METHOD OF CONSTRUCTION OF CONCRETE SHIPS. 17 



mer crew outside of the hull, but this method was found unsatisfactory, for the 

 hammers would at times through error be applied to surfaces where the concrete had 

 been placed and partially set, or they would not be applied where they could most 

 effectively settle the concrete. It was found more satisfactory to use a long shank bitt 

 and apply the hammer from the inside directly to the floor through the concrete as it 

 was being placed, and to the inside surface of the outside shell forms through the 

 windows left in the inside forms, or through small holes made in the inside forms, 

 which were later plugged. 



The ordinary type of hammer, while suitable for vibration of the forms, re- 

 quires the operator to absorb the recoil of the blow, and in consequence it is neces- 

 sary to relieve him at frequent intervals. At San Diego a special type of hammer 

 was devised, with an envelope casing which almost entirely absorbed the recoil. 

 This hammer has proven to be very satisfactory. 



There is a total of approximately 2,800 cubic yards of concrete to be placed 

 in the 7,500-ton ships, distributed approximately as follows: — 600 cubic yards in 

 bottom to top of bilge, 1,200 yards from top of bilge to and including the second 

 deck, 800 yards from second deck to and including main deck, and 200 yards in 

 superstructures. These various quantities were placed in one continuous operation 

 at the rate of from 8 to 15 cubic yards per hour. 



With the large yardage of concrete to be placed in a continuous operation 

 over a relatively large area in small units, it was necessary to organize the crews 

 with considerable care and to plan a systematic operation. In some of the ships 

 the placing of concrete was confined to two groups, either starting at opposite ends 

 of the ship and working toward the middle or starting at the middle of the ship and 

 working toward the ends. In most ships the depositing of concrete was carried on 

 simultaneously in four sections of the hull; either two groups worked from the 

 ends toward the middle and another two worked from the middle toward the ends, 

 or two groups each started from the third points in the length of the ship and 

 worked simultaneously toward the middle and ends of the ship. 



There is a difference of opinion among the contractors as to whether the con- 

 crete should be placed in the bottom floor and allowed to flow under the bottom 

 frame forms before filling the frame forms, or whether the bottom frame forms 

 should be first filled, the concrete being permitted to flow out underneath into the 

 floor before filling the floor forms. Defects have been found in some of the con- 

 crete placed by both methods. The results obtained are apparently dependent upon 

 the care exercised. 



Since the concrete was deposited continuously over periods of from fifty to one 

 hundred hours, the men worked in two or three shifts of eight to twelve hours each. 

 Great care and attention must be given to the placing of each shovelful of con- 

 crete into the forms, or defects of a serious nature will result. All the workmen con- 

 nected with the placing of concrete should be schooled and instructed with regard 

 to their individual responsibilities, for inspection alone cannot insure a good job, 

 and one careless workman in this position can cause major defects. 



