I am installing a concrete driveway on expansive soil. I was planning on going with 6″ of 3,000 PSI concrete w/ rebar 12″ OC. I’m not sure whether to use road base or sand and how to get good compaction. Any advice?
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The primary concern is before you do anything else, set your grade so that the area drains away from the driveway. If you don't control the moisture content of the clay, it really doesn't make a bit of difference what else you do. Seriously. You should see what expansive clay can break.
By pitching the area around the driveway to drain away from the paved area you can help control wide variations in moisture content where it does the most damage.
Next, cut the native soil to finish grade minus 14 inches. Rip this grade up a little, make sure it's not too wet or dry, and compact it with a jumping jack or a pad-foot roller. You need a ramming or kneading action for clay. Do not waste your time with a plate, it will do more harm than good.
Now put down a layer of geotextile, which is not the same as weed barrier. This will increase the shear strength of the finished product (it functions as an assembly), and it will keep your coarse aggregate from disappearing down into the clay.
Next, you don't want sand. Crushed stone (NOT round stone)or an 8" of state-spec base course is good. Compact this thoroughly. In this case a large (like 800 pound)reversible plate works fine. The little 200 pound plates are useless for anything but patching asphalt.
You can rent all this stuff cheap.
Now you can put your concrete down.