Most concrete bridge decks develop transverse cracks soon after construction. The authors recently completed a study for the National Cooperative Highway Research Program (NCHRP) that examined the causes of transverse cracking in new concrete bridge decks. They found that a combination of shrinkage and thermal stresses causes most transverse deck cracking, not traffic loads and vibrations before or during concrete hardening.
Generally, decks are made with high-strength concrete, which is more prone to transverse cracking. This risk may be reduced by selecting a concrete mix that does not excessively exceed the required compressive strength. The lowest possible cement content should be used to minimize the risk of transverse deck cracking by reducing shrinkage, initial hydration temperatures, and thermal stresses. In general, concretes with higher aggregate contents and lower cement paste contents are less likely to develop cracks. Water reducers are recommended to reduce water and paste volume and the risk or severity of early cracking.
The first large stresses in a new concrete bridge deck usually develop during the first 12 to 24 hours, when the concrete temperatures change rapidly during early hydration. To reduce concrete temperatures during this cycle place concrete during cooler, less windy weather, place cooler concrete, mist the concrete during placement and wet cure, and shade the deck. Sunscreens, windbreaks, fog mist, and monomolecular curing films can all reduce evaporation rates.