Nondestructive Determination of Response of Shear Keys to Environmental and Structural Cyclic Loading

Abstract

Adjacent box girder bridges use grouted shear keys to share loads among beams. These keys tend to crack and leak. A full-scale portion of an adjacent box girder bridge is being used to test the performance of grouted shear keys under environmental and cyclic loads. The pulse velocity method is employed to find cracks in the shear keys. Two separate tests were conducted. In the first, shear keys that were grouted in late autumn cracked soon after casting, before any load had been applied. Data from instruments embedded in the beams and shear keys showed large discontinuities in strain caused by freezing temperatures. The strains were much larger than strains that occurred under loads corresponding to the weight of an HS20-44 truck. The beams were subjected to 41,000 cycles of loading, simulating HS20-44 wheel loads. No new cracking occurred from the loading, but cracks caused by temperature propagated under loads. In the second test, the keys were grouted in summer. Higher temperatures caused by the sun’s heat on the top of the beams again caused large thermal strains, which cracked the shear keys. These keys were subjected to 1,000,000 cycles of load corresponding to an HS20-44 wheel load. As in the first test, the load itself did not cause new cracks but the existing thermal cracks propagated under the load.