Cover Image
close this bookJournal of the Network of African Countries on Local Building Materials and Technologies - Volume 1, Number 4 (HABITAT, 1991, 48 p.)
View the document(introduction...)
View the documentContributions to the Journal
View the documentForeword
View the documentHousing in Africa, problems, prospects and strategies*
View the documentKenya: Towards the development of a national code of practice for structural masonry - the Kenyan approach*
View the documentNigeria: Research and development in the promotion of standards and specifications for stabilized soil blocks*
View the documentEthiopia: Light-weight concrete made with Ethiopian pumice*
View the documentMauritius: Use of calcarenite blocks in housing construction in Rodrigues*
View the documentGhana: Optimum firing temperature for some clay bricks*
View the documentEthiopia: Construction of mud houses - an alternative to the traditional methods of house construction*
Open this folder and view contentsIndia: Technology profiles
View the documentPublications review
View the documentEvents
View the documentBack cover

Nigeria: Research and development in the promotion of standards and specifications for stabilized soil blocks*

* By A.O. Madedor and A.O. Dirisu, Nigerian Building and Road Research Institute (NBRRI), Lagos.

Introduction

The wide-scale availability of soils, including clays, in Nigeria, is an indication of a successful promotion of soil blocks, burnt bricks and stabilized soil blocks. Though basic machinery and equipment are indispensable in the production of some of these materials, manual operation can lead to noticeable improvements in the final products. In soil construction, for instance, the Nigerian Building and Road Research Institute (NBRRI) has developed a manually-operated block-making machine which has improved the mechanical properties of soil blocks as compared with hand-moulded blocks. Also, an improvement in stabilized soil has been achieved by NBRRI by adopting simple measuring cans for proportioning of soil and cement and the use of a sieve for grading the soil. In fact, the resource endowments of Nigeria offer a good potential for production of a variety of indigenous building materials. However, this potential has not yet been translated into commercial production, mainly because of lack of standards and specifications.

The common practice in rural areas is for materials to be produced without the application of any standard. Even in those cases where standards have been stipulated, a fundamental gap remains in the sense that there is no means to enforce the standard (A typical example is NIS 74, 1976). Worse still, the general run of building regulations and codes of practice makes little or no reference to locally-adopted standards but, rather, incorporates standards of imported building materials.

Standards and specifications are basic instruments for promotion of acceptable products on the market and, in the context of building materials, they ensure economy, durability, safety and cost-efficiency, as well as health in construction.

It is disheartening to note that, in Nigeria, hardly any standard on indigenous materials has been formulated so far. It is against this background that this paper aims at reviewing the state-of-the-art in the production of adobe or sun-dried blocks, stabilized-soil blocks and burnt-clay bricks. An attempt is also made to highlight gaps in the quality-control procedures and technical characteristics for these materials.

Characteristics of materials

General properties of soil

A clear understanding of the characteristics of a selected soil group is a prerequisite for establishing successful soil-construction practice. To a large extent, failure in earth construction and the general unpopularity of the materials are due to a lack of knowledge on the properties of soils prior to their use in construction (1).

Soils can be classified into six main groups, based on their texture or grain size. The composition of soil, in terms of the grain size distribution, determines the plasticity, compactability, and cohesion which, in turn, determine its fundamental mechanical properties. For the purpose of use in construction, soil classification has been categorized into the three following groups (2):

(a) Fine grained soil, i.e., not less than 90 per cent should pass through a 2 mm sieve;

(b) Medium grained soil, i.e., not less than 90 per cent should pass through a 20 mm sieve;

(c) Coarse-grained soil, i.e., not more than 90 per cent should pass through a 37.5 mm sieve.

Mineral content of soil, if determined, can be a useful criterion for determining the suitability of soils. Investigations carried out at NBRRI reveal that:

(a) Soils with 15 per cent or less clay content are suitable for soil blocks (adobe);

(b) Soils containing expansive clays (black cotton soils) are not suitable for direct use, except when treated with special agents;

(c) Soils with organic matter exceeding 1 per cent and with soluble salts greater that 1 per cent or with excessive mica flakes are not suitable;

(d) Quartz and other siliceum minerals, limestone and marl, are acceptable.


Bricks made of earth and 10 per cent cement

Engineering properties of clay products

Stabilized-soil blocks, sun dried (adobe) and burnt-clay bricks of high strength, with low water-absorption capacity and high durability, require certain basic production practices, notably, a thorough mixing of the ingredients, avoidance of excess water and wet curing.

Compressive strength

It is important to determine the compressive strength of soil blocks as well as the ability of the blocks to withstand strength variation between wet and dry conditions. The behaviour of stabilized-soil blocks depends, mainly, on the mechanism of stabilization. Tables 1 and 2 show the results of collaborative research between NBRRI and the Civil Engineering Department of Ahmadu Bello University (ABU) on strength properties of stabilized-soil blocks. The dry compressive strength of blocks was above 2.0 N/mm2, irrespective of age of curing. On wet compressive strength there is variation on strength, based on a number of days of soaking. This may be due to hydration. The lowest strength (wet) occurred after one day of soaking but no strength was less than 1 N/mm2. If BS 2028, 1970 specifications are to be followed, the average strength of 12 sandcrete blocks should be 2.76 N/mm2. In the dry compressive strength test conducted, this criterion was satisfied by 8 per cent cement stabilization. Generally, there is an appreciable increase in compressive strength with an increase in compaction effort; it appears that most strength is gained after the first seven days of curing, for there is little or no strength increase between 14 and 28 days. An increase in compaction effort can adequately compensate the amount of stabilizing agent used. At 7 N/mm2 with 5 per cent cement, the compressive strength obtained after seven days of curing is almost that obtained with 12 per cent cement at 28 days curing.

Table 1. Dry compressive test of 8 per cent cement-stabilized block

Number of blocks tested

Average compressive strength [N/sq mm]

Age of block

6

3.06

7 days

4

2.88

14 days

4

3.04

28 days

5

3.06

21 days

5

2.87

1 month

5

3.10

2 months

4

2.25

3 months

2

4.55

6 months

6

5.33

9 months

4

3.92

12 months

6

3.87

15 months

6

4.68

20 months

4

4.50

21 months

Table 2. Wet compressive strength of 8 per cent cement-stabilized block

Number of blocks tested

Average compressive strength [N/sq mm]

Age of block

Number of days soaked

4

1.28

28 days

1

4

1.34

28 days

2

5

1.49

28 days

3

4

1.83

28 days

6

5

2.11

28 days

9

4

1.99

28 days

12

5

1.95

28 days

15

4

1.29

1 month

1

4

1.29

2 months

1

4

1.95

6 months

1

4

1.24

9 months

1

4

1.61

12 months

1

4

2.06

15 months

1

6

1.93

20 months

1

6

1.91

21 months

1

In its research activities, NBRRI has considered recommendations made by various research groups (2, 3, 4, 5, 6). Generally, stabilization with 8 to 12 per cent cement has been recommended by the majority of these groups, however, this level of cement consumption makes the stabilized-soil blocks uncompetitive against sandcrete blocks. NBRRI’s investigation was confined to the use of 5 per cent cement. The compaction effort was 1 N/mm2 and the curing period was seven days. Results from the test were in conformity with those of ABU (see table 3).

Table 3. Effect of compaction effort, percentage stabilization and curing time on compressive strength (ABU)

Compaction

8 per cent cement 28 days

10 per cent cement 14 days

10 per cent cement 28 days

12 per cent cement 28 days

1 N/mm2

4.0

3.6

3.6

3.7

2 N/mm2

6.5

7.1

7.8

7.8

3 N/mm2

8.6

9.1

9.2

11.1

4 N/mm2

9.2

9.5

9.8

13.3

Much work has been carried out at ABU on contact layer for hollow stabilized blocks and immersion period. For an assessment of the influence of contact layer on compressive strength, various types of packing materials were selected viz. sand-cement mortar (1:1), 3-ply plywood (3 mm thick), softboard sheet (10 mm thick), soil-cement (1:8) and steel platens. Results for 8 per cent cement-stabilized blocks are shown in table 4.

Table 4. Strength test for 8 per cent stabilized hollow blocks with different contact layers

Stabilized block

Sand cement mortar

Plywood sheet

Soft wood

Soil-cement mortar

Steel platens

8 per cent dry

1.98

2.78

2.82

2.22

2.40

8 per cent wet

1.79

1.53

2.05

1.91

1.41

The above results show clearly that the compressive strength of stabilized-soil blocks is underestimated in comparison with the use of softboard by about 20 per cent when plywood or soil-cement mortar is used and by 25 per cent when sand-cement mortar or steel platens are used. It follows that the soft-board sheets are the most suitable contact layers for the assessment of compressive strength of stabilized-soil blocks.

The compressive strength of stabilized-soil blocks, like that of sandcrete or burnt-day blocks, varies with the moisture content. Since block walls are to resist design loads both in dry and in wet conditions, the lower strength is taken for design purposes. Investigations have shown that during the first day of immersion, the compressive strength of blocks decreases drastically to about 40 per cent of the dry compressive strength. The minimum wet strength test lasts throughout the next day and then increases gradually to about 65 per cent of the dry compressive strength at about 7 to 10 days of soaking. It has been recommended that for the determination of minimum wet compressive test, soaking of samples should not exceed two days.

Durability

The durability test (wetting and drying) on compacted-soil/cement mixtures gives the weather-resistant properties of the stabilized soil block. The recommended limit for minimum durability is not more than 5 per cent loss in weight after 12 cycles of wetting and drying for permanent building developments and for urban areas with more than 508 mm of annual rainfall (5). Other recommendations state that the weight loss should not be more than 0.4 per cent after six cycles (7). From results obtained by ABU and NBRRI, the two recommendations could not be met. As a solution, NBRRI proposed the use of overhangs of the roofs and verandahs in most houses constructed with stabilized-soil blocks.

Test methods

Stabilized-soil blocks of high strength, low water-absorption characteristic and high durability require certain basic production practices, notably, a thorough mixing of the ingredients, avoidance of excess water and good wet curing. It is important to carry out regular tests on samples of blocks for each production batch. To ensure the suitability of blocks for use in construction, the following standard tests are recommended.

Bulk density

A sample of the block is dried and the weight is recorded. The weight is then divided by the volume which gives the bulk density (kg/m3). A high bulk density is an indication of high compressive strength for each batch, an average of three blocks should be measured and any block with a variation in weight exceeding 5 per cent should be rejected.

Moisture content

At the time of use, the moisture content of the blocks should not be more than 4 per cent.

Water absorption

A block is dried and the weight is recorded as A. The block is then immersed in water for 24 hours after which the weight is recorded as B. Water absorption percentage is equal to:

This test should be conducted on three blocks, and the average value recorded should not be more than 25 per cent.

Compressive strength

For the compressive strength test, a sample of three blocks, after each cycle of curing, is immersed in water for 24 hours. The adhering moisture is then wiped off the blocks, and the compressive strength is determined as in normal block testing procedures (NIS 74 and BS 2028). Any block with a compressive strength of less than 15 per cent of average strength should be rejected. Normally, compressive strength of a water-saturated block after 28 days at wet curing should be in the order of 2.1 N/mm2.

However, a strength of 1.4 N/mm2 should be adequate for a single-storey building (5), (7). The Civil Engineering Department of Ahmadu Bello University proposed the minimum average compressive strength of 1.5 N/mm2 and 10 samples of blocks for establishing the average compressive strength.

Durability test

In undertaking the wetting and drying test, the following weights of a block are recorded:

(a) W1 = The weight of a block after being cured and dried ready for use;

(b) W2 = The weight of the block after immersion in water for 24 hours;

(c) W3 = The weight of the block which has been dried in the sun after 24 hours soaking in water, and

(d) W4 = The weight of the sun-dried block after 20 strokes of a wire brush. The process is repeated through six cycles with each cycle representing an entire operation from W1 through W4. After this, the following results are determined:



Weight of water absorbed

= W2 - W1



Weight of loss on drying

= W2 - W3



Weight loss on brushing

= W3 - W4



Porosity of block, per cent

=



Loss in weight per cycle, per cent

=

From the above analysis, a good block is one which shows no sign of erosion or flaking and having a weight loss of not more than 0.4 per cent after six cycles. Table 5 shows test results carried out at NBRRI.

Table 5. Test results on blocks at NBRRI

Properties

Laboratory

Field

Bulk density

1.8 g/cm3

1.8 g/cm3

Compressive strength

1.65 N/mm2

1.47-1.70 N/mm2

Water absorption

13.0 per cent

18.0 per cent


Brick masonry of a modern house

Recommendations

The test specification for precast concrete blocks (BS 2028) as well as that for burnt clay bricks (NIS 74) are not directly applicable for compressive strength tests of stabilized soil blocks. In the absence of a suitable specification, they could, however, be used as a guide with the following modifications:

(a) The pre-conditions recommended by NIS 74 for perforated bricks can be followed:

Immerse the specimen in water at 27.7°C for 24 hours. Remove the specimen from water and drain out any surplus water. No mortar shall be filled in the perforation and no mortar capping shall be provided.

Thus, “preparation of specimen”, recommended by BS 2028, should be neglected, since concrete capping reduces the strength and long-term immersion in water, due to rehydration of cement, and does not allow for testing the minimum wet compressive strength.

(b) “Actual testing” recommended by NIS 74 for perforated bricks should be modified and read as follows:

“Place the hollow faces of the block between two softboards each of 10 mm in thickness and carefully centred between the plates of the testing machine. Apply the load axially and without shock at a steady increasing rate of 15 N/mm2 per minute until failure occurs”.

The use of Plaster of Paris should be examined before it is recommended for compressive strength tests.

(c) Other recommendations, given by both standards (NIS 74 and BS 2028), such as “Test specimens”, “Apparatus”. “Measurement of compressive strength”, and calculation of “Test results”, should be taken into account.

(d) Further tests are to be carried out to gather sufficient results for the preparation of a new specification for stabilized soil blocks as it is urgently needed in the construction industry.

(e) The Department of Civil Engineering of Ahmadu Bello University and NBRRI should carry out further tests on contact layers and minimum wet compressive strength of stabilized-soil blocks.

NBRRI block-making machine

The use of stabilizing agents, notably, cement, in soils, to produce a durable building material, had been recognized for quite a long time. However, until some presses with adequate compaction effort were developed, starting with the Cinva Ram, it was not possible to extend the idea in a practical way to the building industry. One such initiative was used locally to produce cement stabilized bricks of the standard size 9 1/2” x 4 1/2” x 3” (240 x 115 x 71 mm). Full-scale trial walls were built with these standard-sized bricks in Kano, in northern Nigeria. The local masons were unhappy with the rate of production and the extra skill needed to lay them properly, Accordingly, NBRRI decided to develop a block-making machine that would reflect local conditions and practices. In this respect, it should be recalled that for over three decades since the end of the Second World War, sandcrete blocks made from sand and cement of sizes 18” x 9” x 9” (460 x 230 x 230 mm) and 18” x 9” x 6” (460 x 230 x 150 mm) were used as the conventional walling materials in Nigeria.


NBRRI block-making machine

NBRRI developed a block-making machine to produce three blocks in one operation. Normal production of blocks is achieved by a three-person team. One person mixes and loads the soil into the press, another compacts and extracts blocks while the third person transports the block to the drying area. Once the workers are familiar with the production process, 800 blocks can be produced daily.

The machine has been in high demand since its use in the construction of a house at a model village in Kano State in northern Nigeria. The potential of the machine for urban housing has been enhanced by its use in a housing project by the Federal Housing Authority in March-April 1990, in Festac Town, a satellite town of the city of Lagos.

References

1. Rai, M., and Jaisingh, M.P.M., Advances in Building 7. Materials and Construction (1st ed.) 1985.

2. United Nations Centre for Human Settlements (Habitat) and Commonwealth Science Council, Workshop on Formulation of Standards and Specification for Local Building Materials, Nairobi, March 1987.

3. Ola, S.A., “Need for estimated cement requirements for stabilizing lateritic soils”, Transportation Engineering Journal, 10543 (May 1974), p. 379.

4. Smith, R.G., “Building with soil cement bricks”, Building Research Practice, vol. 2 (1974), No. 2.

5. Ola, S.A., “Soil stabilized compressed blocks”, Geotechnical Research Report No. 1 (ABU, Department of Civil Engineering, August 1983).

6. Fitzmaurice, R., Manual on Stabilized Soil Construction (New York, 1958).

7. Lunt, M.G., “Stabilised soil block for building”, Overseas Building Notes No. 184 (1980).