Shear behavior of masonry panels strengthened by FRP laminates

Construction and Building Materials 16 (2002) 409–416

Shear behavior of masonry panels strengthened by FRP laminates M.R. Valluzzi*, D. Tinazzi, C. Modena Department of Constructions and Transportation Engineering, University of Padova, Via Marzolo, 9-35131 Padova, Italy Received 6 July 2001; accepted 31 May 2002

Abstract The present experimental study, performed on brick masonry panels strengthened by Fiber Reinforced Polymer (FRP) laminates, was aimed to investigate the efficiency of an alternative shear reinforcement technique. A series of nine unreinforced masonry (URM) panels and 24 strengthened panels have been subjected to diagonal compression tests. Different reinforcement configurations were evaluated. Experimental results pointed out that FRP reinforcement applied only at one side of the panels did not significantly modify the shear collapse mechanisms (diagonal splitting) of the URM; while double-side configurations provided a less brittle failure and a noticeable ultimate capacity increase. Performances of the different reinforcement configurations are compared in terms of strength and mechanism of failure; finally, experimental results are also used to calibrate existing analytical formulations for ultimate shear strength prediction. 䊚 2002 Elsevier Science Ltd. All rights reserved. Keywords: Brick masonry; Shear behavior; FRP laminates

1. Introduction The Italian traditional masonry works, which represent a large part of its historical heritage, are particularly susceptible to in-plane shear actions. As is known, in URM brittle, shear rupture occurs either as a diagonal splitting or as step-pattern sliding along the mortar joints, depending on the characteristics of the constituent materials (mortar and bricks). Therefore, in order to predict properly the masonry shear capacity, it is necessary to first identify the most anticipated failure mechanism, based on the knowledge of the involved materials. Presently, small diameter steel bars are successfully used as reinforcement in masonry retrofitting, and several field applications on heavily loaded historical structures have been presented w1x. Nowadays, FRPs represent a new opportunity to restoring ambit, with considerable development in URM strengthening. A key problem is represented by FRP’s up-to-failure linear elastic behavior, which prevents the ductility of the system being based on the plastic *Corresponding author. Tel.: q39-049-827-5576; fax: q39-049827-5604. E-mail address: [email protected] (M.R. Valluzzi).

behavior of the strengthening material itself; therefore, redistribution-derived theories are not applicable. Consequently, investigations on alternative mechanisms providing sufficient signals of incipient collapse are required. A certain number of FRP masonry strengthening applications have already been performed, involving either FRP bars and laminates, but few analytical or experimental research works have investigated the effectiveness and reliability of that new technology w2– 4 x. In the present experimental work, which was performed on coupon-size masonry panels, the diagonal compressive test has been chosen to simply simulate the in-plane shear phenomenon. 2. Materials characterization As mentioned, masonry mechanical properties depend on the characteristics of the constituent elements (bricks and mortar), as well as on the workmanship and the interface interaction within the assemblage. Thus, an extensive program of tests, ranging from the units to the assemblages scale, was performed. The main mechanical properties of the bricks were determined by unidirectional compressive and tensile

0950-0618/02/$ - see front matter 䊚 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 9 5 0 - 0 6 1 8 Ž 0 2 . 0 0 0 4 3 - 0

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Table 1 Physical and mechanical characteristics of the FRP laminates Type of fiber

High tensile strength carbon (CFRP)

Glass type E (GFRP)

Polyvinylalcohol (PVAFRP)

Density (kgym3) Equivalent thickness (mm) Characteristic tensile strength (MPa) Tensile modulus of elasticity (GPa) Ultimate strain (%)

1820 0.165 3430 230 1.5

2600 0.115 1700 65 2.8

1300 0.07 1400 29 6

tests. The characteristic compressive strength of five specimens was found to equal 8.83 MPa. Indirect tensile tests (splitting test) on five specimens provided an average value equal to 0.95 MPa; thus, the corresponding tensile strength can be adopted to equal 0.76 MPa. Mortar used for the masonry panels had the following mix composition: 380 kgym3 of inorganic binder; 1140 kgym3 of sand (Dmaxs4.8 mm); and waterybinder ratios0.7. The flexural and compressive standard tests on six specimens (40=40=160 mm3, after a period of 28 days of curing) revealed an average strength equal to 1.48 (hence, the corresponding tensile is approx. 1.19 MPa) and 6.03 MPa, respectively. Interface friction characteristics along the mortar joints are referred to previous works involving the same materials w3x. During those tests, 12 triplets where tested with four different levels of confining stress (0.05, 0.1, 0.3 and 0.5 MPa). Interpolating the experimental points obtained on the friction vs. confining stresses, it was possible to detect, according with the Coulomb criterion, the parameters describing the shear strength f v for sliding along a mortar joint: the cohesion f v0 and the dry friction coefficient m.

Therefore, the Coulomb equation representing the dry friction mechanism during joint sliding is: fvsfv0qms0s0.66q1.36s0

(1)

Compression tests with load cycles performed on masonry panels, having nominal dimensions of 51=51=12 cm, revealed a characteristic compressive strength of 5.56 MPa, a modulus of elasticity of 1400 MPa (referred to the 30% of the ultimate load) and a masonry Poisson ratio equal to nxzsy´x y ´zs0.03 and nyzsy´y y ´zs0.03 (referred to a transversal, x or y, and the vertical direction z, respectively). The FRP laminates involved in the experimental work consist in carbon, glass or polyvinyl-alcohol unidirectional fibers embedded in epoxy resin, according with the wet lay up technique. Their main mechanical characteristics, declared by the manufacturer, are shown in Table 1. Pull-off test were performed in order to evaluate the FRP-brick adhesion strength under actions orthogonal to the bond surface. The average tensile strength of six specimens is equal to 0.44 MPa; in all cases the rupture was due to detachment of the brick superficial skin.

Fig. 1. Single-side strengthening patterns.

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Fig. 2. Double-side strengthening patterns.

Bond strength along the fiber direction was determined by pulling two consecutive bricks connected by a stretched FRP strip. The measured average strength of five specimens was equal to 2.50 MPa; again, detachment occurred within the brick superficial skin. 3. Specimen description A series of 33 masonry panels having nominal dimensions of 51.5=51=12 cm were built. They were made

of solid clay bricks (5.5=25=12 cm) and have 10mm-thick mortar joints. Twenty-four of them have been strengthened by different FRP materials: nine with CFRP; 10 with GFRP and five with PVAFRP (see Fig. 1). Due to the small size of the samples and the bricks friableness, it was necessary to cut the loading diagonal corners, obtaining a plane transferring areas of 12=12 cm.

Table 2 Description of the specimens Unreinforced panels

PNR 1, PNR 2, PNR 3,PNR 4, PNR 5, PNR 6, PNR 7, PNR 8, PNR 9

Squared grid strengthening configuration panels CFRP

Double-side (1 layer of 1.2 cm) Single side (1 layer of 2.4 cm)

GFRP

Double-side (2 layers of 3 cm) Single side (4 layers of 3 cm)

PVAFRP

Double-side (4 layers of 5.5 cm) Single side (6 layers of 7.5 cm)

Diagonal strengthening configuration panels CFRP

Double-side (1 layer of 1.7 cm) Single side (2 layers of 1.7 cm)

GFRP

Double-side (3 layers of 2.8 cm) Single side (6 layers of 2.8 cm)

PR PR PR PR PR PR PR PR PR PR PR PR PR PR PR PR

1 2 3 1 2 1 2 3 1 2 3 1 2 3 1 2

PRD PRD PRD PRD PRD PRD PRD PRD

Carb 2F Carb 2F Carb 2F Carb 1F Carb 1F Glass 2F Glass 2F Glass 2F Glass 1F Glass 1F Glass 1F PVA 2F PVA 2F PVA 2F PVA 1F PVA 1F 1 2 1 2 1 2 1 2

Carb 2F Carb 2F Carb 1F Carb 1F Glass 2F Glass 2F Glass 1F Glass 1F

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412 Table 3 Experimental tests results Panel type

Cracking Load (kN)

Crack pattern

Failure load (kN)

Unreinforced masonry panels PNR 1 – PNR 2 – PNR 3 – PNR 4 – PNR 5 – PNR 6 – PNR 7 – PNR 8 – PNR 9 – Average

– – – – – – – – –

75.5 90.0 91.2 127.9 77.1 93.2 93 121.6 137.3 100.7

Single-side reinforced panels PR 1 Carb 1F PR 2 Carb 1F Average

Sharp diag. Sharp diag.

– –

PR 1 Glass 1F PR 2 Glass 1F PR 3 Glass 1F Average

– 86.0 –

PR 1 PVA 1F PR 2 PVA 1F Average

– –

PRD 1 Carb 1F PRD 2 Carb 1F Average

– –

Sharp diag. Sharp diag.

112.8 119.8 116.3

PRD 1 Glass 1F PRD 2 Glass 1F Average

113.8 108.1

Sharp diag. sharp diag.

115.3 108.3 111.8

Double-side reinforced panels PR 1 Carb 2F – PR 2 Carb 2F 104.6 PR 3 Carb 2F – Average PR 1 Glass 2F PR 2 Glass 2F PR 3 Glass 2F Average

– 120.5 –

PR 1 PVA 2F PR 2 PVA 2F PR 3 PVA 2F Average

– 130.5 –

PRD 1 Carb 2F PRD 2 Carb 2F Average

– 125.3

PRD 1 Glass 2F PRD 2 Glass 2F

– 133.2

Sharp diag. Sharp diag. Sharp diag.

91.6 90.0 90.8

Sharp diag. Sharp diag.

Spread diag. Spread diag. Spread diag. Spread diag. Spread diag. Spread diag. Spread diag. Spread diag. Spread diag. Spread diag. Spread diag. Spread diag. Spread diag.

Average

In order to study the influence of the eccentricity of the strengthening, the strips were applied on both sides or only at one side of the panels; in the latter case, the FRP thickness was doubled to maintain the FRP amount constant.

94.6 99.1 94.0 95.9 102.0 100.0101

100.5 113.3 98.5 104.1 106.7 135.4 103.0 115.0 148.5 150.4 146.0 148.3 143.1 149.4 146.3

%

Splitting Splitting Splitting Splitting Splitting Splitting Splitting Splitting Splitting Ref. Splitting Splitting y9.8 Splitting Splitting Splitting y4.8 Splitting – q0.3 Splitting Splitting q15.5 Splitting Splitting q11.0 DLM at lower corner DLM sup.qRTT centr. DLM lower, sides A and B q3.4 DLM up. and low. corner RTT side A DLM low. side A and B q14.2 DLM at lower corner DLM at lower corner DLM low. side A and B q47.3 DLM up. and low. side A DLM inf.qRTT centr. q45.3

170.3 180.0 175.2

Failure mode

DLM low. side A DLM inf. sides A and B q RTT centr. side A q74.0

Moreover, two configurations of the reinforcing system were investigated: strips as grid arrangement or application of diagonal strips orthogonal to the loaded diagonal. The same reinforcement amount was applied for the two different configurations, for each kind of

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4.2. Single-side strengthening Splitting failure with a clear diagonal crack pattern was also obtained in all single-side reinforced panels, whereas ultimate load was in many cases lower than the reference. The samples exhibited a clear bending deformation during the loading phases along the unreinforced diagonal; as a consequence, the main damage was concentrated on the unreinforced side (see Fig. 3). That bending phenomenon was caused by a noticeable difference of stiffness on the opposite sides as a result of the asymmetrical reinforcement. Among the one-side reinforced specimens, diagonal strengthening configuration always revealed a higher effectiveness than the squared grid set-up. 4.3. Double-side strengthening Fig. 3. Single-side reinforced panel failure mode: diagonal splitting with a single large crack on the unreinforced side. Notice the bending along the free diagonal.

reinforcing material, so that the ‘stiffness by mechanical ratio’ rE can be maintained for the two mentioned arrangements. The design reinforcement criterion of the FRP amount was based on the expectation of an increase of the 50% of the URM ultimate shear strength by applying the principal tensile stress limitation method on the homogenized section (following the Frocht theory, as in Yokel and Fattal w5x). As a consequence, due to the different mechanical characteristics of the fibers, each test condition is characterized by a different width of the strips and different number of layers to be glued (see Figs. 1 and 2). The panel typologies are shown in Table 2. 4. Experimental results The samples were subjected to diagonal compression test, and both vertical and horizontal deformations were measured by displacement transducers. Quantitative results are summarized in Table 3, where abbreviations are as follows: DLM means de-lamination of FRP stripes, RTT means FRP tensile rupture; the location of the possible concentration of the rupture is also indicated. In the following, a description of the different panel typology behaviors is given:

In all these cases, the failure mechanism consisted in sudden loss of collaboration between reinforcement and substrate, due to either de-lamination (peeling) of the superficial part of masonry or rupture of the FRP strips (see Fig. 4). Grid-reinforced specimens determined spread-cracks patterns, whereas a clear splitting crack appeared in all the diagonally reinforced panels. The ultimate strength increase was noticeable in almost all cases; while only the CFRP-reinforced panel was seriously affected by de-lamination. The URM typical sudden failure was noticeably corrected by the FRP strengthening, especially by the grid configuration, where crack wide spreading provided sufficient signals of incipient crisis well before collapse. Deformations increased visibly up to failure, and the global behavior resulted was less brittle, as shown in Figs. 5 and 6 (up to 80% of the ultimate load, when sensors have been removed). 5. Analysis of the results An FEM simulation was implemented by means of a commercially available code to validate the assumptions on stress distributions corresponding to the different strengthening configurations of the panels. Results are

4.1. Plane panels As expected from the material characterization and the triplet test results, all the unreinforced specimens presented brittle failure due to splitting along the loaded diagonal. The average failure load, used as reference value for the comparison with the strengthened specimens results, is equal to 100.7 kN.

Fig. 4. Double-side reinforced panel failure mode: either peeling in the anchoring zones or rupture of the fibers are reached depending on both the reinforcement stiffness and bonding area.

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Fig. 5. Stress–strain diagram of panels reinforced with carbon-FRP strips in a diagonal configuration, either on one or both sides. Values refers to the average measurement on the opposite faces.

shown in Fig. 7 in terms of principal tensile stress (splitting stress). According to the Frocht theory, in URM panels the highest splitting stresses are concentrated in the core, whilst partial stress relocation is given by the diagonal reinforcement. Finally, the squared grid reinforcement configuration presents a more uniform distribution of stress on the panel, while stress peaks are shifted toward anchoring zones. Analytical models available in literature to evaluate the shear strength VRd of URM shear walls are here reported. All of them are based on the linear effects superposition, which derive from the implicit assumption of plastic stress redistribution. Even though the latter assumption is not properly introduced when dealing with FRP, at present no more appropriate approaches are available. Formulations (2) and (4) (ENV, Eurocode 6 w6x and Tomazevic et al. w7x, respectively) are proposed for masonry reinforced with steel bars; whereas formulation (5) w4x is introduced specifically for FRP. Essentially, they are the sum of the original masonry shear

Fig. 6. Stress–strain diagram of panels reinforced with glass-FRP strips in a diagonal configuration, either on one or both sides. Values refers to the average measurement on the opposite faces.

Fig. 7. FEM simulation: highest splitting stress in the core of the URM panel; partial stress relocation with diagonal reinforcement; and shifting of stress peaks to anchoring areas with squared grid reinforcement distribution.

capacity and the reinforcement effect, which is reduced for FRPs to take into account different issues descending from their non-ductile behavior. Particularly, in the mentioned formulas are included the characteristic strength of the masonry f vk and the geometrical and the mechanical characteristics of the masonry wall and of the reinforcement (t is the wall thickness, l its length, ds0.8l is the effective depth; rfrp is the FRP ratio computed on the wall section; Ar is the area of the reinforcement; f tk is the characteristic tensile stress of the FRP; ´frp,u is the FRP tensile ultimate strain). In particular, the formulation (5) considers a factor of efficiency r, which depends on the failure mode (FRP rupture or de-bonding). The expression of r, given by the Eq. (6), was found by Triantafillou w4x for concrete

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Fig. 8. Comparison between experimental and analytically predicted shear strength for URM and FRP-strengthened panels.

members (´frp,e is the effective FRP strain). On the basis of the present experimental database it would be possible to provide a better calibration of the r factor for masonry; in fact, by measuring the effective strain for each different FRP type it is possible to newly determine all coefficients of Eq. (6) by polynomial interpolation. Note that f vk0 wEqs. (2) and (3)x is obtained by sliding tests on triplets, whereas f 9vk0 wEq. (4)x is obtained by diagonal compressive tests. (2)

VRdsfvktdq0.9drfrpftkt where:

(3)

fvksfvk0qms0; B

E

VRdsD0.9tlf9vk0y1.5qy1qs0yf9vk0Gq0.4Arftk

(4)

VRdsfvktdq0.9drfrpEfrpr´frp,ut (rsefficiency factor)

(5)

r´frp,us´frp,es0.0119y0.0205ŽrfrpEfrp. q0.0104ŽrfrpEfrp.2

(6)

C

F

The comparison among the experimental and the predicted values of the shear strength is reported in Fig. 8. The efficiency factor given by Eq. (6) appears to be excessively conservative as it provides very low shear strengths, so a better calibration of the formula is necessary. Anyway, despite the same mechanical parameter rE was maintained for diagonal and grid set-up, as mentioned before, the related shear strength differed of more than 40% for the two configurations. Therefore, for a better calibration of the r factor, also the geometrical reinforcement arrangement should be considered,

as it can noticeably affect the FRP strengthening effectiveness. 6. Conclusions The contribution of FRP strips on the shear behavior of clay brick wallets has been investigated; far from being exhaustive, the results of the present study indicated that asymmetrical applications (single-side reinforcement) on masonry panels offer a limited effectiveness. The diagonal configuration is more efficient in terms of shear capacity than the grid set up; however, the latter offers a better stress redistribution that causes a crack spreading and a less brittle failure. In most cases, less stiff FRP material appeared to be more effective both in terms of ultimate strength and stiffness (not reported for brevity) increase of the panels. That was due to the particular design criterion used (weaker material has a larger adhesion area), and also to the fact that stiffer material is more vulnerable to debonding, especially when the number of plies increase. Presently, our effort is oriented to formulate a relation involving stiffness, thickness, width and number of layers of FRP to quantify the susceptibility of the reinforcement to de-bond. Low increments in the shear strength, in the particular experience here described, are also attributable to the peeling occurring in the portions next to the applied compressive loads (where high stress causes premature cracks) and to the peculiar lower tensile strength of the bricks, which causes splitting failure through them as

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main failure mechanism. Nevertheless, bricks used represent the most common typology in Italy. Alternative anchoring methods appear to be a key issue to evaluate during further experimentations in order to prevent loss of effectiveness due to de-bond. The type of test conducted and the specimen dimensions appear an easy and efficient system to check elementary strengthening configurations. Finally, the application of Triantafillou’s formula wEq. (5)x requires further calibrations, especially for failure mechanisms affecting the efficiency parameter and the configuration of reinforcement. Anyway, until new approaches to predict the shear capacity are available, a better definition of the efficiency factor r appears to be the right direction to follow. Acknowledgments All the materials used in the research, including the fibers and the adhesion system, were supplied by Modern Advanced Concrete (MAC S.p.A.) of Treviso, Italy.

References w1x Binda L, Modena C, Valluzzi MR. Mechanical effects of bed joints steel reinforcement in historic masonry structures. Structural faultsqrepair. Engineering Techics Press, Edinburgh, UK: 1999. w2x Tinazzi D, Arduini M, Modena C, Nanni A. FRP-structural repointing of masonry assemblages. Ottawa, Canada: ACMBS III, 2000. w3x Valluzzi MR, Modena C, Marchetti M. Shear strengthening of masonry panels using FRP. Proceedings of the Twelfth IB2MaC Conference. Madrid, Spain: 2000. w4x Triantafillou TC. Strengthening of masonry structures using epoxy-bonded FRP laminates. J Compos Constr ASCE 1998;2(2):96 –104. w5x Yokel FY, Fattal GS. Failure hypothesis for masonry shear walls. J Struct Div 1976;102(ST3):515 –32. w6x ENV 1996-1-1. Eurocode 6: design of masonry structures. 1996. w7x Tomazevic M, Lutman M, Pertrovic L. In plane behavior of reinforced masonry walls subjected to cyclic lateral loads. Report to the Ministry of Science and Technology of Republic of Slovenia, parts 1 and 2. Liubljana, Slovenia, 1993.