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re: Tying New Concrete to an Existing Foundation - Yes or No?
Posted on 6/24/14 at 12:01 pm to ForeverLSU02
Posted on 6/24/14 at 12:01 pm to ForeverLSU02
Like others have said, do not tie it in. Use a good expansion material and pour it as a separate slab. Chances are your foundation has already settled. The new slab is going to settle while the existing is done. This will cause either cracks or voids under the concrete.
0
Posted on 6/24/14 at 12:01 pm to civiltiger07
Thanks a lot guys 
Posted on 6/24/14 at 12:01 pm to urinetrouble
Agree with others - would not tie into existing foundation. There will be differential settling no matter how it is done. Advantages to doweling into foundation are that you'd always have a flush connection b/t patio and house. Disadvantage is that if differential settling occurs, and the dowel connection is stronger than the reinforcing several feet into the house, your foundation could crack.
Also, I don't think you need 6" for a patio, I'd go 4" with welded wire as reinforcement, maybe #3's on 18" c-c if you really want rebar. For reference, 6" driveway approaches are required in the area I live in (and build roads for 4 years in).
Also, I don't think you need 6" for a patio, I'd go 4" with welded wire as reinforcement, maybe #3's on 18" c-c if you really want rebar. For reference, 6" driveway approaches are required in the area I live in (and build roads for 4 years in).
Posted on 6/24/14 at 12:04 pm to urinetrouble
quote:Probably just wire mesh.
How do you plan on reinforcing the slab?
quote:Not sure yet. The main goal this summer is to just get slab poured.
And how do you plan on covering it in the future?
Posted on 6/24/14 at 12:05 pm to BSG
quote:
Why are you going 6" thick? What's the future plans?
TWSS
Posted on 6/24/14 at 12:07 pm to ForeverLSU02
quote:
Probably just wire mesh.
Probably gonna end up at the bottom.
quote:
Not sure yet. The main goal this summer is to just get slab poured.
Where are the columns going to be? Outside of the patio?
Posted on 6/24/14 at 12:11 pm to urinetrouble
quote:
Probably gonna end up at the bottom
you can pull the mesh up while pouring the concrete or use spacers to hold it off the ground. I would not use rebar. Its and unneeded expense. For a patio that only has people and maybe a column from a lean-to on it the stresses in the concrete will be negligible. No need for rebar reinforcement.
Posted on 6/24/14 at 12:11 pm to Happygilmore
quote:
i dont think you should tie in because the new pad will settle, no matter how much you get the grade packed. when it settles even if its 1/2" you put alot of stress on the existing foundation if you are tied into it.
This makes a lot of sense - treat it as 2 separate slabs - tie them together on the surface with paving stones or something can both add a nice touch and save you from mechanical distortion of either slab.
I concur with 4" unless you're putting a building or semi-regularly leaving vehicles or equivalent weight items. For just walking around, patio furniture, outdoor kitchen, etc., 4" is more than enough - that cuts your concrete materials by 1/3 right there - with a concomitant decrease in weight of the final slab.
Posted on 6/24/14 at 12:16 pm to urinetrouble
quote:
Probably gonna end up at the bottom.
If only there were products that prevented this.
Posted on 6/24/14 at 12:18 pm to civiltiger07
quote:
you can pull the mesh up while pouring the concrete or use spacers to hold it off the ground. I would not use rebar. Its and unneeded expense. For a patio that only has people and maybe a column from a lean-to on it the stresses in the concrete will be negligible. No need for rebar reinforcement.
#4's at 48" o.c with mesh on top wouldn't be much of an added expense.
Posted on 6/24/14 at 12:23 pm to ForeverLSU02
quote:
Ok. That answers my next question. So would you go 4"?
Yeah, 4" will hold up for what you need.
Posted on 6/24/14 at 12:23 pm to urinetrouble
quote:We'll probably anchor columns into the slab itself. Thinking about using galvanized steel columns.
Where are the columns going to be? Outside of the patio?
Posted on 6/24/14 at 12:24 pm to urinetrouble
Ok but now you have to spend time tying all that rebar. What are the #4 bars accomplishing? There is no need to use flexural rebar in for a 4" patio slab. And plus if the rebar is at the center on the slab it will be accomplishing nothing.
Posted on 6/24/14 at 12:24 pm to ForeverLSU02
Something like this
Posted on 6/24/14 at 12:24 pm to ForeverLSU02
quote:
We will be pouring a back patio slab (about 20'x30'x6")
What size footings are you going to have?
4" is plenty thick for what you are doing and no need to dowel in the new to existing.
Posted on 6/24/14 at 12:25 pm to urinetrouble
quote:
#4's at 48" o.c with mesh on top wouldn't be much of an added expense.
But then he'd have to tell people what he did when they ask.Posted on 6/24/14 at 12:25 pm to ForeverLSU02
quote:
We'll probably anchor columns into the slab itself. Thinking about using galvanized steel columns.
So you're going to anchor the columns into a 4 inch slab?
Posted on 6/24/14 at 12:26 pm to civiltiger07
We recently extended our back patio with a slab, and there was no rebar or dowels tied into the existing slab.
Posted on 6/24/14 at 12:26 pm to ForeverLSU02
quote:
Is it a post tension slab?
quote:
How do I find this out?
The easiest way to tell if you have a post tension slab is to look at the exterior and every 36" you should have a round circle about 1-1/2" diameter that has been filled with concrete once the cables have been pulled.
Posted on 6/24/14 at 12:29 pm to civiltiger07
quote:
What are the #4 bars accomplishing?
Keeping the mesh from sinking.
quote:
And plus if the rebar is at the center on the slab it will be accomplishing nothing.
I guess all those elevated structural slabs designed with one layer of rebar at the center of the slab are hanging on by a wing and a prayer.
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