Behavior of Composite Bridges at Early Ages and Under Thermal Loading

Abstract

This paper focuses on deflections and deformations that occur at early ages and under thermal loading in newly rehabilitated bridges comprised of steel girders made composite with concrete deck slabs. A full-scale prototype bridge was constructed at the Bert Cooper Engineering laboratory. The prototype matches the Eagle Chief Creek (Bridge "A") on SH14 in Woods Co. A large array of sensors and instruments were installed to measure strains, deflections, and temperature in both concrete and steel.

Concrete decks cast on steel bridge girders change in volume immediately upon taking initial set. Volume changes are caused by elevated temperature during curing, drying shrinkage, and creep. A large proportion of drying shrinkage occurs at early ages; this has led some to theorize that concrete shrinkage is responsible for poor elevation control of concrete bridge decks. Our research findings indicate that the primary cause for poor ride quality is poor elevation control during construction operations.

Concrete bridge decks are also subject to repeated temperature changes that cause temperature gradients through the depth of the slab producing internal thermal stresses that directly result in bridge deformations. Volumetric changes in concrete lead to upward and downward bridge deflections, differential strains and the resulting internal stresses within the concrete. The cumulative effect of these phenomena can adversely affect the ride quality, cause excessive deflections, decrease durability and reduce the long-term performance of steel girder bridges made composite with concrete decks.