The methods of handling concrete just described are those commonly employed on operations of comparatively small size, where the concrete work can be finished in the course of a few days, or, at most, in a few weeks. The equipment is often of a more permanent character, however, and the methods employed are somewhat different, when constructing the walls and floors of large buildings. We shall not attempt to describe all these methods, but shall confine our attention to two of them which are fairly typical. The distribution of the concrete mixture is accomplished in the same manner in both cases, but the methods of delivering the sand and stone to the mixer are somewhat different.

In one case sand and crushed stone are delivered by truck, trolley, train, or other means, at a convenient place outside of the actual zone of building operations, and are stored in piles where they will not interfere with the work. Tracks are laid on an incline from the storage area to a platform over the concrete mixer, which is situated nearer the work. A cable-drawn "skip" or car, divided into two compartments of proper size, brings the sand and stone in the correct proportion to the mixer platform, and dumps these materials into a hopper which delivers them to the mixer, - the cement and water being added at the same time.

In the other system a large bin is erected over the mixer. This bin is divided into two parts, - one for the sand and the other for the crushed stone. The sand and stone are brought to the job by trucks or by some other means, and are dumped on the ground on either side of the bin. They are there elevated and deposited in the bin by a clam-shell bucket operated by a derrick. The derrick boom is raised high enough to clear the bin and is maintained at this elevation, so that the movements of the derrick are confined to swinging the boom and hoisting and lowering the bucket.

As in the method last described, a platform is erected directly above the mixer, and the sand and stone are delivered to this platform, by chutes leading from the lower part of the bin and thence downward through openings in the floor of the platform to the mixer. The flow of sand and stone is regulated by gates placed in the chutes.

The danger from exposed gears, belts, sprockets and chains, and other moving parts of the mixers, is considerably diminished when using either of these two systems, because most of the work is done on the platforms above the mixers. The machines must be oiled regularly, however, and attention of other kinds is often necessary, so none of the danger points should be left unprotected. The elevated platforms should be guarded by substantial railings, and the men should be cautioned to use all possible care to avoid stepping or falling into the openings in the platforms through which the materials are delivered to the mixers.

Careful and experienced men are required for handling the motors or engines used for hauling the skips or operating the derricks. The exposed moving parts of the motors or engines should be guarded, and all ropes and cables should be inspected frequently and kept in good condition.

At least six equally-spaced guys should be provided for every guy derrick, and particular attention should be given to the fastenings of these guys. Every derrick should be equipped with adequate and effective mechanical brakes, and the brakes should be tested frequently to make sure that they are in good order. (See also section V, with regard to the care and operation of derricks).