16 METHOD OF CONSTRUCTION OF CONCRETE SHIPS. 



Since the hulls in this yard had been completely formed inside to the second 

 deck before any concrete was poured, a system of signals had to be installed in 

 order to keep the men in the various compartments supplied with concrete as needed. 

 For supporting the runway upon which the concrete for the top deck was trans- 

 ported in buggies a hanging scaffold was employed. 



At Wilmington and Jacksonville the concrete was transported from the mixers 

 by bottom dump buckets swung on a whirler or derrick to a number of hoppers 

 mounted on the staging at the top of the forms. The concrete was conducted by 

 gravity through pipes to mortar boxes, from which it was shoveled or "pailed" to 

 the forms. At Mobile the concrete was elevated in a bottom dump bucket by a gan- 

 try crane and swung directly to a mortar box, where it was deposited. From the 

 mortar box the concrete was placed in the forms by shovels and pails. 



The most unique equipment was that employed at San Diego. No. 24 motor- 

 driven Koehring mixers located in pits discharge into Inslee controllable one-yard 

 bottom dump buckets set on push cars that are transferred from under the dis- 

 charge chutes of the mixer to a position on the arc of a circle prescribed by the end 

 of a revolving tower crane boom at a fixed slope. From this position the bucket is 

 elevated and swung into position over the hull, lowered between the trusses, where 

 the concrete is dumped into a hopper which travels along a runway on the center 

 line of the ship. The concrete is then discharged from the hopper at any point 

 along the hull into chutes and delivered directly into forms or into flat boxes, and 

 then handled by shovels and coal buckets into the forms. There are four trav- 

 eling hoppers, operated on wooden tracks secured above the runway, which is hung 

 from the trusses. 



PLACING CONCRETE. 



On account of the large amount of reinforcing steel and small clearances be- 

 tween the forms and steel it was found that the concrete could not be placed by 

 the ordinary method of rodding or tamping unless a very watery concrete mixture 

 of low strength was employed. As can be seen from an examination of the photo- 

 graphs, much of the interior of frames, keelsons and shell could not be reached 

 with a rod. The most thorough and practical means of settling the concrete into 

 the forms and about the reinforcing steel was found to be by vibration. After some 

 experimentation it was found that small air hammers of commercial type (similar 

 to Ingersoll-Rand "Little David" No. D), with blunt bitts from 12 to 36 inches in 

 length, held against the outside or inside of the forms, near the point where the 

 concrete was being placed, would not only cause the concrete to flow, fill the forms 

 and thoroughly embed the steel, but would also increase the density of the concrete 

 by driving out nmch of the entrapped air. Care was exercised not to let the hammer 

 come in contact with the reinforcing steel. From thirty to sixty of these hammers 

 were employed on each ship during the concreting. For horizontal surfaces such as 

 decks, a long shank bitt was placed in the hammer, and in most cases applied to the 

 upper side of the forms through the concrete. 



It was at first attempted to vibrate the outside forms by maintaining a ham- 



