Measuring in wheelbarrows is commonly employed, and fre quently results in very irregular proportioning, as the barrow may not always be equally filled, unless special attention be given to the loading. When this method is employed, it is desirable to have barrows of such form that they may he evenly filled to level surface. When ordinary barrows are used, a bottomless box may be placed in the barrow, filled and removed, before starting with the load. It is worth while to use a method that will give accurate measure ment, even at a small extra cost for labor.
Cement is Measured by counting bags.
The mixing of concrete by hand should be done upon a water tight platform. The cement and sand should first be mixed dry, being turned by shovels, or worked by hoes, until the mixture has uniform color. Water may then be added and the mixture worked into a rather soft mortar, after which the stone is wheeled or shoveled on top of the mortar and the whole turned with shovels until thoroughly mixed. When this method is followed the stone should be wet, to prevent taking the water from the mortar.
After mixing the sand and cement dry, the stone may be immedi ately distributed over the top of the mixture, water added, and the whole mixed by turning with shovels. In mixing concrete the shovels must be turned completely over and the contents deposited bottom side up. It is often difficult for workmen who have used shovels in other work to get the knack of doing this. Until they do, they accomplish very little.
Water should be poured on from buckets and care used to get only the quantity needed to properly mix the concrete. The quantity of water to be used depends upon the character of the work and manner of placing the concrete. An excess of water beyond that necessary to give a plastic consistency is always an element of weakness in the concrete.
Work of this kind always requires close supervision to see that all of the operations are properly performed and that the concrete produced is of uniformly good quality. Economy in hand mixing depends upon the work being so organized that it goes smoothly in all its parts, every man having his regular duties, and the number of men at each kind of work being such that one set of men does not have to stand idle waiting for others.
81. Machine is now used for mixing in practically all large concrete construction, and it is rapidly replacing hand mixing in much of the smaller work. Portable plants, which may readily be moved from place to place, are making this economic ally feasible. There are a large number of mixers on the market, differing more or less in their method of mixing the materials or in their mechanical appliances for handling materials.
Rotating botch mixers are either cubical boxes mounted to rotate about horizontal axes passing through two opposite corners, or cylindrical or conical drums rotating about their geometrical axes. The interior of the mixer is usually provided with blades, causing the materials to be thrown from one part of the mixer to another by the rotation. In using these mixers, the materials in proper pro portions to form a batch of concrete are put into the hopper of the machine, and charged into the mixer at one time. The mixer is then run for a sufficient length of time to mix the ingredients, thoroughly and the concrete is drawn off through the outlet. The amount
of water required should be determined, and measured for each hatch. Automatic appliances for measuring water are provided on some machines.
The proportions of materials are under perfect control by this method, and the thoroughness of mixing may be insured by regulat ing the time of rotation for a batch. Examination of the concrete as it comes from the mixer will show whether it is thoroughly mixed and of proper consistency. Some operators try to speed the work by using more water than necessary and using less time in mixing. This diminishes the strength of the concrete and should not be allowed. The more thoroughly the concrete is mixed, the less the amount of water required to produce a given consistency, and thoroughly mixed materials flow better and have less tendency to separate in placing than those made soft by excess of water. In most cases the run should not be less than one minute properly to mix a batch. Some machines have devices for automatic control of the discharge to prevent shortening the time of mix.
Paddle mixers, consisting of a series of paddles mounted on a horizontal shaft, and working in a trough, are usually employed as mixers. In these, the materials are fed in at one end, forced down the trough by the paddles, and discharged at the other end. These machines have not usually been so satisfactory as the batch mixers, on account of the lack of uniformity in supplying the materials. Some automatic feeding appliances have been found to work fairly well, and when the supply of ingredients can be evenly regulated these machines may do good work.
Gravity 'mixers are those in which the materials are mixed by falling through a vertical or inclined chute, and striking obstructions in the fall which throw them together. This mixer requires ano power for operation, but the materials must be at a considerable elevation to provide the necessary fall.
The cost of machine mixing depends largely upon the appliances used in conveying the materials to and from the mixer and the method of feeding the mixer. The arrangement of a mixing plant must depend upon the character and amount of work to be done and the topography of the site. When the work is large and concentrated within a small area, a plant of permanent character may he erected with derricks or other mechanical appliances for handling the materi als. Sometimes a plant of this kind is made semi-portable by erect ing it on a framework resting upon wheels or rollers, which permit it being moved as the work progresses.
It is very common to have the mixer set so that it may discharge into harrows or carts on the ground, the materials being supplied by wheelbarrows from piles on the ground near the mixer. For this purpose, the rotary mixer, with movable hopper which may be let clown to the ground for falling, is often used, portable plants mounted on wheels being very commonly of this Iype. Wheel barrows with the wheel under the body of the barrow, so that the barrow may be easily clumped over the wheel, are convenient for this kind of work.
In building construction, the mixer is commonly at the surface of the ground and supplied by barrows, the concrete being delivered at required elevations by bucket hoists.