Experimental Evaluation of the Seismic Performance of Retrofitted Masonry Walls

Journal Pre-proofs Experimental Evaluation of the Seismic Performance of Retrofitted Masonry Walls Bahador Bagheri, Jung-Han Lee, Han-Gil Kim, Sang-Hoon Oh PII: DOI: Reference:

S0263-8223(19)33814-0 https://doi.org/10.1016/j.compstruct.2020.111997 COST 111997

To appear in:

Composite Structures

Received Date: Revised Date: Accepted Date:

8 October 2019 18 January 2020 27 January 2020

Please cite this article as: Bagheri, B., Lee, J-H., Kim, H-G., Oh, S-H., Experimental Evaluation of the Seismic Performance of Retrofitted Masonry Walls, Composite Structures (2020), doi: https://doi.org/10.1016/j.compstruct. 2020.111997

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Experimental Evaluation of the Seismic Performance of Retrofitted Masonry Walls Bahador Bagheri1, Jung-Han Lee2, Han-Gil Kim3, Sang-Hoon Oh4*

1 Ph.D., Senior Researcher, Department of Architectural Engineering, Pusan National University, Korea, (E-mail: [email protected]) 2 Ph.D., Senior Researcher, Earthquake Hazards Reduction Center, National Disaster Management Research Institute, Korea, (E-mail: [email protected]) 3 Graduate Student, Department of Architectural Engineering, Pusan National University, Korea, (E-mail: [email protected]) 4*(Corresponding

author) Professor, Department of Architectural Engineering, Pusan National University, Korea, (E-mail: [email protected])

Abstract Unreinforced masonry (URM) buildings are known as seismically vulnerable systems and require retrofitting. In this study, the in-plane seismic behavior of URM walls with and without an opening before and after retrofitting using different developed methods was investigated. Retrofitting methods involving a combination of metal laths, steel plates, connecting steel plates (CSPs) and polymer coating material were considered. Twelve masonry walls, including three non-retrofitted (as reference specimens) and nine retrofitted walls, were tested by subjecting the specimens to cyclic loads. The experimental study was carried out on full-scale specimens, which were tested simultaneously under the conditions of gravity and in-plane cyclic loads. The performance evaluation of each specimen was performed in terms of the lateral strength and deformation, hysteretic response, and energy dissipation. The double side retrofitting/upgrading approach substantially enhanced the lateral strength, displacement, and energy dissipation capacity of the test specimens. Furthermore, it was found that the specimens involving the combination of the polymer coating with the steel plates and CSPs could be an alternative method in terms of high energy dissipation capacity owing to the ultimate strength and displacement enhancement. Keywords: Masonry wall; Cyclic load; Steel plate Retrofitting; Polymer coating material. 1. Introduction Structures with unreinforced masonry (URM) walls can be found worldwide; however, these walls are extremely vulnerable to earthquake loads. In recent earthquakes—e.g., in the 1999 Kocaeli, 2003 Boumerdes, 2008 Sichuan, and 2016 Pohang earthquakes—it has been noted that the structures with URM walls are likely to fail when 1

subjected to earthquake loads. The vulnerability of these walls can be attributed to the weakness of the mortar in tension and sliding [1]. However, as there are an innumerable number of these buildings, the social and economic implications render the demolition and renovation of these buildings impractical. The common causes of seismic inadequacy are structural degradation, insufficient design loads, structural damage sustained in a previous earthquake, flawed design or construction, and changes in the building occupancy [2].In general, the four main failure modes for an in-plane URM wall, as reported in various codes, are those based on rocking, bed-joint sliding, toe crushing and diagonal tension. In ASCE41-06 [3], ASCE41-13 [4], and FEMA-356 [5], equations are proposed to determine the lateral strength of URM walls due to the activation of any of the probable failure modes, taking into account the geometric dimensions of the wall, boundary conditions, and specifications of the used materials. In the ASCE 41-06 [3], the lateral strength of the URM wall is defined only for the bed-joint sliding, rocking, and toe crushing failure modes. The bed-joint sliding and toe crushing failure modes correspond to the force-controlled actions, and the rocking failure mode corresponds to the deformationcontrolled actions. Therefore, in determining the lateral strength of a URM wall under the rocking failure mode, the expected properties of the materials are considered, whereas in the bed-joint sliding and toe crushing failure modes, the lower bound of the material properties is considered. FEMA-356 [5] and ASCE41-13 [4] define the methods of determination of the lateral strength of URM walls for all four abovementioned failure modes. In this case, the failure modes of the diagonal tension and toe crushing are presumed to be forced-controlled, and the rocking and bed-joint sliding failure modes are controlled by deformation. It should be noted that the axial failure mode of a wall under gravity load corresponds to the force-controlled actions in all the standard codes. However, not all the above failure modes involve a collapse of the wall, and the final failure may be a combination of several failure modes. Therefore, a rationally developed failure criterion should be able to predict the tensile, compressive, and shear types of failure [6]. The seismic retrofitting techniques for masonry structures can be classified as reduction in the earthquake forces or upgradation of the existing building to resist the earthquake load by changing the structural system or upgrading the element strength [7]. Different retrofit techniques have been suggested to improve the performance of URM infilled structures. A single- or double-sided shotcrete jacket reinforced with a welded wire or steel mesh may be used [8]; the disadvantages of this method are the addition of a substantial weight to the structure and the negative influence on the aesthetics of the structure. Some other techniques include confinement/jacketing techniques [9, 10], the addition of additional thin surface treatments (plaster with wire mesh and cement mortar, e.g., shotcrete, coatings, grouted steel bars) [11], repointing [12, 13], internal reinforcement [5, 12, 14, 15, 16], tying [17], grout injection [18], filling in the door or window openings [5], seismic retrofitting of enlarged openings using FRPs [19, 20, 22], spraying of FRPs [22], use of steel strips [23], use of shape memory alloys [24], use of epoxy-bonded fiber-reinforced polymer (FRP) laminates [25], and the use of ferrocement and plaster overlays [26]. FRP laminates are highly effective in enhancing the strength of the masonry infill, which can be desirable for certain structures (e.g., steel frames) but undesirable for other structures (e.g., non-ductile reinforced concrete frames). Mander et al. [26] performed a cyclic load test on thick ferrocement overlays with two layers of wire reinforcement 2

and demonstrated the rapid strength deterioration of the steel bounding frame. The utilization of FRPs has been one of the common and practical reinforcement strategies [27, 28]. However, as witnessed in full-scale blast tests, most FRP retrofits have some drawbacks, including premature failure due to debonding and delamination [29]. Some studies were conducted using analytical and numerical techniques to investigate the effect of an explosion on the use of polymer coatings for strengthening the reinforced concrete and masonry structures. Researchers investigated the use of polymer coatings for smaller blast loads and for use in non-load-bearing elements as an alternative to FRP retrofits. [30, 31, 32, 33, 34, 35, 36, 37, 38, 39]. However, the experimental reports of masonry walls retrofitted by a polymer coating under cyclic loading are scarce in the literature. To this end, the purpose of this research was to develop new seismic retrofitting techniques for URM walls under an in-plane static cyclic load and to evaluate their performance. The new retrofit techniques proposed by the authors are specifically for unreinforced masonry walls with and without an opening, and these techniques include various retrofit methods such as those involving the use of metal laths, steel plates with and without a CSP, polymer coatings material, and the combination of these elements. The aim is to improve the seismic performance of such systems by enhancing the ductility and lateral strength as well as delaying the strength deterioration by developing a practical and cost-effective retrofit method. In this paper, first, the experimental observations of the failure modes are described, and the performance evaluation of each specimen is reported in terms of the experimental evaluation of the lateral strength and deformation, hysteretic response, and energy dissipation. The research objectives were achieved through a series of full-scale experimental tests, in which 12 full-scale walls tests involving non-retrofitted and different retrofitted methods were performed to validate the proposed retrofit technique. 2. Experimental program and loading system This experimental in-plane static cyclic program investigates the efficacy of using metal laths, steel plates, and spray coating material as externally bonded retrofitting methods in the URM walls. Twelve full-scale single URM walls including non-retrofitted and retrofitted specimens were tested in the test facility at the Seismic Research and Simulation Test Center (SESTEC) of Pusan National University (PNU). Among these specimens, three walls were tested as the reference specimens (non-retrofitted) to compare the results with those of the retrofitted walls. The experimental program was divided into two stages, and the first and second stages included nine [40] and three specimen tests, respectively. All the specimens were fabricated at the project site. Table 1 summarizes the details of the specimens used in this experimental program. A horizontal force was applied to the steel head beam for all the specimens, which in turn distributed to the wall uniformly. To attach the specimen to the steel head beam at the upper part and to the base girder at the lower part, boundary-steel plates with high strength bolts were installed and fixed into the specimen (Fig. 1(a)). Four stoppers were installed at the top and bottom of the specimens to prevent the slipping of the specimen. The axial force was applied to the specimen by installing an additional mass of 10 t on the upper part of the specimen. The typical cyclic loading sequence adopted for the tests, namely, displacement-controlled cycles using a 1000 kN actuator, is shown in Fig. 1(b) as per ASTM C1717-19 [41] suggested. The force rate was 0.05 mm/s at the starting point, 3

and it was increased gradually at the end of each stage of the displacement. Each stage of loading consisted of three cycles until the selected displacement. The horizontal displacement was measured using an LVDT with a size appropriate for the expected horizontal displacement.

Table 1. Specimens’ descriptions Retrofitting method

Metal laths, steel plates and CSP

No.

Specimen

Description

1

NR-S

Non-retrofitted wall

2

NR-W

Non-retrofitted wall with opening window (Opening size: 1.5×1.2 m)

3

NR-D

Non-retrofitted wall with opening door (Opening size: 0.9×2.1 m)

4

MS-S

Wall retrofitted using metal laths + steel plates with CSP

5

MS-SF

Wall retrofitted using metal laths + steel plates + CSP in fixed condition

6

S-W

Wall retrofitted using steel plates + CSP with opening window

7

MS-W

Wall retrofitted using metal laths + steel plates + CSP with opening window

8

S-DF

Wall retrofitted using only steel plates + CSP with opening door in fixed condition

9

MS-DF

Wall retrofitted using metal laths + steel plates + CSP with opening door in fixed condition

Polymer coating material, steel plates and CSP

10

M-R-SU

Wall retrofitted using steel plates without CSP + polymer coating material

11

MS-SF2

Wall retrofitted using metal laths + steel plates + CSP in fixed condition

12

M-R-SUC

Wall retrofitted using steel plates using CSP + polymer coating material in fixed condition

(a)

4

(b) Fig. 1. (a) Typical test setup (b) Loading history 3. Retrofit methods involving metal laths, steel plates, and CSPs 3.1 Description of the test specimens The retrofitting method of the URM wall using metal laths, steel plates, and CSPs is shown in Fig. 2. The approach is intended to improve the strength and ductility of URM walls and to prevent the falling of the wall components in the out-of-plane direction. The metal lath size is a factory standard width of 0.9 m. The metal lath is placed horizontally and the extension of metal lath is basically overlapped. The steel plates fix the metal lath to the masonry wall and enhances the strength of the masonry wall. Such plates are intended to improve the strength and ductility, and to maintain the whole integrity of the masonry wall during the horizontal deformation. Particularly, in the walls with the small aspect ratio, preventing the slip failure occurrence. In addition, for a wall with openings, the steel plates shown in Fig. 2 can be installed around the openings of the masonry wall to control the generation of shear cracks around the openings. The details of the metal lath, steel plates, and connecting steel plates are shown in Fig. 2(a). Three specimens NR-S (solid wall), NR-W (window opening) and NR-D (door opening) were considered as the reference specimens, and their schematic along with retrofitted methods is as shown in Fig. 3. To perform the retrofitting of the solid URM wall, metal laths with steel plates and CSPs were employed. As given in Table 1, MS-S indicates the retrofitting method using metal laths and steel plates, and MS-SF indicates the same condition with added anchor bolts at the top and bottom of the wall to ensure that the boundary condition is better fixed. Fig. 2(c) reveals more details about the installment of fixed conditions to the top and bottom of the masonry wall. According to the present condition of the existing masonry buildings, the walls are considered to be 4.0 m wide and 2.67 m high (aspect ratio 0.67); the wall thickness is 0.19 m, and the shape and position of the openings are those employed generally. The size of the door is 0.9 m × 2.1 m, and the size of the window was 1.5 m × 1.2 m. For the S-DF and MS-DF specimens (Walls with door opening), the fixed condition is only considered at the 5

bottom of the walls. It was due to the cracks observation made during the NR-D specimen test which mainly concentrated at the bottom. To prevent the out-of-plane displacement that may occur during the loading, ball jigs were installed at the upper part of the specimens. The specimens fabrication are as follow: First, the masonry walls that are required to be retrofitted are arranged, and the metal lath is fixed to the both sides of the wall using concrete nails at intervals of approximately 40 cm. At this time, the metal lath is fixed to the steel plate connecting portion by using an epoxy resin. Next, an anchor hole for fixing the steel plate to the masonry wall is drilled in the wall, and then the steel plate is attached to the wall using the anchor bolt (Fig. 2(b)). In this method, the metal lath and steel plate are attached to the wall by using a concrete nail or bolt after performing a simple surface treatment on the surface of the masonry wall to be retrofitted. Finally, both sides of the wall are plastered. In order to cover the anchor bolts of fixed condition at the top and bottom of the wall, and to maintain the whole integrity of the wall after retrofitted methods, the surface plastering were decided to have the thickness as shown in Fig. 2(c). The plaster type for this experiment is a cement mortar type. In the Korean Standard Association (KS) F 2262 [42], the cement mortar for coating the wall should meet two criteria, crack preventability (not deformability) and water permeability. This means that standard code recognizes that the plaster has no contribution to the strength/deformability of the wall and the effect of plaster is negligible.

(a) Composition of retrofitting elements

(b) Fixing the metal lath and steel plates to URM wall

(c) Details view 6

Fig. 2. Details of retrofitting method using metal lath and steel plates

[NR-S]

[NR-W]

[NR-D]

(a) Reference URM specimens (mm)

[MS-S, MS-SF]

[MS-W, S-W*]

[MS-DF, S-DF*]

(b) Retrofitted URM specimens (* denotes the URM specimens retrofitted using only steel plates) Fig. 3. Schematics of test specimens All the bricks used in the test specimens were concrete type and 190 mm × 90 mm × 57 mm in size as shown in Fig. 4. According to the Korean Standard Association (KS) F 4004 [43], the compressive strength of concrete bricks should be more than 8 MPa. As given in Table 2, three brick samples are tested by the manufacturer and verified that their bricks are satisfying the standard. The thickness of mortar is 10 mm according to the Korean Building Code [44]. According to this, the weight ratio of cement and sand should be 1:3. Due to most workers use a ratio of 1:5 on-site, the ratio of 1:5 was used. The amount of water was adjusted on-site for constructability. As given in Table 3, to get the compressive strength of mortar, 3 mortar samples were tested. The average of compressive strength of mortar test is 4.8 MPa.

7

Fig. 4. Details of bricks and mortar Table 2. Mechanical properties of bricks Test

Size (mm)

No 1 2

190 × 90 × 57

3

Compressive

Absorption

strength (MPa)

rate %

13

8

12

9

12

9

Table 3. Mechanical properties of mortar Test No

Size (mm)

1 2

Compressive

Average

strength (MPa)

(MPa)

4.88 50 × 50 × 50

3.96

3

4.8

5.56

3.2 Experimental results Figs. 5 to 8 and Table 4 summarize the test results for the reference and retrofitted specimens qualitatively and quantitatively. 3.2.1 Reference specimens Figs. 5 and 6 respectively show the final stage of fracture and the load–displacement curve for the reference specimens. The primary findings are as follows: Specimen NR-S: Flexural cracks occurred in the lower part of the specimen at the initial stages of the loading when the load and drift ratio were approximately 70 kN and 0.27%, respectively. Diagonal/sliding cracks were generated at the lower part of the northern side of the wall. These cracks propagated progressively with the increase in loading until a critical bed-joint crack occurred at the southern side when the load and drift ratio 8

reached values of 71.96 kN and 0.5%, respectively. The hysteretic behavior (Fig. 6(a)) indicates the occurrence of the slip phenomenon at the lower part of the curve owing to the bed joint crack generation at one side of the wall, which eventually caused a rocking failure along with sliding at the ultimate state. The maximum drift ratio was 0.89% at the ultimate displacement. Specimen NR-W: The first major diagonal/sliding cracks in the wall were observed at the four corners of the opening when the drift ratio was 0.19% at the step 1. Flexural cracks occurred at the lower middle of the opening at the first cycle of step 2. The nonlinear behavior was initiated by the shear crack generation at the corner of the window. At step 4, the cracks intensified owing to the occurrence of the bed-joint crack at the southern side and started to propagate along the wall. Therefore, the total failure of the wall was dominated by the sliding shear along the bed-joints. The effect of the bed-joint cracks on the hysteretic behavior of the wall is shown in Fig. 6(b), which indicates that these cracks caused sliding in the unloading stage. Finally, the wall failed owing to the rocking, sliding and shear failure modes. The maximum drift ratio was 1.26% at the ultimate displacement. Specimen NR-D: Shear cracks appeared in the upper edge of the opening, and the left and right segments of the wall opening behaved differently owing to the flexural cracks generated at the top of the wall at a drift ratio of 0.13%. As loading progressed, the specimen started to slide, and cracks formed in the toes at the northern side. The propagation of the cracks led to a combination of rocking and sliding failure modes along with the shear. Spalling and crushing of the wall surface occurred at a drift ratio of 0.5%. The load–displacement hysteretic response of the specimen is shown in Fig. 6(c). The asymmetry in the load–displacement diagram of the specimen can be attributed to the occurrence of the rocking failure mode along with the sliding. The maximum drift ratio was 0.88% at the ultimate displacement.

(a) NR-S

(b) NR-W

9

(c) NR-D Fig. 5. Failure patterns at test completion (reference specimens)

(a) NR-S

(b) NR-W

(c) NR-D

Fig. 6. Load–displacement curve 3.2.2 Retrofitted specimens Figs 7 and 8 respectively show the failure patterns of the specimens at test completion and the effect of the seismic retrofitting methods on the hysteretic behavior of masonry walls with or without openings compared with the results of the reference specimens described in the previous section. The test results of each specimen as well as the degree of influence of each method on the seismic retrofitting of the URM wall are described. Specimen MS-S: The first crack was a flexural crack that propagated horizontally and vertically at the bed-joint level (Fig. 7(a)) at a lateral load of 65 kN and drift ratio of 0.25% (Fig. 8(a)). Cycling continued as the specimen started to rock until it reached a lateral strength of 112.74 kN and drift ratio of 0.59%. After reaching the ultimate lateral strength, the masonry wall slid along its base, and this sliding was accompanied by limited rocking. In the subsequent test runs, the rocking led to the failure of the masonry wall at the toes. At this stage, buckling of the steel plate at the bottom side of the wall, owing to compressive stress, was observed. The test was stopped at a drift ratio of 1.53% as the ultimate stage. The specimen finally failed owing to the rocking failure mode along 10

with sliding. The hysteretic behavior of the retrofitted MS-S specimen compared with that of the reference specimen (NR-S) is shown in Fig. 8(a). The maximum strength values are 112.74 and 109.27 kN in the positive and negative directions, respectively. The maximum displacement values in accordance with the maximum load are 15.83 mm and -12.25 mm in the positive and negative directions, respectively. The specimen undergoes buckling at the steel plates in the bottom side of the wall and remains at a constant load for further increase in the displacement. Specimen MS-SF: Cracks were initiated and generated uniformly in the wall at a drift ratio of 0.25% and lateral load of 60 kN. Wall failure finally occurred owing to the rocking failure without slipping at the bottom of the wall (Fig. 7(b)). The hysteretic behavior of the retrofitted MS-SF specimen compared with that of the reference specimen (NR-S) is shown in Fig. 8(b). Owing to the influence of the added connection plates to realize the fixed condition, the load–displacement curve exhibited a stable shape with a larger energy absorption capacity in comparison with the reference specimen (NR-S) until the end of the experiment. The maximum strength reached 166.17 kN and 202.8 kN in the positive and negative directions, respectively. The displacement at the maximum load was 37.43 mm and 30.51 mm corresponding to drift ratios of 1.4 and 1.14 in the positive and negative directions, respectively. The experiment was terminated owing to the fracture of the test specimen in the negative loading direction. Specimen S-W: The specimen with the window opening was retrofitted by the steel plates using a CSP (Fig. 7(c)). Corner cracking gradually occurred at the opening due to the steel plate reinforcement, and at the end, a constant load was attained because of the sliding. Finally, the specimen slipped and fractured owing to the rocking failure mode. The hysteretic behavior of the retrofitted S-W specimen compared with that of the reference specimen (NRW) is shown in Fig 8(c). The hysteretic behavior is similar to that of the MS-S specimen, and the load remained constant owing to the buckling of the steel plates at the bottom side of the wall and the sliding. Specimen MS-W: This specimen was retrofitted using a uniformly distributed metal lath over the entire wall and a steel plate around the opening and edges. Crack initiation occurred at the edge of the opening, and the cracks occurred uniformly on the specimen. Finally, similar load–displacement curve shapes with S_W were obtained. The hysteretic behavior of the retrofitted MS-W specimen compared with that of the reference specimen (NR-W) is shown in Fig. 8(d). In comparison with the hysteresis behavior of the S-W specimen, the change in the stiffness during the cycle was small owing to the effect of the metal lath, and the overall behavior was almost similar to that of the S-W specimen during the final stage. Specimen S-DF: This specimen was retrofitted using a steel plate around the opening and edges of the wall in the fixed condition. The cracks initiated at the lower part of the specimen at a drift ratio of 0.31% and propagated through the wall during the subsequent steps. This specimen finally failed by the rocking failure of the wall due to cracking in the lower part of the specimen. The hysteretic behavior of the retrofitted S-DF specimen compared with that of the reference specimen (NR-D) is shown in Fig. 8(e). Although the initial stiffness was lower, the displacement at the maximum strength was 34.07 mm (drift ratio 1.27%) and 47.64 mm (drift ratio 1.78%) in the positive and negative directions, respectively, which is larger than that corresponding to the reference specimen. 11

In addition, a large difference was noted in the maximum strengths corresponding to the positive and negative forces. This phenomenon occurred because the force was not transmitted smoothly and uniformly due to the generation of premature cracks on the upper part of the opening. The testing was stopped due to buckling of the steel plate at the compression side and crushing of the wall at the maximum displacement in the negative direction. Specimen MS-DF: This specimen was retrofitted using a steel plate around the openings and edges and uniformly distributed metal lath over the entire wall in the fixed condition. Flexural cracks were generated first on the edge of the openings of the specimen at the drift ratio of 0.28% and the cracks were uniformly distributed on the entire surface of the test specimen. The test specimen exhibited a relatively stable load–displacement curve shape compared to that of other specimens in the unfixed condition. During the final action, compressive buckling of the steel plate occurred due to the negative force, and it was then destroyed. The hysteretic behavior of the specimen compared with the reference specimen (NR-D) is shown in Fig. 8(f). A satisfactory performance in the hysteretic behavior is obtained owing to the presence of the steel plates and metal lath in the fixed condition. The difference in the maximum strength values for the positive and negative directional forces is caused by the position of the opening.

(a) MS-S

(b) MS-SF

12

(c) S-W

(d) MS-W

(e) S-DF

(f) MS-DF

Fig. 7. Failure pattern at test completion (Retrofitted specimens)

(a) MS-S

(b) MS-SF

13

(c) S-W

(d) MS-W

(e) S-DF

(f) MS-DF Fig. 8. Load–displacement curves

4. Method involving polymer coating material, steel plates, and CSPs In the method involving the polymer coating material, a polymer coating material having a super tensile performance is applied to the surface of non-structural elements such as a masonry wall to increase the resistance against a lateral force. The polymer coating material has an adhesion strength of 3 MPa and tensile strength of 37 MPa, and it can be sprayed in the form of a paint in various colors. Consequently, the application of this retrofitting method is considerably simple and convenient (Fig. 9). The coating was sprayed in layers until the desired nominal thickness was estimated to have been achieved. The reinforcement effect can be controlled according to the thickness of the spraying of the polymer material, and this method can be used to retrofit the masonry wall in the existing buildings. 14

(a) Application on an existing wall

(b) Spraying of the polymeric coating material

Fig. 9 Polymer coating material 4.1 Test specimens Fig. 10 shows the general test setup for two specimens. M-R-SU is a specimen that was retrofitted with the polymer coating material and steel plates, as shown in Fig. 10(a). After the construction of the URM wall, the steel plates were attached on both sides in a similar manner as for previous specimens. Subsequently, both sides of the wall with the attached steel plates were sprayed with the coating with a thickness of 2–2.5 mm. The construction and setup for the M-R-SUC was similar to that of the M-R-SU except that before spraying the wall, the steel plates were connected using a connecting steel plate (CSP), as shown in Fig. 10(b). Figure 10(c) shows the detail of specimen in terms of polymer coating. The polymer coating thickness however exaggerated in this figure for better understanding. The specimen MS-SF2 was retrofitted in a similar manner as specimen MS-SF; a similar retrofit method was employed to minimize the construction and installment error of the specimen to prevent the occurrence of the dominant rocking failure at an early stage of loading, which occurred in the case of the MS-SF specimen. To this end, the steel plates and anchor bolt at the bottom of the specimen that enable the realization of a fixed condition were carefully installed.

a) M-R-SU

b) M-R-SUC 15

(c) Details view Fig. 10. Schematics of test specimens 4.2 Experimental Results Figs. 11 and 12 respectively show the failure patterns of the specimens at the test completion and the effect of the seismic retrofitting methods on the hysteretic behavior of the masonry walls compared with the results of the reference specimen described in the previous section. The red and blue lines in Fig. 11 represent the propagated cracks on the wall and the polymer stretching or tearing, respectively. The test results of each specimen as well as the degree of influence of each method on the seismic retrofitting of a URM wall is described. Specimen M-R-SU: For the specimen retrofitted using the polymer material along with a steel plate in the fixed condition, stretching of the polymer at the corner of both sides occurred during loading and unloading in the first cycle of step 3 when the displacement reached 12 mm (drift ratio 0.41%) and the load was 117.35 kN and 95 kN in the positive and negative directions, respectively. As the loading proceeded to the second cycle, the strength deteriorated by approximately 18 % in the positive direction. In step 4, the specimen started to lift at the first cycle when the actuator displacement reached approximately 16.2 mm (drift ratio 0.56%) and the load was 107.76 kN. No crack or obvious shear crack was observed on the surface and at the corners. The lifting displacement, as measured by the vertical LVDT, was approximately 1 mm. In step 6, when the displacement reached 24.23 mm (drift ratio 0.84%) and the load was 119 kN and 122 kN in the positive and negative sides of the force-deformation curve, respectively, polymer stretching at the edge of some steel plates was observed, which finally resulted in tearing. The similar observation was noted during steps 7 to 10. In step 10, the corner lifting was approximately 3 mm. Finally, the test was terminated at step 11 after two cycles because of the sudden out-of-plane rotation of the mass. The displacement reached 44.9 mm (drift ratio 1.55%), and the load was 113.9 kN and 123.6 kN in the positive and negative directions, respectively, at the first cycle. Moreover, the steel plates were not integrated, and the cracks underneath the polymer grew larger. The cracks were investigated after the polymer tore in this step. The corner lifting was almost 3.72 mm. The force–displacement relation of this specimen is shown in the form of the force–drift ratio to account for the height difference with the reference specimen and allow a reasonable comparison (Fig. 12(a)). 16

Specimen M-R-SUC: The specimen retrofitted using the polymer along with the steel plate and CSP in the fixed condition started to lift up in the first cycle of step 2 when the actuator displacement reached approximately 7.45 mm (drift ratio 0.27%), and the load was 100 kN. No crack or obvious shear crack was noted on the surface and at the corners until step 3. The loading continued to step 4; when the displacement reached 15 mm (drift ratio 0.56) and the load was 137 kN, the specimen lifted at the corners during both negative and positive loading. The lifting measured using the vertical LVDT was approximately 3.2 mm. In step 5, when the displacement reached 18 mm (drift ratio 0.67%) and the load was 149 kN and 122.6 kN in the positive and negative sides of the force– deformation curve, respectively, slight peeling and protruding of the mortar along with polymer tearing occurred at the top northern side where the actuator head was connected. Furthermore, a small displacement of the steel plates at their connection part along with the stretching of polymer were observed. Consequently, strength deterioration occurred in the upcoming cycles. At the second cycle of this step, the strength deteriorated by approximately 20% and 10% for the positive and negative sides, respectively. The corner lifting was approximately 3.3 mm. In the first cycle of step 6, in which the displacement reached 22.85 mm (drift ratio 0.85%) and the load was 118.4 kN, 45°-inclined cracks were initiated at the lower part of the southern side owing to the polymer stretching. However, a clear investigation of the crack propagation on the wall was not possible owing to the surface being covered by the polymer. Moreover, during the second and third cycles, some bricks that were already peeled at the previous step were broken. The corner lifting was almost 3.85 mm. In the first cycle of step 7, in which the displacement reached 26.11 mm (drift ratio 0.97%) and the load was 96.41 kN, more cracks appeared on the wall on the lower part of both sides. Among these cracks, a main shear diagonal crack extending from the previous step appeared. At the same time, with the continuation of the cycles, the polymer became more stretched, and the tears tended to occur partially. In this step, the corner lifting was approximately 5.83 mm. In step 8, in the first cycle, with a displacement of 32 mm (drift ratio 1.19%), the steel plates at the left and right edges of the wall were partially buckled and deformed. Simultaneously, the cracks on the wall surface continued to extend, and the corner lifting was the same as in the previous step. Finally, the test was terminated in step 10 after two cycles because some bricks at the top of the wall failed, resulting in a sudden out-of-plane rotation of the mass. The displacement reached 38.13 mm (drift ratio 1.43%), and the load was 129 kN at the first cycle. The corner lifting was almost 8.5 mm. The test specimen was severely deformed at the upper side, and the test was terminated. After the end of the test, for further investigating the crack pattern, the steel plates located at the lower one-third part, where the cracks propagated, were removed. It was found that the horizontal sliding shear cracks underneath the steel plate propagated, and the inclined observed cracks were connected. Therefore, the final state was of diagonal sliding shear along with the rocking failure. Specimen MS-SF2: For the specimen retrofitted using a metal lath with a steel plate connection in the fixed condition, small cracks appeared at the bottom of the northern side in step 1 when the actuator displacement reached approximately 3.82 mm (drift ratio 0.14%) and the load was 92.25 kN. In step 2, in the first cycle in the positive direction, when the displacement reached 7.54 mm (drift ratio 0.28%) and the load was 126.6 kN, the cracks from the previous step extended and more small cracks were observed as well. As unloading continued, 17

small horizontal and vertical cracks as well as a horizontal crack at the bottom of the loading direction face appeared at the bottom of the southern side. The loading continued to step 3, and when the displacement reached 11.36 mm (drift ratio 0.42%) and the load was 131 kN, the cracks extended and became wider. As the cycles continued, more cracks were initiated at the corners of the wall. Furthermore, the specimen was lifted at the corners during both the negative and positive directions of loading. The lifting measured by the vertical LVDT was approximately 4 mm at the end of this step. In step 4, when the displacement reached 15.26 mm (drift ratio 0.57%) and the load was 138.8 kN, the wall covers at both the top corners were crushed, and the cracks widened. The corner lifting was approximately 6.8 mm. In step 5, when the displacement reached approximately 19 mm (drift ratio 0.71%) and the load was 140.5 kN, the wall cover was crushed and fell off in pieces at the top northern side where the actuator head was connected. Moreover, new small cracks were observed at the four corners of the wall. The corner lifting was approximately 8.7 mm. In the first cycle of step 6 during positive loading, in which the displacement reached 22.85 mm (drift ratio 0.85%) and the load was 140.9 kN, some bricks at the top corner of the northern side were broken and fell off in pieces. As the unloading proceeded, a horizontal crack was observed in the region where the steel plates are located in the lower middle part of the wall. Moreover, during the second and third cycles, some more cracks appeared at the corners. The corner lifting was almost 8.8 mm. In step 7, when the displacement and load reached 26.6 mm (drift ratio 0.99%) and 144 kN, respectively, more cracks appeared on the wall at the lower of both sides. The unloading cycles led to the extension and widening of the shear sliding crack that had appeared in the previous step. The crack width was approximately 10 mm. In this step, the corner lifting was approximately 10 mm. Finally, the test was terminated in the first cycle of step 8 because some bricks at the top of wall failed, which resulted in the sudden out-of-plane rotation of the mass. The displacement reached 30.45 mm (drift ratio 1.16%), and the load was 140.14 kN. The corner lifting was almost 12 mm. The test specimen was severely deformed at the upper side, and the test was terminated. After the end of the test, to further investigate the crack pattern, the wall cover along with the steel plates located in the lower onethird part, where the cracks propagated, were removed. It was found that the horizontal sliding shear cracks underneath the steel plate propagated. Therefore, the final state was the sliding shear along with the rocking failure.

18

(a) M-R-SU

(b) M-R-SUC

(c) MS-SF2 Fig. 11. Failure patterns at test completion (Retrofitted specimens)

(a) M-R-SU

(b) M-R-SUC

19

(c) MS-SF2 Fig. 12. Load–displacement curves 5 Interpretation of test results 5.1 Response parameters The in-plane wall response parameters were calculated from the force–displacement curves presented in Fig. 12. According to Fig. 13, the backbone skeleton curve of each test was bi-linearized, and the values were drawn and listed in Table 4. The bi-linearized yield force Fy, elastic stiffness Kel and the ductility ratio μ were determined using Eqs. (1) to (3), respectively: Fy=0.9Fu

(1)

Kel=Fy /δy

(2)

μ= δu / δy

(3)

Because of the early failure of most specimens, the horizontal displacement was insufficient to obtain a 20% force reduction in the post-peak phase except for the specimens NR-W in the negative direction, MS-S in both directions, S-W in the negative direction, S-DF in the positive direction and MS-DF in the positive direction. Therefore, to calculate the ductility ratio, either the maximum displacement (δmax) or the displacement at 20% of the force reduction (δu,20%) was considered according to the specimen characteristics. In Fig. 14, the effect of each retrofitting method on the deformation capacity and ultimate lateral strength is shown. The retrofitting and upgrading improve the strength and energy dissipation capacity. However, the retrofitting methods did not increase the initial stiffness of the specimens, and in most cases, the initial stiffness of the retrofitted specimens was less than that of the reference specimens, except in the case of the MS-SF2 specimen that exhibited a slight increase in the initial stiffness. Among the solid wall specimens, the M-R-SUC and MS-SF2 specimens exhibited a considerable enhancement in terms of the strength and ductility in comparison with other specimens. However, although the MS-SF specimen could resist a higher load, the stiffness was lower, which led to dissipate the energy at a higher drift ratio. 20

As shown in Table 2, the maximum strength of the retrofitted specimens increased more compared to the corresponding increase for the reference specimens. However, the increased strength value varied according to the type of the retrofit. The average values of the positive and negative sides were considered for the comparison, and the key findings are as follows: The MS-S specimen exhibited an increase of 138.3% in the lateral strength of the wall. However, the strength of the MS-SF specimen was significantly increased by approximately 235% owing to the fixed condition. Furthermore, the fixed condition of the wall increased the lateral strength by up to 166% compared to that of the MS-S specimen. In terms of the maximum displacement, the MS-S and MS-SF specimens exhibited values with an increase of 201.3% and 186.6%, respectively, compared to the reference specimen values. For the M-R-SU specimen, in step 3 during the second cycle, a strength loss of 18% was observed, likely due to the tearing of the polymer. It is assumed that cracks in the mortar joints occurred in the first cycle of step 3. Thereafter, as the steps progressed, a noticeable increase in the maximum strength was not observed. In addition, the maximum strength and displacement increased by 158% and 227% respectively in comparison with the corresponding value of the reference specimen. For the M-R-SU, M-R-SUC and MS-SF2 specimens, along with the enhanced maximum strength and displacement, an enhanced ductility was also noted, among the retrofitted specimens. The M-R-SUC and MS-SF2 specimens maintained their load carrying capacity without a substantial loss at drift ratios of 1.43% and 1.14%, respectively. For the S-W specimen, the maximum lateral strength was found to increase by almost 189% compared to that of the reference specimen (NR-W). However, the MS-W specimen exhibited a corresponding increase of 130% as well. Although the S-W specimen demonstrate a higher lateral strength, the hysteretic behavior of the MS-W specimen was more stable and capable of a higher dissipation of energy. In term of the maximum displacement, the S-W and MS-W specimens exhibited increases of 117% and 129% respectively compared to the corresponding value of the reference specimen. In the case of a wall with a door opening, the use of the retrofitting methods could effectively increase the lateral strength of and displacement of the wall. In the case of the S-DF and MS-DF specimens, the strength was increased by 179% and 241% respectively compared to that of the reference specimen. In terms of the maximum displacement, the S-DF and MS-DF specimens exhibited increases of 229% and 216% respectively compared to that of the reference specimen. Faulty installation pertaining to the fixed condition in the S-DF specimen could be a possible explanation for the reduced stiffness.

21

Fig. 13. Equivalent bilinear skeleton curve adapted from Magenes and Calvi [45].

a) Solid wall

b) Wall with window opening

c) Wall with door opening Fig. 14. Backbone skeleton curves of the specimens

22

Table 4. Test results

Retrofitted specimens

Reference specimens

Specimen

1

NR-S

2

NR-W

3

NR-D

4

MS-S

5

MS-SF

6

S-W

7

MS-W

8

S-DF

9

MS-DF

10

M-R-SU

11

M-R-SUC

12

Fy

Fu

δy

δu(20%)

δmax

Kel

(kN)

(kN)

(mm)

(mm)

(mm)

(kN/mm)

+

77.12

85.69

4

-

16.78

19.28

4.2

-

65.67

72.97

3.5

-

23.85

18.76

6.81

+

53.71

59.68

4

-

31.74

13.42

7.93

-

56.2

62.45

4.5

35

35.54

12.49

7.77

+

55.55

61.72

5.2

-

23.53

10.68

4.52

-

58.22

64.69

3.8

-

23.73

15.32

6.24

+

100

111.16

7.2

27

39.57

13.89

3.75

-

96.55

107.28

8

19

39.78

12.06

2.37

+

148

164.42

24

-

37.51

6.16

1.56

-

182.93

203.25

24.2

-

35.74

7.55

1.47

+

88.29

98.1

15.9

-

39.24

5.55

2.46

-

120.66

134.06

14.2

19.4

39.24

8.49

1.36

+

73.75

81.95

15.1

-

42.91

4.88

2.84

-

68.96

76.62

15.9

-

43.89

4.33

2.76

Direction

MS-SF2

μ

+

69.46

77.18

22.5

37

54.51

3.08

1.64

-

135.87

150.97

40

-

54.03

3.39

1.35

+

118.61

131.78

15.8

48.1

51.07

7.5

3.04

-

157.46

174.95

17.1

-

51.07

9.2

2.98

+

110.83

123.14

7.5

-

44.69

14.77

5.95

-

114.16

126.85

8.5

-

44.87

13.43

5.27

+

134.79

149.77

7.9

-

38.15

17.06

4.83

Failure mode

Rocking, Shear sliding Rocking, Shear Sliding Rocking, Shear sliding Rocking, Shear sliding Rocking Rocking, Shear sliding Rocking, Shear sliding Rocking Rocking Shear Sliding Rocking, Diagonal, shear

-

110.73

123.03

6.4

-

38.15

17.3

5.96

sliding

+

131.16

145.73

4.9

-

30.54

26.76

6.23

Rocking, Diagonal, shear

-

128.66

142.95

6

-

30.547

21.44

5.09

sliding

5.2 Cumulative absorbed energy A high energy absorption capacity is particularly useful when a structure is exposed to severe earthquakes. The energy absorbed in one cycle by an element is equal to the area of the hysteresis loop of the force–displacement curve for that cycle. Fig. 15 shows the cumulative absorbed energy (CAE) of each test for each cycle. The energy dissipated in the specimens depends on the friction between the seams and cracks, initiation of new cracks, yielding of the steel plates, tensile resistance capability of the polymer, boundary conditions and crushing of bricks. Based on the results, the cumulative absorbed energy of the reference and retrofitted specimens was found to be similar at the beginning of the test. However, the cumulative absorbed energy of the retrofitted specimens at the end of the test increased by up to 7.3 times for the specimens with the door opening compared with that of the reference specimens. For the reference specimen without openings, the final cumulative absorption energy did not increase significantly with the application of the retrofitting method involving the metal lath and steel plate strengthening technique (MS-S). As shown in Fig. 15(a), the M-R-SUC and MS-SF2 specimens dissipated a higher energy compared to 23

that dissipated by the reference specimen as well as the specimens corresponding to other retrofitting methods. The capacities of energy dissipation for both the specimens were nearly similar until the MS-SF2 specimen test was terminated at step 7. However, the M-R-SUC specimen tolerated a larger number of steps, which resulted in a higher dissipation energy. The final cumulative absorbed energy increased by almost 6 times in the case of the M-R-SUC specimen. In the case of walls with the window opening, the cumulative absorption energy pertaining to the retrofitting methods increased with the application of the steel plates around the openings and edges and the metal lath method; however, the S-W specimen exhibited less tolerance pertaining to the absorption energy in the subsequent steps compared to the tolerance of the MS-W specimen.

(a) Specimens without openings

(b) Specimens with window opening

(c) Specimens with door opening

Fig. 15. Cumulative dissipated energy

6. Summary and Conclusions Although several studies have focused on the seismic response of URM walls, the impact of different types of retrofitting methods is required to be further investigated to help architects select the most effective strategies for retrofitting. Furthermore, the appropriate design codes do not indicate the appropriate structural coefficients in the case of retrofitted masonry walls. The objective of the current research was to bridge this knowledge gap. Twelve masonry walls including three non-retrofitted (as reference specimens) and nine retrofitted were tested by subjecting the walls to cyclic loads. The influence of different retrofitting methods on the seismic behavior of the walls was compared. It was noted that considerably different test results can be expected if the walls are retrofitted by applying different retrofitting methods. The key results of the static tests on the masonry wall specimens are as follows. 

Regardless of the presence of openings and applied retrofitted methods, the maximum strength and displacement of the retrofitted specimens were 122–278% and 110–246% larger than those of the reference specimens, respectively. Moreover, the retrofitting methods did not increase the stiffness of the wall significantly. In most cases, the stiffness was lower than in the reference cases. 24



Using the fixed condition can be valuable for the seismic strengthening of URM walls in terms of the maximum strength, ductility and energy absorption capacity. It is, however, advisable that this fixed condition should be installed carefully to be practicable.



Using connecting steel plates can improve the seismic performance of the wall by enhancing the integrity of the entire wall when subjected to displacement. This aspect was noted by comparing the behavior of the M-RSU specimen with that of the M-R-SUC specimen.



The polymer coating material increases the wall displacement, strength and energy dissipation and can delay all the modes of wall failure. Therefore, the use of the polymer along with steel plates using connection steel plates is a suitable solution for reinforcing the walls. This approach can be considered to be an efficient and promising retrofitting technique along with other advantages such as a limited increase in structural weight, cost effectiveness, easy and quick installation, prevention of falling of broken bricks and mortar and reduced influence on the architecture and aesthetics.



The ductility factors in this study cannot be a representative factor of the damaged walls. As the ductility factor is sensitive to the yield displacement, it must be determined with caution. Therefore, the authors believe that the ultimate load, displacement, and energy dissipated by each wall are the key parameters instead of the ductility factor.



In some specimens, the stiffness showed smaller in amount. This is due to possible errors in the construction and installment of the initial walls (walls before retrofitted), and therefore the various retrofitted methods mentioned in this paper do not significantly increase or decrease the stiffness of the walls which results in no change in natural vibration period of the system.

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[2020.01.18]

Author statements

Manuscript title: Experimental Evaluation of the Seismic Performance of Retrofitted Masonry Walls (Ref: COST_2019_3628). Authors Bahador Bagheri, Jung-Han Lee, Han-Gil Kim, Sang-Hoon Oh. Corresponding author: Dr. Sang-Hoon Oh To the best of my knowledge, all persons listed as authors qualify for authorship. Authors’ contributions: All authors have contributed equally in: (a) Conception and design; (b) Acquisition of data; (c) Analysis and interpretation of the data;

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(d) Drafting the article or revising it critically for important intellectual content; (e) Approval of the final version.

Sincerely, Oh Sang Hoon, Ph.D. Professor, Department of Architectural Engineering, Pusan National University, Busandaehak-ro 63-2, Geumjeong-gu, Busan, 46241, Korea Tel: +82-51-510-1009 Fax: +82-51-514-2230 E-mail: [email protected]

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Declaration of interests

☒ 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.

☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

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[2020.01.18]

Dear Dr/ Prof. Antonio Ferreira

Manuscript title is “Experimental Evaluation of the Seismic Performance of Retrofitted Mas onry Walls”. The paper was coauthored by Bahador Bagheri, Jung-Han Lee, and Han-Gil Kim.

The highlights of this paper are as below;

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New retrofitting techniques for improving in-plane seismic performance of URM wall are proposed.

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Twelve full-scale masonry walls were tested.

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The retrofitting techniques can greatly increase the seismic behavior of the URM wall.

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The most suitable retrofitting method is selected by polymer coating material.

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The polymer coating technique is simple and economical for implementation.

Sincerely, Oh Sang Hoon, Ph.D. Professor, Department of Architectural Engineering, Pusan National University, Busandaehak-ro 63-2, Geumjeong-gu, Busan, 46241, Korea 32

Tel: +82-51-510-1009 Fax: +82-51-514-2230 E-mail: [email protected]

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