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The development of the cracks is often a subject that raises numerous questions. Cracks may appear in structures, either at the initial stage or with time. It should first of all be accepted that cracks of some type are unavoidable in any structure.
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The Moisture Change in building materials simply define as materials capable of absorbing water expands and contract on drying. Nearly, all building materials (e.g. Concrete, mortar, burnt clay brick, timber, plywood etc.,) are porous in their structure in the form of inter-molecular space. This type of materials have tendency to expand when absorb moisture from atmosphere and shrink on drying. In addition, it can be said that during monsoon, most of the building materials absorb moisture and vice versa, during winter/summer they shrink.
The above described movements are reversible i.e. cyclic in nature and are caused by increase or decrease in the inter-pore pressure with moisture change. The extent of movement depends upon molecular structure and porosity of a material.
Apart from reversible movement, certain materials undergo some irreversible movement due to initial moisture changes after their manufacture or construction. The incidences of irreversible movement in materials are shrinkage of cement and lime based materials on initial drying i.e. initial shrinkage/plastic shrinkage and expansion of burnt clay bricks and other clay products on removal from kilns i.e. initial expansion.
Consequently, if building materials used possess expanding or contracting natures, then existing cracks increase or sometimes new cracks also get formed.
From consideration of moisture movement of reversible nature, materials can be broadly classified as under:
|01.||Very Small Moisture Movement||Burnt Clay ricks, Igneous Rocks, Limestone, Marble, Gypsum Plaster, Metals etc.||The use of these materials does not require for much precautions.|
|02.||Very Small Moisture Movement||Burnt Clay ricks, Igneous Rocks, Limestone, Marble, Gypsum Plaster, Metals etc.||The use of these materials does not require for much precautions.|
|03.||Small to Moderate Moisture Movement||Concrete, Sand-Lime Bricks, Sandstones, Cement and Lime Mortars, etc||In the use of these materials some precautions in design and construction are necessary.|
|04.||Large Moisture Movement||Timber, Block Boards, Plywood, Wood-Cement Products, Fibrous Boards, Asbestos Cement Sheets, etc.||Special techniques of treatment at joints as surroundings, and protective coats on surface are required.|