TIBET - 19

TECHNICAL INFORMATION OR BUIIDIRG MATERIALS

FOE USE III THE DESIGN OF LOW-COST HOUSIHG

THE RATIONAL BUREAU OF STANDARDS

UNITED STATES DEPART! ISN'T OF COME.TERCS

WASHINGTON, D. 0.

July 13," 1936*

INVESTIGATIONS

PORTLAND

OF

JNT STUCCO CONSTRUCTION

This is a digest in part of Circular No. 311, 'Stucco Investigations at the Bureau of Standards with Recommendations for Portland

Cement Stucco Construction', (December 13, 1926), by ^raM n utcncocv

issued by the Bureau of Standards. The part selected deals with investigations only and will be followed by subsequent digests covering recommendations and finishes and maintenance.

As a result of the rapid and widespread growth in the use of

stucco, the Bureau of Standards, cooperating with interested associa^

tions and manufacturers, conducted numerous exposure and laboratory

tests to determine methods of improving such construct ion.

(1911) Investigations were begun in 1911 in cooperation with

the Associated Fetal Lath Manufacturers in an effort to determine

what factors caused corrosion and consequent failure of metal a embedded in stucco and plaster. Three hundred panels of various

plastering materials such as cement, lime and gypsum applied to

various metal bases were subjected to atmospheric exposure for two

years •

General conclusions reached after examination of these panels

and confirmed by subsequent observations were that oversanded mortar

or the imperfect embedment of the metal lath had more to do with the

development of rust than the nature of the cement itous materials. -- ese panels were retained throughout the period of the entire series of

tests and were still in excellent condition after 15 years exposure.

The metal bases were listed according to merit as follows:

^Available from Superintendent of Documents, Government Printing

Office, Washington, D. 0. (Price 15 cents)

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(1) Lath galvanized after expansion

(2) Lath cut from galvanized sheets

(3) Painted ingot iron lath

(4 } Painted steel lath

(5) Sherrardized steel lath

(6) Plain ingot iron

(7) Plain steel lath

(1915) These tests having aroused interest among manufacturers

and users of stucco, the Bureau of Standards in 1915 undertook a larger and more varied series of tests covering all types of stucco

construction,

Co Hah orating with those interested, a committee was set up to

act in an advisory capacity, and inspect and he responsible for the

satisfactory construction of the test panels,

A building 200’ long, 26 T wide and 25 T high was erected consisting of 56 exterior panels of typical combinations whose construction

and materials compared favorably with those used in good house con- struction, Most of the panels contained, at the center, a window

2* 6” x 3

f 6", The lower story was largely of masonry construction

while the second story was of frame.

The tests included a large variety of bases to which were applied

stucco of various proportions. Observations and reports were made by

the committee and conclus ions of a general nature were reached. Summarizing, ir may be said vhat monolithic concrete not coated with a bituminous compound was most satisfactory as a base, metal lath applied

directly to the studs and back-plastered was second, brick and tile

was third, metal lath over wood sheathing was fourth, wood lath was

fifth, and gypsum block and plasterboard was last.

All the panels over diagonal sheathing developed large and

prominent cracks which invariably first appeared near the corners of

the windows and extended across the wall parallel to the sheathing.

It was felt that these were brought into prominence,, if not actually

caused, by the shrinkage of the sheathing.

Stuccoes on gypsum block and monolithic concrete coated with

bituminous waterproofing compounds were in poor condition, especially

after the second winter.. The results obtained did not indicate that

the stucco was unsuitable, but that the combination of stucco and a base giving a weak bond gave poor results.

The sand-float finish used was, in a great majority of the

panels, floated while the mortar was too soft. This resulted in

bringing to the surface a rich mixture of cement or cement and lime

which was subject to high shrinkage and upon drying, developed ex2 -
eessive crazing and map cracking. It was thought that the trouble

was caused by the use of too much moisture, not necessarily mixing

water, but too liberal soaking of the bases and undercoats.

Cement mortar panels to which small amounts of lime were added

proved most satisfactory, as did the mixes leaner than those commonly

specified at that time. The prominence which the smooth sand-float

finish gave to fine cracks, unevenness of texture, blotches and other

small defects suggested the advisability of finishing with rougher

surfaces, such as

Tf rough-cast M or pebble-dash finishes,

(1916)' In 1916, it was decided to supplement the previous investigations with a series of tests in which more attention would be

given to methods of application and it was also decided to investigate

further the use of the leanex- mixes.

Twenty-two new frame construction panels forming the walls of a penthouse were erected on the roof of the original test structure.

Deductions from these tests may be summarized as follows:

Diagonal sheathing of improperly seasoned wood or wood that has

absorbed considerable moisture is apparently unsatisfactory for

stuccc backing. With only such wood available, horizontal sheathing

appears to be the better construction, provided the wood frame is

sufficiently braced.

Back-plastered construction for frame structures appears to be

more desirable as far as the stability cf the stucco is concerned.

This type of construction demands consideration of the insulating

qualities of a wall so constructed, as well as attention to the rigidity

of the framing,

lean mixtures produced better stucco when the necessary plasticity

and density were maintained by grading the aggregate.

Changes in the method of finishing, partly by the use of less water

in the undercoat, and partly by waiting for the stiffening of the finish

coat to develop before final floating, produced improvement over the

previous work by eliminating crazing, large pattern map cracking was reduced, particularly in those panels where stucco was of a lean mix.

Previous indications of map crazing in cement stucco seemed to be

the result of the over-wetting of undercoats. The desirability of

studying the behavior of various stucco mixtures led to the making of

further laboratory studies following the suspension of activities in this

direction during the World War,

Measurements were made of shrinkage or expansion of mortar before,

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during fend after set. These extended over e. period of more than two

years and resulted in obtaining much valuable information which, when

applied to the actual conditions prevailing in the stucco test panels,

explained why some panels were good and others were poor. They also

enabled the specifying of a procedure that would largely eliminate

erasing and map cracking. It was found that the crazing and cracking,

caused by excessive shrinkage, could be avoided by the proper control

of moisture in the base or undercoats.

(19£3) Through the generosity and cooperation of the National

Lumber Manufacturers Association, the Portland Cement Association, the

Associated Metal Lath Manufacturers and other interested individual manufacturers, a program was drawn up in 1923 which involved the replacing

of 32 panels. These tests were conducted primarily to satisfy certain

unanswered questions which were substantially as follows;

(1) Is wood sheathing or open framing best for stucco?

(2) Is the reinforcing value of the metal lath or fabric sufficiently important to warrant heavier lath and higher cost?

(3) khan metV.d of attaching metal lath or fabric will most

effectively minimize the transmission of strains to the stucco slab?

(4) Is curing of stucco to be recommended and to what extent is

it worth the additional cost?

(5) Is it beneficial to delay the application of the finish coat

for an interval of several days?

Conclusions and answers to the foregoing questions may be stated

as follows:

(1) Back-plastered and possibly paper backed construction seemed

preferable to sheathed construction. Insulation and proper bracing of

the frame should be taken into consideration. If sheathing is to be

used, it should be placed horizontally.

(2) Definite conclusions of the comparative merits of the four

types of metal bases used, each having distinctive features not contained

in any other, could not be drawn. However, indications show that

heavier members have greater reinforcing value, but that further study

of size and spacing of the members is required.

(3) There was no advantage of one type of furring over another.

Some slight indications favored the loose tie. Further study is necessary

before recommending any change in the present methods.

(4) A certain amount of curing is beneficial. If new stucco can

be protected from too rapid drying, the ultimate shrinkage should be less

and an early development of strength obtained.

( 5 J ‘The tests indicated that delays in the application of the

finish coat were not particularly beneficial and that as good, and

possibly better, results would be obtained by applying the coats on successive days.

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