The invention of pozzolanic concrete (opus caementicium) provided ancient Roman engineers with an extraordinarily versatile and durable building material, which made possible the construction of some of the largest and most complex vaulted structures built in antiquity. In 2010, in collaboration with Ingraffea, we conducted an experimental study on Mode-I fracture properties of reproduced Imperial Roman pozzolanic mortar using an ad-hoc arc shaped bending test. In the present study we use these data in conjunction with post-critical compressive response data available from the literature to construct a non-linear damage plasticity formulation for opus caementicium suitable for 3D implementation in Abaqus Explicit. We use this FE formulation to evaluate how the structural design of the vault supporting system of Diocletian’s Frigidarium (298-306 AD), consisting of flanking shear walls and monolithic granite columns, affects the development and propagation of fractures and ultimately the static and seismic stability of the vault.