An improved numerical model for the nonlinear analysis of reinforced concrete (RC) slabs subjected to blast loadings is proposed. Instead of using a plastic-based model, which has been popularly used in the blast analyses but has the limitation in describing the strain-softening behaviors, a strain-rate-dependent orthotropic constitutive model that directly determines the stress state from the stress-strain relations defined from the biaxial strength envelope is considered. To minimize the mesh-dependency of FE analysis, in advance, the ultimate strains in the stress-strain relations are determined on the basis of the fracture energy concept, and additional considerations for the strain rate effect are implemented. Moreover, to cope with the development of large deformation concentrated within the plastic hinge region after yielding of reinforcement, the bond-slip effect is also taken into consideration by introducing modified bending stiffness, which is determined from the energy conservation law. Finally, to verify the validity of the proposed numerical model, correlation studies between the numerical results and experimental data for RC slabs are conducted.