behaviour of low-temperature fired laterite bricks under

8/4/2019 Behaviour of Low-temperature Fired Laterite Bricks Under

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.Construction and Building Materials 16 2002 101112

Behaviour of low-temperature fired laterite bricks underuniaxial compressive loading

Laurent MbumbiaU, Albert Mertens de Wilmars

( )Uni ersite catholique de Louain UCL , Unite de Genie Ci il et Enironnemental, Place du Le ant 1, Bat. VINCI, 1348 Lou ain-la-Neu e, Belgium

Received 20 December 2000; received in revised form 31 July 2001; accepted 25 October 2001

Abstract

It has been shown previously that the properties of some laterite building bricks abundant in tropical areas can be improved bystabilization through heat at low temperatures. Further investigations based on a series of laboratory tests were carried out onthese brick specimens subjected to uniaxial compressive loading at room temperature. The characteristics of the stressstrainrelationship are presented for predicting brick performance. When subjected to deformation, they behaved nonlinearplasticelasticplastic. They were found to exhibit linear elasticity in a domain where strain varied between 2 and 5% according

.to the type. These values of strain were found to be higher than the maximum elastic strain 0.5% observed for many ceramics. 2002 Elsevier Science Ltd. All rights reserved.

Keywords: Stressstrain relationship; Nonlinear plasticelasticplastic behaviour; Low firing temperature; Laterite bricks; Cameroon

1. Introduction

The behaviour of brick masonry as a structural mate-rial under specific loading conditions has been the

w xsubject of some investigations 14 . The case of bricksas a component material of masonry held only little

w xattention 5 . Among the bricks, the most well-knowngroup is fired clay brick, one of the oldest and most

used manufactured building materials. Another class of .bricks, made of lateritic soil and its relative bauxite ,has been the subject of new investigations this last

w xdecade 610 . Laterite is the traditional building rawmaterial available in most parts of tropical areas. Thedifference between laterite and clay lies mainly in theparticle size. Laterite is used as soil containing gravel,sand, silt and clay sizes in various proportions. To be

UCorresponding author. Tel.: q32-10-472-116; fax: q32-10-472-

179. .E-mail address: [email protected] L. Mbumbia .

used as raw material for brick making, laterite needs tobe ground in small particles of cohesive material. Itcould be used intact if the clay fraction is predominant,so as to ensure cohesive property.

In a previous study on the performance characteris-tics of lateritic soil bricks fired at relatively low temper-ature, it has been reported that these products pre-sented compressive strength higher than the minimum

indicated by many standards and building codes 2. w xMPa for building bricks 11 . Furthermore, it has been

reported that the durability of the bricks, measured bycompressive strength and resistance to erosion, is guar-anteed for those fired beyond 550 C. As one of themost important characteristics of a material is its re-

w xsponse to stress 12 , it was essential to understand thebehaviour of these types of bricks when subjected touniaxial compressive loading. Such a test simulates bestthe loading the bricks will undergo in service.

The study aims to determine the fundamental me- .chanical properties stressstrain characteristics and

0950-0618r02r$ - see front matter 2002 Elsevier Science Ltd. All rights reserved. .PII: S 0 9 5 0 - 0 6 1 8 0 1 0 0 0 3 5 - 6


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( )L. Mbumbia, A.M. de Wilmars r Construction and Building Materials 16 2002 101112102

to obtain a better understanding of the behaviour oflaterite bricks, fired at various specified conditions.Mechanical properties describe the way that a materialresponds to forces, loads, and impacts. Laterite bricksfired at low temperature, in the range 550750 Cpresent acceptable compressive strength, low density

and are also resistant to erosion. These propertiesmake the bricks of study attractive structural materialsthat can be used more effectively and economically inthe design and construction of buildings. The lack ofdata on the response of low temperature fired lateritebricks to load makes it difficult to predict the behaviourof the masonry and hence allows for a safe structuraldesign.

In this paper, emphasis is placed on finding the .stiffness modulus of elasticity and the yield stress

obtained by plotting the stressstrain diagram. Weselect these two mechanical properties not as beingmost important but partly because they are familiarand partly because they are the properties which aregenerally considered first in making a structural ormechanical component. The resistance of a material toa permanent deformation is measured by means of thestress necessary to produce a certain strain. Accordingto the fact that stress versus strain curves obtained were not linear, the modulus of elasticity is studiedherein in terms of linear, secant and tangent elasticmodulus.

The rigidity of a material, which does not follow theHookes law, is obviously not constant, but varies withthe stress. Sometimes, average rigidity is the best mea-

surement of this quantity under a given constraint. It iscalled the secant modulus, and represents the average

.slope of the curve or slope of its secant cord. If onewishes to have rigidity associated with a small increase with load, one can find instantaneous rigidity by theslope of the tangent to the curve at the point inquestion. This slope is called the tangent modulus. InSection 5.2, only the linear elastic modulus should beused as Youngs modulus of elasticity.

The main properties used in the theoretical back-ground are presented in Section 2. In Section 3, experi-mental details are described. Test results are presented

in Section 4, and the discussion is presented in Section5.

2. Theoretical background

2.1. Modulus of elasticity or stiffness of bricks

The modulus of elasticity indicates the stiffness orresistance to movement of a material. This property isconventionally measured using standardized tests basedon small specimens subjected to uniaxial compressiveloading.

In practice, the modulus help to choose the size ordimensions of the material. For example, with a highmodulus the dimensions of a section could be reducedthrough a certain point below which it becomes im-practical because of the dangers of elastic instabilityw x13 . Also, a high modulus could limit the ability of the

material to accommodate shock loading, and for mostengineering applications a material needs to be compli-

ant but not too much to avoid large elastic strain.under service stresses rather than stiff and strong.

The experimental study conducted herein aims atcontributing to establish empirical expressions avail-able to estimate the modulus of elasticity of lateritebricks. The modulus of laterite brick, along with themodulus of mortars, will be used to estimate the modu-lus of elasticity of masonry or structural elements made

.of the two components brick and mortar . Beyond this,it is the understanding of stress strain behaviour of aceramic body made with laterite that is in questionhere.

2.2. Definition of properties

The apparent limit of elasticity, the secant modulusand the tangent modulus of elasticity are non-linearproperties commonly used to analyse elasticplasticbehaviour of materials. They are determined experi-mentally from the stress strain diagram.

2.2.1. Linear modulus of elasticityThe linear modulus of elasticity E is referred to

linthe constant of Hookes law. In this paper, it is calcu-lated according to the relation:

yA C . .E s MPa 1lin yA C

where , , , are defined at the nomenclature A A C Cand specified on Fig. 1.

Fig. 1. Schematic representation of stress strain diagram of low-tem-perature fired laterite brick.


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( )L. Mbumbia, A.M. de Wilmars r Construction and Building Materials 16 2002 101112 103

2.2.2. Secant modulus of elasticity

The secant modulus of elasticity E is defined assec .the stiffness from the origin of the sf curve at a

w xcertain point of that curve 14 . Its value depends onthe position of the point, which is generally specified bythe indication of the stress at that point. It is used

when plastic strains are also involved and is calculatedas follows:

w x . .E s r s MPa 2sec I

.where I is any point of the curve sf .

2.2.3. Tangent modulus of elasticity

A tangent elastic modulus E is defined as antanincrement of stress divided by an increment strain for

w xan elastic substance 15 . At any stress or strain level,the tangent modulus of elasticity represents the slopeof the stress strain diagram at that point, or instanta-

neous stiffness. It is calculated as follows:

w x . .E s drd s MPa 3tan I

.where I is any point of the curve sf .When plotting the curve E versus strain, we ob-tan

tained the derivative curve of stressstrain curve. Ex-perimentally, the great importance of derivative curveis that it represents for each value of the variable thevalue of the variation of the function according to thevariation of the variable around the point considered.The position of maximum and minimum is determinedthus with a higher degree of accuracy. A maximum or a

minimum of the derivative curve indicates a point ofinflection of the primitive curve.

2.2.4. Stress

The term stress may be variously interpreted tomean the yield stress or the maximum stress. The yield

.stress is designated herein by MPa and will beAconsidered at the outset where we recorded a plateauin stress. The maximum stress is designated in this

.paper by MPa . It occurs when the slope of theB curve is horizontal at the maximum strength withconventional nominal area.

w x According to Herubin 16 , for the designing pur-pose, it should be better for a material not to bestressed close to the failure stress. Therefore, shouldA

be preferred to which is given herein as an indica-B.tion .

3. Experimental

3.1. Raw materials

The laterite raw materials were collected from twodifferent areas, namely EtougEbe and Cite Verte of

Table 1Summary of some physical and geotechnical properties of samples oflateritic soils from the Yaounde area

Parameters Locationrsite

EtougEbe Cite Verte

.IParticle size %Gravel 2 34Sand 28 29Silt 31 15Clay 39 22

Consistency limits .Liquid limit LL % 58 61 .Plastic limit PL % 38 39

.Plasticity index PI % 20 22 Activity 0.51 1

.Yaounde town 352 N, 1132 E in Cameroon. Thedisturbed samples were obtained at a depth between 50

and 200 cm from a virgin site of each area. Particle sizeanalysis and the consistency limit tests were carried outaccording to BS 1377:1975 to determine the physicaland geotechnical properties of samples. The results arereported in Table 1. Chemical and mineralogical analy-sis were also performed respectively by X-ray fluores-cence spectrography and X-ray diffraction. Their re-sults are presented in Table 2.

3.2. Brick preparation

The specimens of brick were manufactured by an

industrial mechanized process in the laboratories of the

Table 2Results of chemical and mineralogical analysis of samples of lateriticsoils from the Yaounde area

Parameters Locationrsite

EtougEbe Cite Verte

( )Chemical analysis % .Loss on ignition L.O.I 11.41 14.75

.Silica SiO 44.3 45.42 . Alumina Al O 30.4 25.32 3

.Iron oxide Fe O 11.5 12.82 3 .Titane oxide TiO 1.37 0.8692

.Manganese oxide MnO 0.0301 0.107 .Magnesium oxide MgO 0.321 0.15

.Calcium oxide CaO 0.0285 0.0294 .Potassium oxide K O 0.346 0.2972

.Sodium oxide Na O 0.0142 2 .Phosphorous oxide P O 0.0142 0.1172 5

Total 99.8025 99.8194

.Mineral analysis qualitative Quartz QuartzKaolinite KaoliniteGoethite GoethiteHematite Hematite

Illite


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.Belgian Ceramic Research Centre BCRC at Mons.Samples of laterite of each site were dried in electricdrying oven at 105110 C for 48 h, then crushed in agrinder to reduce the particle size. The proportions bymass of the various sizes of particles present in thecrushed samples are reported in Table 3. After process-

ing, the specimens average size was found to be 60mm=33 mm=27 mm. After curing during 21 days,they were then fired at specific temperatures in anelectrical furnace in the Civil Engineering Laboratory

.at the UCL. Two dwell times 4 h and 8 h , i.e. thebearing firing time, were chosen for the specimens of

.EtougEbe, and one dwell time 4 h was retained forthe specimens of Cite Verte. For all the cycles offiring, a rising rate of 2 Crmin and a lowering rate of1 Crmin were maintained. After firing and cooling,seven series of bricks related to the temperature ofcuring or firing, i.e. 27, 110, 350, 550, 750, 850 and 975

C of each of the three types, namely EE4 EtougEbe,. . .4 h , EE8 EtougEbe, 8 h and CV4 Cite Verte, 4 h

were retained for test purposes. The test took on onlyone specimen randomly chosen of each firing tempera-ture. It aimed to give an idea on the aspect of theshape function of the stress strain diagram. More testsbased on five or six specimens of each series areindicated to establish real stressstrain relationship, which is out of the scope of the present paper. Theaverage final product size was found to be 53 mm=30mm= 25 mm. The physical properties of the

w xEtougEbe bricks have been presented 11 .

3.3. Uniaxial compressi e loading

A view of a typical specimen in a testing machine is

Table 3Proportions by mass of the particle sizes of crushed samples

.Sample wt. % Particle size

.Sand Silt Clay fraction -2 m

EtougEbe 20 20 60

Cite Verte 15 45 40

shown in Fig. 2. The uniaxial compressive loading wasdone according to the ASTM C 126:E 4 and E 6standard methods 14 days after firing and cooling. Fewmodifications were brought. They took on the surfaceof specimen where the load was applied. Steel plates of30 mm=30 mm=10 mm and 100 mm=30 mm=10mm were used on the upper and the lower the surfacesof specimen, respectively. A capping layer made of thinsheet of cardboard of 30 mm=30 mm=2 mm was alsoplaced between the specimen and the steel plates, so asto have a uniform and standard compressed surface ofspecimen. The steel plate of 100 mm=30 mm=10mm was pasted on the platens of the test machine. Thespeed of loading by displacement of the platens was 0.4mmrmin and the tests took place on a universal testing

machine. The specimens were tested flat wise i.e. the.load was applied in the direction of depth of the brick

under steadily increasing axial compressive loads up tofailure. At the outset, a 0.001-mm micrometer dialgauge type Mitutoyo, with its point in contact with theplatens of the test machine, was used to measure the vertical displacement. Vertical force, vertical displace-

ment and time were recorded systematically on a com-puter connected to the test machine. The data obtainedpermitted the computation of stresses and strains by

Fig. 2. View of a typical compressive specimen in a testing machine.


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Fig. 3. Stressstrain diagrams of different types of bricks.


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dividing the vertical force by the initial cross-sectional 2 .area 900 mm and the vertical displacement by the

initial height respectively. The computed data permit-ted to plot the stress strain curves.

4. Results

4.1. Characteristics of samples of lateritic soils and crushedmaterials

As shown in Table 1, the lateritic soils from the .Yaounde area are plastic in nature 20-PI-25 . The

samples contain particles of all sizes from gravel downto clay and could be described as gravelly sandy silty

.clay. The activity of the sample of EtougEbe 0.51shows that this laterite raw material is an inactive clay

. activity-0.75 in which kaolinite activity value ap-.proximates 0.4 , as revealed by the mineralogical analy- .sis Table 2 , is the dominant clay mineral, while the

sample of Cite Verte, with an activity of 1, is a normalclay dominated both by kaolinite and illite activitys

. w x0.9 17 . Quartz, goethite and hematite are other min-erals present in the samples. As indicated in Table 2,the chemical analysis shows that the raw materials

. .consist mainly of silica SiO , alumina Al O and2 2 3 .iron oxide Fe O . They contain only a small quantity2 3

.of calcium oxide -0.03% and alkalies.Table 3 gives the proportions by mass of the particle

sizes of crushed materials used to prepare specimens.

According to the values obtained, the EtougEbecrushed material can be considered as a clay ofsiliceous nature, while the Cite Verte crushed mate-rial can be considered as a silty clay of siliceousnature.

4.2. Stress strain characteristics

The stressstrain diagrams of different types of lat-

erite bricks are shown in Fig. 3. Three regions can beidentified on the diagrams:

The first region is represented by the branch OC .Fig. 1 . It corresponded probably to the effect ofcapping layer made of thin sheets of cardboard

partly and non-elastic response of material the otherpart. At test beginning, the capping layer deformedunder the effect of compression until the momentwhen the contact of test machinercardboardrspeci-men became real and effective. From this moment,the effective recording of load and displacement ofmaterial started and a plastic strain began at verylow stresses.

The second region is represented by the linearzone CA. Here, the bricks exhibited a stiffer and

.more linear response Fig. 3 up to a yield stress. Inother words the bricks of all series and all types

behaved as Hookean materials in that region. The width of the elastic domain is measured by the

.quantity as listed in Tables 46 or, inA C .extension by the quantity . Within this do-A K

main, the strain represents approximately 50% inaverage of the strain at the yield stress.

The third region is represented by the branch AB.In that region the bricks exhibited a non-linearcompressive stress response up to a maximum stress at which point large deformations occurredBwithout significant increase in the stress. The transi-tion from elastic to plastic behaviour of the bricks

took place. Bricks of series 27, 110 C for all types .and 350 C for the type EE4 exhibited the exis-tence of a plateau in the stress strain diagram, at which the strain increased rapidly while the stressremained substantially constant.

Talking about the slope of the elastic domain, it wasobserved that bricks of series 27, 110 and 350 C hadlower slopes than bricks of series 550, 750, 850 and 975

C. It meant that the first group of bricks 27, 110 and

Table 4

Specific mechanical properties of series of bricks of type EE4

Series of brick 27 C 110 C 350 C 550 C 750 C 850 C 975 C

. MPa 2.97 3.57 3.87 9.63 10.38 11.27 15.34B . MPa 2.80 3.10 3.48 8.20 10.08 11.40 13.20A . MPa 0.50 0.90 1.00 2.00 3.00 3.00 3.00C

0.032 0.037 0.036 0.047 0.054 0.052 0.050B 0.025 0.030 0.025 0.032 0.040 0.040 0.044A 0.008 0.010 0.017 0.017 0.023 0.020 0.022C 0.004 0.002 0.014 0.012 0.016 0.013 0.015K 0.007 0.007 0.011 0.015 0.014 0.012 0.006B A 0.017 0.020 0.008 0.015 0.017 0.020 0.022A C 0.021 0.028 0.011 0.020 0.024 0.027 0.029A K

.E MPa 135 110 310 413 416 420 463lin


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Table 5Specific mechanical properties of series of bricks of type EE8

Series of brick 27 C 110 C 350 C 550 C 750 C 850 C 975 C

. MPa 2.97 3.57 3.79 9.49 10.36 12.28 14.63B . MPa 2.80 3.10 2.98 7.20 9.00 11.00 14.00A . MPa 0.50 0.90 1.00 2.00 3.00 3.00 4.00C

0.032 0.037 0.043 0.046 0.064 0.063 0.040B 0.025 0.030 0.036 0.037 0.039 0.044 0.034A 0.008 0.010 0.024 0.023 0.022 0.020 0.049C 0.004 0.002 0.019 0.018 0.013 0.014 0.013K 0.007 0.007 0.007 0.009 0.025 0.019 0.006B A 0.017 0.020 0.012 0.014 0.017 0.024 0.015A C 0.021 0.028 0.017 0.019 0.026 0.030 0.021A K

.E MPa 135 110 165 371 353 333 667lin

.350 C deformed more quickly than they resisted theload.

4.3. Modulus of elasticity

The computed values of linear modulus of elasticity .E are presented in Tables 46. The modulus, withinlina type, varied from one series of brick to another. Forthe type EE4 and EE8, when passing the series 27 Cto the series 110 C, the modulus value decreased.Beyond the series 110 C, the value of E increasedlinsignificantly up to the series 550 C from which onecould observe a relative stability up to the type 850 Cand a relative increase at the series 975 C. For thetype CV4, E increased from the series 27 C up tolin

the series 850 C from which the relative decreaseobserved at series 975 C cast some doubt and needsmore tests before it can be confirmed.

Fig. 4 shows the variation of secant modulus of .elasticity E of bricks of type EE4 with strain. Tosec

.plot the curves of variation, the Eq. 2 was used withthe consideration of the curve origin at the point of . diagram where the stress took the value 0.50MPa. This assumption was made to deduct the effect ofcardboard capping layer. The secant modulus increased

with the strain and attained its maximum value at theyield point. Beyond this point, its value decreased withstrain whatever the series of brick. As said earlier, two

groups of bricks were distinguishable on Fig. 4: the firstgroup concerned the series 27, 110 and 350 C withtheir yield point secant modulus at approximately150185 MPa at 1.3% of strain, and the second groupconcerned the series 550, 750, 850 and 975 C withtheir yield point secant modulus between 280 and 345MPa at approximately 3% of strain. Compared with thevalues of E , it can be seen that the values of E atlin secthe yield point were the lowest beyond the series 110C. With more the slope is weak, with more materialapproaches a purely elastic material.

Fig. 5 illustrates the variation of some tangent modu-

.lus of elasticity E with strain in the case of typetanEE4 bricks. The great importance of the diagram is theattached stressstrain curves. In other words, this fig-ure shows the variation of the stress and its derivativewith strain. As the stress and the strain increased, thetangent modulus varied, passing through local andglobal minimumrmaximum. Those particular points canbe seen on the curves by the summits oriented in the

. .sense of concavity minimum or convexity maximum .Few sharply defined summits can be observed on the

Table 6

Specific mechanical properties of series of bricks of type CV4

Series of brick 27 C 110C 350 C 550 C 750 C 850 C 975 C

. MPa 3.22 4.28 5.39 7.97 8.31 8.89 9.61B . MPa 2.90 3.98 4.27 7.48 8.24 8.25 7.24A . MPa 0.51 1.00 1.00 2.00 2.00 4.00 2.00C

0.050 0.055 0.054 0.061 0.062 0.062 0.067B 0.040 0.046 0.037 0.057 0.056 0.051 0.045A 0.010 0.022 0.016 0.028 0.025 0.034 0.023C 0.008 0.020 0.009 0.015 0.015 0.023 0.016K 0.010 0.009 0.017 0.004 0.006 0.011 0.022B A 0.030 0.024 0.021 0.029 0.031 0.017 0.022A C 0.032 0.026 0.028 0.042 0.041 0.028 0.029A K

.E MPa 80 124 156 189 201 250 238lin


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Fig. 4. Secant modulus of elasticity versus strain curves of bricks of type EE4.

. . .curves a , b and c of Fig. 5. The summits indicated .the points of inflection of the primitive curves sf .

5. Discussion

5.1. Beha iour of low-temperature fired laterite bricks

For the different brick types examined in this study,

the cardboard capping layer used in uniaxial compres-sive test appear to give an increased initial strainrelative to the setting up of specimens on testing ma-chine. It can be assumed that one part of the non-lin-

.ear behaviour observed in the first region branch ofFig. 3 is attributed to cardboard effect and another partto the initial cracks or porosity present in specimens.This last assumption is be confirmed by a previous

w xresearch conducted by Watstein 5 . He stated thatduring the test of brick specimens, there were audibleindications of cracking confirmed by the extensibility ofclay products of the order of 200300=10y6 . He made

the assumption that microscopic cracks develop in thebrick, and postulated that the initial cracks were closelyspaced microscopic fissures based on the fact that thestressstrain curves were smooth and showed no sharp

breaks. For the case of 110 and 750 C bricks type.EE4 where the stressstrain curves shown sharply

defined discontinuities, it can be assumed that theinitial cracks were of appreciable width and few in

.number. A similar phenomenon non-linear behaviourobserved in compressive test on low-temperature firedlaterite bricks is be put in evidence furthermore by

w xPaterson 18 who indicated that when subjecting acylindrical test tube of rock at the rupture under triax-

ial compression, one observes in a first period an elasticor an inelastic closing of pores of the material. Thisclosing that translates itself by the inverse curvature ofthe stressstrain diagram tends to disappear when oneapplies a confining pressure to the test tube.

As water absorption is an indication of porosity ofw xbricks, it has been shown in a former paper 11 that,

for the types EE4 and EE8 bricks, the average waterabsorption decreased when passing from the series 350

. .C 2829% to the series 550 C 27% . It increasedslowly from the series 550 C to the series 750 C .28% . From this last series, water absorption de-

.creased up to the series 975 C 26% . For the bricksof type CV4, new investigations show that the averagewater absorption increased when passing the series 350

. .C 22% to the series 750 C 24% through the series .550 C 23% . From the series 750 C, it value de-

.creased up to the series 975 C 23% . As one couldobserve, the difference in average water absorption isinsignificant within the same type of bricks whateverthe series. In return, the difference is appreciable

.between the bricks of EtougEbe EE4 and EE8 and .those of Cite Verte CV4 of approximately 5%,

probably due to their nature clay for EE4 and EE8,.silty clay for CV4 .

For all bricks types examined, it can be postulatedthat the non-linear behaviour observed was due essen-tially to pores, fissures and microscopic cracks presentin bricks. In fact, within the linear zone, the modulus ofelasticity is known to be constant according to thetheory on strength of material. If so, the secant modu-lus versus strain curves of Fig. 4 and the tangentmodulus versus strain curves illustrated in Fig. 5 shouldhave been represented by an horizontal line on the


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Fig. 5. Some curves of variation of the tangent modulus of elasticity with strain, in relation with the stressstrain curves. Series of bricks of type . . .EE4: a 27 C; b 550 C; c 975 C.

diagram. But the variation of secant and tangent modu-lus observed tends to confirm the presence of signifi-cant porosity. In that way, it could be postulated that

.the strain Tables 46 corresponds to strainKfavoured by porosity.

The variation of slopes observed on the stress straindiagram for the two groups of bricks 27, 110 and

. .350 C and 550, 750, 850 and 975 C explained thedegree of firing. The bricks of the first group are lessfired hence less compact than those of the secondgroup. That is why the first group of bricks deformedmore quickly than they resisted the load.

In the first approximation, the stress strain relation-

ship for each laterite brick type could be mathemati-cally characterized in the form of an S shape function.More data from further tests on more specimens arerequired to predict the exact mathematical model.

5.2. Design application: strength-to-density and modulus-to-density ratios

Density, strength and modulus of elasticity are main variables important for the selection of bricks in thesense that they could permit to attain minimum weightor minimum deformation for the component brick ina structure. As an illustration, let us explore two key


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w xconsiderations in design presented by Flinn 12 : how .much load a component brick will carry and how

much it will deform.Design based on stiffness begins with establishing

the relationship between deformation, load, and geo-metric factors, while the relationship between yield

strength, load, and geometric factors is related to de-sign based on strength.

For a brick in axial compression, the formulas belowtaken from the standard texts on mechanics andstrength of materials could be applied:

Ph .s 4

ELb

and

P . s 5y Lb

where is the observed contraction, P is applied load, is yield strength, E is the modulus of elasticity, L isythe length, b is the width and h is the height.

Let us consider the relationship between yieldstrength, , and density for a brick in axial compres-ysion.

The mass

.m sLbh 6

. .Combining Eq. 5 with Eq. 6 yields

.m sPh 7 /y

.Therefore, the mass or the weight is a function ofthe ratio of to . When we must have the lightesty

. weight component brick , we select the material withthe highest ratio r.y

Let us now consider the relationship between themodulus of elasticity E and the density . Combining

. .Eq. 4 with Eq. 6 yields

Ph2 .m s 8 / E

The weight is proportional to the density divided bythe modulus of elasticity. Finding an optimal stiffnessor having the lightest weight component equals tomaximize the ratio Er.

Figs. 6 and 7 show the general tendencies of varia-tion, respectively, of yield strength-to-density ratio andlinear modulus-to-density ratio with the temperature offiring. It may be observed from these figures that, theratios r and Er increased when passing from theyseries 110 C to the series 975 C for the three typesEE4, EE8 and CV4. Furthermore, the difference in

ratios is insignificant when passing from the series 550C to the series 975 C. In other words, higher is thetemperature of firing the brick, higher are its yieldstrength-to-density and linear modulus-to-density ratiosand lower is its weight.

w xThe recommendation made in a previous paper 11for the attractiveness to builders of the series 550 Caccording to its durability is being reinforced here bythis study according to the light-weight of the bricks ofthat series. With this series, it may be possible to

economize energy saving of fuel and the conservation.of resources when manufacturing products, reduce

weight of products while keeping an optimal strengthand an optimal stiffness.

6. Conclusions

On the basis of the study presented in this paper, thefollowing conclusions can be drawn.

Fig. 6. Yield strength-to-density ratio diagrams of the three types of bricks.


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Fig. 7. Linear modulus-to-density ratio diagrams of the three types of bricks.

.1 The stressstrain relationship was curvilinear upto the yield point for all series of bricks of the threetypes. Beyond the yield point, the bricks exhibited anon-linear elasticity. In other words, the laterite build-ing bricks fired at low temperatures exhibit a non-lin-ear plasticelasticplastic behaviour as indicated bythe results of the uniaxial and uniform compressive

loading. Those bricks are non-elastic materials minor.plastic strain begins at low stresses during a short

period of loading corresponding to the time their initialcracks, pores, sharp notches and microscopic fissuresclosed under a certain applied load. Beyond this pe-riod, they exhibit elastic behaviour as the common

ceramics up to the yield stress. From this point largedeformations occurred without significant increase inthe stress corresponding to the transition from elasticto plastic behaviour.

. 2 The mechanical properties studied herein stress.and stiffness are very sensitive to the microstructure of

the bricks. The stress and stiffness of low-temperaturefired laterite bricks vary with their structure porosity,

.degree of firing . .3 The capping layer made of cardboard seemed to

play a small role on the first period of loading duringcompressive testing. But its effect did not modify the

signification given to the non-linear response of speci-men in the first branch of stress strain diagram. Theuse of cardboard presents noticeable advantages easier manipulations, especially weaker dispersion ofthe results, less costly.

.4 The elastic strain at the yield point varied from 2to 5% according to the brick type and was found to be

.higher than the maximum elastic strain 0.5% observed .for brittle materials. This range of values 25% which

seem high must be taken with care. The response ofthe component brick may be modified when assem-bled with the component mortar in masonry. It meansthat, the appreciation of the complete behaviour of a

structure where those low-temperature fired lateritebuilding bricks are a component calls for further inves-tigations on specimen of that structure.

Acknowledgements

The authors are grateful to the Director of theBCRC, Dr F. Cambier and its personnel for the prepa-ration of specimens. They also wish to acknowledge thepersonnel of the Civil Engineering Laboratory at theUCL for their assistance with experiments. This studyhas been held by the Belgian administration in charge

.of cooperation DGCI .

Appendix A. Nomenclature

.E: modulus of elasticity MPa .E : linear modulus of elasticity MPalin .E : secant modulus of elasticity MPasec

.E : tangent modulus of elasticity MPatan . , : stressrstrength at the yield point MPaA y

. : strain at the yield point mmrmmA

. : maximum stressrstrength MPaB . : strain at the maximum stress mmrmmB : stressrstrength at the outset of the linear zoneC

.MPa : strain at the outset of the linear zoneC

.mmrmm : strain at the point of intersection of theK

straight-line portion of the stress strain curve .with the strain axis mmrmm

.: contraction of test brick unit mm 3.: density grcm

.m: massr weight of brick kg .P: applied vertical load on the brick N


12/12


.L: length of brick mm .b: width of brick mm .h: height of brick mm

Subscripts:EE4: related to EtougEbe bricks fired during 4 hEE8: related to EtougEbe bricks fired during 8 hCV4: related to Cite Verte bricks fired during 4 hA: yield point on the stressstrain curveB: failure pointC: outset of the linear branchO: origin point of the stress strain curve

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behaviour of low-temperature fired laterite bricks under

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