Chapter 2

L I T E R A T U R E R E V I E W

2.1 GENERAL

One of the essential preconditions for improving the standard of living of people is the provision of reasonably good housing (Ghavami & Fang, 1984). Rapid and uncontrolled urbanisation in many developing countries have resulted in a severe shortage of houses in recent years, specially in towns and suburban areas. Owing to limited financial resources, the optimum use of locally available materials and the development of associated technical information would be the solution for providing economically viable housing for majority of the low to medium income earners. The promotion of self help schemes is also a priority area (Lim et al., 1984).

In this context, it would be essential to explore the ways and means of optimising the use of resources available locally while paying sufficient attention to the safety and protection of the environment. This is particularly true for Sri Lanka where the shortage of conventional building materials have made good housing too expensive for majority of low to medium income earners.

House construction with alternative building materials poses a challenge to the policy makers, planers, architects and engineers in view of the parameters that have to be looked into, such as social acceptability, adequate strength, security, economy, ease of maintenance, availability of materials, level of technology required, duration of construction and durability (Rao et al., 1983).

In this research, an attempt was made to give a comprehensive coverage to above parameters by using alternative building materials and methods. In a normal house, the basic structure consist of foundations, walls, floor slabs and a roof. The conventional building materials used consist of random rubble, bricks, sand, reinforced concrete and timber. Out of these, bricks, timber (Fernando, 1979) and sand (Dias et al., 1997) are in short supply. Over exploitation of these resources in past two or three decades has resulted in a considerable environmental degradation such as those associated with clay mining, sand mining and deforestation. Concrete is expensive due to high cement content. Steel is locally manufactured using imported raw materials. Thus, minimisation of the usage of these also could be advantageous in reducing the construction cost.

In this research, cement stabilised soil blocks are introduced as an environmentally friendly and cost effective alternative to burnt clay bricks. The use of reinforced concrete is optimised for floor slabs with the aim of minimising the use of concrete and steel.

Since two areas of research is covered in this report, the literature review on alternative building materials can be divided into two main sections namely:

1. cement stabilised soil blocks, and

2. concrete slab systems.

2.2 CEMENT STABILISED SOIL BLOCKS

The shortage of conventional construction materials and the associated environmental problems call for an urgent investigations into the possibility of using economical and environmentally friendly alternative materials that are available locally (Lim et al., 1984). One such material that is abundantly available is soil. Soil is a broad term used in engineering to include all deposits of loose materials in the earth crust. This definition separates soils from rock, from which soils have weathered due to physical, chemical and biological processes. This is a continuous process and therefore all soils are in transition which is in the geological time scale (Bryan, 1988a). This is the process that imparts the properties and great variety to the soils as it is found on the earth crust. The body of the soil fabric will normally contain a proportion of very small particles of clay minerals, generally less than 0.002 mm in effective diameter. Clay particles display two important characteristics; an ion exchange capacity and an affinity for water which includes volumetric and plasticity changes with changes in moisture content (Rigassi, 1995).

One of the main reasons for lack of popularity of soil is its undesirable qualities. Those are (Kateregga 1983):

1. its low loadbearing capacity which makes it unsuitable for supporting heavy roofs of large span buildings,

2. its low resistance to moisture movements and absorption that can lead to structural weakness,

3. its low compressive strength due to low binding strength of particles,

4. its very high shrinkage or swelling ratio resulting in major structural cracks of its products when exposed to different weather conditions and therefore making them unsuitable for building construction purposes, and

5. its low resistance to wear and tear and low durability calling for frequent repairs and maintenance when used in building construction.

These are the main weaknesses which put earth products at a disadvantageous position when compared with other widely used building materials such as concrete and bricks. These weaknesses cause a lot of fear, doubts and hesitations among designers,

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developers, users, decision makers, financiers, etc., in trying to accept soil products for building construction.

However, there are a number of qualities that make soil a good building material. Those are (Kateregga 1983):

1. it has a high resistance to fire, which is one of the most important qualities required in any building material,

2. it has very high thermal insulating value that enables to keep the interior of a building cool when outside is hot; this is specially important for tropical climates such as those prevailing in Sri Lanka,

3. it has good noise absorbent characteristics which is quite suitable for house designs, and,

4. since it is locally available, it is possible to minimise the transport costs.

Despite these advantages, not much attempt has been made to improve soil to minimise its disadvantages, so that it will become an economical, environmentally friendly, durable and strong construction material (Spence & Cook, 1983). One method that has been successfully used to improve soil characteristics is stabilisation with a suitable agent. The stabilisation agents can be cement, lime or bitumen (Norton, 1986). Gypsum has also been used when it is available (Kafescigln et al., 1983). Rise husk ash is another alternative that can be used (Rahman, 1986). Another technique that has been used with soil to obtain desirable characteristics is increasing the density by compaction.

With the advent of Cinva ram compressed block press in 1952 by Raoul Ramirez at the Cinva Centre of Bogota, Colombia, stabilisation and compaction of soil has been used to produce blocks of sufficient strength (Guillaud et al., 1995). Presently, there are a number of manual and motorised machines in use. The compaction pressure of these machines vary between 2 N/mm 2 to 10 N/mm 2 (Bryan, 1988 b).

The most popular stabilising agent has been cement due to its wide availability and its suitability to stabilise laterite soils found in tropical and sub-tropical countries. Lime and rice husk ash also have been used in certain areas. Lime is particularly suitable for stabilising soils with high clay contents.

In Sri Lanka, laterite soil can be found few centimetres below the ground level, beneath the organic top soil. Laterite is a generic name given to a material found in tropical and sub-tropical areas of the world where the weather produces reasonable quantities of warm water filtering through the soil removing the soluble chemical salts, leaving a material which is rich in compounds of iron and aluminium. This accounts for the general reddish appearance of lateritic soil (Lilley & Robinson, 1995). This name has originated from the

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Latin word 'later', which means 'brick', since laterite blocks have been used as bricks in India (Rahman, 1987).

It can appear in many forms with significantly different characteristics, depending on the distribution of particle sizes within an individual soil. A very important feature of lateritic soil is its clay content in relation to the overall particle size distribution, which can vary considerably at sites separated by a few hundred metres (Rigassi, 1995).

It has been suggested by Spence & Cook (1983) that one reason for soil construction being under utilised in developing countries is the enormous variability of the naturally occurring soils. This has created difficulties in making specifications and has led to selection of options such as bricks and concrete as conventional building materials.

2.2.1 Stabilisation of soil

Stabilisation of soil means alteration of its properties in such a way that the soil does not loose strength on saturation. Stabilisation of soil is intended to reduce the volume of voids, fill the voids that cannot be eliminated and increase the bond between the grains. Stabilisation is achieved by (Rigassi, 1995):

a. densification: this is the creation of a dense medium blocking pores,

b. reinforcement: creating an isotropic network limiting movement,

c. cementation: creating an inert matrix of opposing movements,

d. linkage: creating stable chemical bonds between clays and crystals,

e. imperviousness: surrounding soil grains with a water proofing film, and

f. water proofing: eliminating absorption.

2.2.2 Selection of soils for block making

Soils are made up of inert materials (gravel, sand, silt) and active materials (clay). The former acts as a skeleton and the latter acts as a binding agent. The proportion in which each type of material present will determine the behaviour and the properties of different soils. There is a requirement for a small amount of fines (clay and silt), but an upper limit is also necessary to limit shrinkage to ensure effective stabilisation. The grading curve of soils selected for stabilisation should preferably fall within the boundaries shown in Figure 2.1, to ensure a satisfactory compaction and durability (Rigassi, 1995).

It was found by Lasisi & Ogunjde (1984) that the compressive strength of cement stabilised soil blocks increases as the maximum grain size decreases. This could be attributed to the increase in surface area associated with smaller particles that lead to improved cohesion between particles.

In order to determine the suitability of soils for stabilisation, the following soil testing methods can be used (Rigassi, 1995):

1. Grain size distribution: this can be used to determine the grading curve for larger particles of the soil. The soil used for stabilisation should be well graded to form a dense structure upon compaction. A dense structure will give a higher number of contacting particles leading to a better loadbearing skeleton, and a reduced porosity thus reducing the susceptibility to water penetration. Gravel and sand give strength to stabilised soil and clay binds the material together both at green state and at dry state. It is important to ensure that the largest particle size fraction is not too much to cause a poor surface finish. Sufficient fines (silt and clay) should also be present to allow handling just after demoulding. According to Saxton (1995), soils having more than 15% fines would be able to act as a binder between coarse particles.

2. Sedimentation analysis: this can be used to determine the grading of materials finer than 0.08 mm, generally identified as silt and clay.

3. Atterburg limits: this can be used to determine the liquid, plastic limits and plasticity index of the soil intended for stabilisation according to BS 1377 (1975). Plasticity defines the extent to which a soil can be distorted without significant cracking and crumbling. Plasticity of soil is mainly due to the clay and silt content in it. Plasticity Index, PI, which is equal to the difference between Liquid limit and Plastic limit, determines the extent of the plastic behaviour of the soil. For stabilisation of soil, both PL and LL should be within certain limits since these two defines the sensitivity of the soil to variations in humidity. According to Rahman (1987), soils having a liquid limit less than 40% and Plasticity Index less than 18% are suitable for stabilisation. An upper limit of 40% for liquid limit was suggested by Bryan( 1988c) as well on the basis of a comprehensive testing programme.

4. Proctor compaction test: this can be used to determine the optimum moisture content for compaction. The compressibility of a soil defines its maximum capacity to be compressed for a given amount of compaction energy and at a given moisture content. When compacted, the density of soil increases lowering the porosity thus making it difficult for water to penetrate through the block. Hence, compaction makes the block less susceptible to modifications in the presence of water while giving it a higher loadbearing capacity. Optimum water content should be worked out for the soil with cement added but not for the soil alone since the cement changes the particle size profile and would raise the optimum

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water content (Spence & Cook, 1983). However, compaction test results such as Procter Compaction should be used with care since the optimum moisture content for cement stabilised soil blocks may depend on the compaction pressure exerted by the machine used for making blocks (Bryan, 1988 a).

2.2.3 P h y s i c a l i d e n t i f i c a t i o n o f so i ls s u i t a b l e f o r s t a b i l i s a t i o n

In addition to these laboratory tests, there is another set of physical tests that can be used for the selection of a suitable soil. These physical tests can be used while manufacturing of blocks to check the consistency and suitability of the material prepared for block making. Since the soil characteristics vary from location to location, these simple physical tests can be extremely useful because it is not always possible to undertake laboratory testing while constructing buildings at various remote places. The following soils are unsuitable for making blocks and can be easily identified with physical testing (Norton, 1986):

1. soils containing organic matter,

2. soils which are highly expansive, and

3. soils containing excessive amounts of soluble salts such as gypsum, chalk etc.

The following are the simple tests to identify the soil type:

1. By observation: Soils which contain high amount of clay tend to crack when dry. Cracking in dry soil indicate high clay in it. If a damp lump of soil is cut into half with a blade and if the cut surface is smooth and bright it has got clay in it. If the soil is in loose state and grainy, it contains a considerable amount of sand.

2. Touch: Rub the soil between fingers and if the soil is smooth or powdery, then clay is present. Sand is gritty and coarse in hand.

3. Smell: Presence of organic matter could be identified with a musty smell.

4. Cigar test: This test can be used to identify a soil suitable for stabilisation. Cigar test is carried out as follows (Riggasi, 1995). First all gravel from the sample is removed. Then soil is moistened and kneaded well until a smooth paste is obtained. It is left to stand for 30 minutes to allow it to become very smooth and rolled between the palms into a cigar shape to about 3 cm in diameter. Then the "cigar" is placed across the palm of the hand and pushed it gently forward with the other. Finally the length of the piece which breaks off is measured. The above procedure is repeated for several times. If the average length measured is less than 5 cm, the soil contains too much sand. If it is more than 15 cm, it contains too much clay and if it is between 5 and 15 cm, the soil is good.

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5. Sedimentation test (Jar test): This test can be used to determine the fines content. A transparent cylindrical jar of at least Vi litre capacity is used for this test. The jar is filled with soil up to about quarter of the capacity and the rest with water with some salt added to act as a dispersing agent. Then the top of the jar is sealed with the palm and shaken well. The jar is left to stand for at least 30 minutes and the sedimentation is observed. Generally the coarse material like gravel will be deposited at the bottom followed by sands and then silts with clay at the top. The depth of each layer gives an indication of the proportions of each type of material (Rigassi, 1995). The accuracy of jar test results was determined by Perera (1994) using results of sieve analysis. For soils containing fines (clay and silt) more than 20%, jar test could be used to determine the fines content within 5% of the actual value, where the values predicted by the jar test were always overestimates. This means that when the actual fines content is 30%, jar test may predict a value between 30% and 35%.

6. Shrinkage box test: This test can be used to determine the amount of cement required for proper stabilisation. Shrinkage box test can be carried out using a simple shrinkage box which can be made at the site. The procedure given below should be followed:

• oil the internal surface of wooden box of 600 mm x 40 mm x 40 mm; • select the sample of soil intended for stabilisation and add water to its

optimum water content; • tamp the soil into the box with a stick and smoothen the surface; • sun dry the contents for three days and keep for a week in shade; • when the sample is completely dry, push all the soil tightly up to one end

of the box and measure the gap created by shrinkage in the soil.

Interpretation of results of shrinkage box test for the laterite soils available in Sri Lanka is given in Table 2.1 (Perera, 1994):

Table 2.1 Results of Shrinkage box test (Perera, 1994)

Shrinkage Cement: soil by volume < 12 mm 1:18

12 mm - 24 mm 1:16 25 mm - 39 mm 1:14 40 mm - 50 mm 1:12

2.2.4 Methods of stabilisation for soil blocks

In cement stabilised laterite soil blocks, the stabilisation is achieved by three different means. Those are mechanical stabilisation, physical stabilisation and chemical stabilisation as illustrated below.

Stablised soil blocks

Mechnical stabilisation - compaction

Pysical stabilisation

- sieving

Chemical stabilisation

- cement, lime,

bitumen,

Mechanical stabilisation, in the form of compaction, is used to change the structure of the soil, thus improving density and mechanical strength. It will also reduce the porosity and permeability. Physical stabilisation is used to change the composition and texture. For example, large particles are removed by sieving. When the fines content is too high, sand is added. Chemical stabilisation is used by adding products like cement, lime etc. to modify the soil properties.

2.2.5 Chemical stabilisation of soil

Chemical stabilisation can be used successfully along with physical and mechanical stabilisation processes to produce strong and durable soil blocks. A number of materials such as cement, lime, bitumen and pozzolanas can be used for the chemical stabilisation of soil.

2.2.5 .1 Cement

Cement is the most widely used chemical stabilising agent due to its wide availability. When cement is mixed with soil and water, cement reacts with the water in the mixture to form an insoluble cementitious colloidal gel. Cement is able to disperse itself to fill the pore spaces where it sets and hardens to form a continuous matrix which surrounds the particles of soil and binds them together.

The main difference between concrete and soil cement is that in concrete all materials finer than 0.1 mm diameter are excluded as aggregates, where as in soil cement they are tolerated (Riggasi, 1995). Clay forms a continuous matrix through the soil causing swelling and shrinkage of soils. The properties of soil cement are considerably affected

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by the type of soil, compacting effort, density of soil cement mixture and proportion of cement used.

The hydration of cement is a time dependant process. The increase in strength and durability continues at a decreasing rate for some months provided that moisture is available to feed the reaction. Compressive strength is adversely affected by any delay between mixing and compaction. The initial setting time of cement complying with BS:12 should be 45 minutes or more. Thus, it is advisable to use cement soil mix for block making within 45 minutes of adding water. According to Lim et al. (1984), the compressive strength of blocks were reduced by 20% when used for block making with a lapse of 2 hours after mixing cement and water.

Efficiency of cement stabilisation also depends on the efficiency of compaction. According to Bryan (1988a), in clayey soils, the strength properties improve with increasing compaction pressure. However, for sandy soils, this advantage is not so significant.

As cement is used in low proportions (2% to 8%) it is not easy to evenly distribute the cement. Mixing should be done in two stages; dry and wet mixing. The cement will begin to act on contact with water and hence water should be added to the dry mix at the last moment before compacting in order to keep the time before it is used to a minimum; this is called the reaction time ( Riggasi, 1995).

Cement stabilised soil blocks must be kept in a humid environment for at least 7 days. The surface of the blocks must not be allowed to dry out too quickly as this causes shrinkage cracks. The blocks must be sheltered from direct sun and wind and kept in conditions of relative humidity approaching 100% by covering them with waterproof plastic sheets. After about 28 days there would not be a further significant increase in the strength of the blocks. High temperatures will increase the strength obtained and temperature of the stacked blocks can be raised using black plastic sheeting as a covering material (Riggasi, 1995).

It is shown by Rahman (1987) with a comprehensive testing programme carried out using a soil containing 30% fines that for a given compaction effort, the optimum moisture content varies with the cement content. It was also found that the cement content can change the dry density marginally, but compressive strength increases significantly with the cement content. California Bearing Ratio (CBR) values also have been determined at both unsoaked and soaked conditions, which also show a considerable improvement with increasing cement content. The compressive strengths were obtained with 115.5mm high, 105 mm diameter specimen air cured at room temperature. The results reported for the testing programme is presented in Table 2.2.

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Table 2.2 Effect of cement on compaction characteristics, CBR characteristics and unconfined compressive strength of soil (Rahman, 1987)

cement % OMC % based on Procter

compaction test

Air dried density (kg/m3)

Unconfined compressive strength

(N/mm 2)

CBR %

7day 14day 28day unsoaked soaked 0 14.4 1820 1.03 1.03 1.03 27.2 11.7 3 15.48 1810 2.34 2.64 2.86 78.7 52.7 6 15.8 1840 3.04 3.29 3.74 131.7 109.7 9 16.0 1880 3.70 4.72 5.26 195.2 178.0

2.2.5.2 Lime

Unlike cement, which works with coarse particles of soil, lime works with clay minerals in the soil. Lime on its own does not have a cementitious effect. However, it reacts with certain clay minerals at the presence of water and produces a cementing effects which in turn increases the soil's strength and reduces susceptibility to water. This is known as a pozzolanic reaction which is rather slow. The cementitious material produced has lower strength than Portland cement and the strength depends on the presence of suitable minerals. Also, to ensure sufficient dispersion of lime in soil, the proportion of lime needed is considerably in excess of that required for the reaction. Soils with significant amount of clay minerals are suitable for stabilisation with lime, which are less suitable for mechanical compaction and tend to be more prone to dimensional changes (Norton, 1986).

The effects of Lime-pozzolan mixture is considerably enhanced by curing at elevated temperatures. Thus lime may be suitable for stabilised soil blocks if the blocks can be heated in a water tank. There are some additives such as Calcium sulphate (Gypsum), Sodium silicate and Sodium chloride (salt) to improve the pozzolanic reaction (Spence & Cook, 1983). According to Rigassi (1995), lime stabilisation should be considered only if cement stabilisation is impossible.

2.2.5.3 Bitumen

The action of bitumen mixed with soil is to act as a water repellent, reducing the dimensional changes, loss of strength and surface erosion associated with wetting. Better results can be achieved by combining stabilisation with compaction. When making compressed blocks, bitumen acts as a lubricant helping the ejection of the block from the mould.

If bitumen is used as a stabiliser, thorough mixing with soil is essential. Bitumen is too viscous to distribute at room temperature and must be heated or dissolved in an aqueous emulsion. When the dilutes have evaporated, the bitumen is left as a thin coating around

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clay particles. This prevents the soil from absorbing water. Therefore, bitumen can be used to improve the wet strength of stabilised soil blocks (Norton, 1986).

2.2.5.4 Pozzolanas

Pozzolanas can be defined as a material composed mainly of reactive silica which will combine with lime at ordinary temperatures at the presence of water and forms stable insoluble compounds with cementing properties.

Many plant ashes have a high silica content, which can made to be pozzolanic. The prime factor of using any plant material is the amount of ash produced during combustion. Amount of ash produced with rice husks is about 20% but for bamboo leaves and some timber species it is less than 10%. This ash essentially composes of silica (Spence & Cook, 1983).

2.2.6 Process of block making

Since machines are used for making cement stabilised soil blocks, it would be possible to achieve good dimensional accuracy and quality by following a proper block making process. According to Rigassi (1995), the following steps should be carefully followed:

1. Soil preparation: Lumps in soil should be broken manually. This soil is then be ready for screening. The mesh size of the sieve can be either 6 mm or 10 mm.

2. Measuring of quantities: Measuring can be done either by weight or by volume with volume batching being the most common and easiest. In volume batching, it is advisable to use a container of fixed volume.

3. Mixing: Mixing of soil with cement should be carried out in dry condition initially. Attention should be placed to obtain a homogeneous mix. After a thorough dry mixing, water can be sprinkled to bring soil cement mix to a desirable moisture content. The quantity of water to be added can be determined by performing a simple drop test. For the drop test, a fistful of moist material taken and then it is shaped into a ball in the hand. It is then dropped from a height of 1.0 m on to a hard surface. If the ball has completely disintegrated, the mix is too dry. If it has broken into 4 to 5 pieces, the moisture content is acceptable. If it has flattened without breaking, it is too wet.

4. Compressing of blocks: A block making machine should be used for this purpose. It is important to use the correct quantity of soil with correct compaction procedure given for the machine. Use of less soil will result in weaker blocks since the compaction ratio is fixed.

5. Curing: For cement stabilised blocks, continuous presence of water within the block is crucial for development of adequate strength. Any rise in temperature within the block is also helpful. Therefore, green blocks should be carefully stacked and should be completely covered with black polythene so that it would be possible to create almost 100% moisture content around the blocks. This minimises any evaporation of moisture from blocks and also helps to raise the temperature around the blocks. Blocks should be kept covered for at least 7 days and preferably for 14 days.

During block making, as a quality controlling measure, it is possible to use a penetrometer with green blocks where the depth of penetration can be used as an indication of the degree of compaction. Excessive penetration of the penetrometer can be an indication of the use of insufficient soil or the use of too much water. Such penetrometers are generally supplied with the block making machines.

Larger particles from laterite soils left after sieving can be used as coarse aggregates in concrete. Many studies have been carried out on the use of lateritic soil in concrete such as Osunade (1993) and Adepegba (1983). The main emphasis of these studies was on replacing sand with finer fraction of laterite soils. It was shown by Rai (1987) that larger particles could be used as coarse aggregates in concrete and the compressive strengths obtained were in the range of 10-12 N/mm 2 at an age of 28 days. With such applications, it may be possible to further optimise the usage of materials. Such concrete can be used for non structural applications such as mass concrete paved on ground to lay cement rendering.

2.2.7 P r o p e r t i e s o f c e m e n t s t a b i l i s e d so i l b l o c k m a s o n r y

Cement stabilised soil blocks are one of energy efficient and economical alternatives to burnt bricks. Unlike the design of reinforced concrete, where a mathematical model exists to predict moments of resistance from the properties and geometry of concrete and steel, no such model as yet exists which can predict the axial compressive strength of masonry walls from the properties and geometric details of units and mortar. Because of this situation, masonry design codes generally have relied upon tests carried out on mortar and units, and also on panels built with mortar and units to develop necessary strength properties (Render & Phillips, 1986).

The strength properties that would generally be used in masonry design includes compressive strength of blocks, compressive strength of mortar, compressive strength of panels, shear strength of panels and flexural strength of panels (Hendry, 1981). Generally, compressive strength of blocks and wall panels are the most important properties in loadbearing construction.

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In order to determine the strength characteristics of cement stabilised soil block masonry, a comprehensive testing programme had been carried out by Reddy & Jagadish (1989). The properties investigated were:

1. Effects of mortar properties on the compressive strength and modulus of elasticity of cement stabilised soil block masonry.

2. Effects of moisture absorption on the compressive strength and modulus of elasticity of cement stabilised soil block masonry.

The blocks used for this testing programme were made using a manually operated machine called ASTRAM which has a compaction ratio of 1.7. The liquid and plastic limits of the soils used were 36.1% and 22.5% respectively. This soil thus has a plasticity index of 13.6%.

2.2.7.1 Compressive strength of cement stabilised soil blocks

Strength of masonry is influenced by a number of factors such as block strength, mortar bed thickness, mortar strength, nature of bonding and slenderness ratio. In the study by Reddy and Jagadish (1989), six mortar mixes were used to construct masonry prisms of a height of four blocks to give a slenderness ratio of 2.52. The size of the cement stabilised soil block used for the determination of strength properties was 305 mm x 146 mm x 82 mm. The mortar mixes were 1:2, 1:4, 1:6, 1:10, 1:12 of cement and sand. It also included a lateritic soil mortar with 5% cement by weight where the soil was prepared by passing through a 3 mm mesh. The mortar thickness has been maintained at 10mm.

For the determination of elastic modulus, soil blocks of size 230 mm x 190 mm x 82 mm had been used to make the prisms of height four blocks. The properties of blocks cured for 21 days are given in Table 2.3. It can be seen that the wet compressive strength of blocks is about 25% of the dry strength.

Table 2.3 : properties of pressed soil - cement blocks of 5% cement cured for 21 days (Reddy & Jagadish, 1989)

Block size 305 x 146 x 82 230 x 190x 82 Block size length x width x height

(mm) length x width x height

(mm) air dried density (kg/m 3) 1840 1830 dry strength (N/mm") 9.85 10.38 wet strength (N/mm ) 2.51 2.72 water absorption % 15.34 15.63 ratio of wet to dry strength 0.25 0.26

The prism testing was done at an age of 28 days to determine the compressive strength. The prism size was 305 mm in length, 368 mm in height and 146 mm in thickness. The

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prisms were tested both in dry condition and wet condition. For wet strength, the prisms were saturated by sprinkling with water before testing. It was observed that in most of the prisms, the failure was initiated by vertical splitting cracks and completed by crushing of the soil blocks at the mortar block interface. In the case of prisms using soil cement mortar, the mortar joints were crushed before the vertical splitting started, which could be attributed to weaker mortar strength.

When interpreting the results of testing, a new term called masonry efficiency, rj, had been introduced (Reddy & Jagadish, 1989).

T) = masonry efficiency = Masonry prism strength/ Block strength

The results of this testing programme are given in Table 2.4. It can be stated that the wet compressive strength of cement stabilised soil block prisms is less than the dry strength. Generally, wet compressive strength of prisms is about 50% of the dry strength. The masonry efficiency of cement stabilised soil blockwork in the wet state is nearly twice as large as the value in the dry state. This indicates that although there can be a considerable drop in the compressive strength of blocks in wet stage, the compressive strength of wall panels would be affected to a lesser extent.

Table 2.4 Compressive strength of soil cement block masonry prisms (Reddy & Jagadish, 1989)

Mortar mix Masonry Prism strength Masonry ef 'iciency (r|) dry (N/mm 2) wet(N/mm 2) dry wet

1:2 cement sand 3.87 2.10 0.39 0.84 1:4 cement sand 3.33 1.68 0.34 0.67 1:6 cement sand 3.21 1.52 0.33 0.61

1:10 cement sand 2.73 1.37 0.28 0.55 1:12 cement sand 2.55 1.31 0.26 0.52 5% cement soil 2.01 0.90 0.20 0.36

2.2.7.2 Modulus of elasticity of cement stabilised soil block masonry

In the study by Reddy and Jagadish (1989), the modulus of elasticity of cement stabilised soil blocks had also been determined. The modulus of elasticity was obtained by measuring stress-strain characteristics of five block high prisms. The prisms prepared by using blocks of 5% cement by weight were cured for 28 days under cover and then dried in air for 30 days. The stress-strain characteristics of wet specimens were obtained after immersing them in water for 48 hours prior to testing. The moisture content of the dry and wet specimens were 3.61% and 15.84% respectively, at the time of testing. The modulus of elasticity of cement stabilised soil block masonry based on tangent modulus is given at a stress of 0.3 N/mm 2 . These results are given in Table 2.5 .

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Table 2.5 : Modulus of elasticity of soil - cement block masonry at 0.3 N/mm 2 stress (Reddy & Jagadish, 1989)

Mortar mix _

Modulus of elasticity (N/mm ) dry state wet state

1:2 cement sand 1080 630 1:4 cement sand 950 650 1:6 cement sand 550 510

1:10 cement sand 490 350 1:12 cement sand 490 390 5% cement soil 340 210

This indicates that modulus of elasticity is affected by the strength of mortar. This result is predictable since strong mortars will have higher elastic modulus, which would reduce the shortening of the mortar under compressive stresses. For low strength mortars such as 1:6, 1:10 and 1:12, the modulus of elasticity is affected to a lesser degree when the walls are wet.

2.2.7.3 Determination of blockwork strength based on elastic analysis

It can be seen from Sections 2.2.7.1 and 2.2.7.2 that the strength properties of blocks and mortars can affect the strength properties of block walls. A theoretical expression was derived to determine the strength of brickwork in terms of elastic properties of bricks and mortar such as elastic modulus, tensile strength, mortar joint thickness, thickness of blocks and Poisson's ratio (Henry et al, 1981). That is presented here since it would be possible to use the same concepts for cement stabilised soil blocks as well.

This expression is derived on the basis that the strains in mortar and blocks should be equal at the interface between blocks and mortar, and the total lateral forces in mortar and blocks should be equal and opposite as shown in Figure 2.2. It should be noted that there are limitations in expressing block compressive strength on the basis of elastic properties since blocks and particularly mortar will not behave elastically up to the point of failure. However, this expression helps to identify the quantities which can be important for strength of blockwork, such as the ratio of joint thickness to block depth, the ratio of elastic moduli of blocks and mortar, and the tensile strength of blocks.

The horizontal stains in blocks and mortar can be used to determine the equilibrium condition. The tensile strains are taken as positive.

The lateral strain of mortar is due to (see Figure 2.2):

1. a confining lateral stress, am, acting on the mortar which gives rise to a strain of -am/Em in the lateral direction

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2. a lateral strain v,„ .oc /Em due to lateral stress resulting from vertical stress, Oc, acting on the mortar where v„, is the Poisson's ratio.

Strain in mortar: £„, = - om/Em + vm. oc/Em (2.1)

The lateral strain in the block is due to the outward lateral stress, Ob, acting on the block due to confinement offered to the mortar and the lateral strain due to vertical stress, oc, acting on the block. Strain in block:

eb = ab/Eb + vb. ac/Eb (2.2)

where E,m Eb , vm , vb are the elastic moduli and Poisson's ratio for mortar and block respectively as denoted by the subscripts.

For statical equilibrium, the total lateral forces in mortar and block are equal and opposite. Hence, when d = depth of block, t = thickness of mortar joint and d/t = r, and considering a unit length of blockwork:

ct„, . t = ab. d; o,n = r ob

Substituting from (2.1) and (2.2), and rearranging:

ob (1/Eh + r/Em) = Oc (v„/Em - Vi/Eh)

and substituting Ei/Em = m

Ob = Oc (vm.m - vb)/(l + r.m) (2.3)

If the failure criterion for the blocks is reaching the limiting tensile strain given by £ui t, which consist of the strains in blocks due to ab and Vb. crc:

£,,/, = ob/Eb + vb. oc/Eb (2.4)

If the corresponding failure stress is Ojb = Eb . £„/,, then it is possible to determine the stress in blockwork due to the interaction between mortar and blocks as:

Ob = Op - vb • Oc

Substituting and rearranging the above equations, the limiting compressive stress in the vertical direction can be written as:

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This relationship shows that the following are of importance when predicting the strength of blockwork.

1. tensile strength of blocks, 2. Poisson's ratio of blocks and mortar, 3. elastic modulus of block and mortar, and 4. the ratio of mortar joint thickness to thickness of block.

However, it should be noted that this relationship only indicates the factors that influence the interaction between mortar and blocks, but should not be used in practice in predicting the ultimate strength of blockwork. This is because the elastic behaviour would not prevail prior to failure of blockwork.

2.2.7.4 Mortars suitable for cement stabilised soil blocks

For the construction of cement stabilised soil block walls, either cement sand mortar or cement soil sand mortar can be used. Cement sand mortars can be either 1:8 or 1:6. Masons generally use rather high water cement ratios for satisfactory workability leading to low strengths. The higher the sand - cement ratio, the greater the water requirement.

There can be a number of cement soil and sand mortars that can be used for cement stabilised soil block construction. It was reported by Reddy and Jagadish (1989), soil cement mortar with 5% cement by weight appears to be more ductile than the cement sand mortars. Alternatively, Cement, lime and sand in 1:1:6 or 1:1:8 also can be used for cement stabilised soil block wall construction. When it is necessary to reduce the cost of mortar further, stabilised mud mortars such as 1:2:6 and 1:2:8 cement, soil and sand are recommended (Reddy, 1995).

It is reported by Falade (1993) that the compressive strength of lateritic mortars decreases with higher water/cement ratios. Thus, it is important to control the water content to that is just sufficient for giving adequate workability.

2.2.8 M a c h i n e s a v a i l a b l e f o r m a k i n g c o m p r e s s e d b l o c k s

A number of machines have been developed in various parts of the world for making compressed blocks. The compacting pressure of a hand operated simple machines like Cinva ram is about 2 N/mm 2 while a motorised press could provide about 10 N/mm 2

(Bryan, 1988a). The manual machines can have compaction transferred through mechanical means such as in Cinva ram. It could be through hydraulic pistons as in Brepak machine developed at Building Research Establishment of United Kingdom.

25

2

Table 2.6: Details of some of the cement stabilised soil block making machines (Houben and Verney, 1989)

Machine Name Size of block Compaction Number of Output Labour force (cm) ratio blocks per

cycle per hour required

ELLSON a) 29 x 14 x 9 1.7:1 1 90 8 to 12 men (heavy manual) b) 29 x 1 9 x 9 1 80 ASTRAM b) 30 x 14 x 10 1.7:1 1 56 5 men (light manual) c) 23 x 19 x 10 1 56 TARA a) 23 x 11 x 5.5 1.8:1 1 124 5 men BALRAM (heavy manual) CERAMEN a) 22 x 10.7x7 2:1 2 300 5 men (heavy manual) b) 29 x 14 x 8 1 150 DYNATE PRE a) 40 x 20 x 20 2:1 4 250 8-10 men 4M b) 40 x 15 x20 4 250 (Motorised) CINVA RAM a) 29 x 1 4 x 9 1.5:1 1 40 4 men (light manual) Auram Press a) 29 x 14 x 9 1.65:1 1 100 6 men 3000 (heavy b) 24 x 24 x 9 1.65:1 1 manual) AIT a) 29 x 14 x 9 1.5:1 1 40 4 men interlocking (light manual)

2.2.8.1 Cinva ram machine

Cinva ram was designed by Rural Ramirez in Inter-American Housing Centre (CINVA) in 1952 (Guillaud et al., 1995). This is the oldest, low cost portable soil block press. This machine has been used for housing construction in many parts of the world. The compaction ratio is 1.5:1. These blocks can generally satisfy the design strengths required for single storey houses, which are generally in the range of 0.25 - 0.4 N/mm 2

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Such machines can deliver up to 10 N/mm compacting pressure. In Brepak machine, the compression mechanism of Cinva ram machine is replaced with a hydraulic piston. For large outputs, motorised machines have been produced where compaction pressures can be in the range of 10 N/mm 2 (Houben & Guillaud, 1989).

In this section, soil block making machines used in Sri Lanka are described in detail while the characteristics of other machines are given in tabular form in Table 2.6 (Houben & Verney, 1989). These machines are generally categorised as light manual, heavy manual, motorised and industrial units in Auram Press 3000 manual.

»

with light roofing materials. This machine is made entirely of steel and consists of a mould box with a cover. The mould box also has a movable base plate connected to a piston. The whole unit is mounted on a heavy wooden base board to provide stability during operation.

» After greasing the sides of the mould the soil mix is filled in making sure that the corners are properly filled and slightly compressed by hand. When the machine is operated, it will first compress the block and then release the block by ejecting it. Then the green block can be removed and carefully stacked for curing.

A comprehensive block testing programme was reported by Perera (1994) for the blocks manufactured with Cinva ram machine. The variation of block strength with 0%, 2%, 4%, 6%, 8%, 10%, 15% and 20% cement percentages by weight have been investigated. Six soil samples with different fines (clay and silt) percentages were used. The compressive strengths were presented at the ages of 7 days and 28 days. The 28 day wet compressive strengths were also reported. The wet compressive strength of blocks were

I obtained after soaking in water for a minimum of 96 hours. A summary of the test results are given in Table 2.7. The details of the testing programme can be found in Perera (1993).

Table 2.7 Summary of test results for Cinva ram machine (Perera, 1994)

Cement % Average compressive

strength (N/mm 2) 7 day

Average compressive

strength (N/mm 2) 28 day

Average wet compressive

strength (N/mm 2) 28 day

2 0.85 1.31 0.32 4 1.21 1.98 0.49 6 1.60 2.73 0.76 8 1.76 3.20 1.29 10 2.13 3.65 1.70 15 2.47 4.35 1.93 20 3.32 5.50 2.70

In order to determine the wall strengths that can be achieved with Cinva ram blocks, a panel testing programme was carried out by Perera & Jayasinghe (1995). The panel sizes used were two blocks in length and five blocks in height. The blocks used for the construction of panels were not cured, but kept in shade after casting. The results obtained were given in Table 2.8. These results also show that Cinva ram machine gives low compressive strengths. However, these strengths could be sufficient for single storey houses provided with roofs where light roofing materials such as asbestos are used.

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Table 2.8: Characteristic compressive strength of panels made with Cinva ram blocks

Fines % Cement % Average compressive

strength of uncured blocks (N/mm 2)

Characteristic compressive

strength of panels (N/mm 2)

20% 2% 0.955 0.255 4% 0.955 0.333

25% 2% 0.71 0.306 4% 0.79 0.348

30% . 2% 0.43 0.279 4% 0.47 0.278

2.2.8.2 The Auram Press 3000

The Auram Press 3000 is manufactured by AUREKA, at Auroville India. The practical output is about 100 blocks per hour with three men working on the machine and three men for mixing and stacking. During operation, the lid is closed manually and it unlocks and opens automatically with the movement of the lever. Different moulds can be fitted on the frame which are either square or rectangular in shape. The height of block can be adjusted with washers from 5 cm to 10 cm, depending on the compression ratio required. The compacting pressure varies from 2.7 to 5.3 MPa.

In this machine, compression and ejection mechanisms are operated in the same direction, hence more efficient than Cinva ram machine. This leads to a higher output. The compaction ratio is adjustable from 1.6 up to 1.9. It is 1.65 for a block of height 90 mm. The press is self stable without any extra brace and two men are required to move the machine by haulage. It is possible to manufacture block sizes of 290 mm x 140 mm x 90 mm, 240 mm x 240 mm x 90 mm or many other sizes by using appropriate moulds.

2.2.8.3 Modified Cinva ram Interlocking block press

The interlocking blocks do not require any mortar joints in the masonry work. Positive and negative frogs provided on top and bottom of the blocks facilitate interlocking with each other. Grout holes are filled with 1:6 cement, sand slurry to give continuity in the vertical direction of the wall.

A locally manufactured modified Cinva ram interlocking block press was used for a detailed testing programme carried out at University of Moratuwa (Perera & Jayasinghe, 1995). Both bending strength and panel strengths were determined for a number of soil types and different percentages of cement. The panels constructed using blocks from this machine have given rather low strengths. This can be attributed to the long mortar columns formed to give the continuity for the interlocked wall. Since the thin mortar columns formed by using the cement sand slurry of 1: 6 are slender, they tend to fail by

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buckling. Buckling of mortar columns will cause cracking in the blocks thus causing them to disintegrate. This machine may need some modifications such as making the mortar columns discontinuous to obtain satisfactory results. A summary of test results are given in Table 2.9.

Table 2.9: Bending strength of blocks and characteristic compressive strength of wall panels made with interlocking blocks

Fines % Cement % Bending strength (N/mm 2)

Characteristic compressive strength of panels (N/mm 2)

25% 2% 0.104 0.233 4% 0.181 0.197 6% - 0.287

30% 2% 0.095 0.173 4% 0.158 0.219 6% 0.157 0.214

35% 2% 0.104 0.231 4% - 0.274 6% 0.281 0.340

40% 4% - 0.148

2.2.9 Construction of structures with cement stabilised soil blocks

When building with cement stabilised soil blocks, in addition to the design concepts pertaining to the masonry construction, there are few "good practises" that should be followed. These can be summarised as follows (Guillaud et al., 1995):

1. Limitations on the plan layouts and opening sizes: The designer should be willing to adopt simple building systems that are compatible with the properties of the blocks such as good compressive strength, but low tensile, bending and shear strengths.

2. Protection of building elements: The designer should be willing to adopt design principles and building solutions, which are suitable for building with earth. These can be the use of large eaves or water repellent coatings to reduce the excessive moisture movements which is a main cause of degradation.

3. Quality controlling: It is necessary to ensure that the execution of the building work is carefully carried out with certain level of quality controlling. This is specially true for loadbearing cement stabilised soil block construction since the material strengths obtained with economical low cement contents are just sufficient to satisfy the safety margins imposed by the partial factors of safety used in masonry design.

With due consideration to these good practices, cement stabilised soil blocks have been successfully used for single storey houses, two storey houses with loadbearing walls, and multi-storey buildings such as hostel buildings (Guillaud et al., 1995, Middleton, 1985). These structures have been constructed in a number of countries including France, Australia, Morocco, Guyana, Saudi Arabia, India etc.

2.2.9.1 Foundations for cement stabilised soil block buildings

The foundations should fulfil two functions in buildings constructed with cement stabilised soil blocks. Those are as follows (Houben & Guillaud, 1989):

1. provision of adequate distribution of wall loads to prevent failure of soils below the foundation and to provide adequate strength against disintegration of foundation due to settlements or earthquakes.

2. minimisation of ingress of moisture through the foundation since earth is inherently vulnerable to fluctuations in moisture content.

Thus the use of good foundation material such as random rubble masonry will be essential. Proper drainage also should be provided around the foundations.

The possibility of providing a reinforced concrete tie beam at the plinth level should also be considered. It was shown by Jayasinghe & Maharachchi (1998) that provision of a tie beam at window sill level can serve the dual purposes of reducing cracks due to thermal movement while enhancing the resistance of the structure to foundation settlements. It was reported by McHendry & May (1984) that the provision of adequate continuity by using reinforcement can enhance the earthquake resistance of stabilised soil walls.

2.2.9.2 Provision of openings in walls

Size and location of openings should be carefully selected in order to minimise concentration of stresses. The following undesirable features were identified (Guillaud et al., 1995):

1. Making of openings too long thus placing excessive load on lintels. The maximum length of openings should be determined depending on the strength of blocks which depends on cement and fines content, and the compaction ratio.

2. Locating of openings immediately next to the corner of the buildings, making the corner to buckle.

3. Locating two openings too close to each other leading to a pier which may be liable to buckle.

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The following rules have been suggested as good practices for cement stabilised soil block walls (Guillaud et al., 1995):

1. In a wall, the ratio of voids to total surface areas should not exceed 1:3 and voids should be evenly spaced.

2. The overall length of openings should not exceed 35% of the length of the wall. The length of each opening should not exceed 1.2 m.

3. The minimum distance of 1.0 m should be left from the edge of the wall. When two adjacent openings form a pier, it should at least be 0.6 m in length.

It was reported by Chandrakeerty (1991 b) that for blockwork construction, careful consideration should be given at the design stage in dimensioning the structure and openings. Failure to do so would result in considerable inconvenience and higher cost. Wall openings should be designed and constructed to dimensions that minimise the cutting of units at the site as shown in Figure 2.3. This leads to wastage of material and slow down the speed of construction. The same concepts should be adopted for cement stabilised soil blockwork as well.

For cement stabilised soil block buildings, the guidelines given in Table 2.10 were suggested by Middleton (1987) on the distance between two adjacent openings.

Table 2.10: Minimum distance between adjacent openings of cement stabilised soil block walls

Wall thickness (mm) Minimum distance between adjacent openings (mm)

250 1100 300 1000 350 900 400 800 450 700

It was reported by Lilley and Robinson (1995) that in rammed earth walls, openings with curved arch shapes have performed better than rectangular openings where concentrated loads were applied at the centre of the openings. This may be an indication that heavy concentrated loads should be avoided within the span of openings when openings of rectangular shape are used.

2.2.9.3 Plasters and coatings

According to Bryan(1988 b), surface erosion could occur when driving rain or abrasive wind blown sand causes mechanical damage and then wash away the loosened material. Thus, when soil is used for building construction, good detailing and stabilisation could

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eliminate this problem. Many sheltered cob walls, which was a traditional earth construction in United Kingdom, has survived without any surface coatings over long durations sometimes exceeding one hundred years (Saxton, 1995).

For the climatic conditions prevailing in Sri Lanka, resistance to rain penetration is one of the most important functions of a building envelope. This should be achieved by controlling the rain penetration resistance of masonry walls (Chandrakeerthy, 1991 a).

Rain penetration is defined as the penetration of water into a wall either through the surface of the wall or through leakage at openings such as windows and doors. Common entry paths are through pores in the face of the masonry units and mortar, through cracks in masonry units and mortar, or through improperly sealed cracks between masonry and other building elements. Such entry paths can be substantially cut off in walls when external finishes such as renderings are applied. It is also possible to use large overhangs for these buildings.

As regards to rain penetration, bond strength of mortar is more important than its compressive strength. Thus the use of lime in mortar is preferred in this respect since it improves the workability and water retentivity of mortar, which is essential for maximum bond (Chandrakeerthy, 1991 a).

It was reported Jamal & Sheikh (1987) that cement stabilised soil block walls can be made water resistant by painting them with liquid sodium silicate. It was recommended to apply three coats of sodium silicate at two day intervals to the exterior walls.

2.3 CONCRETE FLOOR SYSTEMS

In multi-storey construction, a suitable floor system should be used. The material usually used in Sri Lanka is reinforced concrete. Timber floors also have been used successfully in some houses, but may not be appropriate in present day context since timber is a scarce material. The concrete floor slab systems generally used in multi-storey buildings can be presented diagramatically as below. The detail descriptions of these systems are given in the sub sections to follow.

one two pan way way joist slabs slabs slabs

flat slabs

waffle slabs

precast beam precast & insitu beam &

slabs slabs

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2.3.1 Floor systems used in multi-storey buildings

An appropriate floor system is an important factor in the overall economy of two storey houses. Reinforced concrete floor systems can be grouped into two categories:

1. one-way slabs in which the slab spans in one direction between supporting beams and walls

2. two-way slabs, in which the slab spans in orthogonal directions.

In both systems, advantage of continuity over interior supports is utilised by providing negative moment reinforcement in the slab.

The floor systems that have been used in buildings can be categorised as follows (Smith &Coull 1991):

1. one way slabs on beams or walls,

2. two way slabs on beams or walls,

3. one way pan joists and beams,

4. one way slab on beams and girders,

5. two way flat plate,

6. two way flat slab, and

7. waffle flat slab.

2.3.1.1 One way slabs on beams and walls

A solid slab up to 200 mm thickness, spanning continuously over walls or beams up to 7.5 m apart provides a floor system requiring simple formwork with simple reinforcement (Figure 2.4). The thickness used for residential buildings vary between 115 mm to 150 mm. This system is heavy and inefficient in its use of both concrete and reinforcement (Smith & Coull, 1991).

2.3.1.2 Two-way slab on beams

The slab spans two ways between orthogonal set of beams that transfer the load to the columns and walls (Figure 2.5). The two way system allows a thinner slab and is economical than one way slabs in the utilisation of concrete and reinforcement. The maximum length-to-width ratio for a slab to be effective in two directions is approximately 2 (BS 8110, 1985).

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2.3.1.3 One way pan joists and beams

A thin, mesh-reinforced slab sits on closely spaced cast-in-place joists spanning between major beams which transfer the loads to the columns (Figure 2.6). The slab may be as thin as 60 mm while the joists are in 150 mm to 500 mm in depth and spaced 600 mm to 900 mm. The slab acting in composite with joists form in effect a set of closely spaced T-beams capable of spanning up to 12 m (Smith & Coull, 1991).

2.3.1.4 One-way slab on beams and girders

A one-way slab spans between beams at a relatively close spacing while the beams are supported by girders that transfer the load to the columns (Figure 2.7). The short spanning slab may be thin, from 75 mm to 150 mm thick, while the system is capable of providing long spans up to 14.0 m (Smith & Coull, 1991).

2.3.1.5 Two way flat plate

The uniformly thick, two way reinforced slab is supported directly by columns or individual short walls (Figure 2.8). It can span up to 8.0 m in the ordinary reinforced form and up to 11.0 m, when post-tensioned, specially in apartment and residential buildings where the imposed loads are not large. It can be economical in buildings due to saving obtained with simple formwork and reinforcement (Taranath, 1988).

2.3.1.6 Two way flat slab

The flat slab differs from the flat plate in having column heads and drop panels (Figure 2.9). The column heads increase the shear capacity while the drop panels increase both the shear and negative moment capacities at the supports, where the maximum values occur. Thus, two way flat slabs can carry heavier loads than flat plate (Taranath, 1988).

2.3.1.7 Waffle flat slab

A slab is supported by a square grid of closely spaced joists with filler panels over the columns (Figure 2.10). These joists carry loads simultaneously in both directions. The slabs and joists are pored integrally over square, domed forms that are omitted around the columns to form the filler panels. The forms which are of sizes up to 750 mm square and up to 500 mm deep provide a geometrically interesting soffit, which is often left without further finish (Taranath, 1988).

2.3 .2 Al ternat ive floor sys t ems used for houses in Sri L a n k a

The conventional one way solid slabs, two way solid slabs and flat slabs have been used in residential buildings of Sri Lanka. However, due to shortage of timber and bamboos used for shuttering work, a number of alternative systems have been developed recently. These systems minimise the usage of formwork and falsework. They can be identified as:

34

1. Precast prestressed concrete beam and insitu cast slab system developed at National Engineering Research and Development centre (Kulasinghe,1998).

2. Precast prestressed concrete beam slab system with hollow blocks finished with a screed.

3. Precast reinforced concrete beam and insitu cast slab system adopted for two storey houses constructed at Koralawella where walls were constructed with Auram Press 3000 blocks.

2.3.2.1 Precast prestressed concrete beam and insitu cast slab systems

This system consists of precast prestressed concrete beams of trapezoidal shape as shown in the Figure 2.11. The beams are cast with heights varying from 100 mm to 175 mm. Since prestressed concrete beams are used, those should be manufactured at factory conditions. These beams are recommended for spans varying from 3.0 m to 6.0 m. Those are located at 600 mm centres and a 50 mm slab is constructed by using a shuttering suspended from the beams. The reinforcement used for the slab is only 50 mm x 50 mm square mesh with 3 mm diameter wires. In this system, precast beams spaced at 600 mm centres can be seen from below.

The reinforced concrete slab of 50 mm thickness is cast using prefabricated shutter panels, which are suspended from the precast beams with binding wire. The top of the shutter, which is lined with a removable thin plastic liner, is kept one inch below the top of the precast beam. The reinforcement mesh is supported on top of the precast beams and a 50 mm thick concrete is cast embedding 25 mm of the beam. No props are required except for long spans of over 4.5 m where a prop is placed at the centre of the precast beams to prevent unacceptable deflections (Kulasinghe, 1998).

2.3.2.2 Precast prestressed concrete beam and slab systems with hollow blocks

This floor system consists of precast prestressed concrete beams spaced at 572 mm, which support infill blocks spanning between them. This floor system is similar to that reported by Moss (1993). A 50 mm thick screed is laid on top of this system to give a continuous top surface. The reinforcement is 50 mm x 50 mm square mesh or 6 mm diameter mild steel bars at 200 mm centres in both ways. The arrangement is shown in Figure 2.12. The main advantage of this system is that it gives a flat soffit. No formwork or falsework is required for the construction.

2.3.2.3 Precast reinforced concrete beam and insitu cast slab system

This slab system was used successfully for a few two storey houses constructed using cement stabilised soil blocks as a loadbearing material at Korelawella, Moratuwa. It consists of a precast reinforced concrete beams of size 150 mm x 150 mm with the

35

reinforcement arrangement shown in Figure 2.13. The slab was cast insitu using a set of formwork which was suspended from the beam. Hence, no falsework was required. The shuttering could be reused thus minimising the formwork cost. The reinforcement requirement also could be reduced since the span of slabs was small.

2.3.3 Other floor systems

A precast reinforced concrete beam and plank system was developed in India to use as roof slabs in cost effective houses (Jindal et al., 1984). This system is quite similar to one way pan joists and beam system described in Section 2.3.1.3. The exception is the ability to construct this system without any formwork. The joists are located at a spacing of 1.5 m. The details of the roof slab system is given in Figure 2.14 a and b . In this system, the structural efficiency is improved by using small spans for slabs, thus leading to a reduction in the amount of concrete and reinforcement. This slab system developed for roof slabs could be extended for floor slabs as well. For this, it is necessary to determine suitable member sizes and to evaluate the dynamic behaviour with respect to floor vibrations.

Another precast roof system that could be constructed successfully is long span brick panels (Chitharanjan, 1986). In a concrete slab, the concrete below the neutral axis is used only to locate the reinforcement and to provide sufficient corrosion resistance to reinforcement. This fact was used very effectively in brick panel roof slabs. In this system, slab panels were precast using the following method. The reinforcement cage was laid on the ground supported by cover blocks. Bricks were placed within the reinforcement cage so that adequate cover can be provided for the reinforcement. The spacing between the bricks were filled up with concrete. Then, a thin layer of concrete was laid covering the bricks which will take compression when the panel is subjected to flexure. The bricks will act as an infill material in the tension zone. The advantage is that certain amount of concrete is replaced by a cheaper material thus leading to a cost saving. The cost saving depends on the cost difference between a brick and the cost of concrete for the volume of a brick. It was reported by Rao et al. (1983) that the roof slabs made with long span brick panels have shown corrosion of reinforcement to a certain degree four years after the construction, which was attributed to insufficient cover to reinforcement.

2.3.4 Membrane action in one way slab strips

Generally it is considered that membrane forces will develop in one way restrained slabs only in the post yield stage when the collapse mechanism is formed. However, it was reported by Lahlouh & Waldron (1992) that membrane forces develop right at the on set of cracking in concrete, well before the yield of the reinforcement has been reached. Hence, the benefits of compressive membrane action apply not only to the ultimate l i m i t s state but could also be available under working load conditions. \

36

The development of the membrane action can be explained by considering the movement of the neutral axis, with the on set of micro cracks. When the slab is loaded there can be micro cracking in the slab as shown in Figure 2.15. As soon as these cracks form, the neutral axis will move towards the compression face with a corresponding axial extension of the slab through the tension zone. If these extensions are prevented by external restraints to a certain extent, the movement of the slab neutral axis is sufficient to induce the membrane forces as shown in Figure 2.15.

The reinforced concrete beam and plank system reported in Section 2.3.3 has one way thin slabs spanning on to beams and walls. When this slab system is used for a floor slab, some of the slab panels, specially of those located in the interior of the building could be benefited by the membrane action. The membrane action can enhance the load carrying capacity and also control the deflections associated with over loading.

2.3.5 L o a d t e s t i n g o f s labs

Load testing involves the systematic application of known test loads to a structure or part of it, and an assessment of the measured response of the structure under the influence of those loads. The guidelines for load testing a structure is given in Section 9 of BS 8110 : Part 2 : 1985. The methods of load testing and precautions to be taken are explained with a lot of details in Menzies (1978), Jones & Oliver (1978), Bungey (1982).

The following important observations have been highlighted by Menzies (1978):

1. The strength of floors, as built, does not necessarily correspond with the strength for which those were designed. This often happens since the reinforcement provided will generally depend on the available bar sizes. Hence, often, the area provided could exceed the reinforcement area required.

2. The degree of recovery can be important to assess the response of a slab, when the deflections are within the allowable limits.

3. When the stiffness of a structure is high giving very small deflections, the percentage recovery becomes less significant.

In composite systems, there can be a considerable amount of load sharing between adjacent members. The degree of load sharing can be predicted by using a simple formula which considers that slab behaves as a linear elastic member. The load carried by each member can be predicted with respect to the deflection of the member (Moss, 1993).

Wr = r (2.6)

37

where W r = the load on r beam 5 r = the deflection of the r t h beam LW r = sum of the corresponding beam loads Z5 r = sum of the corresponding beam deflections

When carrying out a load test for a composite slab systems, it is important to consider the effects of movements caused by environmental temperature changes which can alter the thermal gradient across the depth of construction. In some cases these thermal movements can be in excess of the deflections due to applied loads and also could be in opposite direction, thus resulting in a net upward movement (Moss, 1993). Therefore, to make an assessment of the response of the structure under the influence of test loads, compensation needs to be made for the influence of the environmental changes on the load induced deformations. In order to overcome the influence due to environmental temperature changes, the following action can be taken. Before the start of the load test, deflection should be taken for a period of time to establish the extent of any interference such as thermal movements (Menzies, 1978). If substantial movements are measured, it is necessary to make allowance for them in the deflections measured when the load test is in progress.

2.3.6 Stiffness increases in slabs due to non structural screeds

In composite floor systems assembled with different elements, the screeds can have a considerable influence on the structural behaviour with respect to the increased stiffness and greater load distribution characteristics. The most effective type of screeds are those of concrete which are bonded to the slab.

It was shown by Moss(1994) that for one way spanning slab panel, the effective flexural rigidity of the composite section can be calculated using the following equation: /

E W = Z 2 (1 -s) + k (E I u n j t + E I s c r e e d ) /k

where E I u n i t and E I s c reed are the individual flexural rigidities for the structural units and on assumed width of screed. Z is the lever arm between neutral axis position in the screed and the structural unit.

k = [ 1 / E u n j t A u m t + 1 / E s c r e e d A s c r e e d ]

and s = % slippage /100

For fully bonded screeds, slippage can be considered as 0 %.

2.3.7 Vibrat ional characteristics of slab panels

The problem of occupant induced vibrations in buildings is one of growing importance. Until recently, this problem was thought to be confined to floors of timber or steel

38

composite construction. When light concrete floors are used, those can be sufficiently light and flexible to give rise to disturbing levels of vibrations.

Assessment of floor vibrations requires knowledge of the level of vibrations that will I cause disturbance, the dynamic characteristics of the floor, and the response of the floor

to the occupant loads. It was reported by Williams and Waldron (1994) that most of the methods available for determining the dynamic response can underestimate the response of the structure and hence should be used carefully.

According to Schuller (1990), light weight long span floor structures may be susceptible to dynamic excitation, such as that due to people walking or dancing. Such activities cause typical periods of 0.25 to 0.5 seconds. Thus, the floors having a natural period of vibration of 0.2 seconds or more are liable to develop resonance, together with motions objectionable to humans.

I 2.4 SUMMARY

The construction industry is one of the largest consumers of engineering materials. The shortage and rising prices of the traditional materials such as cement, bricks, sand and timber are encouraging those in the construction industry to look for alternative materials. This is necessary not only from the point of view of maintaining a lower construction cost but also to relieve pressure on the existing supply and to reduce associated environmental degradation.

An appreciable saving in cost also can be achieved through the use of locally available materials especially indigenous materials found in the vicinity of the construction. The saving comes both in terms of materials as well as transportation costs. ^QnlTsuch material is laterite soil. It is readily available and hence a cheap material found in most

) parts of Sri Lanka. It is easy to work with, requires less skill, and hence it encourages and facilitates unskilled individuals and groups of people to participate on self help basis. It offers a very high resistance to fire destruction and provides a comfortable built environment due to its high heat insulation properties.

With increasing population and associated rapid rate of urbanisation, there is a growing concern in many developing countries to find cheaper solutions to the problems of providing decent and affordable shelter to the majority of the population. When alternative building materials are proposed to meet the above, it would require a thorough understanding of their properties and detailed investigation to determine the suitability for a particular form of application.

When alternative building materials are selected it is very important to consider the durability of the materials very seriously. If the alternative building materials are not as durable as the conventional materials, life time cost of cheaper materials can be more than the life time costs of more expensive, durable counterparts. The durability requirements

39

for different parts of the building should be stated as the minimum service life expressed in years or as resistance to some environmental agents.

When alternative building materials are introduced there can be some sociological problems to be overcome than technological. This comes with the social acceptance of alternative building materials. For example, many people may not like living in earth houses because they may denote a low social status. Thus, when earth is used to produce cement stabilised soil blocks, those should be as strong, durable and aesthetically appealing as burnt clay bricks.

When alternative building materials are used, they must be able to withstand the environmental agents. In climatic conditions that prevail in Sri Lanka, there can be a number of weathering agents such as solar radiation, rain and air constituents, biological agents such as termites and fungi, and mechanical agents such as winds and floods. Thus, it would be necessary to assess the resistance against each of these agents and to take appropriate protective action to ensure the required service life.

When developing the alternative building materials for Sri Lanka the following strategy could be adopted:

1. If there is a possibility to produce alternative materials which are as good as traditional material by using locally available materials without causing environmental degradation, then those should be developed scientifically so that engineers will be able to use them with confidence.

2. If the present application of traditional materials can be further optimised, it should be pursued with a proper scientific background.

In this research work, the first approach was used with cement stabilised soil blocks using laterite soils. In this case, Auram Press 3000 machine giving a compaction ratio of 1.65 was selected for the experimental investigation since it coukLproduce blocks which are strong enough for loadbearing two storey building construction. The buildings constructed using these blocks can be made as attractive as those built with traditional materials. Since laterite soils are available in most parts of Sri Lanka, the cost of blocks also could be comparatively lower than burnt clay bricks. It was also shown by Ranasinghe (1997) that the levelling of lands containing laterite soil hills could be advantageous with respect to reaching the ground water sources using open wells and also levelling of the land makes it more attractive to house builders. Hence, it can be considered as an activity that dose not degrade the environment very much.

The second approach has been used to optimise the concrete floor slabs. The traditional solid slab construction with thickness ranging from 100 mm to 125 mm, though aesthetically appealing , dose not optimise the use of concrete and steel. This is because the concrete below the neutral axis, which can be about 75-90% of the depth, does contribute to the dead weight while merely providing cover to reinforcement to prevent

40

corrosion and providing depth required for controlling deflections. In lightly loaded solid slabs of low spans, the reinforcement requirement is generally not governed by the flexural requirements, but by the need to control cracking. On the other hand, the reinforced concrete beam slab construction with precast slabs explained in Section 2.3.3 can be an attractive alternative since it optimises the use of concrete while minimising the area of reinforcement required. This has the added advantage of precast construction, which eliminate the use of traditional bamboo and timber falsework and formwork, thus forms a more environmentally friendly alternative. The only drawback of this system is that it could be aesthetically less appealing for some since the beams spaced at 1.5 m are visible. On the other hand, some others might find it more attractive than a flat soffit obtained with traditional construction.

The research information gathered from literature and those found in this research for cement stabilised soil blocks can be illustrated as follows.

Cement stabilised soil blocks

Information available in literature

Selection of soils for blocks

Information found in this research for blocks having a compaction ratio of 1.65

Selection of stabilisers

Manufacturing of blocks

Mortars suitable for blockwork

Strength properties of blockwork

Strength data for blocks and blockwork

Development of quality controlling measures for / blockwork construction

Design methods for blockwork as a loadbearing material

Guidelines for construction of two storey residential buildings

Cost studies for cost comparison purposes

41

The research information gathered from literature and those found in this research for concrete floor systems can be illustrated as follows.

Insitu cast floor systems used for slabs

Information available in literature

Precast floor systems

Precast roof slab systems

Behaviour of floor systems

Concrete floor systems

Information found in this research for reinforced concrete precast beam slab systems

Design methods for precast beam slab systems

Construction techniques for beam slab systems

Behaviour of precast componenets and slab systems

Cost studies for cost comparison purposes

The detailed testing programme carried out for cement stabilised soil blocks is presented in Chapter 3. The detailed experimental programme carried out for the composite precast beam slab system is given in Chapter 4. The design study carried out to determine the feasibility of using the composite beam slab system with cement stabilised soil block loadbearing walls is presented in Chapter 5. A detailed cost study which could be useful for comparison of the proposed alternative materials with traditional materials is presented in Chapter 6.

42

3 ( 5 o z < w

< 0.

100

80 -

p w 60 _ OCX < o 0 - 3 U-O OOC 40

20

200 0,02 0,002

SIZE OF PARTICLES PASSING THROUGH SIEVE (mm)

Figure 2.1: Boundaries of grading curve for soils suitable for stabilisation (Guillaud et al. 1995)

Oc

0 "m Aoi tar

Oc

<jh Block <jh Block <jh Block <jh Block

Figure 2.2: Elastic theory of failure of brickwork

43

1

s a 1 3 3?

I E blocks need cutting

M i l ' I I

I no wastage of blocks

Figure 2.3: Selection of dimensions and locations of opening to minimise cutting of cement stabilised soil blocks

Figure 2.4: One way slabs Figure 2.5: Two way slabs on beams

Figure 2.6: One way pan joists

44

Figure 2.7: One way slab on beams and girders

45

insitu cast concrete

25 mm 50 mm

100- 175 mm

prestressed concrete beam •

600 mm

Figure 2.11 Precast prestressed concrete beam sys tem with insitu cast s labs

rso mm screed

precast beam hollow block

Figure 2 . 1 2 Precast prestressed concrete beam slab sys tem with h o l l o w b locks and insitu cast screed

R6 at 200c/c

Figure 2 . 1 3 : Precast beam and insitu cast slab sys t em used for Koralawel la houses

46

R . C . P L A N K

| 10 | 10 | 10 |

LC-

-6 0 3 Nos.

•6 0 8 20mm c/c

f Clear cover. 1-5 cm ) SEC. 1-1

M 30 4-

S E C 2-2

30C-*>n

-3- 6mm 0 M.S. bar

"6mm 0 M.S. bars

* 2 0 c / c f

30 CIYI

150 / 130 c<v>

3 cm

L- SECTION R.C PLANK - 3-3

I — 1 No. 6mm 0 tie bar 2 Nos. 6mm 0 bars

6mm 0 stirrups « 13 cm. c / c A"*-j

-="5

3 5 0 / 280 / 2 0 0

2 Nos. 12 $ + 1 No. 10 | (For 35 c m joist) Or

2 Nos. 10 $ ( for 200 & 280 cm. joists )

L-SECTION OF R.CJOIST

6 0, Tie bar 6 0 i 13 cm c / c

^ 6 $ 2 Nos.

-10 £ 2 Nos.

SEC A- A FOR 200 4 280 c m LONG R.C JOIST

15 cm

6 0 i Tie bar 6 0 .13 cm c/c 6 0 ,bar 2 Nos. 12 $ . 2 Nos. 10$ 1 No.

S E C - A - A FOR 350 cm.RCJOIST

Figure 2.14(a): Precast reinforced concrete beam and plank system used for roof slabs (Jindal et al., 1984)

47

Figure 2.15(b): Precast reinforced concrete beam and plank system used for roof slabs (Jindal et al., 1984)

48

Figure 2.15: Compressive membrane action in axially restrained reinforced concrete slabs (Lahlouh & Waldron, 1992)