PRESENT STATUS OF THE CONCRETE SHIP. 191 



ward at the rate of about 36 load applications ( 18 complete cycles) per minute. The 

 tensile stress developed in the steel is about 23,000 pounds per square inch, and 

 746,000 applications of the load had been made up to August 18, 1918. There had 

 been no perceptible increase in the strain in the steel or the concrete after 1,000 

 applications of load and very little increase between the first and the thousandth ap- 

 plication. Cracks appear in the lower portion of the beam under downward load and 

 in the upper portion under upward load. The largest cracks on August 18 were 

 about i/ioo inch wide. With continual opening and closing of cracks, if any should 

 occur in the alternate hogging and sagging of a ship, the possibility of a grinding 

 action on the fractured surface has been considered as a possible destructive agency, 

 but thus far in the test no such grinding has been observed. 



During the making of all large test specimens, small specimens of concrete have 

 been made, using the same material as that going into the large specimen. The 

 compression tests of the small specimens show strengths as great as the assumed 

 strength on which the working stresses used in design were based. 



In order to determine whether in the complicated network of reinforcement 

 present in the ship structure the quality of the concrete could be made satisfactory, 

 special tests were made. Slabs 4 inches thick, with a very large amount of reinforce- 

 ment made up into a complicated network, were poured on edge to a depth of 3 feet. 

 All parts of the structure were well filled out with concrete, covering all the rein- 

 forcement satisfactorily, and, in the load test which was applied, failure occurred by 

 tension in the steel before reaching the ultimate strength of the concrete, although a 

 stress of about 4,400 pounds per square inch in compression in the concrete had been 

 developed. 



All the evidence thus far accumulated indicates that a concrete strong enough 

 to warrant the use of a high working stress in compression (1,500 pounds per square 

 inch) can be obtained in all cases where sufficient attention is given to the placing 

 of the concrete. 



Tests are under way to determine the conditions under which leakage through 

 construction joints and through cracks may be expected, and how it may be pre- 

 vented. Thus far hollow beams, having wall thicknesses of 4 inches and construction 

 joints in which as much as seven hours elapsed between the pouring of successive 

 layers of concrete, have been tested under a 30-foot head of water. No leakage 

 occurred through any of the construction joints and apparently none through the 

 side wall except where the wall was penetrated by a steel tie bar. However, leakage 

 did occur through the thicker upper flange of the beam where imperfections in the 

 construction occurred. 



While under a pressure due to a 30- foot head of water, the beams were loaded so 

 as to give a shearing stress of 300 pounds per square inch. It was found that where 

 a diagonal tension crack was as much as 0.002 inch wide, a small amount of leakage 

 occurred, and where it was 0.004 inch wide the leakage was appreciable. The tests 

 are being continued to obtain more complete results. 



An inspection made by the United States Bureau of Standards in 19 17 of rein- 



