Numerical investigation on ultrahigh performance concrete (UHPC) cold joints for steel-UHPC composite bridges at hogging-moment regions

The steel-UHPC composite system has recently used in some bridges due to its light weight and long-term durability compared with steel-concrete composite system. However, the cold joint is required in many applications due to construction or design requirements. The cold joint at the hogging moment area is typically subjected to high tensile stress that may create cracks and therefore the cold joint shape is the key factor that influences the performance. The research numerically investigated the performance of the UHPC composite system using finite element simulation. The investigated parameters were cold joint shapes, reinforcement ratio and UHPC thickness. The FE results agreed very well with the filed results in load-deflections, load-strains, load-slips, and failure modes. It was found that the cold joint shape had negligible effect on the ultimate load with the differences within 4% among the investigated joints and the FE results confirmed that using T shaped joint had better performance. Meanwhile, the reinforcement ratio played a significant role on shifting the crack from the interface to the mid span. This would be beneficial as the fiber is typically less at the interface and as results the crack propagation will be faster at the interface. As the reinforcement ratio increases from 0.98% to 3.91%, the ultimate load increases up to 12% for the joints. The UHPC slab thickness of 55 mm is sufficient as in increase the thickness to 75 mm had negligible effects on the load capacity with a maximum increase of only 1.5%.