11th INTERNATIONAL BRICKJBLOCK MASONRY CONFERENCE

TONGJI UNIVERSITY, SHANGHAI, CHINA, 14 - 16 OCTOBER 1997

MOISTURE TRANSFER BETWEEN MORTAR AND CLAY BruCK UNIT AND ITS EFFECT ON LONG-TERM DEFORMATlONS OF MASONRY

1. P. Forth1 and J. J. Brooks2

I. ABSTRACT

The prediction oflong-tenn movements in clay briek masonry by composite modelling is complicated by the fact that unbonded brick and mortar prisms may not be representative ofthe bonded briek and mortar elements ofthe masonry. In clay brick masonry built with undocked units, this ean be a result of the moisture transfer between mortar and brick which decreases the mortar's potential for creep and shrinkage. This paper compares the -long-tenn movements of bonded bricks and mortar joints within 5-stack high clay masonry walls with those of unbonded bricks and mortar prisms. Modification factors are then applied to previously published eomposite modelling data. Using these factors, ereep and moisture movement strains are predicted to within 10010 whereas previously, with unmodified data, composite modelling had overestimated the movements by between 50010 and 100%.

2. INTRODUCTION

The long-tenn movements of clay brick masonry are influenced by a number of factors, viz. euring conditions, age at loading, geometry, mortar and brick type etc. [1-4]. In addition, the time-dependent movements can be critieally influeneed by an interaction between the clay briek and mortar elements of the masonry composite due primarily to the unit water absorption properties [5,6]. This interaction results from the transfer of moisture between the two elements of the masonry, leading to a reduction in the water content of the bonded mortar, thereby decreasing its potential for creep and shrinkage. Previously, the transfer of moisture has been observed by a number of investigators researching predominantly bond strength [7-10] and mortar bed-joint compressive strength [9-12]. Anderegg [7] suggested that the compressive strength of the mortar bed-joint would increase with an inerease in the initial rate of suction, although for mortars with a high OPC content, the compressive strength would be expected to

Keywords: Clay Brick Masonry; Moisture Transfer; Shrinkage; Creep; Prediction.

lResearch FelIow, School ofthe Environment, Leeds Metropolitan University, Leeds, LSl 3HE, Engtand 2Senior Lecturer, Department ofCivil Engineering, The University ofLeeds, Leeds LS2 9IT, England

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decrease when the 'product cQmbination' involved high absorption bricks. Schubert [11], however, shows that a reduction in compressive strength occurs even with low unit water absorption properties and that this reduction is indepen4ent of the levei of initial rate of suction and water absorption, although the amount of moisture transfer was influenced by unit water absorption properties. The amount of water absorbed also depends on the condition ofbrick (dry or docked), type ofmortar and its w/c ratio [7]. The relationship is further complicated by the variation in pore structure throughout the body of the brick and the interrelationship that exists between the pore properties of the bonded brick and mortar.

The unit / mortar interaction can be significant when predicting the elastic a'1d long-term behaviour of clay masonry from composite models. A number of models have developed over the years [6, 13-15], however, experimental verification has only been achieved on a limited number of test data. More recently, Brooks [16-19] developed alternative composite models for masonry deformations which were successfully validated on a number of occasions [16-19]. However, for certain practical situations the accuracy of the models was considerably reduced because the unit / mortar interaction was not taken into account. The presence of this interaction means, therefore, that the input data obtained from the unbonded brick and mortar samples are not representative of the behaviour of the bonded components.

This paper reports a study of the transfer of moisture between the brick and mortar components from 30 minutes after laying ofbricks to 120 days. The etfects ofmoisture transfer on the long-term movements of Armitage clay masonry are then quantified by comparing the bonded unit and mortar movements with those of equivalent unbonded bricks and mortar prisms. The modification factors developed were then applied to previously obtained unbonded data [3] and time-dependent strains were re-estimated by the model developed by Brooks [16] .

Measurement ofbonded mortar and unit deformations

To isolate the time-dependent movements ofthe bonded brick and mortar ofthe masonry so that they · could be compared with the movements of the equivalent unbonded brick and mortar prism the following procedure was adopted. The deformations of the bed­joint mortar in the masonry were deterrnined by subtracting the deformations of the bonded brick units in the masonry from the overall deformation of the masonry. The strain in the mortar was then calculated from:

&oortu = ~(l_5_0_x_iiDM_""'Y...:..)::.--_(~(_I5_0_-_m_x-,n):-x_6nUD_· =»

where 150

lmortar

lmasonry GUruts m

m x n

length of the Demec gauge used to measure the overall strain ofthe masonry (mm) strain ofthe mortar (J.1s)

mean strain measured on the masonry (J.1s)

mean strain ofthe units (J.1s)

mortar joint thickness (mm)

673

(1)

n number of mortar joints over which the Demec gauge spans

Assuming a nominal mortar joint thickness of 10 mm and by making n equal to 2, Eq. (1) reduces to:

&mortar = 7.5 &rnasoory - 6.5 &units (2)

3. EXPE~NTALPROCEDURE

Figure 1. iJlustrates the 5-stack bonded brick walls together with the loading frames used in this investigation. Four walls were constructed, two walls were monitored for moisture movement strain, while the remaining two walls were used to isolate creep of the masoríry. Immediately after construction, ali walls were sealed with polythene until the application of the load at 14 days, so as to prevent moisture loss to the surrounding

~

I" II

I" I I

~

+ +

+ +

a. Creep wall

í 2 No. Calibrated Load Cells

~

" I II

1-:somm Demec J Gauge

"I , I

~

r--:::: Capping Mortar I (coated .... ith bitumen paint)

+ +

+ + I Steel Plate

I I

b. Control wall

Fig. 1. 5-Stack Masonry Walls and Loading Frames

atmosphere. A single Class (ii) 1 : Y2 : 4Y2 OPC : lime : sand mortar mix, having a w/c ratio of 0.76, was used in the construction of the four walls. From this mix, individual mortar cubes and prisms were cast to determine compressive strength, modulus of elasticity, creep and shrinkage of the unbonded mortar. AlI prisms were partially seaIed to the same volume I exposed surface area (vi s) ratio ofthe bonded mortar ofthe 5-stack masonry. That procedure has been used previously [2] to simulate the rate of external moisture loss from the masonry. Individual clay units were also monitored for moisture movement strain and creep in the header and bed face directions, the bed face creep of the units being obtained from 5-stack unbonded walls. Table 1 gives details of the unbonded brick and mortar properties. Ali strains were measured using demountable Demec gauges and 50 mm acoustic I vibrating wire gauges (bonded I unbonded units).

674

Table 1. Unbonded brick and mortar prism details

Component Compressive Type Strength

(MPa) Armitage cIass 86.4 'B' Eng Brick Mortar prism 17.5

.. • lnitIal rate of suctIon * Bed face I Header face

Modulus of Elasticity 24-hr soaking

(GPa) (%) 27.0/19.0' 5.0

13 .0

Absorption 5-hr boiling IRS·

(%) kgm2min- I

6.9 0.48

- -

To monitor the moisture transfer between brick and mortar, modified unit water absorption tests were carried out using 17 different two-course bonded masonry couplets (Fig. 2). The weight of one unit ofthe couplet was recorded before laying and then after assembling the couplets at the followíng ages: 0.5, 1, 3, 5, 7, 14 and 24 hours, 3, 7, 14, 28, 40, 50, 60, 70, 80, and 120 days. To prevent bonding between the mortar and the brick to be weighed, a layer of polythene mesh was used. The weight of the masonry couplet was checked before the top brick was weighed to ensure no loss of moisture from the couplet during curing. The mQdified or percentage water absorption at the various times was determined as:-

Percentage Water Absorption = M", x 100 Mb

where Mw = weight ofwater absorbed by brick; Mb = weight of the brick prior te construction in the masonry couplet.

Brick (top) ---- ythene Mesh

IPOl

I Brick (banom) - Mo rtar Joint

Figo 2. Bonded Masonry Couplet for Modified Water Absorption Tests

As wíth the other masonry and unbonded tests, ali couplets were stored in a controlled environrnent of 21 ± 1°C and 65 ± 5% relative humidity, after being stored under polythene sheet for the first 14 days.

675

DISCUSSION OF RESUL TS

Moisture Transfer

The pattem of moisture transfer to and from the brick during the curing period and the following period of drying after the polythene is removed at 14 days is shown in Fig. 3. Initially, moisture is rapidly absorbed by the brick from the mortar joint, however, the situation then reverses and the moisture appears to be taken up by the mortar as no moisture is lost to the outside, because the system is sealed. From 14 days, after the polythene was removed, moisture is then able to be lost to the atmosphere from both brick and mortar element, however, Fig. 3 suggests that only a small loss in moisture occurs from the brick.

This pattem of moisture transfer has been observed before [8,10] and is an indication that the mortar is hydrating and is no-Ionger plastic. Sneck [8] commented that the reverse of flow back to the mortar was assisted by the blocking of pores in the brick by cementitious material thus reducing the suction force of brick. Similarly, the suction force of the mortar is influenced as its pores are filIed as the process of hydration continues [20]. The transfer is, therefore, based on a conflict between the capillary suction force ofthe brick and mortar pore structures.

1.2

~

o 1.0 w CII IX:

0.8 o CIJ CII ~

w 0.6 IX: :::> I-CIJ 0.4 o ::li

~ 0.2 ~ I­Z

~ 0.0 IX: W o..

-0.2

o

Cured under polythene for 14 days Exposed to drylng

~~

.1 10 100 1000 TIME (days) log seale

Fig. 3 Percentage moisture absorbed in 2-courae masonry

In this investigation, the overall quantity of water removed by the bonded Annitage units (Fig. 3) from the bonded mortar is only considered to represent the 'excess' water in the system which is present to satisfy the workability requirements of the mortar. As hydration of the bonded mortar is unlikely to be affected by the loss of this 'excess' water, no large influence ofunit water absorption on the mortar properties of elasticity,

676

shrinkage and creep might be expected. This appears to be confinned for the modulus and creep of the masoruy reported in the previous investigation [3], where the sarne types of unit and mortar were used. However, for shrinkage predictions were significantly overestimated by the composite model. This is discussed below.

Time-dependent defonnations

Figure 4 compares the moisture movement strain and creep of the unbonded mortar prisms with the corresponding movements measured in the mortar b~-joint of the masoruy deterrnined from Eq. (1). The shrinkage ofthe mortar bed-joint is 25% less than the shrinkage exhibited by the unbonded mortar prism at 160 days. This apparent reduction in shrinkage is thought to be attributable to the time of start of shrinkage which would occur during the curing period due to the unit absorption property [5], whereas for the unbonded prism the start ofshrinkage was 14 days. From Fig. 3 it is also apparent that the effect ofthe unit absorption property is to reduce creep ofthe mortar joint, but

2000

1600

';--1200 o ~

z

~ 800 I­U)

400

o o 40 80

~ Prlam - Shrinkage

120 160 TIME (daya)

e.d-joint - Shrlnkagl

Prlam - Crup eed-joint - Cre.p

200 240

Fig. 4 Comparison of mortar prism and mortar bed-joint

Movements

only by 5%. at 160 days. From the results of Fig. 4, the mortar bed-joint / mortar prism shrinkage and creep ratios were calculated over the period of 160 days. From Fig. 5 it can be seen that beyond 40 days there is little change in the shrinkage and creep ratios and, therefore, constant values ofO.75 and 0.90, respectively, may be assumed.

677

1.2

2 1.1 u-(/.)

o ã: 1.0 (/.)Q.

Q a: 0.9 1-< ~~ 0.8 w O C!J 2 0.7 <-~ I- 0.6 ~z a: Õ 0.5 :t:...,

(/.)60.4 Cw ~m 0.3

a: :!i ~ 0.2 Wa: a: o 0.1 °2

0.0

@] Creep

@ ~ tQl ~ [GJ

@] @]

~ ~ @l ~ li! fi! fil @ Shrlnkage

o 40 80 120 160 200 240 TIME (daya)

Fig. 5 Creep and Shrinkage Ratios of Mortar Bed-joint I Mortar - Time

The moisture expansion of the unbonded and bonded c\ay units are shown in Fig. 6. Compared with the unbonded brick in the bed direction, the moisture expansion in the header âirection is approximately 1.6 times greater. Although some difference may be explained by the variability that is inherent in c\ay bricks, it is likely that the difference in moisture expansion is also due to anisotropy. This is in agreement with comments made by West [21).

o

-10

~ -20 ... o :::. -3 O z < a: -40 l-(/.)

-50

-60

-70

o Fig. 6

. Unbonded· Bed

............... Bonded·Bed

o Unbonded-Header

40 80 120 160 TIME (daya)

Moisture Expansion - Time Unbonded Armitage Units

678

200 240

of Bonded and

If the moisture expansion of the bonded brick units in the bed face direction is compared with the unbonded brick header face expansion (Fig. 6), it can be seen that after approximately 80 days, the expansion is similar. The difference in behaviour of the bonded and unbonded units in the bed face direction could again be explained by the variability in the units, because the greater expansion in the bonded unit from the initial absorption of water (Fig. 3) would appear to be reversed shortly after measurements were started at 14 days. From Fig. 6, it can be seen that the moisture expansions are small compared with mortar shrinkage, (Fig. 4), and that the differences in moisture expansion will not be significant in the prediction of long-term movements.

Creep of the unbonded and bonded units can be seen in Fig. 7. The levei of creep in the bricks was low and any differences were, therefore, considered to be insignificant. As with moisture expansion of the unbonded unit no modification factor was considered to be necessary to adjust the creep ofthe unbonded unit. Figure 7 also shows the unbonded unit header creep modified by a factor of 0.7. This factor represents the ratio of initial elastic modulus of the unit obtained in the bed face and header face directions. The results confirm that an accurate assessment of creep of the unit in the bed face direction can be made by measuring its header face deformation and then adjusting the data according to the ratio of moduli about these two faces. This is significant, as in practice it is much easier to measure creep in the header face direction.

14

12

10

~ 8 o

a.. 6 w W lI: o 4

2

O

-2

O

Fig. 7

40 80

Creep - Time of Armitage Units

o Unbond.d.H.ader

!§1 Unbonded·Bed ! (0.7 x Unbonded

/ ·Header) I

I.

120 160 200 240 TIME (day.)

Individual Bonded and Unbonded

Modified prediction of creep and shrinkage

The results of the modified predictions can be seen in Figs. 8 to 11, where they are compared to the measured and unmodified predicted movements of masonry constructed

679

200

160

=. 120

40

I o o

I

I I

I

/

/ ~/

I

1[Qj/

40

/ /

/

/~/

Unmodifled· Predlcted

o--/-~----~----~-- @J M88sured / @J --() Modifled·Predlcted

80 120 160 200 240 TIME (days)

Fig . 8 Measured and predicted shrinkage • time for the single ·Ieaf wall constructed with a 1 : 1 : 6 morta r

280

240

200 ~

~

=. 160 o.. w w

120 a:: o

80

40

O

O

Fig. 9

I

I I

//&1' ~/

I

" I

~----&í-/ '-&--- "

U nmodifled· P redlcted @J Modllied·Predicted

_--@-- -- <l! Meaaured

40 80 120 160 200 240 TIME (days)

Measured and predicted creep • time for the single -Ieaf wall constructed with a1 : 1 : 6 mortar

680

280

240 Unmodlfled· Predlcted

G~ 200 o .... w (!J

-< ~ z a:: :J: (I)

G

160

120

80

40

O

O

, I

I ,

t;;'

/ ~ , , , ,

, , , ,

, , , ,

p//

40

,0----0--_ ,0// --0----0----0 Modified·Predlcted

@) Me8lured

80 120 160 200 240 TIME (daya)

Fig. 10 Measured and predicted shrinkage - time for the single - leaf wall constructed with a 1 : 5 GGBS morta r

320

U nmodUled· Predicted 280 ~ Muaured

240

~ 200 .... D..

t!: 160 a:: o

120

80

40

o o 40 80 120 160 200 240

TIME (daya)

Fig. 11 Measur.d and predicted creep - time for the single -Ieaf wall constructed with a 1 : 5 GGBS mortar

681

from 1 : 1 : 6 and 1 : 5 GGBS mortar types [3]. From Figs. 8 to 11 it ean be seen that the intluenee of the Armitage unit absorption property was, therefore, removed by applying the modifieation faetors rieveloped in this investigation (Fig. 5) to the unbonded mortar prism ereep and shrinkage data obtained previously [3]. Typiealiy, composite model predietions whieh originally overestimated moisture movement strain . by 70 %, now prediet movements to within 10% (Fig. 8). Creep predictions also improved fi'om an original overestimation of 15% to predictions now within 5% (Figs. 9 and 11). The error in the prediction of the GGBS masonry moisture movement strain was reduced from approximately 150 % to nearer 50 % (Fig. 10). A possible explanation for the signifieant discrepaney still present in the modified prediction may be the variability in the Armitage unit water absorption property whieh ranged from 4.1 to 5.7%, with the average value being 5.03 % (Table 1). It is expected that by constructing a wall with above average unit absorption properties this would produce an additional decrease in the potential of the bed-joint mortar to shrink, necessitating a larger reduetion factor to modify the eorresponding unbonded mortar prism data. Previous researeh by Tapsir [23] has shown that the modifieation faetor does inerease with an inerease in unit water absorption properties and the relationship is sueh that a small ehange in unit water absorption property eorresponds to a signifieant inerease in the modification faetor, (whieh in this case eould aeeount for the residual overestimation). Additionally, this error in the predietion of moisture movement strain of the GGBS masonry could be linked to the redueed ability ofthe GGBS mortar to retain moisture [20].

4. CONCLUSIONS

The results of this investigation eonfirm the presence of moisture transfer between the bonded briek and mortar elements of masonry. This interaction is a result of the unit water absorption property and ean only be removed by fully saturating the brieks prior to construction. The transfer of moisture is expected to oecur even when units possessing a low water absorption i.e. 1% are used in masonry. The influenee of this interaction on the long-term movements of elay masonry ean be seen from the comparison of the long­term movements of the mortar bed-joint and unbonded mortar prisms. This eomparison illustrates that the effect of the reduetion of water in the mortar by the unit is greatest on shrinkage, as water loss is the underlying cause of drying shrinkage.

The inaeeuraey of a number of previous predictions using the model developed by Brooks [16] is a result of unbonded samples being unrepresentative of the bonded elements. This was due to the action of the unit water absorption properties whieh reduced the water content of the bonded mortar, however, this differenee ean now be aecounted for with modification factors applied to the unbonded mortar prism data. Modified predictions of creep and moisture movement strains for Armitage elass 'B' engineering briek masonry are then possible to within 10 %, although these factors are only relevant to the experimental eonditions of this investigation as they may depend on a number of other faetors sueh as mortar type, euring and age at loading.

Although the results of this investigation highlighted the presenee of anisotropy in the Armitage unit, no faetors were required to modify the behaviour of the briek. The findings also eonfirmed that ereep of the unbonded unit in the bed face direction ean be

682

obtained by measuring its header face deformation and then adjusting the data according to the ratio of moduli about these two faces .

ACKNOWLEDGEMENTS

The authors thank EPSRC for financiaI support and MarshaUs Clay Products Limited for the supply of the materiaIs.

5. REFERENCES

1. Forth, J . P ., Bingel, P. R., and Brooks, 1. J., "Influence of Age at Loading on Long-term Movements ofClay Brick and Concrete Block Masonry", Proc. 7th North American Mas. Conf, South Bend, Indiana, USA, 1996, pp. 811- 821.

2. Brooks, 1.1. and Abdullah, C.S., "Geometry Effect ofCreep and Moisture Movement ofBrickwork", Masonry Inter., Vol. 3, No. 3, 1990, pp. 111-114.

3. Forth, 1. P . and Brooks, 1. J., "Influence ofMortar Type on the Long-term Deformation ofSingle-leafClay Brick Masonry", Proc. 4th Inter. Mas. Conf, No. 7, Vol. 1, London, 1995, pp. 157-161.

4. Brooks, J. 1. and Forth, 1. P., "Influence ofUnit Type on Creep and Shrinkage of Single-leafClay Brickwork", Proc. 3rd Inter. Mas. Conf, No. 6, London, 1994, pp.31-33 .

5. Forth, J . P . "Influence ofMortar and Brick on Long-term Movements ofClay Brickwork", PhD Thesis, The Univ. ofLeeds, Civ. Eng. Dept., Leeds, 1995.

6. Ameny, P. , Loov, R. E. and Shrive, N. G., "Models for Long-term Deformation ofBrickwork", Masonry Inter., No. I, Apri11984, pp. 27-36.

7. Anderegg, F.O., "The Effect ofBrick Absorption Characteristics Upon Mortar Properties", ASTM, Proc. 42, 1942, pp. 821-30.

8. Sneck, T., "Dependence ofMasonry Properties on the Interaction Between Masonry Units and Mortar", Proc. 6th Inter. Brick Mas. Conf, Rome, 1982, pp. 246-57.

9. Morgan, J. w., "Brick Absorption", ArchitecturaI Sci. Rev., Vol. 20, Pt. 3, Sep~.

1977, pp. 66-68. 10. Lawrence, S.J. and Cao, H.T., "An ExperimentaI Study ofthe Interface Between

Brick and Mortar" , Proc. 4th North Am. Mas. Conf., Los Angeles, 1987, pp. 48.1-14.

11. Schubert, P . and Hoffinann, G., "Compressive Strength ofMortar in Masonry Significance, Influences, Test Methods, Requirements", 10th Inter. Brick Mas. Conf., CaIgary, Canada, 1994, pp. 1335-1344.

12. Longfoot, B. R., " In-situ Mortar Strength", Asia-Pacific Conf on Mas., Singapore, 1991 , pp. 121-123.

13 . Shrive, N.G. and England G.L., "Elastic, Creep and Shrinkage Behaviour of Masonry", Inter. Joum. ofMas. Constr., Vol. 1, No. 3, 1981, pp. 103-9.

14. Jessop E.L. , Shrive, N.G. and England, G.L. , "Elastic and Creep Properties of Masonry", Proc. NAMC, Boulder, Colorado, 1978, pp. 12.1-16.

15. Ameny P., Loov, R.E. and Shrive, N.G., "Prediction ofElastic Behaviour of Masonry" , Inter. Jouro. ofMas. Constr., Vol. 3, No. 1,1983, pp. 1-9.

683

16. Brooks,1. 1., "Composite Modelling ofMasorrry Deformation", MateriaIs and Structures, 23,1990, pp. 241-251.

17. Brooks,1. 1. and Abdullah, C. S., ''Composite Model Prediction ofthe Geometry Effect on Creep and Shrinkage ofClay Brickwork". Proc. 8th Inter. Brick/Block Mas. Conf, Vol. 1, Dublin, 1988, pp. 316-323 .

18. Brooks,1. 1. and Abdullah, C. S., "Creep and Drying Shrinkage ofConcrete Blockwork", Magazine ofConcrete Research, 42, No. 150, March 1990, pp. 15-22.

19. Brooks,1. 1. and Abdullah, C. S., "Composite Modelling ofthe Geometry Influence on Creep and Shrinkage ofCaIcium Silicate Brickwork", Proc. Brit. Mas. Soc., No. 4, 1990, pp. 36-38.

20. Neville, A. M., "Properties ofConcrete", 4th Edition, Longman Group Ltd., 844 pages.

21. West, H. W. H., "Moisture Movement ofBricks and Brickwork", Trans. Brit. Ceram. Soc., Apri11967, pp. 137 - 60.

23 . Tapsir, S.H., "Time-dependent Loss ofPost-tensioned Diaphragm and Fin Masorrry WaIIs", PhD Thesis, University ofLeeds, Civil Engineering Dept. , Leeds, 1994.

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