It wasn’t long ago when deciding on a driveway material was easy: asphalt or concrete. Today, the concrete choice has expanded to include a multitude of concrete options.
Plain gray concrete is still the most often installed in our area, but the decorative idea is catching on.
Flat Concrete Slab Construction
Properly Prepared Sub Grade:
The sub grade should be compacted and have an even thickness. A standard is 4" thick – you want 4" thick continuously, not a 3½" to 4½" varying thickness.
The Correct Concrete Mix:
A 4000-psi, .50 water to cement ratio is best for concrete slab construction. This provides better wear ability and a "denser" concrete than the typical 2500-psi mix.
In our cold climate, air entrainment is added to the concrete at the batch plant. This entrained air allows any moisture which does enter the concrete to expand in the microscopic air pockets during a freeze/thaw cycle instead of putting internal pressure on the concrete.
Correctly Placed Joints
Joints should be at least ¼ the concrete thickness – so a 1" deep joint should be used in a 4" thick slab. Joints should also be spaced 2-3 times in feet the thickness of the concrete: so a 4" thick slab should have joints no farther than 8’ – 12’.
If joints are spaced too far apart, cracks will often occur where the joints should have been. We usually saw cut our joints within 24-48 hours of pouring the concrete.
For best drainage, the concrete should slope ¼" per running foot away from the home. If proper drainage is prevented due to the area of concrete being locked between two structures, a drain may need to be installed which will collect the water at a low point in the concrete and feed it down the drain. It is very important you direct water away from the slab. Many slabs have to be replaced due to downspouts emptying directly onto or next to the drive or sidewalk.
Reinforcement can be with either wire mesh, or steel bars placed in a grid pattern. With both methods, effort needs to be taken to keep the reinforcement in the center of the concrete. Note that reinforcement does not eliminate cracks – it simply holds them together.
After concrete is bull-floated, it should be left alone until all the bleed water on top of the concrete has evaporated. Starting the finishing operation too soon can trap surface water and create a weak surface.
Of course, there are many other important steps in building a concrete slab. The items listed here, though, are the most common causes of problems when not done properly.
Word About Maintenance
Plain gray concrete benefits by being cleaned and sealed periodically. But if this basic maintenance isn’t done – well, it still looks OK. Decorative concrete, however, needs to be maintained to continue to look good for years. And thus protect your investment.
Usually simply cleaning thoroughly once a year with a pressure washer, or floor polisher using a degreaser, then sealing the surface, will keep colors looking vibrant for years.