Plastering

PLASTERING

Plastering in buildings refers to the process of applying mortar coats on the surfaces of walls, columns, ceiling to get smooth finish. Plastering covers defective workmanship in the construction of a given piece of masonry; and, also it conceals cheap/unsound quality of material used in building up the m onry. It is only after plastering that an appropriate base is ready to further decorate the surface by white- washing, colour-washing, distempering, or painting. Plastering is done to both the surfaces of a wall including the external face and on the internal face. Plaster is a sort of mortar obtained by mixing together materials like cement, lime, clay with fine aggregate and water. Mortar used for plastering may be lime mortar, cement mortar or lime-cement mortar.

OBJECTIVES AND REQUIREMENTS OF PLASTERING

The main objectives of plastering are:

❖To give smooth finish to the wall and RCC surface.

❖To protect the wall from rain water and other atmospheric agencies.

❖To conceal defective workmanship if any.

❖To protect surfaces against vermin.

❖The requirements of good plaster are:

❖It should adhere to the surface easily.

❖It should prevent infiltration by moisture.

❖It should be hard and durable.

❖It should be cheap.

GENERAL SPECIFICATIONS OF PLASTERING

Lime and cement are the two major constituents of plastering and hence the specifications for these two types of plastering are given below:

Lime Plastering:

Lime that is to be used in combination with cement for the purpose of making mortar, shall be ground dry. Stone lime is measured by weight and unslaked lime is measured when freshly burnt. Sand used in the mix should be clean, coarse and free from any admixture of clay, loam, salts, organic matter etc. Before plastering, the surface to be plastered should be prepared by cleaning out any loose materials resent on the surface. The joints shall be raked out thoroughly. Raking shall be done with a hook to a depth of 1.25 cm. It is easier to rake out the joints before the masonry mortar has set. After raking is done, and loose material washed off by water, the surface to be plastered shall be watered for 24 hours before the plaster is applied. If any chemical retarder has already been applied to the formwork, the surface should be roughened by wire brush, leaving no retarders behind on the surface. All putlog holes shall be filled up in advance of the plastering work as the scaffolding is being taken down. Pure fat lime shall be slaked and then immersed in water for at least 48 hours. Lime shall then be thoroughly stirred with water and strained through muslin. On settling, the surplus water shall be removed, and further water is allowed to evaporate, until the paste is thick enough for use. Lime mortar used for plastering shall have fat lime to sand ratio of 1: 3 or 1: 4. If hydraulic lime is used mix proportion (lime: sand) is 1:2.

Cement Plastering

Specifications of cement plaster shall be the same as for lime plastering. The thickness of cement plastering shall be such as 12 mm, 13 mm, 15 mm, or 20 mm. It shall be finished with a floating coat of neat cement. Smooth finishing shall be done with trowel. In two-coat plastering, when the first coat is set, the surface shall be scratched with a sharp tool to form a key for the next coat. This coat shall be floated and finished, in the same manner as the first coat. Where it is not required subsequently to paint, distemper, or colour wash the plaster, it shall be finished to the final colour required and polished, plasterer's putty being used for the finishing coat. Cement mortar of 1: 4 or 1 : 6 mix is very commonly used for plastering, richer mix being used for outer walls. To combine the cost effectiveness of lime mortar and good quality of cement mortar many use lime-cement mortar of proportion (cement :lime sand) of 1: 1:6 or 1: 1: 8 or 1: 2: 8. Lime mortar is usually applied in 3 coats while cement mortar is applied in two or three coats for the stone and brick masonry. For concrete surfaces cement mortar may be applied in two or three coats. For concrete building blocks only one coat of cement mortar is applied. The first coat provides means of getting level surface. The final coat provides smooth surface. If three coats are used second coat is known as floating coat. If single coat is used its thickness is kept between 6 to 12 mm. Such coats are used on concrete surfaces not exposed to rain.