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SHEAR STRENGTH OF GROUTED REINFORCED MA SO NR Y BE AMS
G, T , SUTER Ph . D. , P . Eng .
Associate Pr ofessor of Civil Engineer i ng
H, KELL ER B. Eng . , M. Eng.
Research Engineer Department of Civil Engineering, Car leton Unive r sity , Ottawa, Canada
SHEAR STRENGTH OF GROUTED REINFORCED
The authors carried out a systematic experimental
study to answer tha question : Can the shear design
of grouted r einforc3d mrlS0nry beams be treated as
a combined case of y·einforced concrete and reinforced
masonr-y? The investl:gation ir,volved a t otal of six
teen beams in which aU test parametel's e:ccept the
ratio of shear span to effe ctive depth were hald con
stant . The papel' presents details of the test pro
gram and shows t hat the shear design of grouted
rrasonry beams can indeed be consider-ed as a combined
case of reinforceâ con:n'ete and reinforced masonry .
RESISTANCE AU CISATLLEMENT
DES POunES EN M4CONNERIE' ARMEE
Les auteurs ont développé W1e étude systématique et
expéri.-rentale pour f'ouvoi r répoildre à la question
suivante: Est-ce que la contra'in te de cisailZemant
de poutres en terre cui te armée peut être calculée
cOmrle un cas corrbiné de bétan arme et de maçonnerie
La recherche portait sur un total de seize poutres
dans lesqu& lZes tous les parametres étaient mainte
nus (?(JYlstants, excepté le rapport entre la portée et
la hauteur e fre cti ve .
L ' art i cle nous montre MS diJtails du prograrrone des
essX1:s et démontr e C/ U ?> le calcul de la cantrainte de
cisaillemant de poutres en t erre cmte année peut en
effet être considépé co= un co,g c.ombiné de béton
arrré e t maç anneri e arrrée.
SCHUBFESTIGl(f;IT A USBETONIERTER
Die Verfasser filhrten eine systematische lIersuchs
s tudie durch , um die Frage zu bean r..Jc rten :
Kann der Schubnachweis von ausbetonie l't en, bewehr
ten Mauerwerkstriigern so gefiJhr·t werde"l , als handle
es sich um einen KombinationsfaZl lion Stch Zb etcrn zmd
bewehrtem Maue rwerk? Die Unters uchung bestand aus
16 Balken, bei we lchen alle Testpal'aJilJ ter , ausgenom
men das Verhãltnis von Schub spanYl1Jeite zup eff ekti ve"l
Hohe, konstant gehalten uJurden. Der Beitrag bringt
Einze lheiten des Ve .1''Buchsprogn:vI1mG !lnd zeigt, doss
der Schubnachweis ausbetonierter !.J,werw9rks triiger
tatsãchlich als KorrbiYl~tiansfaZZ von Stahlbeton
bewehrtem Maw rwer'k betrachtet werden kann.
DE SCHUIFSTERKTE VAN MET BETON GEI'ULDE
De auteurs hebben een systematische experimente?e
studie uitgevoerd om de vr'aag te beantw oor den. :
kan de schuifsterkte van met beton gevu lde ge~pende
metselwer'kliggers behandeZd worden als een geKomb?:
neerd geval van gewapend metse lwerk en geUlapend be
ton? Het onderzoek omvatte in totaal aestien lig
gers, ulaarin aUe testparameters met uitzondering
van de veY'houding overspanning tot effaktieve hoogte
konstant wel'den gehouden .
De mededeling geeft details van het ondeY'zoeksprogram
ma en toont aan da t de berekening van schuifsf'amzingen
deze liggers inderdaad moeten beschouwd wcrden als
een kombinatie van gewapend beton Ziggers en Zigger s
in gewapend metselwel'k .
Gr outed rei.nforced masonry (GRM) beams are genera lly bui 1 t up fro;;; two oro more wythes Df brickwork wi th cavi ties between the wythes; these cavities co nta in the reinf orcement and are filled with grout, the grout normally :::o,l sisting Df ô high slump peôgrôvel concrete p~u red OI' pumped into place , After hardening Df the concrete , t he grouted reinforced beam i5 considered as a com;Josite unit in which both the masonry Ivythes and the grouted c oncrete saction resist applied moments and sh ears. The shear design Df such members is generally bas ed on stresses which are identical to the case Df rei.nforce d masonry (RM) beams, i . e . members that do not contain a grouted section ; in Canada an allovJabl e at ress Df 0 . 7~:: 50 psi (0 . 35 N/mm 2
) is specified for both types mof members , where f' denotes t he compressive strength Df the br ickwork . m
A review Df the shear strength Df r einforced concrete (RC) and RM beams shows that a RC member exhibits a s hea r resistance wh ich is typically two to three times that Df a RM member . The question then arises : Is the shear resis tôn ce Df a GRM beam significantly greater than that of a non -grouted RM beam and moreover·, can this resistance be considered as the combined strength of the brick~o rk wy thes and the grouted section? Since little published evidence i s available on grouted beams 1
, a systematic experimental investigation was undertaken to answer this question o
To determine if the shear resistance of GRM beams is grea ter than that of RM beams and can also be treated as the combined shear strength of the brickwork wythes and the grouted conc rete section, three separate series of beam tests dealing with GRM , RM, and RC beams are requi red . Since r eliab l e published evidence on RC beams is available 2
, the test program described in this papeI' comp rised two beam series of GRM and RM beams . Each be am series consisted of eight beams as shovJn in Figure 1 . For a constant cross section containing a pe rc entage of reinforcement p Df abou t 1.4%, the only test va riable was the ratio of shear span to effective depth a/do The a/d ratio was selected as the single test variab le for the following reasons: the shear resistance of PC beams depends mainly on a/d and p , and to a minor degree on f ', the compressive strength 2 , , Si.nce the shear capgcity of RM beams is infl ue nced primarily by a/d and only to a negligible degree by p' , a / d is the one common key variable on which the shea r strangth of both RC and RM beams depends and r,ence can be assumed to have a major influence also in the case of GRM beams . The a/d ratio was varied between 1 and 7; a f airly large p value of about 1 . 4% was selected to obtain pronounced shear failures.
Test 8aem Oetails
oetails Df the GRM and RM beams cros s sections are shown i n Figure 1. A two - point loading set - up was adopted for alI beam test s except beams 15 and 16 . 8eams 15 and 16 were tested unde r a central single point load to accommodate the se l ong beams in the testing machine. For the GRM beams , the width of the gr out cavity was chosen to ac commodate the steel and to provide space for the plac emen t of web reinforcement in a later investigation .
ThroughOlJ t t he test progr em 3-corod bricks Df the type "Red Supertex " were used . IUthough the front face of the bricks WôS roughly textured , the opposite face, which i n the case of GRM beams serves as the form for the grout, was relati vely smooth . The bricks had
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nominal dimensions of 203 x 95 x 75 mm and a specified comp ressive strerlgth Df 69 N/mm 2
• Average crushing strengths were 92 and 33 N/mm~ for sing le bric ks and five course brick prisms, respectively .
A 1 : 0 . 5 : 4 . 5 normal Portland cement:lime:sand mortar mix by volume was used in accordance with CanBdian re qu irements for GRM construction s . At the time Df beam testing , the average mortar strength was 14 . 7 N/mm 2
• Oesign mortar joint thicknesses we re 22 mm for the first bed joint at the steel leveI and 10 mm for alI othe r joints. For the GRM beams , a grout mix of one part normal Portland cement to four parts sand was used . At the time of beam testing, average grout strength s of 15.1 and 27 . 5 N/mm 2 were obtained for 75 x 150 mm cylinders and 50 x 50 x 114 mm prisms , respectively . Since the prisms we re made by pouring the grout into a space formed by bricks and lined wi th permeable papeI' , the prism results reflect more closely the true grout strength in the beams .
Steel reinforcement was furnished by 6-16 mm bars and 2-16 mm bars for the GRM and RM beam series, respectively . The average yield stress was determined as 445 N/mm 2
• To prevent bond anchorage failure wh ich might obscure the true shear capacity of beams, 10 mm thick anchorage plates (100 x 100 mm for RM be ams and 100 x 305 mm for GRM beams) were welded tc the ends Df the reinforcing steel . This method obviated the need for large beam overhangs to provide sufficient a ncho rage length .
Manufactu re and Testing of 8eams
A typical daily production consi sted of the manufac ture of two baams Df equal a/d together with the associated contraI specimens for compression, mortar and grout st rength tests . For each type of control specimen , five samples were produced . The wo r \<. was done by an experienced bricklayer and the quality Df workmanship was average .
For the manufacture of the GRM beams , two wooden pla nks were attached t o the floor of the lôboratory 305 mm apart and the wythes were built up to the fourth layer along these planks . The grout cavi ty between the wythe s was then cleaned Df mortar fins and droppings, and the reinforcing steel ~laced in po sition . Prior to laying the fifth brick lay@r, closed metal ties (100 x 254 x 4 . 5 mm diameter) Nere p laced in the top mortar joint at 610 mm spacings according to Canadian design requirements s . These ties prevent lateral movement of the wythes upon grouting Orle day after construction of the brickwork . The grout was poured into place with a slump varying betwee n 200 and 250 mm and puddled briefly . Each daily product ion of beams and cont rol specimens was cured under a polyethylene cover for 7 days.
8eams were tested at ages Df 29 to 32 rl ays . Loading was applied in increments Df about one-tenth Df beam capacity and corresponding crack extensions were traced on t he white - washed beam surface. Failure Df a be am generally took place within one to two hours. After achieving e f irst fa i lure in one shear span of a beam it was no r ma l ly possible to bandage the failed end by means Df externaI plates and tie r ods to obtain a second shear failure i n the othe r shear span.
TEST RESULTS ANO OISCUSSIoN
8eam test results are presented in Fig.2 to 4 and Table 1 . Note that the RC beam results are based on Kani ' s work 2 • His work is used here firstly because he tested a large number Df RC beams (over 133) to establish the dependence of RC sheôr resistanc8 on a/d , p, and f~, and secondly because his 150 x 300 mm
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cross section approximates the cross sections of the author~' investigetion . Since Kani used p values of 0 . 8 and 1 . 88% rath e r than th e 1 . 41% of the GRM beams , l jnear lnterpolati on was 8n~loye d to ar r i v8 at the RC beam shear capaci ties shown in laole 1 for p = 1 . 41% . Altllough nis test pro~ram included three f ' val ues and his result s showed merely a minor dependen~e of shea r str8r1gth un f ', only t he 17 beams tests for f ' = 26 N/mm 2
are repres e nt~d in l ab le 1, 5ince tllis corlcrete s trengt h appr Cl xJ mates th e grout st rength of the GRM beams . Kani did not test beams with a n a/d value gr eat er t han 6 , hen ce U,e result for a/d = 7 in lable 1 denot es a shear strRngth ost imat ed by the authors .
CracKing rnd Fai 1 ure Mo des
Sinc8 th e naturc Bnd ex ten t of flexural and shear crackin i of the GRM beams WBS s imilor to that Df RM b8 ems ", no a dd i tional comlnents nec d be made here. It is of i ntereSé , hOlol8ver, to examine fai lure nlodes and unusu=:l behaviour of the GR M bea ms . 1.10 typ es of Shp.6r f ai lu re mode s were observed and classified as 5
s1 and 5
su in l able 1, where
refers to a slow s hear f ai lure where the b8am exhibi ts-addItio nal strength beyond its diagonal cracking capacity , and
refer s to a sudden shear foil ure whe re ultima te beamcapac:Cty is r eaehed upon s udden progression Df a diago nal crack aeross thR web of the shear span.
lhese shear failure rrüdes a re common to RC 2, RM 4, and
[,RM bearre . Frolfl lal1 1e 1 1 t can be seen that in ge r, e r al t hR shorter bealT.s ex!übi.ted a slOloJ typ c and the longer beams a s udden typ e Df failu r e . lhe slow type o"f fai lu re for lJeams of low a/d val ues is olJtairled lJec.3use aft e r s ucld an diagonal el'ack ing s llch members are typically transfo rrned into a ticd a rch which exhibits additional strength .
Unus ua l behaviour was exhjbtted in the final failure Df lJeam GRM-7 with an a/d value of 1 . For beam GRM-7 it was observed th a t just prior to r eaching the ultimate l oad the stee l ane ho ragE plate at one end of the beam started to be nd , resulti ng in a differential ti e force Df th e arc h aeross t he width of ~h e beam . Wi t h additional load and time , the deform~tion Df the plate bacame se large ( up t o 10 mm ) that vi r tually complete separation of the brick wythes from the concrete core r esulted i n that e nd Df t he beam. At this s tage a l I load was transferred to th e r ei nforced conc ret e section and rapid failure ensue d due to web compression failur e . However, the other lJeam e nd s howed no distreso and upon bandaging of the failed en d was able to withstand a high e r l oad . Since no other GRM beam failed prernaturflly in a marlner similar to GRM-7 , the composit e behavi our achievecl by th e a nchorage plates as well as by th e interaction between brickwork s urfa ces with t he gro ut ed cor e was generally satisfac tory. Note that in practicR the plates would l ikely be omitted in favour Df beam extensio ns to develop the reinforcement. 5inc8 beam ends would f rame for instance i nto adjoini ng wall sec tions , the possibility of brea kdown Df comp03ite action vJ ou l d be remote .
Ultimate 5hea r 5tresses
Ultimate s hear stresses for "RM , GRM , and RC beams are present ed in lable 1 and Figu r e 2 . Fo r the RM and GRM beams , twCl separate ultimate values were ob tained for the two shear spans Df eaeh beam with the exception cf beams GRM-7 and 12 .
lhe in fluenee of the main shear parameter a/ d on ultimate s l1 (0)r Bé r ess v is shown in Figure 2 . lhe indivjdua l failurE valu~s and avelBge failure l i nas cle3rly i ndicate that v increases signifi cantly with decreasi ng a/d values fgr al I three types of beams , RM, RC and GRM . Most important , it can also be s een
that the average GRM beam shear resis tanee is greater than that of RM beams and falls between that Df RC and RM beams . lhe increas ed s hear capac ity of GR M beams ove r RM beams amo unt s t o ab out 50 per cant and hence is significant . lhe question still remains if the GRM shear capacity ean be considered as t he combined shear strength Df RM a nd RC beams . lo answer th is question , th e RM beam results of this investigation ware first eompa r ed to other publishe d Evid8nce in Figure 3. It can be seen f r om Fi gure 3 that the authors ' RM results falI with i n the band of other experi mental data and tha t the overall average curv e including the authors ' RM results alters th e average curve defined by 5uter and Hendry4 only alightly . l he nsw ove r all average curve then represent s the depe ndence Df v on a/d for RM beams in general, and in conjunction w~th the average RC c urve Df Figure 2 hos been used to obtain ' derived ' GRM shear capacities . Derived GRM shear strengths , based on the relative widths Df tt. e grout and brick sections, a r e depicted i~ Figure 4 togethe r with the exp e r ilnent61 RC , RM Clnd GRM res ult s . For convenience alI shear capacities are expressed as a functi on Df th e RC strengths . A comparison of the deri ved ver sus t he e xperi mental GRM s hea r strength ratios of Fi gure 4 shows ralôtive ly good agl"eernent for alI a/d values except 1. 5 and 2 . Here it must be recognised that s ince the RM and RC shear st rengths are based on a large rlumb ar :) f test r es ul ts , their respective lines i n Figure 4 a r e esta~ llshed witn much greater certainty t han the experimenta l GRM li ne . Addi tional GRM test data eould be expected to falI with in a scatt er band similar to tl13t SllOV,'11 In Figure 3 f or RM beams . Moreove r', due to the variabi li ty of th e t ied arch strength, the scatter i s known to be widest fo r beams with low a/d va luBs , henc8 thi s may explai n the differenee be t waen experimell "L<.ll "lnd de rived GRM strengths depicted in Flgure 4 fOI' a/d va l ues Df 1.5 and 2.
lhe relatively good agreBment beb~ee n experiment al and deri ved GRM shea r capaeitje s ove r m:J s t of the a/ d r ange then giV 8S riae to the euncl~sj Gn that GRM shear stre ngth ean indeed be considHred as tha direct cornposite strength of the RM and RC portions nf a heam . Since only the re I ative widths of the RM an d RC po r tions need be co nsidered, it con readily be seen in Figure 4 that as the wIdth of th e grouted R~ sect j on dimin i shes , the GRM capacity lines aporoaches tha RM Une, anel a I terr,ati vely , as the gruut widt h irlcreasas , the GR M line moves t owards the RC 1lne. l es t evidence to verify t his GRM behaviOL;r for narrow and wide grout sections wo ul d be Jes i r ab l a .
CDNC LU5I DN5
8ased on tests report ed in thi s papEr, the ulttmate sh ear stress of GR I1 beams increases markEidly wi th decreasi ng a/ d values simjlar to the cases of RC and RM beams . l he shear ca pacity Df GRM beams falls between t ha t of RC and RM beams, and f or th e particular eros s section Df t his investigatio n wa s si gn ificantly greater th an that Df RM bearns . lhe GRM results further indicate s ince composiée action exists betwee n brickwork wythes a nd gruuted conc rete co r e , the GRM sh ea r capacity c an be safely derived from the separ ate s hear capacit ies Df th e grout and brick sections according to their relative width s .
FinanciaI support for t his invest i gation was provided by the National Research Co uncil of Canada. lhe auth ors also wish to t hank Domt a r Construction MateriaI s Ltd . fo r s upplying the bricKs .
1 . Keller , H. "Shear Strength Df Grou t ed Reinf orc ed Mas on r y Beams ", M. Eng . Thesis , Carleton Un i vers ity Septembe r 1975 .
2 . Kan i, G. N. J . "The Riddle Df Shear Failure and its Solution ", ACI Journal , Vo l. 61 , No . 4 , April 1964 , p . 441 .
3 . Bri ti s h Standards Inst i tution , "Code Df Practice f or thB St r uct ura l Use Df Co ncre te ", CP1 10 , Part November 1972 .
4 . Suter , G. 1. and HendT'lJ , A. W. "Sheflr Strength Df Reinforceo Brich'orl< Bearr,s ", Th" Str uctura I Engi neer , Vol . 53 , No . 6 , June 1975 .
5 . Nat i onal Building Code Df Canada. "Supplement No . 4" , Part 1, P lain and Rei nforced Masonry , 1975 .
4 . c . 2- 3
3 ' 2
2 ·0 -
o ~--~----~--~----~--~----~--~--~ o/d
O 2 3 4 5 6 7
Fig . 2 Beam Gross Section and Brick Bonding Arrangement
TABLE 1 - SU MMARY OF BEMI TEST RESIH,TS
f aiJure Mode Total Ultim"te v ~', ~lort ar Brick Prism Grout OI'
'I u Strength St r ength Concr2te Beam Beam p 1. u2
Serie5 No. (perc~nt) a/d f ai l ure Failure CH /mm2 ) (N /mm 2 ) O;/ mm 2 ) (N/mm2 ) fi ( N/mm2 ) c
RI1 1 1.49 1 Ss l Ssl 1. 70 2.13 18 . 58 28.77
2 1. 5 SSl Ssl 1. 19 1. 49 18. 53 28. 77
3 2 Ssl Ssl 0 .93 0.97 18. 05 36 . 31
4 3 S Ssl 0 .80 1. 03 11.711 3'1.15 su
5 4 S S 0.56 0.61 13 . 91 33.37 s u su
6 5 S S 0.44 0.47 14 . 29 31. 97 su su
13 6 S S 0 ,'11 0 . 46 9 . 17 34.54 su su
15 7 S S 0 , 32 0.37 13.53 32.49 su su
-GRI1 7 1.41 1 Ssl - 2.68 >2 . 95",,', 18. 58 28.77 28.82
8 1. 5 Ssl Ssl 2.51 2.75 18. 58 28.77 28.02
9 2 Ssl Ssl 1. 54 1.85 10.05 36.31 26.22
10 3 Ss l Ssl 0.86 0.91 11 . 74 34.15 17.46
11 4 S S 0.71 0.78 13.91 33.37 24 . 37 su su
12 5 S - 0 . 65 >0 . 65'''''' 14.29 31. 97 28 .5 6 su
14 6 S S 0. 57 0 . 62 9.17 34. 54 31.54 su su
16 7 S S 0 , G5 0 . 69 13 ,53 32,49 34 , 22 su su
RC 1.41 1 Ssl 4. 93 25.03
1.5 Ssl 3 . 26 26 . 32
2 Ssl 1. 83 25 . 61
3 S 1. 09 27 .93 su
4 S 0 . 97 25.86 su
5 S 0.09 26.99 su
6 S su
Q, 81 24.17
7 S 0.79 -su
NOTES; I'Shear force div i derl hy brl; includes beam self-wel ght ,",,', Bandaging of f ailed end not sufficiently eHective t o achieve ultima te failure of second end .
-150 a 610 a
RM - BEAM ELEVATION
- 150 a 610 a
I I J 1 I I I I I I 1 J 1 I j
I 1 1 1 I I I 1 l .l J .l .l J .l
1 ] j j 1 1 ]
t GRM - BEAM ELEVATION
1 1 .l j
~ = 1. 4 9%
~ rrn ••• •••
~ = I .'4 I%
Fig . 1 Beam Cross Sect ion and Brick Bonding Arrangement
' Vu N/mrn2
8 OVERALL AVERAGE INCLUDING AUTHORS' RESULTS
T o o o
o",;--,~ o o o
o o 8
Influence of a/i!. on V u f or RM beams
o-e o f= .. o: I 0-6 t-e> z
~ <f> OA -o: .. w I <f>
o /d O O 4 5 6
Fig . 4 Shear strength r atio versus a/d