technical information on building materials

TI3M - 27

FOR USE Ii\T THE DESIGN OF LOW- COST HOUSING

THE NATIONAL BUREAU OF STANDARDS

UNITED STaTSS DEPARTMENT OF COMMERCE

WASHINGTON, D. 0,

November 3, 1936

BIS HARDENED 'TALL PLASTER

This is the third in a series of three sections of a digest

of Circular 0151, ’’Fall Plaster: Its Ingredients , Preparation

and Properties’’, (January 9, 1924), i issued by the Bureau of

Standards, dealing with the physical properties and defects of

hardened plaster.

Pr operties

Eire Resistance: The fire resistance of plaster or plastered

constructions is of importance chiefly where used in segregating

portions of buildings, such as attached garages for dwellings.

Plaster is of value as a fire resistive material only so long as

it remains in place. The use of lime plaster is limited in this

respect because it generally falls off on the side of the partition which is exposed to fire. In plastering to resist fire, the

backing or plaster base is also important.

Acoust ics : The transmission of sound through a wall depends

not only upon the plastering, but also on the entire construction

of the wall. Other tilings being equal, less sound will be transmitted by a heavy wall than by a lighter one. Of two walls of

equal weight, the thicker will transmit less sound. In figuring

either weight or thickness, the plastering should he included as part of the wall. A hollow wall, except for the bridging, is

more effective than a solid wall in preventing transmission, provided the air space is introduced without sacrificing either

weight or thickness of the solid material. The acoustical

properties of a wall (or ceiling) are more complex than described

above, hut the subject is too bread to be covered in this digest.

Available

(Price 10 cents

from Superintendent of Documents, Washington, R* C.

11253S-C 1
.

'
Volume Change: Plaster responds to changing atmospheric conditions by expansion or contraction, even after it has hardened, These

’’movements'1 of plaster are generally slight and not injurious unless

emulative, Extreme expansion will cause the plaster to bulge and

finally to break away from, the backing, Contraction may cause cracks

in the plaster, resulting in separation of the coats.

hardness . fall plaster is frequently obliged to resist impact

or abrasion. Its hardness may oe considered from the standpoint of

its ability to resist such forces. Resistance to impact is a function of the complete wall rather than that of the surface only.

Resistance to abrasion may be considered as a function of the surface finish, hny of the usual cementitious materials should harden

to a satisfactory degree, provided they are sufficiently compacted

by troweling, and that atmospheric conditions do not interfere with

their hardening.

Resistance to hater; The leakage of water, usually resulting

from defective plumbing or flashing, is a kind of abuse to which

plaster is frequently exposed. If the wood lath was not thoroughly

saturated when plaster was applied, it may become so because of

the leak, which “ill cause it to swell beyond its original dimensions and break the keys of the plaster,

water standing in contact with metal lath may cause corrosion,

particularly if stray electric currents are also present. This

effect may be minimized by using painted or galvanized lath, and

by using plaster of such a nature that the water forms an alkaline

solution, this is the case with portiand cement or line plaster,

and with gypsum plaster containing lime. Better protection is

afforded if such lath is Completely embedded in the plaster.

Continued seepage of water through plaster may cause failure

due to the solution and disintegration of the cementitious material,

or local expansion. Gypsum is somewhat soluble under these circumstances, while carbonated lime is little affected, and portiand

cement is improved by water. ’.later issuing from a leak usually

saturates the plaster over a limited area only. The plaster within

the area tends to expand, but is confined by the dry plaster sur- rounding it with the result that the wet plaster bulges out, cracks,

and falls off.

"here continuously damp conditions are anticipated, portiand

cement plaster should be used.

Becor stive finishes: Whether the plaster is decorative of

itself, or whether it acts merely as a base for the decorative

paint or caper, its most important function is that of giving a pleasing appearance to the room.

2
Three-coat plaster should ordinarily be used where a smooth white

finish is desired,

A sand float finish may he obtained with a two-coat plaster job,

omitting the white finish coat, A wall finished. in this way may he

made structurally sound and is considerably cheaper than three-coat

plaster. A large variety of finishes may he obtained with the use of

various wood and steel floats, exposing such aggregates as colored

sand, and mineral pigments, hut care must he taken to select proper

materials and use exact proportions, The use of textured finishes

for a variety of decorative treatments has recently become a popular

practi ce

.

Paint ing :

•*- Painting or papering a wall is the most general

type of wall decoration, but certain conditions restrict and govern

their use. Plaster which is in immediate contact with an exterior

masonry wall should not he painted with an oil paint, as there is

a strong probability that mo is tore ’'/ill find its way through the

wall and into the plaster and cause the paint to peel off, When

plaster is applied on lath, rather than on masonry, it is safer to use oil paint , Plaster should he thoroughly dry when painted. If

painted before the reactions involved in the hardening process are reasonably complete, free lime, will he likely to saponify the

linseed oil, causing the paint film to become brittle, Free lime

is usually present in all green plasters, whether lime, gypsum, or cement

»

Papering : One distinct advantage in papering is that minor

surface cracks affecting the appearance, but not the durability of

the plaster, will not s'. ow through the paper. It is advocated,

however, that when walls are to be re-papered, the old paper be

entirely removed and the plaster scraped and pointed. The pointing of settlement cracks is a distinct aid in prolonging the life

of plaster.

Types and Causes of Defects: Aside from failures caused by

fire, or leaks, or by settlement of the building, well-made plaster

should give satisfactory service indef tritely, "hen failure does

occur, however, the important thing is to ascertain the reason for

such failure in order that it may be effectively repaired.

structural Grachs: A large prominent crack, extending across

the surface and through the plaster, is probably a structural

crack. It is caused b

r movement of the structural members of the

building, and is not a fault of the plaster. Although possible

to construct a building sufficiently massive and rigid so that

this type of crack could not occur , the more practical expedient

is to let the cracks form, and after the build. ing has "found

itself 7 ’, point them up.

1 See TIM - 18

3
Ma c Cracks: This type is loss prominent than are structural

cracks. They go through the plaster, hut do not extend entirely

across the surface. Instead, a system of cracks, running at various

angles, form mere or less geometric figures over the surface. These

figures are usually large and well defined, and are repeated at

various places on the surface. Hence the name, map cracks. They

are usually caused by a lack of uniformity in the bond between the

backing and the scratch coat, and may always be taken as an indication that the bond between the plaster and the backing is not uniform.

Shr inkage Cra

c

ks : Shrinkage cracks resemble map cracks, except

that both the cracks themselves and the areas which they surround

are much smaller. They do not run clear through the plaster, being

confined usually to the finish coat. If the finish coat is sanded,

shrinkage cracks indicate that the plaster was tco rich, that it was permitted to dry too quickly, or that it was not troweled enough.

Shrinkage cracks in a white finish indicate that it was troweled too

soon. Shrinkage cracks have little effect on any of the properties

of plaster except its appearance. A gloss paint should not be used

to cover such a surface, as this type of paint will cause the

shrinkage cracks to stand out in high relief.

Popping; Pops are small conical holes appearing on the plaster

surface. They are caused by the hydration of individual particles

in the plaster. Certain compounds of lime are not acted upon

readily by water, but when exposed for an extended period of time

will hydrate, this hydration being accompanied by excessive expansion. Whenever particles of such materials find their wav into

plaster, they remain apparently inert during the mixing, application, arid hardening. Sometime later the hydration begins. The

expansive force pushes the particle and everything in front of it

out of the well leaving the conical hole, called a "pop/'. If

hydrated lime is used, the responsibility of failure rests with

the manufacturer ; if quick-lime is used, the responsibility rests

either with the manufacturer or with the contractor who did the

slaking; if no lime is used, the quality of sand should be invest iQ C-v UO'U .

Expansion; It has been noted that map cracking is caused by

a differential movement of the plaster and the backing. This movement may cause expansion as well as contraction. The effect is

entirely different, however. If the bond between the scratch coat

and the backing has been broken over a small area and the plaster

is expanding, as a result of rising temperature or increasing humidity,

compressive stresses will be concentrated in the plaster over this

area. 'The first effect will be to cause the plaster to bulge out

from the wall. As this movement is usually quite slow, considerable

bulging may take place before failure actually occurs. Should tho

atmospheric conditions change ana the plaster begin to contract, it

may return to its original condition with no apparent damage. If

expansion continues, however, it -will eventually break the plaster.

The break will not be a clean crack such as caused by contraction,

but will be more in the nature of shattering.
Softness ; Plaster should be hard enough to withstand reasonable

abuse, and not rub off. If the plaster is soft, too little cementitious

material has been used.

Blotches and Efflorescence : Blotches are often caused by certain

salts in the plaster, which are soluble and hygroscopic, ns the mixing

water evaporates, these salts are brought to the surface and deposited.

Their hygroscopic properties cause them to absorb water from the air,

thus keeping' the plaster damp. Calcium chloride is typical of this

kind of salt. It is readily formed by interaction between lime and common salt, and is one reason why sea water or beach sand should not

be used for plastering.

If the salt is soluble but not hygroscopic, it will be deposited

on the surface of the plaster as a white crystalline substance which

may easily be rubbed off.

Sometimes a white finish shows mottled gray and white, the areas

of each color being so small that they are discernible only on close

inspection. This condition is the result of as ing lime and gypsum

of slightly different colors, which were not mixed as thoroughly as they should have been. These spots are permanent, but in :io way

affect the quality of the plaster.

Peeling of Paint; The peeling or blistering of paint is almost

always due to the presence of water in the plaster,! If this occurs,

the plaster may have been painted before it was thoroughly dry, or leaky water pipes may be the cause of the moisture in the plaster

which is causing the paint to peel.

Rec ommend ed Selection of Materials: The choice of materials is

usually a compromise between quality and cost, both initial and

maintenance. Certain constructions predetermine the type of plaster

backing required, but ordinarily the purchaser is froe to select the

backing and other plaster materials. To serve its full purpose,

plaster should be as serviceable and durable as the rest of the con- struction.

Plastering is classified below as four types, A, B, C, and D,

determined by the purposes to be served on the basis of the ratio

of quality to cost. This information may be helpful in the selection

of plastering materials.

1L Plaster used in public buildings, monumental work, etc.,

where quality is placed above cost, and appearance and durability

are paramount: —

1 See Tim - 18 .

_ r _
Exterior masonry walls should be furred with tile or heavy metal

lath on metal strips. Gauged lime or gypsum plaster should be used

throughout, with three coat work 7/8 inch on lath, and two coat work

5/8 inch on masonry and tile. Partitions should be of tilo or heavy

metal lath on metal studs. Veiling plaster should oe put on in thin

coats on heavy metal lath. G11 concrete the aggregate should be

thoroughly exposed. If the masonry ceiling is of gypsurn tile, gypsum

plaster should be used. If the ceiling is of clay tile, either lime

or gypsum may be used.

3 . Plaster used in buildings of very good construction but

designed to earn a reasonable return, saich as office buildings,

apartments, first class industrial buildings, etc.:

Plastering for these buildings should be practically the same

as for class "A ' except that medium weight metal lath may he used

and, in some cases, 3/8 inch plasterboard on partitions. Glass "a”

plastering should be used for ceilings.

C. Plaster used for buildings where economy cf first cost is

the chief consideration and where maintenance costs are to be

minimized

:

This type is used in small office buildings, apartments, some factories and dwellings, and is probably the most commonly used

type. Plastering is the same as for type V except that light weight

metal lath, ho. 1 wood lath, or 3/8 inch plasterboard may be used on

all walls. For ceilings, medium weight metal lath should be used

unless they are of masonry, in which case type "A ’ plastering should

be used.

_D. Plaster used for buildings where minimum quality with the

maximum of satisfactory service is desired:

In this type, furring1 is unfortunately sometimes omitted, and

plastering (two coats 5/8 inch thick) is applied directly to outside

masonry walls. Inside masonry walls may be finished with a single

heavy coat of plaster. A 3/4 inch three-coat or 5/8 inch two-coat

plaster is recommended on light weight metal or wire lath, Wo. 2 wood

lath, or l/4 inch gypsum board on wood furring and studs. Ceilings

should be of either medium weight metal lath or 3/6 inch plasterboard on joists or furring strips spaced not over 16 inches apart

on centers, with plaster similar to that used for walls.

Lime and gypsum plasters represent a large percentage of the

plaster generally used because the use of oortland cement plaster is

limited, being more particularly adapted to places constantly subjected to moisture. Portland cement plaster is not sufficiently

plastic to be economical, and for finish coats, where a white coat

is necessary, white Portland cement should be used.

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