PLACING/FINISHING
CONCRETE:
Before you start the pour or placement, insert isolation joint material
in all locations where the slab meets an existing structure. The
strips, sometimes called expansion strips, are readily available.
The top
of the joint material should be flush with the top of the slab,
or even better, 1/4 inch lower. Never have it above the slab. In
some circumstances, that arrangement could be a hazard to foot traffic.
If you
don't have permanent interior forms to serve as control joints,
cut control joints once the concrete has been floated. The joints
should fall every 5 feet in a walk that is over 3 feet wide. In
a walls from 2 to 3 feet wide, set control joints every 3 feet.
As the
concrete is placed, use a shovel to distribute it as evenly as possible
between the forms. Then, with a straightedge 2X4, screed the top
of the concrete, removing excess that pushes up before the screed
board. This is where a helper comes in handy.
Once
the concrete is level between the forms, use a wood float to smooth
the surface. When the sheen on the concrete is noticeable, you can
finish trowel the surface, although dragging a stiff broom across
the surface gives a pleasing nonskid top.
Keep
the concrete damp for a week or so. You can use a sprinkler on a
garden hose for this. Or, you can cover the concrete with burlap
and keep the burlap damp. After 10 days or so, the forms may be
stripped from the job and any backbilling with earth completed.
Rain
can damage green concrete; the raindrops will pit the surface. Try
to avoid rainstorms, but if one comes along, protect the slab with
tarps or old newspapers. A pitted surface resulting from a heavy
rainstorm can sometimes be troweled over and smoothed-if the concrete
hasn't set up too much. However, once set, the material can't be
troweled.
