Site Built Rebar Chair - Eng-Tips

phamENG

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Feb 6,

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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. Upvote 0 Downvote

XR250

Structural

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What about using bricks to support the chairs? My footing is oversized anyway so the loss of section should not be an issue. Upvote 0 Downvote

Lomarandil

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Jun 10,

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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. Upvote 0 Downvote

phamENG

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Feb 6,

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as long as the brick/block has at least the design compression capacity of the concrete, you're okay. Upvote 0 Downvote

azcats

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Oct 17,

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

[URL unfurl="true"]https://deslinc.com/rebar-saddle[/url]

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KootK

Structural

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Oct 16,

18,689

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CA

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. Upvote 0 Downvote

DoubleStud

Structural

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Jul 6,

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just go to the nearest material tester lab and grab their junked concrete cylinders . Upvote 0 Downvote

XR250

Structural

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Jan 30,

6,196

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US

Thanks for all the wisdom.
Contractor ended up using bricks as chairs for the chairs. Upvote 0 Downvote

Hoping to get the blinding levelled, insulation down, mesh installed and UFH clipped next week ahead of concrete pour.

Link to Trico

Approx 110mm slab, so was going to use 50mm plastic supports for the A193 mesh, but just come across "mars bars" or concrete square bars, which seem like a much easier solution, but cost a lot more! 

1) How many square bars per 4.8x2.4 sheet of mesh do I need? 

2) How much easier/better are they in reality than lots of plastic chairs? 

If it saves me a day of labour, I'll pay the extra, if not, then I won't bother. There will be a lot of walking over the mesh to get the UFH tied to it, so needs to be relatively steady.

Thanks!

Hi we used a combination on ours. Mars bars - these are quite cheap and flexible, you break them up of course into smaller bits, when installing don't create a nice uniform line as apparently that can potentially help induce cracking so go random! We also needed metal chairs (1m & 2m lengths) at different heights to support our 2 levels of mesh in the slab. Not sure if you get 50mm mars bars but don't see why not.

If you are looking for more details, kindly visit Concrete Reinforcing Rebar Support Chairs.

To your questions - we just had a pallet delivered and used as many as we felt was needed for adequate support - they went a looooong way, we had about 200m2 of slab that was plenty

Not used plastic chairs only metal 

Thanks both.

While I'm sure I'll figure out how many I need once I start doing it, that doesn't really help with ordering them to arrive with the mesh.. 

I've ordered 100 square bars for the 20 sheets, that should be enough I'd hope!

When you break them up, presumably it's still into lengths greater than 200mm so it spans more than one rebar?

If we start running low, the garage might be done using broken/cut up pavers - I've got a fair few of those kicking about too! 

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