I have a 20ft x 30ft (East-West x North-South) concrete slab (in two sections: 20x10 piece on the west, and 20x20 piece on the east) that I'd like to enlarge to about 30ft x 50ft in each dimension. Location is NE Arkansas. I want to build a post-beam wooden carport over it (work area, 2 cars, and a mower) possibly eventually walling it in to be a shop building.

It is on a bit of a slope: N side of slab is flush with ground, S side is about 1 ft above the ground with one corner having a bit of an erosion problem (SE corner), slab corner is exposed about 6 inches above the soil and the hole above the ground goes back about a foot or so under the slab.

I was thinking I could reinforce the exposed corner by digging out some around it and pouring a concrete footer underneath it and hard packing some dirt in as best I can. Then building up some earth from that corner going 20ft east to extend the slab to 50 ft. And then extend the north side 10 ft.

I read that I can drill holes and insert rebar horizontally to tie the slabs together, but I'd also like to pour a fresh slab, say 3 inches thick, above the old one. Should I place some rebar and/or concrete wire attaching it securely to the existing slab via drilled holes with rebar inserted vertically?

Trying to avoid the work/expense of destroying the old slab. It's about 25 years old and doesn't have any cracks, just a really rough surface.

Question: Best advice on pouring a new slab over the existing one and also enlarging it?

Slab Diagram

  • Yes, I don't have a problem with that edit. I'm looking for advice on how to enlarge the slab by pouring a new one over it given the details in the post (sloped ground, exposed corner). I've added an image to attempt to make that clearer. – jdods Jul 17 '17 at 19:51
  • Will the entire space be used for this carport? Have you got any drains installed in the old slab. Does the old slab have welded wire mesh and/or rebar? If the old slab has no metal in it I would bust it up and use the pieces for a rough patio. Looks just like flagstone. Do the footing of the slab correctly with expansion joints. Have you considered CMU pavers? Concrete modular unit pavers that are 2" thick? Butted up together on top of 4" crushed gravel and 2" mason sand that has been compacted leveled...makes an incredible surface. When a paver unit cracks, just remove and replace. – stormy Jul 20 '17 at 5:15
  • I don't know anything about the old slab's structure as I've only lived here for 4 years and it is about 25 years old. Is there a way to tell if it has rebar/mesh in it without destroying it? It doesn't have any drainage holes. I guess if I'm going to go through the trouble of paving over it, I can destroy a small section, maybe the exposed corner. I'll look into the CMU pavers, never heard of them. Would really like a complete slab though. – jdods Jul 20 '17 at 14:42
  • I may leave the west 20x10 section uncovered but the rest covered completely. I want space for 2 cars, a mower, and some room to work. I may eventually wall it into a shop building. – jdods Jul 20 '17 at 16:09
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    I am sorry I got the dimensions incorrect; you've got 1500 sq. ft. so for 4" of gravel you'll need 18 yards and for 4" concrete you'll need another 18 yards. Maybe 18.5. Check my math please I've got fried brains right now, grins! I'll am sure that that existing pad needs to be removed. You want your bed to be on subsoil, then gravel 4" thick and compacted. Who knows what is under the old slab and how sturdy it is especially after cars drive on it. Concrete is great stuff but once it cracks it stays cracked. Expansion joints...you'll have to use pressure treated 2X12 for that low side. – stormy Jul 21 '17 at 1:06

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