1. INTRODUCTION. Fig.1 Dimension of test specimen

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1 F1B04 Evaluation of a Shear Wall Reinforced with Glass FRP Bars Subjected to Lateral Cyclic Loading Nayera Mohamed PhD candidate, Department of Civil Engineering, University of Sherbrooke, Sherbrooke, Canada Ahmed Sabry Farghaly Post-Doctoral Fellow, Department of Civil Engineering, University of Sherbrooke, Sherbrooke, Canada Brahim Benmokrane Professor, Department of Civil Engineering, University of Sherbrooke, Sherbrooke, Canada Kenneth W. Neale Professor, Department of Civil Engineering, University of Sherbrooke, Sherbrooke, Canada ABSTRACT With the establishment of several construction applications of FRP reinforcement, there is a need for a system to resist lateral loads induced from wind and earthquake loads in these constructions. Reinforced concrete shear walls have shown effective performance in resisting lateral loads caused by wind and earthquake loads. Therefore, shear walls are frequently used in parking garages and multi-story buildings exposed to high lateral loading. This research involved testing a shear wall totally reinforced with FRP bars. Early information showed the inapplicability of reinforcing shear walls with FRP, but after overcoming the difficulties encountered, the results showed an opposite conclusion. The results obtained demonstrated significantly high utilization levels of the shear wall reinforced with FRP in term of drift, deformability, and failure mode. The large scale shear wall experiment was carried out to examine the strength, stiffness and deformability by observing the degradation in stiffness and strength while resisting in plane reversed loading, and to measure the energy dissipation of the system accounting for the deformability of the shear wall. The shear wall specimen was in the medium-rise wall category where both flexural and shear deformations exist. The specimen was totally reinforced with glass FRP bars to resist flexure, shear, and sliding shear deformations. KEYWORD Glass FRP bar, concrete, shear wall, cyclic lateral loading, ductility, strength

2 1. INTRODUCTION The investigation reported herein addresses the behavior of a shear wall with a medium aspect ratio which is common in moderate-rise buildings. The shear wall aspect ratio is simply defined as the height-to-length ratio (h w /l w ). A large proportion of shear walls constructed in the U.S. and Canada are classified as medium-rise; with wall height-to length aspect ratios (h w /l w ) typically between 2 and 4 [1]. In such shear walls both nonlinear flexural and nonlinear shear deformations significantly contribute to the lateral response. Horizontal construction joints in a shear wall may become the weakest link in the chain of resistance and is considered a poor energy dissipater [2]. From the only previous experimental work on shear walls reinforced with CFRP grids [3], it was found that the vertical reinforcement was pulled out from the base slab causing low energy absorption capacity and high degradation in lateral load capacity in comparison with steel reinforced shear walls. Paulay and Priestley [2] explained that the possibility of failure by sliding shear is a feature of shear walls as large shear stresses are transferred across crack by means of shear friction. Accordingly, construction joints are subject to axial compression or tension and bending moments beside shear forces. Therefore, to ensure the structural resistance of walls and ensure energy dissipation, it is relevant to provide sufficient reinforcement across construction joints [2]. The main objective of the present study was to assess the possibility to construct shear walls totally reinforced with glass FRP bars which can be comparable to steel reinforced shear wall and be used in FRP reinforced structures. This goal is reached by designing sufficient reinforcement, to avoid all brittle failure modes in a shear wall with equal shear and flexure contribution. 2. EXPERIMENTAL PROGRAM 2.1 Design of Wall Specimen The shear wall specimen represents a large-scale moderate-rise shear wall. As illustrated in Fig.1, the height of the tested shear wall (h w ) was 3500 mm. The horizontal length (l w ) was 1500 mm including two 200 mm boundary elements at each end, which were embedded in the wall with the same thickness of the wall. The wall thickness (b w ) was 200 mm. The base slab thickness was 700 mm. The base was used to fix the specimen to the lab rigid floor and as an anchorage length for the vertical bars. The base slab is anchored to the rigid lab floor using four bolts 66 mm diameter, as shown in Fig.1. The axial compression force applied on the wall was taken as 0.07A c f c, simulating a real loading condition. The CSA A23.3 [4] and ACI 318 [5] provisions for minimum dimensions and reinforcement ratio have been applied to the wall specimen. The design against flexure, shear and confinement were satisfied by using the CSA S [6] and ACI 440.1R-06 [7] provisions mm mm Fig.1 Dimension of test specimen 1200 The design of FRP-reinforced concrete members for flexure is analogous to the design of steelreinforced concrete members. Experimental data on concrete members reinforced with FRP bars show that flexural capacity can be calculated based on assumptions similar to those made for members reinforced with steel bars. The design of members reinforced with FRP bars should take into account the uniaxial stress-strain relationship of FRP materials. If FRP reinforcement ruptures, failure of the member is sudden and catastrophic; however, there would be limited warning of impending failure in the form of extensive cracking and large deflection due to the significant elongation that FRP reinforcement experiences before rupture. In any case, the member would not exhibit ductility as is commonly observed for under-reinforced concrete beams reinforced with steel reinforcing bars. The concrete crushing failure mode is marginally more desirable for flexural members reinforced with FRP bars. By experiencing concrete crushing, a flexural member does exhibit some plastic behavior before failure.

3 In conclusion, both failure modes (FRP rupture and concrete crushing) are acceptable in governing the design of flexural members reinforced with FRP bars provided that strength and serviceability criteria are satisfied. To compensate for the lack of ductility, the member should possess a higher reserve of strength. The margin of safety suggested by CSA S [6] and ACI 440.1R- 06 [7] against failure is therefore higher than that used in traditional steel-reinforced concrete design. According to CSA A23.3 [4] and ACI [5], the nominal shear strength of a reinforced concrete cross section V n is the sum of the shear resistance provided by concrete V c and the steel shear reinforcement V s. Compared with a steelreinforced section with equal areas of longitudinal reinforcement, a cross section using FRP flexural reinforcement after cracking has a smaller depth to the neutral axis because of the lower axial stiffness (that is, product of reinforcement area and modulus of elasticity). As a result, the shear resistance provided by both aggregate interlock and compressed concrete is smaller. Research on the shear capacity of flexural members without shear reinforcement has indicated that the concrete shear strength is influenced by the stiffness of the tensile (flexural) reinforcement. The contribution of longitudinal FRP reinforcement in terms of dowel action has not been determined. Because of the lower strength and stiffness of FRP bars in the transverse direction, however, it is assumed that their dowel action contribution is less than that of an equivalent steel area. Further research is needed to quantify this effect. The concrete shear capacity V c of flexural members using FRP as main reinforcement is simply the ACI shear equation for steel reinforcement modified by a factor, which accounts for the axial stiffness of the FRP reinforcement. The ACI method used to calculate the shear contribution of steel shear reinforcement is applicable when using FRP as shear reinforcement. The wall shear capacity was designed higher than the flexural capacity to ensure the flexural failure of the specimen as desired. To prevent sliding shear failure, one layer of grid of 45 inclined GFRP bars was used in each direction as shown in Fig.2. These inclined GFRP bars were anchored in the base and in the wall with length equal to the development length calculated from the CSA S [6] provisions. In addition, to assure wall ductility, the amount of confinement in the boundary elements was adopted according to the previous research [8] which specify less than 110mm length of FRP bar to avoid buckling failure. Therefore, the tie spacing in boundaries was taken as 100 mm. 200 Base steel Boundary element Vertical Reinforcement Horizontal Reinforcement Inclined Reinforcement Fig.2 Reinforcement layout of the wall specimen Boundary Reinforcement Vertical Reinforcement 1500 mm Horizontal Reinforcement Fig.3 Cross section of wall specimen Stirrups 2.2 Materials The wall was constructed using normal-weight, ready-mixed concrete with an average 28-day compressive strength of 40 MPa. The base slab was reinforced with steel bars 25M to assure its rigidity. The wall was reinforced with V-ROD GFRP reinforcing bars [9] (Figs.2,3). For vertical reinforcement, two layers of GFRP #3 bars spaced at 120 mm for web reinforcement were used. Also, 8 GFRP #3 bars were used for each boundary element at both ends of the wall, while horizontal reinforcement was GFRP #4 bent bars spaced at 80 mm. The mechanical properties of the reinforcing bars are shown in Tables 1 and 2. The boundary reinforcement ratio was 1.4% according to the boundary element area (200 x 200 mm), while the vertical reinforcement ratio was 0.81%. The horizontal reinforcement ratio was 1.6%. HM- GFRP #3 was used for the inclined reinforcement, the inclination angle was 45 and the spacing was

4 100 mm. The embedded length of the inclined reinforcement in the base and in the wall was 650 mm. Reinforcement details are shown in Figs.2,3. Table 1 Mechanical properties of vertical bars d Bar b A f E f f fu ε fu (mm)(mm²) (GPa) (MPa) (%) # ± Table 2 Tensile properties of bend bar (horizontal) E Bent bar portions f f fu ε fu (GPa) (MPa) (%) Straight portion 51.9 ± ± Bend portion ± d b = bar nominal diameter, A f = nominal cross sectional area, E f = modulus of elasticity, f fu = guaranteed tensile strength, ε fu = ultimate strain 2.3 Experiment Setup The test setup of the shear wall consists of four main parts as follows (Fig.4) Reaction wall Actuator for lateral loading Steel beam Shear wall specimen 2 steel triangles for bracing system Axial load system Lateral bracing system a. Fixing the base to the lab floor; in preparation for testing, the base was leveled on the laboratory floor, and was fixed to the laboratory floor with pre-stressing 66 mm Dywidag steel bars to prevent uplifting during the application of lateral loading and to prevent horizontal sliding. Wall base Fig.4 Specimen under setup for testing LVDT 1 b. Vertical loading system; simulating the gravity load was applied through two hydraulic jacks applying tension on two high strength 34 mm Dywidag steel bars placed on both sides of the wall. These steel bars were connected at the bottom to a stiff steel plate which is anchored with the base slab to the lab rigid floor. The upper sides of the bars are connected to a stiff steel beam which rests on the top of the wall in order to distribute the axial load. LVDT 10 LVDT 9 LVDT 8 LVDT 7 LVDT 2 LVDT 3 LVDT 5 LVDT 6 hw /2 c. Lateral loading system; load was applied to the wall specimen by a 1000 kn MTS-hydraulic actuator. The actuator was jointly connected to the top steel beam and transmitted its force to the specimen through a 50 mm thick steel bearing plate. The system is designed to be self-supporting and self-aligning during load reversals. d. Lateral bracing system; to avoid out-of-plane movements during testing, a bracing system was provided at the level of the top beam which rested on the wall. Two large steel triangles attached with two bearing rollers were attached to a rigid reaction wall. This system was then attached to the side of the top steel beam. LVDT 4 Fig.5 Locations of LVDTs on the specimen 2.4 Instrumentation Fig.5 shows the location of different LVDTs attached to the specimen. Lateral deformation at the top, mid-height, and bottom of the wall is measured by three LVDTs; LVDT 1, LVDT 2 and LVDT 3, respectively. One LVDT is used to measure the unlikely horizontal sliding of the base slab (LVDT 4). Two LVDTs (LVDT 5 and 6) are installed at the boundary elements to measure the vertical deformation of the boundary. Two further LVDTs (LVDT 7 and 8) inclined at 45 are

5 installed in the lower region of the wall close to the base to measure diagonal shear deformations. Two more LVDTs (LVDT 9 and 10) are attached to the upper steel beam to measure the sway of the shear wall at the top. Then two other LVDTs were installed at the position of the first two cracks to measure the crack width during testing. Displacement (mm) The foregoing system of measurements made it possible to estimate the flexural, shear, and sliding components of the wall displacements, as discussed in following section. These estimates were also based on a series of strain gauge measurements at various positions along the reinforcing bars. Additional strain gauges were attached to measure the concrete surface strain. No. of cycles Fig.6 Sequence of loading displacement Concrete crushing 2.5 Loading For the axial load, this load is slowly increased to the maximum value (840 kn) then maintained at this value throughout the testing. The value of the axial load was calculated according to a compressive stress equals to 0.07 f c. As the effect of loading history is not a variable in the testing, the typical procedure of applying quasi-static reversed loading until failure was used. Displacement control was used throughout the test. The displacement was applied in two cycles at the same amplitude with increments of 2 mm up to first cracking (around 10 mm), followed by increment of 5 mm up 50 mm, then increments of 10 mm to failure. A typical sequence of displacement cycles is shown in Fig.6. Fig.7 Crack pattern and failure mode 3. ANALYSIS OF RESULTS 3.1 Crack Pattern and Failure Mode The cracking pattern at the end of the test is shown in Fig.7. The failure mode of the wall was as expected. The wall started with flexural cracks, followed by shear cracks. The first flexural crack occurred at the lower part of the wall at approximately 120kN. The cracks were horizontal within the length of the boundary elements. Flexural cracks extended to about two thirds of the height of the wall (2/3 l w ). Flexure cracks were followed by shear cracks in the web of the wall. The inclination of the shear cracks was quite higher in the top part than that of the cracks at the bottom part. With cycling to increased deformations, the rhomboidal pieces of concrete between the intersecting cracks gradually deteriorated and spalling of concrete cover occurred at both sides of boundary elements. Fig.8 Concrete crushing Fig.9 GFRP longitudinal bar rupture

6 Thereafter, a major horizontal crack at about 200 mm from the base was clearly evident. A significant loss of strength, leading to failure, was observed when concrete deteriorated in most heavily stressed parts of the boundary elements close to the base (Fig.8). Stirrups were cut and rupture of longitudinal GFRP bars in the boundary element and web occurred under compression (Figs.9,10). Load (kn) Horizontal displacement at top of wall (mm) Fig.11 Lateral load-displacement response 78% of ultimate load Load (kn) Fig.10 GFRP stirrups cut 3.2 Load Deflection Response Lateral load-displacement results, as shown in Fig.11, demonstrate a general similarity to steel reinforced shear wall behavior. Mid-height walls generally produce hysteresis curves that are more pinched and exhibit less energy dissipation than would similar walls with a high aspect ratio. The unloading/ reloading curves seem to demonstrate linearity depending on GFRP behavior. The reloading branches followed a similar loading path but at a lower loading stiffness, resulting in lower peak strength. The unloading path shape seems to be dependent on the strain at the onset of unloading. The load-displacement curves indicated that the first excursion of a new displacement level followed the loading path of the second excursion of the previous displacement amplitude. This suggested that additional cycles at a specific displacement level would produce negligible damage to that experienced by the first unloadingreloading cycle. Two LVDTs monitored the elongation of the boundary elements. This result is used to determine the extent of cracking and deformation of flexural reinforcement. In all cycles, the elongation demonstrated recovery, except from about 78% of the ultimate load (Fig.12). This is an indication of elastic behavior of the specimen as the displacement was not accumulating but rather diminishing with increased lateral displacement. Elongation of boundary element (mm) Fig.12 Elongation of boundary elements The maximum elongation in the boundary element was 13.3 mm (Fig.12). It is observed that the crack width with GFRP reinforcement is less than the crack width experienced in steel reinforced shear walls. This can be due to yielding of the steel which causes a large crack width. 3.3 Stiffness The performance in terms of stiffness of the shear wall is shown in Fig.13. It was found that the stiffness is gradually decreased without any abrupt change even in region of residual strains (at 78% ultimate load) or at ultimate load. Stiffness (kn/m) Drift angle (rad.) Fig.13 Performance in term of stiffness

7 3.4 Deformability and Energy Dissipation Deformability is an essential property of shear walls under reversed loads, including the ability to sustain large deformations and absorb energy by hysteretic behavior. In previous studies on RC shear walls, when a structure or element does not exhibit a clear yield point, it was necessary to go back to the concept of energy absorption and dissipation capacity. The energy dissipation may be evaluated by calculating the area enclosed by the hysteresis loops of the load-displacement curve. The accumulative energy dissipation is then calculated as the sum of the area enclosed by all previous hysteresis loops (Fig.14). Cumulative energy dissipation (J) Displacement Fig.14 Cumulative energy dissipation (J) A more suitable means to compare energy dissipation with other researches in RC shear walls, is the energy ratio. The energy ratio denotes the ratio of the dissipated energy to the introduced energy (Fig.15). The latter corresponds to the area below the graph of the force-displacement relationship. So, the introduced energy has to be calculated for each half-cycle. The energy ratio was found to be equal to The energy ratio reported by other researchers on steel reinforced shear walls [10] shows energy ratios equal to approximately Load (kn) Dissipated energy Introduced energy Horizontal displacement at top of wall (mm) Fig.15 schematic of calculating energy ratio The shear wall specimen achieved a maximum drift equal to 3.1%, which is close to the ultimate drift that may be achieved by medium rise steel reinforced shear walls. The mean drift value in many design codes is taken in the range 1.5-2%, which can be considered when designing an FRP shear wall. The ultimate drift in medium rise steel reinforced shear wall is 4% which is usually admitted in seismic evaluation in order to limit damage of the non-structural elements, [4,11]. 5.6 Stability It was essential to monitor twisting of the specimen due to a torsionally weak wall section. Using two LVDTs placed on both ends at the top of the wall, the twisting was measured. No instability problems were noticed during testing and negligible twisting occurred with slight differences in displacement. As a conclusion, with the simplest definition of deformability; the capability of sustaining a high proportion of their initial strength under large deformations, a shear wall reinforced with GFRP may satisfy considerably ductility/deformability requirements. This is because this shear wall behaves in an elastic manner causing negligible residual deformations which causes degradation in strength. It is worth mentioning that the good confinement of the concrete in the boundary element played a great role in increasing the ductility of the wall. 6. CONCLUSIONS On the basis of results obtained from this experiment on a mid-rise shear wall reinforced with GFRP bars with a moderate reinforcement ratio, and from calculating the strength, stiffness, and ductility of such a shear wall, it can be concluded that the shear wall reinforced with GFRP may be qualified for resisting lateral loadings due to the following findings; (1) The experimental shear wall specimen has shown insignificant strength degradation and a reasonable stability of stiffness during reversed cyclic loading. (2) The failure mechanism of the wall was as expected. As a typical medium-rise shear wall, the wall started with flexural cracks, followed by shear cracks. Then the failure was flexural compression with a major flexure crack associated with rupture in the

8 GFRP vertical bars. (3) Negligible residual strain is experienced up to 78% of the ultimate load. (4) The FRP shear wall experienced less crack width than that experienced in steel reinforced shear wall due to the absence of yielding in the FRP bars. (5) The shear wall reinforced with FRP achieved a drift equal to 3.1%, which is near the limits of steel reinforced shear walls. (6) The good confinement of the concrete played a great role in increasing the ductility of the wall. However, more experimental tests and analytical studies are needed to further validate the present findings and to study the effect of different reinforcement ratios on the performance of shear walls reinforced with FRP bars. ACKNOWLEDGEMENT The authors wish to express their gratitude and sincere appreciation to the Canada Research Chair in Advanced Composite Materials for Civil Structures for funding this research project and the technical staff of the structural lab in the Department of Civil Engineering at the University of Sherbrooke. REFERENCES [6] Canadian Standards Association (CAN/CSA): Design and Construction of Building Components with Fibre-Reinforced Polymers, CAN/CSA S806-11, Canadian Standards Association, 5060 Spectrum Way, Suite 100, Mississauga, Ontario, Canada. [7] American Concrete Institute (ACI), Committee 440: Guide for the Design and Construction of Structural Concrete Reinforced with FRP Bars, ACI 440 1R-06, ACI, Farmington Hills, Michigan, USA, 2006 [8] D. H. Deitz, I. E. Harik, and H. Gesund: Physical Properties of Glass Fiber Reinforced Polymer Rebars in Compression, Journal of Composites for Construction, ASCE, Vol.7 (4), pp , 2003 [9] Pultrall: V-ROD Reinforcing FRP Bars Data Sheet; [10] C. Ggreifenhagen: seismic behavior of lightly reinforced concrete squat shear walls, PhD thesis, École Polytechnique Fédérale De Lausanne, Faculté Environnement Naturel, Architectural Et Construit, Section of Civil Engineering, 2006 [11] T.A. Duffey, C.R. Farrar, and A. Goldman: Low-Rise Shear Wall Ultimate Drift Limit, Earthquake Spectra, Vol. 10, No. 4, pp , 1994 [1] Jiang, H., and Kurama, Y. C.: Analytical Modeling of Medium-Rise Reinforced Concrete Shear Walls, ACI Structural Journal, Vol.107 (4), pp , 2010 [2] T. Paulay and M. J. N. Priestley: Seismic Design of Reinforced Concrete and Masonry Buildings, John Wiley and Sons, Inc, 1995 [3] T. Yamakawa, and T. Fujisaki: A Study on Elasto-Plastic Behavior of Structural Walls Reinforced by CFRP Grids, Proceedings of the Second International Symposium on Nonmetallic (FRP) Reinforcement for Concrete Structures (FRPRCS-2), RILEM proceedings 29, pp , 1995 [4] Canadian Standards Association (CAN/CSA) (2009). Design of Concrete Structures. CAN/CSA A (R2010), Canadian Standards Association, 5060 Spectrum Way, Suite 100, Mississauga, Ontario, Canada. [5] American Concrete Institute (ACI), Committee 318: Building Code Requirements for Structural Concrete (ACI ) and Commentary, ACI, Farmington Hills, Michigan, USA, 2008