Experimental response of double-wythe masonry panels strengthened with glass fiber reinforced polymers subjected to diagonal compression tests

Engineering Structures 39 (2012) 24–37

Contents lists available at SciVerse ScienceDirect

Engineering Structures journal homepage: www.elsevier.com/locate/engstruct

Experimental response of double-wythe masonry panels strengthened with glass fiber reinforced polymers subjected to diagonal compression tests Arsalan Kalali, Mohammad Zaman Kabir ⇑ Department of Civil Engineering, Amirkabir University of Technology, Tehran Polytechnic, Hafez Ave., P.O. Box 15875-4413, Tehran, Iran

a r t i c l e

i n f o

Article history: Received 19 October 2010 Revised 21 January 2012 Accepted 30 January 2012 Available online 3 March 2012 Keywords: Diagonal tension test Shear strength Diagonal tension strength Unreinforced brick panels Strengthening Glass fiber reinforced polymers

a b s t r a c t Many existing unreinforced masonry buildings are seismically vulnerable and require retrofitting. The present experimental study, performed on half scale double-wythe brick masonry panels strengthened by glass fiber reinforced polymers (GFRPs), aimed to investigate the efficiency of a shear reinforcement technique using externally bonded FRP composites. The test specimens were built in a manner to simulate the traditional masonry walls built in Iran. A series of two unreinforced brick panels and five GFRP strengthened panels were subjected to diagonal tension tests. The two unreinforced diagonal tension specimens presented a brittle failure due to splitting along the compressed diagonal. The key parameter was the strengthening configuration. Experimental results pointed out that GFRP reinforcement provided a less brittle failure and a noticeable load bearing capacity increase. Performances of the different reinforcement configurations were compared in terms of strength, deformation capacity, ductility, energy absorption, stiffness and mechanism of failure. Also, near the end of the paper, experimental results of three perforated masonry shear walls are reported. The walls were tested under constant gravity load and incrementally increasing in-plane loading cycles. Two of these walls and two of the diagonal tension specimens had the same geometrical reinforcement ratio in the diagonal direction. Ó 2012 Elsevier Ltd. All rights reserved.

1. Introduction Despite representing a significant portion of the building stock around the world, many of the existing unreinforced masonry (URM) buildings are seismically vulnerable and need to be retrofitted. The main structural elements that resist earthquakes in these buildings are the old URM walls which were designed to resist mainly gravity loads. Under seismic loading, URM walls have two possible failure mechanisms, namely in-plane shear and out-ofplane bending. Therefore, researchers address either retrofitting to improve the in-plane [1] or out-of-plane [2] behavior. This paper focuses on the in-plane behavior of unreinforced brick (URB) walls before and after retrofitting, using glass fiber reinforced polymers (GFRPs). The principal in-plane failure mechanisms of URB walls subjected to earthquake actions can be summarized as shown in Fig. 1 [3]. Numerous conventional techniques (e.g. ferrocement, shotcrete, grout injection, external reinforcement, post tensioning, and center core) are available for retrofitting of existing masonry structures. Several researchers summarized and discussed the advantages and disadvantages of these conventional techniques [4]. The disadvantages of these techniques can be listed as: time consuming to

⇑ Corresponding author. Tel.: +98 9121482273; fax: +98 2166414213. E-mail addresses: [email protected] (A. Kalali), [email protected] (M.Z. Kabir). 0141-0296/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.engstruct.2012.01.018

apply, available space reduction, occupancy disturbance, building operation disruption and affecting the aesthetics of the existing wall. In addition, the added mass can also increase the earthquake induced inertial forces and may require strengthening of the foundations as well. Most of these problems may be overcome using FRP for retrofitting. Other benefits of FRP strengthening systems are: highly durable in many environments; field experiments have shown that retrofitting with FRPs is cost-effective [5]; and in the case of wet lay-up, this strengthening system may be formed in place around any surface shape. However, it is also important to note possible disadvantages of FRP strengthening: removal is extremely difficult; the resins presently used are flammable, and give off toxic vapors during a fire, fire protection is a concern when implementing an FRP system; the susceptibility of the resin in FRPs to ultraviolet light, the resin slowly becomes brittle; the long-term reliability of FRPs is largely unproven; and FRPs are impermeable to moisture transport. Surface preparation is very important to the success of FRP application as unfilled cracks or unsmoothed irregularities can cause premature debonding. The effects of surface irregularity are certainly a concern for masonry and must be investigated to determine the essential requirements for positive results. Various researchers have examined the use of FRPs to enhance the in-plane performance of masonry walls under monotonic, cyclic or seismic loading. Large increases in both load and displacement capacity were observed, with the amounts depending on the

A. Kalali, M.Z. Kabir / Engineering Structures 39 (2012) 24–37

Fig. 1. Failure modes of in-plane loaded URB walls: (a) shear failure; (b) sliding failure; (c) rocking failure; and (d) toe crushing failure [3].

quantity and type of the FRP used. Also, different in-plane failure modes of FRP strengthened masonry walls were reported such as shear failure, flexural failure, FRP rupture, anchorage failure and debonding failure [6–13]. Whereas extensive research has been conducted and reported for retrofitting of reinforced concrete structures using FRP, much less has been reported for retrofitting of URM walls [14–16]. In addition, FRP reinforcement can also be used successfully for retrofitting of masonry arches [17–18] and masonry infill walls in reinforced-concrete or steel frames [19], thus further work is required since so little research has been done but what has been investigated shows exciting promise. For masonry structures, it is worthwhile to point out that in a general sense, relatively large discrepancies between analysis results (numerical results) and test results are observed that can be attributed to the nature of the masonry material. Compared with steel or concrete materials, properties of the masonry material, including strength values and fracture behavior, exhibit a greater degree of uncertainty and deviance due to the uncontrollability of workmanship. As a result, it is more difficult to reach a precise analysis result for these types of structures [20–21]. The numerical modeling becomes more difficult, if the masonry structure has been strengthened by FRPs, because of the complex interaction mechanisms between FRP and masonry material [22–25]. Thus, in this research, an experimental program was undertaken on the strengthening of the existing brick walls by GFRPs with different configurations. This study has been conducted on the existing unreinforced brick shear walls representative of conditions existing in Iran. The main objective of this paper is to assess the in-plane shear performance of these unreinforced brick walls before and after retrofitting using GFRPs with different configurations. A ‘‘direct’’ approach to the estimation of the strength of masonry walls consists of performing experimental tests (shear-compression tests) able to simulate reality as closely as possible, in terms of boundary conditions and acting forces. Although quite accurate and reliable, such an approach is expensive and time-consuming. For this reason, ‘‘indirect’’ approaches, based on simplified theoretical models, are usually adopted. The simplified models present in the literature and codes are oriented to describe specific failure modes which may occur in masonry walls (rocking/crushing, bed joint sliding, and diagonal cracking). In this experimental work, which was performed on masonry panels, the diagonal tension test [26] was chosen to simply simulate the in-plane shear failure mode. Similar to concrete, the behavior of URM walls under shear is much more complex than that under flexure [27]. The shear failure mode is the most common in-plane damage mode in unreinforced brick walls under earthquake loading. In the case of shear failure mode; walls (generally with low aspect ratios and high axial loads) tend to develop a diagonal cracking failure (see Fig. 1). The diagonal cracks developed in the wall either follow the path of the bed and head joints for relatively strong bricks and weak mortar, or may go through the masonry units in case of relatively weak bricks and strong mortar, or both. Obtaining a diagonal shear failure mode of a wall was found to be difficult due to the tendency of isolated walls to rock and, in

25

some cases, diagonal shear failure was obtained by restraining the wall from rocking or by precompressing the wall with a large magnitude axial load that is improbable in common URM buildings [28]. Alternatively, force can be applied along a panel diagonal to obtain a diagonal crack in a so-called diagonal compression test, which has been standardized (ASTM E 519) for URM panels measuring 120 cm  120 cm. Such a test (the diagonal compression test) offers the following main advantages: versatile application to different types of masonry; capability of providing values of mechanical parameters representative of the masonry in the whole wall [29]; laboratory and on-site performability [30]. In recent researches conducted by diagonal tension tests on masonry panels in accordance with ASTM E 519 [26], performances of different shear reinforcement techniques have been compared in terms of strength, ductility and mechanism of failure [28,30–37]. Additionally, according to FEMA 356 [38], the in-plane strength of the diagonal failure mode (shear failure mode) of existing and enhanced URM walls shall be taken in accordance with the following equation:

V dt ¼

fdt0 A

 sffiffiffiffiffiffiffiffiffiffiffiffiffiffi L fa 1þ 0 h fdt

ð1Þ

where L, h and A are geometrical parameters of the wall (L = length of the wall, h = height of the wall, and A = cross-sectional area of the wall), fa is axial compressive stress due to gravity loads and fdt0 is masonry diagonal tension strength (shear strength) which is obtained from diagonal tension tests on masonry panels in accordance with ASTM E 519. The diagonal tension strength (shear strength) for masonry panels is calculated as follows:

fdt0 ¼

0:707Pmax Ap

ð2Þ

where Pmax = maximum applied diagonal compression load, and Ap = cross-sectional area of the panel, calculated as follows:

Ap ¼

  L p þ hp t p  Lp t p 2

ð3Þ

where Lp = length of the panel, hp = height of the panel, and tp = thickness of the panel. Eq. (1) can be successfully used if the diagonal tension masonry panels and the corresponding full-size masonry walls have the same masonry unit and mortar. Also, the diagonal tension masonry panels should be constructed under the same conditions as the corresponding actual masonry walls. Although some experimental results are available in the literature [8,31,33–34,37], only a few of them concern the case in which the diagonal tension test is carried out on FRP retrofitted doublewythe URM panels [28]. Moreover, none of them considers all the properties of masonry constituents in the existing masonry buildings in Iran. The selected section (double-wythe URM panels) in this study for the diagonal tension specimens is more realistic, because it can be similar to that of a masonry shear wall. Near the end of the paper, experimental results of three masonry shear walls are reported. The walls were tested under a combination of vertical compression preload and in-plane horizontal shear loading. Two of the shear walls and two of the diagonal tension specimens had the same geometrical reinforcement ratio in the diagonal direction. This similarity between different types of test specimens can not be found in the existing literature. The purpose was extrapolation of the diagonal tension test data to actual masonry walls. The comparison among results is interesting, because these comparisons can be used to assess the accuracy of existing analytical formulations (for example Eq. (1) provided in FEMA 356) for ultimate shear strength prediction of masonry walls or develop new analytical shear models.

26

A. Kalali, M.Z. Kabir / Engineering Structures 39 (2012) 24–37

Assessment of design formulas for in-plane FRP strengthening of masonry walls based on a test database collected from the available literature can be found in [27,39–40]. According to [39], the analysis of URM walls points out that the design formulas suggested by FEMA 356 for evaluating the lateral strength of URM walls provides a good estimate of the lateral strength. Here, an assessment of the design formula (Eq. (1)) provided in FEMA 356 for computing the in-plane shear strength of strengthened URM walls has been performed. In this experimental study, the specimens were instrumented. The instrumentation arrangement was designed in order to measure applied loads, displacements of the panels and strain of the GFRP strips. The assessment of performance is reported qualitatively through test observations and quantitatively by assessment of load bearing capacity, deformation characteristics, ductility, failure energy and stiffness. The research objectives were met through a series of half-scale experimental tests. 2. Experimental program This static experimental program investigates the effectiveness of GFRPs as externally bonded retrofitting materials for the inplane strengthening of unreinforced brick walls. All the test specimens were scaled down to one-half due to the experimental limitations. All the geometrical parameters in this experimental work (e.g. the dimensions of the bricks, the thickness of the bed and head mortar joints) were scaled down to one-half. On the other hand, the mechanical parameters of all the materials used in the one-half scale specimens were the same as those of the full-scale specimens. Seven half-scale panels were tested at the structural laboratory of Iran Building and Housing Research Center at Tehran (BHRC) as described in the following sections. Forces, displacements and strains were monitored using the provided devices during the tests. Data collected from these tests permit evaluation of performance parameters of the specimens at different limit states. Masonry mechanical properties depend on the characteristics of the constituent elements (brick and mortar), as well as on the workmanship. So the materials used to construct the test panels and their properties as determined by the appropriate ASTM standards are given as follows.

experimental program all of these were considered consistent with those used in the existing unreinforced masonry buildings. The nominal size of each one-half scale specimen was 560 mm by 560 mm by the thickness of 105 mm. The panels were built with 14 courses of bricks. The masonry bond pattern used to construct the panels is shown in Fig. 2. According to [8,33], retrofit using FRPs is able to reduce the inherent variability of URM. Therefore two specimens were unreinforced and considered as control specimens (specimens URBP1 and URBP2). Five specimens were strengthened (with different strengthening configurations) by one layer of unidirectional GFRP, excluding specimen RBP-F2L which had two layers of unidirectional GFRP, as shown in Fig. 3. Table 1 summarizes the tested specimens. The geometrical reinforcement ratio in one direction is defined as percentage of the total cross-sectional area of GFRP in that direction over the corresponding gross sectional area of the specimen. The same reinforcement ratio was applied for the two different configurations (diagonal and grid patterns) in the diagonal direction (compressed diagonal). Glass fiber reinforced polymers were used to retrofit the URB specimens. Application of the GFRP took place after curing of the brick panels for at least 28 days in laboratory conditions. Table 2 shows the glass fiber fabric properties according to manufacturer’s data. The application of the wrap material was a simple and rapid operation. The application method was dry lay-up. An epoxy resin based adhesive (two component epoxy Sikadur 330) was used for bonding the glass fiber sheet. The specimens were left to cure in laboratory conditions for at least 7 days. Cured laminate (GFRP) properties in accordance with ASTM D 3039 [43] after standard cure are given in Table 3 according to the datasheet made by the manufacturer. Glass FRP is excellent for seismic upgrades where the seismic loads only temporarily engage the FRP. In cases where stresses are sustained in the FRP (such as in bending and shear

2.1. Test specimens The Iranian traditional masonry works, which represent a large part of its historical heritage, are particularly susceptible to damage due to in-plane actions such as earthquake. The test specimens were intended to represent masonry structures built during the last 40 years of the 20th century in Iran [41–42]. Since the performance of the masonry panels is influenced by the type of masonry materials (bricks and mortar) and the style of construction; in this

Fig. 2. The brick masonry bond pattern of the test panels.

Fig. 3. The unreinforced and GFRP strengthened brick panels: (a) URBP2; (b) RBPH; (c) RBP-X; (d) RBP-F2L.

27

A. Kalali, M.Z. Kabir / Engineering Structures 39 (2012) 24–37

2.2. Bricks and mortar

Table 1 Description of the GFRP strengthened specimens. Specimen

Strengthening configuration

RBP-H

One layer, symmetrical double-side, grid arrangement (three 7 cm horizontal strips and two 10 cm vertical strips), with end anchorage for the horizontal strips by the thicknesses of the masonry panel

RBP-X

One layer, symmetrical double-side, diagonal arrangement (four 7 cm diagonal strips), with end anchorage for the diagonal strips by the thicknesses of the masonry panel One layer, symmetrical double-side, diagonal arrangement (four 7 cm diagonal strips), without end anchorage for the diagonal strips

RBP-XWO

RBP-F1L

RBP-F2L

The diagonal tension test masonry panels consisted of materials (bricks and mortar) representative of those used in the existing masonry buildings. A commercial firm specially produced the brick units used in the experimental program to be one-half scale of original solid clay bricks. The original full scale solid clay brick is 210 mm  100 mm  56 mm; this resulted in a ½-scale brick nominally measuring 105 mm  50 mm  28 mm. The bricks were completely saturated and soaked in water prior to their use. In addition, during construction the head and bed joints were approximately 6 and 10 mm thick, respectively, to be consistent with the half-scale bricks. The mortar had a composition of 1 part cement to 5 parts sand, by volume. Based on the masonry codes, walls made of bricks shall be constructed in such a way that vertical joints do not fall in one line and to be thoroughly filled with mortar (Standard No. 2800 [44]) which was followed in this study (Fig. 2). For determination of the compressive strength of the mortar, four 50 mm cube specimens were made and tested in accordance with ASTM C 109/C 109M [45]. The average compressive strength of these 50 mm cube samples taken during the construction of the diagonal tension specimens was 5.2 MPa. Then, different types of masonry samples were constructed using one-half scale bricks and mortar joints. For measuring bond strength of the mortar to the bricks, five test specimens consisting of crossed-brick couplets were constructed. The average tensile bond strength of 0.062 MPa was determined by testing these specimens according to ASTM C 952 [46]. An important failure mode of masonry walls is the sliding process in mortar joints. Here, in order to understand better this phenomenon, an experimental study was carried out as shown in Fig. 4. When a compressive stress is applied to a mortar joint, the shear strength results from the combination of two mechanisms: the shear bond strength and the friction resistance. Therefore, the ultimate shear strength (su) can be expressed by Mohr–Coulomb criterion given by the following equation:

Figure

One layer (0°), symmetrical doubleside, full surface, with end anchorage for the horizontal fibers by the thicknesses of the masonry panel Two layers (0°/90°), symmetrical doubleside, full surface, with end anchorage for the horizontal fibers by the thicknesses of the masonry panel

su ¼ c þ rn  tan u

strengthening), glass FRP should be avoided because of creep rupture effects. Carbon is much more suitable in these applications. In previous research (for example in [37]), it was seen that in single-side reinforced panels, shear strength was in many cases lower than the reference value (in the case of unreinforced panels). These nonsymmetrical strengthened panels exhibited a clear twisting deformation during the diagonal tension test. Hence, it was decided to install the same amount of GFRP material on each face of every panel in order to prevent an eccentric stiffness and strength distribution that may cause twisting.

ð4Þ

where c, u and rn are respectively, the cohesion, the internal friction angle and compression stress normal to the mortar joint. In Fig. 4a mortar joints are under pure shear and thus, the cohesion can be obtained simply. The average cohesion (shear bond strength) of the mortar joints was 0.2 MPa. Two steel devices were constructed in this study in order to apply simultaneously compression and shear loads to mortar joints (see Fig. 4b). The average internal friction angle of the mortar joints was determined using these devices and Eq. (4), as 30 degrees. In other words, the average internal coefficient of friction (l = tan u) between brick and mortar was equal to 0.58. The bricks used in panels construction were tested in accordance with ASTM C 67 [47]. The average compressive strength

Table 2 Glass fiber fabric properties used in the experimental program. Commercial name

Fiber orientation

Area weight

Tensile strength

Tensile modulus

Tensile elongation

SikaWrap Hex 430G

Unidirectional

430 g/m2

2.25 GPa

70 GPa

2.8%

Picture

Table 3 GFRP properties used in the experimental program. Tensile strength

Tensile modulus

Tensile elongation

90° tensile strength

90° tensile modulus

90° tensile elongation

Thickness

ASTM test method

537 MPa

26.49 GPa

2.21%

23 MPa

7.07 GPa

0.32%

0.51 mm

D 3039 [43]

28

A. Kalali, M.Z. Kabir / Engineering Structures 39 (2012) 24–37

was 11.7 MPa. Also, the average compressive strength of three masonry prisms was determined, according to ASTM C 1314 [48], as 4.95 MPa. The masonry prisms used for the compression tests had an aspect ratio of 1.84 and had a cross section similar to that of the diagonal tension specimens. The length, width and height of these compression prisms were 161, 105, and 190 mm, respectively. The compression prisms were built using the same construction technique as was used for the diagonal tension specimens. The average chord elastic modulus of these unreinforced prisms was determined in accordance with ASTM C 469 [49] (Corresponding to 5% and 40% of the maximum load), as 676.67 MPa. The above determined values for the masonry material properties are summarized in Table 4. 2.3. Test setup and loading system All the seven one-half scale brick panels were tested in accordance with ASTM E 519 which determines the diagonal tensile or shear strength of brick assemblages by loading them in compression along one diagonal, thus causing a diagonal tension failure with the specimen splitting apart parallel to the direction of load (see Fig. 3). This test method was developed to measure the diagonal tensile (shear) strength of masonry more accurately than was possible with other available methods. The specimen size was selected as being the smallest that would be reasonably representative of a full-size brick wall and that would permit the use of testing machines such as those used by many laboratories. Two steel loading shoes were constructed in accordance with ASTM E 519 (see Fig. 5) to accommodate the test specimens. The test base was rigid. The test setup was similar for all of the test specimens. For example, it is illustrated in Fig. 3 for some specimens. The loading shoes were used to apply the hydraulic jack load to the specimen and support the other (reaction) end. One load cell was installed in series with the hydraulic jack. This jack could only produce compressive load and was mounted on a rigid reaction frame. The length of bearing of the loading shoes was 10 cm. Experimental work has indicated that the minimum length of bearing of the loading shoes should be approximately one eighth of the length of the edge of the specimen to avoid excessive bearing stress. Specimens were instrumented for measuring the applied load and the shortening of the compressed diagonal. Each specimen was loaded continuously and without shock. The loading procedure was displacement-controlled. The load was applied at a constant rate of about 0.3 mm/ min until the failure limit state of the specimen was reached. 2.4. Instrumentations The specimens were instrumented with several devices as shown in Fig. 6. Two linear variable differential transducers (LVDTs) measured the shortening of the vertical diagonal under compressive load

Table 4 Mechanical properties of the masonry materials used in the experimental program. Item

Property

Value

Brick Mortar

Compressive strength Compressive strength

11.7 MPa 5.2 MPa

Brick/mortar interface

Tensile bond strength Shear bond strength Coefficient of friction

0.062 MPa 0.2 MPa 0.58

Masonry prism

Compressive strength Elastic modulus

4.95 MPa 676.67 MPa

during the test. The axial strains in the GFRP were measured using electrical strain gauges. The applied vertical force at the panel top was measured using a load cell. Loads, displacements, and strains were all recorded by a data acquisition system. 3. Experimental observations A summary of the experimental observations regarding the behavior, crack pattern, and failure mode of the test panels subjected to diagonal tension tests is given in the present section. The two unreinforced specimens failed ultimately in classical tensile splitting along the compressed diagonal as shown in Fig. 7. The average peak load, used as a reference value for comparison with the strengthened specimens’ results, is equal to 32.96 kN. The ratio of the weight to the load-bearing capacity for the unreinforced specimens is about 1.81%. This ratio also decreases for the FRP reinforced specimens and therefore the impact of the change in self-weight due to scaling down is negligible. The behaviour of the two unreinforced specimens under load indicated that they failed suddenly. Therefore, the failure of these specimens was very brittle. Here, the response of masonry panels subjected to a diagonal compression load is the core issue; thus, in the case of diagonal compression, distribution of stresses in the reference brick panels is determined. A comprehensive discussion about the selected method for finite element modeling of unreinforced masonry structures can be found in [50]. Homogenization of these panels is done in three steps which are shown in Fig. 8. With regard to the style of arrangement of bricks and bed and head joints of mortar in each brick panel which makes the panel stiffness in different directions uneven; the equivalent homogenous material of masonry is considered as orthotropic and nine independent parameters needed for defining it, are determined according to [50]. This finite element modeling can explain some results and observations in this experimental program. It is possible to introduce the concept of an effective crack direction with the purpose of obtaining a graphic visualization of the cracking pattern in the finite element model of masonry specimens. Different criteria can be adopted for the definition of the

Fig. 4. Mortar joints under (a) pure shear; (b) shear combined with compression.

A. Kalali, M.Z. Kabir / Engineering Structures 39 (2012) 24–37

29

Fig. 7. Failure pattern of specimen URBP2 in the diagonal tension test. Fig. 5. Steel loading shoes.

Fig. 6. An overview of the instrumentation: (a) reference specimens; (b) specimen RBP-H; (c) specimens RBP-X and RBP-X-WO; (d) specimens RBP-F1L and RBP-F2L.

direction of cracking. For example, we can assume that cracking initiates at points where the maximum principal tensile stresses are the greatest. The direction of the vector normal to the crack plane is assumed to be parallel to the direction of the maximum principal tensile stress [50]. According to Fig. 9, the cracking develops diagonally which is similar to the experimental observations. According to Frocht theory [29], in URM panels the highest splitting stresses (maximum principal tensile stresses) are concentrated in the centre which can be seen in Fig. 9. The failure mode of the GFRP strengthened panels was diagonal splitting similar to the unreinforced panels, excluding specimens RBP-X and RBP-F2L. In these strengthened specimens, after the maximum load was achieved, debonding of GFRP overlays developed along the compressed diagonal band of the specimen under increasing displacement. Because of this debonding of GFRP overlays, the applied force decreased until the failure limit state occurred. After full debonding of these GFRP overlays, the failure mode of diagonal splitting took place. Non diagonal GFRP strips (for example in specimen RBP-H) were subject to axial and also

shear stresses during the diagonal tension test. These shear stresses caused wrinkles in the GFRP strips as can be seen in Fig. 10 and acceleration of GFRP debonding. Specimen RBP-F2L, which had the highest amount of GFRP axial rigidity in the current investigation, had the highest strength. This specimen had mixed modes of failure, namely diagonal splitting and sliding with a maximum strength of 142.25 kN, as illustrated in Fig. 11. This means that the GFRP enhanced the shear resistance by a factor of approximately 4.32. In specimen RBP-X, the GFRP strips were subject to only axial stresses and prevented the panel from diagonal splitting. Thus, specimen RBP-X failed locally under compression at its lower corner, near the lower loading shoe at the test end (see Fig. 12). Also, some local debonding of GFRP strips along the compressed diagonal band of the specimen was observed. Due to local crushing in masonry materials, the GFRP strips did not reach their ultimate strain (Table 3) as shown in Fig. 13. Because of the type of end anchorage, the GFRP strips of specimen RBP-X were more effective than those of specimen RBP-X-WO (according to Fig. 13). The GFRP strips in specimen RBP-X-WO did not have any end anchorage. As a result, the failure mode clearly changed from corner crushing to diagonal splitting due to the lack of end anchorage. As shown in the next sections, a significant increase in the performance parameters and more ductile failure can be achieved if end anchorage for the FRP reinforcement is provided. Also, according to Fig. 13, axial strain in middle GFRP strips is bigger than that of upper and lower GFRP strips in specimens RBP-X and RBP-X-WO. Because, in panels subjected to a diagonal compression load, the highest splitting stresses are concentrated in the centre (Fig. 9b). These experimental tests demonstrate the ability of GFRPs to keep the bricks together and maintain the specimens’ integrity despite the heavy damage in the specimens. Debris falling from URB walls during real earthquakes represents a major source of hazard even if the whole building remains safe. Progressive debonding of the GFRP overlays acted as a fuse which averted brittle failure and allowed the composite to remain effective at large displacement levels and to contribute to a highly nonlinear and apparently more ductile system response as shown in Fig. 14. For the particular specimens tested during this investigation, GFRPs increased the specimens’ deformation capacity by a factor of 3.61–22.22. In all of the strengthened panels, rupture of the GFRP strips (those installed on both faces of the panel) did not occur, but failure of the strengthening took place by debonding of the GFRP from the surface of the panel. Debonding of the GFRP from the masonry occurred either at the epoxy–brick interface (between the adhesive and filler layers, possibly due to poor surface preparation), or beneath the surface of the brick. The extent of debonding for a GFRP strengthened panel was detected by tapping the strengthened surface of the panel and noting where it sounded hollow.

30

A. Kalali, M.Z. Kabir / Engineering Structures 39 (2012) 24–37

Fig. 8. Steps of the homogenization of the reference diagonal tension specimens [50].

Fig. 9. (a) Homogenized finite element model of the reference URB specimens in the elastic range. (b) Distribution of the maximum principal tensile stress in the reference specimens under a diagonal displacement of 0.5 mm. (c) Distribution of the maximum principal tensile stress vector in the reference specimens (white vectors).

4. Experimental results and comparison After the tests, the brick panels strengthened using GFRP were evaluated by comparing the performance parameters such as stiffness, strength and ductility with those of the unreinforced brick panels as follows.

The global behaviour of the specimens, described by the applied load vs. deformation along the compressed diagonal curve, is shown in Fig. 15. From this figure, it can be concluded that the use of the GFRP reinforcement is quite effective. The initial stiffness of the curves of the reinforced panels is characterized by a similar slope as one obtained in the case of the unreinforced panels,

A. Kalali, M.Z. Kabir / Engineering Structures 39 (2012) 24–37

31

Fig. 10. Wrinkles in the GFRP overlays due to shear in specimens: (a) RBP-H; (b) RBP-F1L.

regardless of the configuration of the strengthening. On the other hand, the load corresponding to the strength limit state of the reinforced panels is much higher than that of the unreinforced panels. The gain in strength is quite remarkable: 145% for specimen RBP-

Fig. 11. Specimen RBP-F2L at the end of the test.

X-WO, 153% for specimen RBP-F1L, 232% for specimen RBP-H, 280% for specimen RBP-X, and 332% for specimen RBP-F2L. Moreover, we observe an important deformation capability of the reinforced panels. The deformation corresponding to the failure limit state of the GFRP strengthened panels is much higher than those of both unreinforced brick panels. The deformation capacity of the strengthened panels was substantially increased by the GFRP reinforcement on both sides in different configurations (Fig. 15), with an increase in deformation capacity of about 261%, 471%, 655%, 1045%, and 2122% for specimens RBP-X-WO, RBP-H, RBPF1L, RBP-F2L, and RBP-X, respectively. Also, the typical sudden failure of the unreinforced brick panels was noticeably corrected by the GFRP strengthening. Therefore, the structural behaviour of the brick panels was enhanced significantly using the GFRP. It is well-known that the response of brick walls is strongly nonlinear also for low load levels. In order to investigate the main aspects of the inelastic behavior of the test panels, the actual behavior was idealized with a bilinear curve (equivalent energy elastic–plastic curve). Bilinearization of the actual response of the panels represents a useful and common approach followed by code provisions currently available worldwide to assess the structural performance of existing structures by nonlinear static procedures (for example, FEMA 356). Thus, the equivalent energy elastic–plastic (EEEP) curve corresponding to the observed behaviour curve was calculated for each tested specimen, as illustrated for specimen RBP-X-WO in Fig. 16. The equivalent energy elastic–plastic (EEEP) curve is an ideal elastic–plastic curve circumscribing an area equal to the area enclosed by the behaviour curve between the origin, the ultimate displacement, and the displacement axis. The elastic portion of the EEEP curve contains the origin and has a slope equal to the elastic stiffness, Ke. The plastic portion is a horizontal line equal to Py determined by the following equation:

Py ¼

Fig. 12. Corner crushing in specimen RBP-X at the end of the test.

sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi! 2A Ke Du  D2u  Ke

ð5Þ

where A = the area under the behaviour curve from zero to the ultimate displacement (Du). The elastic stiffness (Ke) is the resistance to deformation of a specimen in the elastic range until the occurrence of the first cracks, which can be expressed as a slope measured by the ratio of the resisted load to the corresponding displacement at 0.4Ppeak or 0.6Py (0.54Ppeak for these test specimens) in accordance with ASTM E 2126 [51] and FEMA 356, respectively. The definition of the in-plane load associated with the first sign of structural damage of brick structures is conventional, because response of the brick structures is strongly nonlinear also at low levels of loads. Variations of K/K0 (K = secant stiffness (ratio of the load to the corresponding displacement), K0 = initial stiffness) versus D/Du for the specimens are shown in Fig. 17. From this figure, we can see that the secant stiffness of unreinforced brick panels significantly

32

A. Kalali, M.Z. Kabir / Engineering Structures 39 (2012) 24–37

(a)

Specimen RBP-X

14 Compression Load (tons)

Compression Load (tons)

14 12 10 8 6 4 2 0 0.0

Specimen RBP-X-WO

(b)

0.1

0.2

0.3

0.4

0.5

0.6

0.7

12 10 8 6 4 2 0 0.0

0.8

0.1

0.2

0.3

Axial Strain (%) Top

0.4

0.5

0.6

0.7

0.8

Axial Strain (%)

Middle (Average)

Bottom

Top

Middle (Average)

Bottom

Fig. 13. Compression load versus GFRP axial strain curve for specimens: (a) RBP-X; (b) RBP-X-WO until the strength limit state.

5

21

4

Compression Load (tons)

18

3 2

15

1

12

0 0.00

0.25

0.50

0.75

1.00

9 6 3 0 0

3

6

9

12

15

18

21

24

27

Deformation along the Compressed Diagonal (mm) URBP1

URBP2

RBP-H

RBP-X

RBP-X-WO

RBP-F1L

RBP-F2L

Fig. 14. Comparison of the global behaviour of the unreinforced and strengthened panels.

Compression Load (tons)

15

12

5

9

4 3

6

2 1

3

0 0.00

0.25

0.50

0.75

1.00

0 0

3

6

9

12

15

18

Deformation along the Compressed Diagonal (mm) URBP1

URBP2

RBP-H

RBP-X

RBP-X-WO

RBP-F1L

RBP-F2L

Fig. 15. Comparison of the global behaviour of the unreinforced and strengthened panels until the failure limit state.

dropped suddenly at the failure limit state. It shows that these unreinforced panels had very brittle failure mechanism which is similar to the experimental observations. Performance parameters in the following limit states were calculated for each behaviour curve and the corresponding EEEP curve, as summarized in Table 5. In addition, the increase in the

performance parameters of the GFRP strengthened panels in comparison with the reference panels is presented in this table. The considered limit states are the strength limit state (Dp, Pp), the failure limit state (Du, Pu), and the yield limit state (Dy, Py) as shown for specimen RBP-X-WO in Fig. 16. The failure limit state is the point (Du, Pu) on the behaviour curve corresponding to the last data point

33

A. Kalali, M.Z. Kabir / Engineering Structures 39 (2012) 24–37

Specimen RBP-X-WO 10

Strength limit state

Yield limit state

Load (tons)

Pp 8 Py Pu

6

Ke

Failure limit state

1

4 2 0 0.0

Dy 1.0

0.5

Dp 2.0

1.5

Du

2.5

3.0

Deformation (mm) Behaviour Curve

EEEP Curve

Fig. 16. Behaviour curve and EEEP curve of specimen RBP-X-WO.

Reference specimens

Specimen RBP-H

1.0

1.0

0.9

0.9

0.8

0.8 0.7

0.6

K/K 0

K/K 0

0.7 0.5 0.4 0.3

0.6 0.5 0.4

0.2

0.3

0.1

0.2 0.1

0.0 0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

0.0

D/D u Specimen URBP1

0.0

0.1

0.2

0.4

0.5

0.7

0.8

0.9

1.0

0.7

0.8

0.9

1.0

0.7

0.8

0.9

1.0

Specimen RBP-X-WO 1.0

0.9

0.9

0.8

0.8

0.7

0.7

0.6

0.6

K/K 0

1.0

0.5 0.4

0.5 0.4

0.3

0.3

0.2

0.2

0.1

0.1

0.0

0.0 0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

0.0

0.1

0.2

0.3

0.4

0.5

D/D u

0.6

D/D u

Specimen RBP-F1L

Specimen RBP-F2L

1.0

1.0

0.9

0.9

0.8

0.8

0.7

0.7

0.6

0.6

K/K 0

K/K 0

0.6

D/D u

Specimen RBP-X

K/K 0

0.3

Specimen URBP2

0.5 0.4

0.5 0.4

0.3

0.3

0.2

0.2

0.1

0.1

0.0

0.0 0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

D/D u

0.0

0.1

0.2

0.3

0.4

0.5

0.6

D/D u

Fig. 17. Secant stiffness of the test panels versus displacement variations.

with the load equal to or greater than 0.8Pp. If the behaviour curve contains data points at loads less than 0.8Pp, the failure limit state is determined at 0.8Pp using linear interpolation. The ductility

factor (l) is the ratio of the ultimate displacement (Du) and the yield displacement (Dy) observed in the monotonic test. The failure energy is defined as the total required energy until the occurrence

34

A. Kalali, M.Z. Kabir / Engineering Structures 39 (2012) 24–37

Table 5 Performance parameters of the tested panels. Specimen

PP (kN)

Du (mm)

Failure energy (kN mm)

Ke = 0.4PP/De Ke (kN/mm)

Reference specimens

URBP1 URBP2 Average

RBP-H

Ke = 0.54PP/De

l

Ke (kN/mm)

l

31.17 34.75 32.957

0.872 0.670 0.771

14.705 11.900 13.303

43.350 53.386 48.368

1.489 1.091 1.290

39.701 53.308 46.505

1.191 1.080 1.135

Value Increase (%)

109.57 232.457

4.407 471.377

334.374 2413.523

48.510 0.294

2.168 68.074

44.351 4.632

1.897 67.075

RBP-X

Value Increase (%)

125.24 280.022

17.142 2122.457

1832.685 13676.479

43.723 9.603

6.469 401.556

44.400 4.526

6.578 479.295

RBP-X-WO

Value Increase (%)

80.87 145.392

2.788 261.405

175.991 1222.942

88.182 82.315

3.306 156.329

84.219 81.110

3.124 175.158

RBP-F1L

Value Increase (%)

83.39 153.011

5.826 655.314

372.623 2701.045

41.771 13.639

3.212 149.031

36.689 21.107

2.730 140.418

RBP-F2L

Value Increase (%)

142.25 331.638

8.835 1045.437

859.425 6360.385

34.933 27.777

2.551 97.798

27.713 40.409

1.829 61.069

of the failure limit state of the specimen. It is equal to the area under the behaviour curve from zero to the ultimate displacement. Based on Table 5, the improvement in the performance parameters of the brick panels due to the different GFRP configurations is substantial. Under in-plane shear, experimental results proved that applying FRP strips along diagonal direction is more efficient than the grid configuration [27,37] as can be seen here. A comparison between the maximum strength of the reference and GFRP strengthened specimens is presented in Table 5. The increase in the maximum strength increased from 145.39% to 331.64% due to GFRPs. Table 5 indicates that the GFRP increased the failure displacement by a factor ranging from 3.61 to 22.22. Also, the GFRP increased the failure energy by a factor ranging from 13.23 to 137.76. High energy dissipation is a desirable property when a structure is subjected to a severe seismic event. The energy dissipated by a tested specimen was attributed to (1) formation of new cracks, (2) friction along existing cracks, (3) crushing of masonry materials, and (4) GFRP deformation, gradual debonding and rupture. However, since GFRPs remain elastic until failure it was not expected that much dissipation of energy would take place through the deformation of the GFRPs. Another parameter that is also important in order to assess the effectiveness of the strengthening technique is the ductility, which is conventionally expressed in terms of a ductility factor l. This factor represents a measure of the local ductility of masonry buildings. The correlation between local and global ductility is generally complex; it is fairly straight when shear response is predominant, whereas it depends on the structural configuration and on the potential failure mechanisms in other cases. In general, the higher the ductility ratio the higher the ability of strengthened masonry structures to redistribute the actions and to ensure an increased global deformation capacity and energy dissipation [35]. According to Table 5, the GFRPs had a significant influence on the ductility factor. The increase in the ductility factor of the GFRP strengthened panels in comparison with the reference panels is between 61.07% and 479.3%. Table 5 shows that the GFRPs had no great positive effect on the elastic stiffness. This stiffness is secant elastic stiffness. Application of experimental results of scaled specimens requires particular attention and scaling principles should be considered.

5. Comparison with test results of wall specimens In a parallel experimental research work, the cyclic behavior of three one-half scale perforated unreinforced brick shear walls

before and after strengthening using glass fiber reinforced polymers (GFRPs) was investigated by the authors. These walls were constructed using one-half scale solid clay bricks and cement mortar under the same conditions as the diagonal tension specimens. One brick wall was unreinforced and considered as a reference specimen. Two walls were directly upgraded after construction using one layer of GFRP. Each wall was retrofitted on the surface of both sides. All these walls were tested under constant gravity load and incrementally increasing in-plane loading cycles. During the test, each wall was allowed to displace in its own plane. The length, height and thickness of these walls were 194, 143, and 16 cm, respectively. Thus, the aspect ratio of the test walls was about 0.74. The test walls were constructed on a strong reinforced concrete footing. After allowing the wall to cure (for at least 7 days), a strong reinforced concrete loading beam was built on the top of the brick wall. The foundation and loading beam dimensions were 240 cm  20 cm  24 cm and 194 cm  20 cm  16 cm, respectively. These test walls had a window opening in their center. The length and height of this window were 52 and 47 cm, respectively. The unreinforced and GFRP strengthened walls are illustrated in Fig. 18. In wall RBW-H, the widths of the vertical GFRP strips were 13 and 30 cm and the widths of the horizontal GFRP strips were 10, 15 and 18 cm. In wall RBW-X, the widths of the lower diagonal GFRP strips were 13 and 20 cm and the widths of the upper diagonal GFRP strips were 13 and 30 cm. In this experimental study, a gravity load of 41.2 kN was applied along the top of the wall by a loading beam in a manner consistent with the floor or roof loading. This vertical load generated an average compression stress of 0.13 MPa and 0.18 MPa in each wall and its piers (adjacent to the window), respectively. The ratio of these stresses to the average compressive strength of the masonry material used in the test walls are 2.7% and 3.7%, respectively. For this purpose, a steel loading basket was constructed. This steel loading basket was filled with 210 lead weights and was subsequently placed on the loading beam. The loading beam distributed this vertical load uniformly on the top of the wall. Thus, this axial load acting on the wall was constant during cyclic loading as seen in the walls in real buildings under seismic loading. Horizontal cyclic load was applied manually in the plane of the wall to the loading beam (via steel plates which were connected to the loading beam during the construction) using two hydraulic jacks and hand pumps. These jacks could only produce compressive load and were mounted on rigid reaction frames. The loading beam distributed this concentrated load uniformly along the top of the wall to simulate floor or roof loads used in the actual masonry building construction. The test wall assembly was laterally supported along

A. Kalali, M.Z. Kabir / Engineering Structures 39 (2012) 24–37

35

Fig. 18. The unreinforced and GFRP strengthened test walls: (a) URBW; (b) RBW-H; (c) RBW-X.

Fig. 19. Test setup for the cyclic load test of wall RBW-H: (a) before application of the vertical load; (b) after application of the vertical load.

its top so as to restrict the out-of-plane displacement of the assembly. The test setup was similar for all of the test walls. For example, it is illustrated in Fig. 19 for specimen RBW-H. The force required to push the wall and the corresponding displacement at each load interval were measured. The observed hysteresis response curve for each tested wall specimen is shown in Fig. 20. In this figure, the term ‘drift’ is the ratio of the horizontal displacement to the wall height. Wall specimen RBW-H and panel specimen RBP-H (grid configurations) had the same reinforcement ratio in the diagonal direction. Also, wall specimen RBW-X and panel specimen RBP-X (diagonal configurations) had the same reinforcement ratio in the diagonal direction. As mentioned earlier, the GFRP strengthening increased the shear strength of the unreinforced diagonal tension panels by a factor of 3.32 and 3.80 in the case of grid and diagonal layouts, respectively (see Table 5). By means of these factors along with the shear strength of the unreinforced brick wall (specimen URBW) and Eq. (1), it is here assumed that the shear strength of the GFRP strengthened brick walls (specimens RBW-H and RBWX) can be predicted as follows:

0 V dt;GFRP fdt;GFRP ¼  V dt fdt0

sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 0 fdt0  ðfdt;GFRP þ fa Þ 0  ðfdt0 þ fa Þ fdt;GFRP

ð6Þ

where subscript ‘GFRP’ used in this equation refers to the GFRP strengthened specimens. As mentioned earlier, in the GFRP strengthened test specimens, after the maximum load was achieved, debonding of the GFRP overlays started and then developed. Thus, Eq. (6) is related to the strength limit state (i.e. before the beginning of the debonding of the GFRP). The comparison between the experimental and analytically predicted shear strength for the GFRP strengthened walls is reported in Table 6.where it can be seen that for wall RBW-H, the predicted strength and the actual strength are very similar. The error of 17.82% for prediction of the shear strength of wall RBW-X is logical. Due to the fact that the crushing of the masonry took place in panel RBP-X during the diagonal tension test. The result of this crushing was that the diagonal splitting did not occur and the maximum load which can be carried by the specimen in this failure mode was not achieved. Therefore, the diagonal tension test can be a simple and useful tool in the choice of a suitable reinforcement configuration for real applications.

36

A. Kalali, M.Z. Kabir / Engineering Structures 39 (2012) 24–37

Specimen URBW Drift (%)

Horizontal Load (tons)

-2.6

-1.95

-1.3

-30

-20

-0.65

0

0.65

1.3

1.9

2.6

-10

0

10

20

30

40

12 8 4 0 -4 -8 -12 -40

Horizontal Displacement (mm) Specimen RBW-H

Specimen RBW-X

Drift (%) -1.95

-1.3

-0.65

0

Drift (%) 0.65

1.3

1.9

-2.6

2.6

12

Horizontal Load (tons)

Horizontal Load (tons)

-2.6

8 4 0 -4 -8 -12

-1.95

-1.3

-30

-20

-0.65

0

0.65

1.3

1.9

2.6

-10

0

10

20

30

40

12 8 4 0 -4 -8 -12

-40

-30

-20

-10

0

10

20

30

40

-40

Horizontal Displacement (mm)

Horizontal Displacement (mm)

Fig. 20. Load–displacement curves at the top of the test walls.

Table 6 Comparison among the experimental and predicted values of the shear strength. Wall specimen

Experimental (average) (kN)

FEMA 356 (kN)

Error (%)

RBW-H RBW-X

82.99 108.99

78.77 89.57

5.08 17.82

6. Conclusions The in-plane shear behaviors of one-half scale brick panels with different GFRP strengthening patterns have been studied under diagonal compression loading in a static test facility. Based on the presented results the following remarks are outlined: This experimental study demonstrates the effectiveness of this new strengthening technology (GFRP reinforcement) for unreinforced brick structures. The type of test conducted (diagonal tension test) and the specimen dimensions appear to be an easy and efficient way to check different strengthening configurations. The contribution of GFRP overlays on the shear behavior of clay brick panels has been investigated; the results of the present experimental research work indicate that the symmetrical applications (double-side strengthening patterns) on brick panels offer a significant effectiveness. The shear strength of the GFRP retrofitted diagonal tension specimens was between 2.45 and 4.32 times that of the unretrofitted specimens. Also, the displacement capacity, failure energy and ductility factor of the GFRP strengthened panels were substantially increased by the external reinforcement. The diagonal configuration for GFRP is more efficient in terms of performance parameters than the grid configuration; despite the fact that the same reinforcement ratio was maintained for the two different configurations in the diagonal direction. As expected, the unreinforced diagonal tension specimens presented a brittle failure along the compressed diagonal, with cracks

that appeared suddenly in the mortar joints and in the bricks, producing instantaneous failure of the panels. The URB typical sudden failure was noticeably corrected by the GFRP strengthening, where gradual debonding of GFRP overlays provided sufficient signals of incipient crisis well before collapse. Deformations increased visibly up to failure, and the resultant global behavior was less brittle. The use of glass fiber reinforced polymers (GFRPs) for strengthening masonry structures is a promising technique but further investigations are needed in this area in order to carry out design guidelines. For GFRP strengthened brick panels, the ranges of conditions under which the currently observed modes of failure occur, need to be elucidated: simple analytic methods need to be developed for codification. At present, the authors of this article are developing these simple analytical models for GFRP strengthened brick panels in different conditions. The results of this research will be presented in a subsequent article. Acknowledgment The authors would like to express their appreciation and thanks for the technical and financial supports of Iran Building and Housing Research Center (BHRC) in this experimental research. References [1] ElGawady MA, Lestuzzi P, Badoux M. Shear strength of URM walls retrofitted using FRP. Eng Struct 2006;28(12):1658–70. [2] Hamed E, Rabinovitch O. Failure characteristics of FRP-strengthened masonry walls under out-of-plane loads. Eng Struct 2010;32(8):2134–45. [3] ElGawady MA, Lestuzzi P, Badoux M. Static cyclic response of masonry walls retrofitted with fiber-reinforced polymers. J Compos Construct, ASCE 2007;11(1):50–61. [4] ElGawady MA, Lestuzzi P, Badoux M. Analytical model for the in-plane shear behavior of URM walls retrofitted with FRP. Compos Sci Technol 2006;66(3– 4):459–74.

A. Kalali, M.Z. Kabir / Engineering Structures 39 (2012) 24–37 [5] Smith A, Redman T. A critical review of retrofitting methods for unreinforced masonry structures. In: Proceedings of EWB-UK research conference, University of Bristol; 2009. [6] Alcaino P, Santa-Maria H. Experimental response of externally retrofitted masonry walls subjected to shear loading. J Compos Construct, ASCE 2008;12(5):489–98. [7] Ascione L, Feo L, Fraternali F. Load carrying capacity of 2D FRP/strengthened masonry structures. Compos Part B: Eng 2005;36(8):619–26. [8] El-Dakhakhni WW, Hamid AA, Hakam ZHR, Hamelin M. Hazard mitigation and strengthening of unreinforced masonry walls using composites. Compos Struct 2006;73(4):458–77. [9] ElGawady MA, Lestuzzi P, Badoux M. In-Plane seismic response of URM walls upgraded with FRP. J Compos Construct, ASCE 2005;9(6):524–35. [10] ElGawady MA, Lestuzzi P, Badoux M. A seismic retrofitting of unreinforced masonry walls using FRP. Compos Part B: Eng 2005–2006; 37(2–3):148–62 [11] Marcari G, Manfredi G, Prota A, Pecce M. In-plane shear performance of masonry panels strengthened with FRP. Compos Part B: Eng 2007;38(7– 8):887–901. [12] Santa-Maria H, Alcaino P. Repair of in-plane shear damaged masonry walls with external FRP. Construct Build Mater 2011;25(3):1172–80. [13] Stratford T, Pascale G, Manfroni O, Bonfiglioli B. Shear strengthening masonry panels with sheet glass-fiber reinforced polymer. J Compos Construct, ASCE 2004;8(5):434–43. [14] Moon FL, Yi T, Leon RT, Kahn LF. Recommendations for seismic evaluation and retrofit of low-rise URM structures. J Struct Eng, ASCE 2006;132(5):663–72. [15] Moon FL, Yi T, Leon RT, Kahn LF. Testing of a full-scale unreinforced masonry building following seismic strengthening. J Struct Eng, ASCE 2007;133(9):1215–26. [16] Shrive NG. The use of fibre reinforced polymers to improve seismic resistance of masonry. Construct Build Mater 2006;20(4):269–77. [17] Cancelliere L, Imbimbo M, Sacco E. Experimental tests and numerical modeling of reinforced masonry arches. Eng Struct 2010;32(3):776–92. [18] Tao Y, Stratford TJ, Chen JF. Behaviour of a masonry arch bridge repaired using fibre-reinforced polymer composites. Eng Struct 2011;33(5):1594–606. [19] Chen WW, Yeh YK, Hwang SJ, Lu CH, Chen CC. Out-of-plane seismic behavior and CFRP retrofitting of RC frames infilled with brick walls. Eng Struct 2012;34:213–24. [20] Yi T, Moon FL, Leon RT, Kahn LF. Analyses of a two-story unreinforced masonry building. J Struct Eng, ASCE 2006;132(5):653–62. [21] Yi T, Moon FL, Leon RT, Kahn LF. Lateral load tests on a two-story unreinforced masonry building. J Struct Eng, ASCE 2006;132(5):643–52. [22] Aiello MA, Sciolti SM. Bond analysis of masonry structures strengthened with CFRP sheets. Construct Build Mater 2006;20(1–2):90–100. [23] Grande E, Milani G, Sacco E. Modelling and analysis of FRP-strengthened masonry panels. Eng Struct 2008;30(7):1842–60. [24] Kabir MZ, Farrin M. Numerical analysis of in-plane performance of unreinforced masonry walls retrofitted with fiber reinforced polymer. In: Proceedings of 11th international conference on civil, structural and environmental engineering computing, Scotland; 2007. [25] Willis CR, Yang Q, Seracino R, Griffith MC. Bond behaviour of FRP-to-clay brick masonry joints. Eng Struct 2009;31(11):2580–7. [26] American Society for Testing and Materials (ASTM). Standard test method for diagonal tension (shear) in masonry assemblages, E 519; 2001. [27] Zhuge Y. FRP-retrofitted URM walls under in-plane shear: review and assessment of available models. J Compos Construct, ASCE 2010;14(6):743–53. [28] Mahmood H, Ingham JM. Diagonal compression testing of FRP-retrofitted unreinforced clay brick masonry wallettes. J Compos Construct, ASCE 2011;15(5):810–20.

37

[29] Calderini C, Cattari S, Lagomarsino Sergio. The use of the diagonal compression test to identify the shear mechanical parameters of masonry. Construct Build Mater 2010;24(5):677–85. [30] Borri A, Castori G, Corradi M, Speranzini E. Shear behavior of unreinforced and reinforced masonry panels subjected to in situ diagonal compression tests. Construct Build Mater 2011;25(12):4403–14. [31] Gabor A, Bennani A, Jacquelin E, Lebon F. Modelling approaches of the in-plane shear behaviour of unreinforced and FRP strengthened masonry panels. Compos Struct 2006;74(3):277–88. [32] Faella C, Martinelli E, Nigro E, Paciello S. Shear capacity of masonry walls externally strengthened by a cement-based composite material: an experimental campaign. Construct Build Mater 2010;24(1):84–93. [33] Hamid AA, El-Dakhakhni WW, Hakam ZHR, Elgaaly M. Behavior of composite unreinforced masonry–fiber-reinforced polymer wall assemblages under inplane loading. J Compos Construct, ASCE 2005;9(1):73–83. [34] Luccioni B, Rougier VC. In-plane retrofitting of masonry panels with fibre reinforced composite materials. Construct Build Mater 2011;25(4):1772–88. [35] Prota A, Marcari G, Fabbrocino G, Manfredi G, Aldea C. Experimental in-plane behavior of tuff masonry strengthened with cementitious matrix–grid composites. J Compos Construct, ASCE 2006;10(3):223–33. [36] Turco V, Secondin S, Morbin A, Valluzzi MR, Modena C. Flexural and shear strengthening of un-reinforced masonry with FRP bars. Compos Sci Technol 2006;66(2):289–96. [37] Valluzzi MR, Tinazzi D, Modena C. Shear behavior of masonry panels strengthened by FRP laminates. Construct Build Mater 2002;16(7):409–16. [38] FEMA 356. Prestandard for the seismic rehabilitation of buildings. Washington: Federal Emergency Management Agency; 2000. [39] Prota A, Manfredi G, Nardone F. Assessment of design formulas for in-plane FRP strengthening of masonry walls. J Compos Construct, ASCE 2008;12(6):643–9. [40] Mosallam A, Banerjee A. Enhancement in in-plane shear capacity of unreinforced masonry (URM) walls strengthened with fiber reinforced polymer composites. Compos Part B: Eng 2011;42(6):1657–70. [41] Tasnimi AA. Behavior of brick walls recommended by Iranian code of practice for seismic resistant design of buildings. Tehran, Iran: Building and Housing Research Center; 2004. [42] Tasnimi AA. Behavior of confined and non-confined brick buildings. Tehran, Iran: Natural Disaster Research Institute; 2005. [43] American Society for Testing and Materials (ASTM). Standard test method for tensile properties of polymer matrix composite materials, D 3039; 2000. [44] Standard No. 2800. Iranian code of practice for seismic resistant design of buildings, 3rd ed. Tehran, Iran: Building and Housing Research Center; 2007. [45] American Society for Testing and Materials (ASTM). Standard test method for compressive strength of hydraulic cement mortars, C 109/C 109M; 1999. [46] American Society for Testing and Materials (ASTM). Standard test method for bond strength of mortar to masonry units, C 952; 1997. [47] American Society for Testing and Materials (ASTM). Standard test methods for sampling and testing brick and structural clay tile, C 67; 2000. [48] American Society for Testing and Materials (ASTM). Standard test method for compressive strength of masonry prisms, C 1314; 2000. [49] American Society for Testing and Materials (ASTM). Standard test method for static modulus of elasticity and Poisson’s ratio of concrete in compression, C 469; 1994. [50] Kalali A, Kabir MZ. Modeling of unreinforced brick walls under in-plane shear & compression loading. Struct Eng Mech 2010;36(3):247–78. [51] American Society for Testing and Materials (ASTM). Standard test methods for cyclic (reversed) load test for shear resistance of walls for building, E 2126; 2005.