Vapour barrier
Concrete is pourous, so in order to keep the floor of the shop dry when
the ground it wet, a sealed layer of plastic is put down under the
concrete.
Here 6 mil poly is used, the edges should be overlapped by about 12"
and its all seamed with tuck tape. Where pipes come through, the tape
is used to make a seal around it.
Any accidentaly holes punch in the plastic are fixed up with more tape,
as can just be seen here where a piece of rebar punched a hole in it.
This plastic should extend to the verry edge of the form. It may be
good to save this as a final task before pouring concrete, as it may
get moved around before.