STUDY ON CEMENT STABILIZED EARTH BLOCK (CSEB) MADE OF DREDGED SOIL

ABSTRACT

 

This paper represents an experimental study on Cement Stabilized Earth Block made of dredged soil and stabilized by Ordinary Portland Cement (OPC). The dredged soil was collected from various point of Bangladesh namely, Kapotakkho river, Brahmmaputra river, and Housing and Building Research Institute (HBRI) Campus.

 

The complete project was divided into two broad phase. In first phase, the optimum percentage of stabilizer (cement) was found out. After testing these blocks by water jet, submersion, modulus of rupture, and compression, the 10% cement mix with dredged soil proved to be viable options for economical and durable blocks.

 

In second phase, CSEB was made with adding sand (Brahmmaputra soil) and Jute fiber as reinforcement. Stabilizer (cement) was kept constant for each type of block as 10%. Different type of curing like air curing, sun curing and polythene wrapping curing were incorporated and tested for three days, seven days, fourteen days, and twenty eight days. The observations on different composition and curing condition and change in strength with maturity age sowed that each composition has its own superiority to others on particular area. For example, CSEB made of HBRI soil and cured at natural air can bear more compressive load where Kapotakkho-Brahmmaputra soil cured at sun is less susceptible to moisture absorption.

1.                    INTRODUCTION

1.1.               Background

The use of earth, as building material dates back thousands of years. Dried earth construction is common in some region of world where the specific climate and economic condition dictate and where the earth construction is aesthetically accepted to all. Raw earth has been used for the construction of buildings since the most ancient times, and the traditional housing that exists in many parts of our planet bears witness to this fact. Abandoned and forgotten with the advent of industrial building materials, particularly concrete and steel, it is today the subject of renewed interest in developing countries as well as in industrialized countries. Often criticized for its sensitivity to water and its lack of durability, this building material has in its present form many advantages for the construction of durable, comfortable and low-cost housing. The present formed is termed as Stabilized Compressed Earth Block (SCEB). If logic and modern methods are applied to its use, it can be all of the following:

-          Efficient and durable;

-          Available locally and cheaply;

-          Economical in energy and in foreign currency for developing countries;

-          An encouragement for the development of building trade skills;

-          Job creating;

-          Capital gains generating;

-          A dynamic for the building sector;

-          Ideal for small and medium scale industries.

Earthen building construction materials production techniques varies from very primary to modern sophisticated mechanized and industrial process. The idea of compacting earth to improve the quality and performance of molded earth blocks is however, far from new, and it was with wooden tamps that the first compressed earth blocks were produced. The turning point in the use of presses and in the way in which compressed earth blocks were used for building and architectural purposes came only with effect from 1952, following the invention of the famous little CINVA-RAM press. This was to be used throughout the world.

1.2.               Advantages and Disadvantages of CSEB

The main advantages of using CSEB as walling materials in residential buildings are summarized below

Ø  Soil is available in large quantities in most regions.

Ø  Cheap and affordable - in most parts of the world soil is easily accessible to low-income groups. In some locations it is the only material available.

Ø  Ease of use - usually no very specialized equipment is required.

Ø  Suitable as a construction material for most parts of the building.

Ø  Fire resistant - non-combustible with excellent fire resistance properties.

Ø  Beneficial climatic performance in most regions due to its high thermal capacity, low thermal conductivity and porosity, thus it can moderate extreme outdoor temperatures and maintain a satisfactory internal temperature balance.

Ø  Low energy input in processing and handling soil - only about 1% of the energy required manufacturing and processing the same volume of cement concrete. This aspect was investigated by the Desert Architecture Unit which has discovered that the energy needed to manufacture and process one cubic meter of soil is about 36 MJ (10 kwh), while that required for the manufacture of the same volume of concrete is about 3000 MJ (833 kwh). Similar findings were also reported by Habitat (UNCHS), Technical Note No. 12 comparing adobe with fired clay bricks.

Ø  Environmental appropriateness - the use of this almost unlimited resource in its natural state involves no pollution and negligible energy consumption thus further benefiting the environment by saving biomass fuel.

The main disadvantages of using CSEB as walling materials in residential buildings are summarized below

Ø  Reduced durability - if not regularly maintained and properly protected, particularly in areas affected by medium to high rainfall.

Ø  Low tensile strength – poor resistance to bending moments, to be used only in compression e.g. bearing walls, domes and vaults.

Ø  Low resistance to abrasion and impact - if not sufficiently reinforced or protected.

Ø  Low acceptability amongst most social groups - considered by many to be a second class and generally inferior building material.

Ø  On account of these problems - earth as a building material lacks institutional acceptability in most countries and as a result building codes and performance standards have not been fully developed.

2.                    TEST PROGRAM

The complete project was divided into two broad phase. In first phase, the optimum percentage of stabilizer (cement) was found out. The test procedure and observed result was discussed in previous report on CSEB. Testing blocks with different percentage, it was observed that 10% cement by weight is viable in Bangladesh in all respect. In second phase, CSEB was made with adding sand (Brahmmaputra soil) and Jute fiber as reinforcement. Stabilizer (cement) was kept constant for each type of block as 10%. Different type of curing like air curing, sun curing and polythene wrapping curing were incorporated and tested for three days, seven days, fourteen days, and twenty eight days. The observations on different composition and curing condition and change in strength with maturity age are the discussed matter of this report.

2.1.               Preparation of Test Specimen:

2.1.1.           The Requirements for Preparation

The basic materials required for the production of compressed stabilized earth building blocks are soil, stabilizer, and water. The stabilizer, whether lime or cement or some other material, is usually available in powder or liquid form, ready for use. The soil may be wet or dry when it is first obtained, and will probably not be homogeneous. In order to have uniform soil, it is often necessary to crush it so that it can pass through a 5 to 6mm mesh sieve.

Different soil types may also need to be used together so as to obtain good quality products. For instance, heavy clay may be improved by addition of a sandy soil. It is not only important to measure the optimum proportion of ingredients, but also to mix them thoroughly. Mixing brings the stabilizer and soil into direct contact, thus improving the physical interactions as well as the chemical reaction and cementing action. It also reduces the risk of uneven production of low quality blocks. Various types and sizes of mixing equipment are available on the market.

2.1.1.1.      Weathering

Mostly, the dredged soil is wet and contain high amount of moisture. Breaking into powder and pulverizing the wet soil require complex machineries. To avoid complex machineries, collected dredged soil was kept under a shade for 15 to 25 days. In this period the soil was weathered and dried to ease the breaking operation.

2.1.1.2.      Grinding followed by screening

The material is pressed between two surfaces - a rather inefficient and tedious process in which bigger stones are broken up, however, only simple machinery is required. The broken up lumps of soil are then passed through a screen.

2.1.1.3.      Pulverization of soil

The material is hit with great force so it disintegrates. The machinery required is complex but we used manual labor with wooden mallet.

2.1.1.4.      Sieving

Soil contains various sizes of grain, from very fine dust up to pieces that are still too large for use in block production. The oversized material should be removed by sieving, either using a built-in sieve, as with the pendulum crusher, or as a separate operation.

The simplest sieving device is a screen made from a wire mesh, nailed to a supporting wooden frame and inclined at approximately 45º to the ground. The material is thrown against the screen, fine material passes through and the coarse, oversized material runs down the front. Alternatively, the screen can be suspended horizontally from a tree or over a pit. The latter method is only suitable in the case where most material can pass through easily otherwise too much coarse material is collected, and the screen becomes blocked and needs frequent emptying.

2.1.1.5.      Proportioning

Before starting production, tests should be performed to establish the right proportion of soil, stabilizer and water for the production of good quality blocks. The proportions of these materials and water should then be used throughout the production process. To ensure uniformity in the compressed stabilized earth blocks produced, the weight or volume of each material used in the block making process should be measured at the same physical state for subsequent batches of blocks. The volume of soil or stabilizer should ideally be measured in dry or slightly damp conditions.

Being confirmed from previous phase of the research we used 10% cement as stabilizer and potable water to form a consistent mix.

2.1.1.6.      Mixing

In order to produce good quality blocks, it is very important that mixing be as thorough as possible. Dry materials should be mixed first until they are a uniform color, then water is added and mixing continued until a homogeneous mix is obtained. Mixing can be performed by hand on a hard surface, with spades, hoes, or shovels.

It is much better to add a little water at a time, sprinkled over the top of the mix from a watering can with a rose spray on the nozzle. The wet mix should be turned over many times with a spade or other suitable tool. A little more water may then be added, and the whole mixture turned over again. This process should be repeated until all the water has been mixed in.

A concrete mixer, even if available, will not be useful for mixing the wet soil, since the latter will tend to stick on the sides of the rotating drum. If machinery is to be used for mixing, it should have paddles or blades that move separately from the container. However, field experience shows that hand-mixing methods are often more satisfactory, more efficient and cheaper than mechanical mixing, and are less likely to produce small balls of soil that are troublesome at the block molding stage.

2.1.2.           Block Production

Many aspects should be taken into consideration before launching an operation to produce compressed stabilized earth building blocks:

Ø  Amount and type of stabilizer required,

Ø  Soil properties and its suitability for stabilization,

Ø  Building standards and hence quality of blocks required,

Ø  Load bearing requirements of construction i.e. single storey or more.

Generally, there is a wide variation of acceptable standards that vary according to local weather conditions. Blocks with wet compressive strengths in the range or 2.8MN/m2 or more should be adequate for one and two-story buildings. Blocks of this type would probably not require external surface protection against adverse weather conditions. For one-storey buildings, blocks with a compressive strength in the order of 2.0 MN/m2 will probably be strong enough, but where rainfall is high an external treatment is necessary. Since the wet strength of a compressed stabilized earth block wall may be less than two-thirds of its dry strength. It should be remembered that all compressive strength tests should be carried out on samples which have been soaked in water for a minimum of 24 hours after the necessary curing period.

The final wet compressive strength of a compressed earth block depends not only on soil type, but also on the type and amount of stabilizer, the molding pressure, and the curing conditions.

The soil preparation and pressing operation can be best described by the pictures below.

 

1.1.               Curing

To achieve maximum strength, compressed stabilized earth blocks need a period of damp curing, where they are kept moist. This is a common requirement for all cementitious materials. What is important is that the moisture of the soil mix is retained within the body of the block for a few days. If the block is left exposed to hot dry weather conditions, the surface material will lose its moisture and the clay particles tend to shrink. This will cause surface cracks on the block faces.
 

In practice, various methods are used to ensure proper curing. Such methods include the use of plastic bags, grass, leaves, etc. to prevent moisture from escaping. After two or three days, depending, on the local temperatures, cement stabilized blocks completes their primary cure. They can then be removed from their protective cover and stacked in a pile, as shown in Figure. As the stack of blocks is built up, the top layer should always be wetted and covered, and the lower layer should be allowed to air-dry to achieve maximum strength. Alternatively, freshly molded blocks can be laid out in a single layer, on a non-absorbent surface, and covered with a sheet to prevent loss of moisture.

The required duration of curing varies from soil to soil and, more significantly, which type of stabilizer is used. With cement stabilization, it is recommended to cure blocks for a minimum of three weeks. Compressed stabilized earth blocks should be fully cured and dry before being used for construction.

The sample was demolded immediately after casting and it was then transferred to curing place. Three different curing conditions (Air curing in shade with water spray, Sun curing, and Polythene wrapping curing) were established. Density, unit weight, and water content of the mixture and freshly casted brick was measured and noted.

Casted bricks were marked date wise to conduct different tests after different curing age and curing condition.

1.1.               Tests on CSEB

In this phase of research mainly compressive strength test for individual block was performed for different composition and curing condition. Dry and wet compressive strength was measured for each set of blocks as per BS 3921: 1985, IS: 3495- Part-1-1992 and ARS-683:1996. Finally, masonry test was done with minimum three layers of blocks as per ASTM C 1388 and CRD C 463-01.

The result of dry and wet compressive strength tests are summarized and analyzed in the following sections.

2.                    TEST RESULT ANALYSIS AND DISCUSSION

2.1.   Compressive Strength Test:

5 pcs of 3 days, 7 days and 28 days cured bricks were selected randomly from the lot and compressive strength test was performed as perNEW MEXICO EARTHEN BUILDING MATERIALS CODE 2003. Result of compressive strength test was shown in Table below.

Compressive Strength Test Result

Dry compressive Strength Test Result (psi)					Wet Compressive Strength Test Result (psi)
Sample	Curing Condition	Curing Age (Days)			Sample	Curing Condition	Curing Age (Days)
		3	7	28			3	7	28
KP	*A	288	427	540	KP	A	187	213	229
KB	A	435	489	1100	KB	A	293	240	675
KJ	A	332	424	585	KJ	A	250	275	340
DC	A	890	990	1107	DC	A	362	391	433
KP	*S	294	330	632	KP	S	204	220	307
KB	S	350	538	557	KB	S	374	422	622
KJ	S	360	580	402	KJ	S	280	300	355
DC	S	490	1095	1062	DC	S	431	441	566
KP	*P	304	308	350	KP	P	200	216	300
KB	P	400	447	512	KB	P	353	464	590
KJ	P	360	332	447	KJ	P	242	307	380
DC	P	501	605	769	DC	P	300	354	433

*A= Air Curing (Open air under shade), S=Sun Curing (Under direct sunlight), P= Polythene Curing (Wrapping by polythene paper)

 

For Dry Compressive Strength:


 

For Wet Compressive Strength:

 

1.1.1.           Analysis of compressive strength test result:

From the table and the figure we can see that compressive strength increases with increasing curing age. It was observed that dry compressive strength of CSEB made from HBRI Campus soil (DC) is superior to any other composition. This was obvious because this soil contain high amount of clay. On the other hand, the composition KB (Kapotakkho+Brahmmaputra) perform well in case of wet compressive strength. It is because of the sand content in the soil of Brahmmaputra.  Also from figure it was observed that for 3 and 7 days sun curing shows good result but finally at 28 days air curing provide better result both for dry and wet compressive strength tests. Hopefully all composition of soil gives satisfactory result comparing the codes and standard both for dry and wet compressive strength.

1.2.               Masonry Test:

To determine the ultimate compressive strength (f’m) of masonry wall this test was performed. The test was conducted as per ASTM C1338-98: Standard Test Method for Compressive Strength of Laboratory Constructed Masonry Prisms. For each sample three unit of block was casted by standard mortar (1:5). A total of 15 pieces of sample for each type of block were constructed and the samples were cured and tested for 3, 7, and 28 days. As previously air curing provided good result so in this case only air cured blocks were tested. The average of 5 specimens for each days test was incorporated in the result book. The test result is given in the following table.

Compressive Strength of Masonry Prism

1.1.1.           Analysis of Masonry Test Result:

From the table and graph we see that 28 days compressive strength of masonry prism is higher for clay bricks and HBRI campus soil. However, no prism fall behind the acceptable limit (300psi) suggested by codes and standards.

2.        CONCLUSION

Based on the review of both experimental and filed investigation on clay bricks and stabilized compressed earth blocks, the following concluding remarks can be drawn:

 

(i) Major usage in the world for construction is clay bricks; many researchers are presently looking for newer options because they need low cost materials, which are also environmentally friendly. The process of manufacturing clay bricks also requires high energy to burn due to the emission of CO2 gas from this process.

(ii) Stabilized compressed earth blocks include; uniformed building component sizes, use of locally available materials and reduction of transportation. Uniformly, sized building components can result in less waste, faster construction and the possibility of using other pre-made components or modular manufactured building elements. Such modular elements as sheet metal roofing which can be easily integrated into a CEB structure.

(iii) The use of natural, locally-available materials makes good housing available to more people, and keeps money in the local economy rather than spending it on imported materials, fuel and replacement parts.

(iv) The earth used is generally subsoil, leaving topsoil for agriculture. Building with local materials can provide employment for local people, and definitely considered more sustainable in times of civil economic difficulties.

(v) People can often continue to build good shelters for themselves regardless of the political situation of the country.

(vi) The reduction of transportation time, cost and attendant pollution can also make CEB more environmentally friendly than other materials.

REFERENCES

Adam EA (2001). Compressed Stabilized earth Block Manufacturing in Sudan. United Nations Educational Scientific and Cultural Organization. Technical Note No12 comparing adobe with fired clay bricks.

Aksa Z (1984). A lime-stabilized soil building materials. Institue of Human Settlement. Bandong, Indonesia .

American Society for Testing Materials (ASTM) (1990). Standard Specification for Hollow Brick, (Hollow Masonry Units Made from  Clay or Shale) United States of America, ASTM C652- 89a.

Amjad J (2000). Investigation of phases in soils used for brick –Making. M.Phil Thesis, Center of Excellence in Solid State Physics, University of Punjab pp. 1-6.

Anderson LA, Johansson BJ, Asteland J (1982). Cement–stabilized Torba. Swedish council for building Research and Swedish. Agency for research Cooperation with Developing Countries. Lund University. Lund Sweden. pp. 78-79.

AS, 1225 (1984). Clay Buildings Bricks. Australia Standard. British Standards Institution (1985). British Standard Specification for clay Brick. London, BS, p. 3921.