Anchorage Devices Ud to Improve the Performance of
Reinforced Concrete Beams Retrofitted with FRP
meijiComposites:State-of-the-Art Review
R.Kalfat 1;R.Al-Mahaidi,M.ASCE 2;and Scott T.Smith,M.ASCE 3
Abstract.The anchorage of fiber-reinforced polymer (FRP)composites when applied to reinforced concrete (RC)structures as externally bonded reinforcement is an effective means to achieve higher levels of fiber utilization prior to premature debonding failure.Commonly documented anchorage methods for FRP-to-concrete applications demonstrating encouraging results include FRP U-jackets,FRP anchors (also known as spike anchors,among other names),patch anchors (utilizing unidirectional and bidirectional fabrics),nailed metal plates (also known as hybrid bonding),near-surface mounted rods,mechanical fastening,concrete embedment,and mechanical substrate strengthening.Anchorages applied to FRP systems have been verified through experimental testing and numerical modeling to increa the ductility,deformability,and strength of the member and also prevent,delay,or shift the critical mode of FRP debonding failure.Although the benefits of anchorage solutions have now been widely acknowledged by rearchers,further studies are required in order to e
stablish reliable design formulations to negate the requirement for ongoing laboratory verification by industry.The prent paper is a state-of-the-art review of experimental studies conducted in the area of FRP anchorage systems applied to FRP-strengthened RC flexural members.Available experimental data are compiled and catalogued and an anchorage efficiency factor for each anchorage type under investigation is assigned in order to quantify the anchor ’s efficiency.Finally,current shortcomings in knowledge are identified,in addition to areas needing further investigation.DOI:10.1061/(ASCE)CC.1943-5614.0000276.©2013American Society of Civil Engineers.
CE Databa subject headings:Fiber reinforced polymer;Anchors;Fastening;Concrete beams;Rehabilitation;Composite materials;State-of-the-art reviews.
Author keywords:Fiber-reinforced polymer (FRP);U-jackets;Anchor;Spike;Mechanical fastening;Bidirectional fabric;Substrate strengthening.
Introduction
The retrofitting of existing reinforced concrete (RC)structures has become necessary due to environmental degradation,changes in usage,and heavier loading conditions.In the forefront of retrofi
t-ting technology is the u of advanced fiber-reinforced polymer (FRP)composites applied to structural members as externally bonded reinforcement (Bank 2006;Hollaway and Teng 2008;Karbhari and Abanilla 2007).The suitability of this material when compared,for example,to structural steel is largely due to its light weight,superior tensile strength,and its resistance to corrosion.The FRP materials are typically applied to the concrete surface using epoxy resin after adequate surface preparation of the con-crete,typically involving sandblasting,water jetting,and the appli-cation of a suitable primer.Once applied,up to ven days of curing is typically required to achieve the full bond strength of the system (Hag-Elsafi et al.2001).
However,FRP solutions are not without their inherent short-comings.For instance,it is widely recognized that failure of RC structures retrofitted with FRP almost always occurs by debonding of the FRP from the concrete substrate.To prevent this type of fail-ure,national standards and design guidelines impo strict limita-tions on the allowable strain level in the FRP which may be safely utilized in design.To achieve acceptable levels of concrete-FRP contact bond stress,allowable strains are lower in cas where a higher degree of strengthening is required and can be as low as 10–25%of the material rupture strain (Kalfat and Al-Mahaidi 2011).Low levels of efficiency are often the result of using higher modulus fibers and multiple layers of FRP.In practice the limitations resul
t in vere underutilization of the FRP material properties.Anchor-age of the FRP is one means to significantly improve the efficiency of FRP systems and hence provide a solution to the shortcomings.Extensive rearch has been undertaken to understand the mech-anisms of FRP application and failure and has resulted in design guidelines being published all around the world within the last decade [e.g.,International Federation for Structural Concrete (fib)2001;Japan Society of Civil Engineers (JSCE)2001;Concrete Society 2004;American Concrete Institute (ACI)2008;Oehlers et al.2008].It is understood that the bond strength of FRP materials can be improved when sufficient anchorage is provided and such provisions have been acknowledged to delay or prevent the critical mode of FRP debonding failure (Galal and Mofidi 2010).In addi-tion,anchorage devices can be esntial to transfer the stress from one structural component to another where application is limited by the geometrical configuration.A popular example is the shear strengthening of T-shaped ctions (Ceroni et al.2008).
1Ph.D.Candidate,Swinburne Univ.of Technology,Melbourne,Australia (corresponding author).E-mail:rkalfat@swin.edu.au 2
Professor of Structural Engineering,Swinburne Univ.of Technology,Melbourne,Australia.E-mail:ralmahaidi@swin.edu.au 3
Associate Professor,Dept.of Civil Engineering,Univ.of Hong Kong,Pokfulam,China.E-mail:stsmith@hku.hk
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Note.This manuscript was submitted on June 29,2011;approved on December 15,2011;published online on December 20,2011.Discussion period open until July 1,2013;parate discussions must be submitted for individual papers.This paper is part of the Journal of Composites for Con-struction ,V ol.17,No.1,February 1,2013.©ASCE,ISSN 1090-0268/2013/1-14-33/$25.00.
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The primary obstacle prently preventing the widespread u of FRP anchorage measures is that no rational and reliable design rules currently exist.As a result,FRP design guidelines stipulate that the practical implementation of anchorage devices should be substantiated by reprentative experi
mental testing(ACI2008). However,the guidelines do not specify the types of testing proce-dures that are considered adequate(Grelle and Sneed2011).The repercussions of time and budget constraints on small and large scale industrial projects mean that such testing is rarely carried out in practice.As a result,the potential benefits of FRP anchorages have typically been superded by more conrvative strengthening approaches such as ction enlargement or column inrtion.
Although anchorage devices applied to the ends of FRP rein-forcements have been tested by many rearchers,the results have been limited by ca dependency with relatively small sample sizes being employed for each study.This paper provides a review of reprentative experimental studies conducted on the major anchor-age concepts by drawing upon a wide lection of publications.The paper assumes a largely qualitative style by physically explaining each anchor concept with the aid of appropriate diagrams.Informa-tion about typical experimental investigations undertaken on each anchor type and descriptions of behavior and failure are given. Databas are also asmbled from available test results and effi-ciency factors are calculated for each anchor concept.Such calcu-lations reprent the quantitative aspect of the paper.While it is recognized that anchorages can be of benefit to a variety of FRP-strengthened elements such as connections,wall,and beams members,emphasis has been given in this paper to flexural mem-bers strengthened in flexure and s
hear becau the constitute the most common strengthening situations.Finally,the terms retro-fitting and strengthening are ud interchangeably throughout the paper.
Mechanisms of FRP Failure and Debonding for Flexurally Strengthened Members
To date,veral failure modes for RC beams strengthened in flexure with FRP plates have been identified from experimental investiga-tions and the are shown in Fig.1.The modes are summarized as (1)concrete crushing,(2)FRP rupture,(3)shear failure,(4)con-crete cover paration failure(Yao and Teng2007),(5)plate end interfacial debonding(Leung and Yang2006),(6)intermediate flexural or flexural-shear crack-induced interfacial debonding (otherwi known as IC debonding)Teng et al.2003;Ombres 2010),and(7)shear-induced debonding[also referred to as critical diagonal crack(CDC)debonding](Oehlers and Seracino2004; Wang and Zhang2008).Modes4to7are all premature debonding failures.Of the,modes4and5initiate at or near the plate end, while modes6and7initiate away from the plate end.In addition, modes5and6,and sometimes mode7,occur at the FRP-to-concrete interface(in the concrete),while modes4and7can occur predominantly at the internal steel reinforcement level.Detailed accounts of all failure modes are provided elwhere(Hollaway and Teng2008).
Many factors control the likelihood of a particular debonding failure mode,including(1)the level of internal steel reinforcement, (2)the distance between a plate end and the adjacent beam support (plate end distance),(3)FRP plate length,width,thickness,and modulus of elasticity,(4)shear-to-moment interaction,(5)concrete strength,and(6)ction geometry(Teng and Yao2007).Obrva-tions suggest that as the plate end moves further away from the support,cover paration failure becomes the controlling mode, whereas IC debonding governs when the distance between the plate end and support is relatively small(Yao and Teng2007).In addi-tion,the probability of debonding initiating near the plate end has been found to be the highest when the ratio of maximum shear force to bending moment is high,such as the higher peeling stress generated at the ends of the external plate.Therefore,slender beams with high shear span/depth ratios do not prent a need for plate end anchorage becau failures are initiated in regions of high bending moment well away from the plate ,Garden and Hollaway 1998).The are just some of many qualitative obrvations to be found in the published literature.
Anchorage Devices for FRP Reinforcement Ud to Strengthen Members in Flexure
Three general categories of anchorage type have been investigated to date to prevent debonding in RC members strengthened in flexure with FRP,namely
1.U-jacket anchors(Smith and Teng2003;Al-Amery and
Al-Mahaidi2006;Pham and Al-Mahaidi2006;Yalim et al.
2008);
2.Mechanically fastened metallic anchors(Garden and Hollaway
1998;Spadea et al.1998;Jenn et al.1999;Duthinh and Starnes2001;Wu and Huang2008);
归结
and
Fig.1.Types of FRP debonding(adapted from Pham and Al-Mahaidi2004)
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3.FRP anchors (Lam and Teng 2001;Eshwar et al.2005;Micelli et al.2010;Smith 2010;Zhang and Smith 2012a ,b ;Zhang et al.2012).FRP U-Jacket Anchors
FRP U-jacket anchors involve the application of unidirectional or bidirectional fiber to the ends of flexural FRP reinforcement (Fig.2)to prevent or delay debonding initiating from the plate end.U-jackets can also be placed along the length of the member to prevent or delay debonding initiating away from the plate end.The ultimate function of a U-jacket is to provide the confinement necessary to resist the tensile peeling stress and longitudinal crack propagation at fiber termination points or intermediate cracks.Khan and Ayub (2010)investigated anchorage heights ranging from 100–200mm and suggested that U-shaped ancho-rages were effective irrespective of their height.The study deter-mined that 100mm partial-height U-wraps delivered the same effectiveness as full-height U-wr
aps becau in both cas failure was by concrete crushing.Becau concrete crushing was obrved for the shorter length jackets,the true potential of full-height jackets could not be utilized.
Debonding failure modes can change due to the addition of FRP U-jackets.For example,Smith and Teng (2003)showed that with the addition of plate-end U-jackets,the critical debonding failure mode could be shifted from concrete cover paration to IC de-bonding.Therefore,in an effort to prevent failure by IC debonding,the placement of U-jackets throughout the span or in the flexural-shear zones (at certain spacings)has been investigated by veral rearchers to date (Al-Amery and Al-Mahaidi 2006;Khan and Ayub 2010;Pham and Al-Mahaidi 2006;Yalim et al.2008).Although lacking in material efficiency,this method has been proven to result in FRP rupture.Such an arrangement of U-jackets is also ud for shear strengthening applications.Selected studies are summarized in the following.
IC debonding in beams retrofitted with U-jacket anchors was re-ported by Pham and Al-Mahaidi (2006).The experimental program comprid 260×140mm RC beams tested under three-and four-point bending.Anchorages encompassing unidirectional fibers of 209GPa modulus were placed at the carbon FRP (CFRP)plate ends or at a spacing of 180mm within the shear zone.Each jacket com-prid two plies of fabric that was 0.175mm thick and 50mm wide,which was bonded to the sides an
d the soffit of the concrete beam to form a U-shape.While the end U-jacket proved to be effective in limiting both forms of end ,end cover paration fail-ure and end interfacial debonding,the critical failure mode was en to shift to intermediate-span debonding at a higher load,and it often occurred together with rupture of the end U-jacket.Such behavior was also obrved in Smith and Teng ’s (2003)study.The rupture was due to a sliding action of the CFRP reinforcement underneath
the U-jacket,causing bending of the jacket legs near the soffit.The experiments also confirmed that the placement of U-jackets in the shear span at certain spacing can postpone the occurrence of IC de-bonding.The inclusion of U-jackets in the shear zone had the dual benefits of resisting the opening of flexural-shear cracks and improv-ing the CFRP-to-concrete bond strength by the incread level of confinement underneath the U-jacket.
To further understand the confining action of FRP U-jacket anchors,the vertical strain distribution within the vertical FRP legs was investigated by Sawada et al.(2003).The strains reported reached values of 3;000μεin the cover region of the concrete and at a load level expected to produce debonding.Further load application resulted in 6;000μεbeing recorded at the maximum loading point.This is indicative that the CFRP U-jacket was resisting the stress that typically result in cover
paration failure.Further rearch conducted by Al-Amery and Al-Mahaidi (2006)determined that the u of the CFRP U-jackets at 200mm spacing along the length of the beam reduced the interfacial slip between the CFRP flexural fiber and the concrete ction by up to one-tenth.In this study,the U-jackets lead to the full utilization of the CFRP flexu-ral tensile capacity.The results demonstrated an increa in flexural strength of up to 95%when using CFRP U-jackets to anchor the CFRP fiber.However,when using conventional CFRP fibers alone,an increa of only 15%was achieved.
Yalim et al.(2008)also conducted investigations on the effects of U-jacket configurations placed throughout the span as oppod to only the plate ends.A total of 26beams were tested in 3-point loading with 4,7,11,and continuous U-jacket arrangements.The study utilized FRP U-jackets to anchor both FRP laminates (modulus of elasticity of 131GPa)and FRP sheets (modulus of elasticity of 70.6GPa).In addition,three alternative surface profiles were inves-tigated:smooth,intermediate,and rough.However,each surface pro-file was not appropriately defined (except by broad definition)and as a result,the categorization is not an appropriate definition of surface roughness.The u of four U-jackets at the FRP ends was successful in preventing the end interfacial debonding failure that was obrved in unanchored specimens,and failure was shifted to IC debonding,confirming the findings o
f earlier rearchers.The beams with ven jackets failed in the same way at a higher load together with U-jacket debonding.Specimens with eleven jackets and full continuous jack-ets failed by rupture of FRP.Although the strain utilization levels and ultimate load capacity were improved with the addition of U-jackets throughout the span,it was found that a higher level of anchorage improved the ductility more than it did the strength.However,the ductility measurements were solely bad on the maximum vertical deflection for the beams prior to failure.Ductility can be defined as the RC beam ’s ability to deform under tensile stress and can be de-termined by monitoring deflection,beam curvature,or strain in the tensile reinforcement.Monitoring beam deflection may be indicative of ductile behavior,but the method fails to consider deformability in terms of beam curvature and cracking (measured from tensile reinforcement strain).In addition,most FRP design guidelines check strain of the the tensile reinforcement to ensure ductility.Although the benefit of U-jacket anchors in flexural retrofitting applications is evident,the provision of U-jackets throughout the span to prevent the mechanisms of plate end and IC debonding may not be a materially efficient method to improve the efficiency of FRP strengthening ap-plications becau additional material is required to reach a given strength (Orton et al.2008).Inclined U-Jacket Orientations
Promising results have been achieved bad on the limited rearch conducted on inclined U-jackets at the FRP ends only (Fig.2
).
Fig.2.U-shape anchoring method at 45degrees
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F o r p e r s o n a l u s e o n l y ; a l l r i g h t s r e s e r v e d .
Published findings indicate that in addition to preventing the two mechanisms of end span debonding,inclined anchors readily shift the critical failure mode to concrete crushing or FRP rupture (Duthinh and Starnes2001;Pornpongsaroj and Pimanmas2003; Sagawa2001).
The effects of alternative U-jacket orientations,including perpendicular,inclined,and X-shaped U-jacket anchors,were inves-tigated by Pimanmas and Pornpongsaroj(2004).In this study, 220mm deep and120mm wide RC beams were tested under four-point bending.Beams were retrofitted with1.2mm thick and 100mm wide plates for flexural strengthening with a150GPa modu-lus of elasticity.The plates were anchored at the plate ends with 0.11mm thick carbon fiber sheets over a width of300mm.Ancho-rages consisted of the application of a single ply of CFRP with 230GPa material stiffness.The study investigated two plate-end termination lengths:200mm and420mm away from the supports, which failed by IC debonding and end cover paration failure,re-spectively,where
no anchorage was provided.
Of the numerous anchor configurations tested,it was found that U-jackets placed at the FRP plate-end locations200mm from sup-ports failed by premature concrete crushing and intermediate span debonding,while U-jackets placed420mm away from supports failed by premature concrete crushing and concrete cover para-tion failure.The influence of end termination distance on end de-bonding failure is consistent with current debonding models(Smith and Teng2002;Smith and Teng2003).Inclined and X-shaped an-chor arrangements all failed by concrete crushing.Interestingly,the authors point out that the CFRP plate experienced the highest con-finement near the side faces of the beam and less restraint in the central zone.This implies that U-jacket anchorages lo effective-ness with increasing beam width.Although the authors concluded that the inclined and X-shaped anchors successfully prevented both forms of plate end and IC debonding,premature concrete crushing failure prevented the occurrence of FRP rupture,masking the full potential of the anchorages from being realized.
Duthinh and Starnes(2001)also confirmed that concrete crush-ing was the controlling failure mode in two out of the three specimens that they tested,and the other mode was a combination of U-jacket rupture and intermediate flexural-shear crack debond-ing.The laboratory program comprid2–6laye
rs of200mm wide CFRP jackets placed diagonally on each plate end.The inclined fibers effectively prevented cover paration failure at the plate ends.It was found that two and six layers of jacket anchored the carbon plate to strain levels of8,260and11;000με,respec-tively,without slippage.The above rearch demonstrates the clear advantages of using inclined U-jackets as oppod to perpendicular orientations at the CFRP plate ends.In addition to the jackets providing confinement,an improvement of bonding and resistance to the opening of longitudinal cover paration cracks,the inclined fibers were en to delay the occurrence of IC debonding.This may be due to a reduction of interfacial longitudinal shear stress in the shear-flexural zones and the resulting energy transfer to the jacket anchors via an induced strut-and-tie action resulting from the inclined fibers.The benefits of inclined fibers were also noted by Sagawa et al.(2001).
In addition to the prevention of debonding failure,Smith and Teng(2003)showed that the u of U-jackets can also enhance ductility.This was confirmed by Buyle-Bodin(2004),who inves-tigated veral FRP anchorage devices to prevent concrete cover paration failure.The experimental program involved five beams, each3,000mm long with a rectangular cross-ction150mm wide and300mm deep.Both perpendicular and laterally inclined CFRP shear jackets were ud to restrain the ends of the CFRP flexural plate at130–200mm spacings.Ductility was measured as either deflect
ion ductility or curvature ductility.Deflection ductility was defined as the ratio of ultimate midspan deflection to yield midspan deflection,where as curvature ductility was considered in a similar fashion but utilized the midspan curvature values.Although all specimens strengthened with both perpendicular and inclined shear jackets exhibited greater load-carrying capacity,deflections,and ductility,it was found that perpendicular orientations of U-jacket anchors provided the most noticeable improvement,with increas in curvature ductility of45%and24%for deflection ductility.The improvements were less obvious in the inclined U-jacket anchors. This may be due to the higher postcracking stiffness exhibited due to the inclined U-jacket anchors.Strain in the tensile reinforcement is usually the most common measure of ductility utilized by FRP design guidelines such as ACI440.2R-08(2008).It may be more beneficial for future rearchers to measure the tensile reinforce-ment strain to quantify ductility performance.
Prestresd U-Jackets
Prestresd U-jackets are a method of anchorage on which little rearch has been conducted.The advantages of prestressing stem from the incread level of confinement and the higher shear resis-tance provided by the prestresd U-jackets.In practical applica-tions,prestressing was introduced onto the sides of the CFRP U-jackets by Pham and Al-Mahaidi(2006)by introducing a gap between t
he jacket and the concrete soffit,as prented in Fig.3.
A prestressing strain of500μεwas introduced into the jacket sides by inrting wedges into a preformed gap.Beams with pre-stresd jackets showed no evidence of slippage in the anchorage zone at failure.This was attributed to an increa in concrete shear capacity in the anchorage zone as a result of the compressive stress induced by the U-jackets.The legs of the prestresd U-jackets
did
Fig.3.Two anchorage systems ud by Pham and Al-Mahaidi(2006,©ASCE)
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not rupture,but failed through a combination of IC debonding and debonding of the end jacket.Only a slight improvement of approximately5%in the ultimate capacity was recorded due to prestressing.Debonding of the U-jackets suggests that a more robust form of anchorage is required to anchor the ends of the prestresd FRP U-straps to increa their effectiveness.This may be a subject for further rearch.Although unconfirmed by further experimental studies,the slight advantages obrved from prestressing are outweighed by their labor intensiveness and poor practicality.
Metallic Anchorage Systems
Metallic anchorages are one of the earliest forms of FRP end anchorage devices investigated by ,Sharif et al. 1994;Jenn et al.1999).Investigations have been conducted on adhesively bonded metallic plates with mechanical fasteners (Fig.4),adhesively bonded metallic U-jackets,and U-jackets with end clamping.Rearchers such as Garden and Hollaway(1998), Spadea et al.(1998),Duthinh and Starnes(2001),and Wu and Huang(2008)have found that the u of metallic anchorages provides a significant increa in anchorage strength in addition to ductility enhancement.
Previous experimental testing demonstrated the ineffectiveness of bonded angle ctions for plate-end anchorage due to the lack of a cure plate end fixing to the concrete.Experiments were con-ducted by Garden and Hollaway(1998)with a number of1.0m long plated beams tested in four-point bending.Cantilevers were also tested to demonstrate that the structural benefit of plate-end anchorage diminishes as the shear span/depth ratio of the beam increas.Each beam was strengthened with67mm wide and 0.87mm thick,111–115GPa modulus CFRP plates.The bolted plate-end anchorage system ud comprid a40mm long steel anchorage block of the same width as the composite plate.The block was cured to the composite plate using laminate adhesive and two mild steel bolts.
A comparison was made between the mechanically fastened steel anchorages and where the bonded plate was continued under the supports of the beam,resulting in a clamping force applied nor-mal to the plate.The authors concluded that the main requirements of bolted plate-end anchors were the shear resistance of the anchor bolts and the FRP-steel adhesive bond.The conclusion was bad upon the similarity of the results obtained between clamping and fastening anchors.The authors did not compare fastened steel anchors with unclamped,unfastened anchors,which would be needed to prove that confinement does not improve anchorage effectiveness.Becau the combined
benefits of bolted plates together with clamping pressure were not investigated,the benefits of the application of clamping forces together with mechanical fastening remain to be fully substantiated.
Duthinh and Starnes(2001)tested a ries of ven beams in four-point bending.A single carbon fiber plate(1.2×50mm)with an elastic modulus of155GPa was ud to strengthen the beams in flexure.Three of the beams tested utilized a203mm wide mechan-ically fastened steel anchor over the plate end.Two bolts were torqued to400Nm,resulting in an applied clamping force of 15–25kN.The result of clamping and adhesion enabled the anchored plate to reach an ultimate strain of11;400με(60%of rupture).Failure was by debonding initiating from diagonal shear cracking.The authors stipulated that clamping combined with adhesion can double or triple the anchorage capacity that can be expected from the bond alone.However,no investigations were carried out using bolted anchorages without torque to asss the contribution of clamping force on anchorage enhancement within the context of the test tup.
Spadea et al.(1998)attempted to improve the performance of CFRP-strengthened RC beams by using external steel anchorages designed to control and minimize the bond-slip between the concrete beam and the CFRP plate.The anchorages consisted of U-shaped steel anchors installed at the plate ends,together with four to eight U-shaped steel anchorages distributed throughout the sp
an,The plates were bonded to the concrete using epoxy resin and contained no external bolts or mechanical fasteners.Experi-mental testing measured maximum fiber strain utilizations of 80%(12;000με)for beam specimens with end anchorages at the plate ends,together with eight U-shaped anchorages distributed throughout the span,corresponding to a67%enhancement over the corresponding unanchored specimen.In addition to the enhanced fiber utilization and strength enhancement provided by the steel anchorages,greater ductility and gradual debonding of the plate over an extended time increment were also obrved.
Ductility was evaluated through an examination of deflection (deflection ductility),curvature(curvature ductility),and the area-under-the-load deflection curve at yielding of the tension
steel Fig.4.(a)Typical FRP plate anchored using permanent mechanical anchorage device[Reprinted with permission from Kalfat(2008)];(b)schematic of typical test tup
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