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Interfacial slip can cause rigidity degradation and stress concentration in fiber-reinforced polymer-concrete hybrid beam (FCHB). Therefore, precisely evaluating the composite action between fiber-reinforced polymer (FRP) and concrete of FCHB plays a pivotal role in structural analysis and design. Previous push-out tests showed that most connections for FCHB behave nonlinearly in load-slip relationships even at a low load level. However, existing analytical equations have their limitations due to the assumption of linear load-slip interfacial relationship which is not suitable for FCHB. The originality of this paper is to propose a finite difference method (FDM) to elaborate the interfacial slip and shear stress. FDM agreed well with the analytical solutions of the linear load-slip relationships for connections. Results indicate that higher accurateness can be obtained by using more elements. And 40 elements for half span of FCHB can reduce the error of numerical results to 1%. Then, the proposed FDM was expanded to predict the interfacial behavior of FCHB considering nonlinear interfacial load-slip relationships. It was found that perforated FRP rib connections can ensure nearly full composite action and the bolted connection can lead to a very high slip level. The use of ultra-high performance concrete (UHPC) results in a higher degree of composite action than normal concrete. The deflection considering slip was computed by adding deformation under full composition action and that caused by the slip effect. It was suggested that high strength steel bolts are effective both in normal concrete and UHPC. When the slip modulus is suggested to be larger than 20 kN/mm, the capacity per bolt should be larger than 20 kN. Full article