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Building materials

Stabilised soil building

As the population of the world continues to grow, so does the need for housing, thus cheap, easy to build accommodation for the thriving masses is a big problem in the developing world. Soil has been used as a building material for thousands of years, but unprotected structures seldom withstand wet climates for long periods of time. Relatively new materials such as cement have meant that blocks can be made which will last for centuries, but they are too expensive for most people in developing countries. A possible solution to this would be to make a block using soil that is then stabilised, as this adds strength and durability to the raw material, even in less arid conditions.

Earth building is the most common method of making cheap accommodation since earth or soil is readily available almost anywhere on the planet. To give an idea of how big the earth building field Houben states: "Thirty percent of the world’s population, or nearly 1,500,000,000 human beings, live in a home of unbaked earth. Roughly 50% of the population of developing countries, the majority of rural populations, and at least 20% of urban and suburban populations live in earth homes. Statistics show that there must be an extra 36,000,000 homes built for the urban population in Africa alone, by the year 2000." Note particularly that these figures do not include rural populations and indeed rural areas are where earth homes are becoming more common.

What is stabilised soil and why do we want it?

Stabilisation techniques can be broken down into three categories, Houben (1994): Mechanical, Physical and Chemical. Mechanical stabilisation compacts the soil, changing its density, mechanical strength, compressibility, permeability and porosity. Physical stabilisation changes the properties of the soil by acting on its texture, this can be done by: controlling the mixture of different grain fractions, heat treatment, drying or freezing and electrical treatment. Chemical stabilisation changes the properties of the soil by adding other materials or chemicals. This happens either by a physico-chemical reaction between the grains and the materials or added product, or by creating a matrix which binds or coats the grains.

Stabilisation fulfills a number of objectives that are necessary to achieve a lasting structure from locally available soil. Some of these are: better mechanical characteristics (leading to better wet and dry compressive strength), better cohesion between particles (reducing porosity which reduces changes in volume due to moisture fluctuations), and improved resistance to wind and rain erosion. Using one or more of the stabilisation techniques listed above, many of these objectives may be fulfilled. Optimum methods depend greatly on the type of soil, and a careful study of the local soil is necessary to suggest an effective method of stabilisation.

In the case of mechanical stabilisation, the soil is compacted to a greater density, and there will always be an improvement in its mechanical properties with virtually any soil type. This is not true however with other forms of stabilisation, where different soil mixtures can lead to better or worse properties using the same technique. In the majority of cases mechanical stabilisation is used in conjunction with a common chemical stabiliser, such as cement. If the stabiliser and the soil are mixed together thoroughly and there is a suitable clay fraction in the soil, the compaction process reduces the quantity of chemical stabiliser required in the block. The increased density also increases the effectiveness of the cement matrix, given that the cement is left in a moist environment (the hydration period to let the cement cure) for at least 7-14 days. For details on selection of soils for cement stabilisation see Gooding (1993 - B). More details on the process of cement stabilisation can be found in Houben & Guillaud (1994) and Spence (1983).

DTU research

Stabilised-soil cement building blocks are an established building material in many parts of the Less Developed World. This thesis has been split into three parts. Part A presented an overview of the process of soil-stabilisation and outlined the roles which soil structure and curing play in stabilisation. It examined methods of testing soils, highlighting errors presented in the published literature and presenting corrected testing procedures and unified plans for their implementation. Part B examined the conventional quasi-static block compaction process (slowly applied pressure) and established that no cost-effective increase in the compacted block density can be achieved by altering such moulding configuration as mould-wall roughness, mould-wall taper, number of applied pressure cycles and double sided pressure application. The tests were also used to assess the plausibility of several theoretical mechanisms underlying quasi-static compaction. Cement may be traded against compaction pressure for a given a final cured strength. The relation of compaction pressure and cement content to well-cured strength was established for 50 mm diameter cylinders and used to assess the financial benefit of high-pressure compassion. It was shown that savings in the cost of cement associated with high-pressure compaction were outweighed by the additional cost of such machinery. However there were additional benefits found to be high-density compaction, beyond the saving in stabiliser costs. It was established that a high-density moulding machine in the range £1000 to £1500 would allow these benefits to become cost competitive. Part C examined both experimentally and theoretically an alternative dynamic (impact below) compaction process, establishing that optimised dynamic compaction may produce strength equivalent to quasi-static high-density moulding while requiring only 25-50% of the energy. Five theoretical models of the process were developed and the Combined Airlock/Friction/Compression Wave Model was shown to have the most explanatory power.

Dynamically compacted block

A recent investigation into dynamic compaction of small soil-cement samples has suggested that a substantial improvement in the production of low cost housing may be achieved using this technique. Therefore, similar methods for producing full-size building blocks would have a considerable impact in areas of the third world where adequate housing is limited and existing techniques provide only short term results. The aim of this project is to design and produce a test rig which will enable research into the production of such building blocks. In addition, it will attempt to show that characteristics discovered on small samples can also be found when the technique is extrapolated to larger blocks. During the design of the test rig it was anticipated that if the research was successful, the future machine would need to work in a less developed environment. Consequently, the test rig was designed with this in mind so that it could also serve as an early machine design prototype. Initial specifications were drawn up for the test rig and machine design, then different design ideas were investigated and appraised. One design was selected for realisation and the necessary components were manufactured. The resulting test rig was successfully completed and was able to produce a number of full size building blocks. Moreover these blocks showed similar characteristics to those displayed by the smaller samples.

Creation of basic building blocks for building structures made from processed stabilised soil.

There are two major functions of a block as used in any typical building structure. The block must be able to support its own weight and the weight of any other block above it as well as any other necessary structure such as a roof. This is normally classified as the compressive strength of the block. Once the blocks are in the structure they must as a collective be able to withstand attack from the elements, such as rain, frost and wind erosion for a reasonable period of time, (UK building regulations design for a 60year lifetime).

The first of the two functions has been tested to sufficient levels for use in the developing world. Even unstabilised mud-brick walls are able to withstand the compressive forces necessary for single storey structures. Furthermore using stabilised soil has yielded blocks of sufficient strength to achieve structures that are several storeys high. Further research into this area would be interesting, especially with the new improved performance of dynamic compaction, but difficult to justify as it has been heavily researched in the past.

The second function is in some respects linked to the first, but it is more connected with determining the longevity of the structure in a potentially hostile environment. Once the first function is satisfied the second determines how long the structure will be able survive before it is no longer able to withstand the compressive forces applied to it. In the example of a simple mud-brick hut, once the structure has dried it can easily support its own weight and the weight of the roof. If heavy rains come and the structure becomes saturated with water again, it will lose its structural strength and may suffer collapse. If the structure can be kept dry or the mud-bricks were made impermeable to moisture then the structure would survive for a much longer period.

It is this second function of a block that requires further research particularly in light of the new developments in dynamic compaction. If a block can be made to withstand attack from the elements then it will provide lasting protection, that can be relied upon year after year. Dynamic compaction provides a significant energy saving (about 30%) in the production of homogeneous building blocks of a specified density (between 1950 and 2050 Kg/m³). As the density is increased the permeability of the block decreases and the effects of water penetration are reduced. This suggests that a denser block provides both greater compressive strength and better resistance to the elements.

A greater block density also improves the efficiency of any stabiliser that is added to the soil mixture. In the example of cement a relatively small percentage of cement (between 4-6%) is sufficient to generate a block that is structurally unaffected by moisture attack with a density as described above. This of course assumes a uniform distribution of the stabiliser throughout the block and this, indeed, may not be necessary.

Rendering or painting the external surfaces of the structure can protect the blocks from the elements, but these processes are expensive and usually do not use locally available materials. A stabiliser such as cement is the same in this respect but if the stabiliser is applied selectively a much smaller amount would be necessary. This is the area where further investigation would be worthwhile.

During the research a new machine to dynamically compact full size blocks will be designed and manufactured and this will then be used to test properties of dynamically compacted blocks with known materials. These results will be compared to the results gained from quasi-staticlly compressed blocks to see if the new technique is as effective as initials tests suggest