Q: What should you do if rain hits after the slab has been placed?
A: The important thing is to be prepared. If you're not prepared ahead of time, you can't do much of anything. Have some portable shelters on hand or at least keep a supply of plastic sheeting and lumber onsite that you can use to improvise a shelter quickly. The slab should be covered immediately in a way that keeps the rain away from the surface while draining the precipitation away from the slab. If the rain comes at a time when the slab is ready to be bull-floated, floated, or troweled, setup a portable shelter over a small portion of the slab and then uncover the slab under the shelter long enough to work on it. Move the portable shelter from place to place while carrying out that particular finishing operation. Then repeat the same procedure with subsequent finishing operations for as long as the rain lasts.