In a technique for widening prestressed concrete (PC) deck slabs, the connection between the old and new deck slabs is crossed by reinforcement and subjected to external prestressing force. A total of six PC deck slabs under a concentrated load were tested to evaluate the effect of the initial prestressing level, concrete strength of the new deck slab, and surface roughness of the interface. A three-dimensional FEM analysis was also conducted to complement the experimental results. The results showed that all PC deck slabs failed in a brittle manner when shear cracks developed at the interface between the old and new deck slabs. Thus, the effective area of the interface for the PC deck slabs under a concentrated load is proposed in this study. Furthermore, by using the value of the effective area instead of the total area of the interface, it has been confirmed that predicted shear capacities obtained from JSCE Specification, AASHTO, and fib Model Code 2010 provided a good lower limit for the experimental ultimate capacities.