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Journal of Constructional Steel Research 66 2010 1081 1087 Contents lists available at ScienceDirect Journal of Constructional Steel Research journal homepage Flexural behavior of strengthened steel concrete composite beams by various plating methods H E M Sallama 1 A A M Badawyb A M Sabab F A Mikhailb aCivil Engineering Department Jazan Univ Jazan 706 Saudi Arabia bMaterials Engineering Department Zagazig University Zagazig 44519 Egypt a r t i c l ei n f o Article history Received 11 September 2009 Accepted 4 March 2010 Keywords Strengthening Steel concrete composite beams Welded bonded plate CFRP sheet Electrical strain gauge measurements a b s t r a c t The effect of pre intermediate separation on the flexural behavior of strengthened steel concrete composite beams by either adhesively bonded carbon fiber reinforced polymers CFRP sheet or welded bonded steel plate was studied In the case of strengthened by CFRP sheet two different attachment patterns namely CFRP sheet wrapped around the flange of the I beam and CFRP sheet wrapped around the flange along with a part of the web were examined by testing four different strengthened steel concrete composite beams under four point bending 4PB Two of these beams were strengthened by fully bonded CFRP sheet with the two different patterns while the others are similar but have pre intermediate debonding area of 50 mm length flange width at the bottom surface of the lower flange In the case of strengthened by steel plate three different attachment patterns of steel plate to the soffit of the beams namely discontinuously welded end welded and bonded welded steel plates were also tested under 4PB The experimental results showed that there is no growth of the intermediate debonding before the yield of the lower flange occurred for all strengthened beams by CFRP sheet After yielding the beams with pre debonding area showed lower flexural capacity than those with fully bonding due to the rapid growth of the intermediate debonding On the other hand there is a difference in the yield load between the three different patterns of the welded steel plates with a marginal difference in the elastic stiffness 2010 Elsevier Ltd All rights reserved 1 Introduction Composite steel concrete girders are used widely in bridge and building construction as the main structural elements in flexure Theload carryingcapacityofsteel concretecompositegirderscan be improved by epoxy bonding fiber reinforced polymers FRP laminates to its tension flange Carbon fiber reinforced polymers CFRP have great potential for the strengthening and repair of steel bridge girders due to high strength to weight ratio corrosion resistance and high fatigue properties 1 4 Debonding in FRP strengthened members takes place in regions of high stress concentrations which are often associated with material discontinuities and with the presence of cracks Propagationpathofdebondinginitiatedfromstressconcentrations is dependent on the elastic and strength properties of the repair and substrate materials as well as their interface fracture properties 5 Due to the brittle nature of concrete there are Corresponding author E mail address hem sallam H E M Sallam 1 On sabbatical leave from Materials Engineering Department Zagazig Univ Zagazig 44519 Egypt mainlyfourdebondingfailuremodesinreinforcedconcretebeams strengthened with a bonded sheet plate namely concrete cover separation plate end debonding critical diagonal crack CDC debonding and intermediate crack IC induced debonding 6 8 Ontheotherhand themainlytwodebondingfailuremodesinsteel I beams strengthened with a bonded sheet plate are intermediate debonding due to the yielding of steel flange and plate end debonding 9 11 Choudhary 12 investigated the variation in flexural behavior of CFRP laminate bonded to steel beams due to the presence of surface flaws at the end of the laminate He 12 found that the presence of surface flaw at the plate end reduced markedly the efficiency of bonded CFRP laminates To the best of the authors knowledge this research is the first to study the effect of the pre intermediate debonding area on the structural behavior of strengthened steel beams Therefore the main goal of the present research is to study the growth behavior of intermediate debonding in steel concrete composite beams strengthened with a CFRP sheet To explore this idea the flexural behavior of steel concrete composite beams strengthened with various methods of attaching external steel plates namely end welded discontinuous welded bonded welded were compared 0143 974X see front matter 2010 Elsevier Ltd All rights reserved doi 10 1016 j jcsr 2010 03 005 1082H E M Sallam et al Journal of Constructional Steel Research 66 2010 1081 1087 Fig 1 I beam after welding two rows of shear stud and cross section dimensions a b Fig 2 a Form work and steel reinforcement b Placing of concrete and compaction using mechanical vibrator 2 Reliability of electrical strain gauge measurements Based on the electrical strain gauge measurements Schnerch and Rizkalla 13 found that the strains decreased by increasing thedistancefromtheneutralaxis Theyattributedthattotheshear lag i e the effect of partial interaction between the steel beam and the CFRP strengthening strip due to the flexibility of the adhesive However Sallam 14 attributed this discrepancy to the locations and the different types of strain gauges used Furthermore Schnerch and Rizkalla 15 attributed the discrepancy in the strain profile beyond yield to the short gauge length of the strain gauges used and the exact location of the plastic hinge within the constant moment region relative to the gauge position Dawood and Rizkalla 16 reported based on their electrical strain gauge measurements that modifying the geometry at the two ends of the bonded plate would significantly increase the bond strength of the adhesive The increase in the observed maximum shear stress for beam 800 T2 is possibly due to a localized discontinuity in the adhesive which affected the strain locallyatthelocationofthegauges However thenumericalresults obtained by Seleem et al 9 gave another acceptable explanation that is the shear stress decreases at the two ends of the splice plate at the extension of the adhesive layer beam 800 T2 while the experimental work 16 recoded maximum shear stress at the two ends of the splice plate see Fig 4 b in Ref 9 The absolute value ofshearstressespredictednumericallyatthetwoendsofthesplice plateforthetwobeams 800 Sand800 T2 arethesame whilethe difference between the corresponding absolute values measured experimentally for beam 800 T2 is greater than 40 Therefore reversetaperedplateendsdonotsufferfromthepeakshearstress i e favorite site of crack initiation debonding 9 There are several dissertations 17 20 measured the strains in the lower flanges of strengthened I beams during their yielding and they found that the readings of the electrical strain gauges were almost constant while the load continued to increase A number of authors of these dissertations tried to explain this phenomenon for example Shaat 19 explained the occurrence of this phenomenon due to a slight relative slip between the concrete slab and the steel beam Dawood 17 stated that this phenomenon occurred due to the localized effect of high levels of residualstressesorpossiblyduetolateralmovementofthetension flange of the beam due to web lateral buckling or local instability or due to a combination of these factor It is worth to note that Dawood 17 found the average strain measured by the PI gauge at the same location did not exhibit the same behavior and followed the same trend as the load deflection envelope Therefore one of theobjectivesofthepresentworkistofocusontheelectricalstrain gauge readings during the yielding of the lower flange 3 Experimental program 3 1 Specimen details and material properties A total of eight identical steel concrete composite beams were made Each beam consisted of a standard steel I beam of size 16 tw 6 3 mm tf 9 5 mm b 74 mm h 160 mm attached to a reinforced concrete deck slab 500 mm wide 100 mm thick cast to act compositely with the steel beam The loaded span of all beams was 1800 mm Two longitudinal rows of shear studs 12 mm diameter and 50 mm height welded to the top flange of the steel beams were used to act as shear connectors between the concrete slab and the steel beams as shown in Fig 1 The concrete slab was reinforced using two layers of 12 mm steel bars Each layer consisted of four bars in longitudinal direction and 20 bars in transverse direction After mixing and placing the concrete the concrete was compacted and finally the surface was finished see Fig 2 Theconcretemixwasdesignedtogetacompressivestrength of 40 MPa after 28 days The measured mechanical properties of steel I beam flange and web steel plates steel studs and steel bars are given in Table 1 The externally bonded strengthening systems selected for this study were steel plates 60 mm width 6 mm thickness 1700 mm long and high strength CFRP sheet Sikawrap Hex r 230 C The properties of the used CFRP sheets are given in Table 2 Sikadur 30 adhesive was used for bonding steelplatesandMbraceSaturant BASF wasusedforbondingCFRP sheet to the bottom flange of the beam Eight pieces of 130 60 50 mm wooden blocks were cut and tightly fit between the flanges using cider wedges at the supports and under loading points to prevent the web crippling as suggested by Tavakkolizadeh and Saadatmanesh 1 It is a well known fact that web elements of steel members may crippling that is buckle or yield when subjected to local concentrated loads 21 3 2 Investigated beams One beam was tested as unstrengthened control beam CB The strengthened beams were divided into two groups The first group consisted of three composite beams strengthened by steel H E M Sallam et al Journal of Constructional Steel Research 66 2010 1081 10871083 Table 1 The properties of the used steel materials ElementProperties Yield strength MPaUltimate strength MPa I beam Flange330 7460 5 Web422 7498 55 Steel plate6 mm thickness315455 Shear stud12 mm diameter390636 5 Steel bars12 mm diameter575 2727 a b c Fig 3 The bottom face of the tension flange of the strengthened beams a DW b EW and c BW Fig 4 The CFRP sheets wrapped around the tension flange and a part of the web beam Cfweb Table 2 The properties of the used high strength CFRP sheets as reported by manufacture Tensile strength4100 MPa Tensile E modulus231 GPa Strain at break1 7 Design thickness0 12 mm Density1 78g cm3 plate attached to the bottom surface of the I beam tension flange using three different attachment patterns as shown in Fig 3 In the first beam the plate was attached by discontinuous welding with U shape welding at the two ends of the plate with leg weld length of 350 mm beam DW In the second beam the plate was only attached by U shape welding at the two ends similar to the previous one beam EW In the third beam the plate was attached similar to end welded but after bonded the plate by adhesive material beam BW The second group consisted of four composite beams strength ened with CFRP sheet using two different attachment patterns to studytheeffectofintermediatedebondingineachattachment The firstbeamwasstrengthenedusingtwolayersofadhesivelybonded CFRPsheetwrappingaroundthetensionflangeonly CFfl Thesec ond one was strengthened using two layers of adhesively bonded CFRPsheetwrappingaroundthetensionflangealongwithapartof the web 50 mm long on its both sides CFweb as shown in Fig 4 The other two beams CFfl D and CFweb D were strengthened sim ilar to the beams CFfl and CFweb but with an artificial debonding 50 mm length flange width between the bottom face of the steel flange and the first layer of the CFRP sheet at the midspan of the beam by using a piece of insulated paper One layer of CFRP sheet with a width of 150 mm having fibers normal to the beam axis wrapping around the tension flange and a part of the web was used to anchorage the CFRP sheet at its ends and beneath loads i e belted CFRP sheet All strengthened beams were designed to fail 1084H E M Sallam et al Journal of Constructional Steel Research 66 2010 1081 1087 Fig 5 Failure mode of the control beam due to flexural Based on the design model for the vertical shear strength of simply supported composite beams proposed by Liang etal 22 theultimateloadsofstrengthenedbeamsarenotexceed 90 of the shear capacity of the control beam 3 3 Surface preparation of steel beam and installation of the strengthening systems To ensure good and strong bond the surfaces subjected to the bond of each steel beams was abraded using a steel brush wheel until a white metal surface was achieved and cleaned by air brushing to remove any dust Before applying the strengthening materials acetone was used to clean the surface from any other bond inhibiting materials Then installation of the strengthening systems was applied The two parts of the epoxy BASF were mixed thoroughly using an electric hand mixer for about three minutes epoxy resin was applied directly onto the prepared beam surfaceusingabrush Thefabricwascarefullyplacedontotheresin coating in the required direction with hands then it was pressed tightly to eliminate any irregularities or air pockets until resin squeeze out between the roving of the fabric Additional epoxy resinwasappliedimmediatelyaftertheapplicationoftheprevious layer The second layer was applied as the first one In the case BW beam Sikadur 30 adhesive epoxy was prepared to bond the steel plate The epoxy bonding was applied first to bond the entire steel plate after passing 48 h an end anchorage by welding leg weld length of 350 mm was performed In such case the heat generated at the plate end due to welding will affect the adhesive near the weld line However this effect is marginal on the efficiency of the plate end anchorage For welded beams the I beam surface was cleaned and the steel plate was welded as shown in Fig 3 3 4 Test setup All beams were simply supported with a span of 1800 mm 100 mm of overhang at each support and tested under four points loading with 500 mm spacing between the two concentrated point loads Load was applied using a 5000 kN capacity hydraulic cylinder machine operating in load control at a constant loading rate equals 0 5 kN s In order to observe the behavior of the beam under investigation strains loads and deflections were measured at the desired locations Different measuring devices were used to collect data Strains in the steel were measured using electrical resistance strain gauges 10 mm KFG 10 120 C1 11L1M2R three strain gauges adhered to its underside face two of them were placed at the mid span and the third one was placed under one of the two load points The first strain gauge was located at the mid span beneath the web MS Web The second strain gauge was located at the mid span under flange MS Flange The third one was located beneath the web under one of the two points of load application UL Web The readings of these strain gauges will show the effect of shear lag and the point of yield commencement However strains at the top of the concrete slab were measured Fig 6 The load tensile strain behavior for control beam using two LVDT gauges 25 mm placed over the concrete deck at the mid span of the beam One of them placed at the mid width and the other at 50 mm from the edge of the concrete to measure any distortion of the beam The vertical deflections of the beam were measured using two LVDT gauges 50 mm placed under the web at the mid span and at the load point Data from strain gauges LVDT s and load cell were automatically recorded using a data logger recording 2 readings s 4 Results and discussion 4 1 Control beam The control beam behaved linearly elastic up to yielding of the tension flange at load equals 238 kN and corresponding deflection at the mid span was 4 mm then the plasticity took place and growth in the lower flange making load deflection curve non linear up to an applied load equals 464 kN and the corresponding deflection at midspan equals 24 mm after that the deflection increased without significant change in the load up to a deflection equals to 32 mm then the load dropped with concrete crushing near one of the load location Fig 5 shows this ductile failure mode of the control beam The load versus tensile strain curves for the control beam at the three locations of measurement MS Web MS Flange and UL Web on the bottom face of its lower flange are shown in Fig 6 It is observed the yield started in the three locations at the same time Therefore there is a negligible effect of shear lag As expected unreliable load strain behavior was observed after yielding where the readings of the three electrical strain gauges were almost constant while the load continued to increase Recently Shaat and his supervisor published a scientific paper 3 from his dissertation 19 showing this phenomenon see Fig 5 a in Reference 3 without any explanation for the reasons of this behavior Turning to Fig 5 a in Reference 3 supported the present argument The slope of P curve in this figure deviated when the flange yielded at 85 kN When the load reached 125 kN the steel at the strain gauge position on web was yielded and instability occurred up to failure H E M Sallam et al Journal of Constructional Steel Research 66 2010 1081 10871085 Fig 7 The load compressive strain behavior for control beam The compressive strain on the top surface of the concrete slab is shown in Fig 7 In the elastic range there is no significant difference between the two positions i e no shear lag effect After yielding of the lower flange at about 238 kN the P relation becomes nonlinear and the compressive strain above the center line is greater than that at overhang due to the effect of shear lag 4 2 Strengthened beams by steel plate The mode of failure for strengthened beams by steel plate is similar to the mode of failure of control beam DW Beam was