Building Design and Construction Handbook - Merritt - Ventech!

11.54 SECTION ELEVEN
11.25.2 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-ft 3 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.
11.25.3 Mixing Plaster
A mechanical mixer disperses the ingredients of a mix more evenly and therefore
is to be preferred over box mixing. Recommended practice is as follows: (1) Place
the anticipated water requirements in the mixer; (2) add about half the required
sand (or all required perlite or vermiculite); (3) add all the plaster; (4) add the rest
of the sand; (5) mix at least 2 mm, but not more than 5 mm, adding water, if
necessary, to obtain proper workability; and (6) dump the entire batch at once.
The mixer should be thoroughly cleaned when it is not in use. If partly set
material is left in it, the set of the plaster might be accelerated. For this reason
also, tools should be kept clean.
For hand mixing, first sand and plaster should be mixed dry to a uniform color
in a mixing box, water added, and the plaster hoed into the water immediately and
thoroughly mixed. Undermixed plaster is difficult to apply and will produce soft
and hard spots in the plastered surface.
Plaster should not be mixed more than 1 hr in advance. Nor should a new mix,
or gaging, be mixed in with a previously prepared one. And once plaster has started
to set, it should not be remixed or retempered.
11.25.4 Plaster Drying
A minimum temperature of 55�F should be maintained in the building where walls
are to be plastered when outdoor temperatures are less than 55�F, and held for at
least 1 week before plaster is applied and 1 week after the plaster is dry.