Connection schemes incorporating keyed joints and external prestressing are advantageous for simplifying the construction procedures of precast concrete segmental beams (PCSBs). However, their effectiveness relies heavily on the external prestressing and the shear performance of the joint sections, leading to complex biaxial stresses in the concrete. The Concrete Damaged Plasticity (CDP) model is commonly used to simulate the nonlinear behavior of concrete joints in PCSBs for its adaptability in analyzing the multiaxial stress state of concrete and adjustable parameters in Abaqus. On the other hand, calibrating the CDP model can be challenging if the parameters are not properly set, which results in a significantly different uniaxial and multiaxial mechanical behavior. Consequently, large differences between experimental and numerical results could be obtained. This study aims to conduct parametric studies on CDP modeling of high-strength and self-compacting reactive powder concrete (RPC) ranging from the element scale to the structural scale. A constitutive relation that accounts for size-dependent localized damage is proposed for isotropic CDP models. From the numerical investigation, it is suggested that large viscosity parameters should be avoided for structural modeling calibration, especially with high dilation angles. The damage evolution rules also affect strain softening accuracy, significantly impacting overall structural response, including stiffness and plastic deformation. However, these rules should be determined after conducting a mesh sensitivity study, as finer mesh divisions capture crack initiation and stress concentration more effectively, while coarser meshes can overestimate strength by averaging localized damage. In this study, a 10 mm mesh size stabilizes results, guiding the choice of strain softening relation and damage evolution based on mesh modifications. Further studies are needed to explore these effects in detail. This work aims to enhance the prediction of direct-shear behavior in keyed joints of PCSBs using finite element methods and provides recommendations for parameter selection in the CDP model.