> > > >

1950-51 Theatre Catalog, 9th Edition, Page 128 (108)

1950-51 Theatre Catalog, 9th Edition
1950-51 Theatre Catalog
1950-51 Theatre Catalog, 9th Edition, Page 128
Page 128

1950-51 Theatre Catalog, 9th Edition, Page 128

If measuring is done with wheelbarrows, each barrow should be marked on the inside for 1 cubic foot, 2 cubic feet, etc. This marking can be done by dumping a cubic-foot box or a cement sack full of material in the barrow, leveling and making a mark at that level. This can be repeated with another cubic foot of material, etc., until the barrow is calibrated.

The most accurate method of measuring aggregates is by weight. On large or small jobs, many contractors are using platform scales with short runways on either side of the platform. The filled wheelbarrows are run up on the platform and a few shovelsful are added or removed to adjust the weight. This method has many advantages over measurement by volume. Not only is it more nearly accurate, producing better uniformity from batch to batch, but also it is easily adjusted for necessary changes in proportions and completely eliminates the problem of correcting for bulking of sand.

Measuring Wateraln using the modern method of proportioning-that of adding a definite amount of water to each sack of cementeit is necessary to maintain accurate measurement of mixing water throughout the duration of the job.

Many concrete mixers are now equipped with tanks and measuring devices which give satisfactory results when properly used. These measuring devices can be set to deliver any number of gallons of water in the mixer drum as specified for the work at hand.

It is necessary to check water measuring devices regularly and to see that valves are tight in order to insure accurate measurement of water. '

An ordinary lZ-quart galvanized pail, marked off in gallons, one-half gallons and one-quarter gallons, is used for measuring water when mixers are not equipped with measuring devices. Recommended practice is to measure water from a barrel using a pail kept at the mixer for this purpose-that is, the measuring pail should not be used for other purposes.

Time of Mixing-With a batch-type machine mixer, it is recommended that mixing continue for at least one minute after all materials, including water, are placed in the mixer drum. When every piece of aggregate is completely coated with cement paste, the concrete is well mixed. More uniform concrete also is obtained from thorough mixing,

Improved WorkabilityeThorough mixing also gives improved workability which reduces the labor required in placing and permits the use of slightly larger quantities of aggregate with a given proportion of cement and water. Contractors find that another advantage of thorough mixing (for at least one minute and preferably two) is that it assists in securing watertight concrete.

Small changes in the speed of the mixer have little, if any, effect because thoroughness of mixing is governed largely by the time of mixing and not

DEPICTED AT SIDE ARE: 1. a concrete mixture which contains correct amount of cement-sand mortar: 2. a mixture with excess 0! some: 3. reinforcement is entirely embedded by rodding concrete; 4. runways built around forms enable placement of concrete where needed: 5. formwork left in place on interior wall assists curing.

the rate of rotation of the mixer drum. Loading the mixer above its rated capacity is not recommended as such overloading prevents thorough mixing. If increased output is needed, it .is best obtained by using a larger mixer or additional mixers instead of speeding up or overloading the mixing equipment on hand.

Thorough mixing is economical because the concrete is easy to place. Inspection of well-mixed concrete shows that all particles of aggregate, including sand, are coated with cement paste.


Methods used to move concrete from the mixer to the forms will depend largely upon the job conditions. On small jobs wheelbarrows are the usual means of transportation. On the larger jobs, buggies and chutes are commonly used.

When using barrows or buggies, care is required to prevent segregation (pieces of coarse aggregate separating out) as the concrete is being moved. Segregation is likely to occur when the concrete is handled over rough ground or runways. A rather stiff consistency usually is required to prevent segregation.

When chuting equipment is used, special care should be used to prevent segregation. It is recommended that a slope not hatter than 1 to 3 or steeper than 1 to 2 be use. When long chutes are required, it is best to deliver the concrete into a hopper before it is placed in the forms. In such a hopper the concrete is remixed to a certain extent and any pieces which may have separated out are recombined into the mass. When chutes are flushed with water in cleaning, care is taken that this water does not enter the forms.

Concrete is deposited in level layers, usually not more than 8" deep. As it is placed it is tamped and spaded just enough to settle it thoroughly and produce a dense mass. Working the concrete next to the forms insures an even, dense surface.

If the mixture becomes sloppy as the forms are filled due to water being forced out of the lower layers of concrete, this can be regulated by using stiffer mixtures.

At the end of a days run or where the work has to stop long enough for the concrete to begin hardening, the top surface is roughened just before it hardens, So as to remove laitance or scum and provide a good bonding surface for the next layer of concrete. Just before resuming concreting, the roughened surface is cleaned, thoroughly wetted with water and then brushed with a cementavater paste of a thick, creamy consistency. This paste is applied in a thin brush coat just a few feet ahead of the concreting operation so that it does not have a chance to dry.

Unless care is used in providing a bond between different layers of concrete, there is danger of seams developing that will cause leakage. This precaution to get a good bond between different layers of concrete is very important wherever the concrete, construction is to be watertight. It is also important in securing good bond between the two layers of concrete in two-course floor construction.

1950-51 Theatre Catalog, 9th Edition, Page 128