Experimental study on fatigue behavior of UHPC joint in negative moment region of π-shaped steel–concrete composite beams

To improve the crack resistance of concrete bridge decks in the negative moment region of π-shaped steel–concrete composite beam, a novel ultra-high performance concrete (UHPC) joint was first proposed. Based on an actual bridge, a 1:2 scale model was then designed as the specimen. A fatigue test was conducted to investigate the variation of its structural properties during fatigue loading, and a static test was performed to evaluate its residual bearing capacity. The test results show that the new UHPC joint has good crack resistance under fatigue loading and can meet the engineering requirements. The stiffness of the specimen significantly decreases after 3 million fatigue cycles. However, without significant fatigue damage on the surface of specimen, plenty of stiffness is reserved. It is found that there is no significant slip between the concrete deck and steel girder, and between the UHPC and normal concrete in the static test after fatigue, indicating that the studs inside the π-shaped steel–concrete composite beam are arranged reasonably and the interaction of the overall structure is reliable. Furthermore, the ultimate bearing capacity of the specimen decreased by only 9.4% from its design value after being subjected to 3 million fatigue cycles. It is revealed that the application of UHPC in the composite beam effectively improves its crack resistance, resulting in a smaller reduction in bearing capacity. This paper can provide theoretical foundation for the preparation and application of the π-shaped steel–concrete composite beams with UHPC joints.