For a trowel-finish coat using lime-Keene’s cementplaster, the recommended proportions are, for amedium-hard finish, 50 pounds of hydrated lime or100 pounds of lime putty to 100 pounds of Keene’scement. For a hard finish, the recommended proportionsare 25 pounds of hydrated lime or 50 pounds of limeputty to 100 pounds of Keene’s cement.For a trowel-finish coat using lime-portland cementplaster, the recommended proportions are 200 prodsof hydrated lime or 5 cubic feet of lime putty to94 pounds of Portland cement.For a finish coat using portland cement-sand plaster,the recommended proportions are 300 pounds of sandto 94 pounds of Portland cement. This plaster may beeither troweled or floated. Hydrated lime up to10 percent by weight of the portland cement, or limeputty up to 24 percent of the volume of the portlandcement, may be added as a plasticizer.For a trowel-finish coat using gypsum gauging orgypsum neat plaster and vermiculite aggregate, therecommended proportions are 1 cubic foot ofvermiculite to 100 pounds of plaster.EstimatesThe total volume of plaster required for a job is theproduct of the thickness of the plaster times the net areato be covered. Plaster specifications state a minimumthickness, which you must not go under. Also, youshould exceed the specs as little as possible due to theincreased tendency of plaster to crack with increasedthickness.Mixing PlasterThe two basic operations in mixing plaster aredetermining the correct proportions and the actualmixing methods used.PROPORTIONS.— The proper proportions of theraw ingredients required for any plastering job are foundin the job specifications. The specs also list the types ofmaterials to use and the type of finish required for eacharea. Hardness and durability of the plaster surfacedepend upon how accurately you follow the correctproportions. Too much water gives you a fluid plasterthat is hard to apply. It also causes small holes to developin the finish mortar coat. Too much aggregate in the mix,without sufficient binder to unite the mixture, causesaggregate particles to crumble off. Without exception,consult the specifications prior to the commencementof any plaster job.MIXING METHODS.— As a Builder, you will bemixing plaster either by hand or using a machine.Hand Mixing.— To hand-mix plaster, you will needa flat, shallow mixing box and a hoe. The hoe usuallyhas one or more holes in the blade. Mixed plaster istransferred from the mixing box to a mortar board,similar to that used in bricklaying. Personnel applyingthe plaster pick it up from the mortarboard.In hand mixing, first place the dry ingredients in amixing box and thoroughly mix until a uniform color isobtained. After thoroughly blending the dry ingredients,you then cone the pile and add water to the mix. Beginmixing by pulling the dry material into the water withshort strokes. Mixing is continued until the materialshave been thoroughly blended and proper consistencyhas been attained. With experience, a person squires afeel for proper consistency. Mixing should not becontinued for more than 10 to 15 minutes after thematerials have been thoroughly blended. Excessiveagitation may hasten the rate of solution of thecementitious material and reduce initial set time.Finish-coat lime plaster is usually hand-mixed on a5- by 5-foot mortar board called a finishing board.Hydrated lime is first converted to lime putty by soakingin an equal amount of water for 16 hours. In mixing theplaster, you first form the lime putty into a ring on thefinishing board. Next, pour water into the ring and siftthe gypsum or Keene’s cement into the water to avoidlumping. Last, allow the mix to stand for 1 minute, thenthoroughly blend the materials. Sand, if used, is thenadded and mixed in,Machine Mixing.— For a quicker, more thoroughmix, use a plaster mixing machine. A typical plastermixing machine (shown in fig. 7-11) consists primarilyof a metal drum containing mixing blades, mounted onFigure 7-11.—Plaster mixing machine.7-11
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