Build a form that you can use more than one
time. Decide the size you wish to make
your slabs. I chose a size 16 inches by 17
inches to meet my need for a concrete floor
in my wood shed. I was able to divide the
4 feet by 8 feet plywood sheet form into two
equal sides and fit five slabs on each half of
I used 2 inch by 2-inch struts to make my
form dividers, actually 2x4, cut down the
center. Nail or screw the outer 2x2 perimeter
to the 3/4 inch thick plywood.
Make the cross interior struts removable by
securing them from the outer 2 x 2 perimeter
with screws, (drill pilot holes). Removable
slats make it easy to remove the slabs when
they have cured, they easily fall out of the
form when the dividers are removed.
When you have completed making the
forms, paint the form parts, this helps
prevent the cement from sticking.
Take a black felt pen and number each
slat and form, so the same slat will go back
in the same place, and the retaining screw
hole in the perimeter 2 by 2, will line up
perfectly with the cross slat.
Mixing concrete by hand,
in a wheel barrel.
Oil the complete inside of the form with any
kind of oil, such as used engine oil. This also
helps prevent the concrete from sticking to
Make crosses from rebar and wire
together in the center. I used a stiff
messenger cable that once was used to
support telephone cable to the poles.
Use an angle grinder to cut the messenger
cable to length.
Place one cross in each square. Working
with a full sized wheelbarrow, shovel 15
shovelfuls of sand/gravel into the
wheelbarrow. Add 5 shovelfuls of Portland
Use a garden hoe and mix the two dry
ingredients. Add a bucket full of water and
continue mixing, adding more water as
The correct mixture is one part of
cement to three parts of sand/gravel.
If you use gravel in your concrete slabs, the
mixture is 60% sand, 40% 3/4 inch crushed
gravel. If you use sand/gravel, the mix is
one part cement to 4 parts sand/gravel.
When you can bring up a wave of cement
without it collapsing, it is the right stiffness.
Forms filled with concrete
Shovel the mixed cement from the
wheelbarrow, into the forms, a wide flat
shovel works best. One sack of cement will
make ten concrete slabs. It takes two wheel
barrel mixes to make ten slabs.
Leveled, troweled and edged
After pouring the cement into the forms,
trowel it level. Wait about half an hour for
the cement to firm up, time depends on
Run a straw broom lightly over
the form to create a texture. Use a cement
edger and go around each individual slab.
If it is sunny day, cover with a tarp to
prevent the cement from drying to quickly.
Drying to fast will cause the cement to crack.
Sprinkle the concrete lightly with water two
or three times a day.
Wait two days, then remove the screws from
the divider slats.
Gently tip the form over allowing the slabs
to fall away from the form onto soft ground.
Take care in handling the slabs at this stage,
as they will not be fully cured for another 7
to 10 days.
Stack them upright so the air can
ventilate around them. Sprinkle the slabs
with water, as needed.
Finished slabs left to cure
After each use, scrape away all traces of
cement that may be sticking to the forms.
A garden hoe works well. Scape away what
you can and finish up with the garden hose.
Replace the dividers, in their original
position, and secure with screws. Re Oil the
forms and you are ready for the next mix.
Finished concrete woodshed floor
This view shows concrete slabs in place,
forming the floor for the wood shed.
The slabs were laid over a layer of sand
and weed control fabric.