The unit weight of concrete containing normal aggregates will range 

 from 22.0 to 24.3 kilonewtons per cubic meter (140 to 155 pounds per cubic 

 foot) but can be increased with the use of heavy aggregate to 28.3 (180 

 pounds per cubic foot) usually at some additional cost. The technique of 

 placement and the size of the armor unit will determine if reinforcing is 

 required in dolos or tribar units. Heavy units, exceeding about 178 kilo- 

 newtons (20 tons) will require reinforcing if placed from a landside unit. 

 Placing armor units from floating equipment where the wave action may cause 

 bumping of the units may require reinforcing in armor units as light as 89 

 kilonewtons (10 tons). 



Table 23 lists the concrete armor units in use today and shows 

 where and when the unit was developed. Table 24 lists projects using 

 tetrapods , tribars, quadripods, and dolosse in the United States. Com- 

 monly used types of units are illustrated in Figure 32. 



e. Other Structures . Concrete has been adopted to many kinds of 

 marine structures as monolithic or cast-in-place structures as well as 

 precast or prestressed units. Concrete is an optimum material for marine 

 structures as it combines durability, strength, and economy. The ability 

 to produce concrete in most any geometric form gives it a high adaptability 

 to most any location and condition of use required. 



(1) Navigation Structures . Prestressed concrete piles are used 

 for navigation light standards. Navigation aids located on breakwaters, 

 along the shoreline as lighthouses and radio signal towers are usually of 

 concrete construction. Also included as navigation structures are mooring 

 anchors for bouys of all kinds. 



(2) Piers and Wharves . Concrete is the most used construction 

 material in building piers and wharves located either on the coastline or 

 in protected harbors. All the elements of pier construction such as 

 piles, dock units, pier girders, substructures, or bulkheads are built of 

 concrete or a combination of concrete and wood or steel. Even then concrete 

 may be used to protect the wood or steel from erosion, corrosion, dryrot, 



or marine organisms attack. Figures 33 to 36 are examples of commercial 

 concrete structures located on the coastline. Special piers have been 

 constructed for product loading lines, waste water disposal, and other 

 discharge lines. Figure 37 shows the piling and deck structure of a 

 recreation pier extending from shore to the open ocean. There are, of 

 course, innumerable concrete piers and wharves constructed in bays and 

 protected harbors along the world's shoreline. 



The use of concrete in these structures is most feasible because 

 it is durable, is readily available in most locations, can be pro- 

 duced in virtually any size or shape, and is economical. Recent 

 developments in precast and prestressed concrete units provide a means of 

 fast and simplified construction procedures with the final structure being 

 both stable and durable. 



Concrete piling is easily manufactured in most any length to about 36 

 meters (118 feet), although longer piles have been made and commonly in 

 round, square, octagonal or hollow core cross sections. Either reinforced 

 or prestressed concrete piles can be designed to support very heavy loads. 



157 



