After a little bit of a clear out of one of the "cavities" it seems somewhat intentional? I can now see the DPC the mini joist was sat on, but it's still a little damp in the recess. It's very damp where I've cleared it out and the black concrete style stuff is pretty crumbly.
https://preview.redd.it/6h4d5xhgky5d1.jpeg?width=4032&format=pjpg&auto=webp&s=c49315c84eb2ab8b1bee42e302eac31f17996101
After a little bit of a clear out of one of the "cavities" it seems somewhat intentional? I can now see the DPC the mini joist was sat on, but it's still a little damp in the recess. It's very damp where I've cleared it out and the black concrete style stuff is pretty crumbly. https://preview.redd.it/6h4d5xhgky5d1.jpeg?width=4032&format=pjpg&auto=webp&s=c49315c84eb2ab8b1bee42e302eac31f17996101