The focus of the preceding chapters has been on the innovations in the use of materials and construction techniques involved in the creation of large and technologically advanced concrete vaulted structures in imperial Rome. The period from Augustus to Constantine is one in which the Roman world underwent great transformation, the nature and causes of which are often the subjects of debate. As a means of generating an overview and putting the conclusions into context, I employ the four criteria for technological innovation described at the end of Chapter 1: (1) accumulated knowledge, (2) evident need, (3) economic possibility, and (4) cultural/social/political acceptability. In what follows, I use these four criteria to explore some of the most salient issues involved in understanding the changes that affected vaulted construction during the three and half centuries under investigation.

ACCUMULATED KNOWLEDGE

Perceptible changes can be seen in the way builders approached vaulted construction during the imperial period including the understanding of the properties of materials, of centering construction, and of the effect of form and mass on vault behavior. Much of the increased understanding no doubt came from years of experience with materials. Another, less direct, influence is that of military technology, which manifested itself largely in the use of timber construction and in metallurgy for making tools and connectors used on the building site.

The quality of the mortar gradually improved from the second century b.c. to the first century a.d.