Bridge Expansion Joint for Bridges with Post-Tensioned Decks

Post-tensioned concrete bridges have unique movement characteristics that affect the design of expansion joints. The post-tensioning force causes elastic shortening of the deck, and creep under the sustained prestress causes additional long-term shortening. These movements must be included in the joint gap calculation. Elastic shortening occurs immediately when the post-tensioning force is applied. For a post-tensioned concrete bridge with an average prestress of 10 MPa, the elastic shortening is approximately 10/30000 = 0.033% of the bridge length. For a 100-meter bridge, this corresponds to 33 mm of shortening. The joint must be installed after the post-tensioning is complete to avoid this shortening being imposed on the joint. Creep under the sustained prestress causes additional long-term shortening. The creep coefficient for concrete is typically 1.5-3.0, meaning the creep shortening is 1.5-3.0 times the elastic shortening. For the example above, the creep shortening would be 50-100 mm over 20-30 years. The joint gap must be set to accommodate this long-term creep shortening. Shrinkage of the concrete deck also contributes to long-term shortening. Concrete shrinks by 200-400 microstrains during the first year after casting, with most of the shrinkage occurring in the first 90 days. For a 100-meter bridge, this corresponds to 20-40 mm of shrinkage. The joint must be designed to accommodate the combined effect of elastic shortening, creep, and shrinkage. Gap setting for post-tensioned bridges requires careful calculation of the expected movements at the time of joint installation. The gap must be set to accommodate the remaining creep and shrinkage movements after installation, plus the full thermal movement range. The calculation must consider the age of the concrete at the time of installation and the expected future movements. Monitoring of post-tensioned bridge expansion joints is recommended to verify that the long-term movements match the design predictions. Gap measurement at regular intervals over the first 10 years allows the creep and shrinkage rates to be tracked. Any unexpected movement should be investigated to determine the cause.