Literature Review[edit source]
This is a review of the available literature pertinent to the cob and earth building at HSU library.
Cob basics[edit source]
Cob is made from a combination of water, clay, sand/small stones, and straw. Cob is a very ancient building material and has been used throughout history because its durable, cheap, and easy to apply. [1] Cob, if one were to make it out of pure ingredients, would be 70 percent sand, 20 percent clay, and 10 percent water. [1]One aspect of cob that makes it so unique and special is that it can be made anywhere, as long as the components can be shipped or trucked or flown into the area. Some cool facts about cob is that it is non-toxic and completely recyclable, which can be a nice relief to the health hazards and carcinogens contained in recently utilized building materials like asbestos and lead paint.[1] Also cob is environmentally sound; it makes no contributions to deforestation, pollution, or mining and no power tools or manufactured materials are needed to make a cob structure.[1] The word cob is rooted in Old English, coming from a word meaning "a lump or rounded mass" and are completely built by hand. Since cob structures require no frame, the builder has an infinite amount of freedom and creativity to proceed with in the design of the structure, much like building with clay.
Cob Conerns and Benefits[edit source]
Some concerns of building with cob involve getting the right methods for retrieving soil, [2] so with a few exceptions, earth suitable for construction comes from the subsoil layers. The topsoil layer, because it contains organic matter which will continue to decompose and change, and which sometimes has a negative effect on certain stabilizers, is not normally used for cob construction. Another concern is that cob walls in general, have good strength in compression but they have very weak strength in regards to tension, so consequently they do not have great resistance to bending over time. [2]Also the lack of tensile strength and resistance to bending causes load-bearing earth buildings to have the reputation of being prone to damage and collapse in an earthquake.[2] The lack of tensile strenghth means that when earth is to be used as a load-bearing material, all forces must pass down within the thickness of the structure to the ground. In regards to cob building, the main constraint is the slenderness of the wall, since the lack of resistance to bending has to be compensated for by greater width in the wall to ensure stability.[2] What this means is that load bearing walls not stabilized by wood studs should be thick and massive. More concerns to take into consideration are that cob walls lose much of their compressive strength when exposed to prolonged rain or moisture like what we experience often in Humboldt. Some Benefits of building with cob include climatic comfort, because thick earth structures are good insulators due to earth's low thermal conductivity value.[2] Earth is a building material with the capacity for making buildings of very high quality and great durability and has also been used very successfully for centuries in countries with wet climates(British Isles) as much as it has in dry countries (Niger,Sudan).
Types of Cob Building[edit source]
[3] There are many different popular styles of earth building including: Adobe(which involves bricks that are molded wet and cast small enough to shrink without cracking),Pise De Terre(originating from France),Tapial(which involves packing soil into wooden framework),Wattle and Daub(which tapial evolved from, is a woven framework of twigs and a mud covering applied to the framework)and finally cob, in which clayey soils are mixed into sticky mud, then laid up directly into a free-standing wall. As far as building with earth materials is concerned,human beings have exercisized limitless styls and types of structures for thousands of years.
Designing cob buildings[edit source]
[1] Cob is fireproof, so fireplaces, stoves, and chimneys can be shaped with it. Cob beds and benches can be placed near the stoves and frieplaces to be warmed. Usually the walls of a cob home are whitewashed or stuccoed over. This undoubtedly helps to preserve them, protecting especially against harsh winds.[1] Both the composition of cob and the positioning of the house/cob structure affect the electricity consumed in the structure. It is more energy efficient to position the house so that it faces the south(in the northern hemisphere)and north(in the southern hemisphere), with big windows in this direction.[1]This way the big south/north facing windows and solar panels, if employed, can capture sunlight and then the thermal mass of the cob can absorb and make use of the solar energy.[4]In similar conditions of this study, the stabilisation by cement or lime is inadequacy. First, the stabilisation does not enable the recycling of the material that is not a positive point in sustainable development. In addition, the use of cement or lime meets also the economic problem and the availability of these materials in the market in some countries and regions. That is why the development of new suitable tests to evaluate the durability of rammed earth following different climatic conditions is also important to decide whether the stabilisation is necessary. The method of stereo-photogrammetry used to measure the erosion of rammed earth walls on site may also help to calibrate and develop more pertinent laboratory test to assess the durability of rammed earth wall.
References[edit source]
- ↑ 1.0 1.1 1.2 1.3 1.4 1.5 1.6 Field Guide to Appropriate Technology,first edition, Academic Press, Inc. 2003
- ↑ 2.0 2.1 2.2 2.3 2.4 Norton,John (1997). Building With Earth: A Handbook, Second Edition, Intermediate Technology Publications Limited, London,UK. p.3-8.
- ↑ Easton,David (1996).The Rammed Earth House, Chelsea Green Publishing Co., White River Junction,VT. p.3-146
- ↑ Q.B. Buia, J.C., Morela,B.V. Venkatarama Reddyc and W. Ghayada (May,2009). "Durability of earth rammed walls exposed for 20 years to natural weathering." Building and Environment,Volume 44,Issue 5,p.912-919.
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- ↑ Easton, David (1996). The Rammed Earth House, First Edition, Chelsea Green Publishing Co., White River Junction,VT,p.8-9.