Out of Plane Flexural Behavior of Brick Wall_2006

www.elsevier.com/locate/compositesb

Composites: Part B 38 (2007) 559–574

Out-of-plane flexural behavior of unreinforced red brick wallsstrengthened with FRP composites

Ayman S. Mosallam

Civil and Environmental Engineering Department, University of California, Irvine, CA 92604, USA

Received 20 March 2006; accepted 27 July 2006Available online 27 December 2006

Abstract

This paper presents the results of a study focused on evaluating the out-of-plane flexural behavior of two fiber reinforced polymer(FRP) composite systems for strengthening unreinforced red brick masonry walls. The full-scale tests followed the International CodeCouncil Evaluation Service (ICC-ES) AC 125 procedure. In the experimental program, a total of four full-scale destructive tests wereconducted on UMR red brick walls. One wall specimen was used as control (as-built) specimen without composites, and the remainingthree wall specimens were strengthened with either E-glass/epoxy or carbon/epoxy composite systems with different fiber architecture.The effect of applying a cross-ply laminate on the ultimate failure mode has been investigated. Full-scale experimental results confirmedthe effectiveness of the FRP composite strengthening systems in upgrading the out-of-plane flexural structural performance of URMwalls. In addition, an analytical model was developed to predict the ultimate load capacity of the retrofitted walls. The analytical mod-eling is based on deformation compatibility and force equilibrium using simple section analysis procedure. A good agreement betweenthe experimental and theoretical results was obtained.� 2007 Elsevier Ltd. All rights reserved.

Keywords: A. Laminates; B. Delamination; D. Mechanical testing; E. Lay-up (manual); Infrastructure

1. Introduction

In general, unreinforced masonry (URM) buildings per-form poorly in earthquakes. Two types of failure are com-monly encountered in URM buildings subjected to seismicforces. The first failure mode occurred in in-plane shearthat are designed to form the lateral load resisting systemof the building. The other type of failure is due to out-of-plane bending stresses caused by seismic inertial forces.The excessive out-of-plane bending is also a major reasonfor the loss of load carrying capacity of URM walls.Fig. 1 shows a typical failure of unreinforced red brick walldue to excessive out-of-plane seismic forces. Compositesoffer an attractive strengthening protocol for existing andhistorical unreinforced masonry structures.

1359-8368/$ - see front matter � 2007 Elsevier Ltd. All rights reserved.

doi:10.1016/j.compositesb.2006.07.019

E-mail address: [email protected]

In the past few decades, composites have successfullybeen used in different construction applications includingstrengthening of reinforced concrete, steel and timberstructures. An in-depth review of different applications ofcomposites in repair and rehabilitation is discussed byMosallam [1]. Lately, several studies have been conductedon evaluating the use of polymeric composites for repairand strengthening both unreinforced and reinforcedmasonry walls subjected to seismic, wind and lateral earthpressure. The advantages of using composite materials inthis application are (i) ease of application, (ii) preservationof the geometrical and architectural details of the walls, (iii)their high strength-to-weight ratio, and (iv) their high resis-tance to corrosion as compared to metallic strengtheningsystems. This paper presents a summary of experimentaland theoretical results of a study that was conducted tocharacterize the out-of-plane flexural behavior of unrein-forced masonry walls externally strengthened with fiberreinforced polymeric (FRP) composite laminates.

mailto:[email protected]

Fig. 1. Failure of unreinforced red brick wall due to out-of-plane seismicforces [Nisqually Earthquake, 28 February 2001].

560 A.S. Mosallam / Composites: Part B 38 (2007) 559–574

2. Related work

The use of composites in strengthening masonry startedinitially at the Swiss Federal Laboratories for MaterialsTesting and Research (EMPA) in Dubendorf, Switzerland.One of the pilot studies in the area was reported by Schwe-gler [2]. Based on the results of this pilot study, a testingprogram on load-bearing masonry walls of a six-storybuilding strengthened with carbon/epoxy laminates wasperformed [3]. Gilstrap and Dolan [4] reported the resultsof an experimental study focused on evaluating the struc-tural behavior of unreinforced masonry walls strengthenedexternally with different types of composites. Both small-and large-scale tests were conducted with varying bound-ary conditions. The walls were tested under both line andconcentrated uniform loading conditions. Albert et al. [5]conducted a similar experimental investigation on the fea-sibility of using polymeric composites as an externalstrengthening system for masonry walls. In their study,the performance of both undamaged and slightly damagedwall specimens was evaluated. Several parameters wereused in their study including composite type, amount ofapplied composite, fiber architecture of the overlay andthe loading regime. Ganz et al. [6] studied four types ofcomposites systems for wall strengthening, namely:chopped E-glass/epoxy, chopped E-glass/polyester, E-glasscomposite fabric cloth with epoxy resin, and E-glass fabriccloth with polyester resin. The results of the study showedsignificant non-linearity in load–deflection relationshipsdue to the effect of delamination between the compositesand the masonry. The behavior of tested strengthenedmasonry beams with was studied analytically [7]. The cyclicflexural behavior of masonry walls reinforced with glass/epoxy composites was investigated by Velazquez-Dimaset al. [8] and Kuzik et al. [9]. Tan and Patoary [10] con-ducted a large experimental program on 30 masonry wallsstrengthened using three different fiber-reinforced polymer(FRP) systems. However, the loading regime that was usedwas concentrated on a portion of the wall. Al-salloum and

Almusallam [11] conducted a study on the behavior ofunreinforced masonry strengthened with composites. Sev-eral wall specimens were subjected to out-of-plane andin-plane loads. As reported, a significant strength increasewas observed for all strengthened wall specimens. Turcoet al. [12] evaluated both the flexural and shear strengthen-ing of un-reinforced masonry using FRP bars. The resultsof the study indicated the potential of this technique formasonry strengthening applications. Similar studies wereconducted by other researchers (e.g. El-Dakhakhni et al.[13], Hamoush et al. [14], Hamilton and Dolan [15], Laur-sen et al. [16]). Ghobarah and Galal [17] studied the out-of-plane behavior of FRP strengthened masonry walls withopenings. Recently, Korany and Drysdale [18] developedan unobtrusive composite rehabilitation technique usingflexible carbon/epoxy cables, mounted near the surface ofthe facade walls in epoxy-filled grooves in the bed and headjoints.

3. Objective and motivations

The majority of historical structures including buildings,arches, bridges and chimneys that requires immediaterepair and/or strengthening are made of red clay brickswith low-strength mortar. To date, limited work has beenpublished on the behavior of red brick walls retrofitted withFRP composites. In addition, several studies highlightedthe major influence of the loading pattern on the ultimateperformance of laboratory-tested wall panels. Hence, it iscritical to accurately simulate the inertial forces generatedby seismic activities that are responsible for the excessiveout-of-plane forces applied to the masonry walls. Line- orconcentrated loading of a wall specimen produces stressconcentration fields that accelerate the strength degrada-tion of the weak aging mortar lines resulting in inaccuratelaboratory-simulated performance and premature failureas compared to the actual field performance. The prema-ture failure of the mortar lines due to the application of lineand/or concentrated loads generates high shear stresses onthe laminate. This shear stress concentration adverselyaffects the strength of the composite laminate and increasesthe possibility of initiating premature local laminate frac-ture and/or debonding at these locations. In order toaccurately simulating the applied loads generated fromseismic action, and avoid these potential premature failuremodes, a uniform hydrostatic pressure was employed inthis study.

In most cases, both in-plane shear and out-of-plane flex-ural upgrades are required to upgrade the seismic perfor-mance of old and historical unreinforced masonrystructures. In order to fulfill these demands, multidirec-tional composite systems are required (e.g. cross-ply,angle-ply or quasi-isotropic lamination) to achieve opti-mum retrofit design. To date, no work has been publishedto evaluate the coupling effect of composite reinforcementsapplied in the different directions for each demand. Thiseffect issue is investigated and discussed in this paper. In

A.S. Mosallam / Composites: Part B 38 (2007) 559–574 561

addition, this is one of the first studies where all testprocedures are confirmed with the requirements of theInternational Code Council-Evaluation Service (ICC-ES)Acceptance Criteria AC 125 [19].

4. Experimental program

4.1. Wall specimens

A total of four unreinforced red bricks large-scale wallspecimens were constructed and tested to failure. Thedimensions of the walls were 2.64 m · 2.64 m (8.67 ft ·8.67 ft) and one brick wide. Large wall dimensions wereselected to avoid scale effects and to reflect the actual per-formance of a weak masonry wall under out-of-plane flex-ural loading conditions. Table 1 describes the wallspecimens evaluated in this study. As mentioned earlier,the International Code Council-Evaluation Service (ICC-ES) Acceptance Criteria AC 125 [19] procedures were fol-lowed for all tests.

Table 1Description of wall specimens

Test ID Specimen’s description

Control unreinforced

WCONT-U As-built wall no composites

Retrofit: carbon/epoxy

WC-RET-02 2 Plies of unidirectional carbon/epoxy laminWC-RET-090 1 Ply of unidirectional carbon/epoxy in each

Retrofit E-glass/epoxy

WE- RET-02 3 Plies of unidirectional E-glass/epoxy lamin

Fig. 2. Compression test fo

Fig. 3. Setup and result

4.2. Mortar

Standard Type-S mortar was used in constructing thesewalls. The mortar was mixed to the proportion specifica-tion of ASTM C-270 Standard. The average compressivestrength of the mortar was obtained by testing six50.8 mm (200) diameter · 101.6 mm (400) high cylinderstaken from the same batch used in fabricating the wallspecimens. The average strength ðf 0mÞ of the mortar onthe day of the tests was 21.37 MPa (3100 psi).

4.3. Red bricks

Common red clay bricks (Castaic) readily available frombuilding suppliers were used. The nominal dimensions were20.32 cm (8 in.) · 10.16 cm (4 in.) · 5.72 cm (21

4in.). Two

types of tests were conducted on the red bricks: (i) a unitbrick compression test to determine the ultimate compres-sive strength (refer to Fig. 2), and (ii) a prism test (refer toFig. 3) to determine the combined compressive strength of

ate parallel to edge supports [0�]2orthogonal direction (perpendicular to edge supports direction) [0�/90�]1

ate parallel to edge supports [0]3

r unit red clay bricks.

s of the prism test.


the unit bricks and mortar. The average compressivestrength obtained from unit brick tests is 25.00 MPa[3.63 ksi]. A lower average strength value of 16.0 MPa[2.33 ksi] was obtained from the prism tests.

4.4. Composite materials

Both carbon/epoxy and E-glass/epoxy composite sys-tems were evaluated. Specifications for each system compo-nent including fibers and epoxies, and certifications forrandom sampling in accordance with requirements ofICC-ES (AC85 [20]) were obtained. This procedure is crit-ical to ensure that all the off-the-shelf composites materialsused in laboratory testing are true representation of mate-rials to be used in the field.

4.5. Test setup

All specimens were tested in a water-bag structuralframe as shown in Fig. 4. The hydraulic pressure load

Fig. 4. Out-of-plane water bag wall test rig.

Fig. 5. Typical hydrostatic pressu

was applied uniformly to each specimen until ultimate fail-ure occurred (refer to Fig. 5). The applied pressure wascontrolled via a data acquisition computer program con-nected directly to the loading frame. The load followeda cyclic loading/unloading regime, which was designedspecifically for these tests. In all tests a loading rate of3.45 kPa/min (72 psf/min) was used. The wall specimenswere simply supported only on two parallel sides whilethe other two sides were unsupported (refer to Fig. 4).A calibrated pressure transducer was used to controland measure the applied water pressure for all tests. Alldata was monitored and recorded using a computerizeddata acquisition system, which also provides real-timemonitoring of data during testing. Deflection and straindata were measured using linear variable differential trans-ducers (LVDTs) and electronic strain gages, respectively.Deflection and strain data were collected using a comput-erized data acquisition system. Stress/strain (r/e) andload/deflection (P/d) curves were developed for each spec-imen and both localized and ultimate failure modes wererecorded and then analyzed. Fig. 6 shows the boundaryconditions and the locations of different deflection andstrain gages.

4.6. Strengthening schemes

Three categories of walls were tested: (i) two (as-built)control walls, (ii) two carbon/epoxy retrofitted walls(WC-RET-02 and WC-RET-0/90) with different lamina-tion schedules, and (iii) an E-glass/epoxy retrofitted wall(WE-RET-02). The following sections summarize the find-ings of each test group.

4.7. Control (as built) tests

At first, an as built, unstrengthened brick wall specimenwas tested to provide a baseline for comparison with other

re loading of wall specimens.

Fig. 6. Locations of deflection and strain gages for strengthened wall specimens.

Fig. 7. Ultimate failure mode of the as-built specimen.


FRP strengthened specimens. No external composite rein-forcement is used (except of two 76.2 mm (300) single cross-laminates adhered to the compression surface to avoidfailure while transporting the specimen to the test rig).

The control wall was subjected to several cycles of uni-form pressure. The self-weight of the wall specimen wassubtracted from the values of the applied uniform pres-sures. As the out-of-plane pressure was applied, the ‘‘as-built’’ wall specimen exhibited a near-linear behavior upto a pressure intensity of 3.30 kPa (69.10 psf), after whichbehavior became non-linear until failure occurred. The ulti-mate load capacity of this specimen was 6.5 kPa (136 psf),with an associated mid-height deflection at the maximumload of 45.41 mm (1.7900). After reaching this ultimate load,rapid stiffness and strength degradations were observed,and the ultimate deflection at the total collapse was about77.30 mm (3.0400).

The failure initiated around mid-span and started withdevelopment of a mortar line crack that propagated acrossthe width of the specimen. As the load increased, the size ofthe cracks increased and a total sudden collapse occurred.Figs. 7 and 8 show the ultimate failure of the as-built wallspecimen and the pressure/deflection behavior measured atdifferent locations, respectively.

4.8. Strengthened walls tests

In order to evaluate the FRP composite systems’ effective-ness in strengthening unreinforced brick walls, three full-scale retrofitted specimens with different composite systemsand fiber architectures, described earlier, were evaluated. In


designing the strengthened specimens, the limit-states werebased on the composites systems mechanical properties,especially the rupture strain of both carbon/epoxy ðec

u ¼1:25%Þ and the E-glass/epoxy ðeE

u ¼ 2:20%Þ, as well as theextreme brick fiber compressive strain for strain compatibil-ity requirements.

4.8.1. Unreinforced brick walls retrofitted with carbon/epoxy

composite laminates

A total of two unreinforced/undamaged specimens fabri-cated at the same time, with same materials, and by the samecontractor as for the as-built specimen, were instrumented

Fig. 8. Pressure–deflection curve

Fig. 9. Pressure/deflection curves for unidirectional [0]2 carbo

and tested to failure under the same out-of-plane uniformlydistributed loading condition. Two fiber architectures wereevaluated: (i) two unidirectional laminates covering theentire wall (0�)2, and (ii) two cross-ply laminates (0�/90�)1.The reason of using the second fiber architecture is that ingeneral retrofit cases, the wall will be exposed to both in-plane as well as to out-of-plane seismic loads, and it is notobvious what would be the effect of this multidirectionalfiber architecture on the overall performance of the retrofit-ted wall. A sufficient gap was provided between the walledges and the supporting steel channels to prevent any pos-sibility of developing ‘‘arching action’’ in the test specimens.

s for the as-built brick wall.

n/epoxy strengthened brick wall specimen (WC-RET-02).

0.000 0.001 0.002 0.003 0.004 0.005 0.006 0.007 0.008

Strain

0

500

1000

1500

2000

2500

Pre

ssu

re (

psf

)

0

2500

5000

7500

10000

Pre

ssu

re (

kPa)

SG-1SG-4SG-5SG-6SG-7SG-8

Carbon/Epoxy (0o)2

Fig. 10. Pressure/strain curves for the unidirectional [0]2 carbon/epoxy strengthened brick wall specimen (WC-RET-02).

Fig. 11. The combined failure mode of brick compression failure andlaminate cohesive failure.


4.9. Cyclic and ultimate behavior of [0�]2 carbon/epoxy

strengthened unreinforced brick wall specimen (WC-RET-

02)

The wall specimen was subjected to several loading/unloading cycles up to 39.76 kPa (830.45 psf), after whicha ramp load (3.45 kPa/min or 72 psf/min) was applied upto the ultimate failure of the wall specimen. The first crack-ing sound was heard at a load level of about 27.17 kPa(567.47 psf). This can be attributed to stretching of thefibers due to the unevenness of the wall surface and themortar lines as well the stiffness incompatibility ofthe two materials (composites/bricks). The behavior waslinear from a load level of about 39.76 kPa (830.45 psf)and up to failure as shown in Fig. 9. From this figure,one can see that both LVDT 3 and LVDT 4 reading wereidentical (Refer to Fig. 6 for gages’ locations.) which con-firms the accuracy of the deflection measurements andthe symmetrically applied loading condition. The ultimatelaminate strain at failure was 0.71% as shown in Fig. 10.This strain is 57% of the rupture strain of the carbon/epoxycomposite system (eu = 1.25%).

The ultimate load capacity of this specimen was74.43 kPa (1554 psf) which is 12 times the capacity of theas-built specimen. The associated mid-height out-of-planedeflection was 87 mm (3.422 in.). The ultimate failure modewas a combination of a compressive failure of the bricksfollowed by a cohesive failure of the carbon epoxy lami-nates as shown in Figs. 11 and 12, respectively.

4.10. Cyclic and ultimate behavior of [0�/90�]1 carbon/epoxy

strengthened unreinforced brick wall specimen (WC-RET-

090)

There were two main reasons behind the selection of thisfiber architecture. Although, the wall resists out-of-planeuniform pressure loading in one-way action (in this study,

walls were supported at only two parallel sides as shown inFig. 6), in the common field application both in-plane andout-of-plane reinforcements for an unreinforced wall aretypically required. For this reason, it was decided to usethis cross-ply [0�/90�] lay-up in order to: (i) investigatethe one-way, out-of-plane flexural response of the brickwall specimen when strengthened with one single ‘‘effec-tive’’ laminate of carbon/epoxy system, and (ii) to evaluatethe effect of the presence of the 90� laminate to both theservice performance and ultimate failure mode of brickwalls. From first glance, one may expect that the 90� lam-inate may not contribute to both the structural perfor-mance of the one-way load-resisting path. However, testresults obtained from this study indicated that this schememay alter the ultimate mode of failure by suppressing theexpected longitudinal separation of the ‘‘effective’’ lami-nates (in the 0�-direction) and force these laminate to work

Fig. 12. Ultimate failure mode of the unidirectional laminated (only on one side) red brick wall specimen.


together. The contribution of the 90�-ply was shown to beeffective and is considered to be a contributing factor indetermining the ultimate failure mode of this specimen.The cross-ply actually acts as a cross-support which forcesthe 0-degree laminated strips to deform as a single widelaminate. This prevents the 0-degree separation betweenthe unidirectional laminates that was observed in specimenstrengthened with two plies of unidirectional carbon/epoxycomposites described earlier (refer to Fig. 12).

This specimen achieved the highest performance, withrespect to the resulting ductile failure that was observedfor this specimen. Test results indicated that, even with a

Fig. 13. Cyclic performance of the cross-ply carbon/epox

single ‘‘effective’’ laminate of carbon/epoxy system, anappreciable increase in the wall strength was achieved.For example, the ultimate capacity of this specimen was60.58 kPa (1265 psf) compared to 6.52 kPa (136.2 psf),and 74.43 kPa (1554 psf) of as-built wall, and the wall spec-imen strengthened with two unidirectional layers carbon/epoxy system, respectively. This ultimate capacity is about81% of specimen WC-RET-02 and 9.22 times the strengthof the as-built specimen. The maximum mid-height deflec-tion at failure was 98 mm (3.859 in.).

Fig. 13 shows the pressure/deflection behavior of thisspecimen. The strain at failure of this specimen was 1%,

y strengthened brick wall specimen (WC-RET-090).

Fig. 14. Pressure/strain curves for the cross-ply carbon/epoxy strengthened brick wall specimen (WC-RET-090).


which is about 83% of the measured rupture strain of thecarbon/epoxy system. This is another indication of themerit of using the cross-ply is that it succeeded in increasingthe efficiency of the external FRP composite reinforcementsystem (the ultimate strain is 16.9% higher than specimenWC-RET-02). Fig. 14 shows that pressure/strain curvesfor the composite laminate of specimen WC-RET-090.

The ultimate failure mode was similar to the two-plyunidirectional wall specimen, which is a combination of

Fig. 15. Ultimate failure mode of the cross-ply ‘‘effective’’ laminate on oneside of brick wall specimen (WC-RET-090).

compression failure of the bricks and a cohesive failure ofthe composite laminates. However, and due to the suppress-ing action of the cross-ply, the failure was more ductile andno longitudinal separation between the laminatedstrips (parallel to the unsupported free edges) was observed.Figs. 15 and 16 show the ultimate failure modes of thisspecimen.

Fig. 16. Compression failure of the red bricks at ultimate load of cross-plycarbon/epoxy strengthened wall specimen WC-RET-090.


4.11. Cyclic and ultimate behavior of [0�]3 E-glass/epoxy

strengthened brick wall specimen (WE-RET-02)

For this wall specimen, a total of three unidirectionalplies of E-glass/epoxy were applied covering the entiretension surface of the brick wall specimen. The unidirec-tional fibers were aligned parallel to the free-edges of theunreinforced wall specimen (refer to Fig. 6). The behaviorof this specimen was similar to the retrofitted specimenWC-RET-02. Figs. 17 and 18 show the loading/unloadingpressure/deflection behavior of this specimen at service

loading conditions (In this paper, service load is defined

Fig. 17. Pressure/deflection curves of the unidirectional [0]3 E-glass/epoxy stre02).

0.0000 0.0005 0.0010

Strain

0

100

200

300

400

500

600

700

800

Pre

ssu

re (

psf

)

E-glass/Epoxy - Ser

Fig. 18. Pressure/strain curves unidirectional [0]3 E-glass/epoxy strengthened

as 40% of the ultimate capacity of the wall specimen whichequals to 30.12 kPa (628 psf). At this pressure load level,fine hair cracks were observed at three locations.). For theultimate load tests, specimens were subjected to a constantramp load up to failure. The maximum central deflectionat failure was 91.7 mm (3.61200). The ultimate capacity ofthis wall was 75.29 kPa (1572 psf) as shown in Fig. 19. Thiscapacity is 11.54 times the out-of-plane ultimate capacity ofthe control, unstrengthened wall specimen. The strain atfailure of the mid-height surface laminate was 1.07% (referto Fig. 20), which translates to about 48% of the experimen-tally obtained rupture strain of the E-glass/epoxy FRP

ngthened unreinforced brick wall specimen at low cyclic load (WE-RET-

0.0015 0.00200

500

1000

1500

2000

2500

3000

3500

Pre

ssu

re (

kPa)

SG-1SG-3SG-4SG-5SG-6SG-7SG-8

vice Load

unreinforced brick wall specimen at low cyclic load level (WE-RET-02).

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0Deflection (inch)

0

500

1000

1500

2000

Pre

ssu

re (

psf

)

0 10 20 30 40 50 60 70 80 90 100

Deflection (mm)

0

2000

4000

6000

8000

Pre

ssu

re (

kPa)

E-glass/Epoxy - Ultimate Load

Deflection-1Deflection-2Deflection-3Deflection-4Deflection-5

Fig. 19. Pressure/deflection curves unidirectional [0]3 E-glass/epoxy strengthened brick wall test specimen at high load levels (WE-RET-02).

0.000 0.002 0.004 0.006 0.008 0.010 0.012

Strain

0

500

1000

1500

2000

Pre

ssu

re (

psf

)

0

2000

4000

6000

8000

Pre

ssu

re (

kPa)

SG-1SG-3SG-4SG-5SG-6SG-7SG-8

E-glass/Epoxy - Ultimate Load

Fig. 20. Pressure/strain curves unidirectional [0]3 E-glass/epoxy strengthened unreinforced wall specimen at high load levels (WE-RET-02).


composite system. Although the E-glass/epoxy inherentlyexhibits lower stiffness properties compared to carbon/epoxy-type laminates, the average stiffness increase in thelinear range of this wall, as compared to the as-built spec-imen, was about 60% higher. The failure mode was differ-ent from the carbon/epoxy specimens. For this specimen(in addition to the combined mode of failure of compres-sive failure of the bricks followed by cohesive failure ofthe E-glass/epoxy laminate that was observed in all preced-ing tests), a tensile fracture of the laminate did occur asshown in Figs. 21 and 22. This can be attributed to the rel-atively lower tensile strength of E-glass/epoxy laminates ascompared to carbon/epoxy laminates.

5. Theoretical modeling

An analytical model was developed to predict the ulti-mate load of the retrofitted specimens. The analyticalmodel used in this study is based on simple section analysisprocedures similar to that used for analyzing reinforcedconcrete beams. However, new parameters have been usedfor masonry wall based on available experimental data (e.g.[21,22]). The first part of the analysis is to define the mate-rial properties. The stress–strain curve for brick-mortarblocks under compression is as shown in Fig. 23. The curveconsists of two distinct regions: a parabolic relationship upto the maximum compressive strength, f 0m, and a linear

Fig. 21. Large mid-height deflection of unidirectional [0]3 E-glass/epoxystrengthened brick wall specimen (WE-RET-02).

570 A.S. Mosallam / Composites: P

descending branch up to the ultimate compressive strain,emu. The first region of the stress–strain curve is assumedpolynomial in the form:

fm ¼ Aenm þ Bem þ C ð1Þ

The four unknowns in Eq. (1) are determined from the fol-lowing boundary conditions:

ðiÞ f m ¼ 0:0 at em ¼ 0:0

ðiiÞ f m ¼ f 0m at em ¼ emo

ðiiiÞ dfm=dem ¼ Em at em ¼ 0:0

ðivÞ dfm=dem ¼ 0:0 at em ¼ emo

ð2Þ

The equations of the stress–strain curve have been deter-mined to be:

� For 0 < em < emo:

fc ¼ Emem 1� 1

nem

emo

� �n�1" #

ð3Þ

n ¼ Ememo

Ememo � f 0mð4Þ

� For emo < em < emu:

fm ¼ f 0m � Edðem � emoÞ ð5Þ

Ed ¼0:5f 0m

ðemu � emoÞð6Þ

The parameters of the previous stress–strain curve are gi-ven as [21,22]:f 0m ¼ 31 MPa (4500 psi); emo = 0.002; emu = 0.0035; Em =19.28 GPa (2.8 · 106 psi); and fmf ¼ 0:5f 0m.

Based on experimental evidences, it is appropriate toassume the FRP composites to be linear elastic up to fail-ure as shown in Fig. 24. The properties for both carbon/epoxy and E-glass/epoxy systems used in this study are pre-sented in Table 2.

The section analysis procedures adopted in this studyare based on the following assumptions:

� Tensile strength of the brick-mortar blocks is ignored.� Tensile resistance of the FRP laminates can be neglected

in the transverse direction.� The area of the FRP laminates is enough for the failure

of the specimen to be due to masonry crushing ratherthan fiber fracture.� Plane section before bending remains plane after bend-

ing, and hence a linear strain distribution can beassumed along the section.

In order to perform the section analysis, it is necessaryto develop parameters describing the equivalent rectangu-lar stress block shown in Fig. 25. These parameters canbe determined by integrating the stress–strain curve forbrick-mortar blocks in compression, as follows:

b ¼ 2 1�R emu

0fmem dem

emu

R emu

0fm dem

" #¼ 0:88 ð7Þ

c ¼R emu fm dem

bf 0memu¼ 0:8 ð8Þ

art B 38 (2007) 559–574

6. Numerical example

In the following example, the proposed analyticalapproach is used to predict the out-of-plane capacity of ared brick wall strengthened with two unidirectional pliesof carbon/epoxy composite system. Dimensions, boundaryconditions, loading pattern, composite lay-up and proper-ties are identical to those used for wall specimen WC-RET-02 evaluated in this study (refer to Fig. 6). Thefollowing are the step-by-step analytical procedures forpredicting the flexural capacity of this wall.

6.1. Strengthened wall information

� Wall dimensions: 2.64 m · 2.64 m (10400 · 10400).� Brick wall thickness: 101.6 mm (400).

Fig. 22. Ultimate combined failure mode of unidirectional [0]3 E-glass/epoxy strengthened unreinforced brick wall specimen (WE-RET-02).

Compressive Strain

Com

pres

sive

Str

ess

dE

mo

mf

fm

/

fmf

m

mum

Fig. 23. Stress–strain model for brick-mortar blocks in compression. Fig. 24. Stress–strain model for typical FRP laminates.

Table 2Properties of composite materials

Composite system Ply thickness,tp, mm [in.]

On-axis tensile modulus,Ej, GPa [·106 psi]

On-axis tensile strength,fju, MPa [·103 psi]

On-axis tensile ultimatestrain, eu (%)

Unidirectionalcarbon/epoxy

0.584 [0.023] 103.4 [15.06] 1245.83 [180.7] 1.25

UnidirectionalE-glass/epoxy

1.143 [0.045] 18.47 [2.679] 424.70 [61.6] 2.20


� Composite system: Carbon/epoxy (CFRP) wet lay-upsystem (refer to Table 2).

� CFRP ply unit thickness = tp = 0.584 mm (0.0200).� Number of unidirectional plies = n = 2.

Table 3Summary of theoretical analysis for retrofitted specimens

Specimenconfiguration

Experimentalmaximumload,pExperimental

ultimate ,kPa (psf)

Theoreticalmaximumload, pTheoretical

ultimate ,kPa (psf)

pTheorticalultimate

pExperimentalultimate

(0�)2 Carbon/epoxy

74.4 (1554) 67.0 (1397) 0.90

(0�/90�)1

Carbon/epoxy60.6 (1265) 53.0 (1107) 0.87

Fig. 25. Stress and strain distribution for section analysis.


� Total thickness of CFRP laminate = tj = tp · n.� Ultimate CFRP on-axis tensile strain = ej = 1.25%

(refer to Table 2).� CFRP on-axis tensile modulus = Ej = 103.4 GPa

(15.06 Msi).� CFRP on-axis tensile strength = fju = 1245.83 MPa

(180.7 ksi).� Boundary conditions: Simply supported on the two

opposing sides (sides perpendicular to fiber directionsas shown in Fig. 6).

(0�)3 E-glass/epoxy

75.3 (1572) 55.0 (1148) 0.73

6.2. Analytical procedures

1. Calculation of neutral axis depth (refer to Fig. 25):h = 400 + 0.04600/2 = 4.023 in. (102.2 mm),
a = bc = 0.88c,C ¼ cf 0mab = 0.8 · 3.629 ksi · a · 10400 = 301.93a,T = Ajfj = AjEjej = 10400 ·(2 · 0.02300) · 15,060 · ej = 72,047ej.
From strain compatibility:
ej = 0.0035(h/c � 1) = 0.01239/a � 0.0035,T = 892,663a � 252.17.
From equilibrium:
C = T or301.93a = T = 892,663a � 252.17from which:a = 1.3500 (34.34 mm).
2. Check of CFRP allowable strain:
ej = 0.00567,eju = fju/Ej = 0.0125 > ej ok. Thus, failure is due tomasonry crushing rather than fiber fracture.
3. Calculation of ultimate moment and maximum load:
Mu = ultimate flexural capacity = cf 0mabðh� a=2Þ ¼Ajfjuðh� a=2Þ, orMu = 1366.22 kip-in. (154.36 N m),wu = ultimate unit load = 8Mu/L2 = 1.01 kip/in.(177.055 kN/m),Pu = ultimate load capacity = 1.011 kip/in. · 10400 =105.14 kip (467.44 kN),pu = ultimate uniform pressure = 105.14 kip s/(10400)2 = 1399.2 psf (66.94 kPa).
From this simple analysis, the predicted ultimate pres-sure was slightly less than the ultimate pressure obtained

from the actual test ðpExpu ¼ 1554 psf=74:43 kPaÞ. However,

the 10% deviation from the experimental ultimate pressureis in the conservative side which is desirable.

Similar procedures were used to predict the other twospecimens evaluated in this study. Table 3 presents a com-parison between the predicted and experimentally obtainedultimate pressure values for the three strengthened wallsevaluated in this study.

7. Conclusions and summary of results

The results of this study confirmed the effectiveness ofboth the E-glass/epoxy and carbon/epoxy FRP compositestrengthening systems in upgrading the out-of-plane flex-ural structural performance of unreinforced brick walls.The strength gains resulted from adding the composite sys-tems was appreciable as shown in Figs. 26–28. The failuremodes of specimens were due to a combination of compres-sion failure of red bricks followed by a cohesive failure asdescribed earlier. The coupling effect of in-plane and out-of-plane reinforcements is shown to have positive effectson both the out-of-plane capacity and the ductility of theretrofitted wall specimen. Furthermore, due to the sup-pressing action provided by the orthogonal ply (appliedin the direction parallel to the support), end-of-strip longi-tudinal separation (parallel to unsupported free edges asshown in Fig. 12) observed in unidirectional reinforcedwall (specimen WC-RET-02), was eliminated. Based onthese observations, it is recommended that in order toachieve optimal out-of-plane performance of strengthenedbrick walls, cross-ply lamination schedule should be used.This will be satisfied in the case where both out-of-plane

Fig. 26. Ultimate capacity comparison for all wall specimens.

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

Ulti

mat

e M

id-H

eigh

t Def

elct

ion

(inc

h)

As-Built E-glass (0) Carbon (0) Carbon (0/90)123

Fig. 27. Comparison of mid-height deflection at ultimate for all wall specimens.


and in-plane composite reinforcements are provided. How-ever, if only out-of-plane reinforcement is required, it isrecommended to add a lighter orthogonal ply (about 10–15%) of the major flexural composite reinforcementdemand. Additional research is needed in order to accu-rately determine the optimum percentage of orthogonalpolymer composites reinforcements.

The simple analytical approach developed in this studywas successful in predicting the experimental ultimateout-of-plane flexural behavior of the walls. However, theaccuracy of predicting the experimental results varied (referto Table 3). For example, the highest correlation betweenanalytical and experimental results was achieved for thewall specimen strengthened with unidirectional carbon/epoxy composites. This can be attributed to the straight-ness of the carbon fabrics, used in this study, resulting in

a better representation of the composite mechanical prop-erties that were used in the analytical modeling. On theother hand, the analytical results obtained for wallsstrengthened with cross-ply carbon/epoxy and E-glass/epoxy composite laminates were relatively less accurate ascompared to the experimental results. The possible reasonbehind this deviation for the cross-ply carbon/epoxystrengthened wall is the fact that the effect of the orthogo-nal ply was ignored in the analysis. As shown in Table 3,the analytical results for the E-glass/epoxy strengthenedwall was about 30% less than the capacity observed inthe test. This can be attributed to ignoring the kinkingeffect and fiber misalignment of the E-glass fabrics usedin this study. More research is needed to develop analyticalmodels capable of including both the effect of the orthogo-nal lamination as well as kinking effects.

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

Max

imum

Lam

inat

e T

ensi

le s

trai

n (

)

As-Built E-glass (0)3 Carbon (0)2 Carbon (0/90)1

Fig. 28. Comparison between mid-span tensile strains at ultimate load for all wall specimens.


Acknowledgement

The FRP materials used in the study was provided byEdge Structural Composites Inc.

References

[1] Mosallam AS. Composites in construction. Handbook of materialsselection. NY, USA: John Wiley Publishing Co.; 2002, 53 p [Chap-ter 45].

[2] Schwegler G. Verstarken von Mauerwerk mit Faserverbundwerkst-offen in seismisch gefahrdeten Zonen, Dissertation ETH Zurich No.10672, Published by Eidgenossische Materialprufungs- und Fors-chungsanstalt, CH-8600 Dubendorf, as EMPA-Report Nr. 229, 1994.

[3] Schwegler G. Verstarkung von Mauerwerkbauten mit CFK. Lamel-len, Sonderdruck aus, Schweizer Ingenieur und Architekten, Nr. 44,1996.

[4] Gilstrap JM, Dolan CW. Out-of-plane bending of FRP-reinforcedmasonry walls. Compos Sci Technol J 1998;58:1277–84.

[5] Albert LM, Elwi AE, Cheng JJ. Strengthening of unreinforcedmasonry walls using FRPs. ASCE J Compos Constr 2001;5(2):76–84.

[6] Ganz HR, Kiss RM, Jai J, Kollar LP, Krawinkler H. Masonrystrengthened with fiber reinforced plastics subjected to combinedbending and compression. Part I – Model. J Compos Mater 2001.

[7] Kiss RM, Kollar LP, Jai J, Krawinkler H. Masonry strengthenedwith FRP subjected to combined bending and compression. Part II:Test results and model predictions. J Compos Mater 2002;36(9):1049–62.

[8] Velazquez-Dimas J, Ehsani MR, Saadatmanesh H. Cyclic behaviorof retrofitted URM wall. In: Proceedings of the 2nd int conf oncomposite in infrastructure (ICCI), Tucson, Arizona, 1998.

[9] Kuzik MD, Elwi AE, Cheng JJ. Cyclic flexure tests of masonry wallsreinforced with glass fiber reinforced polymer sheets. ASCE JCompos Constr 2003;7(1):20–30.

[10] Tan KH, Patoary MKH. Strengthening of masonry walls against out-of-plane loads using fiber-reinforced polymer reinforcement. ASCE JCompos Constr 2004;8(1):79–87.

[11] Al-Salloum YA, Almusallam TH. Load capacity of concrete masonryblock walls strengthened with epoxy-bonded GFRP sheets. J ComposMater 2005;39(19):1719–44.

[12] Turco V, Secondin S, Morbin A, Valluzzi MR, Modena C. Flexuraland shear strengthening of un-reinforced masonry with FRP bars.Compos Sci Technol 2006;66:289–96.

[13] El-Dakhakhni WW, Hamid AA, Hakam ZHR, Elgaaly M. Hazardmitigation and strengthening of unreinforced masonry walls usingcomposites. Compos Struct 2006;73:458–77.

[14] Hamoush SA, McGinley MW, Mlakar P, Scott D, Murray K. Out-of-plane strengthening of masonry walls with reinforced composites.ASCE J Compos Constr 2001;5(3):139–45.

[15] Hamilton III HR, Dolan CW. Flexural capacity of glass FRPstrengthened concrete masonry walls. ASCE J Compos Constr2001;5(3):170–8.

[16] Laursen PT, Seible F, Hegemier GA. Seismic retrofit and repair ofreinforced concrete with carbon overlays. Report no. SSRP-95101,University of California, San Diego, 1995.

[17] Ghobarah A, Galal K. Out-of-plane strengthening of unreinforcedmasonry walls with openings. ASCE J Compos Constr2004;8(4):298–305.

[18] Korany Y, Drysdale R. Rehabilitation of masonry walls usingunobtrusive FRP techniques for enhanced out-of-plane seismicresistance. J Compos Constr 2006;10(3):213–22.

[19] International Code Council-Evaluation Service (ICC-ES). Acceptancecriteria for concrete and reinforced and unreinforced masonrystrengthening using fiber-reinforced polymer (FRP) composite systems(AC125), July 2003 [http://icc-es.org/criteria/pdf_files/ac125.pdf].

[20] International Code Council-Evaluation Service (ICC-ES). Accep-tance criteria for test reports (AC85), July 2003 [http://icc-es.org/criteria/pdf_files/ac85.pdf].

[21] Fattal SG, Gattaneo LE. Structure performance of masonry wallsunder compression and flexure, National Bureau of Standards(NIST), Washington, DC, 1976.

[22] Triantafillou TC. Strengthening of masonry structures using epoxy-bonded FRP laminates. ASCE J Compos Constr 1998;2(2).

http://icc-es.org/criteria/pdf_files/ac125.pdf



Out of Plane Flexural Behavior of Brick Wall_2006

Documents

Transcript of Out of Plane Flexural Behavior of Brick Wall_2006