14 METHOD OF CONSTRUCTION OF CONCRETE SHIPS. 



amount of reinforcing steel in the structural members it is very difficult to remove 

 sawdust, shavings and other debris vifhich may collect at points of intersection of 

 the steel such as where the keelsons, frames and bulkheads join the floor slab, and in 

 the haunches in frames. If such debris has been permitted to accumulate at these 

 points, both water and air, even under considerable pressure, have been found to 

 be ineffective for its removal. It is practically impossible by visual inspection to 

 detect such accumulations after the steel is erected. 



Large openings should be left in the outside forms under all keelsons, bulkheads 

 and the like, and at frequent intervals elsewhere over the bottom forms, and con- 

 stant vigilance should be maintained to keep the forms clean during the erection of 

 the steel and inside forms. No unnecessary carpenter work should be done within 

 the hull, and such wood-working as is done should be over a work-box so that the 

 waste will not fall into the forms. It is particularly difficult to see the accumula- 

 tion of debris under such masses of steel at the frames, bulkheads, etc. The form 

 was washed and supposedly cleaned, and an inspection before pouring did not 

 detect the dirt. 



All forms were thoroughly wetted just before placing concrete, and in some 

 cases it was necessary to spray the reinforcing steel in order to reduce its tempera- 

 ture from exposure to the sun. 



MATERIALS USED FOR CONCRETE. 



A concrete was required which would have a compressive strength of not less 

 than 4,000 pounds per square inch at 28 days and be of the lightest weight possible. 

 While in the plastic state, it must be of such a consistency that it is possible to 

 work it into place, thoroughly imbedding the reinforcing steel and completely fill- 

 ing the forms. 



. In order to obtain the highest possible strength with a maximum quantity of 

 aggregate, the specifications called for a special high-grade Portland cement, which 

 required that it be ground so that at least 90 per cent passed the 200-mesh sieve. 

 The value of this fine grinding is shown by the results of tests given in Table 4, 

 Plate 3. 



It was necessary to develop some new type of aggregate in order to reduce 

 appreciably the unit weight of the concrete. After much investigation an aggre- 

 gate was developed with which it was possible to decrease the weight of the con- 

 crete from 145 pounds per cubic foot to 105 to 120 pounds per cubic foot. Since 

 there are 2,800 cubic yards of concrete in each of the 7,500-ton ships, each pound 

 reduction in the unit weight per cubic foot of the concrete represented an added 

 carrying capacity of approximately 32.5 long tons in the ship. Thus a saving of 

 30 pounds per cubic foot represents approximately 1,000 long tons additional carry- 

 ing capacity in the ship. 



The aggregate developed is a vesicular slag* made by burning suitable clay in 



*See Engineering News-Record, April 24, 1919, for complete description of product and method of manu- 

 facture. 



