A treatise on concrete, plain and reinforced : materials, construction, and design of concrete and reinforced concrete; 2nd ed. . Fig. 93.—Portable Mixing Plant. () Boston Tunnel employed a derrick bucket. The stone was first filled into a height determined by a gage, then the sand was shoveled on top of itand struck off with a different gage, and finally the required number ofbags of cement emptied on top of the sand. The bucket was taken by aderrick and dumped into a duplex mixer. Cambridge Bridge Piers. When the quantity of concrete to be laidwarrants the installation of the necess
A treatise on concrete, plain and reinforced : materials, construction, and design of concrete and reinforced concrete; 2nd ed. . Fig. 93.—Portable Mixing Plant. () Boston Tunnel employed a derrick bucket. The stone was first filled into a height determined by a gage, then the sand was shoveled on top of itand struck off with a different gage, and finally the required number ofbags of cement emptied on top of the sand. The bucket was taken by aderrick and dumped into a duplex mixer. Cambridge Bridge Piers. When the quantity of concrete to be laidwarrants the installation of the necessary machinery, economy requiresthat the stone and sand shall not be handled at all by laborers. If thestone is crushed on the spot, it may be raised to bins above the mixer 27: A TREATISE ON CONCRETE. MIXING CONCRETE 273 by bucket elevators or belt conveyors, while a similar plan for elevatingthe material may sometimes be advantageously followed where giavel isused. In building the substructure of the Cambridge Bridge, Boston,Mass.,* the concrete plant was located on a pier resting on piles. Thegravel for the concrete was dredged from the harbor and dumped fromscows into the water close to the pier. An orange peel bucket, operatedfrom a dredging machine on a scow, lifted the gravel, and dropped it intoa hopper whence it ran by gravity upon the combination inclined screendescribed on page 240, which separated the sand, pebbles, and the coarsewaste material. Bucket elevators raised the sand and pebbles to binsabove the mixer, and from the bins, which were \-shaped, the materialsfell by gravity into the measuring hoppers. These were arranged in twosets, an essential requirement for maximum output, so that one batchcould be measured while another was being dropped into the mixer. Theba
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