Performance of UHPC shear keys in box girder bridges-field and finite element study

Abstract

The typical shear key in some box girder bridges historically featured longitudinal cracks on the key-girder interface as a result of temperature gradients. Ultra high-performance concrete (UHPC) is currently used as a shear key filling material in some box girder bridges as an alternative to a non-shrink grout. However, field data on long term performance of UHPC shear keys is lacking in available literature and therefore a long term study is essential to understand its performance under temperature gradients. This study investigated the long term performance of UHPC shear keys by collecting data from a bridge for a period of 2 years. The field results indicate that UHPC shear keys experienced satisfactory performance with no indication of crack due to temperatures fluctuating between 40 °C to −30 °C. During the winter months, the tensile stress reached about 95 % of the cracking threshold of UHPC while the stresses in the girders exceeded the concrete tensile strength which indicates cracking. The results from the finite element model confirm the field behavior and indicate that damage and subsequent debonding initiates at the top of the shear key and progresses further down the shear key under additional increments of cold temperature. After debonding, the dowel experiences substantial increase in stress due to thermally caused degradation and therefore the presence of dowels is essential for preventing separation between the shear key and the girder.