Gravel for a base

by Vicki
(Fallbrook, CA )

Question:

Should there always be gravel in the base of a concrete patio? If there is enough drainage in the soil, is it OK to not have gravel as a base?

Answer:
Hi Vicki, thanks for visiting...thank you for the photo, look like you have a pretty good size pour coming.

With the Simpson Tie column base, you're well on the way for a nice wood patio cover, that's cool!

Great questions because the advice with regard to this matter is all over the map. I guess it depends on who you ask.

As to the first question my answer is No, and Yes to the second question.

My opinion is based on what I see in the field which is different from what I read on paper. I have torn and replaced a lot of cracked concrete patios and driveways. Some had gravel base some didn't. Some had welded wire mesh, rebar, fiber mesh as reinforcements, and some were unreinforced. Seeing these led me to believe that it doesn't really matter what we do.

The only time I transport material for concrete slab, patio, or driveway is if I need to raise the existing ground elevation. If the ground elevation is good and undisturbed, I pour directly on it.

There is no doubt that compacting to stiffen the disturbed portion of the soil or newly transported material is good. Going beyond that is something I don't recommend.

When prepping concrete patios, we are fooling around with the top 3 to 8 inches may be 12 inches max if we have the money. I believe that the factors that affect the soil behavior which will in turn affect the concrete is a lot deeper. May be 3 ft or 10 ft below who knows. The only way to know is to do soil testing but even with tested soil I still see concrete cracked prematurely.

From the photo, you are ready to pour. Make sure your concrete crew pull the wire mesh upward as they pour. By design wire mesh should be at mid height of slab, mesh at the bottom of the slab is useless.

During concrete pours, everyone is focus on leveling the slab, very rare someone pull up the wire mesh, even when they do it's too high,too low, very inconsistent. This will lead to a wave like wire mesh reinforcement.

The only time I recommend reinforcements if the slab is for strength i.e. truck loading. When that happen rebar is the way to go other than that I pour patio and driveways unreinforced.

More important than reinforcements on this type of slab are the control joints. They should be 10 ft square max.

Hope this help
Tonga