RESILIENT INFRASTRUCTURE June 1 4, 2016

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1 RESILIENT INFRASTRUCTURE June 1 4, 2016 PUNCHING SHEAR BEHAVIOUR OF HIGH STRENGTH CONCRETE SLAB-COLUMN CONNECTIONS REINFORCED WITH GFRP BARS Ahmed M. Hussein MS Student, University o Manitoba, Canada Ehab El-Salakawy Proessor and CRC, University o Manitoba, Canada ABSTRACT The atastrophi nature o punhing shear ailure exhibited by lat plate system requires a great attention and robust preditions o the behaviour o slab olumn onnetions. This paper presents an experimental study arried out to investigate the punhing shear behaviour o ibre-reinored polymer (FRP) reinored onrete (RC) interior slabolumn onnetions made o high strength onrete (HSC). Three ull-sale HSC speimens were onstruted and tested up to ailure. The three onnetions were reinored with GFRP sand-oated bars with reinorement ratios o 1.0, 1.5 and 2.0% without any shear reinorement. The typial dimensions o the test speimens were mm with a 300 mm square olumn extending 1000 mm above and below the slab, representing the region o negative bending moment around an interior supporting olumn o a parking struture. All speimens were simplysupported along all our edges with the orners ree to lit. The onnetions were subjeted to vertial load and unbalaned moment that were monotonially applied through the olumn tips. The behaviour o the speimens in terms o the deormation and strength harateristis is disussed. Inreasing the reinorement ratio inreased the punhing shear apaity and dereased the reinorement strains and deletions at the same load level. The test results were also ompared to the preditions o the relevant North Amerian odes where appliable. Keywords: Punhing Shear, Flat Plate, GFRP, Interior Connetion, High Strength Conrete 1. INTRODUCTION The inreased inidene o durability problems in steel-reinored onrete (RC) strutures due to orrosion o steel is well known. The problem is even worse or strutures in old limati onditions suh as parking strutures, leading to eetively higher hloride ingress due to the use o de-iing salts. Even with protetive measures suh as, inreasing the onrete over, dereasing the permeability o onrete by using appropriate dosages o supplementary ementitious materials, orrosion inhibitors and/or the use o dierent kinds o steel reinorement (i.e., stainless steel, epoxy-oated steel and galvanised steel), tedious repair proedures annot be avoided and the eonomi impliations an be quite large. On the other hand, replaing the orrodible steel with non-orrodible iber-reinored polymers (FRP) reinorement provides a promising solution to the orrosion problem. In North Ameria, a large number o parking strutures is onstruted using lat plate systems, taking the advantage o the absene o beams. However, lat plate strutures are suseptible to punhing shear ailure due to large shear ores and unbalaned bending moments, whih is a dangerous mode o ailure due to its suddenness and brittleness, thus it is onsidered a major drawbak. The unbalaned moment ould result rom uneven loading onditions, uneven lengths o adjaent spans and/or eentri loading. Also, the use o high strength onrete (HSC) has inreased in the onstrution industry, not only or its higher ompressive strength, also or other desired properties that ome along with higher strength, suh as inreased stiness or high abrasion resistane. However, it exhibits a dierent behaviour ompared to normal strength onrete (NSC), whih drew the attention to the perormane o suh elements. Several researhers have studied the eet o HSC on the punhing shear behaviour o steel-rc onnetions. Test results showed that, HSC delayed the appearane o lexural raks, inreased the stiness ompared to NSC, the ubi root o onrete ompressive strength yields better results than the square root in STR-837-1

2 prediting the shear strength and in general the ailure o HSC slabs is more brittle (Gardner 1990; Marzouk and Hussein 1991; Ramdane 1996; Ozden et al. 2006). On the other hand, relatively little work has been onduted on HSC RC slab-olumn onnetions reinored with FRP. It was reported that inreasing the onrete strength enhaned the punhing apaity and the initial stiness, however it has little eet on the post-raking stiness (Zhang et al. 2005; Hassan et al. 2013; Gouda and El-Salakawy 2016). This paper presents the results o three ullsale HSC interior slab-olumn onnetions reinored with GFRP bars and subjeted to a ombination o shear ores and unbalaned moments. 2. EXPERIMENTAL PROGRAM 2.1 Test Connetions In this study, three ull sale interior slab-olumn onnetions were onstruted and tested under ombined shear and unbalaned moment, with a moment-to-shear ratio o 0.15 m, up to ailure. The slab dimensions and reinorement were deined by perorming an elasti analysis o a typial multistory parking garage system onsisting o three 6.5 m long square bays in both diretions, resulting in mm square slab with a entral olumn. A onnetion o these dimensions simulates, with a good approximation, the region o negative bending moment around an interior olumn, bounded by lines o ontra-lexure whih are assumed to be 0.2 times the bay length. However, mm slabs were ast to allow or supporting learane as shown in Figure 1. The entral 300-mm square olumn was adequately reinored with 4-20M onventional steel reinorement and 10M stirrups to prevent premature ailure, the olumn stub extends 1000 mm above and below the slab. Figure 1: Connetions geometry and reinorement layout or a typial onnetion (all dimensions in mm) The three onnetions were reinored with sand-oated GFRP bars. All onnetions were reinored in tension side only using one orthogonal mesh at an average eetive slab depth o 160 mm or all onnetions, and none o the onnetions had shear reinorement. This study aims to investigate the eet o lexural reinorement ratio on HSC slabs. The designation o the onnetions onsists o two haraters; the irst harater indiating the type o reinorement (G or GFRP) while the seond harater is a number indiating the lexural reinorement ratio (1.0, 1.5 and 2.0%). The details o the onnetions are listed in Table 1. Connetion Table 1: Details o Test Connetions Slab Thikness Reinorement Conrete Strength (mm) Type (MPa) Reinorement Ratio (%) G GFRP G G STR-837-2

3 2.2 Material Properties All speimens were ast using high-strength, ready-mixed onrete using a maximum aggregate size o 19 mm. The onrete ompressive strength was determined on the day o testing based on standard ompression tests on mm onrete ylinders. The obtained onrete ompressive strengths are listed in Table 1. Sand-oated GFRP reinoring bars were used as slab reinorement in both diretions. The harateristis o the GFRP reinoring bars are summarized in Table 2. Bar Size Diameter (mm) Table 2: Properties o Reinoring Bars Tensile Modulus (GPa) Eetive Area (mm 2 ) Ultimate Strength (MPa) Ultimate Strain (%) No. 15M , Instrumentation Eah speimen was provided with 15 eletrial strain gauges attahed to our reinoring bars passing through the olumn (two bars in eah diretion) at ritial loation. Also, the deletion proile o the slab was aptured in both diretions using a total o 12 linear variable displaement transduers (LVDTs). All instrumentation was onneted to a data aquisition system (DAQ) to reord the readings during the test. 2.4 Test Setup and Proedure Figure 2: Test setup The speimens were tested in the Strutures Laboratory at the University o Manitoba. The onnetions were simply-supported on a supporting rame as shown in Figure 2, onsisting o our heavy steel I-beams assembled together, along all our edges with the orners ree to lit. The onnetions were tested in an upside-down position with respet to the position o a real struture. In addition, neoprene strips were inserted on top o the supporting STR-837-3

4 rame to ensure a uniorm distribution o the loads along the edges. This arrangement allows the vertial shearing ore to be applied rom top to bottom, by a 1000 kn hydrauli atuator, thereore, tension raks appeared at the bottom side o the slab. The unbalaned moment was aused by two lateral ores applied at the tips o the upper and lower olumns through two hydrauli jaks. During the test, the propagation o raks was areully drawn at 20 kn inrements. 3. TEST RESULTS AND DISCUSSIONS 3.1 Craking Pattern and Mode o Failure In all tests, the irst rak was observed passing through the olumn orner on the tension side o the slab, i.e. loation o maximum bending moment, along the bars, ollowed by similar raks in the orthogonal diretion. As the load inreased, the irst radial rak developed rom the orner o the olumn towards the orner o the slab, as more raks around the olumn irumerene were ormed. At a relatively higher load, radial raks beame wider and a series o irumerential raks appeared at dierent distanes rom the olumn periphery onneting the radial raks together. Finally, as the ultimate load was reahed, punhing ailure ourred with the olumn penetrating through the slabs, haraterized by a sudden drop in the vertial load with the ormation o a wide irumerential rak deining the ailure one as shown in Figure 3. For all slabs, neither onrete rushing on the ompression side nor bar rupture was evident whih onirms that the mode o ailure was pure punhing shear ailure. a) G-1.0 b) G-1.5 ) G-2.0 Figure 3: Craking pattern at ailure STR-837-4

5 3.2 Reinorement Strains The relationship between the strain in the reinoring bars at the olumn ae in the diretion o unbalaned moment and the vertial load is presented in Figure 4. Generally, strains in the reinoring bars start to inrease rapidly ater the irst rak in eah speimen, whih depends on the relative loation o the rak and the gauge. The maximum measured reinorement strain was 8,590 μɛ in the onnetion with the lowest reinorement ratio (G-1.0). This measured strain represents 33% o the ultimate tensile strain o the GFRP bars, whih onirms that no rupture ourred in the reinoring bars. At the same load level, inreasing the reinorement ratio dereased the reinorement strain. All slabs showed a similar reinorement strain proile. Figure 5 represents the strain proile in the diretion o unbalaned moment at inrements o 25% o the ailure load or onnetion G-1.5., it an be notied that, strains are dereasing as moving urther rom the olumn ae, whih indiates that no bond slippage ourred during the test, also, higher strains orresponds to the diretion o moment appliation, due to the unbalaned moment. Figure 4: Load-strain relationship 3.3 Deletions Figure 5: Reinorement strain proile or G-1.5 Figure 6 shows the relationship between the vertial load and maximum deletion in all slabs measured at 50 mm rom the olumn ae in the diretion o unbalaned moment. Beore initiation o raks, the behaviour o the three onnetions was omparable, as it depends on the mehanial properties o the onrete. Ater raking, the behaviour depends on the post-raking stiness up to ailure, whih is a untion o the axial rigidity o the STR-837-5

6 reinorement, ρe. Thereore, inreasing the reinorement ratio by 50 and 100% inreased the post-raking stiness by 46 and 100%, respetively. Moreover, the deletion dereased at servie load level by 52 and 77% and at ailure by 24 and 43%, respetively. Again, this is attributed to the inreased axial stiness 3.4 Punhing Shear Strength Figure 6: Load deletion relationship The ultimate apaity was normalized by multiplying the ultimate load by 3 ' 84 to aount or the variation o the onrete ompressive strength as listed in Table 3, where 84 MPa is the average ompressive strength o onrete or the three speimens and the ubi root o the onrete strength was used rather than the square root to ollow the provisions o the Canadian standard CSA/S Inreasing the reinorement ratio by 50 and 100% inreased the normalized apaity by 16 and 28 %, respetively. This is attributed to better ontrol o the raks whih inreases the ontribution o the unraked onrete, also inreasing the reinorement ratio inreased the ontribution o dowel ation. Connetion First Crak Load (kn) Failure Load (kn) Table 3: Test Results Normalized Maximum Deletion Failure (mm) Load (kn) Servie Failure Reinorement Strain at Failure (μɛ) G ,590 G ,660 G , Code Comparison To evaluate the punhing shear resistane o FRP-RC slab-olumn onnetions without shear reinorement, the CAN/CSA S (CSA 2012) provided equations similar to those o steel-rc strutures, with modiiations to aount or the axial stiness o FRP, ρ E. Also, it assumes a ubi-root relationship between the punhing resistane and the onrete ompressive strength instead o square-root relationship. The punhing shear resistane is the least o the ollowing three equations: 2 [1] vr v E 1 ' 3 (MPa) STR-837-6

7 d [2] vr v s E b 1 ' 3 (MPa) 1 ' 3 (MPa) [3] vr v E Where, v is the atored shear stress resistane provided by onrete, β is the ratio o long side to short side o the olumn, λ is a ator to aount or onrete density, φ is the resistane ator or onrete, E and ρ are the elasti modulus and the lexural reinorement ratio or the FRP reinorement, respetively, is the onrete ompressive strength (shall not exeed 60 MPa) and α s is a ator takes into aount the support ondition (α s = 4 or interior olumns), d is the average slab depth and b is the ritial setion perimeter. In ontrast, the ACI 440.1R-15 (ACI Committee ), gives only one equation to alulate the punhing shear resistane as ollows: 4 ' [4] V b 5 (MPa) Where V is the nominal shear strength provided by onrete, is the onrete ompressive strength, b is the ritial setion perimeter and is the raked transormed setion neutral axis depth, and may omputed as: [5] kd k 2 [6] n n n 2 Where ρ and n are the FRP reinorement ratio and the ratio o modulus o elastiity o FRP bars-to-modulus o elastiity o onrete, respetively. The ailure loads were ompared to the preditions o CSA S and the ACI 440.1R-15 as listed in Table 4. CSA S provided reasonable yet slightly onservative preditions with an average V exp. / V pred. o 1.17 ± 0.03 and a COV o 2.6%. On the other hand, ACI 440.1R-15 highly underestimates the apaities with and average V exp. / V pred. o 1.79 ± 0.06 with a COV o 3.35%, this underestimation is due to that the punhing shear equation o ACI 440.1R-15 only aounts or the unraked onrete ontribution to resist the applied shear stresses. Connetion Failure Load, V exp. (kn) Table 4: Code Comparisons Punhing Shear Capaity Preditions, V pred. CSA/S ACI 440.1R-15 V pred. V exp. / V pred. V pred. V exp. / V pred. G G G Mean SD COV (%) STR-837-7

8 4. CONCLUSIONS 1. All onnetions exhibited a brittle punhing shear ailure, where the olumn punhes through the slab, with no signs o onrete rushing on the ompression side o the slab or FRP rupture 2. Inreasing the reinorement ratio has a onsiderable eet on the post-raking stiness; inreasing the reinorement ratio by 50 and 100% inreased the post-raking stiness by 46 and 100%, respetively. 3. No bond slippage was observed sine the strains in the reinorement were inversely proportional to the distane rom the olumn. 4. Inreasing the reinorement ratio by 50 and 100%, inreased the punhing apaity by 16 and 28%, respetively 5. The CSA/S standard gives reasonable preditions to the experimental work with an average V exp. / V pred. o 1.17 ± 0.03 and a COV o 2.6%. On the other hand, ACI 440.1R-15 highly underestimates the apaities with and average V exp. / V pred. o 1.79 ± 0.06 with a COV o 3.35%. REFERENCES ACI Committee Guide or the Design and Constrution o Strutural Conrete Reinored with FRP Bars, ACI 440.1R-15, Amerian Conrete Institute, Farmington Hills, MI, USA. Canadian Standards Assoiation (CSA) Design and Constrution o Building strutures with Fibre- Reinored Polymer, CSA/S806-12, Canadian Standards Assoiation, Toronto, Ontario, Canada. Gardner, N. J Relationship o the Punhing Shear Capaity o Reinored Conrete Slabs with Conrete Strength. ACI Strutural Journal, 87(1): Gouda, A. and El-Salakawy, E Punhing Shear Strength o GFRP-RC Interior Slab Column Connetions Subjeted to Moment Transer. Journal o Composites or Constrution, 20(1): Hassan, M. Ahmed, E. and Benmokrane, B Punhing-Shear Strength o Normal and High-Strength Two-Way Conrete Slabs Reinored with GFRP Bars. Journal o Composites or Constrution, 17(6): Marzouk, H. and Hussein, A Punhing Shear Analysis o Reinored High-Strength Conrete Slabs. Canadian Journal o Civil Engineering, 18(6): Ozden, S. Ersoy, U. and Ozturan T Punhing Shear Tests o Normal- and High-Strength Conrete Flat Plates. Canadian Journal o Civil Engineering, 33(11): Ramdane, K. E Punhing Shear o High Perormane Conerte Slabs. 4th International Symposium on Utilization o High-Strength/High-Perormane Conrete, Paris: Zhang, Q., Marzouk, H., and Hussein, A A Preliminary Study o High Strength Conrete Two-Way Slabs Reinored with GFRP Bras. Proeedings o the 33rd CSCE Annual Conerene: General Conerene and International History Symposium: GC 318: STR-837-8