The conventional method of constructing mud walls by the rural people includes adding water to the available soil and piling lumps of the resulting mud, one over the another, until the wall is raised up. The mud wall is plastered with the paste of the same soil inixed with small quantities of cowdung and straw. This method of raising the wall facilitates early deterioration, and the wall often is not strong enough to support a roof. This is why the roof is independently supported by 6 or more bamboo or casurina posts. This type of wall construction should, therefore, be discouraged.
Soil is used as a construction material in two other types of construction, namely, rammed earth and adobe or sundried bricks or blocks.
In the rammed earth type of construction the soil is rammed between wooden moulds to construct a solid mass of wall with the help of side shuttering. The shuttering can be moved horizontal y or vertically until the wall is completed. When earth is rammed into a confined space at the optimum moisture content, its resistance to erosion by rain is increased. Furthermore, the wall will be of uniform thickness and attain versatility. But, experience shows that point or such a wall may crack under concentrated loads coining from the thatch or tiled roofs. This type of construction with mud is best avoided.
Thc term adobe originated from the spanish word meaning a building material consisting of loam, straw and such other materials. In adobe type of construction, the usual practice is to mould the earth into blocks and build the wall with these blocks. This method provides for the shrinkage of these small units (blocks) to take place before being laid into a wall. This type of construction is quite popular in Mexico and some South American countries. However, Indian construction practice has, for some reason, avoided adobe constructions.
Sun-dried bricks are popular in India for semi-permanent buildings in certain areas. Sun-dried bricks are moulded from clay and dried in the sun. These are, however, incapable of withstanding the softening and erosive action of rain water. Tests have indicated that if coal tar is added at the rate of 0.5 to 1 percent (by weight of soil) to the clay, both the erosion resistance and compressive strength of walls stand improved. This brings us to the concept of stabilisation of soil with small quantities of tarhitumen or lime/cement.
Stabilized Mud Blocks
An appropriate type of chosen soil, when compacted at optimum moisture content, can be made strong and durable by the addition of a stabilizing agent. Stabilisation enables the soil to retain its shape and a significant proportion of its strength even when wetted to the point of saturation. Various types of stabilising agents have been used. Some of them are substances which act as water-proofing agents; the most important of these being bitumen, asphat and certain resins. Also, there are admixtures which acquire and retain rigidity on the addition of water and provide a rigid skeleton to the compacted soil; of this type, portland cement is the most commonly used. There is now a large fund of experience with cement as a stabilising medium. Moreover, there are substances such as lime whicli react chemically and physically with certain constituents of the soil and strengthen it.
The quantity of cement added to soil for stabilisation varies from 2.5 to 5 percent by volume. Sandy soils require less amount of cement for stabilisation, while clayey soils need more. It has been found that the use of 1% lime and 1% cement in a stabilised soil mass gives better results than the addition of 2.5% cement.
Out of the various industrial organic products tried so far, the use of molasses (jaggery) has been found to be the most effective for improving the compressive strength and reducing permeability of soils. For stabilisation purposes, the total quantity of material normally added to the soil mass is 5% by weight.
It is important to note that naturally available building materials such as mud can be improved by the addition of small quantities of costlier factory made materials. This is cost and energy wise, cheaper than using burnt bricks. Here, it is important to note that the load coining from thatched and tiled roofs on the wall does not require the use of high-strength masonry made in lime or cement mortar.
Stabilised soil is generally made into blocks of 30 cm x 15 cm x 10 cm or 30 cm x 20 cm x 10 cm in a hand operated block-making press, such as the Ellson Block Master, which is made by Kathiawar Metal mn Works Ltd., Rajkot, and its cost is of the order of Rs. 15,000. More recently, an improved version has been designed and marketed by Aeroweld Industries, B-9 HAL Industrial Estate, Bangalore - 560 037. The size of the blocks is 30.5 cm x 14.4 cm x 10 cm or 23 cm x 19 cm x IOcm. One such machine can be utilised to make 300 - 500 blocks per day by four unskilled workmen. Average number of blocks required for a two-roomed house is about 2500.
If these machines can be hired out to the house builder for one week, he can make the required number of blocks for building his rural house. The compressive strength of these blocks varies from 1.5 to 3.5 m' which is adequate to carry the weight of tiled-roofs. These houses can be plastered with combination mortar of 1 cement : 1 lime : 6 sand on the outside and then, white washed. On the inside, stabilised mud plaster can be used. As a civil engineer, you can acquire one such machine and propagate this system of construction in rural areas.
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