Base under concrete patio

[Bolson32]

We have, essentially, a ground level deck coming out of the back of our house and we hate it. It's 30 years old, ****** shape, paint flaking everywhere, and low and behold my stupid dog got stuck under it today so I had to tear up some boards to get her out. Anyway, tearing it out and pouring a patio has been on the todo list for awhile now, so I guess it's getting bumped to the top.

My question is around base for a detached patio slab on grade. Anyone skipped the hauling in material and compacting it? My cousin works for a concrete company and he's saying that they're not actually hauling in base material and compacting it for patios anymore. Depending on the type of soil they're working with, a lot of the time, disturbing it is worse than just leaving it be. The logic being, that you can't actually compact anything near as well as mother nature and time has done. So just scrape off the top soil, and pour right on what's there to minimize disturbance to the already in-place soil.

In my head, this makes a ton of sense, our soil drains pretty well, and I'm not worried about moisture seeping through as it's a patio, not a basement or a garage. He's willing to do it, but it'll obviously cost more, and if there's no real reason, I'd just as soon skip it. Save the time and money.

What do you guys think?

 

[corvette43056]

The most basic question how long do you want it to last.

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[nadogail]

IMHO, because ships have been made of concrete, the amount of moisture coming through a cured patio slab will be almost nothing.

 

[ConCretin]

It's a prevalent misconception that you must remove and replace existing soils under a slab on grade. Obviously you want to remove topsoil and organics but once that is complete, if the native soils are dry and stable and the elevation works, you are good to go.

It's a good idea to proof roll the sub base before starting. A few inches of stone will provide a capillary break and keep the surface from getting rutted or muddy from rain and construction traffic if that's a concern, but it's not structural and not absolutely necessary especially for a patio.

For any free floating exterior pad, I'd recommend wire reinforcing to help hold everything together due to the likelihood of movement.

 

[Bolson32]

It's a prevalent misconception that you must remove and replace existing soils under a slab on grade. Obviously you want to remove topsoil and organics but once that is complete, if the native soils are dry and stable and the elevation works, you are good to go.

It's a good idea to proof roll the sub base before starting. A few inches of stone will provide a capillary break and keep the surface from getting rutted or muddy from rain and construction traffic if that's a concern, but it's not structural and not absolutely necessary especially for a patio.

For any free floating exterior pad, I'd recommend wire reinforcing to help hold everything together due to the likelihood of movement.

This is pretty much what I'm looking for. Shouldn't be much of a delay between prep and pour so the stone is probably unnecessary. I'll get the organics scraped off and give it a look, but I can't imagine any elevation change to be required.

Also plan on using rebar and fiber-reinforced so hopefully it lasts a good long while.

 

[ConCretin]

Also plan on using rebar and fiber-reinforced so hopefully it lasts a good long while.

I'd skip the fiber - it's irrelevant unless this is a very large patio. Thoughtful and timely control joints are all you need.

The key to longevity is a sound concrete surface. Slabs fail due to a deteriorating surface far more often than structural failure.

If the slab will be exposed to freeze thaw, it should be air entrained and get a broom or float finish. No steel trowels. Control the mix water carefully to avoid a whole range of potential surface defects. Allow a max slump of 4" or use a mid range water reducer if you need more workability. No vapor barrier to minimize bleed water. Cure the slab well to ensure a strong durable surface.

Good luck!

 

[Bolson32]

I'd skip the fiber - it's irrelevant unless this is a very large patio. Thoughtful and timely control joints are all you need.

If the slab will be exposed to freeze thaw, it should be air entrained and get a broom or float finish.

What would you consider very large? It'll be 624sq ft.

Also, is a float finish acceptable in Minnesota? I know broom finish is for traction and I don't want to be slipping everywhere. But, I do like the look of a smoother finish than the sandpaper finish you get from brooming.

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[ConCretin]

624 sf (25x 25 ish) is pretty manageable from a crack control standpoint. Fiber is most beneficial in large commercial slabs.

I like the look achieved by sawing the slab in a regular pattern, either square or diagonal to create the look of large tiles. You control cracks and create a decorative look. Use a colored concrete and you're stylin.

Air entrained concrete is more resistant to freeze thaw damage when moisture is present but it can delaminate if you finish it with a steel trowel before all the air and bleed water escape. Using a broom, magnesium or wood float avoids the problem. Not to mention how slick a steel trowel finish is.

 

[NUTTSGT]

If the guy does brooming right, give it a winter or two and it'll be nice. The first winter freeze/thaw seems to knock off the high stuff, leaving a nice textured finish.

 

[yeldogt]

With a concrete flat work .... I like to use a stronger mix and do exposed aggregate. Small river stones ,,, nice looking ..still flat .. no slip.

 

[Bretny]

Your in MN. You get freezing temps so will want to prep the area. Good drainage under will keep it from moving in the frost.

 

[Bolson32]

Your in MN. You get freezing temps so will want to prep the area. Good drainage under will keep it from moving in the frost.

No offense, but this just seems like such an old husband's tale to me and just a prevalent misconception as LLWillysFan has already stated. Frost heaves are definitely a thing here but they occur, essentially no matter what soil is around and they occur much deeper in the ground than a few superficial inches that the base material would be.

I would think, in order for a few inches of base to have any impact at all you would have to have a ton of freeze/thaw cycles within the top few inches of ground. Which, really doesn't happen once winter sets in here. Sure, you'll get snow melt when you shovel, but that moisture actually evaporates, it doesn't somehow magically seep through your pad and into the base material that is frozen solid. Once the frost sets in, it's typically in the first few inches of soil for most of the year.