New Porch Slab off of existing workshp

[ustmd]

Looking for some information from the brain trust.

I have an existing workshop that is slab on grade. Here is the build thread:

http://www.tractorbynet.com/forums/projects/184711-not-big-not-pretty-but.html?highlight=


I am thinking about pouring a 8 x 10 slab in front of the french doors. Eventually I build some type of shade structure over the slab (either a free standing pergola or a porch roof tied into the shop).

We are on black gumbo clay which moves significantly (during the summer we get 4-6" cracks that go down to infinity and beyond).

My question is how should I handle the joint between the new slab and the old slab? Do I try to tie them into together (re-bar drilled into the existing slab) or keep them separate? If I keep them separate, how should I handle the joint between the new slab and old slab to account for the inevitable movement?





Thanks

 

[davesl708]

When my new driveway was poured the contractor drill the garage floor and inserted rebar then placed some 3/4" closed cell foam between the garage floor and the new pour as an expansion joint.

 

[bigtiller]

I would pour the slab about 1/2" lower than the shop floor is, and separate them with a foam expansion joint. Let them move independently. Make sure to caulk the expansion joint with a hi-quality caulking made for concrete.

More importantly, I would prepare the base with 6" of crushed 3/8" rock and compact it very well.

 

[handirifle]

If you tie them together, use rebar, drilled and epoxied into the old slab. If you do not, make sure the roof (if any) is NOT tied to the existing building, If it moves enough it will pull away from the wall. I agree with bigtiller on the gravel base. Here in CA EVERYTHING is built for earth quakes. And everything needs a strong footing. Depth and width of the footing depends on the weight to be added above.

Either way will work. With that kind of soil I would ask some local contractors about the type and amount of reinforcing (rebar) to use. Here the rebar for the slab is buried into the footing, and tied to the footing rebar as well and left standing about 4ft above the footer. Then when the footing cures, the rebar is bent to the ground, and all additional rebar is wired to those pieces, and the slab is poured as a cap over it all. Not sure if it's done like that everywhere, but this does reduce the number and size of the cracks we get from ground movement.