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I have a concrete patio that I'd like to use as the foundation for a CMU wall. I'd like the CMUs to be tied into the patio slab with rebar. I get that the gist is to drill a hole in the slab, put rebar in the hole, and then fill it in with concrete to anchor the rebar into the hole. What size rebar should I use, and how big a hole do I need to drill? If I use 1/2" rebar, can the hole be 5/8"? 3/4"? Does it have to be 1"? etc.

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    Often patio slabs are not constructed suitably to serve as a foundation. To be effective as reinforcing rebar must be monolithically bonded to the concrete. Simply sticking it in a hole will not achieve this irrespective of hole size. – user23752 Sep 1 '14 at 3:36
  • Not even pouring concrete in the hole to hold the rebar to the slab? – iLikeDirt Sep 1 '14 at 3:42
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    Nope. This is why plans and permits and inspection are required in many places. Typical method is epoxy. It requires technical rigor. It won't turn a patio slab into a foundation, however. – user23752 Sep 1 '14 at 3:49
  • Gotcha. Guess I won't do that. – iLikeDirt Sep 1 '14 at 3:53
  • Is your patio an add on or under the main part of the roof, like a lanai? If a lanai, the perimeter might have thick enough concrete to serve as a foundation. – Edwin Sep 1 '14 at 16:47
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It's been a long while since I worked construction, but I think I remember dowelling into concrete in a similar manner with a 1/2" drill bit and #4 rebar. We used a sledgehammer to drive the rebar pegs in. It MAY have been a larger bit, but I think I remember it being the same size.

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  • For the record, this was for adding an extra few inches width to the foundation for a brick shelf, when the home buyer decided they wanted brick instead of siding after the foundation had already been poured. So the extra foundation was poured beside the existing slab, with the rebar pegs tying the two together. – Doresoom Dec 1 '14 at 15:06
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I have been working with and around concrete for 20 years now. One thing is you cannot put cmu block or any other stucture on just a thin concrete pad. You must have a load bearing surface (footer) , and the specs will vary depending on soil type, frost line, and load. Second, if you wanted to dowel in to concrete you must use epoxy to anchor them. If using #4 rebar, i would go slightly larger on the bit (5/8") to allow enough epoxy for bonding and an easier initial fit.

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I happened upon this as I was shopping for a new rotary hammer bit. A couple of the posts here are correct. If you intend to construct a wall, the foundation component that will bear the load of such a wall will have to be designed to be up to the task. Furthermore, if you're tying into another concrete component -- like a footing or a reinforced concrete wall -- you must use a bit that allows for the placement of the rebar and the anchoring epoxy. As a mason and a builder, I've used 5/8" rotary hammer drill bits to epoxy #4 rebar to reinforced concrete as a "tie in" or an anchor. in general, it's been my experience that drilling the hole, blowing it out with compressed air and inserting a round "pipe" brush into the hole to loosen the concrete dust and debris is essential for proper bonding of the anchoring epoxy. To clarify, the round pipe brush is mounted to a cordless drill and used with compressed air, repeatedly, until the hole is dust free. Once you see no more concrete dust being forced out of the hole with the compressed air, you're ready to pump the anchoring adhesive into the hole. We generally fill said holes roughly 1/3rd full, using a caulking gun, before inserting the rebar into the hole in a rotating fashion. Any excess that comes out upon insertion of the rebar is simply wiped flat with a putty knife and a paper towel (to clean the putty knife). >> Caution, if the anchoring epoxy isn't coming out of the hole during insertion of the rebar, quickly pull the rebar out and add more anchoring epoxy. You must have the appropriate amount of anchoring adhesive to ensure a strong bond between the steel rebar and the reinforced concrete. Bear in mind that the epoxy is beginning to set/harden as soon as it's mixed in the application nozzle, so speed is of the essence when doing this type of work.

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