342

ELASTICITY AND STRENGTH OF CLAY BRICKWORK TEST UNITS

J . J . BROOKS and M.A. AMJAD Department of Civil Engineering

University of Leeds Leeds LS 2 9JT , U. K.

ABSTRACT

Compressive strength and modulus of elasticity of variou s sizes of single leaf walls and piers , constructed from Class B e ngineering clay bricks and a 1:Y,:4Y, mortar , have been determined . In addition , strength and modulus data have been obtained for brick units , brick core samples and mortar units . lt appears that both strength and modulus of brick­work are independent of concrete platen restraint for height/wid th ratios up to 9 . 5 . The accuracy of predicting modulus of brickwork is improved when moduli of brick and mortar are incorporated in composite models.

INTRODUCTlON

The influence of test machine plate~ restraint on the compressive strength

of masonry units is not clear . Platen restraint can cause an apparent

increase in strength when the height/width ratio of the test unit is

less than 2 , but t he types of platen, brick or block and mortar are

relevant factors . When the height/width ratio exceeds about 6 , the

strength of masonry decreases due to the slenderness effect . Platen

restraint may affect modulus of elasticity and general stress- strain

behaviour as well as strength . Clarification of such effects would be

beneficiaI when comparing reported test data and when translating resul t s

of small units into s t rength etc . of full size masonry members .

In the design of masonry, modulus of elasticity is estimated from

empirical functions of brick strength (1 - 3) . Frequently , such estimates

are not very accurate probably because the confluence of mortar type is

not taken into account. AIso, such relationships are restricted to

certain types of brick, whereas it would be desirable to have a universal

method which is applicable to any types of brick or block and mortar.

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Such an approach is feasible by composite modelling of the two main

phases of masonry . Several authors have proposed such models (4- 8) , but

these have not bee n verified in su ffi cient detail, possibly because of

the inconvenience of measuring the moduli of brick and mortar units.

The overall aims of thi s investigation are to investigate the stress­

strain behaviour through to failure of various masonry units made from

a range of bricks , blocks and mortar, and to check the validity o f the

composite model approach for predicting the modulus of elasticity of

masonry . The present paper presents the initial findings of the research ,

and is conc erned wi t h compressive strength and modulus of elasticity of

various sizes of brickwork made from the types of clay brick and mortar .

EXPERIMENTAL DETAILS

Eleven different sizes of brickwork were constructed in stretcher bond

from a Class B engineering perforated clay brick and a 1:Y,:4Y, mortar ;

the maximum height of brickwork was 13 courses . For each size , two units

were built on reinforced concrete slabs and cured under polythene sheet­

ing for approxima tely 25 days . The t op surfaces of each unit were

then capped with reinforced concrete slabs using high alumina cement

mortar. One day before testing at the age o f 28 days , the units were

positioned in the tes t machine and the jack header plate was bedded and

levelled by Fondu cement ; Oemec strain and LVOT attachements were then

positioned and fixed.

One of the aims of the proj ect was to investigate the distribution

of strain over each unit by means of Oemec strain gauges . Since the

time required for readings at each level of load was lengthy , the effects

of creep were minimised by adopting the procedure used for determining

the static modu lus of elasticity of concrete (BS 1881 : Part 121: 1983),

viz . by load cycling the unit twice before taking strain readings at

stresses of 10 , 20 an d 30% of the estimated strength . On comp leti on of

the Demec strain measurements , the strain through to failure was recorded

on an X-Y plotter using LVOTs .

The campressive strength and madulus af elasticity af brick and

martar units were alsa determined at the age af 28 days . Campress ive

strength was measured an full size bricks between bed faces, accarding

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to BS 3921: 1974 , and between headers . For the modulus of elasticity

of brick five different tests were adopted to yield data for the compos ite

models : (a) s ingle brick between b ed faces ; (b) three- stack unbonded

brick between bed faces; (c) five - stack unbonded brick between bed

faces; (d) single brick between headers ; (e) 50 x 25 mm dia. brick

cores be tween alI three faces. In alI these tests , units were tested in

a dry state in order to attach electrical resistance strain gauges, and in

tests (b) and (c) the same brick was used as in test (a) . The compressive

strength of 75 mm mortar cubes was determined according to BS 4551: 1980 ,

whi le 100 x 100 x 250 mm mortar prisms were used for both modulu s of

elasticity and compressive strength .

DlSCUSSION OF RESULTS

Table 1 lists compressive strengths and moduli of elasticity of brick­

work and mortar units ; general ly, the modulus was determined from the

linear part of the stress- strain wave, the limit of proportionality

being approximately 30% of the strength .

Test Masonry Height No. size* widthl

ratio

1 3 x lP 1.10 3 x lP 1.10

2 5 x lP 1. 72 5 x lP 1. 74

3 1 3 x 2P 2.20 13 x 2P 2 . 40

4 7 x lP 2 .34 7 x lP 2.41

5 9 x lP 3 . 00 9 x lP 3 . 14

6 3 x lW 2.26 3 x IVI 2 .30

7 5 x lW 3.84 5 x lW 3.84

8 5 x 2W 3.84 5 x 2\-) 3 . 86

TABLE 1 Test results

Masonry Masonry strength elast i c

modulus (MPa) (GPa)

22 . 9 12.4 24 . 5 16 . 8

23 . 8 16 . 6 28 . 6 16 . 8

14 . 8 15 . 8 15.3 16.7

21.3 15 .5 28 . 7 15 . 5

19.0 15 . 3 22.7 15.8

24 . 8 17 . 3 29 . 0 19.2

25 . 0 16 . 7 28 . 0 18 . 7

20 . 2 17 . 0 22 . 8 19 . 0

Mortar cube Mortar strength elastic

modulus (MPa) (GPa)

6 .1 6 . 6 8 . 8 7 .4

13.5 5 . 0 7 . 6 4 . 6

5 . 6 4.9 6 . 4 6.0

5.4 7 . 4 20 . 2 12 . 6

6 . 9 6.5 7 .2 6.0

6.1 6 . 6 8 . 8 7 .4

13 . 6 5.0 7 . 6 4.6

12 . 5 5 . 0 7 . 6 4.6

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Table 1 cont'd

Test Masonry Height Masonry Maso:1ry Mortal' cube Mortal' No. size* width/ strength elastic strength elastic

ratio modulus modulus (MPa) (GPa) (MPa) (GPa)

9 7 x lW 5 . 22 23 .8 15.5 5 . 4 7 . 4 7 x lW 5 . 26 25 . 5 15 . 9 20 . 2 12 . 8

10 13 x 2W 9.60 23 . 5 15 . 9 6 . 7 6.7 13 x 2W 9 . 62 24 . 2 1 7 . 8 7 . 3 6 . 0

11 13 x 4VJ 9.64 18 . 2 15 . 6 5.6 4 . 9 13 x 4W 9.68 19 . 7 17 . 5 6 . 4 5 . 9

NB * number of brieks high x number of b ricks wide ; P ~ pier , W = wal1.

There was a very large variation in mortar eube strength although mixes had

similar eonsistenee; the mean and standard dev i ation was 8 . 9 ± 4 . 3 MPa.

For the range of sizes of brickwork investigated , there appears to ·be no

significant influe nce of height / least lateral dimension ratio on either

eompressi ve strength or modulus of elast. i city (see Figs . 1 a nd 2) ,

Figure 1 .

<U n. ~

;:: +' Dl) e (!)

l. +' Ul

(!)

> • .-<

Ul Ui (!)

l. c. E o

U

ao r------------------------------------------,

95% C.L. 30

• ° • o O o r"lean 8 --- .-.- -. ---0--- 0 ________ __ _

• 20 • ° 8

- - - - - •• - - ____ --.9s!' ~ J... ____ _

:0

Ü ~li----2~----~----~---~~---~

Height/l e ast late,'al cime nS lon ratlo Compressive strength o f c1ay brickwork as a function of heighV

width ratio

although the large variation in mortal' strength could have been a f actor.

Consequently, i t may be infe rred tha t th e re is little e ffect of concrete

platen re s t rain t on strength and e1astiei ty of briekwork units used in

laboratory tests. Confirmation of this finding is di ff icult as other

C\l ~ o

.>, -'-' .,.., () .,..,

.j.J

ljJ

C\l rl Ql

"-' o

ljJ ;:l rl ;:l 'O o "'"

346

20, _ 0 __ - 8 _.Jl~ Q..,. L...:.. ---° 8 8 __ e_I _ e__ Mean 1

151_ ... ee 8 '0 __ 2 5% 5:..:L ___ _

e

10

5

I oi

2 4

e Pier

° 'iJall

5 8 Height/least lateral dimension ratio

10

Figure 2 . Modulus of elasticity of clay br i ckwork as a function of height/width ratio

investigators' results are sometimes contradictory as far as strength is

concerned, and i t should be remembered that the type of platen may be an

important factor. However, the non - influence of geometry on modulus of

elasticity agrees with that of Lenczner (9).

The compressive strengths of the single brick units and core samples

are shown in Table 2. There is some anisotropy of strength as indicated

TABLE 2 Strength (MPa) of clay brick units and cores

Parameter Bed face Header face Stretcher face

brick* core brick core brick core

f·1ean 105 . 0 134 . 1 15.0 122 . 5 - 113.9

Standard 12 . 0 15.7 1.7 14.9 - 19 . 2 deviation

Nurnber of I 10 6 6 6 - 6 samples

*BS 3921 : 1974 testo

by the cores, the order of strength being bed face> header face> stretcher

face . The presence of perforations clearly affected the strength of

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full-size bricks, especially between headers for which the strength was

only 14 per cent of that between bed faces. Corresponding modulus of

e l asticity data are given in Table 3; the moduli of cores foll ows the

TABLE 3 Modulus of elasticity (GPa) of brick units and cores

Bed face Header face St retcher face Parameter

l-brick* 3-brick 5-brick core brick core brick core

Mean 29.3 34.4 40.4 31.8 17.8 28.9 - 26.0

Standard 3.8 7.3 3.2 4.2 1.3 2 .2 - 2.1

deviation

Number of 10 4 4 6 6 6 - 6

samples I I

*BS 3921: 1974 test

same p attern as that of strength, i.e. there is some degree of anisotro py .

For t he full size bricks, there were difficulties in determining t he

modulus of the middle brick in three- and five - stack unbonded tests b e -

cause the stress - strain curve was non- linear. The reasons were attribut ed

to the uneveness and incomplete contact area of the bed faces, despite

careful preparation by grinding beforehand. The moduli of these tests

(secant modulus of 3 MPa) were greater than that of either the single

brick test or the core test, the moduli of the latter tests being simi lar.

This finding is encouraging since it implies that modulus can be relate d

t o 'standard' brick strength testo

The modulus of elasticity of mortar (see Table 1) was 6.5 GPa with

a standard deviation at 2.2 GPa. The values were obtained from prisms

whose strengths were the same as the mortar cubes, a result which might

s eem surprising because of the platen effect but the prisms were tested

in a dry state which has the effect of increasing strength. As before,

this finding is encouraging because there is a possiblity of relating

the modulus of mortar directly with the 'standard' cube strength.

The average experimental data of this investigation were used to

compare the various methods of predicting elasticity, as prcposed by

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different investigators. For those methods based on brick unit strength,

the accuracies were: Plowman (1), +28%, Sinha and Hendry (2) -24% and

Lenczner (3) +41%. Using the composite model approach, an improvement in

accuracy is achieved, viz. Ameny et alo (4) - 15%, Ameny et alo (5) +6%,

Jessop et ai. (6) +18%, Sahlin (7) +12%. The model used by Brooks (8) was

developed for general application as follows:

Ewy

where Ewy

b y

e

H

A w

Ab

A m

Eby

Em

~

b y . e ~

Eby ·~ + Em·Am

+ r1y. (e + 1).

H E m

modulus of masonry under axial loading;

height of brick or block unit;

number of courses;

height of masonry;

cross- sectional area of masonry;

cross- sectional area of bricks or blocks;

cross-sectional area of vertical mortar joints;

modulus of bricks or blocks (between bed faces);

modulus of mortar;

thickness of mortar bed joints

The above expression yielded a precicted modulus to within 1% of the

average measured modulus.

CONCLUSI ONS

Based on the initial findings of this investigation, the conclusions are:

1 . For the range of clay brickwork geometries of this investigation, com­

pressive strength and modulus elasticity of brickwork are independent

of concrete platen restraint, as measured by the height/least lateral

dimension ratios between 1 and 9.5.

2. Tests on core samples of brick unit showed anisotropy. Strength and

modulus decreased in the order testing between bed faces, h3ader faces

and stretcher faces . The presence of perforations greatly affects the

anisotropy of full - size brick units.

3. The modulus measured by the 'standard' strength test for brick units is

similar to the 'unrestrained' modulus as given by the core sample testo

.....

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4 . Composite mode ls yield more accurate estimates of modulus of brickwork

than emp irical equations based on brick unit strength .

REFERENCES

1 . Plowman , J .M ., The modulus of elast icity of brickwork . Pr oc . Brit . Ceram . Soc. , 1965 , 4 , pp . 37- 44 .

2 . Sinha , B.? and Hendry , A . ~ . , The effect o f brickwork bond on the l oad­bearing capacity o f model brick wa l ls . Proc . Brit . Ceram . Soc ., 1968 , 11, pp . 55- 67 .

3 . Lenczner , O., The effect of strength an d geometry on the elastic and creep properties o f masonry members . Proc . Amer . Masonry Con f ., Boulder , Co l o rado, 1978 , pp . 23 . 1- 23 . 15 .

4 . Ameny, P ., Loove , R. E . and Jessop , E . L ., Strength , elast ic and creep properties of concre te . lnt . J . Mas . Constr ., 1980 , 1 , 1 , pp . 33- 39.

5 . Ameny , P ., Loove , R . E . and Shri ve , N., Prediction of elast ic behaviour of masonry . lnt . J. ~l as . Constr . , 1983 , 3 , 1 , pp . 1- 9.

6 . Jessop , E . J . , Shrive , N. G. and England , G. L ., Elastic and creep proper­ties of masonry . Proc. N. Amer . Mas . Con f ., Boulder , Colorado , 1978 , pp . 12. 1- 12 . 17 .

7 . Sah li m, S ., Structura1 Masonry , Prent ic e - Hall Inc ., New Jersey , 1 971.

8 . Brooks , J . J ., Composite models for predicting e1astic and 1ong- te rm movements in b r ickwork wa11s . Proc . Brit . Mas . Soc ., 1986 , 1 , pp . 17- 19 .

9. Le nczner , O. , Brickwork - guide to creep . Structura1 C1ay Products Ltd ., SCP17 , 1977 , 26 pp .

ACKNOWLEDGEMENT

The authors acknowledge George Armi tage & Sons PLC who supp1ied the br i cks.