If creep cavities on grain boundaries grow by the constrained diffusive mechanism, partly cavitated boundary facets act mechanically like microcracks. Two cell models, one based on a cylindrical cell and the other on a regular tetrakaidekahedron, are worked out numerically to explore the influence of a distribution of microcracks on the constitutive response of a creeping solid. The results confirm the predictions of analytical estimates based on the differential self-consistent method of Rodin and Parks . The Rodin and Parks model is then combined with the Robinson model  to provide a comprehensive model covering primary, secondary, and tertiary creep under arbitrary loading conditions. The combined model is implemented in the finite element code ABAQUS. The model is adjusted to a set of creep curves for a 12% Cr steel (X 20 CrMoV 12 1), and tests on compact specimens are successfully modeled.