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Introduction
Building with earthen materials is well known to be an ancient technique, occurring more frequently when human race began evolving from a nomadic life style to an agricultural one. In recent years people seek more sustainable building materials and natural building methods. Developments prove that earth construction is still found to be a viable economic and environmental conscious technique.
To see this, first the materials used for the construction has to be studied, then the method of construction should be examinedand finally the physical properties of the structures should be analyzed. Compiling all this, it will be evident that rammed earth construction is a viable construction technique, yet safer for the environment.
Raw Materials
As the name implies, the primary material used in rammed earth construction is the earth itself. There are five basic types of soil (gravel, sand, silt, clay, and organic), and the dirt in a given location is generally some combination of all or most of these types. Historically, the longest lasting rammed earth walls were made of soil that was 70% sand and 30% clay. Like a concrete that contains gravel, sand and cement as a binder, a soil contains gravel, sand, and, silt & clay which act as binders as well.
The earth materials in order to last long, should have certain physical properties such as Granularity or texture, compressibility, plasticity and cohesion. The particle size distribution should range according to the table given below in order to get a proper output.
Soil Stabilization is important to increase the soil’s resistance to destructive weather condition in the following ways.,
1. By cementing the particles of the soil together.
2. Reducing shrinkage and swelling of the soil when its moisture content varies.
3. Making the soil less permeable.
Desirable qualities for soil construction materials include:
• Strength
• Low Moisture Absorption
• Limited Shrink/Swell Reaction
• High Resistance to Erosion and Chemical Attack
• Availability
Method of Construction
This consists of mainly 6 stages, 1. Preparing the site, 2.Laying the Foundation, 3.Analyzing the soil, 4.Framing the walls, 5.Tamping the soil, 6.Finishing the walls.
The construction of the wall begins with a temporary frame (formwork), usually made of wood or plywood, to act as a mould for the desired shape and dimensions of each wall section. The form must be sturdy and rigid, and the two opposing wall faces clamped together, to prevent bulging or deformation from the large compression forces involved. Damp material is poured in to a depth of 10 to 25 cm (4 to 10 in) and then compacted to around 50% of its original height. The material is compressed iteratively, in batches, gradually building the wall up to the top of the frame.
Tamping was historically done by hand with a long ramming pole, and was very labor-intensive; modern construction can be made less labor-intensive by employing pneumatically powered tampers. After completion it is strong enough for the frames to be immediately removed. This is necessary if a surface texture is to be applied (e.g., by wire-brushing, carving, or mould impression), since the walls become too hard to work after about an hour. Construction is best done in warm weather so that the walls can dry and harden. The compression strength of the rammed earth increases as it cures; it takes some time to dry out, as much as two years for complete curing.
Compressive Strength
C. Jayasinghe and N.Kamaladasa[1] conducted a comprehensive study on compressive strength for rammed earth walls constructed with three laterite soil (Hard,Clayey,Sandy). There were two identical panels tested to determine the average compressive strength of rammed earth wall. The wall thickness was 160mm, length was 1000mm and the height was 650mm. The load versus deformation curves were plotted. Chart 1 shows the load-deformation curves for three different types of soil. Chart 2 explains the strength increase with cement percentage.
The tests proved the possible use of laterite soils, hard laterite or clayey compositions for rammed earth walls. It was advised to maintain fines content below 20% and cement content more than 6%. The overall factor of safety is to be taken above 5 since the failure of rammed earth wall is of compressive crushing nature and the elastic modulus was found to be around 0.5kN/mm2.Generally, use of cement stabilized rammed earth as walling material is advised after the research findings, especially for single story houses.
Thermal Comfort
Xiang Dong, Veronica Soebarto et all.,[3] considered three Australian climate zones representing hot arid, warm temperature and cool temperature climates to achieve thermal comfort in naturally ventilated rammed earth houses. The test was based on the design of the houses considering the window size, shading and ventilation.
In hot arid climate 70% of the time acceptable indoor operative temperature can be achieved in a house built with 300mm thick solid rammed earth walls.
In warm temperature climate with the same house design nearly 68% of time acceptable temperature prevailed. To get better results window facing the north accounted nearly 50% of the wall area to allow solar heat entering the house
In cool temperature the window size were maximized and window shading were minimized, which resulted in acceptable climate 45% of the time. Comfortable temperature were found to be improved by 13% if 300mm insulation was installed in external rammed earth walls.
The studied proves that rammed earth has a great potential to be used for construction of naturally ventilated houses in hot arid and warm temperature climates, but in cool temperature climates, naturally ventilated rammed earth houses were found not suitable.
4.Seismic Behavior
Rammed earth buildings have shown poor seismic behavior in earthquakes occurred over the last 50 years. So the study conducted by Luis E. YAMIN, Camilo A. PHILLIPS et all.,[4] summarizes the characteristics and mechanical properties of earth made structures and the proposed rehabilitation alternatives to withstand seismic hazards.
Rammed earth were constructed to determine the behavior of main structural elements in buildings constructed with traditional techniques. A total of six cyclic in-plane loading tests were performed on rammed earth wall with different levels of vertical loading. The result showed that the failure were caused by combination of shearing stresses in the wall plane and bending stresses. These results showed that the buildings constructed with traditional techniques are highly prone to seismic failure.
To counter act these failures two alternatives were selected.,
1. Strengthening with wire mesh
In this method wire mesh were placed horizontally and vertically in critical zones. After connecting the meshes it is covered with lime and sand mortar. This prevents lateral instability that appears suddenly in non-reinforced constructions which are heavily damaged during earthquakes.
2. Strengthening with boundary wooden elements
This involved installing wooden elements in the wall plane on both faces. These were interconnected by means of bolts whose hole was filled with cement mortar. This showed promising alternative for seismic retrofit.
These alternatives were found to improve seismic behavior, this provided limited structural continuity and generate certain level of confinement that reduced expected failure and delayed the collapse.
Durability
The study of durability of rammed earth was determined by measuring the mean erosion depth, conducted by Q.B. Bui, J.C.Morel et all.,[6] Stereo-photogrammetry method was used to measure the erosion of rammed earth.
The specimens were built in 1985, the method involved two photos taken from two different view point. The measurement on the image allowed an accuracy of T0.005mm. The wall was 1.1m high and 600mm on the sides. The erosion of the erosion of a rammed earth wall is not a linear function of time. During the first time after construction, the wall shows more erosion on the surface then the erosion stabilizes. This is because the non-linearity is due to the loss of compaction energy.