New one to me

[bootlegend]

This is a foundation for a large aggregate bin and crusher. Contractor is planning on using the blocks as spacers for top and bottom mat. We are saying no. Has anyone else seen this method used before? I'm also not thrilled with the bottom mat support but it doesn't bother me as much as the blocks.

 

[bootlegend]

From ACI 301-99: (Anyone with the latest version want to chime in if there been changes to this?)

3.3.2.4 Reinforcement supports— Unless otherwise permitted, use the following reinforcement supports:
3.3.2.4.a Place reinforcement supported from the
ground or mud mat on precast concrete reinforcement supports.
3.3.2.4.b Place noncoated reinforcement supported
from formwork on reinforcement supports made of concrete,
metal, or plastic.

My interpretation of 3.3.2.4a is that the bottom mat bricks should be precast rather than broken pieces. Although, I guess they were precast before they were broken. No mention of chair supports?

3.3.2.4b says that supports should be made of concrete, metal, or plastic. So, not a lot of clarity in that.

Regardless, I'm going to document my concerns to the owner and push for the stands. Thanks for the input everyone.

 

[Agent666]

I've seen a contractor try this, the argument we used for replacing masonry units was simply the fact that the blocks at around 17.5MPa were no where near the 40MPa concrete pour and the fact that your typical block is far more porous than solid compacted concrete (possible longer term durability issue).

It was a heavy cage on uneven rock in our case, even the typical hollow cellular masonry blocks were breaking. So they were resolving this by stacking all the broken bits into a tower each time one broke!

It was resolved by pouring some concrete cubes of similar strength to main pour and substituting these instead. They did try commercially available concrete spacers (sim to this type of thing), but they also just crushed under the weight of the cage.

 

[Aesur]

I have seen this a few times, and each time voiced my concerns but the EOR was ok with it; on the other hand I also have concerns with plastic or metal supports because depending on the bond of the concrete to the support it could cause a gap allowing water to penetrate and have easier access to the reinforcing.

301-10 states:

3.3.2.4 Reinforcement supports -- Unless permitted otherwise, use reinforcement supports indicated in 3.3.2.4.a through 3.3.2.4.i.

3.3.2.4.a - Use precast concrete reinforcement supports to support reinforcement above ground or a mud mat.​

3.3.2.4.b - Use reinforcement supports made of concrete, metal, or plastic to support uncoated reinforcement.​

3.3.2.4.c - Use wire reinforcement supports that are galvanized, coated with epoxy or another polymer, or made of plastic to support zinc-coated (galvanized) reinforcement.​

The rest is in regards to galvanized and epoxy coated.

The way I read this is use, Dobies or other precast concrete reinforcement supports for bottom mat. For the top mat you can use concrete, metal or plastic to support the reinforcing. However I agree, I don't like the idea of using a full CMU block and believe a better option would be a standee made of bent reinforcing.

 

[AskTooMuch]

I dont understand why can't this contractor just bend reinforcing as standee in the field. That shouldnt be hard to do.

 

[TLHS]

I specifically call out requirements for reinforcement supports now in my notes, after having a contractor that gave me broken pieces of precast concrete as bar spacers off of grade for lower level construction. I decided it was fine but also realized I didn't have a good hammer to use as a reason if I hadn't been happy with it.

 

[bootlegend]

I dont understand why can't this contractor just bend reinforcing as standee in the field. That shouldnt be hard to do.

I did not see the photo until the day after it was taken, so by the time we contacted the contractor the entire top mat was installed. The contractor has said he will correct it but it will delay the pour. I just wanted a second opinion before pushing the issue.

Thanks for all the input. I think most agree that it isn't the best idea.