Retrofitting Earthquake-Damaged RC Structural Walls with Openings by Externally Bonded FRP
- 格式:pdf
- 大小:1.89 MB
- 文档页数:12
Retrofitting Earthquake-Damaged RC Structural Walls with Openings by Externally Bonded FRP Strips and Sheets
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.©2013American 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 chang-ing 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 Wight1991;Marti1985),the effects of the regular and irregular openings on the seismic performance of RC walls are still not fully understood.Thus,two3-story reinforced concrete model walls,scaled to one-third,were tested under reversed cyclic lateral load.Sspecimens W1and W2were 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 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,col-umns,and beam-column joints(Lam and Teng2001;Teng and Lam2002;Pampanin et al.2007;Teng et al.2009;Li and Chua 2009;Li and Kai2011;El-Maaddawy and Chekfeh2012),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 fi-bers 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 reinforce-ment.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 approx-imately30%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 medium-rise 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.
1Associate Professor and Director,Natural Hazards Research Centre at Nanyang Technological Univ.,Singapore639798(corresponding author). E-mail:cbli@.sg
2Research Associate,Natural Hazards Research Centre at Nanyang Technological Univ.,Singapore.E-mail:qiankai@.sg 3Lecturer,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 February24,2012;approved on September25,2012;published online on September27,2012.Discus-sion period open until September1,2013;separate discussions must be submitted for individual papers.This paper is part of the Journal of Com-posites for Construction,V ol.17,No.2,April1,2013.©ASCE,ISSN 1090-0268/2013/2-259-270/$25.00.
The available research conducted on the rehabilitation of walls using FRP was promising;however,the repaired walls were with-out 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(ACI2008).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 com-pressive 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
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 fail-ure 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 W1had irregularly distributed openings.The dimensions of Specimen W1are given in Fig.1and had three subassemblies as follows:(1)the top beam,(2)the web,and (3)the foundation beam.W1was2,000mm wide,2,300mm high, and120mm thick,with an aspect ratio of approximately h w=l w¼1.27,where h w¼2;540mm was the vertical distance from the lateral loading point to the wall base(Fig.1),whereas l w¼2;000mm was the width of the wall.The size of each irregu-
larly distributed opening was600mm×600mm.Two deep flanges(120mm×400mm)were added to the side edge of the wall.The reinforcement details are also presented in Fig.1.The reinforcements applied in a certain place are denoted according to their quantity,steel types,and diameters as illustrated in Fig.1. For example,6T10means there are six T-bars whose diameters are 10mm.High yield strength steel deformed bars and the mild steel plain bars are indicated as T-bars and R-bars,respectively.
Control Specimen W2had regularly distributed openings.The size of each regularly distributed opening was400mm×400mm. Similar to W1,W2also had three subassemblies:the top beam,the web,and the foundation beam.W2was2,600mm wide,2,300mm high,and120mm thick,with an aspect ratio of1.0.Thus,both
W1Fig.1.Dimensions(mm)and detailing of Specimen
W1 Fig.2.Dimensions(mm)and detailing of Specimen W2
and W2are low-rise or squat walls.However,the cross section of Specimen W2is rectangular and without flanges.The vertical and horizontal reinforcement details are shown in Fig.2.The web of the wall was divided into beam,column,nodal,and panel zones based on the openings,as shown in Figs.1and 2.Material Properties
Ready-mix concrete,which had a characteristic strength of 30MPa,13mm maximum size aggregate,and a slump 100mm,was used
to cast the specimens.The measured compressive strength f 0c
of Specimens W1and W2are 36.9MPa and 39.1MPa,respectively.A high yield steel bar with nominal yield strength of 460MPa and a mild steel bar with nominal yield strength of 250MPa were used.The properties of the steel bars and FRP are shown in Tables 1and 2,respectively.Test Setup
The testing rig,shown in Fig.3,consisted of two systems:an in-plane 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 2,000kN in compression and 1,200kN in tension.The stroke of the jack was 405mm.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 foot-ings were provided to balance the lateral loading.A prestress scheme was applied to every steel rod so that the rotation and slid-ing during the test could be restrained efficiently.No axial force was applied in the test because the lateral force transfer mecha-nisms in the walls were the focus in this study and low axial load levels are common for low-rise shear walls in practice.
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 distrib-uted 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 dis-placement-controlled in which the top displacement was expressed as a factor of the vertical height of 2,540mm 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=50multiplied by the vertical height of 2,540mm from the wall base to the point where the load is applied.The typical loading procedures are illustrated in Fig.4.
Seismic Behavior and Failure Modes of the Control Specimens
Control Specimens W1and W2had been tested to failure and sus-tained severe damage.Specimen W1developed a sliding failure at the bottom panel,but the sliding face was approximately 200mm above the base of the wall.The damage sustained by Specimen W1included severe concrete crushing and spalling at the bottom
Table 1.Measured Steel Bar Properties Types Yield strength f y (MPa)Ultimate strength
f u (MPa)Specimen W1W2W1W2R6308293428405R10385382502481T10480467545541T13493493581581T20
512
512
607
607
Table 2.Tyfo Fiberwrap Composite System
Parameter
Properties a
GFRP with epoxy,Tyfo SEH-51A composite CFRP with epoxy,Tyfo SCH-41composite Type of FRP
Unidirectional GFRP sheet
Unidirectional CFRP sheet
Ultimate tensile strength in primary fiber direction
575.0MPa 986.0MPa Ultimate tensile strength 90degrees to the primary fiber direction 25.8MPa 40.6MPa Elongation at break 2.2%
1.0%
Tensile Modulus 26.1×103MPa
95.8×103MPa
Laminate thickness
1.3mm
1.0mm
a
Property values given are based on test value by supplier (FYFE Asia Pte.Ltd in
Singapore).
Fig.3.Typical experimental setup
flanges and the right bottom corner of the bottom panel.Further-more,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 W1could 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 W1is illus-trated in Fig.5.On the other hand,Specimen W2had 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 W2is illustrated in Fig.6.Strut-and-Tie Models
As mentioned previously,strut-and-tie models were utilized to aid in designing the strengthening schemes of the damaged specimens
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 compres-sive 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.1and 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 deter-mined by keeping the concrete compressive stress in it lower than the strength limitations suggested by Schlaich et al.(1987),which
is 0.68f 0c
for a concrete strut with cracks parallel to it or 0.51f 0c 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 posi-tion of the openings.For example,the details of the strut,nodes,and compressive stress of each strut of Specimen W1are 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 W1and 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 W1and W2are illustrated in Figs.7and 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 W1in positive and negative loads were different owing to the exist-ence of an irregular opening.However,the strut-and-tie models of Specimen W2were symmetrical in these two load directions as shown in Fig.8.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.
-60
-40-2002040
600246
8101214161820
Cycle number
T o p L a t e r a l D i s p l a c e m e n t (m m )
1/20001/10001/600
1/4001/300
1/200
1/150
1/100
1/75
1/50
Fig.4.Typical loading
procedures
Fig.5.Typical cracking patterns of Specimen W1after
test
Fig.6.Typical cracking patterns of Specimen W2after test
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 than0.3mm but less than20mm,epoxy resin was injected to seal them.For cracks with crack width larger than 20mm but less than50mm,patch repair using a bonding agent and polymer modified cementitious mortar was adopted.The depth of each layer of the patch repair could not exceed20mm.For regions where the depth of the concrete removed exceeded50mm, 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 for1day because the ep-oxy resin required a minimum of24h 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 RW1and RW2,respectively.Figs.9 and10show the proposed FRP-strengthening schemes of Speci-mens RW1and RW2,respectively.
As shown in Fig.9(a),the tie-strengthening scheme of Speci-men RW1is 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#1was determined as follows:
W s¼
T s=2
f FRP×d FRP
¼
248.8×103
2×ð575×1.3Þ
¼166mmð1Þ
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#1is248.8kN);
f FRP=tensile strength of the FRP sheet;and d FRP=thickness of the FRP sheet(One layer of FRP was assumed in the precedin
g calculation.If the spacing was restrained,more layers of FRP could be designed.)
Thus,a one-layer GFRP sheet with150mm width was applied on both faces of the wall(#1strip).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(0.68f0c)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
Fig.7.Strut-and-tie models for Specimens W1
Fig.8.Strut-and-tie models for Specimens W2
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.
However,for strut strengthening,RW2was 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.5shows the failure mode of Specimen W1,indicating that sliding failure was observed at the bottom panel,which was ap-proximately 200mm 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 400mm 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 RW1and RW2are shown in Figs.11and 12,respectively).The fiber anchor con-sisted of two parts:the anchor bolt and the protruding fibers.The total length of the fiber anchors used was approximately 110mm.The length and diameter of the anchor bolt were approximately 50and 7mm,respectively.First,holes (10mm in diameter)were drilled at a depth of 50mm 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 60mm 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
Failure Modes and Response under Cyclic Loading
After repairing and strengthening of the damaged control speci-mens,the repaired Specimens RW1and RW2were retested under a similar reversed cyclic lateral load,and the test results were illus-trated.For repaired specimens,the lateral displacement was re-peated 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
(a)
(b)
(c)
Fig.9.Proposed FRP strengthening schemes for Specimen RW1:(a)tie-strengthening scheme;(b)currently used strut-strengthening scheme;(c)refined strut-strengthening scheme
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 RW1had a flexural fail-ure,which was concentrated between the base and the foundation.
For Specimen RW2,the breaking of epoxy resin was heard at the drift ratio of0.25%.At the first cycle of the drift ratio of0.50%, 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 of1.00%,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 of150mm.When the maximum strength was reached,the whole wall tilted forward in the out-of-place direction.
Load-Displacement Hysteresis Responses
Fig.13shows the load-displacement hysteresis loops of the control and repaired specimens.The hysteretic behavior was evaluated in
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
Fig.11.Proposed fiber anchorage schemes in the base of the wall of Specimen RW1。