Another concern is the fact that the bridge will be standing in a saltwater bath for at least 75 years. After it hardens, excess water evaporates, and the concrete shrinks and begins to crack. As the steel rusts, it breaks up the concrete, creating other pathways for corrosive salt. Here in the north, the situation is made worse by freeze-thaw cycles. Then it freezes.
Frozen water expands, cracks it some more, and lets more water seep in.
The constant stream of cars and trucks crossing the bridge, pounding over potholes, can add to the cracking. There are many ways bridge builders can lessen the amount of cracking that occurs. The two large piers that flank the drawbridge piers on the new bridge, for example, are sheathed in granite, which is impervious to saltwater. The granite sleeves also protect the piers from blows by ice floes and wandering tugboats. Other piers have precast concrete sleeves.