17.-The Effect of Mortars on the Strength of Brickwork Cubes

by S. B. A. HOATH, H. N. LEE and K. H. RENTON Tilling Construction Services Ltd., Gerrards Cross, Bucks.

ABSTRACT

The compressive strength of mortar cubes is used as a means of classifying mortars. The present work, using matured Buxton lime putty, has shown that this is not a reliab/e means for predicting the strength of brickwork. Mortars containing lime may not a/ways have the greatest strength in a /aboratory mortar cube but when tested in the form of brickwork pro­duce a higher ratio of brickwork strength to mortar strength. This is a/most certain/y due to the effect of difference in bond strengths between lime and non-lime mortars.

L'lnfluence des Mortiers SUl' la Résistance Mécanique de Cubes de Maçonnerie en Brique

La résistance à la compression de cubes de mortier est utilisé pour la classification des mortiers. Le présent travai!, pour lequel on s' est servi d'un enduit de chaux de Buxton, a montré que cette propriété ne constitue pas un bon critere pour l' évaluation de la résistance mécanique des maçonneries en brique. Les mortiers contenant de la chaux ne présentent pas nécessaire­ment le maximum de résistance mécanique sur un cube de mortier de laboratoire, mais donnent un rapport plus élevé: résistance de la maçon­neriejrésistance du mortier, quand I' essai est fait sur une maçonnerie en brique. Ceci est presque certainement dú à I'effet des différences d'adhérence entre les mortiers avec chaux et les mortiers sans chaux.

Der Einjluft des Mõrtels auf die Festigkeit von Mauerwerkswürfeln Zur Klassifizierung von Morteln benutzt man die Druckfestigkeit der aus ihnen hergestellten Probewülfel. Die vorliegende Arbeit zeigt am Beispiel von abgebundenem Buxton­Kalk-Binder, dajJ diese Methode nicht zur Vorherbestimmung der Mauerwerksfestigkeit geeignet isto Kalkhaltige Mortel erreichen im wür­felformigen Probekorper nicht immer ihre grojJte Festigkeit. Bei der Prü­fung im Mauerwerk ergibt sich ein grosseres Verhiiltnis der Mauerwerks­festigkeit zur Mortelfestigkeit. Das ist hochstwahrscheinlich auf die Aus­lVirkung der unterschiedlichen Bin­dungskriijie bei Kalkmorteln und anderen Mortelarten zurückzuführen.

1. INTRODUCTION The introduction of standard test methods for mortars 1

has enabled workers in the UK to adopt a more system­atic study of the properties of different types of mortars. Results of tests from different sources can now be inter­changed and are beginning to have practical importance. This is particularly the case for wet properties of mortars. Some reserve, however, must be applied when using the compressive strength tests on morta r samples to predict the ultimate wall strength. Such a claim is implied in the specification for load-bearing brickwork2 as shown in Table 1.

factors to wall strength, in particular adhesion and the effect of air entrainment. If a Iime-based mortar has a strength of 400 Ibfjin2 the uItimate wall strength should be higher than a non-lime mortar of equal strength, and similarly a lime-based mortar which is marginally below this value may provide greater wall strength than a non­lime mortar showing a strength test above 400lbfjin2•

It is considered by the authors that the strength test on mortars does not allow for the contribution of other

The wall strength is dependent upon the strength of a mortar and of the units and the adhesion between the two. Adhesion is a/fected by a number of mortar properties and the presence of water at the mortar-unit interface is necess­ary. A mortar of high water retentivity is, therefore, im­portant. RITCHIE and DAVISON 3 prepared mortars to con­form with M, S, N, O and K types, as designated by the

TABLE I - MINIMUM AVERAGE COMPRESSIVE STRENGTHS OF LABORATORY SPECIMENS OF MORTAR

Mix· Min . compressive strength Grade Description (vol.) (IbJlin 2)

CoLoS 7 days 28 days

] Cement:sand 1 :0---!-: 3 1600 2400

11 Cement: lime: sand : 1 :-!-:4-!- 800 1200

JTl Cement: lime: sand 1 1 5- 6 } Cement :sand with plasticizer 1 O 5- 6 400 600 Masonry cement: sand 1 O 4-!- :

IV Cement :lime:sand 1 2 8- 9 "I I Cement: sand with plasticizer 1 O 7- 8

J 150 225 I Masonry cement:sand 1 06

Hydraulic lime:sand 012 I

* C :L:S= Cement :lime:sand 113

114 The Effect of Mortars on the Strength of Brickwork Cubes 5000 ~-----.------~-----,------,------, ASTM4 and from the results obtained plotted graphs as

shown in Figure 1. In these tests dry hydrated lime was used and it is stated that higher water retentivity results are expected when matured lime putty is used.

~ -"" 4000

~,ooo ri

Loo-\!) z W cr l­V)

W > , ih 1000 r---V) ,

W cr <l. ~ o u

87

86

85

84

--, 83

j 82 ~

~ 81 W

cr cre.

1 80

\ 1 76 ~ --- COMF' STRENGTH ~o l ~ ----- WATER R~ENTIV~T~ __ ~ 75

Similar results were obtained from mortars made from seven different types of lime and standard sand, which were tested by the latest test methods 1 and are shown in Figure 2.

It can be seen from the results of these two groups of workers that the replacement of cement with lime reduces strength but improves water retentivity.

To determine the relationship between brickwork cubes, built and tested in accordance with the requirements of the specification,2 and different types of mortars, a series of experiments were made. A programme using fuIl-size waIl panels would be beyond the scope of these labora­tories, but the relationship:

Compressive strength of storey-height walls = 205 ·45 + 0·60 x compressive strength of brickwork cubes

has been shown to exist. 6 Figure 3 shows the experimental results on which this relationship is based .

100C 80C 60C 40C 20C OC 2. EXPERIMENTAL o L 20 L 40 L 60 L 80 L 100 L

PROPORTlON OF CEMENT (c) LIME (L) IN MORTAR (c + L) : SANO = 1:3 BY VOLUME

FIGURE I- Relationship between mortar composition, compressive strength and water retentivity. After RITCHIE and DAVISON . .1

In these experiments the following bricks were used (mean compressive strength in brackets): 1. Fletton-type clay bricks (4750 Ibf/in2).

4000 ,..·--------------.,----------,.---------,------------------, 100

~ ~ ~ 3000 o

_-0------_0---

-- COMF' STRENGTH - ---- WATER RETENTIVITY

CEMENT - LIME - SANO

o W Z

95 ~ W cr ~

90 ~ :> 1=

~ 85 ~ cr W I-~

FIGURE 2- Relationship between mortar composition, average compressive strength , water retentivity for morta r made with different types of lime. After GILLARD and LEE.5

I

2000 ,

>-W a: O

I o

l-V)

LL0 o

O .~ 1500- bO"l-x- o

1-"" O'

~~ <,)(

<, ." W...J

o a:...J '" ,,'j. I-c( o V) 3; o W >1- o -x ~~ 1000 !--LIJLIJ a:x <l. ~ O u

1500 2000 2500 3000

MEAN COMPRESSIVE STRENGTH OF LABORATORY-MAOE BRICKWORK CUBES Dbf/ i,,~ FIGURE 3- Relationship between wall st rength and laboratory brickwork cube strength .

After WEST. EVER ILL and BEECH 6

S. B. A. Hoath, H. N. Lee and K. H. Renton 115

2. Selective hard pressed clay bricks (5250IbfJin2).

3. Repressed wire-cut clay bricks (8750IbfJin2).

4. Calcium silicate bricks (white) (4420IbfJin2).

All the mortars were made from two batches of com­mercial sand from the same source. The sieve analyses are given in Table 2. Both comply with BS 1200:1955.

TABLE 2-SIEVE ANALYSES OF SANDS

Weight (%) passing sieves B.S. sieve

SandA Sand B

3fl6in. 100 100 No. 7 95 97 No. 14 91 90 No. 25 79 73 No. 52 39 26 No. 100 9 3 No. 200 c1ay

and silt 1·80 1·26

One large batch of mortar was prepared from sand A, a matured lime putty and ordinary Portland cement and used for all the clay brickwork cubes. A second batch of mortar was prepared from sand B, a matured lime putty and ordinary Portland cement and used for the calcium silicate brickwork cubes.

In all cases six brickwork cubes, 9 x 9 x 9 in. nominal, and six 4-in. cubes ofmortar were prepared.2 The mortar cubes were stored at a temperature of 20 ± 2°C and a rei ative humidity of greater than 90 %. The brickwork cubes were stored in an unheated store.

The results obtained are given in Table 3.

3. DISCUSSION

It can be seen from Table 3 that in all cases the mortars satisfy the strength requirements of the standard,2 but for Grade IH mortars in all cases the strength of the cubes built in non-lime mortars is considerably lower than for lime mortars. When the ratio of brickwork cube strength to the strength of the bricks and mortars is calculated the results shown in Table 4 are obtained.

In all these results the traditional cement: lime: sand mortars have the highest ratio of brickwork cube strength to mortar strength. It is interesting to note that although the addition of a plasticizer to a I: I : 6 mortar does not reduce its compressive strength, it does affect the strength of brickwork cubes made with it. The effect, however, is much less than that of air-entrained non-lime mortar or non-lime masonry cement. For instance, the ratio of brickwork cube to mortar strength at 28 days for a I: 1: 6 mortar was found to be 1'82, 2 ·58 and 4·06 for the three types of c1ay bricks and 2 ·16 for the calcium silicate bricks. The addition of a plasticizer to the

T ABLE 3-CLAY AND CALCIUM SILlCATE BruCKS

I Mortarmix Mortar compressive Brickwork cube

strength compressive strength Mortar I by volume* (lbfJin 2) (lbfJin2) grade (C:L:S)

7 days 28 days 7 days 28 days

Clay bricks (4750 IbfJin 2)

I 1 :-!-:3 2220 2590 1100 1290 11 1 :t:4t 955 1570 1015 1345 III 1: 1:6 440 680 1060 1235

1:1:6+P 450 645 855 950 1:0:6+P 645 950 385 1110 1:0:4tMC 770 875 740 945

IV 1 :2:9 180 245 635 660

Clay bricks (5250 lb.ff in 2) I 1 :-!-:3 2220 2590 1480 1875 11 1 :t:4t 955 1570 1360 1590 III 1: 1:6 440 680 1310 1680

1:1:6+P 450 645 1075 1215 1:0:6+P 645 950 925 1275 1:0:4tMC 770 875 890 1140

IV 1:2:9 180 245 895 1050

Clay bricks (8750 IbJI in2) I 1 :-!-:3 2220 2590 2860 3360 11 l :t:4t 955 1570 2410 2700 III 1: 1:6 440 680 2450 2770

1:1:6+P 450 645 1940 2140 1:0:6+P 645 950 1725 1900 1:0:4t MC I 770 875 1650 2080

IV 1 :2:9 180 245 1520 1810

Calcium silicate bricks (4420 IbfJin2) I 1 :!-:3 2370 2830 1980 2230 II l :t: 41- 800 2100 1615 1725 III 1: 1:6 410 920 1935 1980

1:1:6+P 400 750 1350 1410 1:0:6+P 645 1265 1320 1360 1:0:4tMC 760 1465 1435 1420

IV 1 :2:9 I 180 325 1585 1730

* C:L:S=Cement:lime:sand; P=plasticizer ; MC=masonry cement.

116 The Etfect of Mortars on the Strength of Brickwork Cubes T ABLE 4 - STRENGTH RELATlONSHIP ; BRICKWORK CUBES TO MORTA R ANO BRICKS

Type of morlar · (C:L :S)

Cio)' bricks (4750 Ibflil11)

1:.:3 I :j:4j I : 1:6 1:1:6+ P 1:0:6 + P 1:0: 4j MC 1 :2:9

Cla)' bricks (5250 Ih/lin1)

1:.: 3 I : j:4j I : 1:6 1: 1:6 + P I :0:6 + P I :0:4j MC 1:2:9

C/ay bricks (8750 Ibfl;"l) 1:.:3 1:1 :41 O: 1:6 1: 1:6 + P 1:0:6 + P I:O:4j MC 1:2:9

Co/dum silicate bricks (4420 Ibll;/I ' ) 1:.: 3 I :j:41 I : 1:6 1: 1:6 + P 1:0:6 + P 1:0:41 MC 1:2:9

I

Rario of bricklt'ork cube lO: Bricks Mor/an

7 days 28 days 7 days 28 days

0·23 0·27 0 '50 0'50 0 ·21 0·28 J-06 0 '86 0·22 0·26 2·40 1·82 0'18 0 ·20 1·90 1·47 0 ·08 0 ·23 0·60 1·17 O-lO 0 ·20 0·96 1·08 0·1l 0·14 3·52 2·79

0'28 0-36 0 ·67 1·10 0·26 0 ·30 1·42 1·01 0 ·25 0·32 2·97 2·58 0·21 0·23 1·67 1·88 0 · 18 0·24 1·43 1·20 0 ·17 0·22 1·16 1·30 0 ·17 0·20 4 ·96 4 ·28

0·33 0-38 1-29 1·30 0·27 0 ·31 2·52 1·72 0 ·28 0 ·32 5'56 4·06 0 '22 0 ·25 4-32 3-32 0-20 0 ·22 2·67 1·99 0·19 0·24 2·14 2·07 0 ·17 0 '21 8-45 7'40

0 ·45 0 ·50 0·84 0·79 0·37 0 ·39 2·01 0 -82 0-43 0 ·45 4·72 2·16 0·31 0·32 3·37 1·88 0·30 0 ·31 2-05 1·08 0·33 0-32 1·89 0·97 0 ·36 0 ·39 8·83 5-32

• C:L:S=cement :llme:sand; P= plasllclzer; MC=masonry cemenl.

I: 1:6 mortar gives an average reduction of about 20 %. A cernent: sand plast icizer mortar gives an average reduction af 48 % and rnasonry cernent morta r 49 %.

SKEEN' compared lhe compressive strength af morta r cubes and 9 x 9 x 36 in. brickwork piers aI an age of 28 days. The results when high-strength bricks were used showed, with one exception, that lhe fatio af mortar slrenglh to brickwork piers was always higher for lime mortars (l:t:3) Ihan non-lime mortars (i :0:3). In lhe case af lhe other bricks no relationship can be seen, but in his work Skeen used dry hydraled lime and not matured lime putt:-' as in the present work.

4. CONCLUSIONS Although Ihis work requires further amplification it does show thal c1assification of a mortar by a laboratory strength test alone could lead to incorrect assumptions. The brickwork cube slrength lesl using the bricks and mortar to be used on lhe si te appears to give a more

realislic resul!. The resulls of the work also show the validity 01' the advice given by mosl calcium silicale brick manufacturers Ihal their bricks should be laid in rich lime morlars.

REFERENCES I. BRITlSH STANOARDS INSTITUTION, Methods ofTesting Mortars.

8th Dfaft 68/3502. 2. BRITISH CERAMI C RESEARCH ASSOCIATION, Model Specification

for Load-bearing Brickwork. B. Ceram. R.A. Spec. Publ. 56, 1967. (Available from lhe Nationa l Federalion of Clay Industries, London).

3. RITCHIE. T. and DAvlsoN J. 1. . Cement-Lime Mortars . Buildillg Research. Ouawa. TecI! . Papo (183), 10. 1964.

4. AMERICAN SOCIETY FOR TESTI NG MATERIALS, Mortars for Unit Masonry . C270- 68.

5. GILLARD, R. and LEE. H. N., Testing of Building Mortars Using lhe New British Standard Methods. Paper presented to A .S.T.M. Symposium, Al lantic City NJ., 1969.

6. WEST, H. W. H., EVERILL, J. B. and BEECH, D. G., Experiments in the Use of lhe 9-in. Brickwork Cube for Site Control Testing. Proe. Brit. Ceram. Soe. (lI), 135, 1968.

7. SKEEN . J. W .. The Strength of Brickwork BuiJt with Plasticized (Aerated) Cement- Sand Mortars. Trans. Brit . Ceram. Soe. 62, (8). 631. 1963.