Investigation on mechanical properties of steel-ECC/UHPC composite girders in negative moment regions

In order to improve the concrete cracking defect in the negative moment regions of steel-concrete composite girders, the engineered cementitious composite (ECC) and ultra-high performance concrete (UHPC) were introduced to replace the normal concrete (NC) to form the steel-ECC/UHPC composite girders. The tests of static mechanical properties in the negative moment regions were carried out, involving one steel-NC composite girder, one steel-ECC composite girder, and two steel-UHPC composite girders. The finite element analysis method was utilized to compare the strain, crack propagation, and distribution characteristics of different types of concretes. The influences of concrete type and reinforcement on the failure mode, bearing capacity and deformation capacity of steel-concrete composite girders were analyzed. Research results show that the steel-concrete composite girders have good overall cooperative performance under negative moments, and the failure modes are all bending failure. Cracks in the ECC and UHPC are delicate, especially in the ECC. Compared with the steel-NC composite girder, the cracking loads of steel-ECC and steel-UHPC composite girders increase by 2.00 and 2.75 times, the flexural stiffnesses increase by 17.23% and 35.73%, and the flexural capacities increase by 9.00% and 6.81%, respectively. Therefore, the UHPC has stronger crack resistance, effectively improving the crack resistance of bridge decks in negative moment region of the steel-concrete composite girders. Moreover, using the ECC and UHPC to replace the NC can enhance the flexural stiffness and bearing capacity of steel-concrete composite girders. There is no significant difference in the cracking load and early stiffness between the reinforced and unreinforced steel-UHPC composite girders. When the unreinforced steel-UHPC composite girder fails, the through cracks form, and its bearing capacity decreases by 13.39% compared to the reinforced steel-UHPC composite girder. As the ECC strength increases, the bearing capacity of steel-ECC composite girder improves significantly. The influence of UHPC strength on the bearing capacity of steel-UHPC composite girder is not obvious. The impact of reinforcement ratio on the bearing capacity of steel-UHPC composite girder can be divided into two stages. When the reinforcement ratio is below 1.6%, the bearing capacity increases significantly, and when it exceeds 1.6%, the growth rate of bearing capacity slows down.