Performance of UHPC in shear keys of box beam girders

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Precast prestressed concrete box girder bridges have been used for bridge construction for decades. The cross section of this type of bridge consists of number of prestressed concrete box girders. The construction process usually entails the following steps: girders are individually erected on site, transversely post-tensioned together, and then connected using small longitudinal voids that are formed by the two outside walls of adjacent girders, called shear keys that are cast on site using high strength concrete. The depth of the shear keys is just a fraction of the depth of the girder. The bond between the precast concrete of the girders and field cast high strength concrete plays a significant role to assure monolithic behaviour and adequate load transfer between girders. Load transfer between the girders is essential for proper function of the bridge structure. The bond strength at the interface needs to be strong enough to prevent cracking. If cracks develop, they reduce service life of the structure by allowing water and chlorides to penetrate into the connection and start slowly deteriorating the concrete. Cracks in the joints need to be prevented at all costs, and therefore the industry partner for this project, WSP, is using a new high strength durable material, ultra-high performance concrete (UHPC), in one of their bridges they designed for Manitoba Infrastructure in Morris, Manitoba. WSP would like to design a shear key that does not need to be post-tensioned, to reduce cost, speed up construction and increase service life of the bridge. This project will study the behaviour of shear keys in the Little Morris Bridge. The shear keys and the box girders will be instrumented, the bond development between the girders and shear keys will be tested over time, and the durability of the structure with particular emphasis on the load sharing between girders and the shear key performance will be studied. The data collected from the site will make it possible to decide if design of shear key without post-tensioning is feasible in future structures. The project will train three HQP, and will enable the industry partner to further their expertise in design of box girder bridges with durable shear keys.