Cracking Tendency and Drying Shrinkage of Silica Fume Concrete for Bridge Deck Applications

This paper presents the results of a study of the cracking tendency and drying shrinkage of silica fume concretes designed for full-depth and overlay placements of highway bridge decks. Mixtures for each application were prepared over a range of silica fume contents and water-cementitious material ratios (w/cm). Concrete laboratory specimens were prepared for testing of unrestrained drying shrinkage and tendency to crack using a restrained ring geometry. Regression analysis was performed on data developed from the laboratory testing and response surface models, and analyses of variance were carried out. The results demonstrate that the tendency of concrete to crack is influenced by the addition of silica fume only when the concrete is improperly cured. When concrete is cured for 7 days under continuously moist conditions, there is no significant effect of silica fume on the tendency of the concrete to exhibit early-age cracking. Silica fume has little effect on the ultimate (long-term) shrinkage of concrete; concretes containing silica fume do not shrink more than otherwise identical concretes without silica fume. At early ages, however, silica fume concrete mixtures show somewhat higher shrinkages than their conventional counterparts. Concrete containing silica fume is more sensitive to changes in w/cm than conventional concrete in terms of the shrinkage that develops at early ages. It is recommended that specifications for silica fume concretes in bridge deck construction include a provision for 7-day continuous moist curing of exposed surfaces.