The appropriateness of Earth as a building material is undisputed. However, its acceptance as pucca construction is still limited, due to its susceptibility to water erosion and resultant loss in compressive strength.

In regions with low or average rainfall, protection against rain can be provided simply by taking suitable design precautions as shown in the diagram; however in areas with high and driving rain, stabilisation of the wall material and/ or protective coverings on the walls become essential. Stabilised earth blocks stand up better to moisture ingress but very often their cost becomes a limiting factor. Plasters, on the other hand, being surface treatments are cheaper in terms of initial capital investment.

Moreover, plasters provide a wide scope for surface treatment and decoration and are a very popular form of self-expression.

Many traditional forms of plasters have been developed for the protection of earth walls.

Traditional plasters of soil, straw and cowdung which are very effective and popular even today, need recurring maintenance. This has never been a problem in the rural areas where annual/biannual replastering (lipai) of the house by the houseowners themselves is a rule rather than an exception. However, lifestyles are changing fast and people are looking more and more towards building which need lesser maintenance and care. Therefore, longer lasting plasters which keep looking good while being economical are required.

Plasters increase longevity and performance of earth walls by:

Forming a weather barrier against moisture ingress.

Increasing the resistance of the wall to wear and tear.

Improving appearance and decreasing maintenance.

The performance of a plaster depends upon the type of the wall surface as well as the materials used in the plaster. A difficulty with conventional cement based plasters is that they are not compatible with earth walls. Earth structures also react significantly to temperature changes and levels of humidity. Alongwith thermal movements in the walls, they absorb and release moisture. Conventional plasters of cement and sand are relatively rigid and unable to respond to the movements of the wall material. The bond between the two eventually weakens and the plaster tends to crack and fall off. Therefore, a plaster mix should perform its function of a good protective cover while accommodating movements within the wall. In essence it should ‘allow the wall to breathe’. It has been found with experience that soil based plasters behave best on soil walls and are inexpensive too.

A variety of plaster mixes and techniques have been developed to improve the quality of soil based plasters. These soil based plasters work in the following three ways:

By reducing the absorption of the material so that it prevents the moisture from reaching the wall underneath even while getting wet itself. This id done by decreasing the surface pores or by applying a thicker coat. A thick plaster may however, slump off the wall due to its own weight and in areas with continuous rainfall the plaster still transmits moisture to the wall.

Stabilisation of the plaster makes it more water resistant. Stabilisers like cement, bitumen and lime are added to soil in small quantities. Such plasters are longer lasting and more flexible than conventional cement plasters. Limitations to the use of these plasters are the availability and cost of the stabiliser and the skill involved in its use.

Application of a surface coat over the plaster or exposed mud wall surface to make the surface non-absorbent. In this category are industrial paints and polymers, which tend to flake and peel off over time due to ultraviolet radiation. Natural substances like resins and cowdung-slurry need routine maintenance. Recently developed silicon based materials get absorbed in the wall and reduce its surface tension making it hydrophobic. These materials have shown very promising results but their long term performance and life span is still under test.

Whatever be the type of plaster, general rules of plaster application have to be followed. These include :

Preparation of the wall surface to receive the plaster coat. This may be done by grooving all horizontal mortar joints to a depth of 1 cm.. to provide keys for a greater wall to plaster bond and allow direct load transfer of the plasters’ self weight as opposed to simple adhesion between the two vertical surfaces. The same could also be achieved by facing the wall with bits of broken potter, tiles etc. which project from the wall surface.

Brushing the surface clean to remove loose pieces, dirt etc.

Filling cracks with mud slurry and allowing the surface to dry.

Applying a thin coat of the slurry of the binding agent (in case of stabilised plasters) prior to the application of the plaster.

Moistening the surface before plastering

The plaster is then applied with force and spread smooth in plumb by the mason’s phanti. Horizontal joints are normally avoided and vertical joints are never made at the corners. Two thinner layers of soil based plasters work better than one thick layer. The plaster must be allowed to dry slowly. Therefore, it is best to apply the plaster when the wall is shaded from direct sunlight and to avoid plastering on very hot or windy days. It is very clear that workmanship and quality of the raw material are as important a determinant of plaster quality and its performance as the compatibility of the mix to the wall. Improved plaster mixed and techniques can significantly upgrade the overall performance of an earth wall, but cannot completely replace the need for maintenance.

A variety of plasters have been developed by R&D institutions and individuals active in this field. Some of these recipes are:

Bitumen cutback plaster (CBRI): The bitumen cutback is prepared by mixing hot bitumen (having 80/100 penetration) with kerosene in a ration of 5:1. One part of this cutback is mixed with 20 parts by volume of a previously fermented (for 10 days or so) soil and bhusa mix (1:1 in ratio). Water is added and it is well pugged by feet. It is applied in two layers, the second after the first has dried. The plaster has shown very good results with a life of at least 7 years.

Lime-soil plaster (ASTRA, IIS, Bangalore):One part of hydrated lime is mixed to 2 parts of soil and to 3-6 parts of sand depending on the clay content of the soil. Before application the surface has to be well prepared and a base coat of lime wash applied. After plastering the surface must be rendered with lime. The performance of this plaster depends on the quality of lime available and the type of soil. It works best with red soils.

Traditional lime-sand-surkhi plasters in 1:2:4 ration also work very well on earth walls. Their use is however limited by the non-availability of good quality lime, the skill required to use it and the cost of the plaster.

Surface Coats: Most soil based plasters absorb moisture to a greater or lesser degree. When a suitable time large is available between two consecutive rain showers the wet plaster gets time to dry out. But, during continuous and long duration rains, even good plasters moisture upto the wall underneath. In such a case a surface coat applied over the plaster proves helpful. The function of the surface coat towards the plaster is the same as that of the plaster to the wall. This is a thinner coat and is cheap and easy to maintain; it also helps avoid frequent plaster replacements.

Traditionally a lipai of cowdung and soil in a ration 1:1 has been the most popular surface coat used over soil walls in rural area. Besides the strong belief in the rural areas on its hygienic qualities, it is easy to apply and easily available. It is redone as frequently as every month. Other natural surface coats include the resin from the Accacia arabica tree (very commonly found in India), certain gums milk products like cassien (and even blood and urine of some animals) and mixed in soil.

The in-situ calcium soap coating developed by ASTRA is also very effective and economical.

Industrial paints and polymers too, are gaining acceptance; but their use will be limited because of cost. The silicon based compound Repellin S-101 has shown very promising results. It is not very expensive and being transparent does not spoil the natural colour and character of earth walls. Its long term performance and life are under test.

by Zeenat Niazi

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