142 



work because of the uncertainty of the reactions of these compounds with sea water. 

 Sufficiently impervious mixes of concrete can be obtained by using the proper quantities 

 of cement as discussed below. To insure the use of the full quantity specified, however, 

 it is good practice to require the cement to be furnished by the Owner. 



The use of sea water for mixing is detrimental to structures containing steel, 

 because of accelerated rusting action. While sea water does not ajJi^reciably reduce 

 the strength of the concrete itself, it produces an excessive amount of laitance and 

 increases the likelihood of non-uniform, inferior work in all classes of structures. 



Proportions 



Proportioning of fine and coarse aggregate to produce workable mixes of high 

 imperviousness requires skill and experience. In order to insure certainty of results, it 

 is safer to provide properly graded screened aggregates as specified and combine them 

 than to economize by using natural aggregates of unsuitable size and grading. Pro- 

 portions should be determined with a view to keeping the cjuantity of fine aggregate 

 as low as is consistent with the production of a workable mix. 



"Simple" concrete structures having a "harbor" exposure may be considered 

 permanent, for commercial purposes, if properly constructed with concrete pro- 

 portioned one part Portland cement to seven and one-half parts aggregate. 



Similar structures having an "ocean" exposure should be constructed of concrete 

 proportioned one part Portland cement to six parts aggregate. If such structures 

 are of sufficient section, they may be constructed of one-to-seven-and-one-half con- 

 crete with a facing of from 6 inches to 12 inches of one-to-five concrete. This veneer 

 should be placed at the same time as the interior concrete. 



Tremie concrete should be proportioned one part cement to four parts aggregate. 

 It may be used for interior fill inside of impervious exterior walls. It should not be 

 used for facing walls exposed to sea water, because of uncertainty with regard to 

 quality. 



"Composite" concrete structures having a "harbor" exposure should be com- 

 posed of concrete proportioned one part cement to five parts aggregate. If the ex- 

 posed surface of these structures above high tide is protected with periodic painting, 

 they constitute a high-class fire-proof construction which is well suited for commercial 

 purposes. Similar structures under the more severe conditions of "ocean" exposure 

 should be proportioned one part cement to four parts aggregate. 



As cracks and checks from impacts and tension account in a large measure for 

 the disintegration of "composite" structures, it is not advisable to increase the cement 

 above the proportions specified; additional protection being better obtained from a 

 waterproof and air tight surface coating of such a composition that it will stretch 

 or heal over small cracks. 



Manipulation 



Faulty manipulation in conjunction with excessive proportions of fine aggregate 

 and mixing water account for almost all instances of disintegration of concrete in 

 sea water. Durability is largely dependent on the imperviousness of concrete, which 

 prevents sea water from freely entering the mass and breaking down the interior 

 structure of the set cement. Mixes which are loaded with excessive quantities of fine 

 aggregate and water to make them flow readily and save labor in manipulation produce 

 absorbent concrete unsuitable for use in marine structures. On the other hand, ex- 

 cessively dry mixes or mixes with too little fine aggregate cannot be worked into a 

 plastic mass under commercial conditions of concreting and are equally undesirable. 



