A pozzolan, as partial replacement of the cement, is another 
effective precautionary measure. The pozzolanic silicate combines, in 
the presence of moisture, with any excess calcium hydroxide present in 
the concrete. This reaction precludes any leaching of the lime, due to 
evaporation at the concrete surface, since additional cementitious 
compounds (calcium silicates) are created within the mass of concrete. 
These additional silicates strengthen the concrete to resist cracking; 
they improve its durability by increasing its resistance to the effects 
of the sulfates present in the seawater. 
Cement content, water/cement ratio, and workability are the funda- 
mental factors relative to durability of concrete. The least permeable 
concrete is usually the most durable concrete. The cement content 
should not be less than 8 bags per cubic yard of concrete. The aggre- 
gate/cement ratio should not be less than 2.6 (by weight). 
The durability of concrete exposed to freezing and thawing in sea- 
water is enhanced by incorporating an air-entraining admixture. As the 
effects of freezing and thawing are much more severe in seawater than 
in freshwater, the optimum amount of entrained air should be from 10 to 
20% by volume (Lyse, 1961). The maximum size of aggregate is of minor 
importance relative to frost resistance of the concrete in a seawater 
environment. 
The resistance of the concrete to abrasion varies directly with 
both cement content and compressive strength of the concrete, and 
inversely with the water/cement ratio. This relation is valid regard- 
less of the aggregate quality or the combinations of aggregate used 
@GSmicheedo53)- 
If the precast units are cured in a high-pressure steam environment, 
their potential volume change (shrinkage due to drying and swellage 
due to wetting) is minimized and their strength (flexural, torsional, 
compressive) is maximized. Note, however, that improving the strength 
by increasing the cement content tends to increase the potential shrink- 
age, whereas improving the strength by complete compaction of the freshly 
mixed concrete serves in reducing the potential shrinkage. 
The concrete inspection program should provide not only quality 
control of concrete at the mixing plant, but should be so comprehensive 
that no discrepancies occur in the formwork, reinforcement, prestressing, 
concreting, curing, and post-tensioning. ACI Standard 311 contains 
recommended practice. 
The following are apparent advantages of prestressed concrete in a 
floating structure: The concrete is in compression, high prestress 
tends to increase durability of the concrete, fatigue resistance is high, 
elasticity is equally good in cold or warm water, and the small steel 
area warrants thinner members which are more flexible than conventionally 
reinforced concrete members. 
Precast prestressed concrete structural elements are desirable 
because they can be standardized, but it is vital that the strength/ 
weight ratio always be high. Prestressed deck slabs can be connected 
transversely by post-tensioning to make them act as a lateral girder in 
