IMPROVING SHORT SPAN PRECAST CONCRETE BRIDGEPERFORMANCE BY MAKING SIMPLE SPANS CONTINUOUS

摘要

A large number of precast concrete bridges with multiple simple spans were constructed inAlberta from 1950 to 1980. These bridges typically consisted of standard precast girder sections erectedside by side and connected either by continuous longitudinal grout keys or discrete connections. Thestandard girders were typically box or inverted channel shapes eliminating the need for a cast in placedeck.Many of these bridges have fallen under duress due to corrosion damage from chloride exposure at thegirder ends and through cracked longitudinal joints between girders. Waterproof deck joints and thinoverlays with waterproofing have been marginally successful in recessing the corrosion damage.However, deck joints tend to be high maintenance items and both concrete and asphalt overlays exhibitreflective cracking along the longitudinal seems within a few years allowing continued exposure of thegirders to chlorides.An effective solution employed in Alberta has been to make the girders continuous for live loads byinstalling a continuous reinforced concrete overlay and a pier diaphragm. The reinforced overlayeliminates intermediate deck joints and provides crack control over the longitudinal joints. The pierdiaphragm provides continuity and protects the girder ends. The overlay is extended down the abutmentends of the girders to protect the ends from chlorides. In some cases the overlay improves live loaddistribution sufficiently to offset the effects of the overlay's dead load on the bridge load capacity. In othercases, shear strengthening is required.This technique was first developed for fairly short spans (up to 12m) more than 15 years ago. It is stillmost commonly used in this span range, however it has been more recently applied to longer spans up to30m in length. While some problems have developed on specific bridge types this system is workingeffectively. Making a simple span bridge continuous has a slightly higher initial cost than installing deckjoints and adding a protective wearing surface. However, the long term benefits of eliminating these repairprone measures along with reducing serviceability stresses makes the system attractive on a life cyclecost basis.