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STRENGTHS OF NATURAL SOIL BRICKS

By Dr-Ing. John K. MakunzaUniversity of Dar es Salaam

Department of Structural Engineering12th December 2006

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1.0 IntroductionAn adequate shelter is a basic human need, yet about 80% of the rural population in developing countries still live in spontaneous low quality settlements, as they cannot afford the high cost of building materials which could produce better shelters. One alternative for the expensive materials is to use natural soil stabilized bricks because they have been identified as low cost material with the potential of reducing the problem of living in poor or sub-standard houses. The technology uses the available soil on site, which is stabilized with a small amount of cement or/and lime depending on the characteristics of the soil so as to improve the engineering properties of the produced bricks.

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Problem StatementResidents of most areas of Southern Highlands in Tanzania have been using mud bricks and interwoven timber, saplings or bamboo daubed with mud to build their houses and sometimes they apply cement plaster on the walls. Problems of the houses:- vulnerable to weather especially during rain season- soil material can expand and loose cohesiveness, particularly with cement plaster

- they are not sufficiently strong, although they somehow last long even for more than ten years.

- it has been learnt that thieves can easily chop-out some bricks or part of the mud wall and break-in then steal properties.

Therefore the houses are not safe and are less reliable. Examples of the houses are shown in Figures 1, 2 and 3.

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Figure 1: Traditional house constructed of mud-walls and roofed with grass

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Figure 2:House built of pure soil bricks (from non-stabilized soil)

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Figure 3: Pure soil brick-wall in which part of the cement

plaster has fallen away

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2.0 Objectives

The main objective of this study was to assess the suitability of bricks made of stabilized soil, as to whether they can be used for the construction of strong, safe and reliable low cost houses, or otherwise.

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3.0 Adopted Methodology

The methodology adopted in carrying out the study included the following: • literature review: Through reading different

books, journals, some publications, and information obtained about properties of soils and soil stabilization,

• site visits and sample collection in which samples S1 and S2 were collected

• laboratory tests and data analysis in which the samples were tested and the data obtained analyzed to check the characteristics and adequacy of the soils as well as the bricks.

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4.0 Soil StabilizationIn many developing countries especially in rural areas, soil is used for the construction of houses. Natural compacted soil has good insulation and fire resistant properties[1]. It is, however, vulnerable to moisture and the erosive effects of weather. Wallsconstructed out of well compacted soil, have adequate compressive strength under dry conditions; however they will loose their strength under adverse moisture content. Soil durability and strength can also be improved by:(i) Changing the distribution of grain size; gradation control(ii) Compacting the soil; (iii) Adding stabilizers or chemicals;(iv) Mixing all of the above.Soil can be improved and used as a building material for varioustypes of structures by adding substances known as stabilizers, and the product is called stabilized soil. A properly stabilized, consolidated, well-graded soil that is adequately moisturized, mixed, and cured will provide a strong, stable, waterproof and long-lasting building bricks.

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Stabilizer material in the soil will do the following;(i) cementing/binding the soil particles together making

the product stronger(ii) water proofing - reducing the amount of voids and

water which can be absorbed by the soil(iii) reducing the shrinkage and swelling properties of soil(iv) increase the tensile strength of soil.

Functions Stabilizers in Soils

Some Commonly Used Stabilizers are;(i) cement(ii) lime(iii) combination of lime and cement(iv) combination of lime and Pozzolana(v) asphalt(vi) burning clay bricks***

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5.0 Soil Tests and Results5.1 Bottle testThe aim of this test was to determine the approximate amount ofclay, silt, sand and gravel presence in the soils.

Table 5.1 Summary of bottle test results Soil description S1 S2Height of clay/silt content (mm) 250 220Total height of the content (mm) 480 480Percentage of clay/silt (%) 52.1 48.5

5.2 Atterberg limits Soils can have various states of consistency, liquid, plastic or solid. The boundaries separating the limits and indices, are expressed as percentages by weight of the moisture content. Table 5.2: Summary of Atterberg limit test

Soil sample S1 S2 Liquid limit (wL) % 43 37.6 Plastic limit (wP)% 21 20 Plasticity index (IP)% 22 17.6

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5.3 Box (linear shrinkage) test This test is used to determine the amount of cement or lime to be used for stabilizing a particular soil. The test results for soil samples S1 and S2 were as follows; Table 5.3: Summary of linear shrinkage test results

Sample soil S1 S1 Shrinkage (mm) 12 18

5.4 Grain size distribution Table 5.4: Summary of particle size distribution

Soil description S1 S2 Clay (%) 34 25 Silt (%) 23 23 Sand (%) 40 51.3 Gravel (%) 3 0.7 Solid density (kg/dm³) 2.65 2.65 Soil description Gravelly silty

clayey SAND Gravelly silty clayey Sand

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6.0 Production of Bricks

(a) (b) brick size: 300 x 114 x 145 mm

Figure 4: (a) Hand steel mould, (b) Stabilized soil bricks

Mix Ratio: Cement: lime: soil 1:1:15 batch by volume

cement: water 1:4 (water cement ratio)

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7.0 Testing of Bricks

Compressive Strength TestThis test was done in order to determine the compressive strength of the bricks against vertical loading. The procedure for testing each brick was done as follows:à External dimensions of each sampled brick were taken and the brick was marked for identification• The brick was weighed and recorded in grams • The brick was placed into the machine and all set-up procedures were properly done• The brick was gradually loaded until it failed• The ultimate load was recorded.

Fig. 5: A Brick under test

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Densities of soil Bricks - S1

11.11.21.31.41.51.61.71.8

1 2 3 4 5 6 7

Sample No []

Des

nity

[g/c

m³]

Figure 6: Densities of bricks – sample S1

Results – Sample S1

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Com pre ssive strength o f S oil Bricks - S 1

1

2

3

4

5

6

1 2 3 4 5 6

Sa m ple No

Str

ess

[N/m

m²]

BS 5628 Va lueTest Re sults

Figure 7: Compressive strength for bricks from sample S1

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Results - Sample S2

Densities of Soil Bricks - S2

1

1.2

1.4

1.6

1.8

2

1 2 3 4 5 6

Sample No []

Dens

ity [g

/cm

³]

Figure 8: Densities of soil bricks from soil sample S1

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Compressive strength of Soil Bricks - S2

0.00

1.00

2.00

3.00

4.00

5.00

6.00

1 2 3 4 5 6

Sample No []

Stre

ss [N

/mm

²]

BS 5628 ValueTest results

Figure 9: Compressive strength for bricks from sample S2

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Water absorption test resultsThe test was carried out in order to assess the water absorption of the bricks.

Results:

Average water absorption for bricks from sample S1 = 12.3%

Average water absorption for bricks from sample S2 = 9.86%

Specification control values : not greater than 20%

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8.0 Compressive Strength Test of Sample Walls

Load bearing walls may be designed to carry in plane horizontal loads induced by wind, bracing effects or earthquake; the loads are transferred to the walls primarily via diaphragms such as floors or roofs.

Figure 10: Wall specimen under test

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Results:Average maximum load: 137.00 kN

Average compressive strength: 1.22 N/mm2

From practical point of view, the strength of walls for normal residential buildings is less than 0.60 N/mm2

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Application of stabilized soil bricks

Figure 11: Guard’s hut built from stabilized soil bricks

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9.0 Concluding Remarks• Basing on the results obtained in this study, it has been

found that bricks made from stabilized soil with a mix ratio of 1:1:15 for cement: lime: soil have compressive strengths greater than that specified in the British Standard (BS 5628 Part 1) of 2.80 N/mm2.

• It has also been learnt that this technology uses less amount of cement in the production of bricks when compared with cement sand blocks

• Building with stabilized soil bricks is a technology which offers a good possibility for enabling low income groups to build their own houses at low cost

• stabilized soil bricks have more advantage when compared to burnt bricks because they are environmentally friendly since there is no felling of trees for burning the bricks.

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Burnt Bricks

Figure 12: Burnt brick site near Ifunda – Iringa region

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Recommendations

• Promoting stabilized soil bricks through advertising and pilot housing, so that many people could have much knowledge about this technology.

• To prepare operation manual for the soil preparation, use of the bricks press and building process.

• Training people about the production of these bricks.• Further research on use of stabilized soil bricks for low-

rise buildings is required. In this case a study on the strength of walls against lateral loading and earthquake effects is deemed necessary.

To achieve a successful dissemination of this technology, the following recommendations should be implemented.

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Steel Mould for Bricks Production

Figure 13: Steel mould for bricks production

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Fig. 14:

Map of Tanzania