TECHNICAL INFORMATION ON BUILDING MATERIALS

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EOR USE IN THE DESIGN OE LOW-COST HOUSING

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THE NATIONAL BUREAU OF STANDARDS

UNITED STATES DEPARTMENT OF COMMERCE

WASHINGTON, D. C.

July 17. 1Q36.

C nyCRF.TE MASONRY UNITS

This is a digest in part of Circular No. 304, "Properties and

Manufacture of Concrete Building Units", (June 10, 1936), V ^ank A.

Hitchcock, issued hy the Bureau of Standards, and abstract of Feaeral

Specification SS-C-621 "Concrete Units; Masonry Hollow.

Comparison of concrete units with other materials should take

into consideration structural requirements, architectural purposes,

economical features, maintenance and fire insurance rates.

Relative costs of laying such units, as frequently set forth in

advertising and other literature, must not he definitely applied

without investigating costs under local conditions. Size is an important factor which may affect savings over other types of masonry

construction, hut it does not necessarily follow that a larger unit

would he proportionately cheaper. Serious objections to the handling

and laying of very large and heavy blocks may materially affect the

attitude of local labor.

Color in cement blocks is largely governed by raw materials

used and the processes of manufacture. Portland cements .may vary from

light yellowish gray to slate due principally to the varied content of

iron oxide or, in some cases, manganese.

^•Available from Superintendent of Documents, Government Printing

Office, Washington, D. C. (Price 20 cents)

Available from Superintendent of Documents, Government Printing

Office, Washington, D. C. (Price 5 cents)

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The merits of special cements for which valuable nro-oerties are claimed can only he determined by actual tests and investigations.

This iS particularly true where it is claimed certain cements would

produce a concrete which is impervious to moisture and non-absorbent.

The essential physical requirements for all aggregates are that

they shall be clean and durable, of sizes governed by the quality of

the concrete, type and dimensions of the unit, and desired surface

effects. The aggregates should be graded from fine to coarse within

the limiting size, bearing in mind that the workability of the mix,

the amount of water required to obtain proper consistency, the density,

and particularly the required amount of cement to attain given strength

are greatly affected by wide variations in the sizes of the aggregates.

They should pass a sieve having openings not larger than l/2 the thick- ness of the thinnest web or shell 'of the unit. The use of "pit-run" or "run of the crusher" material may be undesirable because such material

tends to segregate into sizes with handling. Desired proportions of

the relative sizes are obtained by separating the fine and coarse aggregate and recombining them in proper proportions.

Sand is generally understood to be an aggregate which will pass

through a ITo. 4 sieve (openings 0.187 inch square). It should be

clean, hard, durable, of uncoated grains, graded from coarse to fine

with the coarse grains predominating. It should be free from organic

matter or other harmful imourities, and may contain small amounts of

silt but not in excess of 10 percent by weight. Mica may be present

in quantities not greater than 1 percent, larger amounts proving

detrimental, especially in sand for use in surface treatments, which

would cause them to dust and peel.

Gravel to be used as coarse aggregate is assumed to be material

retained on a Ho. 4 sieve and should be clean, durable, free from soft,

flat or elongated pieces.

Trap rock,- sandstones, and limestones, when crushed, make good

concrete material and are often used as fine and coarse aggregate while

slates, shales and some forms of sedimentary rocks, lacking durability,

are not recommended for such use.

Blast furnace slag, affording an economical source of aggregate,

can be classified as air-cooled and granulated, and is produced by the

respective air and water cooling of molten slag. Its use in concrete

should be confined to the air-cooled type, which should be uniform in

quality, free from metallic iron and other objectionable matter and

the material should weigh no less than 65 pounds per cubic foot. As

in other materials, graduation of sizes must be considered and dust

should be eliminated if present in excessive quantities.
All cinders are not equally suitable for concrete but, generallyspeaking, those from power plants provide the best material. They

should be crushed to the necessary size and usually -require more thorough mixing than other material

.

Impurities iii the gaging water as well as its temperature may

seriously affect the strength and setting time of concrete. It is,

therefore, recommended that the water be free from oils, acids, strong

alkalies, vegetable matter, or factory wastes and its temperature should

not be lower than 6(3° Fahrenheit.

The value of various admixtures to increase workability, early

strength, water resistance properties and the curing of concrete can only be determined by investigation, analysis and tests. These properties of concrete have been discussed in TIBM numbers 5» 6 and 8.

Certain requirement s for concrete building units are imposed by

conditions of use, while others, such as architectural and structural

requirements, are imposed either by local ordinances or by commonly

used standards. Generally accepted sizes have been established and are

set forth in Simplified Practice Recommendations R 32-32,-*- published by

the National Bureau of Standards.

The following is a summary of the requirements set forth in Federal

Specification SS-C-621, (April 28, 1931 » amended May 1935) The

specification provides for hollow masonry wall construction units manu- factured from portland cement or other hydraulic cement and aggregates

consisting of natural sand and gravel, crushed rock, slag, burnt clay

or shale, cinders, or other inert materials. It also requires that the

units be sound and free from cracks or other defects which would interfere with the setting or permanence of construction, and if intended

for use as a base for ulaster or stucco, the units should be sufficiently rough to insure good adhesion .

The specification covers 2 types of units, type I or load-bearing

and type II or non load-bearing. The specification requires a 3/*+

inch minimum for shell and web thickness, and permits tolerances of

plus or minus 3 percent in over-all dimensions.

The average percentage of moisture in the units at time of

delivery shall not exceed 4o percent of the total absorption of the

units when tested as provided for in the specification.

Available from Superintendent of Documents, Government Printing

Office, Washington, D. C. (Price 5 cents)

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Type I (Load-bearing)

Requirements for Compressive Strength

Minimum Thickness of Shells

(Outer shells of unit parallel to

exposed surface of wall)

Compressive Strength

(Pounds per square inch

of gross area)

Average of 5 Individual

or more units minimum

1 l/4 inches or more 700 600

3/U to 1 1/4 inches 1000 S00

The absorption shall not exceed l6 pounds per cubic foot of

concrete. This requirement shall be waived when the units are used

in construction not exposed to soil or weather if so stated in invitations for bids.

Type II or non load-bearing units should have a gross area

compressive strength of 350 pounds per square inch. These units are intended for use in interior partitions, backing for exterior walls,

and fireproofing in skeleton- frame buildings, where the only strength

requirements are that they be sufficient to prevent excessive breakage

in delivery and handling.

Detailed information of tests for determining compressive

strength, absorption, etc., is given in Federal Specification SS-C-621.

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