Pour
concrete over a solid, well-drained base, especially if it must
support heavy loads like vehicles. Because concrete slabs “float” on the soil,
soft ground or voids underneath may cause unsupported areas to crack under
heavy weight. Pack about 4 in. of sand or gravel over clay and other poorly
draining soils to provide even support.
Embed 3/8- or
1/2-in. rebar (steel reinforcement bar) in the concrete to
strengthen it. Don’t rely on steel mesh or plastic fibers. They help prevent
cracks but do little to improve strength.
Cut control joints
about 1 in. deep (one-fourth the thickness of the slab) when
finish-troweling. Concrete shrinks slightly as it cures and will crack at
regular intervals. You want the cracks to occur at the control joints. Space
the joints about every 4 ft. on walks and at least every 10 ft. on larger
slabs. If the concrete hardens before you can cut the grooves, cut them the
next day with a circular saw and diamond blade.
Finish-trowel the
surface after the water on top disappears. If you trowel concrete
too soon, too much cement (paste) will rise to the surface, weakening the
surface layer. But once the concrete begins to firm up and the water
disappears, be prepared to move quickly with your troweling. Concrete can
harden fast.
Click image to enlarge.
Use as dry a mix as you can
handle. Adding water to the concrete makes it flow down the chute
and fill your forms more easily, but it also weakens the final slab. A drier
mix is more difficult to pack into your form, especially around the edges, but
makes for a stronger, more crack-resistant slab.
Click image to enlarge.
Keep the concrete
damp during the curing period (about a week). In cool and moderate
conditions, cover it with plastic. Under hot conditions, above 80 degrees F,
frequently water it down with a garden hose to keep it moist. If kept wet,
concrete will reach about 90 percent of its maximum strength after about a
week.
