11th INTERNATIONAL BRICK/BLOCK MASONRY CONFERENCE

TONOJI UNIVERSITY, SHANOHAI, CHINA, 14 - 16 OCTOBER 1997

STRESSES IN RESTRAINED eLA Y MASONRY P ANELS

J.P.Forthl, P.R.Bingd and J .J.Brooks3

1. ABSTRACT

Axial stresses developed in fired clay masoruy panels under conditions of restraint to free vertical moisture expansion are reported. Panels were constructed using bricks drawn from the same batch but taken from the top and the bottom ofthe kiln, respectively. The age of the bricks at the start of testing was four days. In each case, complementary panels were constructed to determine free moisture expansion and creep ofthe masoruy.

Compressive stresses were induced in the restrained panels as a result of suppressing the irreversible moisture expansion of the masoruy. The induced stresses were significantly lower than the stresses predicted from a knowledge of the unrestrained irreversible moisture expansion and creep ofthe masoruy. The effect ofbrick location within the kiln was found to have a negligible influence on the leveis of stress induced in the restrained panels.

2. INTRODUCTION

Within the UK ali clay brickwork is considered to undergo long-term irreversible expansion as a result of adsorption of moisture vapour from the atmosphere by the brick component of the masoruy[ I ]. Externai restraint to free moisture expansion may lead to stresses being developed in the masoruy. The role of creep of masoruy in relieving the levei of these stresses is important in this respect. From rheological modelling of relaxation (a creep process) Lenczner[2], for example, showed that the build-up of stress in a clay brickwork infill panel, restrained from expanding freely by an unyielding frame, was significantly relieved by creep. The results were not confirmed experimentally, however.

K.EYWORDS : Clay Brick Masonry; Irreversible Moisture Expansion; Creep

I Research Fellow, School of the Environment, Leeds Metropol itan University, Leeds, LS I 3HE, England

2 Lecturer, Schocl of the Envi ronment, Leeds Metropolitan University, Leeds, LS I 3HE, England

J Senior Lecturer, Department ofCivil Engineeririg, The University ofLeeds, Leeds LS2 9JT, England

685

This paper presents the experimental results of a preliminary investigation to determine the leveis of compressive stress that develop in clay masonry panels when long-term irreversible vertical moisture expansion is restrained. Comparisons are made with the estimated leveis of induced stress predicted from a knowledge of the free (unrestrained) vertical expansion, modulus of elasticity and creep, using analytical methods previously verified for masonry[3].

As moisture expansion of clay bricks is known to be very sensitive to changes in firing temperature[4] panels were constructed using bricks taken from the same batch, but drawn from the top and bottom of the kiln, respectively. The manufacturer's data indicated that the difference in firing temperature between these locations was approxirnately 40°C. In this respect, Wyatt[5] previously noted that the average long term expansion of bricks drawn from the bottom of the kiln, from the same firing and using the same clay body, was four times greater than that of bricks drawn from the hotter part ofthe kiln even though the temperature difference did not exceed 60°C.

To maximise irreversible moisture expansion the masonry panels were constructed using four day old bricks which had been sealed in polythene immediately after leaving the kiln.

3. THEORY

A number of analytical methodsare available to predict time-dependent stress in rnasonry from a known strain history. These have been thoroughly reviewed elsewhere[3,6] and only the relevant equations are presented below:

(1) Effective Modulus Method (EM Method)

0"(/) = E(/o) (&(/» [1 + ;(t,to)]

(1)

where o(t) = stress at time t; Erta) = modulus of e)~sticity at the age at first application of load to; «t,la) = creep coefficient (ratio of creep at time t to initial elastic strain at time to); 8(t) = moiSture movement strain at time t

(2) Trost - Bazant Method (Age adjusted EM Method)

0"(/) = E(to) (&(t» [1+ %(/,to) ;(t,/o)]

(2)

where % (t,ta) is the ageing coefficient (assumeci to be 0.82 [6])

686

(3) Rate ofCreep Method (RC Method)

tJ.& +~ E(to)

(J 2 = ---,----'......-<--

_tJ._1/1_ + _ 1_ (3)

E(to) E(to)

where, (JJ and (J2 are the mean stresses during two consecutive tinite intervals oftime Li/J

and Li/2, and t11/1 is the corresponding change in creep coefficient.

(4) Improved Dischinger Method (ID Method)

(4)

where, I/1da> is the lirniting value of the ratio of delayed elastic strain to initial elastic strain (assumed to be 0.4 [6])

4. EXPERIMENTAL DETAll..S

13 course high x 2 brick wide single-Ieaf panels were constructed using 'Melford Yellow' bricks from the same batch but taken from the top and bottom of the kiln, respectively. Panels were constructed in sets ofthree. The fust panel was prevented from expanding vertically by periodic adjustment of the load on the top plate of the loading frame (Fig. I). The four calibrated bars of the loading frame acted as load cells and enabled the levei of stress in the panel to be determined. A second panel was subject to constant stress to determine creep whilst the third panel was unloaded and acted as a control to determine the free (unrestrained) vertical expansion ofthe masonry.

r 4 No. Calibrated Load Cells

A A "

11

oJc:=J[ o -n

11

Jc=::::J[ K

JL

11 oJc:=J[ o -

JL "

~ a. Creep Wall

"

~

T 750mmDemec

Gauge

o

Il

J o U

b. Control Wall

Fig. I Masonry Details and Loading Arrangement

687

•

r 4 No. Calibrated Load Cells

A A " "

Il oJc:=J[o

11

II 11

Jc=::::J[ H

H

11

H

" " y ~ c. Restrained Wall

AlI the panels were sealed with polythene immediately after construction. This was removed at the start of the tests at an age of two days. Six panels in total were constructed.

To eliminate any play between the nuts and threads of the calibrated bars of the loading frame the restrained panels were initially subject to a very slight precompression by hand­tightening the nuts. The levei of stress imposed on the panel was negligible but this procedure was considered necessary to ensure that irreversible vertical expansion of the panels was totally restrained from the outset.

Strains in the panels were measured using a demountable, mechanical 'Demec' gauge. Readings were taken daily for the first few days and then at less frequent intervals. After 2 months readings were taken only every two to three weeks. AlI tests were carried out in a controlled environment at a temperature of 2l± 1 ac and a relative hurnidity of 65±5%. In addition to the strain measurements on the masonry panels the moisture movement strain of unbonded bricks between header faces was also measured. Readings are still continuing.

The same mortar was used throughout, namelya 1:Y2:4 \/2 (by volume) OPC: hydrated lime: sand mixo The 28 day mean compressive strength of the mortar was 15. O MP'a . .

5. DISCUSSION

Table 1 gives details ofthe compressive strength and unit water absorption properties of the bricks used, tested in accordance with BS3921 [7]. The results are consistent with the findings of previous research[5,8-10] which showed that strength increases, and water absorption decreases, with an increase in firing temperature.

Table 1. Unbonded Brick Details

Brick Position Conípressive Water Absorption Initial Rate of inKiln Strength (%) Suction

(MPa) 5-hr boiling 24-hr soaking (kgmm-2 mino!)

Bottom 44.0 21.4 17.8 1.78

Top 47.4 19.2 16.0 1.63

...

Figure 2 shows the unrestrained vertical expansion of the control walls and the expansion of individual unbonded bricks between header faces. Whilst there is little difference in the expansion of the unbonded bricks after 240 days, the masonry panel constructed with bricks from the bottom of the kiln expanded approximately 70% more than the panel constructed with bricks taken from the top of the kiln. The ratios of brickwork to brick expansion after 240 days were 9.4 and 5.6 for the panels constructed with bricks from the bottom and top of the kiln, respectively. Previous research has shown that irreversible moisture expansion of an extruded clay brick in the bed face direction is of a similar order to that in the header face direction[ 11-13]. The bigh leveis of irreversible

688

vertical expansion recorded in the control walls of this investigation are, therefore, difficult to explain in tenns of the expansion of individual bricks and the shrinkage of mortar. Similar 'enIarged' vertical moisture expansions of c1ay brickwork have been reported previously[l1,14, lS]. This phenomenon is presently under investigation by the authors.

100

O "? ~ -100

:z -200 o ~ -300 cC

~ -400 w w -500 a: i= -600 UJ

Õ -700 :::::E

-800

-900

"it"~~~:="~~~~>--~-{}--...6=-=l\ B r I c k -T o p Brlck-Bottt m

a Maaonry-Top

o

G

o 40 80 120 160 200 240 280 320 TIME Cdays)

Fig. 2 Moisture expansion of unbonded bricks and masonry panels

Figure 3 shows axial creep of the panels constructed using bricks from the top and bottom ofthe kiln, respectively. Whilst the total time-dependent defonnation ofboth

1200

.;-1000 o

o... 800 w w a: o 600 o u. o 400 w o... UJ

200

O

O 40

Fig. 3.

G

o

o

Brlck klln posltlon :-o Bottom

~_j!-"1Y To P

E;oltom • 5.97 GPa

E;op • 6.04 GPa

80 120 160 200 240 2.80 320 TIME CdaYI)

Specific creep 01 masonry panels Co Initlal elaatlc modull)

689

panels was broadly similar, creep of the panel constructed using bricks from the bottom of the kiln is higher due to the larger vertical moisture expansion of the associated control wall (Fig. 2), which has to be added to the measured strain of the loaded wall in order to isolate creep.

Figures 4 and 5 show the compressive stresses developed in the panels when irreversible vertical expansion was restrained. Over the 240 day test period the leveis of stress were very low, typically ranging between 0.05 and 0.2 MPa for each pane!. Figures 4 and 5 also show the stresses in the restrained panels predicted from the free vertical expansion of the associated control walls, modulus of elasticity and creep, using Eqs. (1) to (4). In each case ali the methods overestimated the measured stresses, which is contrary to the findings for concrete[ 6]. Whilst for design purposes the use of these analytical methods will provide a upper limit to the leveis of compressive stress induced in restraint situations, the accuracy of such predictions is low with the leveis of stress being over­predicted by between 400% and 800%. It is, therefore, apparent that the 'enlarged' vertical expansions measured in the control panels of this investigation do not generate significant leveis of stress in masonry panels under conditions of restraint to irreversible vertical moisture expansion. Further research into the response of clay brick masonry under conditions of restraint is current1y in progresso

1.2

1.0

0.8 as o-~

m 0.6 m w a: l-m 0.4

0.2

0.0

o 40 80 120

Ijl"--B"--o---B---o Rate of Creep t--S- -- e.- - o- --~ Improved Dischinger

0---6 ....ó---<>--~--o------<> T r o s t • B a z a n t

...... _o---111---0----0 E ff e c t I v 8 Mo d u I u s

160 200 240 280 320 360 TIME (days)

Fig. 4. Measured and Predicted Stress in restrained wall using bricks taken from the bottom o, the kiln

690

1.0

0.8

~

ai

~ 0.6 fi) fi) w a: ~ 0.4

0.2

0.0

D---g Rate 01 Cr88p n- __ g---o o I d DI h' D---B---5---g---' .. _ ~. __ g-- -o mprove se Inger

I ó- -.- S" D---~' -- g--- ó-- ' - 'O Trost·Bazant

I ~ - o Efleetlve Modulus

-"'I--@---121 M e a a u r e d

o 40 80 120 160 200 240 280 320 360 TIME (daya)

Fig. 5. Measured and Predicted Stress in restrained wall using bricks taken from the top of the kiln

6. CONCLUSIONS

1. The compressive stresses induced in the restrained panels of this investigation were very low, typically ranging between 0.05MPa and 0.2MPa, and appeared to be unaffected by the location of the brick within the kiln.

2. Analytical methods overestimated the leveis of stress in the restrained panels by between 400% and 800%.

3. For the clay brick used in this investigation a difference in firing temperature of 40°C has negligible influence on irreversible expansion between header faces of unbonded units.

4. After 240 days the free vertical expansion of a masonry panel constructed with bricks from the bottom of the kiln is approximately 70% greater than that of a panel built with bricks from the top of the same kiln batch.

5. From a comparison ofthe expansion ofindividual bricks with that ofthe associated masonry control panel it appears that the vertical expansions of the control panels ofthis investigation include an additional 'enlarged' expansion. When restrained, these ' enlarged' vertical expansions did not generate significant leveis of stress in th~ masonry panels.

691

ACKNOWLEDGEMENTS

The authors would like to thank Hanson Brick Company for supplying the bricks and the BDA for sponsoring this ongoíng programme of research.

7. REFERENCES

1. Britísh Standards Instítutíon, BS 5628:Part3 : 1985.Use ofMasonry:Part3: Materiais and Components, Desígn and Workmanship, BSI, London, 1985.

2. Lenczner D. "Stresses ín brickwork ínfill panels due to moísture expansíon", Masonry InternatíonaI, No. 7, 1986.

3. Bíngel, P.R , "Stress relaxatíon, creep and straín under varying stress in masonry", PhD Thesís, Universíty ofLeeds, England, December 1993 .

4. Freeman I.L. and Smith RG. "Moísture expansíon of structuraI ceramics", Building Research Statíon, Garston, 1966.

5. Wyatt K. l "DimensíonaI change and its control in clay masonry construction", Proc. North American Masonry Conference, Universíty ofColorado, 1978.

6. Neville A.M., Dílger W.H. and Brooks l l Creep of plaín and structuraI concrete. Constructíon Press, London and New York, 1983.

7. Britísh Standards Instítutíon, BS3921 : 1985 .Britísh Standard Specification for Clay Bricks, BSI, London, 1985.

8. Bonner D.G.R and Butterworth R, "Clay building bricks ofthe UK", Minístry of Works, NBAC, H.M.S.O., 1950, 170 pages.

9. Hoskíng lS., Huebar H.V. , Waters E.H. and Lewis RE., "The permanent moísture expansíon of clay products 1. Bricks", CSIRO Aust. Dív. Build. Res. Tech. Papo No. 6,1959

10. Ritchie T., "Residual stress in clay bricks", Proc. Brit. Ceram. Soc., No. 27, 1978, pp 1-6.

11. Forth lP. and Brooks ll, "Influence ofthe clay unit on the moisture expansion of masonry", Proc. 4th Int. Mas. Conf, Proc. Brit. Mas. Soe., No. 7, 1995, pp 80-84.

12. West H.W.H. "Moisture movements of bricks and -brickwork", Trans. British Ceramic Socíety, 66 (4), 167, pp 137-160.

13. Freeman I. L. and Smith R G. "Moisture expansion of structuraI ceramics". Buíldíng Research Statíon, Garston, 1966.

14. Brooks l l and Bingel P. R "Moísture expansion ofFletton brickwork". Proc. 1st Int. Mas. Conf, British Masonry Socíety, No. 2, pp 8-11, Stoke-on-Trent, 1988.

15. Beard R , Dínnie A. and Richards R "Movement ofbrickwork". Proe.ofthe British Ceramic Socíety, Spring 1966.

692