Paul wrote:
Thanks both. Blackjack is a tar-like bitumen that I assume acts as a
waterproof barrier between the two layers of concrete. To be fair, the
two ends of the run are dry, and where it wasn't covered with carpet
it is now dry, so I assumed that his method worked OK, and it was just
concrete under the carpet that was not dry as it was covered.

6-8 weeks is normal in these conditions less if you heat it.

Juts be patient and leave the carpet up for now.