Most of the approximately 350,000 concrete bridges in the United States have concrete decks that develop avoidable premature cracking due to initial shrinkage soon after the pour. While the useful life of concrete can exceed 50 years, outdoor installations frequently do not achieve that. Obviously, the cement used in concrete can have a lasting impact on the life span of new concrete bridge decks. One growing solution is the use of Type K cement for new bridge construction and rapid hardening cement for repairs and new overlays. Proven, yet still under-specified, these special cements can help extend the useful life cycle of concrete bridge decks.
A good portion of the approximately $8 billion spent annually repairing, rehabilitating, strengthening, and waterproofing bridge decks is allocated for deck repair. However, using concrete products that help make the concrete more durable in the first place, such as shrinkage-compensated concrete for new construction, could eliminate much of the need for deck repair.
Because bridge decks are exposed top and bottom, they ice up faster in the winter and often are 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, forming shrinkage cracks, which provide a path for chloride ions to reach the reinforcing steel.
Concrete has a pH of about 11 to 13. Upon contact with such a highly alkaline material, reinforcing steel develops a passivating film that protects it against corrosion. However, from bleeding of excess water, chlorides can enter the concrete via cracks and water channels. If this happens, 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.
While suitable for many applications, when used for roads and bridges, portland cement concrete (PCC) has one limitation that contributes to need for repairs: it shrinks as it dries. Shrinkage is the bane of longevity of a reinforced concrete bridge deck because it leads, eventually, to corrosion of the reinforcing steel.
Although some shrinkage is due to formation of hydration products, most results from the bleeding of excess water. Portland cement requires a water-cement (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 surface channels.
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 and therefore cracks develop as a result of the shrinkage stresses.
Research conducted recently at the University of Kansas supports the conclusion that cracks in concrete are the major source of entry of chlorides. Test results show that at cracks, the average chloride concentration at a depth of 3 inches can exceed the corrosion threshold (1 pound/cubic yard) of uncoated reinforcing steel within one year. After two years, the threshold will be exceeded in most decks. Yet away from cracks, at a depth of 3 inches, the chloride concentration is less than the corrosion threshold even after a dozen years. The research also showed that chloride concentration increases as the bridge deck ages.
Using shrinkage-compensating cement can extend the useful service life of concrete. The most effective way to help ensure durable concrete bridge decks is to eliminate cracking of the concrete from drying shrinkage that results in tensile stresses during a time when the concrete still has little or no tensile strength. There is a solution to this problem: a shrinkage-compensating cement that can offset the tensile stresses caused by shrinkage.