This article provides instructions to design, build, mix, and pour cement, to make concrete sidewalk slabs. Welcome to Shebang, your free information site.
Sidewalk plywood forms
two forms side-by-side
Concrete Slabs Instructions
Slab Forms
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 woodshed. I was able to divide the 4 feet by 8 feet plywood sheet into two equal sides and was able to fit five slabs on each half of the plywood. I used 2 inch by 2-inch struts to make my form dividers (actually 2x4, cut down the center). Nail the outer 2x2 perimeter to the 1/2 inch thick plywood. Make the cross interior struts removable by securing them from the outer edge 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.
Tip
When you have completed making the forms and all the dividers are in place, paint the form, this helps prevent the cement from sticking and aids in replacing the dividers in the same place, after each use. The outline will show where the dividers fit. 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 in a wheel barrel by hand
Oil the complete inside of the form with any kind of oil, such as used engine oil. This helps prevent the concrete from sticking to the form. 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. 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 all-purpose cement. Use a garden hoe and mix the two dry ingredients. Add a bucket full of water and continue mixing, adding more water as needed. The correct mixture is one part of cement to three parts of sand.
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, you have 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, trowel led and edged
Finishing
After you pour the cement into the forms, trowel it level. Wait about half an hour for the cement to firm up, time depends on temperature. 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 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. 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. Replace the dividers, in their original position, and secure
with screws. Oil the forms and you are ready for the next mix.
Finished woodshed floor
This view shows concrete slabs in place, forming the floor for the wood shed.
The slabs were laid over weed control fabric.
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