the plane of the deck. Shortening of beams and slabs may occur because 
of creep and may result in troublesome joints and undesirable tensile 
stresses. This problem can be minimized by using high-strength concrete, 
avoiding extreme prestress, and curing the concrete adequately (at least 
one month) before installing the components. Connections must be de- 
signed to distribute stresses caused by shortening and thus can minimize 
cracking. A girder designed as a simple beam must not be raised or 
supported at the center since tensile cracking will develop due to neg- 
ative moment. Units must always be raised or supported at the designed 
bearing points. 
Lightweight prestressed concrete is not used as extensively in 
marine structures as in buildings located inland. Gerwick (1959) states 
that excellent durability in seawater is attainable if the proper type 
of lightweight aggregate is used; supposedly, he is referring to expanded 
shale aggregate in pellet form (i. e., aggregate that has been fired 
after, rather than before being graded). The use of natural sand with 
lightweight coarse aggregate is recommended because concrete containing 
all lightweight aggregate has a lower modulus of elasticity, undergoes 
greater deflection in bending, and creeps more than conventional sand- 
and-gravel concrete; blending natural sand with lightweight coarse 
aggregate serves to counteract these effects. The submerged weight of 
lightweight concrete is about half that of conventional concrete. 
Increasing the thickness of concrete cover over prestressed strands 
causes considerable increase in cost. Gerwick (1959) considers a 2-inch 
cover adequate for most American harbor structures, but recommends further 
research to ascertain the most economical use of prestressed concrete in 
marine structures. 
Loss of prestress in prestressed concrete may be due to relaxation 
of prestressing steel, steam-curing of concrete, shrinkage of concrete, 
elastic shortening of the structural member, creep of concrete, anchor- 
age slip, and friction of the steel tendons during post-tensioning. In 
an assembled structure some of these losses occur simultaneously and 
affect each other. The structural designer must know the causes and 
magnitudes so that the completed assemblage will perform satisfactorily 
(Podolny, 1969). 
Lightweight-aggregate concrete usually undergoes greater shrinkage 
and swellage, due to moisture change, than does normal-weight-concrete. 
Nevertheless, corrosion of steel reinforcement is no greater in fully 
compacted structural grade lightweight concrete, incorporating chemically 
inert aggregate such as expanded shale, than in conventional concrete. 
Moreover, such lightweight-aggregate concrete is more fire resistant 
than is concrete containing sand and gravel. 
“Portland Cement Association (Design and Control of Concrete Mixtures, 
llth Edition, 1968) defines structural lightweight concrete as having a 
28-day compressive strength in excess of 2,500 psi and an air-dry unit 
weight of less than 115 lb. per cu ft. 
