DSpace at VNU: Retrofitting Earthquake-Damaged RC Structural Walls with Openings by Externally Bonded FRP Strips and Sheets

DSpace at VNU: Retrofitting Earthquake-Damaged RC Structural Walls with Openings by Externally Bonded FRP Strips and She...

Retrofitting Earthquake-Damaged RC Structural Walls with Openings by Externally Bonded FRP Strips and Sheets Downloaded from ascelibrary.org by University of Alberta on 03/13/15 Copyright ASCE For personal use only; all rights reserved Bing Li1; Kai Qian, A.M.ASCE2; and Cao Thanh Ngoc Tran3 Abstract: The majority of research studies on the behavior of reinforced concrete members with externally bonded fiber reinforced polymer (FRP) sheets have been focused on beams, columns, and beam-column joints However, limited experimental studies have been conducted to investigate the performance of structural walls retrofitted by wrapping FRP strips or sheets, especially on structural walls with openings The validated retrofitting schemes for strengthening damaged walls without openings may not be suitable for walls with openings Therefore, a series of experimental studies were carried out at Nanyang Technological University, Singapore, to study the effectiveness of the proposed repair and strengthening schemes in recovering the seismic performance of the damaged walls with irregular or regularly distributed openings The strut-and-tie approach was utilized to design the repair schemes The repaired walls managed to recover their strength, dissipated energy, and stiffness reasonably, indicating that the strut-and-tie approach can be a good design tool for FRP-strengthening of structural walls with openings Moreover, the shear and sliding capacities of repaired walls were enhanced by using fiber anchors The repaired walls failed primarily because of debonding of the fiber reinforced polymer at the base of the walls DOI: 10.1061/(ASCE)CC.1943-5614 0000336 © 2013 American Society of Civil Engineers CE Database subject headings: Earthquakes; Seismic effects; Fiber reinforced polymer; Walls; Openings; Bonding; Rehabilitation Author keywords: Repair; Fiber reinforced polymers; Wall; Opening; Strut-and-tie; Reinforced concrete Introduction RC structural walls play a very important role in carrying lateral loading and resisting drift in tall buildings Piercing a wall with openings may significantly influence its behaviors, such as changing its force transfer mechanism, deducting its strength and stiffness, and decreasing its ductility level Although walls with openings have been studied by some researchers (Yanez et al 1992; Ali and Wight 1991; Marti 1985), the effects of the regular and irregular openings on the seismic performance of RC walls are still not fully understood Thus, two 3-story reinforced concrete model walls, scaled to one-third, were tested under reversed cyclic lateral load Sspecimens W1 and W2 were designed with similar dimensions and details as Yanez et al (1992) and Marti (1985), respectively Moreover, for detailed results of these two control specimens refer to Wu (2005) and Zhao (2004), respectively The goal of this paper is to investigate whether the damaged walls with openings could restore their seismic performances after proposed retrofitting Fiber reinforced polymers (FRP) were utilized in this study because of their high strength-to-weight ratios, corrosion resistance, ease of application, and tailorability In addition, the Associate Professor and Director, Natural Hazards Research Centre at Nanyang Technological Univ., Singapore 639798 (corresponding author) E-mail: cbli@ntu.edu.sg Research Associate, Natural Hazards Research Centre at Nanyang Technological Univ., Singapore E-mail: qiankai@ntu.edu.sg Lecturer, Dept of Civil Engineering, International Univ., Vietnam National Univ., Ho Chi Minh City, Vietnam E-mail: tctngoc@hcmiu edu.vn Note This manuscript was submitted on February 24, 2012; approved on September 25, 2012; published online on September 27, 2012 Discussion period open until September 1, 2013; separate discussions must be submitted for individual papers This paper is part of the Journal of Composites for Construction, Vol 17, No 2, April 1, 2013 © ASCE, ISSN 1090-0268/2013/2-259-270/$25.00 orientation of the fiber in each ply can be adjusted to meet specific strengthening objectives (Engindeniz et al 2005) Although numerous research studies had been conducted to strengthen or repair the structural components, such as beams, columns, and beam-column joints (Lam and Teng 2001; Teng and Lam 2002; Pampanin et al 2007; Teng et al 2009; Li and Chua 2009; Li and Kai 2011; El-Maaddawy and Chekfeh 2012), there are limited experimental studies that were conducted to investigate the effectiveness of FRP retrofitting the damaged RC structural walls, especially for walls with openings Neale et al (1997) have tested wall-like columns that were strengthened using FRP, including wall-like columns with different arrangements of externally bonded FRP reinforcements subjected to uniaxial compression only Lombard et al (2000) performed rehabilitation of structural walls using carbon fiber reinforced polymer (CFRP) externally bonded to the two faces of the wall to increase its flexural strength The use of unidirectional carbon fibers with the fibers aligned in the vertical direction increased the flexural capacity and precracked stiffness and the secant stiffness at yield Several nonductile failure modes of the wall were attributed to the loss of anchorage or tearing of the fibers Antoniades et al (2003) tested squat RC walls up to failure and then repaired them using high-strength mortar and lap-welding of fractured reinforcement The walls were subsequently strengthened by externally bonded FRP sheets as well as by adding FRP strips to the wall edges FRP increased the strength of the repaired walls by approximately 30% compared with traditionally repaired walls However, the energy dissipation capacity of the control walls could not be restored completely Li and Lim (2010) retested four seismically damaged structural walls (two low-rise walls and two mediumrise walls) after conventionally repaired and strengthened by wrapping with FRP sheets It was reported that the repaired and strengthened walls were able to restore the performance of the damaged RC walls This repair method is relatively easy JOURNAL OF COMPOSITES FOR CONSTRUCTION © ASCE / MARCH/APRIL 2013 / 259 J Compos Constr 2013.17:259-270 Downloaded from ascelibrary.org by University of Alberta on 03/13/15 Copyright ASCE For personal use only; all rights reserved The available research conducted on the rehabilitation of walls using FRP was promising; however, the repaired walls were without openings in the previous tests and the effectiveness of repairing RC walls with openings (irregularly and regularly distributed) by a similar method needs to be further investigated Understandably, some of the repair and strengthening schemes for walls without openings may not be suitable for the repair of damaged walls with openings For example, wrapping integrated FRP sheets on the face of the walls cannot be used in retrofitting of walls with openings Bonding discrete FRP strips or sheets could be a good alternative to recover the seismic performance of the damaged walls with an opening However, the direction, width, and the number of layers of each FRP strip had to be determined properly The strut-and-tie model was a truss model of a structural member, or of a D-region in such a member, made up of struts and ties connected at nodes and capable of transferring the factored loads to the supports or to adjacent B-regions (ACI 2008) Thus, this study employed the strut-and tie model to determine the direction, width, and number of layers of the individual FRP strip Strut-and-tie models have been used intuitively for many years in design work, whereby complex stress fields inside a structural member arising from applied loads are simplified into discrete compressive and tensile force paths With the aid of the strut-and-tie model, a better visualization and understanding of the distribution of internal force and the mechanism of force transfer can be achieved The research program in this study taps this advantage to propose FRP-strengthening techniques for RC structural walls with openings Experimental Program Fig Dimensions (mm) and detailing of Specimen W1 The first part of the paper briefly presents the seismic behavior of two one-third scaled RC structural walls as control specimens that were tested under reversed cyclic load After studying the failure modes of the control specimens, the damaged RC walls were repaired by epoxy injection, the loose concrete was replaced by high strength mortar and subsequently strengthened by externally bonded FRP strips, which were designed according to the proposed strut-and-tie models Then these repaired specimens were retested under similar loading conditions Description of Control Specimens Control Specimen W1 had irregularly distributed openings The dimensions of Specimen W1 are given in Fig and had three subassemblies as follows: (1) the top beam, (2) the web, and (3) the foundation beam W1 was 2,000 mm wide, 2,300 mm high, and 120 mm thick, with an aspect ratio of approximately hw =lw ¼ 1.27, where hw ¼ 2; 540 mm was the vertical distance from the lateral loading point to the wall base (Fig 1), whereas lw ¼ 2; 000 mm was the width of the wall The size of each irregularly distributed opening was 600 mm × 600 mm Two deep flanges (120 mm × 400 mm) were added to the side edge of the wall The reinforcement details are also presented in Fig The reinforcements applied in a certain place are denoted according to their quantity, steel types, and diameters as illustrated in Fig For example, 6T10 means there are six T-bars whose diameters are 10 mm High yield strength steel deformed bars and the mild steel plain bars are indicated as T-bars and R-bars, respectively Control Specimen W2 had regularly distributed openings The size of each regularly distributed opening was 400 mm × 400 mm Similar to W1, W2 also had three subassemblies: the top beam, the web, and the foundation beam W2 was 2,600 mm wide, 2,300 mm high, and 120 mm thick, with an aspect ratio of 1.0 Thus, both W1 Fig Dimensions (mm) and detailing of Specimen W2 260 / JOURNAL OF COMPOSITES FOR CONSTRUCTION © ASCE / MARCH/APRIL 2013 J Compos Constr 2013.17:259-270 and W2 are low-rise or squat walls However, the cross section of Specimen W2 is rectangular and without flanges The vertical and horizontal reinforcement details are shown in Fig The web of the wall was divided into beam, column, nodal, and panel zones based on the openings, as shown in Figs and Downloaded from ascelibrary.org by University of Alberta on 03/13/15 Copyright ASCE For personal use only; all rights reserved Material Properties Ready-mix concrete, which had a characteristic strength of 30 MPa, 13 mm maximum size aggregate, and a slump 100 mm, was used to cast the specimens The measured compressive strength f c0 of Specimens W1 and W2 are 36.9 MPa and 39.1 MPa, respectively A high yield steel bar with nominal yield strength of 460 MPa and a mild steel bar with nominal yield strength of 250 MPa were used The properties of the steel bars and FRP are shown in Tables and 2, respectively Fig Typical experimental setup Test Setup The testing rig, shown in Fig 3, consisted of two systems: an inplane loading system and an in-plane base beam reaction system The in-plane loading system included a double action hydraulic jack and four steel beams The jack had a capacity of 2,000 kN in compression and 1,200 kN in tension The stroke of the jack was 405 mm The steel frame was arranged in a configuration such that two-direction lateral loadings could be applied to the wall Four high strength steel rods were preset in the loading beam to enable lateral loading to be applied on the top loading beam The base beam reaction system was designed to resist the rotation and sliding of the wall specimen when the load was applied They were attached to a strong floor by high strength rods Reaction footings were provided to balance the lateral loading A prestress scheme was applied to every steel rod so that the rotation and sliding during the test could be restrained efficiently No axial force was applied in the test because the lateral force transfer mechanisms in the walls were the focus in this study and low axial load levels are common for low-rise shear walls in practice Table Measured Steel Bar Properties Types Yield strength f y (MPa) Ultimate strength f u (MPa) Specimen W1 W2 W1 W2 R6 R10 T10 T13 T20 308 385 480 493 512 293 382 467 493 512 428 502 545 581 607 405 481 541 581 607 Instrumentation and Test Procedure To record data from the experimental setup, a dynamic actuator, strain gauges, linear variable displacement transducers (LVDTs), and displacement transducers were utilized An LVDT was set up at the center of the top beam to measure the top drift LVDTs were arranged vertically in the walls to detect the flexural deformation The panel shears deformations measured by the LVDTs distributed along diagonal directions of the panels Local strains in the reinforcement bars were measured by electric resistance wire strain gauges (TML FLA-5-11-5LT), which were installed on the bars before casting of the specimens The specimens were tested under cyclic lateral loading, which was applied to the top of the wall The loading cycles were displacement-controlled in which the top displacement was expressed as a factor of the vertical height of 2,540 mm from the base of the wall to the point where the load was applied The factors used were 1=2;000, 1=1;000, 1=600, 1=400, 1=300, 1=200, 1=150, 1=100, 1=75, and 1=50 multiplied by the vertical height of 2,540 mm from the wall base to the point where the load is applied The typical loading procedures are illustrated in Fig Seismic Behavior and Failure Modes of the Control Specimens Control Specimens W1 and W2 had been tested to failure and sustained severe damage Specimen W1 developed a sliding failure at the bottom panel, but the sliding face was approximately 200 mm above the base of the wall The damage sustained by Specimen W1 included severe concrete crushing and spalling at the bottom Table Tyfo Fiberwrap Composite System Propertiesa Parameter Type of FRP Ultimate tensile strength in primary fiber direction Ultimate tensile strength 90 degrees to the primary fiber direction Elongation at break Tensile Modulus Laminate thickness a GFRP with epoxy, Tyfo SEH-51A composite CFRP with epoxy, Tyfo SCH-41 composite Unidirectional GFRP sheet 575.0 MPa 25.8 MPa 2.2% 26.1 × 103 MPa 1.3 mm Unidirectional CFRP sheet 986.0 MPa 40.6 MPa 1.0% 95.8 × 103 MPa 1.0 mm Property values given are based on test value by supplier (FYFE Asia Pte Ltd in Singapore) JOURNAL OF COMPOSITES FOR CONSTRUCTION © ASCE / MARCH/APRIL 2013 / 261 J Compos Constr 2013.17:259-270 Top Lateral Displacement (mm) 60 P (+) 1/50 40 1/75 1/100 20 1/2000 1/1000 1/600 1/400 1/300 1/200 1/150 -20 -40 -60 Downloaded from ascelibrary.org by University of Alberta on 03/13/15 Copyright ASCE For personal use only; all rights reserved 10 12 Cycle number 14 16 18 20 Fig Typical loading procedures Fig Typical cracking patterns of Specimen W2 after test flanges and the right bottom corner of the bottom panel Furthermore, the shear force had to be sustained by the bottom panel because the bottom right column, where the concrete cracked severely, could not bear the shear force When the sliding face was formed, the strength of the wall decreased significantly The severe sliding shear failure that occurred in W1 could be attributed to several causes First, low-rise (squat) shear walls had a much higher propensity for shear failure compared with slender walls Second, boundary elements (deep flanges) significantly increased the flexural strength of a wall but simultaneously jeopardized the shear strength compared with a rectangular wall Third, piercing the wall with openings further weakens its shear strength Moreover, most of the steel bars in the flanges buckled, and some steel bars were fractured The final crack pattern of Specimen W1 is illustrated in Fig On the other hand, Specimen W2 had obvious movement observed along the diagonal cracks that appeared on every panel The four bottom columns were damaged severely but no sliding shear failure was observed In particular, extremely severe concrete crushing was observed at the two columns near to the bottom edges Significant spalling of the concrete cover and buckling of the steel bars were also observed The final crack pattern of Specimen W2 is illustrated in Fig Strut-and-Tie Models As mentioned previously, strut-and-tie models were utilized to aid in designing the strengthening schemes of the damaged specimens P(+) Fig Typical cracking patterns of Specimen W1 after test because of the openings, which changed the load path and stress distribution significantly In general, a strut-and-tie model would simplify a structural member as a hypothetical truss The compressive concrete struts and tensile steel ties joined together at the nodal zones The following assumptions were made in the development of the strut-and-tie models of Specimen W1: (1) the beam and column zones (as shown in Figs and 2) were subjected to axial tensions when the entire zones were under tension in the load paths; (2) all reinforcements in the beam or column zones were lumped into one tie, and its position was located in the centriodal axis of the reinforcement lumped into it; and (3) the strut position was determined by keeping the concrete compressive stress in it lower than the strength limitations suggested by Schlaich et al (1987), which is 0.68 f c0 for a concrete strut with cracks parallel to it or 0.51 fc0 for a concrete strut with skew cracks According to these assumptions, the tie area and its influence region can be easily determined However, the real geometry of a strut may sometimes be difficult to illustrate because the strut may represent a bottle-shaped stress field In normal practice, the strut can be idealized into a prismatic or uniformly tapered shape according to the geometry of the nodal zone For a concentrated node, its geometry can be clearly defined by the boundary of the bearing plate or tie In the case of walls with an irregular opening where generally no bearing plate presents, the geometries of the nodes and struts are determined based on the divisions of the beam and column zones and the position of the openings For example, the details of the strut, nodes, and compressive stress of each strut of Specimen W1 are presented in the Appendix The greater details of the experimental results of the strut-and-tie models are described in Wu (2005) and Zhao (2004) for Specimens W1 and W2, respectively As the load path and force magnitude of the struts and ties are the most important factor considered in the FRP retrofitting design, the load path and force magnitude of Specimens W1 and W2 are illustrated in Figs and 8, respectively In the models, the compression struts were shown as dotted lines, whereas the tensile steel ties were shown as solid lines The load path of the strut-and-tie model of each specimen was determined based on the crack pattern observed from the tests and the principal stress flows obtained from the numerical analysis As shown in Fig 7, the strut-and-tie models of Specimen W1 in positive and negative loads were different owing to the existence of an irregular opening However, the strut-and-tie models of Specimen W2 were symmetrical in these two load directions as shown in Fig The widths of the FRP strips were calculated according to the load magnitude and were placed in the direction and location of the load paths in the strut-and-tie models 262 / JOURNAL OF COMPOSITES FOR CONSTRUCTION © ASCE / MARCH/APRIL 2013 J Compos Constr 2013.17:259-270 O A S T G kN W 6.4 U F 42 379.2 kN Y kN kN 353.6 kN 600 X 600 Opening 70.8 kN 114.4 kN E Q 84 343.2 kN R 282.8 kN Z kN 3.2 34 34 kN 600 X 600 Opening 13 4 N D 329.2 kN kN M 600 X 600 Opening C 66 218.0 kN B 600 X 600 Opening 600 X 600 Opening H I J Fig Strut-and-tie models for Specimens W1 P (+) 305.0 196.0 20 163 51.4 2.8 24.9 0.5 45.7 4.3 19.7 16 8.9 29.9 9460.2 400 X 400 Opening 75.2 86.0 7.8 14 19.7 180 63.8 20.1 16 11 14.2 9.7 17.1 84.4 1.1 5.8 22188.1 17140.6 5.9 37 283.2 55 14 26.9 6.6 40.2 63.3 22.4 4.5 22 104.8 98 24.4 0.7 5.3 16 13 26.0 10.9 78 254.7 Downloaded from ascelibrary.org by University of Alberta on 03/13/15 Copyright ASCE For personal use only; all rights reserved kN 379.2 kN kN 53 600 X 600 Opening L 248.8 kN 314.8kN 50 P (-) 104.8 kN P (+) K Fig Strut-and-tie models for Specimens W2 Repair and Strengthening Schemes The walls were first visually inspected for cracks and loose concrete Then, the loose/spalled concrete was removed using hammers and chisels before repairing the cracks For cracks of width larger than 0.3 mm but less than 20 mm, epoxy resin was injected to seal them For cracks with crack width larger than 20 mm but less than 50 mm, patch repair using a bonding agent and polymer modified cementitious mortar was adopted The depth of each layer of the patch repair could not exceed 20 mm For regions where the depth of the concrete removed exceeded 50 mm, it was repaired by pumping grout into the damaged region using a pressurized grouting method The grout consisted of prepacked 20-mm-diameter aggregates mixed with cement treated with super plasticizer, and grout fluidifier was used to achieve high strength and workability The specimens were then left to cure for day because the epoxy resin required a minimum of 24 h for curing Subsequently, as mentioned previously, the damaged control specimens were strengthened by externally bonded FRP strips aligned along the load path of the strut-and-tie models The repaired and strengthened wall specimens were denoted RW1 and RW2, respectively Figs and 10 show the proposed FRP-strengthening schemes of Specimens RW1 and RW2, respectively As shown in Fig 9(a), the tie-strengthening scheme of Specimen RW1 is described as follows: The width and number of layers of the FRP sheets were determined based on the tensile force of the tie determined by the strut-and-tie models (refer to Fig 7) The tensile force of the tie was assumed provided by the externally bonded FRP strips only Thus, for example, the design width of the glass fiber reinforced polymer (GFRP) strip #1 was determined as follows: Ws ¼ T s =2 248.8 ì 103 ẳ 166 mm ẳ f FRP ì dFRP ì 575 ì 1.3ị 1ị where W s = design width of the FRP sheet; T s = tensile force of the tie along the FRP sheet (the tie force of strip #1 is 248.8 kN); fFRP = tensile strength of the FRP sheet; and dFRP = thickness of the FRP sheet (One layer of FRP was assumed in the preceding calculation If the spacing was restrained, more layers of FRP could be designed.) Thus, a one-layer GFRP sheet with 150 mm width was applied on both faces of the wall (#1 strip) For tie strengthening, the designed FRP strips were applied on the wall in sequence as shown in Fig 9(a) For strut strengthening (see the Appendix), the strut stresses are normally less than the limitation (0.68fc0 ) suggested by Schlaich et al (1987), except for strut QW However, the influence of the flanges has not been considered in the strut width of the strut QW Thus, no crushing of the struts was observed in the failure mode of Specimen W1 Similar behavior was observed in Specimen W2 Thus, the strut strengthening primarily relied on the epoxy injection to fill the cracks and external bonded the FRP sheets [as shown in Fig 9(b)] These FRP sheets along the cracks not only improved the effectiveness of the epoxy repairing but also delayed the crack development during the test because the fibers perpendicular to the primary fibers could provide a slight tensile strength (as given in JOURNAL OF COMPOSITES FOR CONSTRUCTION © ASCE / MARCH/APRIL 2013 / 263 J Compos Constr 2013.17:259-270 S1 (Width x layers) Unit (mm) 150 x (G) S2 S3 Note: G=Glass fiber C=Carbon fiber =Strain gauge 200 x (G) S4 S5 S6 200 x (G) 250 x (G) C-Shape (1 Layer) (C) Downloaded from ascelibrary.org by University of Alberta on 03/13/15 Copyright ASCE For personal use only; all rights reserved Front Face of Tie Strengthening C-shape Wrap (1 Layer) (a) 280 x (C) 290 x (C) (Width x layers) Unit (mm) C-Shape (1 Layer) (G) 360 x (C) 240 x (C) Note: G=Glass fiber C=Carbon fiber =Strain gauge C-Shape (1 Layer) (G) 14 10 190 x (C) 150 x (C) 12 190 x (C) 15 11 13 Front Face of Strut Strengthening (b) However, for strut strengthening, RW2 was slightly different from that of Specimen RW1 For RW1, diagonal FRP sheets were applied along the direction of the diagonal struts For RW2, to evaluate whether resolving the diagonal compressive force of the strut into horizontal and vertical components and applying the FRP strips according to the magnitudes of these components could be an effective alternative method, part of the diagonal struts were strengthened through a pair of vertical and horizontal FRP strips Fig 10(b) shows the final strut-strengthening scheme on both faces of the wall Fig shows the failure mode of Specimen W1, indicating that sliding failure was observed at the bottom panel, which was approximately 200 mm above the base of the wall Thus, a one-layer L-shaped GFRP sheet was applied on both faces of the base of Specimen W1 (as shown in Fig 11) Similarly, L-shaped GFRP sheets were applied on both faces of the base columns of Specimen RW2 Moreover, for Specimen RW2, GFRP sheets with 400 mm width were utilized to wrap each of the bottom columns because severe damage and concrete crushing was observed in these columns (as shown in Fig 6) Moreover, to prevent premature delamination of the L-shaped GFRP sheets, a series of fiber anchors were mounted (the locations of the fiber anchors of RW1 and RW2 are shown in Figs 11 and 12, respectively) The fiber anchor consisted of two parts: the anchor bolt and the protruding fibers The total length of the fiber anchors used was approximately 110 mm The length and diameter of the anchor bolt were approximately 50 and mm, respectively First, holes (10 mm in diameter) were drilled at a depth of 50 mm on the wall faces before the application of the FRP sheets After the application of the FRP sheets, the anchor bolts were inserted through the epoxy resin into the holes The protruding fibers were bent and spread out in circles with a radius of approximately 60 mm to act as the base of anchorage To ensure good adherence between the FRP sheets and concrete, the surface was first cleared of dust and any deleterious substances that might act as bond barriers Sharp edges and protrusions were also removed by mechanical grinding Results and Discussion 10 Failure Modes and Response under Cyclic Loading 12 15 11 13 14 (c) Fig Proposed FRP strengthening schemes for Specimen RW1: (a) tie-strengthening scheme; (b) currently used strut-strengthening scheme; (c) refined strut-strengthening scheme Table 2) In the future, to further restore the seismic performance of damaged walls with openings, tie strengthening in conjunction with concrete tensile strength strengthening [bond FRP strips along the direction of principal tensile strength, as shown in Fig 9(c)] was recommended For Specimen RW2, similar to Specimen RW1, tie-strengthening strips were first applied on the surface of the wall The tensile strips were designed based on the magnitude of the tie force as determined by the strut-and-tie models (refer to Fig 8) Fig 10(a) illustrates the final tie-strengthening scheme of Specimen RW2 After repairing and strengthening of the damaged control specimens, the repaired Specimens RW1 and RW2 were retested under a similar reversed cyclic lateral load, and the test results were illustrated For repaired specimens, the lateral displacement was repeated for two cycles before the drift ratio reached 1.0% After that, only one cycle of the lateral displacement was applied However, for the control specimens, the lateral displacement was repeated for two cycles until the failure of the specimens For Specimen RW1, no visible cracks or debonding were observed during the initial cycles of the test At the drift ratio of 0.25%, hairline cracking was distributed along the height of the wall Because the RC walls at this point were wrapped by FRP sheets, the presence of the cracks was inferred from the thin marks in the epoxy resin at the exterior surface of the FRP sheets At the drift ratio of 0.33%, the breaking of epoxy resin was heard and a horizontal flexural crack formed at the interface of the repair mortar and the anchor block at the right base of the wall Subsequently, the crack opened substantially During the first cycle at the drift ratio of 0.67%, the GFRP sheet at the right base of the wall delaminated at the edges, exposing the concrete underneath Horizontal flexure cracks also started to form at the center of the wall base near the opening In the second cycle, the GFRP sheet at the left base of the wall 264 / JOURNAL OF COMPOSITES FOR CONSTRUCTION © ASCE / MARCH/APRIL 2013 J Compos Constr 2013.17:259-270 Note: =Strain gauge (Width x layers) Unit (mm) Front Face of Tie Strengthening S1 S3 S4 S5 S7 S2 40x1 S6 20x1 S8 S10 S11 S12 S16 S13 S14 S17 S18 S20 S21 C-shape 20x1 40x1 Wrap S9 30x1 50x1 S15 S19 20x1 40x1 20x1 C-shape 60x1 Wrap C-shape Wrap (1 Layer) Downloaded from ascelibrary.org by University of Alberta on 03/13/15 Copyright ASCE For personal use only; all rights reserved (a) P(+) P(-) 70x3 60x3 25x1 25x1 45x1 35x1 35x1 50x1 45x1 45x1 P(+) P(-) 70x2 60x3 70x1 70x1 90x1 70x2 50x2 60x2 80x2 50x4 80x1 50x1 70x1 70x2 50x4 70x2 40x1 50x3 50x1 50x3 70x2 Front Face of Strut Strengthening 60x3 70x1 70x1 90x1 70x2 50x2 60x2 80x2 50x4 80x1 50x1 70x1 70x3 60x3 25x1 25x1 45x1 35x1 35x1 50x1 45x1 45x1 Back Face of Strut Strengthening Note: (Width x layers); Unit (mm) (b) Fig 10 Proposed FRP strengthening schemes for Specimen RW2: (a) tie-strengthening scheme; (b) currently used strut-strengthening scheme Back Face Front Face Locations of Fiber Anchorage in the Bottom Locations of Fiber Anchorage in the Bottom Fig 11 Proposed fiber anchorage schemes in the base of the wall of Specimen RW1 debonded As the drift ratio increased, loud cracking resulting from the debonding of the FRP sheets was heard At the drift ratio of 1.00%, the GFRP sheet delaminated completely from the base The FRP sheet at the right base of the wall was then completely delaminated It was obvious that Specimen RW1 had a flexural failure, which was concentrated between the base and the foundation For Specimen RW2, the breaking of epoxy resin was heard at the drift ratio of 0.25% At the first cycle of the drift ratio of 0.50%, visible epoxy resin cracks were observed at the base of the wall Cracks started to appear at the wall base at the drift ratio of 0.67% At the drift ratio of 1.00%, diagonal cracks on the wall were observed, and the cracks were primarily at the lower part of the wall As the drift ratio increased, the FRP strips debonded from the wall to the base A vertical crack was formed, propagating from the base edge upwards, with a crack length of 150 mm When the maximum strength was reached, the whole wall tilted forward in the out-of-place direction Load-Displacement Hysteresis Responses Fig 13 shows the load-displacement hysteresis loops of the control and repaired specimens The hysteretic behavior was evaluated in JOURNAL OF COMPOSITES FOR CONSTRUCTION © ASCE / MARCH/APRIL 2013 / 265 J Compos Constr 2013.17:259-270 600 0.5 % 1.0 % 1.5 % 2.0 % 500 Drift ratio 400 300 200 100 -100 -200 -300 -400 W1 Drift ratio -500 RW1 -2.0 % -1.5 % -1.0 % -0.5 % -600 -60 -50 -40 -30 -20 -10 10 20 30 40 50 60 Horizontal Displacement (mm) 600 0.5 % 1.0 % 1.5 % 2.0 % 500 Drift ratio 400 300 200 100 -100 -200 -300 -400 W2 Drift ratio -500 RW2 -2.0 % -1.5 % -1.0 % -0.5 % l -600 -60 -50 -40 -30 -20 -10 10 20 30 40 50 60 Horizontal Displacement (mm) Fig 13 Load-displacement hysteretic loops for control and repaired specimens terms of lateral resisting capacity and maximum displacement The lateral resisting capacity of Specimen RW2 was significantly higher than the corresponding control Specimen W2 in the positive and negative loading cycles, respectively The maximum peak strengths in each positive and negative cycle of control Specimen W2 were 334 and 348 kN, respectively, and were reached at the drift ratio of 0.50% For repaired Specimen RW2, the maximum peak strengths Load-Displacement Envelopes To study the lateral resisting capacity and ultimate displacement capacity of the specimens, a comparison between the envelopes of hysteretic loops of the tested specimens is shown in Fig 14 Lateral Load (kN) Lateral Load (kN) Fig 12 Proposed fiber anchorage schemes in the base of the wall of Specimen RW2 in each positive and negative cycle were 456 and 411 kN, respectively This indicated that the proposed repair and strengthening schemes were effective for repairing the damaged walls with regularly distributed openings However, for repaired Specimen RW1, the maximum strength was reached at the drift ratio of 0.50% for both the control and repaired specimens At this drift ratio, the maximum strengths in the positive and negative direction of control Specimen W1 were 385 and 394 kN, respectively, whereas those of repaired Specimen RW1 were 411 and 308 kN, respectively Thus, the lateral resisting capacity of the repaired specimens was only slightly higher than that of their corresponding control specimens in the positive loading cycles, and repaired Specimen RW1 did not reasonably recover the strength of Specimen W1 in the negative loading cycles The strengthening schemes of Specimen W1 were asymmetrical owing to the irregularly distributed openings In addition, as shown in Fig 9(b), the FRP sheets were applied on the face of the wall according to the strengthening scheme of the positive load cycle first Thus, the FRP sheets that were subsequently applied for strengthening the wall in the negative loading cycle helped to anchor the previously applied ones and indirectly improved the effectiveness of the strengthening sheets for the positive loading cycle Lateral Load (kN) Locations of Fiber Anchorage in the Bottom Lateral Load (kN) Downloaded from ascelibrary.org by University of Alberta on 03/13/15 Copyright ASCE For personal use only; all rights reserved Front Face 600 0.5 % 1.0 % 1.5 % 2.0 % 500 Drift ratio 400 W1 300 200 RW1 100 -100 -200 -300 -400 Drift ratio -500 -2.0 % -1.5 % -1.0 % -0.5 % -600 -60 -50 -40 -30 -20 -10 10 20 30 40 50 60 Horizontal Displacement (mm) 600 0.5 % 1.0 % 1.5 % 2.0 % 500 Drift ratio 400 W2 300 200 RW2 100 -100 -200 -300 -400 Drift ratio -500 -2.0 % -1.5 % -1.0 % -0.5 % -600 -60 -50 -40 -30 -20 -10 10 20 30 40 50 60 Horizontal Displacement (mm) Fig 14 Comparison of load-displacement envelope of control and repaired specimens 266 / JOURNAL OF COMPOSITES FOR CONSTRUCTION © ASCE / MARCH/APRIL 2013 J Compos Constr 2013.17:259-270 Energy Dissipation Capacity RW2 reached the maximum dissipated energy of about 280 kNm, whereas that for Specimen W2 only reached 60 kNm Stiffness Degradation The stiffness of both the control and repaired walls was estimated based on the secant stiffness of the plots of force against displacement Fig 16 shows the comparisons of stiffness degradation for each tested specimen The comparison of the repaired Specimen RW1 curve with the corresponding curve for control Specimen W1 shows that the initial stiffness of Specimen RW1 was significantly higher than that of Specimen W1 The repaired Specimen RW1 was not as stiff as the original wall in considering the negative loading cycles; whereas, generally speaking, the repaired Specimen RW1 had recovered the stiffness reasonably On the other hand, the repaired Specimen RW2 not only had much higher initial stiffness but also had delayed stiffness degradation compared with the corresponding control Specimen W2 This is a desirable property in an earthquake-like situation It was observed in the past earthquake that most of the RC structures failed owing to the sudden loss of stiffness of structural joints with increasing lateral movement of the structure FRP Strains The readings in the strain gauges attached to the FRP strips are shown in Figs 17 and 18 for Specimens RW1 and RW2, 250 120 200 100 Secant Stiffness (kN/mm) Energy (kNm) The energy dissipated was based on the cumulative energy dissipated calculated by summing up the energy dissipated in consecutive loops throughout the test Fig 15 shows the comparison of the cumulated energy dissipated for the control and repaired specimens The repaired specimens not only restored the energy dissipation capacity of the control specimens, its energy dissipation capacity also increased by more than twofold For Specimen RW1, after the drift ratio of 0.4%, the energy dissipated was more than double that of the control specimens At every drift ratio for Specimen RW2, it had very significant improvement in the energy dissipated compared with Specimen W2 At the final stage, Specimen 150 100 50 0.0 W1 RW1 0.2 0.4 0.6 0.8 1.0 Drift Ratio (%) 1.2 1.4 80 60 40 300 120 250 100 200 150 100 50 0.0 W1 RW1 20 -2.0 1.6 Secant Stiffness (kN/mm) Energy (kNm) Downloaded from ascelibrary.org by University of Alberta on 03/13/15 Copyright ASCE For personal use only; all rights reserved The lateral resisting capacity of Specimen RW2 was larger than its corresponding control Specimen W2 in both the positive and negative loading cycles by 36.5 and 18.1%, respectively However, its strength at the drift ratio of 1.33% decreased by 52.0 and 25.4% in the positive and negative loading cycles, respectively This faster strength degradation was not favorable for a rehabilitation objective In the positive loading cycles, repaired Specimen RW1 only enhanced the strength of Specimen W1 by 6.8% Specimen RW1 also only recovered the strength of Specimen W1 by approximately 78.2% in the negative loading cycles Similar to Specimen RW2, it was not reasonable for Specimen RW1 to recover the ductility of Control Specimen W1 -1.5 -1.0 -0.5 0.0 0.5 Drift Ratio (%) 1.0 0.2 0.4 0.6 0.8 1.0 Drift Ratio (%) 1.2 1.4 1.6 Fig 15 Comparison of energy dissipation capacity of control and repaired specimens 2.0 80 60 40 W2 RW2 20 W2 RW2 1.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 Drift Ratio (%) 1.0 1.5 2.0 Fig 16 Comparison of secant stiffness between control and repaired specimens JOURNAL OF COMPOSITES FOR CONSTRUCTION © ASCE / MARCH/APRIL 2013 / 267 J Compos Constr 2013.17:259-270 1200 according to Eqs (2 and 3), it was understandable that the majority of the tensile force of the ties was still provided from the steel reinforcement -6 Strain (10 ) 1000 800 ð2Þ Fcontribution ¼ Ftie − Fcontribution steel frp ð3Þ 600 S1 S2 S3 S4 S5 S6 400 200 0.0 0.2 0.4 0.6 0.8 1.0 Drift Ratio (%) 1.2 1.4 where Fcontribution = FRP’s contribution; Fcontribution = steel rebar’s frp steel measured contribution; εfrp = measured strain of the FRP strip; Gfrp = tensile modulus of the FRP; Afrp = cross-sectional area of each FRP; and Ftie = tie force obtained from strut-and-tie models Fig 17 FRP strains on Specimen RW1 Conclusions 300 -6 Strain (10 ) 250 S1 S2 S3 S4 S5 S6 S7 S8 S9 S10 S11 200 150 100 50 0.0 0.2 0.4 0.6 0.8 1.0 Drift Ratio (%) 1.2 1.4 350 300 250 Strain (10-6) Downloaded from ascelibrary.org by University of Alberta on 03/13/15 Copyright ASCE For personal use only; all rights reserved ẳ measured ì Gfrp × Afrp Fcontribution frp frp 200 S12 S13 S14 S15 S18 S19 S20 S21 150 100 50 -50 0.0 0.2 0.4 0.6 0.8 1.0 Drift Ratio (%) 1.2 1.4 Fig 18 FRP strains on Specimen RW2 respectively As shown in the figures, the recorded strain in the fiber was relatively low compared with their fracture strain of 1.0% (10; 000 με), although the strain gauge reading along the tiestrengthening fibers displayed tensile reading This was possibly because debonding of the FRP at the wall base prevented the full development of the strength of the fiber in tie strengthening Moreover, because no special anchorage was designed for these FRPs in the wall body, it was predictable that the strength of the FRP strips could not be fully developed For Specimen RW2, similar to Specimen RW1, all strain readings fell below 350 με, which was much lower than the allowable value of 1.0% Although some of the fiber readings were initially compressive, the majority of the fibers obtained tensile value As the tie force of each tie only had two sources (steel reinforcement or FRP for tie strengthening), In the present paper, the strut-and-tie models were utilized to help in designing the strengthening schemes for repairing damaged structural wall with openings Based on the observations and the experimental results of this study, the following conclusions can be made: • The proposed strengthening schemes designed by the strut-andtie models can effectively recover the overall behavior (strength, stiffness, and energy dissipation capacity) of damaged specimens with irregularly or regularly distributed openings It indicates that the strut-and-tie models are effective to help design the strengthening schemes for walls with openings • Most of the FRP-strain readings indicated that the FRP strips were not fully utilized because the tensile strain was relatively low compared with the fracture strain of the FRP strips This indicated that the majority of the tie force was still provided by the steel bars This was primarily because the tests were stopped as a result of debonding of the FRP sheets in the connection in between the base wall and the foundation Moreover, no special anchorage was provided in the FRP strips for tie and strut strengthening • In future practice, replacing the integral FRP sheets along the diagonal cracks with several short FRP strips bonded perpendicular with the diagonal cracks will strengthen the strut and increase the effectiveness of the FRP strips to delay the reopening of the diagonal cracks • Fiber anchors were generally effective in improving the sliding capacity of the walls because few areas on the wall base showed failure in anchorage However, the performance of the repaired specimens could be further improved if the delamination of the FRP sheets could be delayed or prevented, such as by using a steel plate anchorage to replace the fiber anchor in the base of the wall Moreover, special anchorage provided in the FRP strips for tie strengthening could improve the strengthening effectiveness Appendix Details of the Strut-and-Tie Model of Specimen W1 Fig 19 presents the details of the strut and nodes of Specimen W1 The regions of extended nodal zones are also depicted Tables and list the primary properties of the ties and struts, respectively Moreover, Table gives the strut width of Specimen W1 The strut width is measured at the narrowest segment of the strut as shown in Fig 19 and is the smallest length of a line from a point at the strut boundary extending perpendicular to the axis of the strut 268 / JOURNAL OF COMPOSITES FOR CONSTRUCTION © ASCE / MARCH/APRIL 2013 J Compos Constr 2013.17:259-270 O K P (+) P (-) A L B Downloaded from ascelibrary.org by University of Alberta on 03/13/15 Copyright ASCE For personal use only; all rights reserved D Y N M C Z R S T Q E F W G H I J U Fig 19 Details of the modified strut-and-tie models of Specimen W1 Table Main Properties of Ties of Strut-and-Tie Models of Specimen W1 Directions Ties Lumped reinforcement Capacity (kN) Loading factor Negative OD DJ AI FH BD EF 8R10+2T10 8R10+2T10 6T10 4T10 6T10 4T10 313.2 313.2 220.0 146.6 220.0 146.6 0.26 0.95 0.62 0.27 0.82 0.18 KM MR NS LT NQ 8R10+2T10 8R10+2T10 4T10 6T10 6T10 313.2 313.2 146.6 220.0 220.0 0.79 1.30a 0.86b 0.55 0.71 Positive a Assume strut MU carrying all force of tie NQ Assume strut NU carrying all force of tie NQ b Table Primary Properties of Struts of Strut-and-Tie Models of Specimen W1 Directions Struts Angle (°) Area (mm2 ) Predicted σc max load factor (N=mm2 )d σc max =fc0 Negative AB BEa DG FG 41.5 78.7 39.8 58.3 31,800 10,560 37,200 20,400 1.34 0.90 1.07 0.34 11.2 22.9 7.70 4.42 0.30 0.62 0.21 0.12 Positive KL LQ QWa NU MU 38.2 53.1 77.6 50.5 35.7 29,640 21,600 11,040 19,200 13,920 1.27 1.67 1.36 1.11b 0.87c 13.2 23.8 38.1 17.8 19.3 0.36 0.64 1.03 0.48 0.52 a The strut angle is determined by the shear taken by the column it represents b Assume strut NU carrying all force of tie NQ c Assume strut MU carrying all force of tie NQ d Strength developed in the strut when maximum strength of model is reached Table Widths of Struts of Specimen W1 Strut AB BE DG FG KL LQ QW NU MU Width (mm) 265 88 310 170 247 180 92 160 116 Acknowledgments This research was made possible through the support of and collaboration with FYFE Asia Private Limited in Singapore The significant assistance from Jeslin Quek and Ow Meng Chye of FYFE Asia are gratefully acknowledged References Ali, M., and Wight, J K (1991) “RC structural walls with staggered door openings.” J Struct Eng., 117(5), 1514–1531 American Concrete Institute (ACI) (2008) “Building code requirement for structural concrete (ACI 318-08) and commentary (318R-08).” Farmington Hills, MI Antoniades, K K., Salonikios, T N., and Kappos, A J (2003) “Cyclic tests on seismically damaged reinforced concrete walls strengthened using fiber-reinforced polymer reinforcement.” ACI Struct J., 100(4), 510–518 El-Maaddawy, T., and Chekfeh, Y (2012) “Retrofitting of severely shear-damaged concrete t-beams using externally bonded composites and mechanical end anchorage.” J Compos Constr., 16(6), 693–704 Engindeniz, M., Kahn, L F., and Zureick, A H (2005) “Repair and strengthening of reinforced concrete beam-column joints: State of the art.” ACI Struct J., 102(2), 187–197 Lam, L., and Teng, J G (2001) “Strength of RC cantilever slabs bonded with GFRP strips.” J Compos Constr., 5(4), 221–227 Li, B., and Chua, H Y G (2009) “Seismic performance of strengthened reinforced concrete beam-column joints using FRP composites.” J Struct Eng., 135(10), 1177–1190 Li, B., and Kai, Q (2011) “Seismic behavior of reinforced concrete interior beam-wide column joints repaired using FRP.” J Compos Constr., 15(3), 327–338 Li, B., and Lim, C L (2010) “Tests on seismically damaged reinforced concrete structural walls repaired using fiber-reinforced polymers.” J Compos Constr., 14(5), 597–608 Lombard, J., Lau, D T., Humar, J L., Foo, S., and Cheung, M S (2000) “Seismic strengthening and repair of reinforced concrete shear walls.” Proc., 12th World Conf on Earthquake Engineering, (CD-ROM), New Zealand Society for Earthquake Engineering, Silverstream, New Zealand, Paper No 2032 Marti, P (1985) “Basic tools of reinforced concrete beam design.” ACI Struct J., 82(1), 46–56 Neale, K W., Demers, M., Devino, B., and Ho, N Y (1997) “Strengthening of wall-type reinforced concrete columns with fiber reinforced JOURNAL OF COMPOSITES FOR CONSTRUCTION © ASCE / MARCH/APRIL 2013 / 269 J Compos Constr 2013.17:259-270 Teng, J G., and Lam, L (2002) “Compressive behavior of carbon fiber reinforced polymer-confined concrete in elliptical column.” J Compos Constr., 128(12), 1535–1543 Wu, H (2005) “Design of reinforced concrete walls with openings-strut-andtie approach.” Ph.D thesis, Nanyang Technological Univ., Singapore Yanez, F V., Park, R., and Paulay, T (1992) “Seismic behavior of reinforced concrete walls with irregular openings.” Earthquake Engineering: 10th World Conf., International Association for Earthquake Engineering, Madrid, Spain Zhao, Y M (2004) “Strut-and-tie modeling of two-dimensional reinforced concrete structural elements.” M.S thesis, Nanyang Technological Univ., Singapore Downloaded from ascelibrary.org by University of Alberta on 03/13/15 Copyright ASCE For personal use only; all rights reserved composite sheets.” Structural failure, durability, and retrofitting, K C G Ong, J M Lau, and P Paramasivam, eds., Singapore Concrete Institute, Singapore, 410–417 Pampanin, S., Bolognini, D., and Pavese, A (2007) “Performance-based seismic retrofit strategy for existing reinforced concrete frame systems using fiber-reinforced polymer composites.” J Compos Constr., 11(2), 211–226 Schlaich, J., Schäfer, K., and Jennewein, M (1987) “Toward a consistent design of structural concrete.” PCI J., 32(3), 74–150 Teng, J G., Chen, M., Chen, J F., Rosenboom, O A., and Lam, L (2009) “Behavior of RC beams shear strengthened with bonded or unbonded FRP wraps.” J Compos Constr., 13(5), 394–404 270 / JOURNAL OF COMPOSITES FOR CONSTRUCTION © ASCE / MARCH/APRIL 2013 J Compos Constr 2013.17:259-270  ... wrapping integrated FRP sheets on the face of the walls cannot be used in retrofitting of walls with openings Bonding discrete FRP strips or sheets could be a good alternative to recover the... compressive and tensile force paths With the aid of the strut -and- tie model, a better visualization and understanding of the distribution of internal force and the mechanism of force transfer... openings It indicates that the strut -and- tie models are effective to help design the strengthening schemes for walls with openings • Most of the FRP- strain readings indicated that the FRP strips were