Plaster Finishes

Prepared on the building site, plaster finishes are classified as wet-type construction.
They take longer to complete than dry-type construction, because they are field mixed with water and require curing before decorative materials, such as paint or wallpaper, can be applied. Plaster finishes are selected nevertheless because they are hard, abrasion resistant, rigid, incombustible, and provide a monolithic (unseamed) surface, even at corners. They are relatively brittle, however, and must be properly applied to avoid cracking when movements due to drying shrinkage or thermal changes are restrained.

Components of Plaster

A plaster finish consists of a supporting base, such as masonry or lath, and one or more coats of plaster or mix of plaster and other ingredients with water that is troweled or machine sprayed over the base.
The principal ingredient of plaster usually is gypsum but may be portland cement.
(Portland-cement plaster, or stucco, is discussed in Art. 11.19.) Gypsum plasters generally are formulated to meet the requirements of Standard Specification for Gypsum Plasters, ASTM C28, or Standard Specification for Gypsum Veneer Plaster, ASTM C587.
C28 gypsum plasters include ready-mixed, neat, wood-fiber and gaging plasters.
They may be applied over a masonry or lath base, generally in two or more coats, with a total thickness exceeding 1⁄2 in. They are required to contain 66% or more by weight of CaSO4  1⁄2H2O.
Veneer plasters must be applied over a special gypsum base, meeting the requirements of ASTM C588, and are limited in thickness to a maximum of 1⁄4 in.
They are usually selected because of low cost, rapid installation (permitting application of decorative materials 24 hr after plastering), and high resistance to cracking, nail popping, and impact and abrasion failure. The finishes are, however, not so rigid as conventional lath and plaster. Also, veneer plasters are more susceptible than gypsum plasters to ridging and cracking at joints when dried too rapidly because of low humidity, high temperature, or exposure to drafts.
Application of gypsum and veneer plasters should meet the requirements of the following ASTM specifications:
C841. Installation of Interior Lathing and Furring
C842. Application of Interior Gypsum Plaster
C843. Application of Gypsum Veneer Plaster
C844. Application of Gypsum Base to Receive Gypsum Veneer Plaster
See also Art. 11.27.

Plaster Mixes

Plaster coats other than veneer plasters are generally composed of gypsum plaster, lime, an aggregate (sand, vermiculite, perlite), and water.
Sisal or synthetic fibers, such as nylon, may be added to some scratch-coat plasters for application to metal lath, to limit to what is needed for good bond the amount of plaster that passes through the lath meshes. Fibering, however, adds no strength.
Sand should comply with ASTM C35. It should be clean, free of organic material, more than about 5% clay, silt, or other impurities, and should not contain salt or alkali. The proportion of sand in the plaster has an important bearing on the characteristics of the product. Oversanding results in considerable reduction in strength and hardness. A mix as lean as 4:1 by weight should never be used.
The Gypsum Association suggests that a 1-ft3 measuring box be used for preparing mixes. Some plasterers use a No.2 shovel, which holds about 16 lb of moist sand, for maintaining proper proportions. Thus, with each 100-lb bag of plaster, a 1:2 mix requires 12 shovels of sand and a 1:3 mix 18 shovels (Manual of Gypsum Lathing and Plastering).
Water should be clean and free of substances that might affect the rate of set of the plaster. It is not advisable to use water in which plasterers tools have been washed because it might change the set. Excessive water is undesirable in the mix, because when the water evaporates, it leaves numerous large voids, which decrease the strength of the plaster. Hence, manufacturers recommendations should be observed closely in determining water requirements.
Perlite and vermiculite are manufactured lightweight aggregates that are used to produce a lightweight plaster with relatively high fire resistance for a given thickness.
Both aggregates should conform with ASTM C35, and the mix should be prepared strictly in accordance with manufacturers recommendations.

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