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1950-51 Theatre Catalog, 9th Edition, Page 127 (107)

1950-51 Theatre Catalog, 9th Edition
1950-51 Theatre Catalog
1950-51 Theatre Catalog, 9th Edition, Page 127
Page 127

1950-51 Theatre Catalog, 9th Edition, Page 127

strong, durable, and watertight. The qualities of portland cement, mixing water, and aggregates which make them suitable for use in a concrete mixture are listed here.

Portland Cement-Although there are five types of portland cement, the one used for general concrete construction is usually entirely suitable for theatre floor construction purposes where no unusual atmospheric or other conditions exist.

It is of prime importance that cement be kept dry at all times prior to use. Cement which contains lumps that cannot be readily pulverized by striking lightly with a shovel should not be used; however, cement that is caked due to the weight of cement piled on it can be reconditioned by rolling the sack on the fioor.

Mixing Watereln general, water that is fit to drink is suitable for mixing with cement. Water that is clean and free from, oil, alkali, or acid is most desirable.

AggregateseAggregates usually are divided into fine and coarse, sand being the most common form of fine aggregate, and pebbles, crushed stone or crushed slag, the most common form of coarse aggregate. Sand or other fine aggregate, such as stone Screenings, includes all particles from very fine (exclusive of dust) up to thOSe which will just pass through a screen having meshes 14-inch square. Coarse aggregate includes pebbles, crushed stone or crushed slag ranging from 1,4 to 11/2 inches or larger.

The largest size of coarse aggregate to use is governed by the nature of the work. For example, aggregate up to 11/2 inches or more in size may be used in a thick foundation wall or heavy footing. In thin slabs or walls it is recommended that the largest pieces of aggregate should never exceed one-third the thickness of the concrete being placed.

Aggregates which are sound, hard and durable are best suited for use in concrete. Aggregates which are soft, naky and which will wear away rapidly through exposure to weather, generally are unsatisfactory.

In addition to being sound, hard and durable, the best aggregates are clean and free from fine dust, loam, clay or vegetable matter. These materials are objectionable because they prevent the cement paste from binding together the particles of sound, durable aggregates, thereby reducing the strength of the concrete and making it more porous. Concrete made with dirty aggregates hardens slowly and may never harden enough to serve its intended purpose.

In gene a], the best aggregate available in the area will be the most satisfactory. Aggregates obtained from established plants with adequate washing and screening facilities ordinarily will insure uniformity in grading, cleanliness and quality.

Well-graded fine aggregate, in which the particles are not all fine or all coarse but any from fine up to those particles which will just pass a screen having meshes 1.4-inch square, is recommended. If the sand is well graded, the finer particles help to fill the spaces (voids) between the larger particles, thus resulting in the most economical use of cement paste in filling the voids and binding the


aggregate particles together to form strong concrete.

Coarse aggregate is well graded when the particles range from 14-inch up to the largest size which may be used on the kind of work to be done.

The natural mixture of fine and coarse aggregates as taken from a gravel bank or crusher seldom is suitable for concrete unless first screened to separate the fine material from the coarse and then recombined in the correct proportion for the class of concrete being made. Most gravel banks contain either more sand or more coarse material than is desirable for best results. Usually, bank-run material contains too much sand.


It is an easy matter to proportion concrete by methods which control the total amount of water that is mixed with each sack of cement and thus control the quality of the concrete. The first thing to do is to determine the quality of cement paste required for the particular job.

Concrete that places and finishes readily is known as workable concrete. In a workable mixture there is sufiicient cement paste to bind the pieces of aggregate together so they will not separate when the material is transported to or placed in the forms. There also is sufficient cement paste and sand to give good, smooth surfaces free from rough spots, called tthoneycombingfi In other words, there is just enough cement paste to fill completely the spaces between the particles of aggregate and to insure a smooth, plastic mix that finishes easily.

A mixture that is workable for one job, however, may be too stiff for anotherethat is, concrete to be placed in thin sections must be more plastic than if used in heavier work. However, it is best to avoid very mushy, sloppy mixtures at all times because they separate in handling, with the larger pieces of aggregate sinking in the mass. This kind of concrete will be of poor quality, and may also be very uneconomical.


Accurate measurement of all materials, including water, is necessary to insure production of uniform batches of concrete of the quality desired.

The best way to make sure that all batches will be uniform, is to use weighing or measuring equipment that is designed to give accurate measurements at all times.

Measuring Aggregates e When measuring is done with shovels, each man in the mixing crew should be required to check the number of shovelsful he takes in handling exactly 1 cubic foot of material. This is done by counting the number of shovelsful of each material required to fill a cubic foot box or a cement sack which holds 1 cubic foot. This test should be made at least once

each day, particularly if new loads of materlal are delivered on the job.

PICTQRIAI. REVIEW of do's and don'ta in construction of concrete tloor tinishes continues With: 1. uniform distribution of coarse aggregate particles: 2. avoid weak cement and sand mortar toppings; 3. crating frequently results: 4. a poor distribution at aggro ates,- 5. a poor mixture with insutficiant tiller or spaces.
1950-51 Theatre Catalog, 9th Edition, Page 127