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Table 7-1.Base Coat Proportions for Different Types of Work
Handling  Materials

Builder 3&2 Volume 02 - Construction manual for building structures
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For a trowel-finish coat using lime-Keene’s cement plaster,  the  recommended  proportions  are,  for  a medium-hard finish, 50 pounds of hydrated lime or 100 pounds of lime putty to 100 pounds of Keene’s cement. For a hard finish, the recommended proportions are 25 pounds of hydrated lime or 50 pounds of lime putty to 100 pounds of Keene’s cement. For  a  trowel-finish  coat  using  lime-portland  cement plaster, the recommended proportions are 200 prods of  hydrated  lime  or  5  cubic  feet  of  lime  putty  to 94 pounds of Portland cement. For a finish coat using portland cement-sand plaster, the  recommended  proportions  are  300  pounds  of  sand to 94 pounds of Portland cement. This plaster may be either   troweled   or   floated.   Hydrated   lime   up   to 10 percent by weight of the portland cement, or lime putty up to 24 percent of the volume of the portland cement,  may  be  added  as  a  plasticizer. For  a  trowel-finish  coat  using  gypsum  gauging  or gypsum neat plaster and vermiculite aggregate, the recommended  proportions  are  1  cubic  foot  of vermiculite to 100 pounds of plaster. Estimates The total volume of plaster required for a job is the product of the thickness of the plaster times the net area to  be  covered.  Plaster  specifications  state  a  minimum thickness,  which  you  must  not  go  under.  Also,  you should exceed the specs as little as possible due to the increased  tendency  of  plaster  to  crack  with  increased thickness. Mixing  Plaster The  two  basic  operations  in  mixing  plaster  are determining  the  correct  proportions  and  the  actual mixing methods used. PROPORTIONS.— The proper proportions of the raw ingredients required for any plastering job are found in the job specifications. The specs also list the types of materials to use and the type of finish required for each area. Hardness and durability of the plaster surface depend  upon  how  accurately  you  follow  the  correct proportions.  Too  much  water  gives  you  a  fluid  plaster that is hard to apply. It also causes small holes to develop in the finish mortar coat. Too much aggregate in the mix, without  sufficient  binder  to  unite  the  mixture,  causes aggregate particles to crumble off. Without  exception, consult  the  specifications  prior  to  the  commencement of any plaster job. MIXING METHODS.— As a Builder, you will be mixing plaster either by hand or using a machine. Hand Mixing.— To hand-mix plaster, you will need a flat, shallow mixing box and a hoe. The hoe usually has one or more holes in the blade. Mixed plaster is transferred from the mixing box to a mortar board, similar to that used in bricklaying. Personnel applying the plaster pick it up from the mortarboard. In hand mixing, first place the dry ingredients in a mixing box and thoroughly mix until a uniform color is obtained. After thoroughly blending the dry ingredients, you then cone the pile and add water to the mix. Begin mixing by pulling the dry material into the water with short  strokes.  Mixing  is  continued  until  the  materials have  been  thoroughly  blended  and  proper  consistency has been attained. With experience, a person squires a feel  for  proper  consistency.  Mixing  should  not  be continued for more than 10 to 15 minutes after the materials have been thoroughly blended. Excessive agitation  may  hasten  the  rate  of  solution  of  the cementitious material and reduce initial set time. Finish-coat lime plaster is usually hand-mixed on a 5-  by  5-foot  mortar  board  called  a  finishing  board. Hydrated lime is first converted to lime putty by soaking in an equal amount of water for 16 hours. In mixing the plaster, you first form the lime putty into a ring on the finishing board. Next, pour water into the ring and sift the gypsum or Keene’s cement into the water to avoid lumping. Last, allow the mix to stand for 1 minute, then thoroughly  blend  the  materials.  Sand,  if  used,  is  then added  and  mixed  in, Machine Mixing.— For a quicker, more thorough mix, use a plaster mixing machine. A typical plaster mixing machine (shown in fig. 7-11) consists primarily of a metal drum containing mixing blades, mounted on Figure  7-11.—Plaster  mixing  machine. 7-11







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