rcedComposite Grid Connectors Used in Concrete Sandwich Panels
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Technical NoteTesting and Acceptance Criteria for Fiber-ReinforcedComposite Grid Connectors Used inConcrete Sandwich PanelsMahmut Ekenel,Ph.D.,P.E.,A.M.ASCE 1Abstract:Precast concrete sandwich panels have been used in building construction since the 1960s.These sandwich wall panels are constructed with two wythes of concrete separated by a layer of thermal foam plastic insulation and a shear connector mechanism between concrete wythes.Recently,more research is focused on fiber-reinforced composite grids as shear connectors due to their superior thermal performance compared to other connectors such as steel or solid concrete.However,while there are guidelines for design of concrete sand-wich panels,there is no standardized guidance that provides testing and capacity determination for composite shear connectors.Therefore,testing and acceptance criteria were developed to evaluate the shear transfer capacities of fiber-reinforced composite grid connectors used in combination with rigid insulation in concrete sandwich panel construction.The criteria are intended to standardize testing and evaluation of the interaction between concrete and composite shear connectors so manufacturers can provide consistent results,and the data pool generated by standardized testing can support the formulation of design assumptions.DOI:10.1061/(ASCE)MT.1943-5533.0000915.©2014American Society of Civil Engineers.Author keywords:Concrete sandwich panels;Shear connectors;Building codes.IntroductionProgressive innovations in science and technology allow manufac-turers to develop new construction materials and structural systems that are user and environmentally friendly as well as energy effi-cient.One of these innovations is the use of alternative materials to develop a precast concrete sandwich panel with both high thermal resistance and high structural performance.These panels are typ-ically used for building envelopes,cladding,bearing walls,and shear walls.Panels generally span vertically between foundations and floors or roofs to provide the permanent wall system,but they may also span horizontally between columns.Concrete sandwich panels provide a combination of relatively superior structural and energy performance compared to many other conventional precast concrete wall panels by positioning the most vulnerable part of the assembly,the insulation,between two durable layers of concrete (PCI Committee Report 2011).The structural behavior of the sandwich wall panel depends greatly on the strength and stiffness of the connectors,whereas the thermal resistance of the insulation layer governs the insulation value of the panel (Benayoune et al.2008).The arrangement and spacing of shear connectors vary depending on factors such as de-sired composite action,applied load,span of the panel,and type of shear connector used (Benayoune et al.2008).Examples for tradi-tional connector types are C-tie,Z-tie,M-tie,welded-wire-truss connectors,bent-wire connectors,plastic or fiber-composite pins,or solid zones of concrete penetrating the foam core (Rizkalla et al.2009;PCI Committee Report 2011).Whereas all of them have been successfully used in concrete sandwich panel construction in thepast,these traditional connectors have the disadvantage of ther-mally bridging the two concrete wythes,thus negatively affecting the thermal efficiency advantage of such panels.Recently,researchers focused on finding an alternative type of shear connector that would enable composite action between con-crete wythes without any compromise on insulation properties of the system;one is fiber-reinforced composite grid connectors.For example,because carbon fibers have a thermal conductivity of ap-proximately 14%of that of steel,connecting concrete wythes with a carbon-fiber composite grid allows a sandwich panel to develop composite action without compromising thermal insulation proper-ties (Rizkalla et al.2009).Also,glass fibers are well-known as a high tensile strength and low-conductive material (Pong et al.2005).This new concrete sandwich panel using glass or carbon fibers as connectors effectively eliminates thermal bridges and undesirable forced compatibility strains because of differential tem-peratures in the concrete.It is reported that carbon-fiber-reinforced composite connector technology has enabled concrete sandwich wall panel constructions that have R -values as high as 32,com-pared to common values ranging from 11to 16for sandwich panels with steel or concrete connectors for similar thicknesses (Gleich 2007).Composite grid connectors also have the advantage of elimi-nating corrosion and reducing cover requirements because of their superior corrosion resistance compared to steel connectors.The main disadvantage of carbon or glass fibers is their high cost compared to steel or concrete;however,this can be offset by the thermal advantages.Another disadvantage is that most compo-sites typically have low shear strength.To avoid shear-related fail-ures in composite structures,it is necessary to prescribe provisions that can be used to evaluate shear-related limit states,including evaluation of proper strength-reduction factors to be used in the design.The typical shape of the partially continuous grid connec-tors is shown in Fig.1.These composite grids are manufactured using a continuous process in which the fiber strands are aligned in a 0-and 90-degree configuration and impregnated with an epoxy1Senior Staff Engineer,International Code Council Evaluation Center,5360Workman Mill Rd.,Whittier,CA 90601.E-mail:mekenel@ Note.This manuscript was submitted on February 25,2013;approved on August 2,2013;published online on August 5,2013.Discussion period open until August 11,2014;separate discussions must be submitted for individual papers.This technical note is part of the Journal of Materials in Civil Engineering ,©ASCE,ISSN 0899-1561/06014004(5)/$25.00.D o w n l o a d e d f r o m a s c e l i b r a r y .o r g b y S h a n g h a i J i a o t o n g U n i v e r s i t y o n 07/17/15. C o p y r i g h t A S CE .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 .resin.The fiber sheets are then cut into strips at a 45-degree angle (Fig.1)to produce the composite grids (Insel et al.2006).An important component of a concrete sandwich panel is rigid insulation.Concrete sandwich panel construction with rigid insu-lation enables substantially smaller total wall thicknesses as well as substantially faster and more economical implementations (Gastmeyer and Donahey 2003).Although there are many insula-tion types on the market,insulated concrete sandwich panels utilize a cellular (rigid)insulation because it provides the material proper-ties that are most compatible with concrete,such as moisture absorption,dimensional stability,coefficient of expansion,and compressive and flexural strength (PCI Committee Report 2011).Rigid insulations may be thermoplastic (expanded polystyrene and extruded polystyrene)or thermosetting (e.g.,polyurethane).Although the use of insulation material is not for the purpose of shear transfer between concrete wythes,early bonding between cer-tain insulation types (e.g.,expanded polystyrene)and the concrete wythes may provide sufficient shear transfer for composite action during handling of panels.It was reported that a stronger insulation would increase the overall load capacity of the panels;however,for design purposes,this bond is considered unreliable for the long term because freeze-thaw or thermal cycles may cause bond deg-radation (Insel et al.2006).There is currently no legally adopted building code in the United States that defines design methods for concrete sandwich panels,especially when fiber-reinforced grid connectors for shear transfer between wythes are used.The PCI document titled “State of the art of precast/prestressed concrete sandwich wall panels ”(PCI Com-mittee Report 2011)provides guidelines for design of such sandwich wall panels;however,this document is not referenced by any legally adopted building code,and this document does not provide any guid-ance in regard to capacity determination of composite shear connec-tors.It only states that composite shear connector capacities may be obtained from the connector manufacturers.The lack of codes and test methods as well as the complex nature of concrete sandwich panels with composite shear connectors and rigid insulation in be-tween has led the engineers to rely on experimental investigations backed by analytical studies.However,it was extremely important to unify these methods so manufacturers can provide consistent re-sults for shear capacities of such connectors.Because the building codes in the United States do not provide requirements for testing and determination of capacities of such product,an acceptance cri-teria was developed in accordance with International Building Code Section 104.11(International Code Council 2012)to establish guide-lines for the evaluation of shear transfer capacities of fiber-reinforced grid connectors used in combination with rigid insulation in concrete sandwich panel construction (ICC Evaluation Service 2010).This paper is intended to explain the testing and conditions of acceptance set forth in this criteria.Scope of Acceptance CriteriaThe fiber-reinforced composite grid connectors addressed by this criteria (AC422),used in conjunction with rigid insulation,are usedto provide structural composite action between the panel facers of concrete sandwich panels that are used as walls to resist wind pres-sure acting on the panel surface.This acceptance criteria also ad-dresses connectors used to transfer shear forces between the concrete panel facers that are due to gravity loads on sandwich pan-els used as exterior bearing walls.The fiber-reinforced grid connec-tors are factory cast into the concrete facers of the sandwich panels.This type of composite grid is oriented diagonally between the concrete wythes,normal to the wall surface,allowing for a truss mechanism to develop.The shear transfer capabilities of the fiber-reinforced grid connecters and rigid insulation addressed by this acceptance criteria are for shear transfer parallel to the length of the connector,with the grids installed in concrete sandwich panels and with the grid element continuous or semicontinuous along the panel length.The grid connectors are intended to transfer the shear flow induced by composite action between two concrete elements subjected to flexural bending in one or more directions.The discontinuous connectors along the length of the panel are typically spaced at a maximum separation of 152mm (6in.)on center.Test and Performance Requirements under AC422Material TestsIt is important that material properties be determined so that the results can be used in the design or used as benchmarks for material performance sustainability test results.Acceptance Criteria AC422requires that concrete mix design and testing follow ACI 318(American Concrete Institute 2011)requirements so a consistent concrete mixture can be used in all tests.Insulation material is the other crucial material for which the properties must be deter-mined.It was reported that overall load capacity of the panels may also depend on the type of insulation used,and a stronger insulation would increase the overall load capacity that is due to interlocking between concrete and the insulation material (Insel et al.2006).Therefore,AC422requires that the compressive strength,flexural strength,and density properties of the insulation material used in the connector performance test specimens be determined.In addition,AC422requires that ultimate tensile strength,ultimate tensile strain,and tensile modulus of elasticity of the strand of the connector grid material be determined in accordance with ASTM D3039(ASTM 2007).Twenty specimens must be tested and results must exhibit a coefficient of variation (COV)of 6%or less;other-wise,the number of specimens must be doubled.The AC422also requires tests on the effects of a wet concrete environment and moisture on aging of shear connectors to be conducted to determine the durability characteristics of fiber-reinforced connector material.Testing requirements,relevant specifications,minimum number of specimens,and percent of retention of average tensile strength are summarized in Table 1.Testing requirements and acceptance con-ditions presented in this table are based on and consistent with AC125(ICC Evaluation Service 2012)and ACI 440FRP material specifications (ACI 2013).Connector Performance TestsThe goal of the connector performance test is to determine the shear flow capacity of fiber-reinforced shear connectors used in concrete sandwich panels.Each grid connector design alternative to be rec-ognized must be tested.A connector design alternative consists of three elements,which are connectors,insulation,and concrete.The parameters are the combinations of connector grid,insulationFig.1.Typical shape of fiber-reinforced grid connectorD o w n l o a d e d f r o m a s c e l i b r a r y .o r g b y S h a n g h a i J i a o t o n g U n i v e r s i t y o n 07/17/15. C o p y r i g h t A S CE .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 .minimum embedment of the connector in the concrete,concrete type,and minimum concrete strength that are being tested for rec-ognition.The connectors must be tested in multiple parallel rows that are uniformly spaced.Except for end joints,the grid connec-tor length must be the same as the length of the concrete elements.The rigid insulation must extend the full length of the concrete elements.The width of rigid insulation,concrete embedment of the grid connectors,and thickness of the concrete element must be the minimum recommended by the grid connector manufac-turer.Test result values for concrete thicknesses that differ from the tested thicknesses can be interpolated from results of other thicknesses tested,provided all other connector design alterna-tives are the same;however,extrapolation is not acceptable under AC422.Connector Shear TestsShear tests must be performed with push-through specimens con-sisting of three layers of concrete and two layers of rigid insula-tion with connectors as shown in Fig.2.A test must consist of loading the center layer and supporting the outer two layers.The load must be applied parallel to the connector rows and con-centric to the test specimen.Test specimens,supports,and loading must all be symmetric about the center plane of the test specimen.The test specimen must contain at least two rows of semicontin-uous grid connector material.If a continuous grid is to be used in the panel manufacturing,tests specimens must contain a continu-ous grid throughout the length of the test panel.Test measure-ments must include applied load and deflection of the center layer of concrete relative to the outer two layers.Five specimens must be tested for each connector design alternative.The relative displacement at supports should be prevented using frictionless lateral support.Freeze-Thaw and Temperature Cycle TestsIt was reported that a stronger insulation would increase the overall load capacity of the panels;however,for design purposes,this bond is considered unreliable for the long term (Insel et al.2006).This is because in some locations,freeze-thaw or thermal cycles at the concrete/insulation interface may produce forces on the insulation that may break individual cells,and insulation may start to disintegrate.Therefore,if the bond between concrete and insulation material will be taken into consideration for shear transfer between concrete wythes along with shear connectors,they must be tested to determine long-term resistance to bond degradation.Currently there is no standardized test specification that is available to test and qualify freeze-thaw and thermal deg-radation of such systems.Subsequently,a test method was de-veloped and included in the acceptance criteria.The main goal of this testing regime is to achieve the volume increase of the water trapped between concrete and insulation that is due to transformation from the liquid state to a frozen state and then observe the degree of bond degradation.It is also in-tended to evaluate the thermal-shock effect on the interface of insulation material and concrete by exposing the specimens to high-low temperature cycles that may cause bond degradation that is due to internal stresses caused by temperature differentia-tion.The AC422requires that freeze-thaw and thermal cycle tests be performed with push-through specimens consisting of three layers of concrete and two layers of rigid insulation similar to Fig.2with the difference that the specimen dimensions must be L ¼609mm and w ¼609mm with two rows of grid connec-tors parallel to the dimension L spaced at dimension s apart.For the purposes of this test,the exterior concrete thickness is set to 51mm and the interior layer thickness is set to 102mm.Insu-lation thickness is set at 51mm minimum.The panel edges must be sealed to prevent direct infiltration of water and prevent direct exposure of the insulation material to the thermal chamber.A minimum of three specimens must be tested as control specimens maintained in laboratory conditions (21Æ2°C).A minimum of three specimens are required for cyclic-aged specimens that are cycled between −29and 60°C for 15times.See Fig.3for the details of the testing regime.The number of cycles was selected based on an existing criteria in use since 2001for composite wall and roof panel systems with an expanded polystyrene core and spray-or trowel-applied cementitious facings (ICC Evaluation Service 2001).After fifteen environmental cycles,the samples must be load tested in a fashion similar to connector shear tests as shown in Fig.2.The relative displacement at supports should be prevented using frictionless lateral support.The condition of acceptance defined in AC422to determine the effect of freeze-thaw and high-low temperature on performance of shear connectors is set as the ratio of shear modulus of cyclic-aged specimens to the shear modulus of control specimens being equal to or greaterFig.2.Typical double shear specimen (all dimensions are in mm)Table 1.Environmental Durability Test Matrix [Data from AC422(ICC Evaluation Service 2010)]Environmental durability test Relevant specificationTest conditions Test duration Minimum number of specimens Percent (%)retention of average tensile strength 1,000h 3,000h Water resistance ASTM D2247(ASTM 2002)100%,37Æ2°C 1,000and 3,000h 20for each duration 9085Alkali resistanceASTM C581(ASTM 2003)Immersion in alkali solution of pH ¼12at 23Æ2°C1,000and 3,000h20for each durationD o w n l o a d e d f r o m a s c e l i b r a r y .o r g b y S h a n g h a i J i a o t o n g U n i v e r s i t y o n 07/17/15. C o p y r i g h t A S CE .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 .Connector Design PropertiesConnectors are intended for design based on ultimate strength rmation obtained from tests must be used to determine nominal shear flow,q n ,and shear modulus,G ,for each connector design alternative and are intended to be used in design in accor-dance with the published guidelines of the PCI (PCI Committee Report 2011).Nominal Shear StrengthThe nominal shear strength must be calculated using the average ultimate load minus three standard deviations,which provides a 99.87%probability that the actual strength will exceed these statistically based design values for standard sample distribution (ACI 2008).The nominal shear strength,q n ,can be determined using Eqs.(1)–(3)q i ;max ¼V i ;max NL ð1Þq a ;max ¼Σq i ;maxnð2Þq n ¼q a ;max −3σpð3Þwhere V i ;max =peak test load for each test specimen (kN);N =number of pieces of equal-length grid segments in the individual test specimen,with a minimum of four pieces of grid located con-centric to the applied load,as shown in Fig.2;L =length of the grid segment and specimen (minimum of 2,100mm,as shown in Fig.2);q n =nominal shear flow (kN =mm);q a ;max =mean shear flow of the specimens (kN =mm);q i ;max =shear flow for each test specimen (kN =mm);σp =standard deviation of the peak test loads of the test specimens;and n =number of test specimens.Determination of Shear ModulusThe shear modulus,G ,is a function of the shear-deformation curve and must be based on the deformation measurement (Δ),which is the deflection of the center layer in relation to side layers in double-shear specimens (Fig.2).It was accepted by the industry and re-searchers to be determined at 50%of the peak load level because ofthe assumption that most materials show elastic response between 30and 50%of the ultimate load;for example,ASTM E564(ASTM 2006)uses 35%of maximum load to determine the shear modulus of the wall assembly.The shear modulus of the connector,G ,used to determine the shear component of the deflection of sandwich panels using the grid connector,can be calculated based on the average nominal shear modulus minus three standard deviations in accordance with the Eqs.(4)–(6)G i ¼0.5V i ;max A sa ×t Δi ;v ¼0.5V i ;max 2Lw ×tΔi ;vð4ÞG a ¼ΣG inð5ÞG ¼G a −3σsð6Þwhere V i ;max =peak test load for each specimen (kN);G i =shear modulus for each test specimen (kN =mm 2);G a =mean shear modulus of the test specimens (kN =mm 2);t =thickness of the rigid foam insulation (mm);L =length of the grid segment and specimen (minimum of 2,100mm,as shown in Fig.2);A sa =total contact surface area between the insulation material and both surfaces of the central concrete wythe (mm 2);w =width of the specimen (mini-mum of 1,200mm,as shown in Fig.2);Δi ;V =relative displace-ment between the central concrete core and the two outer concrete wythes of each specimen evaluated at 50%of the peak load level (mm);and σs =standard deviation of the shear modulus of the five test specimens.Strength Reduction FactorsThe following strength reduction factors are applicable for the design of the sandwich panel construction with grids.A strength reduction factor of 0.75,which was adopted from ACI 318(ACI 2011),must be used to determine the nominal design shear flow strength (ø).Because of a lack of a data pool to justify reliability analyses,which is one of the main purposes of this cri-teria so industry can use standardized test methods and collect data for further analyses,an additional strength reduction factor of 0.85(Ψf ),which was adopted from ACI 440.2R (ACI 2008),must also be applied to improve the reliability of strength prediction,which yields a total strength reduction factor of 0.64as given in Eq.(7)ϕψf q n ¼0.75×0.85×q nð7ÞFor the design of the sandwich panels for sustained loads,an industry-accepted strength reduction factor of 0.15(∅s )must be used to determine the nominal design shear flow strength.This number is considered to be conservatively safe because ACI 440.2R (ACI 2008)requires 0.20for glass fibers and 0.55for car-bon fiber;ACI 318Appendix D (ACI 2011)requires 0.55for steel members.This can be further justified when more data is available in accordance with this criteria.∅s q n ¼0.15q nð8ÞStiffnessDesigning the deflections of sandwich panels using the construc-tion method under consideration must be based on the combination of flexural deformation and shear deformation.The total deforma-tion must be based on the following:ðδF λΔþδS ξÞ¼δLongTeamð9ÞFig.3.Freeze-thaw and high-temperature aging cycles.Time is shown for illustrative purposes onlyD o w n l o a d e d f r o m a s c e l i b r a r y .o r g b y S h a n g h a i J i a o t o n g U n i v e r s i t y o n 07/17/15. C o p y r i g h t A S CE .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 .where δF =flexural deflection of the panel due to moment applied to the panels by sustained gravity loads;λΔ=long-term flexural deformation creep factor due to sustained loads in accordance with ACI 318Section 9.5;δS =shear deflection of the panel due to shear in the panel due to the sustained gravity loads;ξ=shear deforma-tion creep factor due to sustained loads in accordance with ACI 318Section 9.5(ACI 2011).SummaryBecause there are no standardized guidelines that provide testing and capacity determination for composite shear connectors,testing and acceptance criteria were developed to evaluate the shear trans-fer capacities of fiber-reinforced composite grid connectors used in combination with rigid insulation in concrete sandwich panel construction and are intended to unify and standardize testing and evaluation of such systems.Based on the test results from com-ponent material testing,environmental aging tests,and connector performance tests,the shear-flow capacity of a fiber-reinforced grid connector can be calculated.For future work,the data pool generated using the standardized testing provided by AC422can support the formulation of design assumptions and strength reduc-tion factors.ReferencesAmerican Concrete Institute (ACI).(2008).“Guide for the design and con-struction of externally bonded FRP systems for strengthening concrete structures.”ACI 440.2R ,Detroit,MI.American Concrete Institute (ACI).(2011).“Building code requirements for structural concrete and commentary.”ACI 318,Detroit,MI.American Concrete Institute (ACI).(2013).“Material specification for car-bon and glass fiber-reinforced polymer (FRP)materials made by wet layup for externally reinforcing concrete structures (under development,2013).”ACI 440,Detroit,MI.ASTM.(2002).“Standard practice for testing water resistance of coatings in 100percent relative humidity.”D2247,ASTM International,West Conshohocken,PA.ASTM.(2003).“Standard practice for determining chemical resistance of thermosetting resins used in glass-fiber-reinforced structures intendedfor liquid service.”C581,ASTM International,West Conshohocken,PA.ASTM.(2006).“Standard practice for static load test for shear resistance of framed walls for buildings.”E564,West Conshohocken,PA.ASTM.(2007).“Standard test method for tensile properties of polymer matrix composite materials.”D3039,West Conshohocken,PA.Benayoune,A.,Abdul Samad,A.A.,Trikha,D.N.,Abang Ali,A.A.,and Ellinna,S.H.M.(2008).“Flexural behavior of pre-cast concrete sand-wich composite panel:Experimental and theoretical investigations.”Constr.Build.Mater.,22(4),580–592.Gastmeyer,R.,and Donahey,R.C.(2003).“GFRP connector and partially precast concrete sandwich panel system.”SP-215:Field applications of FRP reinforcement:Case studies ,American Concrete Institute (ACI),Detroit MI,103–119.Gleich,H.(2007).“New carbon fiber reinforcement advances sandwich wall panels.”Struct.Mag.,61–63.ICC Evaluation Service.(2001).“Acceptance criteria for composite wall and roof panel systems with an expanded polystyrene core and spray-or trowel-applied cementitious facings.”AC176,Los Angeles,CA.ICC Evaluation Service.(2010).“Acceptance criteria for semicontinu-ous fiber-reinforced grid connectors used in combination with rigid insulation in concrete sandwich panel construction.”AC422,Los Angeles,CA.ICC Evaluation Service.(2012).“Concrete and reinforced and unreinforced masonry strengthening using externally bonded fiber-reinforced polymer (FRP)composite systems.”AC125,Los Angeles,CA.Insel,E.,Olsen,M.D.,Tanner,J.E.,and Dolan,C.W.(2006).“Carbon fiber connectors for concrete sandwich panels.”ACI Concr.Int.,28(10),33–38.International Code Council.(2012).International building code ,Country Club Hills,Chicago,IL.PCI Committee Report.(2011).“State of the art of precast/prestressed concrete sandwich wall panels.”PCI J.,Spring,131–176.Pong,W.A.,Morgan Girgis,A.F.,Tadros,M.K.(2005).“Proposed GFRP connectors in sandwich panels.”SP-230,7th Int.Symp.on Fiber-Reinforced (FRP)Polymer Reinforcement for Concrete Structures ,American Concrete Institute (ACI),Detroit,MI,21–37.Rizkalla,S.H.,Hassan,T.K.,and Lucier,G.(2009).“FRP shear transfer mechanism for precast,prestressed concrete sandwich load-bearing panels.”SP-265,Thomas T.C.Hsu Symp.:Shear and Torsion in Concrete Structures ,American Concrete Institute (ACI),Detroit,MI,603–625.D o w n l o a d e d f r o m a s c e l i b r a r y .o r g b y S h a n g h a i J i a o t o n g U n i v e r s i t y o n 07/17/15. C o p y r i g h t A S CE .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 .。
