Guidelines for Adjacent Precast Concrete Box Beam Bridge Systems (2023)

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1   Guidelines for Adjacent Precast Concrete Box Beam Bridge Systems Adjacent box girder bridges are a solution of choice for short span (less than 100 ) bridges. e favorable span-to-depth ratio makes these bridges ideal when clearance is an issue. e adjacent box girder bridge can have either a composite, concrete deck; a non-composite, asphalt deck; or have no deck at all. Load is transferred between girders using grouted shear keys. Unfortunately, these shear keys crack and leak. Salt-laden water leaks through the joints and penetrates the girders leading to corrosion of the prestressing strands and non- prestressed reinforcing steel. Previous research has shown that the cause of the cracking is thermal movements. Shear keys are usually cast during the times of the year when the weather is warm. e girders naturally expand due to the warm temperatures, but the tops of the girders are heated during the day and expand even more. In the colder months, the girders contract placing high stresses on shear key grout leading to cracking. Research shows that live loads typically do not cause cracking but may propagate cracks caused by thermal stresses. NCHRP Project 12-95A, “Proposed AASHTO Guidelines for Adjacent Precast Concrete Box Beam Bridge Systems,” sought to nd a solution to the problem of shear key cracking and leakage. e solution focused on four major issues: nding a more crack-resistant shear key shape, improving the bond between the shear key material and the prestressed girder, nding a way to assess this bond, and evaluating the materials used for shear keys. e main tasks of the research were: (1) reviewing the literature for possible solutions; (2) performing a nite element analysis to understand the stresses that occur under both thermal and live load conditions and to understand the parameters that aect shear key performance; (3) proposing solutions to improve shear key performance; (4) performing testing to verify proposed solutions; and (5) creating suggested specications and details for the proposed solutions. e conclusions are: 1. Use of deeper shear keys is suggested, with full-depth shear keys being the optimal solution. Deeper shear keys provide more bonded area between the shear key and the girder. 2. Do not ll the top 4 inches of the shear key with grout. e thermal gradient is the most severe over the top 4 inches of the girder. Not lling the top 4 inches with grout, combined with the use of a deeper shear key, moves the grouted area of the shear key out of the areas of greatest thermal stress and reduces the overall stress in the shear key. 3. Roughen the sides of girder and prewet them before placing the shear key material. Sand- blasting to a Concrete Surface Prole (CSP) of at least 4 or using an exposed aggregate S U M M A R Y

2 Guidelines for Adjacent Precast Concrete Box Beam Bridge Systems surface is suggested to improve bond. Prewetting the girder surface prior to placing the shear key material also improves bond. 4. Use a grout or concrete with high bond properties to fill the shear key. Grout or concrete used to fill shear keys should have a bond strength of at least 200 psi (pounds per square inch) when tested using the ASTM C1583 pull-off test. 5. Cast shear keys during cooler weather or at night to limit thermal stresses.