Implementation of an Improved Shear Key Detail in the Buffalo Branch Bridge

About the report:

Adjacent box beam bridges are economical bridge systems for accelerated bridge construction.  The box beams are constructed at precast plants and are traditionally connected by a shear key filled with grout.  This system is typically used for short spans with low clearance restrictions.  However, due to the grout deteriorating and debonding from the precast concrete in the shear key, reflective cracking propagates through the deck, which allows water and chemicals to leak down into the joints.  This can lead to corrosion of the reinforcing and prestressing steel inside the precast member.  This necessitates the bridge being rehabilitated or replaced, which negates some of the economic advantage it had to begin with. 

This research project aimed to design a rehabilitation plan for an adjacent box beam bridge with deteriorated joints using very high performance concrete (VHPC).  VHPC was chosen as an economical alternative to the proprietary ultra high performance concrete (UHPC) and extensive material tests were performed.  The less expensive VHPC generally performed slightly below UHPC; however, compared to conventional grout, VHPC had higher compressive and tensile strengths, a higher modulus of elasticity, gained strength faster, bonded better to precast concrete, was more durable over time, and shrank less.  The rehabilitation also included pockets cut into the beams across the joints, which are referred to as cutouts.  A short reinforcing bar was placed in each cutout, and the cutouts were filled with VHPC along with the shear key.

The repair method developed in this research project was used to rehabilitate the Buffalo Branch Bridge.  Live load tests were performed before and after the rehabilitation to determine if the new connection detail resulted in better load distribution and smaller relative displacements of adjacent beams.  Strain and displacement measurements indicated that the soffit beams were more engaged in carrying truck loads after the repair, and relative vertical displacements of adjacent boxes were much smaller.

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