That's a very interesting question. Unfortunately, thanks to both the general scarcity of sources for the early medieval period and the complete indifference of those few sources we do have to building methods, we have little sense of the era's architects thought. It is likely, however, that after the fifth century, and especially outside the major Mediterranean cities, few builders would have known anything about Roman concrete.

First, a bit of background. Concrete, ancient or modern, has two basic components: aggregate – the more or less finely-ground stone or ceramic fragments that make up most of its volume – and the lime-based cement that binds these particles together. By trial and error, the Romans discovered that mixing pozzolana (a volcanic ash found near Rome and in Campania) with lime created an immensely durable mortar. Over time, they began to use this mortar as a building material in its own right, and quickly discovered two novel uses for it. Thanks to mineral properties the Romans knew nothing about, their variety of concrete could harden underwater, and was in fact strengthened by interaction with seawater (see this (jargon-filled) article). Equally importantly, the Romans realized that concrete could support vaults and domes of unprecedented size - whence the Pantheon and the great halls of Roman baths.

So: making true "Roman" concrete required access to Italian pozzolana or an equivalent material; when Herod built his massive harbor at Caeserea, for example, he imported vast quanities of pozzolana from the vicinity of Naples to create a proper hydraulic cement. As the imperial government (always the most important sponsor of large-scale building programs) weakened and (in the west) collapsed, access to pozzolana became increasingly difficult. This was compounded by the fact that few cities undertook monumental building projects in late antiquity - churches, the largest structures in most early medieval settlements, typically made no use of concrete.

Procopius (Buildings, 1.11-18-20) mentions Justinian using Roman hydraulic concrete to build a new harbor in Constantinople. But after this, aside from a few mentions in Isidore of Seville's Etymologies (e.g. 15.8.1), concrete virtually vanishes from the literary record. Early medieval builders had no occasion to use concrete, and no access to the materials they would have needed if they had.