mm50802φ6mm CFRP laminate(1.4 x 9.4mm2)515 (a) 3001800 mm300Suspension yoke300300LVDT 61472LVDT 19906300ControlLVDT 3468LVDT 3558LVDT 63796300F2F2h (b)  Figure 1. (a) Slab cross-section dimension and disposition of the steel bars and CFRP laminates and (b) load configuration and arrangement of the LVDTs (dimensions in mm).  Laminate strip1800 mm850SG5150SG4SG3150 150150SG7SG6300F2F2SIDE VIEWh50 (a) 9001800150SG5502005050150100 150SG3 SG4Support lineSG2300SG6 SG72 Laminate stripsLoad lineSG1Load lineBOTTOM VIEW75150752φ6mmSupport line (b) 1800900Support line900150150Load lineLoad lineSG8SG9SG82005050TOP VIEWSupport linewith overlaywithout overlay300 (c)  Figure 2. Disposition of the strain gauges: (a) side, (b) bottom and (c) top views.  Everaldo Bonaldo, Joaquim A. O. Barros and Paulo B. Lourenço 4 Table 1. Characteristics of the plain concrete (obtained experimentally9)  Control Slabs  NSM Strengthened Slabs NSM & SFRC Strengthened SlabsProperty SL01 SL06 SL03S SL04S SL08S SL02SO1 SL05SO1 SL07SO2  fcm (MPa) 45.65 49.39 43.13 32.41 49.35 47.76 49.56 47.80  fctm,fl (MPa) 4.98 6.10 5.92 4.73 5.80 5.86 5.59 6.04  fctm,ax a (MPa) 2.75 3.37 3.27 2.61 3.21 3.24 3.10 3.32  Ec a (GPa) 35.67 36.61 35.00 31.82 36.60 36.21 36.66 36.22 a Derived from CEB FIP model code 1990  Table 2. Summary of the characteristics of the steel reinforcement, CFRP and its adhesive, and SFRC and its adhesive (obtained experimentally9)  Steel reinforcement CFRP laminate Laminate adhesive SFRC overlaya Overlay adhesive O1fcm = 38.93MPa feqm,2 = 5.00MPa feqm,3 = 4.12MPa φs = 6mm Es = 217.3GPa σsy = 548.8MPa εsy = 2.70‰ tf = 1.41mm Wf = 9.37mm Ef = 156.1GPa σfu= 2879.1MPa εfu= 18.45‰ Ea = 7.47GPa σau= 33.0MPa εau= 4.83‰ O2fcm = 53.10MPa feqm,2 = 4.83MPa feqm,3 = 3.86MPa Ee = 3.62GPa σeu= 26.56MPaεeu= 10.74‰ a Evaluated according to RILEM TC 162 TDF recommendations as reported in Barros et al.10   2.2 Results and comments  Table 3 includes the maximum load, the maximum concrete compression strain, the maximum strain in the CFRP laminates and the failure modes of the tested slab strips. Due to problems with the data acquisition system, the strains in the SL04S were not measured. The maximum load of the strengthened slabs was about five times higher than the maximum load of the corresponding unstrengthened slabs. The maximum concrete compression strain has exceeded the strain corresponding to the concrete strength. The maximum strains recorded in the CFRP laminates are about 80% of its ultimate strain, but these values do not correspond to the maximum load since the strain gage data acquisition was interrupted for the load values included in brackets. Due to the significant increase of the slab load carrying capacity, provided by the NSM strengthening technique and the hybrid system, some slabs have failed in a flexure/shear combined mode, but for a deflection that was several times the deflection at the yielding of the reinforcement. Strengthening of RC slabs with CFRP laminate and SFRC overlay 5Table 3. Summary of the slab test results  Strengthening Slab I.D. Average ultimate load (kN) Strength increasing ratio (%) Concrete compression strainc (‰) CFRP laminate straind (‰) Type of Failure SL01 2.26[5.35] Flexure Reference SL06 5.03 (9.00%) NA 1.96[4.71] NA Flexure SL03S 3.40[24.24] 14.10[23.13] Flexo-shear SL04S NE NE Flexure CFRP laminate strengthening SL08S 24.48 (1.59%) 386.68 2.90[24.00] 12.70[18.70] Flexure SL02SO1 2.66[35.42] 12.95[34.42] Flexo-shear SL05SO1 2.53[31.66] 13.50[31.50] Flexo-shear CFRP + SFRC strengthening SL07SO2 33.79 (5.56%) 571.77a 38.03b NE 12.58[26.84] Flexure (value) Coefficient of Variation (COV) = (Standard deviation/Average) x 100 a With respect to the reference; b With respect to the CFRP laminate strengthening; c Value in square brackets is the maximum load; d Maximum value recorded in SG7 and corresponding load in square brackets; NA: not applicable; NE: not evaluated  Using the strains recorded in the strain gauges installed on the laminates, the average laminate-concrete bond stresses ( RLbmτ ) developed along the CFRP laminate strips was evaluated (refer to Figure 3(a)). A typical bond stress variation in the CFRP laminate strips is shown in Figure 3(b).  σ fσ f + dσ fτbmεLεRLRLSGRSGLwftf(at the two faces)RLRLτbm = ECFRP ACFRP ∆εRL2  Wf  LRL(MPa)0 5 10 15 20 25 30012345020406080100850SG3150150 300150 150SG5SG4 SG62FSG72F50Laminate stripTotal load, F (kN)SL08S Bond stress, τRLbm (MPa) SG4-SG3 SG5-SG4 SG6-SG5 SG7-SG6 Mid-span deflection (mm)(a) (b)  Figure 3. (a) average bond stress calculation and (b) typical bond stress variation in monitored CFRP laminate strip for the slab SL08S.  Figure 3(b) shows that up to crack initiation, the CFRP laminates were not yet mobilized. In general, at service load, corresponding to a deflection of 7.2 mm at mid-span, the bond stress did not exceed 1.0 MPa. Despite the fact that in some strengthened slabs it was not possible to evaluate the Everaldo Bonaldo, Joaquim A. O. Barros and Paulo B. Lourenço 6 average bond stress variation up to the ultimate load, it can be noticed, however, that the bond stress did not surpass 5.0 MPa. This maximum value is much lower than the bond stress limit value (12 MPa), registered in pullout-bending tests11.   3. NUMERICAL ANALYSIS  Previous works3 have shown that, using a cross-section layered model that takes into account the constitutive laws of the intervening materials and the kinematic and the equilibrium conditions, the deformational behavior of structural elements failing in bending can be predicted from the moment-curvature relation, M - χ, of the representative sections of these elements, using the algorithm described in a former paper12. To evaluate the M - χ relationship, the slab cross section was discretized in layers of 0.5 mm thickness. The slab tangential stiffness matrix was determined evaluating the tangential stiffness matrix of the two nodes Euler-Bernoulli beam elements discretizing the slab (a mesh of 60 elements). The values of the parameters for the models defining the behavior of plain concrete, SFRC, steel bars and CFRP laminates are given elsewhere12. Figure 4 shows that the developed numerical strategy is able of fitting with enough accuracy the registered experimental load-central deflection curves of the tested slabs.  0 10 20 30 40 50 60 70 80 90 100 11005101520253035400.0 2.4 4.8 7.2 9.6 12.02.55.07.510.012.515.0  Experimental  NumericalSL03SSL04SSL08SSL03SSL01Total load, F (kN)Mid-span deflection (mm)SL02S01SL05S01SL07SO2    Figure 4. Experimental versus numerical load-central deflection curves.  Strengthening of RC slabs with CFRP laminate and SFRC overlay 7As a consequence of the increase of the post-cracking stiffness, provided by CFRP laminates, the service load has increased 54% for the slabs strengthened with NSM (see inset of Figure 4). The hybrid strengthening lead to an increase of about 212% in the service load with respect to the reference (see inset of Figure 4).   4. CONCLUSIONS  The testing program carried out demonstrated that the hybrid strengthening technique has great potential application towards flexural strengthening of RC slabs. A percentage of 0.12% of CFRP laminates (CFRP reinforcement to the conventional steel reinforcement ratio close to 50%) has increased about 54% the service load of the 1.8 m RC slabs with a steel reinforcement ratio of 0.24%. However, the slabs strengthened with NSM technique and SFRC showed an increase of approximately 212% in the service load, with respect to the reference slabs. Comparatively to the NSM technique, the hybrid strengthening strategy has lead to an increase of about 103% in load at the service load level. The hybrid strengthening system has also lead to an increase of about 570% in the RC slab maximum load carrying capacity with respect to the reference slabs and an increase of about 40% in comparison with the slabs strengthened only with NSM technique. When compared to the reference case, about 390% of increase in the load carrying capacity was attained by the strengthening with NSM technique. The cracking spacing calculations and crack features observations, at the bottom of the slabs, indicate that a significant improvement in the crack behavior of RC slabs can be achieved with the NSM technique. When compared with the bond stress limit recorded in pullout-bending tests, a very low bond stress profile was observed through the interfaces CFRP laminate-epoxy adhesive-concrete, along the laminate strips in slabs where NSM strengthening was applied. The NSM strengthening system has also provided a significant increase in the stiffness and deformation at failure, which are consistent with the high stress redistribution owing to prominent composite action between the CFRP reinforcement and concrete. Since the hybrid strengthening system has lead to substantial increase in flexural load, the shear capacity of the composite slabs has limited their deformability; however, the stiffness of the slabs has strongly increased and high ductility was maintained. The numerical model developed to simulate the load-deflection relationship of RC elements reinforced strengthened with CFRP laminate strips has reproduced with high accuracy the force-mid span deflection of the carried out tests.   Everaldo Bonaldo, Joaquim A. O. Barros and Paulo B. Lourenço 8 ACKNOWLEDGMENTS  The authors acknowledge the Portuguese Science and Technology Foundation (FCT) for the PhD grant number SFRH / BD / 11232 / 2002. Thanks also for the companies “Companhia Geral de Cal e Cimento S.A. (SECIL)”, Sika S.A., “Central do Pego”, “Pedreiras Bezerras”, Bekaert NV, “Degussa Construction Chemicals Portugal S.A.”, S&P® Reinforcement, which have supplied cement; overlay bond product; fly ash; aggregates; steel fibres; superplasticizer and CFRP adhesive; and CFRP laminate, respectively.   REFERENCES  1. L. De Lorenzis, A. Nanni, and A. La Tegola, Flexural and shear strengthening of reinforced concrete structures with near surface mounted FRP rods, In Proceeding of the 3rd Int. Conf. on Advanced Composite Materials in Bridges and Structures.J. Humar and AG Razaqpur, Editors, Ottawa, Canada, pp.521-528, 15-18 Aug. (2000). 2. A. Carolin, Carbon fibre reinforced polymers for strengthening of structural elements, Doctoral Thesis, Division of Structural Engineering, Luleå University of Technology, Luleå, Sweden, p. 190, Jun. (2003). 3. J. A .O. Barros, and A. S. Fortes, Flexural strengthening of concrete beams with CFRP laminates bonded into slits, Cement. Concr. Compos. 27(4), 471-480 (2005). 4. R. Kotynia, Strengthening of reinforced concrete structures with near surface mounted FRP reinforcement, 5th International Conference - Analytical models and new concepts in concrete and masonry structures AMCM 2005, p. 8, Gliwice - Ustron, 12-14 Jun. (2005). 5. J. A. O. Barros, and S. J. E. Dias, Shear strengthening of reinforced concrete beams with laminate strips of CFRP, International Conference Composites in Constructions - CCC2003, Cosenza, Italy, pp. 289-294, 16-19 Sept. (2003). 6. J. A. O. Barros and J. M. Sena-Cruz, Strengthening a prestressed concrete slab by epoxy - bonded FRP composites and SFRC overlayer, 7th Int. Conf. on Inspection Appraisal Repairs & Maintenance of Buildings & Structures, Nottingham Trent University, UK, 11-13 Sept. (2001). 7. E. Bonaldo, J. A. O. Barros, and P. B. Lourenço, Bond characterization between concrete base and repairing SFRC by pull-off tests, Report 04-DEC/E-13, May (2004). 8. E. Bonaldo, J. A. O. Barros, and P. B. Lourenço, Bond characterization between concrete substrate and repairing SFRC using pull-off testing, Int. J. Adhes. Adhes. 25(6), 463-474 (2005). 9. E. Bonaldo, J. A. O. Barros, and P. B. Lourenço, Steel fibre reinforced concrete and CFRP laminate strips for high effective flexural strengthening of RC slabs, Report 05-DEC/E-14, Oct. (2005). 10. J. A. O. Barros, V. M. C. F. Cunha, A. F. Ribeiro, and J. A. B. Antunes, Post-cracking behaviour of steel fibre reinforced concrete, Mater. Struct. 38 (275), 47-56 (2005). 11. J. M. Sena-Cruz, and J. A. O. Barros, Bond between near-surface mounted CFRP laminate strips and concrete in structural strengthening, J. Compos. Construct. 8(6), 519-527 (2004). 12. E. Bonaldo, J. A. O. Barros, and P. B. Lourenço, Concrete slabs strips reinforced with epoxy-bonded carbon laminates into slits, 3rd Int. Conf. on Construction Materials: Performance, Innovations and Structural Implications, Vancouver, CA, 22-24 Aug. (2005). 

Efficient strengthening technique for reinforced concrete slabs - SFRC and CFRP laminate strips

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Efficient strengthening technique for reinforced concrete slabs - SFRC and CFRP laminate strips

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Universidade do Minho: RepositoriUM