Q.: If you are pouring a large supported slab, 60 feet by 100 feet, and it starts to rain, what measures can you take to protect the slab?
A.: You really have to be ready ahead of time for the possibility of rain. You should have a good supply of polyethylene sheet on hand—enough to cover the slab and direct the runoff away from the slab surface.
Just in case a rain comes at a time when finishing operations ought to be taking place, you should have a portable shelter big enough for finishers to work under, or at least have the means of improvising a shelter with polyethylene and lumber. The slab should be entirely covered with polyethylene, and the polyethylene picked up only when and where the shelter is being used. Then the slab is re-covered before the shelter is moved to the next station.