Site Built Rebar Chair

I'm not a fan of metal rebar chairs for bottom bars for this reason. Corrosion in the exposed steel provides a path for moisture to get to the primary bars. A lot of foundation contractors here will drive rebar into the middle of the footing with a piece of tape on it to mark the top of footing elevation, and pull it out before the concrete sets. But chairs are either concrete or plastic. In your case, probably plastic on concrete would have been the best route.

What about using bricks to support the chairs? My footing is oversized anyway so the loss of section should not be an issue.

Yeah, bricks or concrete "dobies" (small blocks of cast concrete) are a better practice for rebar chairs in contact with earth.

Dobies are also a good use for that last half yard of concrete left in a truck at the end of a placement. I used to see them formed up in thin wall PVC or plastic cups in the developing world. Wet set a length of tie wire in the top for connecting to the mat later, and you're all set.

as long as the brick/block has at least the design compression capacity of the concrete, you're okay.

These provide isolation for your situation. No idea on availability.

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The blocks are what is commonly done here as well. For most applications, I don't sweat the strength of them and it is often lower than the concrete I'm sure . The block strength doesn't affect rebar tension capacity other than local bond and, in many situations, also doesn't meaningfully impact the capacity of a flexural compression block. Losing a few inches of compression block in a footing or raft slab is negligible. If it was a hard working beam, that might be a different story.

If I have a concern with the blocks, it's probably more to do with their potentially acting as a vector for corrosion. If the blocks fall loose, or deteriorate themselves, then you basically have an access panel for water to get in. But, alas, nothing is perfect and the blocks are "what is done" here, apparently without issue.

just go to the nearest material tester lab and grab their junked concrete cylinders .

Thanks for all the wisdom.
Contractor ended up using bricks as chairs for the chairs.

Concrete bricks...

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