Archaeological, structural, and compositional observations of the concrete architecture of the Basilica Ulpia and Trajan’s Forum

New archaeological excavations begun in 1998 have exposed the complexity of construction design and planning of the Basilica Ulpia and Forum of Trajan, raising new research questions and providing a unique opportunity to reexamine sectors of these structures that were the object of older excavations dating to 1812 and 1924-1933. In this article, we integrate archaeological, geological, and mechanical perspectives to reconsider diverse aspects of Roman building technology and the construction of the Forum. In particular, we investigate forms and structural stability of the integrated structural elements that constitute the upper stories of the Basilica Ulpia. Furthermore, we document the compositions of brick faced, opus latericium walls and the wall core, or nucleus, of these structures, as well as those of concrete vaults and staircases, focusing on the petrographic and material characteristics of their coarse aggregates (caementa) and the fine aggregates (harenae fossiciae) of their pozzolanic mortars. With this interdisciplinary approach, we evaluate the technical expertise of Roman builders in designing the complex concrete elements in the architectures of the Basilica Ulpia and Forum of Traja