More recent units are 30 feet long by 8 feet wide by nearly 3 feet high 
and have walls 3 inches thick. Though pontoons of normal weight concrete 
(145 pef) possess the advantage of inertia, lightweight concrete (50 to 
115 pcef) has been used. Compressive strengths range from 2,000 to 6,000 
psi at age 28 days. 
Apart from structures ashore, prestressed concrete is a promising 
material for use in pontoons and flat-bottom barges. Ferro-cement- 
constructed vessels, described previously, are generally thin curved 
hulls although a few prestressed ferro-cement flat-bottom hulls have 
been built in England and New Zealand. Prestressed concrete flat-bottom 
barges, prestressed as well as conventional, are known to have been built 
in the Philippines, China, and the Soviet Union. Prestressed concrete 
barges used in the Philippine Islands are 200 feet long by 55 feet wide 
by 14 feet high. 
Twenty years ago Freyssinet (1950) stated, "All methods of assembly 
by prestress of units, whether themselves prestressed or not, into 
larger structures can be used for the assembly of floating units of 
dimensions dependent upon the available means of floatation. By further 
prestress it is simple to link such assemblies together once they are 
floating, thus enabling the building up of structures whose dimensions 
are independent of the means of floatation. This method is applicable 
to any floating works: docks, ships, quays, and breakwaters, whether 
floating or fixed; floating platforms monolithic over very large areas; 
power-stations utilizing energy derived from the sea (thermal, potential, 
or dynamic); as well as floating structures with or without hollow 
interiors later anchored to the ground (locks, docks, etc.)." 
Recently a British firm of consulting engineers prepared a report 
for Shell United Kingdom Ltd relative to concrete seadromes at London, 
New York, Chicago, Los Angeles, Tokyo, and Caracas in Venezuela (Civil 
Engineering and Public Works Review, 1970). They are proposing a float- 
ing cellular prestressed post-tensioned concrete raft to provide an 
aircraft runway length of 14,000 feet for accommodating jumbo jet aircraft. 
The structure is designed to withstand aircraft exerting a take-off load 
of 1,000,000 pounds. Precase prestressed concrete cells, each 100 x 100 
x 3 1/2 feet in size and filled with expanded polystyrene, would be 
floated into position and joined together by post-tensioning laterally 
and longitudinally. Protection against waves would be provided by a 
floating breakwater. More recently, another floating airport (FLAIR) 
concept of precast, prestressed concrete has been suggested (Weidlinger, 
1970). Obviously, these concepts reflect the structures foreseen by 
Freyssinet 20 years ago. 
FRESHWATER STRUCTURES 
Notable exceptions, i. e., structures not in a marine enviornment 
per se, include Lake Pontchartrain Bridge (fixed) and the two Lake 
Washington Causeways (floating). 
