the rit;i(lit>- or llcxil)ility "f its nicnihcrs; tliu interior structure of members; the depth 

 of concrete encasements over steel or wood and the general form or shape of structures 

 and members. Effects of exposure include such considerations as location with respect 

 to wind and waves; the elevation with reference to tide; the nature of foundation sub- 

 strata and the height above bottom; the nature and intensity of loadings, whether 

 impact or static loads; the fluctuation of temperatures, both of air and sea water; 

 the humidity of the air; and the frequency of wetting and drying out of the members. 



While neither space nor data are available for a discussion of all these phases of 

 type and exposure, structures may broadly be classified as "simple" or "composite" 

 in regard to type, and exposure may be roughly judged as "Harbor" or "Ocean" 

 exposure in regard to the intensity of disintegrating conditions. This classification 

 covers in a broad way the essential differences in type considered in relation to dur- 

 ability, and visualizes the actual extremes of exposure to which structures are commonly 

 subjected. 



In this discussion, "Simple" structures will be understood as including homo- 

 geneous concrete structures greater than 12 inches in minimum section and having no 

 steel, wood or other structural material embedded at less than 6 inches from the surface. 



"Composite" concrete structures will be understood as including structures having 

 members of reinforcing steel, structural steel or wood, embedded at less than six 

 inches from the exposed surfaces. Complex members of concrete having sections less 

 than 12 inches in dimension will for purposes of design and construction be classified 

 with "composite" structures. 



"Harbor" exposure will be understood as referring to structures located in pro- 

 tected harl)ors where the members are only occasionally exposed to sea water spray 

 and are well braced and anchored against impacts. 



"Ocean" exposure will be understood as referring to structures located in the 

 ocean and exposed to continual spray from surf and heavy impact of waves. 



Materials 



Irrespective of type or exposure, the materials used for marine concrete construc- 

 tion should be high grade commercial products. The aggregate should of itself be 

 dense, imperxious sand, graxel or stone, not subject to disintegration in sea water. 



Special attention should be given to the grading of aggregates and to the pro- 

 portion of fine to coarse aggregate, with a view to securing the most impervious 

 mortar consistent with the quantity of cement specified. This is accomplished by 

 cutting down the quantity of mortar through using a properly graded coarse aggregate, 

 thereby increasing the proportion of cement to fine aggregate, and by further increasing 

 the quality of the mortar by using a properly graded fine aggregate. The uniformity 

 with which aggregates hold their grading is also important. Materials having variable 

 gradings which require frequent changes in proportioning should not be used. Coarse 

 aggregate from J4-inch to 1 1 2 inch has been specified as the common and most de- 

 sirable grading. Aggregate of larger size than 1 1 2 inch recjuires so much care to prevent 

 separation that its use is not recommended. F"or special conditions where sections of 

 concrete are very narrow, coarse aggregate ma>' ad\antageously be reduced to 34- 

 inch maximum size in order to facilitate depositing. 



While it has been the practice in some cases to make special reciuirements as to 

 magnesia and alumina contents of the cement and to increase the fineness of grinding 

 beyond the requirements of the standard specifications, no cases of deterioration 

 specificalh- attributable to cement of the quality- specified have been reported. The 

 mixing with the cement of chemical waterproofing compounds is not advised in marine 



