Because bridge decks are exposed on top and bottom, they ice up faster in the winter and are often treated with deicing salts and/or other aggressive chemicals to keep the surface free of snow and ice. Concrete shrinks as it dries and hardens, leading to the formation of drying shrinkage cracks. These provide a path for chloride ions to reach the reinforcing steel.
Concrete has a pH of about 11-13. In response to contact with such a highly alkaline material, reinforcing steel develops a passivating film that protects it against corrosion. But, if chlorides entering the concrete via cracks and water channels (occurring from bleeding of excess water) reach the steel, the protective film may be destroyed, which in turn allows corrosion to begin.
The products of corrosion take up a greater volume than the constituents. An internal pressure develops, eventually leading to cracking and spalling of the concrete cover above the steel. This reduces the bond and anchorage of the steel to the concrete, negatively impacting the structural properties of the deck and its contribution to the bridge structure.
To minimize the need for early bridge repair, that corrosion must be prevented.
Though suitable without reservation for many applications, portland cement concrete (PCC) has an underlying limitation when used for roads and bridges that contributes to the need for repairs: It shrinks as it dries. Shrinkage is the bane of longevity of a reinforced concrete bridge deck because it eventually leads to corrosion of the reinforcing steel.
Although some of the shrinkage is due to hydration products forming, most is a result of the bleeding of excess water. Portland cement requires a w/c ratio of only about 0.22 for complete hydration. However, the ability to place the concrete where it needs to go requires about twice that. The bleeding of the excess water is a loss of some volume and leaves voids both in the mass and in channels to the surface.
As the PCC hardens and shrinks, tensile stresses develop, caused by restraint from reinforcing steel, forms, aggregate interlock, and other factors. While the concrete is still relatively fresh, the tensile strength is practically negligible. Therefore cracks develop as a result of the shrinkage stresses.