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Effect of FRP stitching anchors on ductile performance of shear-deficient RC beams retrofitted using FRP U-wraps
Structures 23 (2020) 407–414
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Effect of FRP stitching anchors on ductile performance of shear-deficient RC beams retrofitted using FRP U-wraps
T
⁎
Abolfazl Eslamia, , Alireza Moghavema, Hamid R. Shayeghb, Hamid R. Ronaghc a
Department of Civil Engineering, Yazd University, Yazd, Iran School of Civil Engineering, Iran University of Science and Technology, Tehran, Iran c Centre for Infrastructure Engineering, Western Sydney University, Sydney, Australia b
A R T I C LE I N FO
A B S T R A C T
Keywords: FRP Shear strengthening End anchorage Debonding Failure mode Ductility Displacement capacity
Premature debonding failure of fiber-reinforced polymer (FRP) composites has remained a problem hindering the efficient utilization of FRP U-wraps. While implementation of end anchorage systems is recommended to overcome this issue, its influence on enhancing the flexural performance of shear-strengthened reinforced concrete (RC) beams has not yet been fully explored. In an attempt to address this, a novel end anchorage system is developed and its efficiency in improving the behavior of shear-strengthened RC beams is evaluated using experiments. The proposed anchorage technique involves an FRP stitching anchor embedded in holes drilled at the end of a groove that is carved out at the terminal point of a U-wrap FRP retrofit along the shear span of the beam. The experimental program is performed using six RC beams, five of which are strengthened with FRP Uwrap shear retrofits. The as-built test specimens are intentionally designed to be shear-deficient prior to the application of FRP composites sheets and that undergo a flexural failure if the FRP shear retrofits used are successful. All the beams are tested monotonically under four-point bending. The results are discussed in terms of load–displacement behavior, ultimate strength, displacement capacity, and ductility factor to show that the proposed anchorage technique is capable of converting the failure mode of the retrofitted specimens from a brittle shear to a ductile flexure-shear one. Finally, the efficiency of the adopted end anchorage system in improving ductility in shear-retrofitted RC beams is explored in detail.
1. Introduction A well-established method for strengthening reinforced concrete (RC) members is to use externally bonded (EB) fiber-reinforced polymer (FRP) composites. The widespread use of FRP composites is attributed to their advantageous properties (e.g., low weight, high strength, corrosion resistance, and versatile application) when compared with other strengthening alternatives. A comprehensive description of different applications of FRP systems for the retrofit and repair of RC structural members are provided by Manos and Katakalos [1]. Among their applications, FRP composites are extensively used for enhancing shear capacity in RC beams [2–7]. FRP shear retrofitting of beams can be performed in such varied configurations as complete wrap, U-wrap, or two-sided [7–9]. Full wrapping, however, is often not possible due to the presence of integral slabs in real field applications [10]. On the other hand, debonding of FRP sheets off the concrete substrate may occur in their other configurations, which can lead to premature failure of the strengthened member
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[7,9,11–13]. Major design guidelines and code specifications [8,14] have attempted to address rather conservatively the debonding issue in shear-strengthened members. Although using anchorage systems has been recommended for shear retrofitting using U-wrap or two-sided schemes, the design details of such anchors are rather scant. Attempts to explore the efficiency of anchorage systems for FRP shear strengthening of RC beams dates back to their early applications. Shear cracks generally initiate from the beam top to propagate downward at an angle of around 45°, whereby they cross FRP wraps at various locations. Thus, anchorage systems used for shear retrofits are typically adhered at the free ends of FRP wraps to facilitate the installation process. These end anchors are beneficial for developing higher strength in FRP wraps as a result of reduced bond stress between the FRP sheets and the concrete substrate which at the same time postponed debonding failure. The anchorage systems generally used for FRP shear strengthening of beams may be classified into the three categories of mechanical [15,16], groove-based [17,18], and FRP anchors [19–22]. Mechanical anchors normally consist of steel plates or metallic
Corresponding author. E-mail address: [email protected] (A. Eslami).
https://doi.org/10.1016/j.istruc.2019.11.007 Received 13 September 2019; Received in revised form 16 November 2019; Accepted 18 November 2019 2352-0124/ © 2019 Institution of Structural Engineers. Published by Elsevier Ltd. All rights reserved.
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proposed, only FRP ones are made from the same materials as the original retrofits are. This consistency in material is useful for eliminating durability problems due to corrosion or deterioration. Nonetheless, FRP anchors have their own shortcomings such as problems with fabrication and installation. For instance, the FRP anchors studied in the past needed to be inserted through the FRP U-wrap into the bonded concrete and flared onto the surface of the FRP U-wraps. This was not only tedious but could also result in misalignment of the fibers in the main FRP sheets, leading to stress concentration. On the other hand, the conventional end anchorage systems used for shear strengthening with wet lay-up FRP composites all required prior concrete surface preparation that was often dusty and hard to accomplish. Moreover, there was almost no reported research on the combined effect of grooving and FRP anchors. Thus, it is essential to develop an end anchorage system that not only transfers a major portion of the forces exerted on the FRP anchor to concrete depths but that also relaxes the need for surface preparation. The present study proposes a novel anchorage system that benefits from the combined effect of FRP anchors and grooves. Furthermore, it will evaluate the efficiency of the proposed end anchorage system in terms of ductility factor and displacement capacity of the shear retrofitted beams – that is, hitherto unfathomed properties that play important roles in the seismic behavior of RC beams.
devices installed at the terminal points of FRP shear retrofits on both beam sides. Although efficient, mechanical anchors suffer from such shortcomings as heavy weight, incompatibility with FRP composites, difficult installation process, and corrosiveness. Groove-based and FRP anchors are more attractive from a practical point of view and have been the focus of a few studies that used shear strengthening with externally bonded FRP composites. In groove-based anchors, FRP shear retrofits are inserted into longitudinal grooves carved at the re-entrant corner of the slab and the beam. Anchorage strength is then increased by pressing an FRP rebar into the groove [17,23]. FRP anchors have also been extensively used in FRP flexural strengthening systems [22,24–26]. These typically comprise strands of bundled fibers with one end embedded into the holes pre-drilled within the concrete substrate and the other end fanned out on the composite sheet. Koutas and Triantafillou [21] examined the effects of a variety of parameters including orientation, number, spacing, and fiber properties of anchors on improving the performance of FRP U-wraps used in shear strengthening of T beams. Their experimental findings indicated the superior performance of FRP anchors placed inside the slab rather than the beam’s web. In addition, anchors of different fiber types have been found to exhibit a behavior similar to that of FRP anchors. In a comparative study, Baggio et al. [19] investigated the effects of commercially manufactured anchors made of carbon fibers and those made of glass fibers. The use of anchors at the terminal points of U-wrap shear retrofits was found to improve the shear capacity of retrofitted beams and to postpone FRP debonding. In addition, Glass FRP (GFRP) anchors were found to be more effective than carbon FRP (CFRP) anchors used in shear strengthening systems. More recently, Bourget et al. [22] developed an innovative technique using CFRP ropes to transform the Uwrap FRP scheme to a closed form stirrup for use in large-scale T beams. In this technqiue, CFRP ropes were inserted into holes drilled at the web-flange re-entrant corner and their free ends were flared onto the two free ends of the CFRP U-wraps. It may be inferred from the literature that end anchorage systems are capable of successfully enhancing the performance of FRP shear retrofits in RC beams. However, the effects of end anchorage on displacement capacity and ductility behavior of FRP strengthened RC beams under shear yet remain to be investigated. What’s more, previous end anchorage techniques commonly require preparation of the concrete substrate onto which the wet lay-up FRP U-wraps are to be installed. As surface treatment is labor intensive, an anchorage technique is required that eliminates the need for surface preparation. The present study aims to investigate experimentally the efficacy of the novel end anchorage technique initially proposed by Eslami et al. [26] for end anchorage of FRP flexural retrofits, in improving the structural behavior of shear-deficient RC beams retrofitted with FRP U-wraps. The effect of surface preparation in the presence of such anchors is also investigated.
3. Experimental program For the purposes of this study, a series of tests were carried out on shear deficient beam specimens in order to determine the effect of the proposed anchorage technique on the ductility performance of RC beams strengthened with FRP shear retrofits. The retrofitting strategy is intended to change the failure mode of the specimens from one of brittle shear to a ductile flexural one. The details of the test specimens and the apparatus used follow. 3.1. Specifications of the test specimens In total, six RC beams with cross-sectional dimensions of 250 × 250mm were used in the experimental program. The overall length and span of each beam specimen measured 1700mm and 1500mm , respectively. The geometric and reinforcement details of the as-built specimens are presented in Fig. 1. The concrete covers on the beam top and bottom were around 20 mm to the surface of the shear reinforcement. The flexural and shear designs of the “as-built” beams were accomplished according to ACI 318-14 [27] with shear failure provisioned to precede flexural failure. To this end, one shear span in each beam (hereafter referred to as ‘the testing shear span’) was reinforced with stirrups 8mm in diameter and spaced 325mm from each other. To provide the requisite shear capacity in the other shear span even after FRP shear retrofit intervention, stirrups 10mm in diameter and spaced at 80mm from each other were added. Three steel rebars, each 22 mm in diameter, were laid at the bottom to serve as the tensile flexural reinforcement while the compressive reinforcement comprised two rebars 10 mm in diameter. It is worth mentioning that the concrete
2. Research significance A number of studies have been conducted in the past on end anchorage of U-wrap FRP shear retrofits. Among the different anchors
A
φ 10@80mm
250 2Ø10
3Ø22
A 100
1500
SEC A-A 100
Fig. 1. Details of the geometry and the reinforcement of the beams (all dimensions in mm). 408
250
200
209
φ 8@325mm
P 2 35
P 2
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70
SA-30
CB
40
A. Eslami, et al.
SAG-0
40
WA
70
40
210
60
40
SAG-0-NP
70
40
70
SA-0
210
210
60
60
210
210
60
Fig. 2. Schematic illustration of the retrofitting configuration of the six tested specimens.
Fig. 2. (continued)
retrofitted all the beams except in the case of those intentionally retrofitted without surface preparation or end anchorage: 1) removing the wood pieces placed to form grooves, 2) drilling holes to insert FRP stitch anchors, 3) grinding the specimens on the surface a hand grinder where FRP U-wrap strips were to be bonded, 4) removing dust residues using an air jet, and 5) rounding all the corners to a minimum radius of 15 mm in order to avoid stress concentration at FRP sheets. To bond the FRP U-wraps, a thin layer of a two-component epoxy resin (mixed at a weight ratio of 1:4) was first applied to the substrate concrete. FRP sheets were then impregnated with the same epoxy resin and bonded onto the concrete surface. During the installation of the FRP sheets, a roller was used to ensure full saturation and to remove any entrapped air bubbles. Finally, excessive resin was removed from the FRP surface. Where a groove was used as part of the end anchor, the FRP fabric was squeezed at the ends into the groove. The proposed anchorage is a combination of FRP stitches and grooves. Briefly, FRP stitches were inserted into holes predrilled into the groove longitudinally carved out along the length of the beam at the terminal points of the FRP U-wrap (Fig. 2). In the case of integrated slabs or T-beams, the grooves and holes can be made at the re-entrant corners of the slab and the beam; in this study, they were created in the beam web. Considering the susceptibility of the beam web in real field applications, hole depth needs to be kept within feasible limits. Not only groove depth and width but also FRP stitch depth and diameter are significant parameters that need due consideration. However, to limit the number of test specimens used in this study, all the grooves had the same rectangular cross section about 20 mm deep and 30 mm wide. Moreover, all the FRP stitch anchors were 10 mm in diameter and 60 mm in length. To allow for proper penetration of the resin around the FRP stitches, holes 14 mm in diameter and 80 mm in depth were drilled into the web (adjacent to the FRP shear strips). As shown in Fig. 3, an FRP stitching anchor was fabricated by rolling a piece of resin-impregnated unidirectional CFRP fabric (100 × 220 mm) around a thin wire and bending to a U shape. Each hole was then filled up to half its depth with a low viscosity epoxy resin before the U-shaped FRP stitch was inserted under slight pressure into it. The groove was then filled with a mixture of epoxy resin and sand filler mixed at a weight ratio of 1:1 to provide full support across the width of the FRP sheet inside the groove.
compressive strength can increase in small scale beams which in turn can result in improving the concrete contribution to shear strength. Nonetheless, as beam specimens of similar dimensions were made in the experimental program, the contribution of concrete in shear capacity can be assumed to be similar for all of them. Thus, the effectiveness of proposed anchorage technique can be assessed through comparison of different specimens in the test matrix. The testing shear spans in five beams were externally strengthened with four strips of FRP U-wraps with their fibers perpendicular to the longitudinal direction of the beam. All the U-shaped strips comprised one layer of unidirectional CFRP sheet 100 mm in width and spaced at 183.3 mm center to center. A schematic illustration of the retrofitting configurations adopted for the test specimens is provided in Fig. 2. The control beam, labelled “CB”, lacked any external FRP shear strengthening. The beam strengthened with FRP shear retrofits but lacking end anchorage was designated by “WA”. The beam designated by “SAG-0” was strengthened with FRP shear retrofits and stitching anchors horizontally embedded into grooves. The grooves had been created by placing two pieces of shaped wood at both sides of the beam web before casting. In field applications, grooves can be carved out through the cover. The beam designated by “SAG-0-NP” was retrofitted in the same way as “SAG-0” was, but with no surface preparation prior to installation of FRP shear strips. “SA-0” represents the beam retrofitted in the same way as “WA” was, but whose FRP shear strips were anchored using only horizontal stitching anchors rather than being inserted into the groove. “SA-30” was retrofitted in the same way as “SA-0” was but whose FRP stitches were inserted at an inclination angle of 30° with respect to the beam width. The main reason for inserting FRP stitches at an inclined angle was to compare their effects on anchorage resistance and prevention of premature debonding failure. The 30° angle was selected for practical reasons as higher angles could have damaged the concrete cover. The test parameters consisted of presence/absence of grooves, surface preparation, and inclination angle of FRP stitches.
3.2. Retrofitting procedure and end anchorage Prior to installation of FRP sheets, the following steps were taken for 409
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(a)
(b)
(c)
(e)
(f) Fig. 3. Fabrication of an FRP stitching anchor.
3.3. Material properties
Table 2 Mechanical properties of the composite materials.
All the test specimens were cast on the same day using a normal weight, ready-mix concrete provided by a local supplier. The maximum aggregate size of the concrete was about 14 mm and its slump measured about 95 mm. The average compressive strength of three cylinder specimens (150 × 300mm ) was reported as concrete compressive strength, which measured about 31.3 MPa and 32.4 MPa after 28 days and at the testing day, respectively. The mechanical properties of the steel reinforcement reported in Table 1 complied with ASTM A370 [28] as determined by direct tensile testing of three samples for each diameter. The CFRP fabrics used as shear retrofits and FRP stitching anchors were of unidirectional SikaWrap-300C 0.171 mm in thickness with an ultimate tensile strength of 3800 MPa, an elastic modulus of 242 GPa, and an ultimate strain of 1.55%. A two-component epoxy resin (mixed at a weight ratio of 1:4) was also used to impregnate the CFRP fibers. Table 2 summarizes the mechanical characteristics of the fabric, the cured FRP sheets, and the resin components as provided by the manufacturer. As observed, the reported properties of the cured FRP composite were lower than those determined based on the rule of mixture.
Material Fiber Cured FRP Resin
Tensile strength (MPa) 3800 470 30
Ultimate strain (%) 1.55 1.30 0.90
Tensile modulus (GPa) 242 36 4.5
Thickness (mm) 0.171 1 –
Hydraulic jack
Load cell LVDT Test specimen
Rigid floor
3.4. Instrumentation and test setup Fig. 4. Test setup and instrumentation.
The test specimens were loaded under a four-point bending apparatus as schematically illustrated in Fig. 4. The shear span of all the beams was taken to be 650 mm corresponding to a shear span to overall height ratio of 2.6. A monotonically increasing load was applied at the mid-span of the specimens using a 400 kN hydraulic jack up to failure. The accuracy of load measurement was within 0.1 kN. Mid-span displacement was measured using two linear variable differential transformers (LVDTs) mounted on both sides of the web with an accuracy of
0.01 mm. During the experiment, beam mid-span load and deflection were recorded using a computerized data acquisition system. 4. Experimental results 4.1. Failure modes and observations Except for the control beam (CB), all the others exhibited a flexuralshear failure as they were retrofitted to behave. The retrofitted specimens could not, however, exhibit identical displacement capacity and ultimate behavior. Fig. 5 plots the variations in the total applied load versus mid-span displacement for all the test specimens while Fig. 6 depicts the status of each test specimen at failure. It should be noted that the asymmetric arrangement of internal stirrups resulted in an asymmetric failure; hence, maximum displacement would not occur at
Table 1 Mechanical properties of the steel reinforcement. Bar diameter (mm)
8 10 22
Stress (MPa) Yield
Ultimate
Elastic modulus (GPa)
461 465 445
650 633 664
196 198 205
410
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300
Total Load (kN)
250
200
150
100 CB SA-0 SAG-0
50
WA SA-30 SAG-0-NP
0 0
5
10
15
20
25
30
35
40
Displacement (mm) Fig. 5. Mid-span load–displacement behaviors of the control and retrofitted specimens.
(a)
(b)
(c)
(d)
(e)
(f)
Fig. 6. Failure modes of the test specimens: a) CB, b) WA, c) SA-0, d) SA-30, e) SAG-0, and f) SAG-0-NP. 411
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continued with increasing mid-span displacement until the third FRP Uwrap (from the support) ruptured progressively at the bottom. Concurrently, a shear crack developed from the loading point towards this FRP strip, leading to the total failure of the beam (Fig. 6e). A final point to note about this beam is that its end anchorage system and those parts remained intact throughout the test. The beam SAG-0-NP, in which FRP sheets were bonded without surface preparation, exhibited a behavior identical to that of specimen SAG-0. The maximum load attained during the test and the corresponding displacement were 217.2 kN and 12.99 mm, respectively. During the test, the longitudinal steel reinforcement yielded and underwent remarkable plastic deformations before failure. These deformations led to several flexural and shear-flexural cracks along the beam at higher loads. After yielding of the longitudinal steel reinforcement, concrete crushing was also observed in the compression zone. At the onset of the failure, a flexural-shear crack formed between the loading point and the third FRP U-wrap, which was accompanied by the rupture of this FRP strip (Fig. 6f).
Table 3 Ultimate loads and displacements in all the test specimens. Specimen
Maximum load (kN)
Increase (%)a
Displacement capacity (mm)
Increase (%)b
CB WA SA-0 SA-30 SAG-0 SAG-0-NP
196.1 261.3 271.9 271.0 267.5 271.2
– 33 39 38 36 38
11.45 11.34 17.60 23.60 36.43 34.90
– – 55 108 221 207
a b
Compared to specimen CB. Relative to that of WA.
mid-span. The control beam (CB) experienced its designed shear failure. During loading, a crack formed in this specimen at the testing shear span between the support and the loading point, which later widened with increasing load up to failure. CB recorded a maximum load carrying capacity of about 196.1 kN corresponding to a displacement of 6.95 mm. Fig. 6(a) shows the failure mode of specimen CB. WA, retrofitted without any end anchorage, showed a flexural-shear failure rather than a shear one. However, this beam failed to provide a ductile behavior due to the debonding of FRP shear retrofits off the concrete substrate. Moreover, it recorded a maximum load of 261.3 kN at a displacement of 8.19 mm. During loading, flexural and shear flexural cracks in WA initially developed in the vicinity of the constant moment region and near the loading points. These cracks then propagated to the middle of the section. As displacement increased, a major shear crack began to develop from the loading point towards the third FRP strip (from the support) leading to the debonding of the crossing FRP strips with a popping sound; this is illustrated in Fig. 6(b). This debonding occurred at parts of the FRP strips located above the shear cracks. Beam SA-0, which was anchored using horizontal stitching anchors, underwent a flexural-shear failure. The maximum load carrying capacity of about 271.9 kN was recorded during the test at a displacement of 10.86 mm. During loading, a couple of shear cracks formed in the testing shear spans of this specimen in addition to a number of flexural cracks in its constant moment region. With increasing load, these cracks widened until FRP strips slipped and ruptured the stitching anchors causing the FRP sheets to debond off the concrete substrate. Fig. 6(c) shows the beam after its failure. Beam SA-30, with anchors embedded at an inclination angle of 30° towards the beam top side, sustained larger displacements and underwent a more ductile flexural-shear behavior. The beam exhibited a maximum load of 271 kN at a displacement of 14.99 mm. During loading, flexural cracks initially formed in its constant moment region which subsequently propagated towards the compression zone throughout the test. At higher displacements, however, a main shearflexural crack developed from the loading point towards the testing shear span that crossed two FRP shear strips and caused these strips to slip out of the stitching anchors (Fig. 6d). These slips are represented by the small drops in the relevant load–displacement curve. Failure in beam SA-30 was accompanied by both concrete crushing at the compression zone and rupturing of the FRP stitching anchor. The specimen SAG-0 underwent a more convincingly flexural failure with a maximum strength of 267.5 kN corresponding to a displacement of 12.04 mm. As observed in Fig. 5, the load–displacement curve for this beam recorded during experimentation indicates a significant plastic deformation of the longitudinal reinforcement. During the test, both flexural and shear-flexural cracks formed symmetrically not only in the constant moment region but in both shear spans as well, which propagated towards the compression side of the beam with increasing load. At a flexural crack depth of around 75% the beam’s height, the concrete at the compression side began to crush. This behavior
4.2. Comparison and discussion The FRP shear retrofits were able to change successfully the behavior of the as-built beams from a shear to a flexural-shear one even in the absence of end anchors. However, the end anchors were found to have significant effects on the structural behavior of the beams after the retrofitting intervention, particularly as regards displacement capacity, and ductility. In what follows, these parameters are individually explored. 4.2.1. Strength and displacement Comparison of the results obtained for the control and retrofitted specimens indicates a significant improvement in ultimate load carrying capacity due to the application of the FRP U-wrap strips. Table 3 reports the maximum loads and ultimate displacements recorded by all the test specimens. Except for beam WA, the retrofitted beams recorded identical increases in their total applied loads. The lower percentage of load increase in beam WA relative to those of other retrofitted beams is clearly due to the debonding of its FRP U-wraps. While it would be possible to achieve higher strengths using this system, it was the objective of the present study only to investigate the effect of end anchorage on the flexural behavior of RC beams shearretrofitted with FRP composites. Evidently, yielding of longitudinal reinforcement impeded higher increases in total applied load in the retrofitted test specimens. Moreover, failure in the specimens SAG-0 and SAG-0-NP was accompanied by the fracturing of FRP wraps. However, these fractures were attributed to local stress concentration in FRP composites due to the high magnitude of bending deformations as a result of the successful prevention of debonding failure by the adopted anchorage system. The displacement capacity of each specimen was defined as that beyond which a 20% drop was observed in the maximum sustained load; this level of displacement would be accompanied by a considerable damage to the specimen. Table 3 reports the relevant values of displacement for all the test specimens. It may be noted that the displacement response was irrelevant in the case of the control specimen since it underwent a shear failure. Hence, the increase in displacement capacity for the retrofitted specimens was calculated relative to that of beam WA. The lowest increase in displacement capacity (55%) was attained only with FRP stitching anchors employed as the end anchorage system. On the other hand, ultimate displacement in beam SAG-0 increased by 221% relative to that of WA retrofitted with no end anchors. It is also worth mentioning that the failure of SAG-0 was attributed to the rupture of FRP shear retrofits, indicating that larger displacement capacities may be achieved by applying more layers of FRP shear retrofits. Comparing the results obtained for specimens SAG-0 and SAG-0-NP 412
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anchors at an inclined angle led to the orientation of one component of the anchor resisting force in the direction of the U-wrap fibers. This component then reduced the bond stress between the CFRP U-wrap and the concrete substrate to postpone debonding failure. However, FRP stitches inserted at an angle of zero degrees would have only augmented the friction resistance between the FRP U-wrap and the concrete substrate.
Table 4 Ductility factors and related parameter for the test specimens. Specimen
Yield dis. (mm)
Ultimate dis. (mm)
Ductility
Increase (%)a
WA SA-0 SA-30 SAG-0 SAG-0-NP
4.90 4.71 4.84 6.00 5.24
11.34 17.60 23.60 36.43 34.90
2.3 3.7 4.9 6.1 6.7
– 61 113 165 191
a
5. Predicted flexural capacity
Relative to that of WA.
Ultimate flexural capacity was calculated for the tested beams using the conventional sectional analysis based on the widely-used Hognestad’s model [29] for simulating the nonlinear behavior of concrete as described in Park and Paulay [30]. Accordingly, the depth of the neutral axis, c , was determined by satisfying the force equilibrium equation given by:
indicates that surface preparation had only a negligible effect on displacement capacity when the proposed end anchorage system was employed. This finding is interesting not only from a fabrication viewpoint but also from economic one as surface preparation as a difficult and tedious part of FRP application, particularly in large projects, can be eliminated altogether.
αf c' bc + As' f s' = As fs
(1)
f c'
is the cylinder compressive strength of concrete and b represents the width of the section. Also, f s' and fs denote the stresses developed while As' and As represent the areas of compression and tension reinforcements, respectively. In addition, the parameter α is defined as: in which,
4.2.2. Ductility factor On a par with strength, ductility is a prominent factor influencing the flexural performance of RC beams. Displacement ductility is defined as the ratio of ultimate displacement, Δu , to yield displacement, Δy . Due to the nonlinear behavior of RC members, their load–displacement behavior typically lacks a well-defined yielding point. However, yield displacement can be either identified as the displacement corresponding to the first yield of the longitudinal steel reinforcement in tension or as the intersection of two linear parts on an idealized bilinear curve. In the present case, the latter approach was adopted and the relevant idealized bilinear curve was derived using the equal energy rule. The bilinear and the original curves were found to intersect at a point corresponding to 60% of the yield load. Equal energy rule was manifested by equating the areas below the original and bilinear curves. Table 4 reports the values for ductility factors and the related parameters for all the retrofitted specimens. Ultimate displacement was defined in the same way displacement capacity was. It should, however, be noted that it is not logical to determine the ductility factor values for the control beam (CB) as it experiences a shear failure without its longitudinal steel reinforcement having yielded. The ductility parameter for the specimen WA is about 2.3. This is while those for specimens SAG-0 and SAG-0-NP are calculated to be around 6.1 and 6.7, respectively. This confirms that implementation of the proposed anchorage system led to significant enhancements in ductility factor. Inserting FRP U-wraps into grooves resulted in additional strength of the bond between the FRP sheets and the concrete similar to what is observed in the typically-used 90° standard hooks for steel reinforcement. It should be noted that all the corners in these specimens had been rounded to a minimum radius of 15 mm in order to prevent stress concentration and facilitate the transfer of force from the U-wrap into the groove. On the other hand, FRP stitches had been implemented at the end of the groove aligned with the fiber orientation, which might have created both resistance force and a fiber strength but in different directions. In fact, the groove was made to exploit all the possible resistance of the stitching anchor by aligning the main retrofitting fibers and the stitching anchors. Interestingly, the highest ductility value was observed in the specimen SAG-0-NP that had been retrofitted with end anchors but without surface treatment. The lowest rise in ductility belonged to the specimen SA-0 anchored only with horizontal FRP stitches; it actually exhibited a ductility enhancement of 61% relative to that of WA. In addition, it was found that inserting FRP stitching anchors at an inclined angle improved the ductile performance of the FRP-shear-retrofitted beams that would, otherwise, experience a flexural failure. This improvement can be explained with recourse to the system of forces at the intersection of U-wrap and anchor along the following lines. Inserting stitching
α=
εc ε (1 − c ) ε0 3ε0
ε0 = 1.71
(2)
f c' (3)
Ec
where, εc and Ec denote the maximum strain and elastic modulus of concrete. Ultimate flexural capacity is thus given by:
Mu = αf c' bc (d − γc ) + As' f s' (d − d')
(4)
'
in which, d and d represent effective depths of compressive and tensile reinforcements, respectively. In addition, γ is a coefficient for locating the centroid and is equal to:
γ=1−
(2/3 − εc /(4ε0)) (1 − εc /(3ε0))
(5)
Given that a ductile flexural failure precedes the shear failure in beams, ultimate flexural capacity of beams was determined to be 88.1 kN m corresponding to a total load of 271 kN. This load is roughly equal to the maximum value sustained during experimental test by all the retrofitted specimens except for WA whose failure was dominated by shear due to the premature debonding of its FRP U-wraps. It should be noted that a negligible difference (1.3%) was observed between the analytical and experimental loading capacities of specimen SAG-0 which might be attributed to use of the average properties of steel reinforcement and concrete in analytical calculations. Overall, it could be concluded that the adopted retrofitting technique was efficient in fully exploiting the ultimate flexural capacity of beams with shear deficiency and improving their ductility behavior. The adopted anchorage system was aimed at changing the failure mode of beams from a brittle shear to a ductile flexural mode. In an ideal condition, the anchorage could be remained intact under monotonic loading until achieving the ultimate flexural and displacement capacities. 6. Conclusions The results of an experiment investigating the effect of end anchorage on flexural performance of shear-deficient RC beams retrofitted with FRP U-wrap were reported. For the purposes of this study, a novel anchorage technique was proposed and its efficiency was evaluated through experimental testing. While FRP U-wrap strips were found capable of shifting beam failure mode from a shear to a flexural one in 413
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all the retrofitted specimens, their post-yield performance was found to depend mostly on the performance of the end anchorage system employed. All the retrofitted specimens recorded almost identical loading capacities since they underwent a flexural failure mode. However, the end anchorage was found able to provide substantial improvements in displacement capacity, and ductility factor of the shear-retrofitted beams. The most desirable post-yield behavior was observed in beams retrofitted with the proposed end anchorage technique but even subjected to no surface treatment. Comparison of the beams SAG-0 and WA revealed that displacement capacity, and ductility factor increased by 221%, and 165%, respectively, once the proposed anchorage technique was implemented. A similar performance was also observed in beam SAG-NP that had been retrofitted using the proposed end anchorage system but no surface preparation. The present experiment is expected to provide the primary impetus for further investigation and evolution of the proposed anchorage system. Within its scope, the experiment confirmed the efficiency of the proposed technique in improving the flexural performance of sheardeficient RC beams retrofitted with FRP U-wraps. However, these findings are based on static monotonic loading and anchorage devices may not retain the same level of effectiveness under cyclic load reversals. Thus, the influence of cyclic loading on the effectiveness of the proposed anchorage devices needs to be also scrutinized in details. Finally, the efficiency of the technique in increasing the loading capacity of shear-deficient RC beams may be suggested for future research.
bonded FRP composites. ACI Struct J 1998;95:107–15. [7] Manos GC, Katakalos K, Papakonstantinou CG. Shear behavior of rectangular beams strengthened with either carbon or steel fiber reinforced polymers. Appl Mech Mater 2011;82:571–6. [8] ACI Committee 440. Guide for the design and construction of externally bonded FRP systems for strengthening concrete structures (ACI 440.2R-17). Farmington Hills, Mich.: American Concrete Institute; 2017. [9] Khalifa A, Gold WJ, Nanni A, MI AA. Contribution of externally bonded FRP to shear capacity of RC flexural members. J Compos Constr 1998;2:195–202. [10] Eslami A, Ronagh HR. Experimental investigation of an appropriate anchorage system for flange-bonded carbon fiber–reinforced polymers in retrofitted RC beam–column joints. J Compos Constr 2014;18:04013056. [11] Bousselham A, Chaallal O. Shear strengthening reinforced concrete beams with fiber-reinforced polymer: Assessment of influencing parameters and required research. ACI Struct J 2004;101:219–27. [12] Cao S, Chen J, Teng J, Hao Z, Chen J. Debonding in RC beams shear strengthened with complete FRP wraps. J Compos Constr 2005;9:417–28. [13] Manos GC, Katakalos KV. Investigation of the force transfer mechanisms for open hoop FRP strips bonded on RC beams with or without anchoring devices. Open J Civil Eng 2013;03:143–53. [14] CSA. Design and construction of building structures with fibre-reinforced polymers. CSA-S806-12. Mississauga, Ontario: Canadian Standards Association; 2012. [15] Mofidi A, Chaallal O, Benmokrane B, Neale K. Performance of end-anchorage systems for RC beams strengthened in shear with epoxy-bonded FRP. J Compos Constr 2012;16:322–31. [16] Tzoura E, Triantafillou TC. Shear strengthening of reinforced concrete T-beams under cyclic loading with TRM or FRP jackets. Mater Struct 2016;49:17–28. [17] Khalifa A, Nanni A. Improving shear capacity of existing RC T-section beams using CFRP composites. Cem Concr Compos 2000;22:165–74. [18] Mofidi A, Thivierge S, Chaallal O, Shao Y. Behavior of reinforced concrete beams strengthened in shear using L-shaped CFRP plates: experimental investigation. J Compos Constr 2014;18:04013033. [19] Baggio D, Soudki K, Noël M. Strengthening of shear critical RC beams with various FRP systems. Constr Build Mater 2014;66:634–44. [20] Kim Y, Quinn KT, Satrom CN, Ghannoum WM, Jirsa JO. Shear strengthening RC Tbeams using CFRP laminates and anchors. Special Publ 2011;275:1–18. [21] Koutas L, Triantafillou TC. Use of Anchors in Shear Strengthening of Reinforced Concrete T-Beams with FRP. J Compos Constr 2013;17:101–7. [22] Bourget S, El-Saikaly G, Chaallal O. Behavior of reinforced concrete T-Beams strengthened in shear using closed carbon fiber-reinforced polymer stirrups made of laminates and ropes. ACI Struct J 2017;114:1087. [23] Grelle SV, Sneed LH. Review of anchorage systems for externally bonded FRP laminates. Int J Con Struct Mater 2013;7:17–33. [24] Smith ST, Hu S, Kim SJ, Seracino R. FRP-strengthened RC slabs anchored with FRP anchors. Eng Struct 2011;33:1075–87. [25] Smith ST, Zhang H, Wang Z. Influence of FRP anchors on the strength and ductility of FRP-strengthened RC slabs. Constr Build Mater 2013;49:998–1012. [26] Eslami A, Shayegh HR, Moghavem A, Ronagh HR. Experimental and analytical investigations of a novel end anchorage for CFRP flexural retrofits. Compos B Eng 2019;176:107309. [27] ACI Committee 318. Building code requirements for structural concrete (ACI 31814) and commentary (ACI 318R-14). Farmington Hills, Mich.: American Concrete Institute; 2014. [28] ASTM A370-17a. Standard Test Methods and Definitions for Mechanical Testing of Steel Products. West Conshohocken, PA ASTM International; 2017. [29] Hognestad E, Hanson NW, McHenry D. Concrete stress distribution in ultimate strength design. ACI J Proc 1955;52:455–80. [30] Park R, Paulay T. Reinforced concrete structures. New York: Wiley; 1975.
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. References [1] Manos GC, Katakalos K. The use of fiber reinforced plastic for the repair and strenghtening of existing reinforced concrete structural elements damaged by earthquakes. Published in the book “Fiber Reinforced Polymers - The Technology Applied for Concrete Repair”, edited by Martin Alberto Masuelli, ISBN 978-953-510938-9, 2013. [2] Mofidi A, Chaallal O. Shear strengthening of RC beams with EB FRP: influencing factors and conceptual debonding model. J Compos Constr 2010;15:62–74. [3] Mofidi A, Chaallal O. Shear strengthening of RC beams with externally bonded FRP composites: effect of strip-width-to-strip-spacing ratio. J Compos Constr 2011;15:732–42. [4] Monti G, Liotta MA. Tests and design equations for FRP-strengthening in shear. Constr Build Mater 2007;21:799–809. [5] Mosallam AS, Banerjee S. Shear enhancement of reinforced concrete beams strengthened with FRP composite laminates. Compos Part B-Eng. 2007;38:781–93. [6] Triantafillou TC. Shear strengthening of reinforced concrete beams using epoxy-
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Effect of FRP stitching anchors on ductile performance of shear-deficient RC beams retrofitted using FRP U-wraps
T
⁎
Abolfazl Eslamia, , Alireza Moghavema, Hamid R. Shayeghb, Hamid R. Ronaghc a
Department of Civil Engineering, Yazd University, Yazd, Iran School of Civil Engineering, Iran University of Science and Technology, Tehran, Iran c Centre for Infrastructure Engineering, Western Sydney University, Sydney, Australia b
A R T I C LE I N FO
A B S T R A C T
Keywords: FRP Shear strengthening End anchorage Debonding Failure mode Ductility Displacement capacity
Premature debonding failure of fiber-reinforced polymer (FRP) composites has remained a problem hindering the efficient utilization of FRP U-wraps. While implementation of end anchorage systems is recommended to overcome this issue, its influence on enhancing the flexural performance of shear-strengthened reinforced concrete (RC) beams has not yet been fully explored. In an attempt to address this, a novel end anchorage system is developed and its efficiency in improving the behavior of shear-strengthened RC beams is evaluated using experiments. The proposed anchorage technique involves an FRP stitching anchor embedded in holes drilled at the end of a groove that is carved out at the terminal point of a U-wrap FRP retrofit along the shear span of the beam. The experimental program is performed using six RC beams, five of which are strengthened with FRP Uwrap shear retrofits. The as-built test specimens are intentionally designed to be shear-deficient prior to the application of FRP composites sheets and that undergo a flexural failure if the FRP shear retrofits used are successful. All the beams are tested monotonically under four-point bending. The results are discussed in terms of load–displacement behavior, ultimate strength, displacement capacity, and ductility factor to show that the proposed anchorage technique is capable of converting the failure mode of the retrofitted specimens from a brittle shear to a ductile flexure-shear one. Finally, the efficiency of the adopted end anchorage system in improving ductility in shear-retrofitted RC beams is explored in detail.
1. Introduction A well-established method for strengthening reinforced concrete (RC) members is to use externally bonded (EB) fiber-reinforced polymer (FRP) composites. The widespread use of FRP composites is attributed to their advantageous properties (e.g., low weight, high strength, corrosion resistance, and versatile application) when compared with other strengthening alternatives. A comprehensive description of different applications of FRP systems for the retrofit and repair of RC structural members are provided by Manos and Katakalos [1]. Among their applications, FRP composites are extensively used for enhancing shear capacity in RC beams [2–7]. FRP shear retrofitting of beams can be performed in such varied configurations as complete wrap, U-wrap, or two-sided [7–9]. Full wrapping, however, is often not possible due to the presence of integral slabs in real field applications [10]. On the other hand, debonding of FRP sheets off the concrete substrate may occur in their other configurations, which can lead to premature failure of the strengthened member
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[7,9,11–13]. Major design guidelines and code specifications [8,14] have attempted to address rather conservatively the debonding issue in shear-strengthened members. Although using anchorage systems has been recommended for shear retrofitting using U-wrap or two-sided schemes, the design details of such anchors are rather scant. Attempts to explore the efficiency of anchorage systems for FRP shear strengthening of RC beams dates back to their early applications. Shear cracks generally initiate from the beam top to propagate downward at an angle of around 45°, whereby they cross FRP wraps at various locations. Thus, anchorage systems used for shear retrofits are typically adhered at the free ends of FRP wraps to facilitate the installation process. These end anchors are beneficial for developing higher strength in FRP wraps as a result of reduced bond stress between the FRP sheets and the concrete substrate which at the same time postponed debonding failure. The anchorage systems generally used for FRP shear strengthening of beams may be classified into the three categories of mechanical [15,16], groove-based [17,18], and FRP anchors [19–22]. Mechanical anchors normally consist of steel plates or metallic
Corresponding author. E-mail address: [email protected] (A. Eslami).
https://doi.org/10.1016/j.istruc.2019.11.007 Received 13 September 2019; Received in revised form 16 November 2019; Accepted 18 November 2019 2352-0124/ © 2019 Institution of Structural Engineers. Published by Elsevier Ltd. All rights reserved.
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proposed, only FRP ones are made from the same materials as the original retrofits are. This consistency in material is useful for eliminating durability problems due to corrosion or deterioration. Nonetheless, FRP anchors have their own shortcomings such as problems with fabrication and installation. For instance, the FRP anchors studied in the past needed to be inserted through the FRP U-wrap into the bonded concrete and flared onto the surface of the FRP U-wraps. This was not only tedious but could also result in misalignment of the fibers in the main FRP sheets, leading to stress concentration. On the other hand, the conventional end anchorage systems used for shear strengthening with wet lay-up FRP composites all required prior concrete surface preparation that was often dusty and hard to accomplish. Moreover, there was almost no reported research on the combined effect of grooving and FRP anchors. Thus, it is essential to develop an end anchorage system that not only transfers a major portion of the forces exerted on the FRP anchor to concrete depths but that also relaxes the need for surface preparation. The present study proposes a novel anchorage system that benefits from the combined effect of FRP anchors and grooves. Furthermore, it will evaluate the efficiency of the proposed end anchorage system in terms of ductility factor and displacement capacity of the shear retrofitted beams – that is, hitherto unfathomed properties that play important roles in the seismic behavior of RC beams.
devices installed at the terminal points of FRP shear retrofits on both beam sides. Although efficient, mechanical anchors suffer from such shortcomings as heavy weight, incompatibility with FRP composites, difficult installation process, and corrosiveness. Groove-based and FRP anchors are more attractive from a practical point of view and have been the focus of a few studies that used shear strengthening with externally bonded FRP composites. In groove-based anchors, FRP shear retrofits are inserted into longitudinal grooves carved at the re-entrant corner of the slab and the beam. Anchorage strength is then increased by pressing an FRP rebar into the groove [17,23]. FRP anchors have also been extensively used in FRP flexural strengthening systems [22,24–26]. These typically comprise strands of bundled fibers with one end embedded into the holes pre-drilled within the concrete substrate and the other end fanned out on the composite sheet. Koutas and Triantafillou [21] examined the effects of a variety of parameters including orientation, number, spacing, and fiber properties of anchors on improving the performance of FRP U-wraps used in shear strengthening of T beams. Their experimental findings indicated the superior performance of FRP anchors placed inside the slab rather than the beam’s web. In addition, anchors of different fiber types have been found to exhibit a behavior similar to that of FRP anchors. In a comparative study, Baggio et al. [19] investigated the effects of commercially manufactured anchors made of carbon fibers and those made of glass fibers. The use of anchors at the terminal points of U-wrap shear retrofits was found to improve the shear capacity of retrofitted beams and to postpone FRP debonding. In addition, Glass FRP (GFRP) anchors were found to be more effective than carbon FRP (CFRP) anchors used in shear strengthening systems. More recently, Bourget et al. [22] developed an innovative technique using CFRP ropes to transform the Uwrap FRP scheme to a closed form stirrup for use in large-scale T beams. In this technqiue, CFRP ropes were inserted into holes drilled at the web-flange re-entrant corner and their free ends were flared onto the two free ends of the CFRP U-wraps. It may be inferred from the literature that end anchorage systems are capable of successfully enhancing the performance of FRP shear retrofits in RC beams. However, the effects of end anchorage on displacement capacity and ductility behavior of FRP strengthened RC beams under shear yet remain to be investigated. What’s more, previous end anchorage techniques commonly require preparation of the concrete substrate onto which the wet lay-up FRP U-wraps are to be installed. As surface treatment is labor intensive, an anchorage technique is required that eliminates the need for surface preparation. The present study aims to investigate experimentally the efficacy of the novel end anchorage technique initially proposed by Eslami et al. [26] for end anchorage of FRP flexural retrofits, in improving the structural behavior of shear-deficient RC beams retrofitted with FRP U-wraps. The effect of surface preparation in the presence of such anchors is also investigated.
3. Experimental program For the purposes of this study, a series of tests were carried out on shear deficient beam specimens in order to determine the effect of the proposed anchorage technique on the ductility performance of RC beams strengthened with FRP shear retrofits. The retrofitting strategy is intended to change the failure mode of the specimens from one of brittle shear to a ductile flexural one. The details of the test specimens and the apparatus used follow. 3.1. Specifications of the test specimens In total, six RC beams with cross-sectional dimensions of 250 × 250mm were used in the experimental program. The overall length and span of each beam specimen measured 1700mm and 1500mm , respectively. The geometric and reinforcement details of the as-built specimens are presented in Fig. 1. The concrete covers on the beam top and bottom were around 20 mm to the surface of the shear reinforcement. The flexural and shear designs of the “as-built” beams were accomplished according to ACI 318-14 [27] with shear failure provisioned to precede flexural failure. To this end, one shear span in each beam (hereafter referred to as ‘the testing shear span’) was reinforced with stirrups 8mm in diameter and spaced 325mm from each other. To provide the requisite shear capacity in the other shear span even after FRP shear retrofit intervention, stirrups 10mm in diameter and spaced at 80mm from each other were added. Three steel rebars, each 22 mm in diameter, were laid at the bottom to serve as the tensile flexural reinforcement while the compressive reinforcement comprised two rebars 10 mm in diameter. It is worth mentioning that the concrete
2. Research significance A number of studies have been conducted in the past on end anchorage of U-wrap FRP shear retrofits. Among the different anchors
A
φ 10@80mm
250 2Ø10
3Ø22
A 100
1500
SEC A-A 100
Fig. 1. Details of the geometry and the reinforcement of the beams (all dimensions in mm). 408
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200
209
φ 8@325mm
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P 2
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SA-30
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40
WA
70
40
210
60
40
SAG-0-NP
70
40
70
SA-0
210
210
60
60
210
210
60
Fig. 2. Schematic illustration of the retrofitting configuration of the six tested specimens.
Fig. 2. (continued)
retrofitted all the beams except in the case of those intentionally retrofitted without surface preparation or end anchorage: 1) removing the wood pieces placed to form grooves, 2) drilling holes to insert FRP stitch anchors, 3) grinding the specimens on the surface a hand grinder where FRP U-wrap strips were to be bonded, 4) removing dust residues using an air jet, and 5) rounding all the corners to a minimum radius of 15 mm in order to avoid stress concentration at FRP sheets. To bond the FRP U-wraps, a thin layer of a two-component epoxy resin (mixed at a weight ratio of 1:4) was first applied to the substrate concrete. FRP sheets were then impregnated with the same epoxy resin and bonded onto the concrete surface. During the installation of the FRP sheets, a roller was used to ensure full saturation and to remove any entrapped air bubbles. Finally, excessive resin was removed from the FRP surface. Where a groove was used as part of the end anchor, the FRP fabric was squeezed at the ends into the groove. The proposed anchorage is a combination of FRP stitches and grooves. Briefly, FRP stitches were inserted into holes predrilled into the groove longitudinally carved out along the length of the beam at the terminal points of the FRP U-wrap (Fig. 2). In the case of integrated slabs or T-beams, the grooves and holes can be made at the re-entrant corners of the slab and the beam; in this study, they were created in the beam web. Considering the susceptibility of the beam web in real field applications, hole depth needs to be kept within feasible limits. Not only groove depth and width but also FRP stitch depth and diameter are significant parameters that need due consideration. However, to limit the number of test specimens used in this study, all the grooves had the same rectangular cross section about 20 mm deep and 30 mm wide. Moreover, all the FRP stitch anchors were 10 mm in diameter and 60 mm in length. To allow for proper penetration of the resin around the FRP stitches, holes 14 mm in diameter and 80 mm in depth were drilled into the web (adjacent to the FRP shear strips). As shown in Fig. 3, an FRP stitching anchor was fabricated by rolling a piece of resin-impregnated unidirectional CFRP fabric (100 × 220 mm) around a thin wire and bending to a U shape. Each hole was then filled up to half its depth with a low viscosity epoxy resin before the U-shaped FRP stitch was inserted under slight pressure into it. The groove was then filled with a mixture of epoxy resin and sand filler mixed at a weight ratio of 1:1 to provide full support across the width of the FRP sheet inside the groove.
compressive strength can increase in small scale beams which in turn can result in improving the concrete contribution to shear strength. Nonetheless, as beam specimens of similar dimensions were made in the experimental program, the contribution of concrete in shear capacity can be assumed to be similar for all of them. Thus, the effectiveness of proposed anchorage technique can be assessed through comparison of different specimens in the test matrix. The testing shear spans in five beams were externally strengthened with four strips of FRP U-wraps with their fibers perpendicular to the longitudinal direction of the beam. All the U-shaped strips comprised one layer of unidirectional CFRP sheet 100 mm in width and spaced at 183.3 mm center to center. A schematic illustration of the retrofitting configurations adopted for the test specimens is provided in Fig. 2. The control beam, labelled “CB”, lacked any external FRP shear strengthening. The beam strengthened with FRP shear retrofits but lacking end anchorage was designated by “WA”. The beam designated by “SAG-0” was strengthened with FRP shear retrofits and stitching anchors horizontally embedded into grooves. The grooves had been created by placing two pieces of shaped wood at both sides of the beam web before casting. In field applications, grooves can be carved out through the cover. The beam designated by “SAG-0-NP” was retrofitted in the same way as “SAG-0” was, but with no surface preparation prior to installation of FRP shear strips. “SA-0” represents the beam retrofitted in the same way as “WA” was, but whose FRP shear strips were anchored using only horizontal stitching anchors rather than being inserted into the groove. “SA-30” was retrofitted in the same way as “SA-0” was but whose FRP stitches were inserted at an inclination angle of 30° with respect to the beam width. The main reason for inserting FRP stitches at an inclined angle was to compare their effects on anchorage resistance and prevention of premature debonding failure. The 30° angle was selected for practical reasons as higher angles could have damaged the concrete cover. The test parameters consisted of presence/absence of grooves, surface preparation, and inclination angle of FRP stitches.
3.2. Retrofitting procedure and end anchorage Prior to installation of FRP sheets, the following steps were taken for 409
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(a)
(b)
(c)
(e)
(f) Fig. 3. Fabrication of an FRP stitching anchor.
3.3. Material properties
Table 2 Mechanical properties of the composite materials.
All the test specimens were cast on the same day using a normal weight, ready-mix concrete provided by a local supplier. The maximum aggregate size of the concrete was about 14 mm and its slump measured about 95 mm. The average compressive strength of three cylinder specimens (150 × 300mm ) was reported as concrete compressive strength, which measured about 31.3 MPa and 32.4 MPa after 28 days and at the testing day, respectively. The mechanical properties of the steel reinforcement reported in Table 1 complied with ASTM A370 [28] as determined by direct tensile testing of three samples for each diameter. The CFRP fabrics used as shear retrofits and FRP stitching anchors were of unidirectional SikaWrap-300C 0.171 mm in thickness with an ultimate tensile strength of 3800 MPa, an elastic modulus of 242 GPa, and an ultimate strain of 1.55%. A two-component epoxy resin (mixed at a weight ratio of 1:4) was also used to impregnate the CFRP fibers. Table 2 summarizes the mechanical characteristics of the fabric, the cured FRP sheets, and the resin components as provided by the manufacturer. As observed, the reported properties of the cured FRP composite were lower than those determined based on the rule of mixture.
Material Fiber Cured FRP Resin
Tensile strength (MPa) 3800 470 30
Ultimate strain (%) 1.55 1.30 0.90
Tensile modulus (GPa) 242 36 4.5
Thickness (mm) 0.171 1 –
Hydraulic jack
Load cell LVDT Test specimen
Rigid floor
3.4. Instrumentation and test setup Fig. 4. Test setup and instrumentation.
The test specimens were loaded under a four-point bending apparatus as schematically illustrated in Fig. 4. The shear span of all the beams was taken to be 650 mm corresponding to a shear span to overall height ratio of 2.6. A monotonically increasing load was applied at the mid-span of the specimens using a 400 kN hydraulic jack up to failure. The accuracy of load measurement was within 0.1 kN. Mid-span displacement was measured using two linear variable differential transformers (LVDTs) mounted on both sides of the web with an accuracy of
0.01 mm. During the experiment, beam mid-span load and deflection were recorded using a computerized data acquisition system. 4. Experimental results 4.1. Failure modes and observations Except for the control beam (CB), all the others exhibited a flexuralshear failure as they were retrofitted to behave. The retrofitted specimens could not, however, exhibit identical displacement capacity and ultimate behavior. Fig. 5 plots the variations in the total applied load versus mid-span displacement for all the test specimens while Fig. 6 depicts the status of each test specimen at failure. It should be noted that the asymmetric arrangement of internal stirrups resulted in an asymmetric failure; hence, maximum displacement would not occur at
Table 1 Mechanical properties of the steel reinforcement. Bar diameter (mm)
8 10 22
Stress (MPa) Yield
Ultimate
Elastic modulus (GPa)
461 465 445
650 633 664
196 198 205
410
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300
Total Load (kN)
250
200
150
100 CB SA-0 SAG-0
50
WA SA-30 SAG-0-NP
0 0
5
10
15
20
25
30
35
40
Displacement (mm) Fig. 5. Mid-span load–displacement behaviors of the control and retrofitted specimens.
(a)
(b)
(c)
(d)
(e)
(f)
Fig. 6. Failure modes of the test specimens: a) CB, b) WA, c) SA-0, d) SA-30, e) SAG-0, and f) SAG-0-NP. 411
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continued with increasing mid-span displacement until the third FRP Uwrap (from the support) ruptured progressively at the bottom. Concurrently, a shear crack developed from the loading point towards this FRP strip, leading to the total failure of the beam (Fig. 6e). A final point to note about this beam is that its end anchorage system and those parts remained intact throughout the test. The beam SAG-0-NP, in which FRP sheets were bonded without surface preparation, exhibited a behavior identical to that of specimen SAG-0. The maximum load attained during the test and the corresponding displacement were 217.2 kN and 12.99 mm, respectively. During the test, the longitudinal steel reinforcement yielded and underwent remarkable plastic deformations before failure. These deformations led to several flexural and shear-flexural cracks along the beam at higher loads. After yielding of the longitudinal steel reinforcement, concrete crushing was also observed in the compression zone. At the onset of the failure, a flexural-shear crack formed between the loading point and the third FRP U-wrap, which was accompanied by the rupture of this FRP strip (Fig. 6f).
Table 3 Ultimate loads and displacements in all the test specimens. Specimen
Maximum load (kN)
Increase (%)a
Displacement capacity (mm)
Increase (%)b
CB WA SA-0 SA-30 SAG-0 SAG-0-NP
196.1 261.3 271.9 271.0 267.5 271.2
– 33 39 38 36 38
11.45 11.34 17.60 23.60 36.43 34.90
– – 55 108 221 207
a b
Compared to specimen CB. Relative to that of WA.
mid-span. The control beam (CB) experienced its designed shear failure. During loading, a crack formed in this specimen at the testing shear span between the support and the loading point, which later widened with increasing load up to failure. CB recorded a maximum load carrying capacity of about 196.1 kN corresponding to a displacement of 6.95 mm. Fig. 6(a) shows the failure mode of specimen CB. WA, retrofitted without any end anchorage, showed a flexural-shear failure rather than a shear one. However, this beam failed to provide a ductile behavior due to the debonding of FRP shear retrofits off the concrete substrate. Moreover, it recorded a maximum load of 261.3 kN at a displacement of 8.19 mm. During loading, flexural and shear flexural cracks in WA initially developed in the vicinity of the constant moment region and near the loading points. These cracks then propagated to the middle of the section. As displacement increased, a major shear crack began to develop from the loading point towards the third FRP strip (from the support) leading to the debonding of the crossing FRP strips with a popping sound; this is illustrated in Fig. 6(b). This debonding occurred at parts of the FRP strips located above the shear cracks. Beam SA-0, which was anchored using horizontal stitching anchors, underwent a flexural-shear failure. The maximum load carrying capacity of about 271.9 kN was recorded during the test at a displacement of 10.86 mm. During loading, a couple of shear cracks formed in the testing shear spans of this specimen in addition to a number of flexural cracks in its constant moment region. With increasing load, these cracks widened until FRP strips slipped and ruptured the stitching anchors causing the FRP sheets to debond off the concrete substrate. Fig. 6(c) shows the beam after its failure. Beam SA-30, with anchors embedded at an inclination angle of 30° towards the beam top side, sustained larger displacements and underwent a more ductile flexural-shear behavior. The beam exhibited a maximum load of 271 kN at a displacement of 14.99 mm. During loading, flexural cracks initially formed in its constant moment region which subsequently propagated towards the compression zone throughout the test. At higher displacements, however, a main shearflexural crack developed from the loading point towards the testing shear span that crossed two FRP shear strips and caused these strips to slip out of the stitching anchors (Fig. 6d). These slips are represented by the small drops in the relevant load–displacement curve. Failure in beam SA-30 was accompanied by both concrete crushing at the compression zone and rupturing of the FRP stitching anchor. The specimen SAG-0 underwent a more convincingly flexural failure with a maximum strength of 267.5 kN corresponding to a displacement of 12.04 mm. As observed in Fig. 5, the load–displacement curve for this beam recorded during experimentation indicates a significant plastic deformation of the longitudinal reinforcement. During the test, both flexural and shear-flexural cracks formed symmetrically not only in the constant moment region but in both shear spans as well, which propagated towards the compression side of the beam with increasing load. At a flexural crack depth of around 75% the beam’s height, the concrete at the compression side began to crush. This behavior
4.2. Comparison and discussion The FRP shear retrofits were able to change successfully the behavior of the as-built beams from a shear to a flexural-shear one even in the absence of end anchors. However, the end anchors were found to have significant effects on the structural behavior of the beams after the retrofitting intervention, particularly as regards displacement capacity, and ductility. In what follows, these parameters are individually explored. 4.2.1. Strength and displacement Comparison of the results obtained for the control and retrofitted specimens indicates a significant improvement in ultimate load carrying capacity due to the application of the FRP U-wrap strips. Table 3 reports the maximum loads and ultimate displacements recorded by all the test specimens. Except for beam WA, the retrofitted beams recorded identical increases in their total applied loads. The lower percentage of load increase in beam WA relative to those of other retrofitted beams is clearly due to the debonding of its FRP U-wraps. While it would be possible to achieve higher strengths using this system, it was the objective of the present study only to investigate the effect of end anchorage on the flexural behavior of RC beams shearretrofitted with FRP composites. Evidently, yielding of longitudinal reinforcement impeded higher increases in total applied load in the retrofitted test specimens. Moreover, failure in the specimens SAG-0 and SAG-0-NP was accompanied by the fracturing of FRP wraps. However, these fractures were attributed to local stress concentration in FRP composites due to the high magnitude of bending deformations as a result of the successful prevention of debonding failure by the adopted anchorage system. The displacement capacity of each specimen was defined as that beyond which a 20% drop was observed in the maximum sustained load; this level of displacement would be accompanied by a considerable damage to the specimen. Table 3 reports the relevant values of displacement for all the test specimens. It may be noted that the displacement response was irrelevant in the case of the control specimen since it underwent a shear failure. Hence, the increase in displacement capacity for the retrofitted specimens was calculated relative to that of beam WA. The lowest increase in displacement capacity (55%) was attained only with FRP stitching anchors employed as the end anchorage system. On the other hand, ultimate displacement in beam SAG-0 increased by 221% relative to that of WA retrofitted with no end anchors. It is also worth mentioning that the failure of SAG-0 was attributed to the rupture of FRP shear retrofits, indicating that larger displacement capacities may be achieved by applying more layers of FRP shear retrofits. Comparing the results obtained for specimens SAG-0 and SAG-0-NP 412
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anchors at an inclined angle led to the orientation of one component of the anchor resisting force in the direction of the U-wrap fibers. This component then reduced the bond stress between the CFRP U-wrap and the concrete substrate to postpone debonding failure. However, FRP stitches inserted at an angle of zero degrees would have only augmented the friction resistance between the FRP U-wrap and the concrete substrate.
Table 4 Ductility factors and related parameter for the test specimens. Specimen
Yield dis. (mm)
Ultimate dis. (mm)
Ductility
Increase (%)a
WA SA-0 SA-30 SAG-0 SAG-0-NP
4.90 4.71 4.84 6.00 5.24
11.34 17.60 23.60 36.43 34.90
2.3 3.7 4.9 6.1 6.7
– 61 113 165 191
a
5. Predicted flexural capacity
Relative to that of WA.
Ultimate flexural capacity was calculated for the tested beams using the conventional sectional analysis based on the widely-used Hognestad’s model [29] for simulating the nonlinear behavior of concrete as described in Park and Paulay [30]. Accordingly, the depth of the neutral axis, c , was determined by satisfying the force equilibrium equation given by:
indicates that surface preparation had only a negligible effect on displacement capacity when the proposed end anchorage system was employed. This finding is interesting not only from a fabrication viewpoint but also from economic one as surface preparation as a difficult and tedious part of FRP application, particularly in large projects, can be eliminated altogether.
αf c' bc + As' f s' = As fs
(1)
f c'
is the cylinder compressive strength of concrete and b represents the width of the section. Also, f s' and fs denote the stresses developed while As' and As represent the areas of compression and tension reinforcements, respectively. In addition, the parameter α is defined as: in which,
4.2.2. Ductility factor On a par with strength, ductility is a prominent factor influencing the flexural performance of RC beams. Displacement ductility is defined as the ratio of ultimate displacement, Δu , to yield displacement, Δy . Due to the nonlinear behavior of RC members, their load–displacement behavior typically lacks a well-defined yielding point. However, yield displacement can be either identified as the displacement corresponding to the first yield of the longitudinal steel reinforcement in tension or as the intersection of two linear parts on an idealized bilinear curve. In the present case, the latter approach was adopted and the relevant idealized bilinear curve was derived using the equal energy rule. The bilinear and the original curves were found to intersect at a point corresponding to 60% of the yield load. Equal energy rule was manifested by equating the areas below the original and bilinear curves. Table 4 reports the values for ductility factors and the related parameters for all the retrofitted specimens. Ultimate displacement was defined in the same way displacement capacity was. It should, however, be noted that it is not logical to determine the ductility factor values for the control beam (CB) as it experiences a shear failure without its longitudinal steel reinforcement having yielded. The ductility parameter for the specimen WA is about 2.3. This is while those for specimens SAG-0 and SAG-0-NP are calculated to be around 6.1 and 6.7, respectively. This confirms that implementation of the proposed anchorage system led to significant enhancements in ductility factor. Inserting FRP U-wraps into grooves resulted in additional strength of the bond between the FRP sheets and the concrete similar to what is observed in the typically-used 90° standard hooks for steel reinforcement. It should be noted that all the corners in these specimens had been rounded to a minimum radius of 15 mm in order to prevent stress concentration and facilitate the transfer of force from the U-wrap into the groove. On the other hand, FRP stitches had been implemented at the end of the groove aligned with the fiber orientation, which might have created both resistance force and a fiber strength but in different directions. In fact, the groove was made to exploit all the possible resistance of the stitching anchor by aligning the main retrofitting fibers and the stitching anchors. Interestingly, the highest ductility value was observed in the specimen SAG-0-NP that had been retrofitted with end anchors but without surface treatment. The lowest rise in ductility belonged to the specimen SA-0 anchored only with horizontal FRP stitches; it actually exhibited a ductility enhancement of 61% relative to that of WA. In addition, it was found that inserting FRP stitching anchors at an inclined angle improved the ductile performance of the FRP-shear-retrofitted beams that would, otherwise, experience a flexural failure. This improvement can be explained with recourse to the system of forces at the intersection of U-wrap and anchor along the following lines. Inserting stitching
α=
εc ε (1 − c ) ε0 3ε0
ε0 = 1.71
(2)
f c' (3)
Ec
where, εc and Ec denote the maximum strain and elastic modulus of concrete. Ultimate flexural capacity is thus given by:
Mu = αf c' bc (d − γc ) + As' f s' (d − d')
(4)
'
in which, d and d represent effective depths of compressive and tensile reinforcements, respectively. In addition, γ is a coefficient for locating the centroid and is equal to:
γ=1−
(2/3 − εc /(4ε0)) (1 − εc /(3ε0))
(5)
Given that a ductile flexural failure precedes the shear failure in beams, ultimate flexural capacity of beams was determined to be 88.1 kN m corresponding to a total load of 271 kN. This load is roughly equal to the maximum value sustained during experimental test by all the retrofitted specimens except for WA whose failure was dominated by shear due to the premature debonding of its FRP U-wraps. It should be noted that a negligible difference (1.3%) was observed between the analytical and experimental loading capacities of specimen SAG-0 which might be attributed to use of the average properties of steel reinforcement and concrete in analytical calculations. Overall, it could be concluded that the adopted retrofitting technique was efficient in fully exploiting the ultimate flexural capacity of beams with shear deficiency and improving their ductility behavior. The adopted anchorage system was aimed at changing the failure mode of beams from a brittle shear to a ductile flexural mode. In an ideal condition, the anchorage could be remained intact under monotonic loading until achieving the ultimate flexural and displacement capacities. 6. Conclusions The results of an experiment investigating the effect of end anchorage on flexural performance of shear-deficient RC beams retrofitted with FRP U-wrap were reported. For the purposes of this study, a novel anchorage technique was proposed and its efficiency was evaluated through experimental testing. While FRP U-wrap strips were found capable of shifting beam failure mode from a shear to a flexural one in 413
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all the retrofitted specimens, their post-yield performance was found to depend mostly on the performance of the end anchorage system employed. All the retrofitted specimens recorded almost identical loading capacities since they underwent a flexural failure mode. However, the end anchorage was found able to provide substantial improvements in displacement capacity, and ductility factor of the shear-retrofitted beams. The most desirable post-yield behavior was observed in beams retrofitted with the proposed end anchorage technique but even subjected to no surface treatment. Comparison of the beams SAG-0 and WA revealed that displacement capacity, and ductility factor increased by 221%, and 165%, respectively, once the proposed anchorage technique was implemented. A similar performance was also observed in beam SAG-NP that had been retrofitted using the proposed end anchorage system but no surface preparation. The present experiment is expected to provide the primary impetus for further investigation and evolution of the proposed anchorage system. Within its scope, the experiment confirmed the efficiency of the proposed technique in improving the flexural performance of sheardeficient RC beams retrofitted with FRP U-wraps. However, these findings are based on static monotonic loading and anchorage devices may not retain the same level of effectiveness under cyclic load reversals. Thus, the influence of cyclic loading on the effectiveness of the proposed anchorage devices needs to be also scrutinized in details. Finally, the efficiency of the technique in increasing the loading capacity of shear-deficient RC beams may be suggested for future research.
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