units, and loading complete units for shipment to the construction site. 
Barges were used for transporting the precast members to the site be- 
cause this was most economical. Equipment for driving the piles and for 
setting the structural members had to accomodate 25-ton loads with 
precision and speed. All personnel were well-experienced in fast and 
accurate contructional procedures required in meeting the schedules of 
erection. Each precast deck slab was 20 feet long by nearly 7 feet 
wide and weighed 13 tons. The precast piles were 1.5 by 1.5 feet in 
cross-section and lengths were as great as 115 feet. 
Several years later, during the period 1953 to 1955, hollow precast 
concrete units about 12 feet high were used in constructing the piers 
for the Richmond-San Rafael Bridge in San Francisco Bay (Gerwick, 1954). 
The 47 precast units, which were pairs of hollow truncated cones in 
configuration and named '"2-bell" or '4-bell" units, had walls 8 inches 
thick. Each 130-ton unit was lowered underwater to rest on previously 
placed precast concrete grids and filled with tremie concrete. The 
cost of the substructure for the 4 mile long bridge was $14,234,000. 
During 1956 and 1957 the Eisenhower Lock and the Snell Lock were 
built on the St. Lawrence Seaway. These concrete structures began 
operating in 1958 and in the subsequent 10 year accomodated nearly 
50,000,000 tons of shipping which involved over 7000 lockages at each 
lock. Winter weather is severe and long; mean minimum air temperature 
is 7°F and termperatures as low as -44°F have been experienced. Deteri- 
oration of concrete in the Eisenhower Lock first appeared in 1962 and 
has been undergoing distress to the present (Waugh, 1969). The nearby 
Snell Lock, built under similar specifications, remains sound. It is 
believed that water forced into all voids in the concrete by successive 
filling and emptying, followed by freezing while dewatered during winter, 
is the primary cause of trouble. As late as 1968 cracking was being 
corrected by post-tensioning of steel tendons anchored in the good con- 
erete of each 44-foot monolith and replacing all deteriorated concrete 
with new concrete. The history of this structure is an example of the 
combined effects of stresses caused by contraction of concrete due to 
thermal decrease, differential shrinkage due to moisture changes, and 
tension due to live loads. 
The most recent precast concrete work of major magnitude, and 
probably the most significant in the category of fixed maritime concrete 
structures, began in 1966 when the first of 57 tube sections was fab- 
ricated for use in the San Francisco Bay Area Rapid Transit System 
(BART). The precast tube sections (See Figure 2.1) vary in length between 
273 and 367 feet; 36 are straight, 19 have either horizontal or vertical 
miters, and two have both types of miter. Mitered tube sections allow 
for vertical and horizontal curves in the nearly 4 mile long underwater 
tube alignment. Each section is 24 feet high and 47 feet wide, and in 
cross-section resembles the outer end of binoculars; each hole, which 
serves to accommodate a one-way track,is 17 feet in diameter (Murphy & 
Tanner, 1966). The installation of the last tube section was completed 
