damp cement floors under joists

Hi I have lifted up my joists to replace rotted soil plates and such and
have noticed that the ground underneath the joist (consisting of lightly concreted rubble) seems to remain damp (not puddling water, just cold and damp to the touch) most of the time. Should I cover the ground with a thick plastic membrane or maybe add another 1 inch thick level of waterproof concrete? I just don't want to have dry/wet rot or wood worm etc. issues in the future. What would you do ? Thanks.

Is this in an extension or internal part of the house ?.

Do you have breather blocks installed in the external walls ?.

I'm no expert but it sounds like you may need to get more air circulating under there.