Performance of UHPC in shear keys of box beam girders

预制预应力混凝土箱梁桥用于桥梁建设已有几十年的历史。该类型桥梁的横截面由若干预应力混凝土箱梁组成。施工过程通常需要以下步骤：在现场单独竖立大梁，横向后张拉在一起，然后使用由相邻大梁的两个外墙形成的小纵向空隙连接，称为剪切键，在现场使用高强度混凝土浇筑。剪切键的深度只是主梁深度的一小部分。梁的预制混凝土与现场浇筑的高强度混凝土之间的粘结对保证整体性能和梁之间足够的荷载传递起着重要的作用。梁间的荷载传递是桥梁结构正常工作的关键。界面处的粘结强度需要足够强以防止开裂。如果出现裂缝，水和氯化物会渗透到连接处，并开始缓慢恶化混凝土，从而降低结构的使用寿命。需要不惜一切代价防止接缝裂缝，因此该项目的行业合作伙伴科进在为马尼托巴省莫里斯的马尼托巴基础设施设计的一座桥梁中使用了一种新的高强度耐用材料——超高性能混凝土（UHPC）。WSP希望设计一种不需要后张的剪切键，以降低成本，加快施工速度，增加桥梁的使用寿命。本项目将研究小莫里斯大桥的剪切键的行为。剪切键和箱梁将被测量，随着时间的推移，将测试梁和剪切键之间的粘结发展，并研究结构的耐久性，特别强调梁之间的荷载分担和剪切键性能。从现场收集的数据将有可能决定在未来的结构中是否可行的设计剪切键不后张拉。该项目将培训三名HQP，并将使行业合作伙伴能够进一步提高他们在设计具有耐用剪切键的箱梁桥方面的专业知识。

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.