LITERATURE REVIEW

INTRODUCTION

This chapter literature related to stabilized interlocking blocks. This could be from previous studies and textbooks. This gives an insight into subject matter and provides guide for the study and researcher. It also suggests stakeholders of what needs to be done.

2.1 DEFINITION OF STABILIZED BLOCKS

According to Gooding and Thomas (1995), cement-stabilised building block is defined as one formed from a loose mixture of soil and/or sand, cement and water (a damp mix), which is compacted to form a dense block before the cement hydrates. After hydration the stabilised block should demonstrate higher compressive strength, dimensional stability on wetting and improved durability compared to a block produced in the same manner but without the addition of cement. This definition includes a range from hand-tamped soil blocks containing only enough cement to enhance their dry strength a little (but not to achieve any long term wet strength) to close-tolerance high-density concrete blocks, mechanically mass produced and suitable for multi-storey construction without a render. The spectrum of cement-stabilised building blocks has been split traditionally into two distinct fractions, sandcrete and soil-cement.

The key differentiating factors between soil-cement and sandcrete are then cohesion/strength of the freshly demoulded block and the block size. During the course of a survey it was found that block size effectively determined the marketed name, large blocks were sold as sandcrete while smaller blocks were sold as soil-cement.

The exceptions to this were in South Africa and Botswana where cement stock bricks are common. However these are typically smaller than soil-cement blocks, 100x225x87.5mm (width x length x depth) compared to 140x290x100mm for soil-cement and 150x460x230mm for sandcrete.

Stabilization is also possible with alternative cementitious binders such as lime. At present ordinary portland cement is the most widely available and quality-consistent stabilizer and is likely to remain so for the foreseeable future. Even if lime were to become widely available with assured quality, lime stabilization requires at least twice as long for initial curing. As quick curing has a significant economic value in block production, lime use is likely to remain less common than ordinary portland cement.

2.2 SOIL CEMENT BLOCKS AS A BUILDING MATERIAL

Research and development of stabilised soil as a building material is not new. The use of CSBs can be traced back 50 years (Fitzmaurice, 1958; Enteiche & Augusta, 1964; Fathy, 1973; Webb, 1988). From the early 1950s attempts were made to develop the material as an alternative walling unit to the modern and more expensive fired bricks and concrete blocks. The promotion of the material was originally introduced via the United Nations (UN Bulletin No. 4, 1950; Fitzmaurice, 1958).

The idea of compacting earth to improve its quality and performance in the form of moulded blocks however dates back to the 18th Century (Houben & Guillaud, 1994). The addition of a binder to stabilize the soil is more recent. Apart from the early work of the United Nations, the history of the spread of the CSB is not well documented. During the 1950s use of the material was widely disseminated worldwide. The 1960s and early 1970s were however stagnant years.

This was to change with the 1976 Vancouver Assembly of the United Nations Conference on Human Settlements (UNHCS, 1976; UNIDO, 1980). Noting with concern that the worlds population was expected to double by the year 2000, and worse still, to quadruple by the year 2030 (representing the largest single population growth in human history), the conference resolved to focus on the development of low-cost housing. Further momentum was to be given 12 years later following the declaration of the year 1987 as the International Year of Shelter for the Homeless (UN/IYSH, 1987). Subsequent proclamations were to follow in 1988 under the theme 'Global Strategy for Housing by the Year 2000'. The key targets of these resolutions were the guaranteed access to decent and durable housing for all from the beginning of the new millennium. Renewed world-wide interest was soon to provide an immense impetus that has ensured the now vibrant spread of CSBs throughout the developing world (Okello, 1989; Schmetzer & Kerali, 1994; Kerali, 1996).

Continued interest in CSBs will in future evolve around the several merits and attractions associated with its use. Firstly, as the basic raw material is soil, its source will remain abundant. This facilitates direct site-to-service application, thereby lowering costs normally associated with acquisition, transportation and production.

Home ownership can then be delivered at comparatively low costs. Secondly, the initial performance characteristics of the material such as the wet compressive strength (WCS), dimensional stability, total water absorption (TWA), block dry density (BDD) and durability are technically acceptable. They are also comparable to those of rival materials (ILO, 1987; Houben & Guillaud, 1994; Houben et al, 1996).

Houses constructed of CSBs also uniquely proffer better internal climatic conditions than other modern materials (Fullerton, 1979; Hughes, 1983). Thirdly, promoting the use of CSBs generates more direct and indirect employment opportunities within the local populace than would be the case with other materials. Fourthly, use of the material contributes directly to the social, cultural and educational advancement of the population (Schumacher, 1973; Anderson et al, 1982; Aksa, 1984). Their use also contributes to the training and re-training of artisans and to the provision of new skills. Use of the material through the provision of local infrastructure such as schools, community centres, health centres and administrative units results in the promotion of human interactions and social development. Finally, use of the material is environmentally friendly, appropriate and correct since it utilises the otherwise unlimited natural resource in its natural state. Moreover, this is achieved with little resultant depletion of other resources, or pollution and requires no excessive energy consumption and wastage as is the case with clamp fired bricks. The elimination of the need for wood fuel resources is seen as a major attraction over such bricks. The use of CSBs is thus in keeping with current sustainable development strategies (VTA, 1977; Plinchy, 1982; Lawson, 1991; Perera, 1993; Norton, 1997).

2.3 DURABILITY OF CSBs

Cement based building materials like CSB's and concrete were originally promoted as having an indefinitely long service life, and that they would require only minimum maintenance. Many cement based materials have indeed given excellent service. However, as these structures continue to be left exposed, it is becoming evident over time that even normal exposure conditions are actually more deleterious than originally thought (Baker, 1991; Sjostrom, 1996). Occurrences of undesirable, unpredicted premature deterioration where defects are clearly visible even to the casual observer, are becoming common. Defects in CSB structures are mainly presented as surface erosion, volume reduction, cracking and crazing, surface pitting and roughening, and detachment of render. These deterioration phenomena have been predominantly witnessed in the wet humid tropical regions of the world. No similar adverse reports have been documented from the hotter and drier regions (Spence & Cook, 1983).

2.3.1 Durability

The word durability originates from the Latin word 'durabilis' which means 'lasting' (Franklin & Chandra, 1972). It can be used in the context of most building materials to mean resistance to weakening and disintegration over time. The term has been described in various ways by different authors although the substance appears to remain the same in all cases. According to BS 7543: 1992, durability is defined 'as the ability of a building and its parts to perform its required function over a period of time, and under the influence of agents'. But according to BSI CP3 1950, 'durability is a measure, albeit in an inverse sense, of the rate of deterioration of a material or component'. More recent definitions state that 'durability may be regarded as a measure of the ability of a material to sustain its distinctive characteristics, and resistance to weathering under conditions of use for the duration of the service lifetime of the structure of which it forms part' (Baker et al, 1991; Sjostrom et al, 1996; Glanville & Neville, 1997). These definitions are too general to be of any practical use with CSBs.

2.3.2 Deterioration

Deterioration has been defined by several authors as 'the time-related loss of quality of a material, usually under the influence of environmental agents' (BS1 CP3 1950; BRE, 1980; Baker et al, 1991). Premature deterioration has also been defined as 'failure to achieve the predicted service life' (BS 7543, 1992). The predicted service life of a block can be obtained from recorded performance or from accelerated tests. Unfortunately, such records are not available.

Failure due to the inability of a newly made block to fulfil its functions has to be clearly distinguished from failure brought about by alterations in properties over the service lifetime of a block. Indeed most building materials will have some of their properties altered over time although their durability may not always be called to question. The durability of a block can therefore be regarded as its ability to resist deterioration. It can be treated as the reciprocal of deterioration under pre-defined conditions (Sjostrom et al, 1996).

Due to deterioration however, the durability of a block is unlikely to remain constant.\It may in fact change considerably. The implication is that durability of a block and its deterioration are likely to influence each other mutually but negatively. As can be expected, the more a block deteriorates, the less durable it is likely to become over time. For example bulk properties of a block such as water absorption and permeability are related to the type of microstructure and density of the block.

However, the microstructure and density of a block may alter appreciably due to weathering (deterioration). This alteration can in turn increase the water absorption and permeability of the block. Such increases are likely to accelerate the rate of deterioration due to softening and dissolution of any unbound soil particles in the block. Further loss of performance can then be expected. The limit at which the loss of performance can be considered unacceptable is not yet well defined in CSBs.

Unfortunately, even if it was, the limit may not be easily applicable without further qualification. This is because depending on the constituent materials used in a block, and on the quality of the processing methods used, no two blocks might be easy to compare. Unacceptable deterioration will therefore vary from block to block, and from property to property. Block properties that diminish over time reflect the past history of the block, both during and after manufacture.

2.3.3 Compaction and Densification

It was shown by Lunt (1980) that higher compaction pressure up to 10 MPa, generated quasi-statically, has beneficial effects on compacted density and cured strength (research conducted on lime-stabilised soil). Subsequently several machines were produced, e.g. the Brepack, which used a hydraulic circuit to achieve compaction pressures up to 10 MPa. Higher density blocks are easier to handle between moulding and curing, have a higher compressive strength after curing and also an improved surface hardness. The first is important to reduce the incidence of damage during handling and to permit the stacking of green blocks during curing (thereby facilitating good curing and reducing the size of any curing yard). The second is important because standards for building materials are usually expressed in terms of bulk compressive strength. Surface hardness is important as lack of it results in rapid rain or wind erosion or requires a render to be applied to protect the blocks.

By increasing the compacted density of the block, whether soil-cement or sandcrete, the stabiliser content may be reduced for a given strength, thus reducing the cost of the block. However experimental research conducted by the DTU has shown that the cement saving resulting from higher compaction pressures is not enough to offset the increased capital cost of a quasi-static high pressure machine, unless production output is dramatically increased. Additional advantages of high density production were noted by the DTU, namely an increase in the allowable range of particle grading for the material to be stabilised and improved resistance to poor curing as a result of reduced block porosity. These factors do counteract the greater cost of high density compaction but not sufficiently to encourage the use of manual quasi-static high pressure machines.

Block density is not solely determined by the maximum compaction pressure that the forming process could exert. In the case of fixed-volume compaction the amount of soil placed in the mould is highly significant.

Too little material and a low density block is produced, while too much material and the machine is over-stressed and liable to jam. Moreover if the material is not compacted at its optimum moisture content, lower density will result. If too little water is present, internal friction is high and densification prematurely ceases. If too much water is added then hydrostatic conditions may be generated where the applied compaction force increases the pressure of the material's pore water but does not result in particle rearrangement and densification. Variable water content causes a further complication as the volume occupied by a damp soil also depends on its moisture content. A dry soil initially expands as water is absorbed, up to a point known as the fluff point, more than this amount of water and the soil volume again decreases.

2.4 STABILIZATON OF LATERITE BLOCKS

2.4.1 EFFECT OF CEMENT CONTENT

The re-birth of pressed earth brick construction was largely stimulated by the work of Fitzmaurice in the late 60’s for the United Nations (Fitzmaurice, 1958). In his report Fitzmaurice examined unstabilised earth buildings in France and England and found that although there were many splendid examples of earth buildings more than 100 years old the maintenance costs were extremely high. He pointed out that as societies became more mechanised unstabilised earth walls became more uneconomical “The labour cost in original construction might not be excessive, but the high maintenance cost was quite impractical.” (Fitzmaurice, 1958)

Fitzmaurice recognised the potential of cement stabilised earth construction based on research into stabilised soil roads in the 1930’s. Sheets and Catton (1938) set out the basic principles of cement stabilised earth, which were established at that time. Fitzmaurice suggested that what was needed was a clear definition of stabilised soil walling as many partially stabilised walls “inevitably have begun to decay and are often disfigured by many cracks within a few years of their completion”. He proposed that the term “stabilised soil walling” should be restricted “to work of such quality that the walls will remain fully durable throughout the expected life of the building” (Fitzmaurice, 1958)

Cytryn (1957) quotes instances of soils where reasonable stabilization has been achieved with cement contents as low as 2.5%. The ACI “State of the Art Report on Soil Cement” (ACI, 1991) suggests that for road pavements “Cement requirements vary depending on desired properties and types of soils. Cement contents may range from as low as 4 to a high of 16 per cent by dry weight of soil. Generally, as the clayey portion of the soil increases, the quantity of cement required increases.”

The author has conducted many tests over a long period of time using various soils and has noted that for durability in a Sydney climate a minimum of 5% cement (by weight) stabilization should be considered, with the suitability of the soil being seriously questioned if more than 10% cement is required. “Most soils can be stabilised with about 7.5% of cement compared to around 12% required in concrete blocks” (Heathcote, 1991).

Heathcote and Piper (1994) proposed that there is a minimum cement content for a particular soil below which the clay-cement skeleton cannot fully develop, and their tests on a particular soil showed this to be the case for cement contents less than 1%. Herzog (1964) notes, “one of the most significant aspects of soil cement strength is its durability under adverse climatic conditions. The semi-impervious skeleton structure with an almost impervious hardened cement core could be expected to hinder considerably the movement of water to and from the enclosed matrix. This is probably the main reason for the stability of clay cement under all weather conditions” (Herzog, 1964).

The effect of cement on the durability of soil cement mixtures has been widely investigated in the road construction industry. For example Abrams (1959) conducted “wire brush” durability tests on two soils with varying cement contents. Soil No 1 (AASHO Group A-1) was a granular soil with minimal fines while Soil No 2 (AASHO Group A-6) was a clayey soil with about 35% passing the 200 sieve. Ngowi (1997) carried out wire brush tests on two Botswana soils made into pressed earth blocks. The first soil (Mahalapye) was a clay soil with 48% clay content and 27% sand content whilst the second soil (Tsabong) was a sandy loam with 14.5% clay content and 63% sand content. Both the Abrams and the Ngowi tests show extremely good correlation between cement content and weight loss as determined by the wire brush test. They show a definite power relationship with 95% confidence limits for the power factor of 1.11 – 4.28 for granular soils and 0.69 – 2.83 for clayey soils.

2.5 FACTORS AFFECTING THE DURABILITY OF EARTH WALL BUILDINGS

The durability of building materials can be defined as their resistance to functional deterioration over time. Durability can conveniently be divided into three sub-sections

a) Physical durability – deterioration caused by physical processes such as abrasion or reversal of stress

b) Chemical durability – deterioration caused by chemical reactions such as rusting of steel.

c) Biological durability – deterioration caused by organic breakdown such as dry rot in timber.

The predominant cause of loss of functionality in earth walls is loss of surface area due to erosion (physical attack) by wind-driven rain, and present test methods such as the Australian Bulletin 5 spray test (Middleton, 1952) are a reflection of that form of deterioration. In this investigation therefore the terms “durability” and “erodibility” are sometimes used interchangeably, as they are in the literature, although it is recognized that erodibility is the more narrow definition. In general earth wall units having a high erosion resistance are resistant to other degradation factors such as heat and static water penetration.

There are many factors which contribute to the breakdown of the surfaces of earth wall units. They are

Influence of material properties –whilst this is most certainly a major factor in mud brick buildings, where increased clay content generally leads to greater erosion resistance, it does not appear to be as big a concern in cement stabilized pressed earth or rammed earth buildings. For these buildings choice of soil is usually related to its ability to be pressed or rammed into a form and the clay contents are usually less than 15%. There is generally little variation in performance between soil types suitable in this case (for the same cement content), although occasionally a particularly reactive clay might be present and cause some problem.

Influence of stabiliser – without stabilisation most pressed earth or rammed earth wall units would not be very durable. They generally have a low clay content and therefore there is little to hold particles together.

Influence of compaction – loose material provides very little resistance to the erosive power of rain. In general durability increases exponentially with degree of compaction.

Influence of freeze thaw and/or chemical attack by airborne salts – it has been shown by Sherwood (1962) that sulphate attack can cause deterioration of clays and it is conceivable that freeze thaw attack could be a problem in some countries. The effect of these will be to destabilise the surface of units and make them more susceptible to attack by wind-driven rain.

Effect of micro-climate and position of element in façade – Real buildings generate their own micro-climate due to their size and shape, which results in considerable variation in raindrop impact velocities and directions on particular facades. Additionally the local effect of projections such as window sills and splashing at the base of walls will all lead to variations in erosion from that predicted using the simple model developed herein.

Influence of surface texture - The surface texture of earth wall units can vary significantly depending on the manner in which they have been formed, and this can significantly affect their erosion resistance. Some units have a surface which is akin to a steel trowelled concrete surface whilst other surfaces are deliberately scabbled to achieve aesthetic effects.

Influence of wetting/drying cycles – cycles of wetting and drying increase surface stresses in the units and will lead to a more rapid breakdown than that due to a constant stream of rain.

Effect of physical deterioration caused by structural effects such as differential shrinkage - any tructural cracking of units will lead to a weakening of their ability to resist attack by wind-driven rain, and will therefore make them less durable.

Influence of surface coating – many earth walls are coated with protective coatings such as renders which improve their durability.

2.6 TEST METHODS

The various test methods used in earth wall construction can generally be classified into three categories

1. Indirect Tests, where a test, which bears little or no relation to the degradation mechanisms, is carried out, such a test having been shown from experience to be a reasonably reliable predictor of the performance of the material under in-service conditions.

2. Accelerated Tests, where an attempt is made to model the in service degradation process, with the intensity of the degradation factors increased to compensate for the reduced time frame.

3. Simulation Tests, where an attempt is made to exactly model in-service conditions

2.6.1 Wire Brush Tests

2.6.2. ASTM D559 Wire Brush Test

The ASTM D559 test is probably the most widely recognised test for durability of cement stabilised earth and was developed following research into the use of soil- cement as a paving material. It was developed towards the end of the Second World War as ASTM D559 “ Methods of Wetting and Drying Test of Compacted Soil-Cement Mixtures” (ASTM, 1944).

The test was developed “to determine the minimum amount of cement required in soil cement to achieve a degree of hardness adequate to resist field weathering” (ASTM D559- Section 3). In this test, stabilised soil is compacted in three layers at optimum moisture content in 102 mm diameter moulds to a height of 116 mm. Each layer is compacted with 25 blows of a 50 mm diameter rammer (weighing 2.5 kg) falling a distance of 300 mm.

Following compaction the specimens are stored for 7 days in an atmosphere of high humidity and then submerged in tap water for a period of 5 hours. The specimens are then placed in an oven at 71􀁱C for 42 hours and removed. The specimens are then given two firm strokes on all areas with a wire scratch brush to remove all material loosened during the wetting and drying cycle. The wire brush is similar to a butchers brush and has 50 groups of ten bristles, each bristle being 50 mm by 1.6 mm by 0.46 mm. The firm stoke corresponds to a pressure of 13 Newtons and eighteen to twenty vertical brush strokes are required to cover the sides of the specimen twice and four strokes are required on each end. Twelve cycles of wetting and drying are carried out and the weight loss at the conclusion of the test determined as a percentage of the original dry weight.

This test is extensively used in the road construction field. The Portland Cement Association (1956) provides guidance as to the limits of soil loss considered acceptable for various types of soils in road construction, ranging from 7% for clayey soils to 14% for granular soils. In South Africa Webb et al. (1950) carried out tests on stabilised pressed earth bricks and fired bricks using a modification of ASTM D559 and concluded that earth bricks made from suitable soils were equivalent to medium quality fired stock bricks.

In 1958 Fitzmaurice carried out a comprehensive study on the condition of existing earth wall buildings for the United Nations (Fitzmaurice, 1958). In his detailed study of the properties of stabilised earth the ASTM D559 wire brush test for testing stabilized earth was employed. Based on his observations regarding the condition of buildings, Fitzmaurice set out guidelines for the maximum weight loss that should be considered acceptable in relation to this test.

Although considered by Walker (1995) to be a severe measure of durability resistance for earth walls the wire brush test has the advantage of being an ASTM standard specifically relating to the durability of soil- cement mixtures. It was adopted for earth building standards in Kenya (UNCHS, 1989) and India (Indian Standard 1725, 1960), although recently the wire brush component has been omitted in tests in India (Reddy and Jagadish, 1995) . The main disadvantage of the wire brush test is the length of time required to perform it (24 days) and the associated expense. Secondary problems are the possibility of variations in results due to inconsistent application of the brush pressure.

2.6.3 Spray Tests

2.6.4 Bulletin 5 Spray Test

In response to an increased interest in earth construction in the 70's the Commonwealth Experimental Building Station in Australia developed a refined version of the spray test (Schneider, 1981). This test is further referred to herein as the “Bulletin 5” spray test as that is the name of the document in which it is contained. This spray test is called up in the Building Code of Australia and a modified version was included in the New Zealand Code of Practice on earth wall buildings (NZS 4297,1998).

The Bulletin 5 spray test involves water being sprayed horizontally out of a special nozzle at a pressure of 50 kPa. The sample is placed 470 mm from the nozzle and after an hour the sample is examined. The depth of erosion is determined using a 10 mm diameter rod. The impact area is within a circle of 150 mm diameter. The nozzle has 35 holes each of which is 1.3 mm in diameter. The total cross-sectional area of flow is therefore 46.5 mm2 . Flow pressure is 50 kPa, which would give a theoretical exit velocity of 10.0 m/sec.

Measurements carried out indicate a discharge of 29.6 l/min for this test which yields a total volume of water in the one hour test of approximately 100 metres or approximately 85 years rainfall in Sydney. The corresponding jet velocity of 9.9 m/s, based on the above cross-sectional area, indicates very little head loss through the nozzle. The maximum allowable erosion is 60 mm per hour. Frencham (1982) reported that “The present thinking is that soils having an erosion rate of 0.5 mm/minute [30 mm total] for the duration of the test are beyond doubt; those having a rate between 0.5 and 1.5 need further evaluation; and those having rates in excess of 1.5 are either unsuitable or need stabilisation to be made suitable.” Such thoughts do not appear in Bulletin 5 and do not seem to coincide with the opinion of many that 60 mm max erosion is too severe.

The kinetic energy associated with the nozzle is approximately 5000 MJ/m2. To obtain some idea of the relevance of the Bulletin 5 spray test, this value can be compared to the value for the annual kinetic energy of rainfall obtained by using the formula of Morgan et al. (1984) in which kinetic energy is related to annual rainfall.

KE (J/m2) = Annual Rainfall (mm) x K ……….(2.2)

Assuming K = 21 J/m2/mm, as suggested by Morgan for temperate climates, and an annual rainfall of 1200 mm, gives an annual kinetic energy of around 25 MJ/m2. Assuming further that the test should relate to a 50-year life span then the total kinetic energy over those 50 years would be 1250 MJ/m2. If this were then related to a limiting erosion of 15 mm then the corresponding kinetic energy for a limiting erosion of 60 mm (that stipulated in the Bulletin 5 test) would be 5000 MJ/m2 , which is consistent with the Bulletin 5 test (assuming of course that all the kinetic energy of the rain was directed at the earth wall).

2.6.5 Dad’s Spray Test

As part of his research work into cement stabilised soil Dad (1985) developed a spray test based on one developed by Meyer and McCune at Purdue University for soil erosion. In this test a particular 2.5 mm diameter Veejet nozzle (No 80100) was placed vertically above a specimen, which was aligned at an angle of 30 degrees to the falling spray of water. (Veejet nozzles are manufactured by Spraying Systems Company in Illinois, USA and are typically used for washing coal on a conveyor. They produce a flat elliptical spray pattern, with lower intensities at the ends). Dad found that using a drop height of 2.45 metres and a nozzle pressure of 42 kPa he was able to simulate the terminal velocity of most raindrops.

The cured test specimens were placed at different positions on a grating below the spray, depending upon whether a high intensity (3125 mm/hour) or low intensity (145 mm/hour) rainfall was being simulated. To simulate the annual rainfall in Bangladesh (3500 mm/year) over a 25-year period, specimens were continuous spraying for 35 hours at a high intensity location and 30 days at a low intensity location. At high intensity, spraying continued for 12 hours after which the specimen was removed and oven dried at 60􀁱 C for 24 hours and then this cycle was repeated.

Some of the conclusions of his work were

Cement content and compaction pressure were the two most important variables for determining durability

The effect of cement content was practically linear.

Spraying at an angle of 90 degrees to the block face produces about 30% more erosion than at 30 degrees.

There was a small difference in erosion between horizontal and side faces

There was a similar magnitude of erosion in the high intensity test compared to the low intensity test, with the high intensity test being marginally higher.

Dad suggested that the high intensity test should be used because of its much shorter duration (35 hours).

Erosion increased with time but at a marginally increasing rate, especially towards the end of the test.

2.6.6 Ogunye’s Spray Test

Ogunye (1997) followed on from the work of Dad using a simulated spraying chamber approximately 1 metre square. He also apparently used a Veejet 80100 (6.4 mm jet diameter) and experimented with various pressures and drop heights. His aim was to simulate rainfall of 150 mm/hr and to do this he found that he needed to spray at a pressure of 50kPa with the nozzle suspended 2000 mm above the specimens, which were placed on a sloping rack.

The sloping rack contained “boxes” for eight specimens and the angle was adjustable, although in his tests an angle of 30 degrees was adopted. Specimens were painted on all sides, except the exposed face, with a heat resistant paint. Specimens were sprayed for a total of 120 hours and rotated at 15-hour intervals to offset any variation in intensity produced by the VeeJet spray pattern. The time of exposure and intensity of rainfall was chosen to reflect the amount of rain experienced in tropical Nigeria.

From data that appeared in his work it seems that the discharge was around 2.3 l/min giving a nozzle velocity of 1.2 m/s, which equates to an impact velocity of around 6.5 m/s. This impact velocity would correspond to a drop size of around 2 mm. Ogunye used his test to examine the change in various parameters (eg wet strength) with weight loss according to his test procedure for five artificial soils, made of combinations of sand, silt and clay, ranging from a sandy loam (Soil 1) to a clay loam (Soil 2). The soils were stabilized with cement (6, 8 and 10%), lime or lime/gypsum. Weight losses for the 120-hour tests ranged from 0.14 to 5.82%, with around 70% of the specimens having weight losses less than 1%.

2.6.7 Drip Tests

2.6.8 Yttrup Drip Test

Yttrup, Diviny and Sottile (1981) developed the first drip test at Deakin University. Their motivation was to devise a simple test that could be used by adobe owner-builders to determine the suitability of their soil. The test involves releasing 100 ml of water in the form of drops onto sample bricks, which are inclined at an angle of 27 degree from the horizontal. Following the test the pitting depth is recorded and the “Depth of Pitting” is taken as the average pitting depth for both sides of the sample.

According to Morris (1994) this test produced water drops of around 6 mm with quite varying frequency. On average he found that 833 drops fell over a period of around 60 minutes (approximately 14 drops per minute). The theoretical velocity of the drops is 2.8 m/s and the kinetic energy 0.44 mJ per drop (Total kinetic energy = 833 x 0.44 = 0.37J). On the basis that each drop impacts on an area of 25 mm by 25 mm this equates to a kinetic energy of 0.6 MJ/m2

Frencham (1982) developed the approach further and related the depth of pitting following the test to an Erodibility Index based on a correlation of drip test results with the observed performance of 20 buildings in the Western District of Victoria, which have existed for periods between 60 and 120 years. He categorised these buildings into four categories : Non Erodable, Slightly Erodable, Erodable and Very Erodable based on their surface appearance. Samples of each category were subjected to the drip test and the results were then used to define four classifications of “Erodibility Indexes”, as shown in Table 2.4.

The rainfall in this area averages around 500 mm per year. It appears that samples were taken from the least exposed areas of the buildings: “the east wall was used as a source of the sample bricks …. because of its minimal exposure to adverse conditions “ and the buildings had little or no eaves overhang. Frencham also proposed a formula to take into account differing wall exposures and rainfall than that experienced by the sample bricks used to calibrate the Erodibility Index. For differing exposure conditions he proposed an Exposure Factor (Ex), which was derived intuitively from the observed effects of weathering.

For differing rainfall rates Frencham proposed the use of a Rainfall Factor RF, which is zero if the rainfall is less than 520 mm and 1 if the rainfall was greater than 520 mm. Frencham combined the above two factors with the Erodibility Index (EI) of a test block to give a “Wall Erodibility Index” (EWI ) which was meant to give a guide to the actual performance of an unprotected wall in a specific location. The Wall Erodibility Index was defined as follows.

EWI = EI (Sample) + Exposure Factor + Rainfall Factor ………. (2.3)

Frencham suggested that soil samples and locational factors which produce a Wall Erodibility Index greater than 4 imply that the proposed material is not suitable for use without stabilisation, i.e. the soil is not suitable in its present form. Frencham further proposed relating Erodibility Indices as determined above to the expected loss of structural wall thickness (Table 2.6)

2.6.9 Comparison Between Drip Tests and Bulletin 5 Spray Test

Weisz et al. (1995) collected sample bricks from three sites in Victoria. 19 of the samples were adobe bricks (from 2 sites) and the remaining 9 were pressed earth bricks from the remaining site. They then subjected them to the above three tests. It appears that the drip tests were applied to the two side faces and the spray test to the flat face, although the latter is not clear in their paper. To avoid premature wetting each brick was first tested by the Yttrup drip test, then by the SAET drip test and finally by the Bulletin 5 spray test.

It appears that the pressed earth bricks (Balnarring) were the weakest and in some cases eroded right through before the 60 minutes was up in the spray test. In this case the pitting depth was calculated based on the average rate of erosion. In some cases the values for the Yttrup drip test were very small, and in this case an arbitrary value of 0.5 mm was assigned to these bricks. Using regression analysis Weisz et al. derived the following relationships between the tests

Yttrup Drip Test Pit Depth = 0.1312 x SAET Drip TestPit Depth (r = 0.78)…..….(2.4)

Bull 5 Spray Test Pit Depth = 5.29 x SAET Drip Test Pit Depth (r = 0.93)….…(2.5)

Bull 5 Spray Test Pit Depth = 37.42 x Yttrup Drip Test Pit Depth (r = 0.90)……..(2.6)

The correlations using the Yttrup drip test contained 15 tests where nominal 0.5 mm erosions appear to have been specified and the values for both the SAET and Bulletin 5 test were low for all the 19 adobe bricks. The values for the pressed earth bricks were much larger. The author carried out a regression analysis on these results only and, although the relationships above remained substantially the same, the correlation coefficients were significantly lower (eg. Bull 5 vs. SAET test r~0.5), when the nominal values were not included.