In-Plane Strengthening of Unreinforced Concrete MasonryWallettes Using ECC Shotcrete

Yi-Wei Lin1; David Biggs, Dist.M.ASCE2; Liam Wotherspoon3; and Jason M. Ingham, M.ASCE4

Abstract: An experimental program was conducted to investigate the viability of using engineered cementitious composite (ECC) shotcreteas in-plane shear reinforcement for concrete masonry wallettes. Twenty-six mortared or dry-stacked ungrouted and unreinforced concretemasonry wallettes were strengthened through the application of an ECC shotcrete mix, applied either by spraying with a shotcrete machine orby hand trowelling. To determine the effect of applicator experience, ECC was applied by both a professional plasterer and amateur plasterers,with and without supervision. The in-plane shear strength of the strengthened wallettes increased between 340 and 3,471%, depending on theECC application method. The variability of the strength increase was smaller for wallettes that had mortared joints and an ECC overlaytrowelled by a professional plasterer. A set of design equations was also developed to determine the shear strength of a strengthened wallette.The design displacement ductility can be conservatively set equal to 1.9 because of the high variability in the experimentally obtainedductility values. DOI: 10.1061/(ASCE)ST.1943-541X.0001004. © 2014 American Society of Civil Engineers.

Author keywords: Fiber-reinforced materials; Shotcrete; Masonry; Earthquake; Shear strength; Concrete and masonry structures.

Introduction

The need for earthquake-resistant structures is widely recognized indeveloped countries that are prone to earthquakes, and require-ments for the design and strengthening of structures against earth-quake forces are well documented in numerous earthquake designstandards [European Committee for Standardization (CEN) 2005a;Building Society Safety Council (BSSC) 2000; Ministry ofConstruction (MOC) 2004]. However, it is typically the buildingstock in developing countries that is most vulnerable to earthquakesbecause of a combination of factors, including an absence or lackof enforcement of building code requirements, poor availability ofquality engineering construction materials (such as reinforcingsteel), poor availability of advanced equipment that is often neces-sary for effective construction of earthquake-resistant structures,and scarcity of well-trained construction personnel. Haiti is a de-veloping country with the aforementioned building stocks that arevulnerable to earthquake loadings. On January 12, 2010, a momentmagnitude scale (Mw) 7.0 earthquake occurred in the town ofLeogane, 25 km away from Haiti’s capital city of Port-au-Prince.Preliminary damage surveys conducted by Eberhard et al. (2010)reported that in Port-au-Prince and Leogane, respectively, 28and 62% of the buildings collapsed, with an additional 31–33%

suffering structural damage. This significant damage (Fig. 1) wasprimarily a result of the absence of local building codes andregulations, and resulted in an estimated 316,000 fatalities (CBCNews 2012). Further investigations by Mix et al. (2011) reportedthat a significant proportion of building damage was attributableto concrete masonry buildings constructed with no provision forearthquake forces. This observation was the specific motivationfor the selection of ungrouted unreinforced concrete masonry wallsas a structural element requiring earthquake strengthening, al-though the results are relevant to countries worldwide in which thisform of construction exists.

To improve the earthquake resistance of a structure, a keyobjective is to improve the in-plane response of structural walls,because these structural elements are often critical for maintainingstructure integrity (Gambarotta and Lagomarsino 1997). Severalexternally bonded overlay materials are available to improve thein-plane earthquake resistance of masonry walls, with textilereinforced mortar (TRM) used to strengthen brick masonry wallelements (Papanicolaou et al. 2007; Ismail and Ingham 2013) andtuff masonry wallettes (Faella et al. 2010). Fiber-reinforced poly-mer (FRP) sheets were used by Mahmood and Ingham (2011) andby Valluzzi et al. (2002), and shotcreting was used by Kahn (1984)to strengthen clay brick masonry wall panels. Aldea et al. (2005)used a cement based plastering system to increase the in-planestrength of concrete masonry walls. Another type of such an over-lay system is engineered cementitious composite (ECC) shotcrete,which is a cement composite reinforced with synthetic fiber.When loaded in tension, ECC exhibits a strain-hardening character-istic through the process of developing multiple microcracks,with stresses carried by fibers that bridge the cracks. The strain-hardening characteristic of ECC makes it an ideal material forearthquake strengthening because ECC can add both strengthand ductility to an existing structure.

Improving the displacement ductility capacity of a structureresults in an increased amount of energy dissipation capabilityduring an earthquake, because the structure is able to maintainits load-carrying capacity while undergoing inelastic deformations.Consequently, many earthquake design standards [New ZealandStandards 2006; CEN 2005a] allow a reduction in design

1Ph.D. Candidate, Dept. of Civil and Environmental Engineering, Univ.of Auckland, 3 Grafton Rd., Auckland 1010, New Zealand (correspondingauthor). E-mail: ylin126@aucklanduni.ac.nz

2Principal, Biggs Consulting Engineering, 740 Hoosick Rd., Troy, NY12180. E-mail: biggsconsulting@att.net

3EQC Research Fellow, Dept. of Civil and Environmental Engineering,Univ. of Auckland, 3 Grafton Rd., Auckland 1010, New Zealand. E-mail:l.wotherspoon@auckland.ac.nz

4Professor, Dept. of Civil and Environmental Engineering, Univ. ofAuckland, 3 Grafton Rd., Auckland 1010, New Zealand. E-mail:j.ingham@auckland.ac.nz

Note. This manuscript was submitted on February 24, 2013; approvedon November 13, 2013; published online on May 19, 2014. Discussionperiod open until October 19, 2014; separate discussions must be submittedfor individual papers. This paper is part of the Journal of Structural En-gineering, © ASCE, ISSN 0733-9445/04014081(13)/$25.00.

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earthquake acceleration for structures that are expected to behavein a ductile manner during an earthquake. Improvement in theductility capacity of ECC strengthened masonry elements has beenpreviously demonstrated in studies reported by Kyriakides andBillington (2008), Mechtcherine et al. (2011), and Lin et al.(2010). An advantage of the ECC overlay system is the capabilityreported by Mechtcherine et al. (2011) of bonding sufficiently withthe masonry surface without the need for bonding agents, which arenecessary for FRP overlay systems. In addition, the tensile strengthand strain-hardening characteristics reported by Li et al. (2001)allow for the reduction or omission of steel reinforcement that isneeded in traditional shotcrete. Because ECC does not contain aseparate mesh reinforcement, such as that used in TRM, anchorsthat are typically inserted to masonry surfaces to secure the meshin place (Ismail and Ingham 2013) can also be omitted, reducing thelevel of formwork required during application. The objective of thisstudy was to investigate the effectiveness of an ECC shotcrete mixas an in-plane lateral strengthening system for ungrouted concretemasonry wallettes, because there is currently no reported researchon this topic.

Experimental Program

The effectiveness of using an ECC shotcrete mix as in-planereinforcement for concrete masonry was investigated by usingeither mortared wallettes having dimensions of 1,180 mmheight ×1,200 mm length or dry-stacked wallettes having dimensions of1,120 mmheight × 1,170 mm length, with thickness of 140 mmfor both wallette types. Wallettes are laboratory specimensdesigned such that they represent a section of a masonry wall ina building, using constituent materials that are nominally identicalto those adopted in actual building construction. Investigation ofmasonry in-plane behavior using wallette specimens has beenwidely conducted in numerous studies, such as those conductedby Tumialan et al. (2001), Corradi et al. (2003), Ismail et al.(2011), Dizhur et al. (2013), and Dizhur and Ingham (2013).The dimensions of the concrete masonry units used to constructthe wallettes are outlined in Fig. 2. ASTM-C270 (2012a) type Smortar was used, with a mix ratio of two parts cement to one partlime to nine parts sand (by volume). The compressive strengths ofthe block (f 0

c), mortar (f 0j), and masonry (f 0

m) were determined byusing ASTM-C67 (ASTM 2001a), ASTM-C109 (ASTM 1999),and ASTM-C1314 (ASTM 2001b), respectively, and are detailedin Table 1, along with the ECC material tensile (f 0

yECC) andcompressive (f 0

ECC) strengths. Fig. 3 shows the idealized average

ECC tensile response. All wallettes were ungrouted because this isa plausible scenario for concrete masonry construction in manydeveloping countries, and is also the expected construction typeto be adopted in the reconstruction of Haiti. The lateral compressivestrength (f 0

lc) of the concrete masonry units (load applied along thelongest dimension of the block) is also summarized in Table 1.

Twenty-six wallette specimens were constructed to investigatethe following primary parameters:

0

1

2

3

4

5

0 0.1 0.2 0.3 0.4

Tens

ile s

tres

s (M

Pa)

Tensile strain (%)

Idealised average ECC tensile response

Fig. 3. Idealized average ECC tensile response

Table 1. Masonry and ECC Material Properties Used in this Study

Concrete masonry ECC

Vertical f 0c

(N=mm2)Lateral f 0

lc(N=mm2)

f 0j

(N=mm2)f 0m

(N=mm2)

Meanf 0yECC

(N=mm2)f 0ECC

(N=mm2)

18.0 10.7 4.4 9.4 2.7a 40b

aValue obtained from uniaxial tensile testing of 50 cylindrical specimens.Additional test details are reported by Lin (2013).bValue reported from ECC supplier and adopted in this study. Additionalcompressive tests conducted in accordance with ASTM (ASTM 2012b) oncylinder specimens (100 mm high and 50 mm diameter) resulted in anaverage compressive strength of 39.2 MPa, which is 98% of theadopted value.

Fig. 1. Example of building damage following the 2010 Haitiearthquake (image by Andrew Wright, Tonetti Associates Architects,reprinted with permission)

(a) (b)

Fig. 2. Dimensions of the concrete masonry units used for walletteconstruction: (a) top view; (b) cross section

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• Strength increase between as-built and strengthened wallettes;• Difference in characteristics of strengthened wallettes and

variability between mortared and dry-stacked wallettes;• Influence of shotcrete application methods (either sprayed or

hand trowelled); and• Difference in strengthened wallette characteristics between

those obtained when using a professional plasterer and thoseobtained when using an amateur plasterer following an identicaltrowelling procedure, but on occasion requiring additionalpatching to achieve a uniform thickness of ECC. This parameterwas investigated because of the potential shortage of skilledlaborers in developing countries such as Haiti; it is expected thatin such construction projects there will be one skilled laborerinstructing several amateur laborers.In this study, a professional plasterer was defined as a person

whose primary trade is plastering and who has been certified bythe supplier of the ECC mix material as an approved applicator.Conversely, an amateur plasterer was defined as a person whohas no previous experience with plastering and is merelymimicking the procedures of the professional plasterer. A supplierapproved applicator and shotcrete machine was used when the ECCwas applied by spraying.

The ECC mix proportions used in this study were developed fora shotcrete mix and are summarized in Table 2, with all ECC usedin this study derived from a single mix. These mix proportions aresimilar (in terms of the cement–sand–fly ash ratio, but with minordifferences in additive dosage to account for local temperaturevariations) to those used by previous researchers (Kim et al.2004; Lin et al. 2010). Preliminary investigations using thisECC mix indicated that a thickness of 10–15 mm in a single spraycould be achieved; therefore, any specified shotcrete thickness inexcess of 10 mm was applied successively in overlays of 10 mm,after the previously applied layers had hardened sufficiently. Nodebonding between successive ECC layers was observed for anytest specimen.

An overview of the full testing configuration is provided inTable 3, with the wallette configurations designated as SX-A-B-C,where:• X represents the thickness of the ECC overlay applied, in mm;• A represents the as-built characteristics, which were DS if the

wallette was dry-stacked or M if the wallette had mortar in thejoints;

• B represents the application method of the ECC shotcretemix, which was either sprayed on by using a shotcrete machine(S), hand trowelled by a professional plasterer (professionaltrowelled: PT) or hand trowelled by an amateur (amateurtrowelled: AT); and

• C represents the sample number of the tested configuration,ranging between 1 and 4.If the application method was trowelled by an amateur, it is also

stated in parentheses whether the amateur was supervised (S) orunsupervised (US). An example of the designated code for awallette is S20-DS-AT(US)-3, representing 20 mm of ECC overlayhand trowelled onto a dry-stacked wallette by an amateur

without supervision, which was the third wallette tested in thisconfiguration.

The wallettes were constructed and tested in two stages in thisstudy to investigate a range of parameters. The first stage of testingfocused on the influence of the unstrengthened wallette type (mor-tared or dry-stacked) on the effectiveness of strengthening usingeither sprayed ECC or trowelled ECC overlay applied by an unsu-pervised amateur plasterer. The second stage of testing investigatedthe effectiveness of strengthening when using ECC that was ap-plied by either a professional plasterer or an amateur plasterer undersupervision. In the second stage, all wallettes were dry-stacked.

ECC Application Procedures

Following wallette construction, those specimens having mortaredbed and head joints were air cured for 14 days. Prior to applying theECC shotcrete mix, the wallette surfaces were brushed to removeany loose material and polystyrene planks were nailed to the sidesof the wallettes to serve as an indicator of the thickness of ECC thathad been applied. The ECC shotcrete mix was supplied in baggedform, and if sprayed, was added to a two stage mixer (Fig. 4) inwhich the ECC material was mixed dry in the first mixing cham-ber, then mixed again with added water in the second mixingchamber. The mixed ECC was pumped through a hose andsprayed onto the masonry wallettes. If the application methodwas trowelling, a generic concrete mixer was used instead ofthe two-stage mixer to mix the prebagged ECC material, andwater was gradually added to the dry material during the mixingprocess. Three mixed (with water added) batches were producedfrom a single batch of dry mix (bagged) material for the applica-tion methods under investigation: one batch for the shotcrete ap-plication and one batch for each of the trowelled series (Stages 1and 2 indicated in Table 3). For the two batches used in thetrowelling application, identical dry mix material, mixing proce-dures, and water content was used to ensure consistent mechani-cal properties. The use of a generic concrete mixer was adopted toreplicate the likely mixing equipment available in developingcountries such as Haiti, so that the ECC mix that was producedhad representative material properties instead of the moreidealistic properties that would have resulted from the use ofspecialized mixing equipment. Because the adopted ECC mixwas originally developed for the two-stage mixer, the mixedproduct produced by the two-stage mixer is likely to have betteruniformity in fiber and aggregate dispersion than the productfrom the generic concrete mixer. However, to quantify the specificinfluence of different mixer types would have required a

Table 2. ECC Mix Proportions Used in this Study

Materials kg=m3

Sand (beach sand) 640Cement (general purpose portland cement) 800Fly ash (type F) 240Water 374Fiber (polyvinyl acetate) 26Additives (superplasticizer and stablizer) 0.3

Table 3. Summary of Wallette Testing Configurations

Wallette configurationThickness of ECCoverlay (mm)

Number ofsamples

S0-DSa 0 3S20-DS-Sa 20 3S20-DS-AT(US)a 20 3S10-DS-PTb 10 4S10-DS-AT(S)b 10 4S0-Ma 0 3S20-M-Sa 20 3S20-M-AT(US)a 20 3aStage 1 test investigated the influence of wallette type (mortared or dry-stacked) and the influence of ECC application by spraying and amateurhand trowelling without supervision.bStage 2 test investigated the effect of hand trowelled ECC application by aprofessional plasterer and an amateur plasterer with professionalsupervision.

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dedicated study of ECC material properties based on extensivesample population, which was beyond the scope of the currentstudy. Nevertheless, because sprayed and trowelled applicationshave their corresponding mixer types, the experimental resultscaptured the overall influences of the two scenarios (ideal andrepresentative). For thicknesses in excess of 10 mm, ECC wasapplied in successive layers of 10 mm, with an interval of approx-imately 45 min provided between the applications of each succes-sive layer to allow the layer to harden. Each applied layer wastrowelled flat so that the successive layer of ECC was appliedto a flat surface. Once the total thickness was applied, theECC surface was hand trowelled flat to achieve a uniformECC thickness (including wallettes that had ECC applied viaspraying). Finally, the polystyrene planks were removed and aconstant water mist was applied to the wallette until the testingdate, which was 14 days after ECC application.

Test Setup

ASTM-E519 (ASTM 2002) is a testing standard adopted by manyresearchers (El-Dakhakhni et al. 2006; Prota et al. 2006; Petersenet al. 2010) for determining the shear strength of strengthenedmasonry wallettes. The testing procedure involves rotating thewallette by 45° and applying forces vertically through wallettecorners [Fig. 5(a)]. A modified version of the ASTM-E519(ASTM 2002) test was adopted in this study [Fig. 5(b)], in whichthe wallette was not rotated 45° to prevent damage that may occurfrom rotating the untested specimen. The modified test methodwas previously used by Corradi et al. (2002) and Brignola et al.(2008) to investigate the in situ in-plane responses of concrete,stone, and clay brick masonry wall sections. To apply the diago-nal compression force, two steel loading shows were placedon opposing corners of the wallette and connected by using ahigh-tensile steel rod. A load cell was positioned between thehydraulic actuator and the top steel channel to measure theapplied force. Two potentiometers spanned across the wallettecorners to measure the diagonal shortening and extension ofthe wallettes.

Experimental Results and Discussion

The diagonal compression force (P) applied to the wallette wasconverted into shear stress (τ ) by using Eq. (1):

τ ¼ P cosαt × 0.5ðH þ BÞ ð1Þ

where α = angle between the horizontal axis of the wallette and thediagonal load applied, equal to 44° in this study; t, H, and B =thickness, height, and length of the wallette, respectively. The dis-placements measured by the two potentiometers (spanning betweendiagonal corners of the wallettes) were converted into a percentageof horizontal drift by using Eq. (2) [adapted from Mahmood andIngham (2011) and Russell (2010)]:

δ ¼ ΔSþΔL2g

ðtanαþ cotαÞ ð2Þ

where ΔS = diagonal shortening and ΔL = diagonal elongation ofthe wallette, as measured by potentiometers placed parallel and

Fig. 5. Wallette test setup: (a) ASTM E519 (ASTM 2002) test setup;(b) modified test setup

Fig. 4. Two-stage shotcrete machine utilized in this study

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perpendicular to the diagonal compression load, respectively;g = gauge length measured by each potentiometer. Using theseequations, the shear stress-drift responses of the tested walletteswere established and are presented in Figs. 6 and 7. The maximumapplied diagonal force (Pmax), maximum horizontal shear force (V)and corresponding shear strength (τmax), ratio of strengthened toas-built wallette shear strength (τmax=τo), horizontal drift at yield(δy), horizontal drift at failure (δu), ductility (μ), shear modulus (G),and Young’s modulus (E) are summarized in Table 4. The averagevalues of τmax=τo, μ, and E for each wallette series are summarizedin Table 5.

General Response of Tested Wallettes

The measured shear stress drift responses of the dry-stackedas-built wallettes are shown in Fig. 6(a). All dry-stacked as-builtwallettes exhibited the bed-joint shear sliding damage pattern[Fig. 6(b)], which is the expected damage pattern when a wallettehas a high brick to mortar strength ratio (or in this study, had nomortar at all) and a low axial force. Dry-stacked wallette shearstrength is a result of the contribution from the unmortared bed-joint frictional resistance and the vertical component of the appliedload imposed on the wallette. The shear force resisted by the wal-lettes remained relatively constant until the potentiometers reachedtheir maximum elongation capacity and testing was terminated.Because the primary contribution to as-built dry-stacked walletteshear strength is from interfacial friction between the concrete ma-sonry blocks, it is likely that Wallette S0-DS-2 had blocks withsmoother surfaces, which resulted in lower shear strength thanthe other dry-stacked as-built wallettes.

The as-built mortared wallettes exhibited a brittle damage pat-tern [Fig. 6(c)], with an almost linear response up to the peak shearstress, at which point a stepwise diagonal crack [Fig. 6(d)] occurredthrough the mortared bed and head joints and the load decreasedsignificantly. This type of damage pattern is similar to that reported

by Dizhur and Ingham (2013) when testing clay brick masonrywallettes constructed by using low strength mortar. The peak shearstresses resisted by the as-built mortared wallettes were signifi-cantly higher than for the as-built dry-stacked wallettes, with400–1,800% higher peak stresses observed.

The majority of the strengthened wallettes developed a similarresponse, with shear stress increasing linearly up to the firstcracking stress, after which the stress level remained relativelyconstant [Figs. 7(a–e)] as cracking developed along the bed andhead joints, and the shear stress was transferred and distributed intothe ECC overlay. Several types of damage pattern were observed;the most common damage pattern was initial splitting tensilecracking of the concrete block webs caused by the diagonal com-pression load at the loading corners, followed by partial debondingof the ECC overlay. The S20-M-S series (20 mm sprayed ECC onmortared wallettes) exhibited a brittle response [Fig. 7(f)] in whichno debonding or cracking of the ECC layer was observed and onlylocalized crushing of the concrete masonry corners occurred. Thisbrittle response for the S20-M-S series of tests was attributed to thevisibly increased compaction of the ECC overlay (and thereforeincreased shear modulus) compared with the visual condition ofthe hand trowelled wallettes, because the shotcrete machine appliedthe ECC at an increased pressure compared to that associated withmanual application by the plasterers. The increased ECC overlaycompaction, in conjunction with the mortared bed and head joints,resulted in a reduced level of stress redistribution from the walletteinto the ECC overlay and led to localized failure of the walletteloaded corners. Wallette S10-DS-AT(S)-2 also developed nodebonding of the ECC overlay and instead had multiple shearcracks through the ECC overlay.

Fig. 8 provides an example crack pattern for each strengthenedwallette series. The tensile ductility of the ECC overlay led tothe development of multiple cracks in the majority of thestrengthened wallettes. The development of multiple cracks onstrengthened wallettes resulted in a significant improvement in

Fig. 6. Shear stress drift plots of tested as-built wallettes and corresponding example wallette crack patterns: (a) as-built dry-stacked wallettes, shearstress-drift plots; (b) as-built dry-stacked wallette example crack pattern (S0-DS-1); (c) as-built mortared wallettes, shear stress-drift plots; (d) as-builtmortared wallette example crack pattern (S0-M-2)

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energy dissipation [Figs. 7(a–e)] compared to the as-builtwallettes (Fig. 6) that failed after the development of a single diago-nal crack.

Shear Strength of Strengthened Wallettes

All strengthened wallettes had much higher strength than theirunstrengthened counterparts (Tables 4 and 5), with increases of814–3,471% for dry-stacked wallettes and 340–520% for mortaredwallettes. This strength increase clearly demonstrated the beneficialeffect of the ECC strengthening for a variety of applicationmethods. The next objective was to quantify the effects of thedifferent methods of application. The results of Series S20-M-Sand S20-M-AT(US) showed that when applying ECC to a mortaredwallette, spraying led to a more consistent strength increase than ifthe shotcrete was hand trowelled by an unsupervised amateurplasterer; the 8.6% coefficient of variation (COV) of S20-M-Swas approximately half the 14.8% COV derived from the resultsof the unsupervised amateur trowelled wallettes (Table 5). Theresults of Series S20-DS-S and S20-DS-AT(US), in which ECCwas applied to a dry-stacked wallette, also indicated similarreductions in COV between spraying and unsupervised amateurplastering, with COVs of 22.3 and 41.6%, respectively (Table 4).

Comparison of the results for the dry-stacked wallettes on whichECC was hand trowelled by a professional plasterer (S10-DS-PTseries) and for the dry-stacked wallettes on which ECC was handtrowelled by an amateur plasterer with supervision [S10-DS-AT(S)series] showed that the average shear strengths were identical be-tween the two series (Table 4). However, the COVof the supervisedamateur trowelled wallettes (30.8%) was much higher than thecorresponding COV for wallettes trowelled by a professionalplasterer (11.9%). The difference between the COV values in-creased when comparisons were made between applications bythe professional plasterer and applications by unsupervised amateurplasterers, with the latter application method resulting in a COVof41.6%. Different thicknesses of ECC were applied to the wallettesunder comparison, but the specimen characteristics were similar.These results clearly indicate that the experience and skillset ofthe applicator has a significant effect on the consistency of thestrength increase achieved by the trowelled ECC. However, the re-sults also show that despite the inclusion of the scenario in which ageneric mixer and an amateur applicator were used to strengthensome of the wallettes, a minimum strength increase of 373%was obtained compared to the strength of the as-built wallette.Further quantification of the specific influences of mixer typeand applicator ability on the strength gain associated with ECC

Fig. 7. Shear stress-drift plots of tested strengthened wallettes: (a) 20 mm ECC overlay sprayed on dry-stacked wallettes; (b) 20 mm ECC overlayapplied on dry-stacked wallettes by an unsupervised amateur plasterer; (c) 10 mm ECC overlay applied on dry-stacked wallettes by a professionalplasterer; (d) 10 mm ECC overlay applied on dry-stacked wallettes by an amateur plasterer with professional supervision; (e) 20 mm ECC overlayapplied on mortared wallettes by an unsupervised amateur plasterer; (f) 20 mm ECC overlay sprayed on mortared wallettes

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overlays applied to ungrouted concrete masonry will require addi-tional studies, such as in situ testing of ECC strengthened concretemasonry walls constructed in developing countries, which wasbeyond the scope of this study.

The range of COV values for each set of wallettes is similar tothose reported in other studies that include masonry wallette testingin accordance with ASTM-E519 (ASTM 2002). Test results fromAlecci et al. (2013), Marcari et al. (2007), and Petersen et al. (2010)showed COV ranges of 13.9–43.0% for as-built wallettes, whereasProta et al. (2006) and Faella et al. (2010) reported test results that

indicated COV ranges of 8.5–38.2% for strengthened wallettes.Notably, all dry-stacked wallette series had higher COV values thantheir mortared counterparts, indicating that masonry jointconditions had a significant influence on the uniformity of walletteresponses.

Because multiple damage patterns (debonding, shear cracking,and concrete block crushing) were observed in the strengthenedwallettes, several equations were collectively utilized to predictthe damage patterns of the wallettes. For equations that accountfor the strength of the ECC overlay, no account is given to the

Table 4. Summary of Test Results from Dry-Stacked Wallette Testing

Wallette configurationSamplenumber

Pmax(kN)

V(kN)

τmax(N=mm2)

τmax=τo(%)

δy(%)

δu(%) μ

S0-DS 1 7.3 5.2 0.03 N/A 0.2 1.1 6.22 2.8 2.0 0.01 N/A 0.004 0.2 4.43 7.8 5.5 0.03 N/A 0.3 1.7 6.7

Series summary Average τmax=τo (%) COV (%) Average μ COV (%)N/A N/A 5.8 17.1

S20-DS-S 1 141.6 100.1 0.61 2,614 0.2 0.8 4.62 191.5 135.4 0.46 1,971 0.1 0.1 1.13 186.8 132.1 0.80 3,429 0.2 0.6 3.1

Series summary Average τmax=τo (%) COV (%) Average μ COV (%)2,671 22.3 2.9 48.9

S20-DS-AT(US) 1 92.6 65.4 0.37 1,586 0.4 0.7 1.92 85.8 60.7 0.35 1,500 0.2 0.8 4.93 197.4 139.5 0.81 3,471 0.3 0.6 1.9

Series summary Average τmax=τo (%) COV (%) Average μ COV (%)2,186 41.6 2.9 48.8

S10-DS-PT 1 81.9 57.9 0.32 1,371 0.1 0.9 8.72 55.8 39.5 0.24 1,029 0.08 0.9 13.73 78.4 55.4 0.32 1,371 0.1 0.5 4.24 69.4 49.1 0.27 1,157 0.2 1.0 6.3

Series summary Average τmax=τo (%) COV (%) Average μ COV (%)1,232 11.9 8.2 43.0

S10-DS-AT(S) 1 46.5 32.9 0.19 814 0.01 0.3 29.22 69.7 49.2 0.28 1,200 0.05 0.5 13.13 102.8 72.7 0.43 1,843 0.2 0.6 3.64 64.6 45.7 0.25 1,071 0.2 1.1 5.9

Series summary Average τmax=τo (%) COV (%) Average μ COV (%)1,232 30.8 13.0 77.3

Table 5. Summary of Test Results from Mortared Wallette Testing

Wallette configuration Sample number Pmax (kN) V (kN) τmax (N=mm2) τmax=τo (%) δy (%) δu (%) μ

S0-M 1 49.3 34.8 0.12 N/A 0.0004 0.002 4.72 40.0 28.3 0.15 N/A 0.001 0.001 1.03 45.5 32.1 0.18 N/A 0.002 0.002 1.3

Series summary Average τmax=τo (%) COV (%) Average μ COV (%)N/A N/A 2.3 71.9

S20-M-S 1 201.3 142.3 0.51 340 0.2 0.7 3.82 156.0 110.3 0.63 420 0.08 0.1 1.83 145.0 102.5 0.59 393 0.03 0.03 1.0

Series summary Average τmax=τo (%) COV (%) Average μ COV (%)384 8.6 2.2 53.5

S20-M-AT(US) 1 133.1 94.11 0.56 373 0.002 0.5 2392 153.6 108.6 0.60 400 0.1 0.4 3.13 185.5 131.1 0.78 520 0.04 0.6 14.5

Series summary Average τmax=τo (%) COV (%) Average μ COV (%)431 14.8 85.5 127.0

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supplementary, but minor, strength attributable to the ungroutedconcrete masonry; hence, it is expected that the predicted strengthwill be slightly conservative. The predicted shear strength of theECC reinforced wallette, Vn;ECC, is determined through Eq. (3):

Vn;ECC ¼ minðVECC;VτECCÞ ð3Þ

Vn;ECC is equal to the minimum of the ECC overlay shear strength(VECC) and the ECC debonding strength (VτECC). For shear failure

of the ECC overlay, Eq. (4), derived by Japan Society of CivilEngineers (JSCE 2006, 2008), is used:

VECC ¼ tECCðzf 0yECC þ 0.18

ffiffiffiffiffiffiffiffiffiffif 0ECC

qlwÞ ð4Þ

The ECC shear strength includes the contribution from the fiberreinforcement (tECCzf 0

yECC, similar to the contribution from distrib-uted steel reinforcement in concrete masonry design) and thecomposite matrix shear strength (0.18

ffiffiffiffiffiffiffiffiffiffif 0ECC

plwtECC, similar to

Fig. 8. Examples of typical strengthened wallette crack patterns: (a) dry-stacked strengthened by spraying (S20-DS-1); (b) dry-stacked strengthenedby amateur trowelling under no supervision [S20-DS-AT-3(US)]; (c) dry-stacked strengthened by professional trowelling (S10-DS-PT-1); (d) dry-stacked strengthened by supervised amateur trowelling [S10-DS-AT(S)-3]; (e) mortared strengthened by amateur trowelling [S20-M-AT(US)-3];(f) mortared strengthened by spraying (S20-M-S-3)

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equations used to determine concrete shear strength). In Eq. (4),VECC is the shear strength of the ECC section in kN, f 0

yECC andf 0ECC are the ECC tensile and compressive strength, respectively,

in MPa, and z is the moment lever arm distance in mm, whichis recommended by JSCE (2008) as 0.87 of the wall length thatECC is applied to, exclusive of any wall length that intersects wallopenings.

For the debonding strength, the equation proposed by Lin et al.(2013) to predict ECC bond strength to clay brick masonry[Eq. (5)] was modified for concrete masonry [Eq. (6)]:

VτECC ¼ Kwt0.12f 0bEECCMABond ðclay brick masonryÞ ð5Þ

VτECC ¼ 0.6ffiffiffiffiffif 0c

pEECCMABond ðconcrete block masonryÞ ð6Þ

Eq. (5) was originally developed by the Japan Concrete Institute(JCI 2003) to predict FRP bond failure to concrete, but was latermodified and calibrated by using experimental results of ECCstrengthened vintage clay brick masonry wallettes to predictECC bond strength when adhered to vintage clay brick masonry.The original factor that is used to calculate the brick tensile strength[12% of the brick compression strength (f 0

b)] was modified to0.6

ffiffiffiffiffif 0c

p[recommended by New Zealand Standards (2006)] to

represent the concrete tensile strength in terms of the concretecompression strength (f 0

c) in MPa. The Kwt factor, used to accom-modate the influence of clay brick masonry wall thickness (mea-sured in number of leafs or wythes for a multileaf wall) on theeffectiveness of an ECC overlay bonded on one wall surface only,was omitted, because the concrete masonry wallettes constructed inthis study had a thickness of only one leaf. The shear bond strength(VτECC) was calculated by using Eq. (6), where EECC is the Young’smodulus of ECC in MPa and M is a function that describes thesaturated increment in shear strength provided when additionalECC overlays are applied, calculated by using Eq. (7):

M ¼ 0.0033ð1 − e−0.08tECCÞ ð7Þ

where tECC = thickness of ECC, in mm. Finally, ABond representsthe bond area in m2, which is equal to half the 1,120 × 1,170 mm

ECC area in this study because of the diagonal crack that splits thewallette into two approximately equal areas.

For the final damage pattern, Eq. (8) is used to check that thehorizontal shear force (V) applied to the concrete block at thewallette corners does not exceed the lateral compressive strengthof the block (f 0

lc) and cause tensile splitting of the block webs:

V ≤ Vlc ¼ f 0lcAlN ð8Þ

where Vlc = maximum compression force, in kN, that the concreteblocks can sustain when loaded laterally; Al = lateral cross-sectional area of the concrete blocks exclusive of any core areas,in mm2; and N = number of concrete blocks over which the lateralcompression load will be distributed, equal to approximately 1.2 inthis study (because the loading shoes span over the height of 1.2concrete masonry blocks). Eq. (8) reflects the upper bound limitof the wallette strength, rather than a prediction of the actual failureload.

A comparison between the measured individual wallettestrengths and the predicted strengths using Eqs. (4) and (6)–(8)is provided in Tables 6 and 7. For wallettes that were handtrowelled by professionals or sprayed, the measured strengthtypically exceeded the predicted strength, with the exception ofone wallette (S10-DS-PT-2), which failed at 98% of the predictedload. For wallettes that were hand trowelled by amateur plasterers,an increased number of wallettes was below the predicted strength,with Wallette S10-DS-AT(S)-1 having a measured strength that wasonly 81% of the predicted strength. The measured strengths wereall lower than the lateral compression strength stated in Table 1; thelateral compression strength of the block represents the upperbound of the load that can be applied. Some wallettes had crushedcorner blocks, although the applied load was less than the lateralcompressive strength of the blocks, which was attributed to concen-tration of stresses at the loading corners because it was difficult toensure that a uniform stress distribution was achieved over the bear-ing area. The discrepancies between the predicted and measuredstrengths were attributed to the partially heterogeneous nature ofthe ECC overlay, which is influenced by the skill and experienceof applicators when applying the ECC overlay, and the effect thatvariations in the procedure used to apply the ECC mix to the wall

Table 6. Comparison between Measured and Predicted Strength of Tested Dry-Stacked Wallettes

Walletteconfiguration

Samplenumber

MeasuredV (kN)

Predicted strength (kN) MeasuredV/lowestpredictedVn;ECC (%)

MeasuredV/revisedpredicted

ϕAVn;ECC (%)DamagepatternVECC VτECC Vlc

S0-DS 1 5.2 N/A 142.9 N/A N/A2 2.0 N/A 142.9 N/A N/A3 5.5 N/A 142.9 N/A N/A

S20-DS-S 1 100.1 81.0 71.0 142.9 141 141 D+C2 135.4 81.0 71.0 142.9 191 191 C3 132.1 81.0 71.0 142.9 186 186 D+C

S20-DS-AT(US) 1 65.4 81.0 71.0 142.9 92 115 D+C2 60.7 81.0 71.0 142.9 85 107 D+C3 139.5 81.0 71.0 142.9 196 246 D+C

S10-DS-PT 1 57.9 40.5 49.3 142.9 143 143 D+C2 39.5 40.5 49.3 142.9 98 98 D+C3 55.4 40.5 49.3 142.9 137 137 D+C4 49.1 40.5 49.3 142.9 121 121 D+C

S10-DS-AT(S) 1 32.9 40.5 49.3 142.9 81 102 D+C2 49.2 40.5 49.3 142.9 122 152 S3 72.7 40.5 49.3 142.9 180 224 D+S4 45.7 40.5 49.3 142.9 113 141 D+C

Note: C = crushing failure of the concrete blocks at the loading point; D = debonding failure between the ECC overlay and the concrete masonry wallette;S = shear cracking of the ECC overlay.

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surface have on the resulting density and distribution of fibers, andconsequently, the associated variability of measured materialstrengths.

Based on the discrepancies observed between the various pre-dicted and measured strengths, it is proposed that when predictingthe shear strength of strengthened wallettes, an applicator strengthreduction factor (ϕA), as listed in Table 8, should be adopted for thevarious configuration and application types. The proposed valuesof strength reduction factors are based on the data shown in Tables 6and 7, in which the percentage from the lowest measured strengthof each application method was used to derive the reduction factor,which was rounded to one significant figure. Eq. (9) shows theupdated equation with the recommended reduction factor. Thecrushing failure of the corner blocks attributable to the diagonalcompression force was considered to be an unrealistic damagepattern, and instead was attributed to the test setup; therefore, itis expected to not occur in actual structures. A revised predictedstrength was calculated by using Eq. (9) and the ratio of themeasured strength to the revised predicted strength is reportedin Tables 6 and 7, showing that, except for one wallette that failedat 98% of the predicted strength, the strength of all wallettesexceeded the revised predicted strength:

Vn;ECC ¼ ϕA minðVECC;VτECCÞ ð9Þ

Ductility Capacity

The determination of ductility capacity allows the behavior of astructural element beyond its elastic range to be described. In thisstudy, the calculation of ductility followed the method adopted byProta et al. (2006), Marcari et al. (2007), Ismail et al. (2011), andMahmood and Ingham (2011). Eq. (10) was used to determine theductility capacity (μ) of the wallettes, with Fig. 9 providing agraphical representation of the parameters on a τ–δ curve:

μ ¼ δuδy

ð10Þ

where δu = ultimate drift, defined as the point at which the strengthhad degraded to 80% of the peak strength. The yield drift (δy) isdefined by using a secant modulus from the origin to 0.7τmax, suchthat the resulting bilinear response has equivalent energy absorp-tion to the measured response. An example secant modulus isshown in Fig. 9, intersecting the experimental data at a stress of0.24 MPa, which is 70% of the maximum measured stress of0.35 MPa, with a corresponding strain of 0.13%, resulting in a yielddrift of 0.17%.

A large variation in the ductility capacity was calculated acrossall tested wallettes (Tables 4 and 5), and with the exception ofSeries S0-DS, the associated COVs of all series exceeded 40%.The high COV value is a reflection of the sensitivity of thecalculated ductility capacity to the yield drift because thisparameter is defined by the peak stress, which is a value thatcan vary significantly. For both series in which the shotcretewas sprayed, the ductility was lower than the hand trowelledcounterpart. No clear trend was identified in terms of how the skillof the applicator influenced the ductility capacity when theshotcrete was hand trowelled, as observed in Series S10-DS-PT,S10-DS-AT(S), and S20-DS-AT(US), in which professionallytrowelled wallettes had less ductility capacity than wallettes trow-elled by amateur applicators under supervision, but higher ductilitycapacity than for wallettes trowelled by amateur applicators with-out supervision. Because some of the wallettes (S20-DS-S-2 andS20-M-S series) exhibited the artificial damage pattern of corner

Table 7. Comparison between Measured and Predicted Strength of Tested Mortared Wallettes

Walletteconfiguration

Samplenumber

MeasuredV (kN)

Predicted strength (kN) MeasuredV/ lowestpredictedVn;ECC (%)

MeasuredV/ revisedpredicted

ϕAVn;ECC (%)DamagepatternVECC VτECC Vlc

S0-M 1 34.8 N/A 142.9 N/A N/A2 28.3 N/A 142.9 N/A N/A3 32.1 N/A 142.9 N/A N/A

S20-M-S 1 142.3 83.1 76.7 142.9 179 232 C2 110.3 83.1 76.7 142.9 137 180 C3 102.5 83.1 76.7 142.9 125 167 C

S20-M-AT(US) 1 94.11 83.1 76.7 142.9 123 153 D+C2 108.6 83.1 76.7 142.9 140 177 D+C3 131.1 83.1 76.7 142.9 167 214 D+C

Note: C = crushing failure of the concrete blocks at the loading point; D = debonding failure between the ECC overlay and the concrete masonry wallette;S = shear cracking of the ECC overlay.

Table 8. Recommended Strength Reduction Factors for Different ECCApplicators

Application method and applicator capabilities

Amateurtrowelled

Professionaltrowelled

Sprayed byprofessionalapplicators

Applicator strengthreduction factor (ϕA)

0.8 1.0 1.0 Fig. 9. Graphical demonstration of the parameters used to determinevalues of ductility

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blocks crushing, these wallettes were discounted when determin-ing a recommended ductility factor for the ECC reinforcedwallettes. Furthermore, the data were insufficient to enable a rig-orous statistical evaluation of the lower 5% characteristic value.Consequently, the lowest ductility factor calculated for the ECCreinforced wallettes of 1.9 is recommended based on the results ofthis study. In earthquake standards such as the New Zealand Stan-dards (2006), a wall with a ductility value of 1.9 results in a 27%reduction in design seismic acceleration over walls that have aductility value of 1.0.

Design Methodology

Based on the results obtained in this study, the following designprocedure is proposed for the in-plane strengthening of concretemasonry wallettes using ECC and having similar dimensionsadopted in this study.1. Determine the expected in-plane damage pattern and the shear

strength of the unretrofitted wallette by using an appropriateguideline such as CEN (2005b), ASCE (2007), or Ingham(2011). If the expected damage pattern is diagonal crackingor shear sliding, and the shear strength of the masonry walletteis less than the design shear force, proceed to the next step. Ifthe expected damage pattern is rocking or toe crushing, analternative strengthening technique is required.

2. Calculate the ECC section shear strength (VECC) usingEq. (4) by assuming the total thickness of ECC overlay thatwill be applied over the wallette surface (either on a singlesurface or both surfaces). Use an appropriate applicatorstrength reduction factor, as listed in Table 8, and increasethe thickness of the ECC overlay (and adjust the design shearforce accordingly because of changes in expected wallettemass) until the factored ECC section shear strength exceedsthe design shear force (ϕAVECC ≥ V�) and proceed to thenext step.

3. Determine the shear bond strength (VτECC) calculated byusing Eqs. (6) and (7) with an appropriate bond area(doubled if ECC is to be applied on both surfaces). If theshear bond strength exceeds the design shear force afterbeing reduced by the appropriate applicator strength reduc-tion factor (ϕAVτECC ≥ V�), the design is complete. Other-wise, mechanical connections such as anchorage bolts thatcan be partially embedded into the concrete masonrywallettes will be required to increase the bond strengthand alternative equations should be adopted to calculatethe new bond strength.

Recommendations for Future Studies

Further testing is recommended to investigate the in-plane perfor-mance of ECC strengthened full scale concrete masonry walls andadditional analytical study of the ECC-wall composite behaviorwill be beneficial in extending the design methodologies recom-mended for wallettes in this study. Investigations into how ECCmaterial properties are influenced by the application method willalso be helpful. In situ testing of concrete masonry walls locatedin developing countries and strengthened by using local mixingequipment and personnel will provide additional insight into theinfluence of ECC mix quality on the response strengthened walls.Investigations into the types of mechanical anchorage available indeveloping countries and their applicability in improving the ECCoverlay-to-wall bond characteristic are also areas that requirefurther research.

Conclusions

The effectiveness of using an ECC shotcrete mix was investigatedfor the in-plane strengthening of concrete masonry walletteshaving dimensions of approximately 1,180 mmheight×1,200 mm length × 140 mm thickness, with the intent of determin-ing the suitability of applying this technique in developing coun-tries, with a specific focus on the suitability of applying themethodology during the reconstruction of Haiti. The effect ofthe absence or presence of mortared joints was also investigated.The influence of the method of ECC shotcrete mix application(either sprayed with a machine or hand trowelled) and the skillof the applicator (professional plasterer and amateur plasterer)on the strength gain provided by the ECC overlay were otherparameters that were studied. Based on the results of the investi-gation, the conclusions described in the following were made.1. Strengthened dry-stacked wallettes had shear strength that was

between 814 and 3,471% of their as-built counterparts. Formortared wallettes, the strength increase was between 340and 520%. Mortared joints reduced the variability of thestrength increase provided by the ECC overlay by an averageof 37%, regardless of the application method;

2. The application method and applicator skill level have signif-icant effects on the strength consistency of the ECC reinforceddry-stacked wallettes. Hand trowelling of the shotcrete by aprofessional plasterer had the lowest strength variability, withCOVof 11.9%, followed by spraying of the ECC, with COVof22.3%. Hand application of ECC shotcrete mix by an amateurresulted in the largest variability in the strength measured, withCOV of 30.8% when the application was made under thesupervision of a professional plasterer, and 41.6% when theamateurs were unsupervised. Identical ranking of the consis-tency of each application method was also observed for themortared wallettes, with sprayed ECC having a higher strengthconsistency than hand application by an amateur withoutsupervision.

3. Based on the discrepancies between the measured strength andthe predicted strength of the various application methods, a setof strength reduction factors was proposed to better predict theminimum ECC reinforced wallette strength.

4. The ductility capacity across all wallette configurations washighly variable and no clear trends were established betweenany of the testing series (application method or wallette jointtype) and the ductility capacity. It is recommended that a duc-tility capacity of 1.9 be adopted until further testing can justifya higher ductility value.

5. An in-plane design procedure has been proposed for ECCstrengthened unreinforced concrete masonry wallettes. Thisprocedure provides design equations for checking the shearstrength of the ECC overlay and the debonding strengthbetween the ECC overlay and the wallettes.

Acknowledgments

The authors thank Derek Lawley, David Nevans, Richard Leary,Michael Barry, Gareth Williams, and Mason Pirie from ReidConstruction Systems for the funding and assistance in providingthe ECC shotcrete mix material and related equipment, Matt Pronkfrom Vijay Frame & Truss for providing the space for the wallettespecimens, and Bob Fleming from Fleming Plasterers andReinstatements for the ECC application. The authors also thankAnthony Le Dain, Bing Zhang, and Karl Yuan for their assistancein ECC application and testing the wallette samples. Lastly, theauthors thank the New Zealand Ministry of Science and Innovation

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for the funding of Yi-Wei Lin’s doctoral study and thank theNew Zealand Earthquake Commission for the funding of Dr.Wotherspoon’s position at the University of Auckland.

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