Microsoft Word - 2232 E. Grande et alii, Frattura ed Integrità Strutturale, 47 (2019) 321-333; DOI: 10.3221/IGF-ESIS.47.24 321 Fracture and Structural Integrity: ten years of ‘Frattura ed Integrità Strutturale’ Numerical simulation of the de-bonding phenomenon of FRCM strengthening systems Ernesto Grande University Guglielmo Marconi, Department of Sustainability Engineering, via Plinio 44, 00193-Roma, Italy e.grande@unimarconi.it, http:// orcid.org/0000-0002-3651-1975 Maura Imbimbo, Sonia Marfia University of Cassino and Southern Lazio, Dep. of Civil and Mechanical Engineering, via G. Di Biasio 43, Cassino, Italy mimbimbo@unicas.it, http://orcid.org/0000-0003-3163-3073 marfia@unicas.it, http://orcid.org/0000-0002-2166-9788 Elio Sacco University of Naples Federico II, Department of Structures in Engineering and Architecture, Via Claudio 21, Naples. Italy Elio.sacco@unina.it http:// orcid.org/0000-0002-3948-4781 ABSTRACT. Aim of the paper is to present a one dimensional simple model for the study of the bond behavior of Fabric Reinforced Cementitious Matrix (FRCM) strengthening systems externally applied to structural substrates. The equilibrium of an infinitesimal portion of the reinforcement and the mortar layers composing the strengthening systems allows to derive the governing equations. An analytical solution is determined solving the system of differential equations. In particular, in the first part of the paper a nonlinear shear-stress slip law characterized by a brittle post-peak behavior with a residual shear strength in the post peak phase is introduced for either the lower reinforcement-mortar interface (approach 1) or both the lower and the upper interface (approach 2). In the latter approach, a calibration of the shear strength of the upper interface is proposed in order to implicitly account for the effect of the damage of the mortar on the bond behavior. In the second part of the paper it is presented the solution of the problem in the case of softening behavior by approximating the shear-stress slip law throughout a step function. Comparisons with experimental data, available in literature, are presented in order to assess the reliability of the proposed approach. KEYWORDS. FRCM; De-bonding; Analytical model; Interface. Citation: Grande, E., Imbimbo, M., Marfia, S., Sacco, E., Numerical simulation of the de- bonding phenomenon of FRCM strengthening systems, Frattura ed Integrità Strutturale, 47 (2019) 321-333. Received: 23.10.2018 Accepted: 24.11.2018 Published: 01.01.2019 Copyright: © 2019 This is an open access article under the terms of the CC-BY 4.0, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. http://www.gruppofrattura.it/VA/47/2232.mp4 E. Grande et alii, Frattura ed Integrità Strutturale, 47 (2019) 321-333; DOI: 10.3221/IGF-ESIS.47.24 322 INTRODUCTION he reinforcement of existing structures has always been a relevant problem both in the technical and scientific civil engineering community. Lately, the study and design of new reinforcement materials is a challenging issue. In particular, fabric reinforced cementitious matrix (FRCM) is an emerging strengthening system obtained embedding a grid of the carbon, glass or aramid reinforcement in an inorganic matrix. In general, the matrix is applied as a double layer incorporating the reinforcement. Nowadays, FRCM systems are used in the current practice to reinforce concrete and masonry structures. Some experimental investigations [1-7] and theoretical/numerical studies ([2, 3, 8-16] on FRCM strengthening systems are available in the recent literature. They testify the efficacy and advantages of FRCM systems together with the need to investigate aspects specifically characterizing the bond behavior of this new family of strengthening systems. The experimental investigations are mainly shear-lap tests that analyze the local bond behavior of FRCMs. From the experimental evidence different failure mechanisms can occur, such as a cohesive failure of the substrate, de-bonding at the reinforcement/substrate interface, de-bonding at the reinforcement/matrix interface, sliding of the reinforcement, tensile failure of the reinforcement in the un-bonded portion and tensile failure of the reinforcement within the mortar. The above mechanism occurrence depends on the characteristics of the strengthening system as well as of the support, such as the mechanical properties of the materials, the thickness of the mortar layers and the configuration of the reinforcement. These mechanisms particularly underline the role of additional phenomena to be necessarily considered for the study and the development of theoretical models/design formulas specific for FRCMs. Regarding the theoretical and numerical studies, the approaches available in literature show particular interest to the derivation of simple laws able to simulate the de-bonding phenomenon and, moreover, models able to account for additional phenomena specific of FRCMs. In [2] the shear stress-slip law at the interface level was obtained throughout a procedure applied to steel and carbon FRCM strengthening systems externally embedded on masonry supports. The procedure was carried out by directly considering the experimental data and in particular the strain gauge measurements. A procedure for the derivation of the shear stress-slip law for FRCMs based on the experimental data was also proposed in [3]. In particular, the authors firstly estimated the fracture energy by using the well-known formula derived by the theory of linear fracture mechanics and, subsequently, they performed numerical FE analyses to detect the optimal values of the parameters of the cohesive shear stress-slip law. In[4] the bond behavior of FRCM-to-concrete was analytically examined by using a general approach applied to the case of FRP-materials. The results emerged from this study particularly emphasized the role of the pronounced descending branch of the calibrated laws in leading to large values of the effective anchorage length. In addition, lower values of bond shear stresses on the concrete surface with respect to those typically characterizing FRP strengthening systems were observed. A similar approach was also presented in [3] for the case of carbon-FRCM materials externally applied on masonry supports. In [13] two approaches for numerically studying the bond behavior of masonry specimens strengthened with FRCMs were proposed. The first one, consisted of an analytical-numerical approach specifically accounting for the interaction between the reinforcement and the mortar; the second approach consisted of a full 3D-FEM non-linear approach obtained as an extension of the procedure originally adopted in [13] and in [15]. A recent study presented in [9,16] was mainly devoted to investigate the influence of the upper mortar layer on the bond behavior of FRCM-strengthening systems applied on structural supports. In particular, the authors carried out a theoretical modeling approach based on the solution of a system of differential equations obtained by introducing equilibrium considerations. From the study emerged interesting aspects concerning the role of the upper mortar layer on the debonding process of FRCMs. Among these, it was observed that increasing the applied load after the occurrence of the de-bonding between the reinforcement and the upper interface it does not lead to further increases of the peak value of normal stresses of the upper mortar. On the other hand, after the occurrence of the first crack at the upper mortar, only the peak value of slips at the lower interface continues to increase whilst the peak value of slips at the upper interface does not significantly increase. In [10] it was proposed a simple approach for the study of the bond behavior of FRCM applied to concrete supports able to enable the use of a common interface modeling strategy by implicitly introducing the effect of the damage of the matrix into the shear behavior of the reinforcement/mortar interface layer. In this paper a one dimensional simple model, based on the one presented in [9] and in [16], is proposed for the study of the bond behavior of FRCM strengthening systems externally applied to masonry substrates. The model is mainly T E. Grande et alii, Frattura ed Integrità Strutturale, 47 (2019) 321-333; DOI: 10.3221/IGF-ESIS.47.24 323 characterized by the derivation of the explicit solution of a system of differential equations obtained by considering the equilibrium of an infinitesimal portion of the reinforcement and the mortar layers composing the strengthening systems. In order to model the slip between the reinforcement and the upper and lower mortar layers, two approaches are considered. The first approach (denoted in the following approach 1), considers a nonlinear behavior of the lower reinforcement/mortar interface only, by considering a shear stress-slip constitutive law characterized by a linear brittle behavior with a residual strength in the post-peak phase. On the other hand, the approach 2 assumes a nonlinear behavior for both the lower and the upper reinforcement/mortar interface, still considering a shear stress-slip constitutive law characterized by a linear fragile behavior with a residual strength in the post-peak phase. Moreover, in the latter approach, a calibration of the shear strength of the upper interface is proposed in order to implicitly account for the effect of the damage of the mortar on the contribution of this component of the strengthening system. In addition to these approaches, in the second part of the paper is presented the analytical solution in case of a shear stress-slip law characterized by a linear softening behavior in the post-peak phase. In particular, a step function approximating the law together with the procedure carried out from the approach 2 are used in order to derive the analytical solution. The proposed approaches are validated in the paper by considering experimental results derived from the literature. Moreover, the results are also compared with the ones obtained by the model recently proposed by [9,16], where, differently from the proposed approaches, the damage of the upper mortar was explicitly introduced in the model by assuming a nonlinear behavior in terms of normal stress-strain for the upper mortar layer. Although this assumption allows to account for the phenomena generally observed, it leads to a computational effort significantly greater than the one characterized the two approaches proposed in this paper. ACCOUNTED MODEL AND APPROACHES he model here considered for the study of the bond behavior of FRCM systems externally applied on masonry or concrete supports is based on the work in [9, 16]. Indeed, considering the scheme shown in Fig. 1, the main components characterizing the model are: a cohesive support, a lower mortar layer, a lower interface, the strengthening, an upper interface and an upper mortar layer. Introducing a reference axis x in the direction of the reinforcement system and fixing the origin in correspondence of the unloaded section, the equilibrium of forces characterizing an infinitesimal portion of the reinforcement and the upper mortar layer (see Fig. 1) leads to the following system of differential equations governing the problem of the bond behavior:       0 0 p e e i i p p p e e e ec p c p d b t s s b dx d b t s b dx                 (1) where p and e c are the normal stresses in the reinforcement and in the upper mortar, respectively; pt and e ct are the thicknesses of the reinforcement and the upper mortar, respectively; i and e are the shear stresses at lower and upper interfaces, respectively, both depending on the corresponding slips is and es ; pb is the width of the reinforcement. Introducing the following hypotheses: - the support and the lower mortar layer are assumed to be rigid; - the (lower and upper) mortar/reinforcement interfaces are modeled as zero-thickness elements with only shear deformability; - the upper mortar layer and the reinforcement are assumed deformable only axially. it is possible to write the displacements of both the reinforcement and the upper mortar layer (namely pu and e cu , respectively) as functions of the slip of the lower and upper interfaces: i p e i e c u s u s s    (2) T E. Grande et alii, Frattura ed Integrità Strutturale, 47 (2019) 321-333; DOI: 10.3221/IGF-ESIS.47.24 324 Figure 1: Schematic of an infinitesimal portion of the strengthening system and the upper mortar component used for performing the equilibrium of the involved forces. Considering a linear-elastic behavior for both the reinforcement and the mortar: i p p p p e i e e c c c c du ds E E dx dx du ds ds E E dx dx dx             (3) the system of differential Eqns. (1) becomes:       2 12 2 2 22 2 0 0 i e e i i i e e e d s K s s dx d s d s K s dx dx                   (4) where 1K and 2K are two constants equal to: 1 2 1 1 , e p p c c K K E t E t   (5) Considering the system (4), the explicit solution is here derived by introducing different shear stress-slip laws characterizing the behavior of the reinforcement/mortar interface. Approach 1: nonlinear behavior of the lower interface A preliminary approach is based on the assumption of a linear-fragile behavior with a residual shear strength in the post- peak stage only for the lower interface: 1( ) ( ) otherwise i i i i i i i i res s G s s s s         (6) where ires is the residual value of the shear strength in the post-peak stage, and iG is the shear stiffness of the lower interface in the pre-peak stage. Differently, a linear-elastic behavior is assumed for the upper interface ( )e e e es G s  , where eG is the shear stiffness of the upper interface. P p pd p c cd c e e isupport lower interface strengthening upper interface upper mortar lower mortar x E. Grande et alii, Frattura ed Integrità Strutturale, 47 (2019) 321-333; DOI: 10.3221/IGF-ESIS.47.24 325 On the basis of these assumptions it is evident that, after the attainment of the slip threshold value at the lower interface, two different parts characterize the behavior of the specimen: part “1” where the upper mortar and the interfaces are both in the pre-peak stage and part “2” where the upper mortar and the upper interface are both in the pre-peak stage while the lower interface is de-bonded for a length a, representing an unknown of the problem. Consequently, four differential equations govern the problem. The first two equations are derived by considering the equilibrium involving an infinitesimal portion of the strengthening system in the part “1”: 2 1 3 1 12 2 2 1 1 4 12 2 0 0 0 i e i i e e d s K s s dx x L a d s d s K s dx dx                   (7) where: 3 1 4 2, , i e e e G K K G K K G G    . The other two equations are obtained through the equilibrium involving an infinitesimal portion of the strengthening system of the part “2”: 2 2 3 22 2 2 2 2 4 22 2 0 0 i e i e e d s K s dx L a x L d s d s K s dx dx                   (8) where i res eG    . The system of differential Eqns. (7) and (8) has an analytical solution that depends on eight constants of integration determined by introducing suitable boundary conditions. In particular, the following conditions are indeed enforced:                         1 1 2 1 2 1 2 1 2 1 2 1 1 0 0 0 0 0 P e e c c e e c c e e p p i i e e i L L a L a L a L a s L a s L a s L a s L a s L a s                         (9) The solution is graphically reported in Fig. 2 by considering a length value of the part “2” equal to a=50 mm, a residual value of shear strength equal to zero and the data reported in Tab. 1. Approach 2: nonlinear behavior of both the interfaces As shown in [11,17], the damage of the upper mortar generally occurs before the slipping of the reinforcement/mortar interfaces by particularly influencing the shear stress transfer mechanism. This phenomenon is here simple introduced by considering an elastic-fragile behavior also for the upper interface and assuming for this component of the strengthening system a bond strength equal to the shear stress corresponding to the attainment of the tensile strength of the upper mortar layer. In other words, the effect of the damage of the upper mortar is implicitly introduced into the behavior of the upper interface. E. Grande et alii, Frattura ed Integrità Strutturale, 47 (2019) 321-333; DOI: 10.3221/IGF-ESIS.47.24 326 a) b) c) Figure 2: Approach 1: a) shear stress developing at the interfaces; b) slip of the interfaces; c) normal stresses at the upper mortar layer. symbol [unit] value Young’s modulus of the reinforcement Ep [MPa] 206000 Young’s modulus of the mortar Ec [MPa] 7000 equivalent thickness of the reinforcement tp [mm] 0.054 thickness of the mortar tc [mm] 4 width of the reinforcement bp [mm] 60 width of the mortar bc [mm] 60 bond length L [mm] 1000 Table 1: Data accounted for numerical analyses. Considering this assumption, the system of equations governing the problem has to account for the development of three possible parts characterizing the behavior of the specimen: part “1”: 0> /ColorImageDict << /QFactor 0.15 /HSamples [1 1 1 1] /VSamples [1 1 1 1] >> /JPEG2000ColorACSImageDict << /TileWidth 256 /TileHeight 256 /Quality 30 >> /JPEG2000ColorImageDict << /TileWidth 256 /TileHeight 256 /Quality 30 >> /AntiAliasGrayImages false /CropGrayImages true /GrayImageMinResolution 300 /GrayImageMinResolutionPolicy /OK /DownsampleGrayImages true /GrayImageDownsampleType /Bicubic /GrayImageResolution 300 /GrayImageDepth -1 /GrayImageMinDownsampleDepth 2 /GrayImageDownsampleThreshold 1.50000 /EncodeGrayImages true /GrayImageFilter /DCTEncode /AutoFilterGrayImages true /GrayImageAutoFilterStrategy /JPEG /GrayACSImageDict << /QFactor 0.15 /HSamples [1 1 1 1] /VSamples [1 1 1 1] >> /GrayImageDict << /QFactor 0.15 /HSamples [1 1 1 1] /VSamples [1 1 1 1] >> /JPEG2000GrayACSImageDict << /TileWidth 256 /TileHeight 256 /Quality 30 >> /JPEG2000GrayImageDict << /TileWidth 256 /TileHeight 256 /Quality 30 >> /AntiAliasMonoImages false /CropMonoImages true /MonoImageMinResolution 1200 /MonoImageMinResolutionPolicy /OK /DownsampleMonoImages true /MonoImageDownsampleType /Bicubic /MonoImageResolution 1200 /MonoImageDepth -1 /MonoImageDownsampleThreshold 1.50000 /EncodeMonoImages true /MonoImageFilter /CCITTFaxEncode /MonoImageDict << /K -1 >> /AllowPSXObjects false /CheckCompliance [ /None ] /PDFX1aCheck false /PDFX3Check false /PDFXCompliantPDFOnly false /PDFXNoTrimBoxError true /PDFXTrimBoxToMediaBoxOffset [ 0.00000 0.00000 0.00000 0.00000 ] /PDFXSetBleedBoxToMediaBox true /PDFXBleedBoxToTrimBoxOffset [ 0.00000 0.00000 0.00000 0.00000 ] /PDFXOutputIntentProfile () /PDFXOutputConditionIdentifier () /PDFXOutputCondition () /PDFXRegistryName () /PDFXTrapped /False /CreateJDFFile false /Description << /ARA /BGR /CHS /CHT /CZE /DAN /DEU /ESP /ETI /FRA /GRE /HEB /HRV (Za stvaranje Adobe PDF dokumenata najpogodnijih za visokokvalitetni ispis prije tiskanja koristite ove postavke. Stvoreni PDF dokumenti mogu se otvoriti Acrobat i Adobe Reader 5.0 i kasnijim verzijama.) /HUN /ITA /JPN /KOR /LTH /LVI /NLD (Gebruik deze instellingen om Adobe PDF-documenten te maken die zijn geoptimaliseerd voor prepress-afdrukken van hoge kwaliteit. De gemaakte PDF-documenten kunnen worden geopend met Acrobat en Adobe Reader 5.0 en hoger.) /NOR /POL /PTB /RUM /RUS /SKY /SLV /SUO /SVE /TUR /UKR /ENU (Use these settings to create Adobe PDF documents best suited for high-quality prepress printing. Created PDF documents can be opened with Acrobat and Adobe Reader 5.0 and later.) >> /Namespace [ (Adobe) (Common) (1.0) ] /OtherNamespaces [ << /AsReaderSpreads false /CropImagesToFrames true /ErrorControl /WarnAndContinue /FlattenerIgnoreSpreadOverrides false /IncludeGuidesGrids false /IncludeNonPrinting false /IncludeSlug false /Namespace [ (Adobe) (InDesign) (4.0) ] /OmitPlacedBitmaps false /OmitPlacedEPS false /OmitPlacedPDF false /SimulateOverprint /Legacy >> << /AddBleedMarks false /AddColorBars false /AddCropMarks false /AddPageInfo false /AddRegMarks false /ConvertColors /ConvertToCMYK /DestinationProfileName () /DestinationProfileSelector /DocumentCMYK /Downsample16BitImages true /FlattenerPreset << /PresetSelector /MediumResolution >> /FormElements false /GenerateStructure false /IncludeBookmarks false /IncludeHyperlinks false /IncludeInteractive false /IncludeLayers false /IncludeProfiles false /MultimediaHandling /UseObjectSettings /Namespace [ (Adobe) (CreativeSuite) (2.0) ] /PDFXOutputIntentProfileSelector /DocumentCMYK /PreserveEditing true /UntaggedCMYKHandling /LeaveUntagged /UntaggedRGBHandling /UseDocumentProfile /UseDocumentBleed false >> ] >> setdistillerparams << /HWResolution [2400 2400] /PageSize [612.000 792.000] >> setpagedevice