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1950-51 Theatre Catalog, 9th Edition, Page 132 (112)

1950-51 Theatre Catalog, 9th Edition
1950-51 Theatre Catalog
1950-51 Theatre Catalog, 9th Edition, Page 132
Page 132

1950-51 Theatre Catalog, 9th Edition, Page 132

placed over two of the blocks and moved with a sawing motion to compact the concrete. The straightedge is not moved horizontally.

Thus a line of compacted concrete the Width of the straightedge (usually about 1") is provided between the two blocks, and this forms the screed strip. The process is repeated between the next two blocks, and so on, giving screed marks every 8/ or 10' in two directions. Additional screed marks are made every 4' to 5' in the same way, using the screeds already placed as guides.

A scraper is then used to strike od' the concrete to the level of the screed strips. The scraper should be about 5' long, slightly beveled on the bottom and have a strip of steel on the face. The blocks are then removed and the spaces filled with concrete.

When doating is done by hand, wood screed strips are often used. These are placed at the proper level with the aid of a surveyorls level or spirit level. The straightedge is moved across the strips in a sawing motion and, at the same time, is advanced horizontally to strike off the concrete. The strips are then removed and the spaces filled with concrete.

Floating-Floating is done to compact the surface, fill up the hollows, and iron out the humps left after screeding and tamping or rolling. As previously stated, the power doat machine will permit the use of a much stid'er, harsher mixture than can be used when doating with wood or cork doats by hand. The machine consists of a steel disk 20" to 24" in diameter on which a motor is mounted. By means of a handle the machine is operated over the surface of the door. The rotating of the disk compacts the concrete and floats out the topping to a smooth surface. With the proper mixture only enough mortar will be brought to the surface for steel troweling.

Troweling*Troweling is an extremely important operation and one which requires experience and skill for the best results. It should be done at the proper time, which is after the concrete has hardened sufficiently to prevent drawing moisture and fine materials to the surface. When the mechanical doat is used, the first troweling may be done immediately after doating. When deating is done by hand it is necessary to use a more plastic mixture and therefore it is necessary to wait for a period after floating until the surface becomes fairly hard.

Cement or mixtures of cement and sand should not be spread on the surface to absorb excess water nor should water be added to facilitate troweling. Final troweling should be done after the concrete is so hard that no mortar accumulates on the trowel and a ringing sound is produced as the trowel is drawn over the surface. This will polish the surface to a smooth finish.

More About CuringeProper treatment of the door after it has been troweled is too often neglected. As stated previously, the concrete must be kept moist so that the cement will continue to combine chemically with the water. This curing process should be started as soon as possible. If it is delayed so that rapid evaporation takes place in the early stages, the surface may crack, craze or dust. The longer the concrete can be kept wet, the stronger, more impervious, and more wear-resistant it will be.

There are several methods of curing concrete doors. The ponding method is sometimes used, in which the door slab is surrounded by small dikes of sand and the enclosure kept filled with water to a depth of an inch or so. As stated previously, frequent sprinkling of the surface and covering the exposed surface with wet sand or wet burlap are other ways of providing curing. Such cover SOUND CONCRETE FLOOR CONSTRUCTION involves a number of painstaking steps. Note roughness of base slab and stiffness of concrete mix being spread with shovel and rake. Concrete is then tumped. screeded, and floated. usually by machine. The floor is then trowelled and cured.

ings should be placed as soon as this can be done without marring the surface and then should be kept continuously wet.

Heavy paper impregnated with asphalt to make it waterproof is also used for curing. This is placed as soon as it can be done without marring the surface and will protect the door from dirt and debris resulting from other building operations. All seams should be lapped and sealed with glued tape to provide a continuous waterproof covering. It will be recalled that colorless membrane curing compounds are also used.

In cold weather construction, when artificial heating devices are used, special precautions are required. The high temperatures near the heating devices cause rapid drying unless the concrete is well protected. Heaters should be raised, and the door underneath for a distance of several feet on all sides of the heater should be covered with 3" or 4" of sand. The sand should be kept saturated with water through the curing period.

Cold Weather Precautions - Concrete hardens Very slowly at temperatures below 500 F., and the hardening practically ceases at freezing temperature. Special precautions are required for all concrete Work in cold weather, but, due to the relatively thin layer of concrete and large area of exposure in door finish, such precautions are particularly important. All concrete should be protected from freezing until it has gained sudicient strength so that it will not be damaged. When necessary, heat should be furnished.

On leaving the mixer, the fresh concrete should be free from ice or frozen lumps and should have a temperature of not less than 70 nor more than 800 F. Heating only the mixing water is often sudicient; in other cases it may be necessary to heat both mixing water and aggregate to meet these requirements. The concrete temperature should then be maintained above 700 F. for at least 3 days, or above 50': F. for at least 5 days when using normal portland cement and above '700 F. for at least 2 days or above 500 F. for at least 3 days when using high early strength portland cement. The door should be kept wet during this period.

The temperature of the hardened slab should be as near that of the new concrete as possible. Warm concrete placed on a very cold, hardened slab will not bond well and when the top course is cooled it may shrink enough to break away from the slab.

Grinding-Some concrete doors are finished by grinding. Mechanical grinders remove the thin film of cement paste that covers the surface after troweling, thereby exposing the aggregates. Such finish needs only one troweling.

Grinding should not be started until the concrete has cured and hardened sudiciently so that aggregate particles will be cut and not torn from the surface. Large double-disk electrically operated grinding machines, such as those used for finishing terrazzo doors, have been found economical. The door is generally kept saturated during grinding. When necessary, air holes and pits may be filled with a cement grout of creamy consistency.

Cleaning and Finish-The new door finish should be protected from accu THEATRE CATALOG 1950-5!
1950-51 Theatre Catalog, 9th Edition, Page 132