My first post here. Question is: We poured a slab for my new garage yesterday. Well, it is raining today and supposed to rain really hard in the afternoon. Is it going to bother my slab? It has been 15 hrs. since pouring of the slab to the first rain , it is 4000 psi concrete and temp around 45 deg, should I be thinking of covering it, or ? Thanks for the answers.

It is a little late to be thinking of covering the slab. What you have now is what you get. Chances are things will be OK. Come back and tell us how it looks.

Temp is fine... a 15 hour cure should be OK... covering it now would be the same as installing a smoke detector after your house has burnt down.
Mike