1. ABSTRACT

COMPRESSlVE STRENGTH OF MASONRY PARALLEL TO THE BED JOINTS

Gisbert Hoffinann1 and Peter Schubert2

In this paper test results are presented from a research project /1 / on the compressive strength of masonry paralIel to the bed joints. The test programme covers alI masonry unit types commercially available in Germany - clay units, calcium silicate units (CS), lightweight aggregate concrete units (LC) and autoclaved aerated concrete units (AAC), selected in accordance with the masonry unit industry. The testing walls had been carried out with general purpose mortar and thin layer mortar with mortared and unfilled perpend joints. The test results are discussed in order to explain the failure mechanism and with regard to Eurocode 8 "Structures in seismic regions-design" and Eurocode 6 "Design ofMasonry Structures".

2. INTRODUCTION

In various applications, masonry compounds (e.g. retammg walls) are subjected to flexural stresses. In certain circumstances, masonry may be used in earthquake risk zones. In both cases, resistance to compressive loads parallel to the bed joints can have a decisive effect on load bearing capacity, especially for masonry units with low compressive strength, a high perforation ratio and/or an unfavourable perforation configuration.

Keywords: Masonry; Walls; Longitudinal Compressive Strength; Failure Mechanism; Perpend 10int.

lDipl.-Ing., Institut fur Bauforschung, RwrH Aachen, SchinkelstraJ3e 3, D-52056 Aachen, Germany 2Dr-Ing. , Institut fur Bauforschung, RwrH Aachen, SchinkelstraJ3e 3, D-52056 Aachen, Germany

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3. COLLATING AND ANALYZING RESULTS OF PREVIOUS STIJDIES

Results of all available German and intemational studies conceming masonry and masonry units were collated and analyzed. A literature study had already been canied out in 1987121.

Available results from the literature for masonry units are summarized in Table 1.

A distinction has been drawn between longitudinal compressive strength and compressive strength along the axis ofunit width. The strength values are given as ratios related to mean compressive strength with and without shape factor in the form of statistical indices. Table 1 also indicates the range of mean compressive values for the masonry units in the studies. The very few foreign results were not included in this analysis.

There are very few results for masonry compressive strength parallel to the bed joints. Owing to differing test parameters and in some cases due to the use of masonry units with little current relevance, these are not readily comparable with the studies performed in the project. For this reason they will not be discussed in greater detail here.

Table 1: Masonry units Compressive strength: - fc~un: parallel to the unit length, - fet un: parallel to the unit width - feo un: according standard without shape factor, - fc ~: according standard with shape factor; anãlysis is confined to available German test results

Compressive strength parallel to the unit length

n fe,un fc~un 1 fcO,un fc~un 1 fe,un masonry unit Range x min x max x v x minx maxx v

N/mm2 N/mm2 % N/mm2 %

clay Mz 2 21 ,9122,7 0,67 0,64 0,70 - 0,67 0,64 0,70 -HLz 9 8,0 .. 82,0 0,31 0,13 0,49 34 0,31 0, 13 0,49 34

LHLz 4 8,0 .. 25,7 0,25 0,07 0,46 78 0,20 0,06 0,39 79

CS KS 4 24,1..31,4 0,68 0,68 0,75 6 0,68 0,68 0,75 6

KSL 4 8,9 .. 26,9 0,42 0,32 0,56 24 0,41 0,32 0,56 24

LC V 3 4,1..23,1 0,77 0,64 0,83 14 0,77 0,64 0,83 14

Vbl 2 3,6/4,7 0,69 0,36 1,02 - 0,61 0,36 0,85 -Hbl 6 1,8 .. 8,6 0,70 0,42 0,92 31 0,62 0,35 0,83 30

Hbn 1 15,6 0,55 - - - 0,46 - - -AAC G, GP 8 2,9 .. 9,1 0,72 0,61 0,84 10 0,69 0,51 0,84 16

n: number oftest series; x: mean value; max x: maximum value; min x: minimum value; v: coefficient of variation

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Table 1: Continuation

Masonry units Compressive strength: - fe~un: paralle1 to the unit length, - fCt un: parallel to the unit width - feo un: according standard without shape factor, - fe~: according standard with shape factor; analYsis is con.fined to available German test results

Compressive strength parallel to the unit width

n fet ,un / feo,un fet,un / fe,un masonry unit x min x maxx v X min x maxx

N/mm2 % N/mm2

c1ay Mz 2 0,74 0,73 0,75 - 0,74 0,73 0,75

Ill.,z 6 0,44 0,20 0,65 38 0,44 0,20 0,65

LIll.,z - - - - - - - -CS KS 2 0,69 0,56 0,83 - 0,69 0,56 0,83

KSL 2 0,67 0,55 0,79 - 0,67 0,55 0,79

LC V - - - - - - - -Vbl - - - - - - - -Hbl - - - - - - - -Hbn - - - - - - - -

AAC G, GP 1 0,66 - - - 0,66 - -

n: number oftest series; X : ruean value; max x: maximum value; min x: minimum value; v: coefficient ofvariation

4. TEST PROGRAMME

v

%

----------

The test prograrume covers alI masonry unit types corumercially available in Germany. Unit types with the greatest possible practical relevance and with a low bulk density and/or high penoration ratio were se1ected. Individual units with higher strengths and bulk densities were inc1uded in the tests for purposes of comparison. The unit formats (length x width x heigth) were generally 300 rum x 240 mm x 238 rum (10 DF) or 490 rum x 240 rum x 238 rum (16 DF), yielding a walI thickness of240 rum for one-unit-thick masonry. In order to maintain a 240 rum wall thickness, smaller formats 240 rum x 115 mm x 113 mm (2DF) were laid in stretcher-header bond. Table 2 contains an overview of the test series inc1uded in the study.

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Table 2: Masonry test Overview oftest series performed

Series Unit type Mortar type1)

NMna NMill

LC-units (DIN 18151 / DIN 18152)

A 3K Hb14 - 0,9 - 16 DF U

B 2K Hb14 - 0,9 - 16 DF V

C Vbl S-W 4 - 0,7 - 16 DF U

CS-units (DIN 106)

D KS L - 12 - 1,4 - 10 DF U

E KSL-12-1,4-10DF V

F KS L - 12 - 1,4 - 10 DF

G KS L - 12 - 1,4 - 10 DF

H KS L - 12 - 1,6 - 2 DF V

I KS - 12 - 1,8 - 2 DF V

AAC-units (DIN 4165)

K GP 2 - 0,5 - 499 x 250 x 249

L GP 6 - 0,7 - 499 x 250 x 249

Vertical perforated clay-units (DIN 105)

M HLzB 12 - 0,8 - 8 DF V

N HLzB 12 - 0,8 - 8 DF U

1) NM: General purpose mortar; mortar group na: Lime-cement-mortar, mortar group ill: Cement-mortar

DM: Thin layer mortar

U

DM

U

V

V

U: Test with unfilled perpendjoints V: Test with mortared perpendjoints

As shown in the overview, three different mortars and nine different types ofunits were used in the tests. The mortar parameters, flexural tensile strength and cube compressive strength of the mortars at ages ofl 14 d and 28 d (only in individual cases) and compressive strength in contact with the masonry unit (except DM) were tested for each unit type using the ibac method /3/. In the case of the general purpose mortars, compressive strength was also tested according to /4/ on large prisms and the longitudinal strain modulus and transverse strain coefficient were deterroined.

For the masonry units; the properties which must be tested according to the applicable standard, i.e. unit longitudinal compressive strength and, in the case of the small-format units (2DF), transverse compressive strength were investigated. During testing, the longitudinal and transverse deformations of the large-format units (~ lODF) were recorded and the longitudinal strain modulus and transverse strain coefficient at 1/3 of maximum stress were subsequently deterroined. Table 3 summarizes the test results.

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Table 3: Tests on masonry units Dry bulk density Pd un, compressive strengtb:

- feo un: mean, without shape factor, - fe~~ : 10ngitudina~ - fet,un: transverse, - fe,un,N: net, - fe~un,N : net 10ngitudin~ - fet un N: net transverse, - f* c~un: converted for equal slendemess,

longitudm;U strain modulus E~un and transverse strain coefficient !l~un

Unit Series Pd,un fco,un fcl,un fct,un fc,un,N fcl ,un,N f" cl,un E1,un !ll,un (s. table 2) kgIm3 N/mm2 [-]

LC A 768 4,94 3,70 - 6,68 5,69 5,00 3460 0,25 B 837 5,59 5,17 - 8,22 7,60 7,00 3780 0,20

C 672 3,95 4,04 - - - 5,46 2920 0,14

CS D-G 1240 12,2 5,47 - 19,4 13,0 5,92 5260 -

H 1451 24,4 9,63 13,4 31,7 15 ,0 13,1 - -I 1627 24,1 15,6 13,4 26,2 21,4 21,3 - -

AAC K 535 3,48 3,50 - 3,48 3,50 4,66 2130 0, 18

L 718 6,77 4,57 - 6,77 4,57 6,09 2590 0,17

c1ay M,N 858 21,5 4,75 - 38,3 10,7 4,78 - -

5. MASONRY TESTS: TEST PROCEDURE AND RESULTS

5.1 Test Specimens

t F ( ) Measunng pOtrots at rear

Fig.!: Masonry specimen with measuring points

The masonry specimens were erected on a steel structure developed at the institute which allows specimens to be turned; they can be erected in the usual way and then turned through 90° for incorporation in the test machine. Fig. 1 shows the inductive displacement sensor array used to measure longitudinal and transverse deformation of the specimens.

5.2 Fundamental Remarks on Testing, Loading and the Failure Mechanism

Testing masonry compressive strength along the bed joint axis is much more difficult than mean testing perpendicular to the bed joints, since the procedure used to construct the masonry specimen and incorporate it in the test machine is much more complex and much more liable to errors. An additional difference is to be found in the greater influence of the perpend joints i.e. the force-transmitting contact surfaces between the

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masonry units along the load axis. Unevennesses in the perpend joint surface zone may entail an uneven distribution of stresses. In bonded masonry (stretcher-header bond) it should also be noted that in each second 1ayer (stretcher 1ayer) a longitudinal joint is extending throughout the specimen heigth.

The following cases may be differentiated for the failure of masonry under compressive loads parallel to the bed joints. In masonry with unfilled perpend joints, force must be transmitted between the individual units via the bed joint. The adhesive shear strength between the masonry unit and the bed joint mortar then becomes decisive for resistance to compressive loads. Beyond that the disproportiuate production of the units bears a greater influence upon the masonry strength parallel to the bed joints than in case of mortared perpendjoints. In this case early separation ofthe masonry unit layers from one another is conceivable if there is a very great difference in stiffuess between the masonry unit layers and the bed joint mortar. In such cases, at least localized separation of the individual masonry unit layers will result in lower bearing capacity, due essentially to the greater influence of slenderness.

5.3 Test Results

0<1.mo [N/mm']

4 . O or---------------,----------,-----------------------------,

1.0

• o CJ series D: unfilled perpendjoints ... â ~ series E: mortared perpend joints

O.O~------~------~~----~------_,------_,r_------r_----~ -4.0 -2. 0 0.0

E. [mmlm]

Fig. 2: Investigations on masonry Stress ( cr) - strain (E) - curves CS-units KS L 12, NM lIa

2.0 4.0 6.0 8.0 10.0

E, [mmlm)

As is evident from Fig. 2, there are the expected significant differences between the stress-strain curves for the two illustrative test series with perforated CS-units and general pupose mortars using mortared and unfilled perpend joints respectively. The stress-strain curves for the series with mortared perpend joints are considerably steeper than those for the other series, implying that the longitudinal and transverse strains under identical compressive stresses are much higher ifthe perpendjoints are unfilled.

The powerful influence of perpend-joint mortaring is confirm.ed by the test results in Table 4. Longitudinal compressive strength for the series with unfilled perpend joints was only about half that for the comparable series with mortared perpend joints. The differences in the modulus of e1asticity parallel to the bed joints are still greater.

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Table 4: Analysis ofmasonry tests Compressive strength: unit : - Íco un: according standard, without shape factor,

- Íc~~: longitudin~ - f* cl,un: converted for equal slenderness,

mortar: - Íc mo 14: cube at an age of 14 d, -: ~ m~ji in the joint (ibac method),

masonry : - Íclma: longitudinal in the bed joint axis, - cal Íc~ma : calculated longitudin~ and ratio cal fc~ma 1 fc~ma

Series ~O,un ~1,un f* c1 ,un ~,mo, 14 ~,moji ~1,ma cal ~1,ma1 ) (see

table 2) N/mm2 Eq N /mm2

A 4,94 3,70 5,00 6,58 9,31 1,79 1 2,68

B 5,59 5,17 7,00 6,60 7,78 3,36 1 3,37

C 3,95 4,04 5,47 5,28 8,25 1,25 1 2,82

D 12,2 5,47 5,92 5,87 9,50 1,42 2 3,22

E 12,2 5,47 5,92 5,37 9,50 3,01 2 3,21

F 12,2 5,47 5,92 16,4 - 2,21 3 2,87

G 12,2 5,47 5,92 11 ,9 6,53 1,86 2 3,34

H 24,4 9,63 13,1 5,85 4,93 3,58 4 4,46

I 24,1 15,6 21,3 6,72 4,58 4,27 5 7,96

K 3,48 3,50 4,66 16,8 - 2,34 7 2,58

L 6,77 4,57 6,09 17, 1 - 3,64 7 3,23

M 21,5 4,75 4,78 6,55 11 ,4 1,56 6 3,16

N 21,5 4,75 4,78 6,15 11,4 1, 13 6 3,07

cal ~1 ma 1 ~1 ~a

[-]

1,50

1,00

2,26

2,27

1,07

1,30

1,80

1,25

1,86

1, 10

0,89

2,03

2,72

1) Equations used to calculate masonry longitudinal compressive strength cal Íc~ma

Eq Equation Source

1 Ícl = 0,99 * ~I 0,69 * ~ 140,05 ,ma ,un ,mo, 181

2 ~I = 0,992 * ~I 0,64 * ~ 140,051 ,ma ,un ,mo,

3 ~I = 0,396 * ~1 0,934 * ~ 140,141 ,ma ,un ,mo,

4 ~I = 0,853 * ~I 0,5 74 * ~ 140,199 ,ma ,un ,mo, /7/

5 ~I = 0,703 * ~I 0,740 * ~ 140,207 ,ma ,un ,mo,

6 Ícl = 0,554 * ~I 0,561 * ~ 140,459 ,ma ,un ,mo,

7 ~I = ° 90 * ~1 0,84 ~ma ' ,un 181

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In a further analytical step an attempt was made to calculate the masonry longitudinal compressive strength as a function of unit and mortar compressive strength with reference to the test results. The formula used was the same as that for masonry compressive strength perpendicular to the bedjoints as used in EC 6 /5/.

(1)

By knowledge ofthe compressive strength ofmasonry, unit and mortar the factor K of the equation has been recalculated for each test serie in order to get general factors for further investigations. Test results and recalculated factor K are summarized in Table 5.

Table 5: Analysis ofmasonry tests Compressive strength: - feI,un: unit longitudinal, - fe mo 14: mortar cube at an age of 14 d, - felma': masonry longitudinal , - fk: masonry longitudinal calculated characteristic, according to EC 6 and calculated factor K

Unit Series f'cl,un f'c,mo,14 f'cl,ma fkl ) K2)

Type (see table 2) N/mm2 [-]

LC A 3,70 6,58 1,79 1,43 0,38

B 5,17 6,60 3,36 2,69 0,58

C 4,04 5,28 1,25 1,00 0,27

CS D 5,47 5,87 1,42 1, 14 0,24

E 5,47 5,37 3,01 2,41 0,53

F 5,47 16,4 2,21 1,77 0,29

G 5,47 11,9 1,86 1,49 0,27

H 9,63 5,85 3,58 2,86 0,42

I 15,6 6,72 4,27 3,42 0,36

AAC K 3,50 16,8 2,34 1,87 0,41

L 4,57 17,1 3,64 2,91 0,53

clay M 4,75 6,55 1,56 1,25 0,28

N 4,75 6, 15 1, 13 0,90 0,21

2)

Also various equations from the literature /6,7,8/ were used to calculate masonry longitudinal compressive strength numerica1ly from unit longitudinal and mortar compressive strength. Detailed explanations to the equations and discussions of the test results are to be found in /1 /.

In the case of AAC-masonry, ie. for this masonry with thin-Iayer mortar and mortared perpend joints, results indicated that masonry longitudinal compressive strength can be described with sufficient accuracy using the existing method of calculation for masonry compressive strength perpendicular to the bed joints.

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When using the equations from literatur developed for normal compressive strength to describe masonry longitudinal compressive strength, the difference between the bearing mechanism of masonry under compressive loading paralle1 and perpendicular to the bed joints must be taken into account as a restriction.

cal f ...... in N/mm'

lO~------------~----------~------------------------------' perpendjoint NM n:.&rtar DM

mortared •• ..

6

.. 4

o o • o ............. ~. o o • •

2

.. .... ... ..... ~~.) O~~----~~.~--------~--------~--------~--------r-------~

O 1 2 3 4 5

cal t:,.... in N/mm'

Fig. 3: Relationship between calculated masonry longitudinal compressive strength cal fcl,ma and tested masonry longitudinal compressive strength obs fcl,ma

6

Figure 3 shows the corre1ation between calculated masonry compressive strength parallel to the bed joints and the test results. In nearly all cases the ratio between calculated and tested compressive strength is clearly above 1. In these cases the longitudinal compressive strength of masonry could not be exactly calculated by the compressive strength of the masonry components (unit and mortar). The compressive strength of masonry parallel to the bed joints is overestimated.

That is not surprising because in case masonry with unfilled perpend joints or with stretcher-header bond there are much more influences on the compressive strength parallel to the bed joints and the failure mechanism as written in item 5.3. For regarding these influences futher theoretical and practical investigations have to be required.

This means also that the analysis made here can be regarded only as a first approach quantifying masonry longitudinal compressive strength as a function of unit and mortar strength.

7. LITERA TURE

/ 11 Schubert, P .; Hoffinann, G.: Mauerwerkdruckfestigkeit in Lagerfugenrichtung. Aachen: Institut fur Bauforschung, 1993. Forschungsbericht Nr. F 408

/2/ Glitza, H. : Druckbeanspruchung parallel zur Lagerfuge. Berlin : Emst & Sohn - In: Mauerwerkkalender 13 (1988), S.489-505

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/3/ Schubert, P; Schmidt, St.: Bestimmung der Druckfestigkeit im Mauerwek. Aachen: Institut fur Bauforschung, 1990.-Forschungsbericht Nr. F 304

/4/ DIN 18 555, Teil 4 03.86. Prüfung von Mõrteln ruit mineralischen Binderuitteln; Festmõrtel; Bestimmung der Lãngs- und Querdehnung sowie von Verformungskenngrõfien von Mauermõrteln im statischen Druckversuch

/5/ EC 6, Design ofMasonry Structures

/6/ Mann, W.: Druckfestigkeit von Mauerwerk; eine statistische Auswertung von Versuchsergebnissen in geschlossener Darstel1ung ruit Hi1fe von Poten.zfunktionen. Berlin : Emst & Sohn - In: Mauerwerk-Kalender 8 (1983), S. 687-699

/7/ Kirtschig, K ; Meyer, J.: Auswertung von Mauerwerksversuchen zur Festlegung von zulãssigen Spannungen und charakteristischen Mauerwerksfestigkeiten, Hannover: Institut fur Baustofllrunde und Materialprüfung der Dniversitãt Hannover. - In: Mitteilungen aus dem Institut fur Baustofllrunde und Materialprüfung (1987), Nr. 54-1 , Nr. 54-2 (1988), Nr. 54-3

/8/ Schubert, P. , Meyer, D.: Druckfestigkeit von Porenbeton- und Leichtbetonmauerwerk. Berlin : Emst & Sohn - In: Mauerwerk-Kalender 18 (1993), S. 627-634

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