How To Prepare Soil For A Concrete Patio?
Posted on
January 16th, 2010
by Jerry
I am planning to have a concrete patio poured at my new house. How can I be sure the contractor is properly preparing the “underlayment” before pouring the concrete? I know that a certain amount of dirt should be removed (about 8″), that 3/4″ gravel and fine gravel should add back about 4″ when compacted, and that there should be a pre-soil treatment as well as a vapor barrier installed before the rebar goes down, but what else should I be sure gets done before the 4″ thick concrete gets poured?
Also, what PSI rating should the concrete be for this type of application?
I am in the Southern California area, in case weather is a concern.
The newly updated standards for ground-supported monolithic concrete flooring is to ONLY use vapor barrior when it is an area that is air-conditioned. So, if your patio is “open-air” like a porch and garage, and will not have conditioned air above it, then do not use any vapor-barrior.
Stand firm on this, as vapor barrior pushes too much water to the surface during concrete curing and can cause more cracks then needed.
Dampen the ground before placing the concrete so that the dirt does not wick moisture away from the fresh concrete. This will also help in cracking.
Vibrate the concrete, if you can, by tapping on the form-boards with a hammer. Place #4 rebar at a minimum of 12 inches on center, and tied at intersections with wire-ties. Sufficient chairs will support the steel if placed one-chair per each 4 square feet.
Consider “control joints” if the longest deminsion is longer then 10 feet in length. These will help the cracking.
Do not pour in the heat of the day, if possible. Early morning is best.
Good luck.
When I had mine built, they built the frame first, of course. Then it rained big time, and the frame contained the water about 6″ deep for a couple of days. This compacted the soil really well, such that even without rebar, it has held up for 30 years without shifting of cracking. It is 14 x 16.
2500 to 3000 psi concrete should be more than adequate. Keeping the water content of the concrete as it is poured to a low level is a good idea to minimize crack later. It it is real soupy when it is poured, there is a greater chance of cracks from shrinkage later. A four inch slump should be about right. It is workable but not too soupy.
sounds like you got it covered. just compact if all before the concert comes
have you ever tried to move a slab of concrete?? just flatten the ground so it is solid