The dynamic structural behaviour and failure mechanisms of the reinforced concrete (RC) slabs under the combined blast and impact loading is numerically evaluated using the software LS-DYNA. The developed material model for concrete includes the plasticity behaviour under dynamic loading. The developed model includes the modified equation of state (EOS) which accounts for the loss of cohesive property and increase in compaction strength of the concrete material. The failure surface includes the effect of strain rate along with the lode angle and damage variable. The behaviour of concrete at low and high hydrostatic pressure is predicted with the developed model. The model has been formulated using a user-defined material model (UMAT). The predictions of the developed UMAT and in-built Winfrith concrete model (WCM) is compared with experimental results available in the literature. The developed concrete model can predict the failure mechanisms and the dynamic response of the slab accurately when compared with the results of WCM. Only limited experiments are available on the behaviour of RC slabs under combined blast and impact loading. Hence, the validation for analysis of the impact and blast loading scenario is carried out separately with the experimental results. The performance under the combined loading scenario is analysed by varying the sequence of applying the impact and blast loads. The slab subjected to impact-blast loading showed an increased dynamic response. The slab shows severe spallation, scabbing because of the flexural shear mode of failure caused by impact loading before the initiation of subsequent blast loading. The RC slabs subjected to longer impact duration before the initiation of subsequent blast loading under impact-blast loading scenario exhibited a more dynamic response. The failure of RC slabs under combined blast and impact loads shows increased localized damage and the stress concentration at the support caused by impact mass. The effect of slab thickness and reinforcement percentage is also studied by varying the velocity of the impact mass. The statistical analysis is carried out to study the uncertainty in the stress response of the developed model using the random variation of the input material parameters. The analysis is carried out to find the most influential input material parameter and the parametric study considered to study the combined impact and blast loading response of the slab. © 2022 Elsevier Ltd