Concrete barrel vaults reinforced with stone voussoirs arches in the western provinces of the Roman empire (Mauretania and Baetica)

In the Roman towns of Sala (Morocco) and Baelo Claudia (Spain) concrete barrel vaults reinforced with stone voussoirs arches were used in the Capitolia as well as a cistern. At both sites the vaults represent an exception in the regional panorama of building techniques and this paper attempts to explain their use in relationship to the wider diffusion of vaulting techniques in the Roman world, and to assess why they were introduced in these specific projects. Both vaults at Sala (beginning of 2nd c. AD), as well as the vault at Baelo Claudia (AD 50-65) supported an upper floor level and this probably determined the need to reinforce them. Other explanations, however, for the use of reinforced vaults at these sites are possible. One concerns the adaptation of Roman concrete that, due to the use of local materials, needed a long setting time before acquiring its final strength. In addition, the adoption of reinforced vaults could aid the construction process, since small centerings could be used to first build the arches, followed by the concrete sections of the vault. Parallels for this kind of construction are found in the barrel vault of the temple of Diana at Nîmes, France (Augustan or Trajanic-Hadrianic period) and the Arch of Trajan in Mérida, Spain (ca. AD 30). In the case of the Capitolium of Sala, the arches also acted as vaulting ribs, such as those used in the architecture of Rome, in order to divert the loads of the upper levels of the building and channel them to the foundations. The spread of reinforced vaults could have been aided by the wide diffusion of a particular vaulting technique used in the heated rooms of the bath complexes of the western Empire, made of spaced-out arches in order to allow hot air to circulate in the remaining hollow spaces. Finally, these arches represent the importance of the diffusion of Roman monumental architectural models as a vehicle for the transfer of construction technologies to the provinces of the Roman Empire.