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CHAPTER XIX 



THE OCCURRENCE OE ROCK BORLNG MOLLUSKS IN CONCRETE 



Rock boring mollusks of tlic famih' Pholadidac are of very widespread occurrence. 

 Their ability to penetrate rocks of various kinds and often of considerable hardness 

 has been a matter of both scientific and popular interest. It has been supposed that 

 concrete in sea water might be subject to attack liy such borers, but specific instances 

 of the occurrence of these organisms in concrete marine structures are rare. A few 

 cases of such attack have lately been reported, which it is of interest to consider in 

 some detail. 



In the work of widening the channel in Los Angeles Harbor, about November 

 13, 1922, it became necessary to remove some old wooden piling which had been 

 jacketed some years previously with concrete. In looking over these piles, Mr. D. E. 

 Hughes, M. Am. Soc. C. E., observed that some of the jackets had been attacked by 

 borers, and he immediately investigated further. Of 18 jackets examined at this 

 location, known as the Old F"ish Cannery Wharf, across the channel from the foot of 

 5th Street, San Pedro, 16 were found to be more or less attacked; about 5 were con- 

 sidered badly attacked (6 borers or more per square foot of exposed surface) ; the other 

 contained fewer, and some only an occasional, borer. The two jackets which did not 

 contain borers stood in shallower water than the others. 



The exact date at which these piles were driven could not be determined, but it 

 was probably several years prior to 1909, at which time it became necessary to jacket 

 them with concrete to protect them from Limnoria or other wood borers. The jackets 

 had accordingly been in place 14 years. The length of time during which they were 

 actually exposed to attack by the rock borers, however, is probably considerably less 

 than this, as the form lumber was left in place outside the jackets and would deter 

 the pholad borers from entering the concrete until the encasing wood was destroyed 

 by wood borers. 



One of these piles was available for examination a month later by one of the 

 authors. The jacket was about 7 feet long, having extended from the mud line up to 

 about mean low water. It consisted of cement mortar with no coarse aggregate (see 

 screen test below), averaging 2'^ o inches in thickness, and sufficiently hard that some 

 difficulty was experienced in breaking it up with a 15-pound iron bar to secure samples. 



The outside form lumber was still partK- In place. It had originally consisted of 

 1-inch redwood, as seen at one place where it had been protected by a cleat. Else- 

 where it had been badly attacked by Limnoria and Teredo diegensis, so that only a 

 thin shell of it remained adhering to the concrete. This thin veneer of wood, however, 

 still covered all of the jacket except about 1 foot at the top, and one corner where 

 the form had sprung apart, leaving a gap through which the rock borers could enter. 

 The entire surface exposed to rock borers was not more than 5 square feet. In this 

 area nearly 40 of the borers occurred, averaging 7 or 8 to the square foot. 



The mollusks in general were a little larger than a man's thumb. The largest 

 one found occupied a burrow measuring 1^^ inches in diameter at its widest portion. 

 Two borers in this jacket had penetrated the concrete until they came in contact with 

 the wood within, but none had actually bored into the wood. One indeed had turned 

 and continued boring in the concrete parallel to the surface of the wood, to avoid 

 entering the latter. 



