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New one to me

New one to me

New one to me

(OP)
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.

RE: New one to me

I have never seen that, certainly not the best, but I can't think of anything specific that could cause a problem here.

What is it that worries you about this?

RE: New one to me

(OP)
My first concern was concrete bond around the bars. It appears some of the laps are over the blocks. There are several column pedestals that are to be formed in between the blocks. I see possible issues with punching shear strength depending on location. I would consider the whole block area to be discontinuous from the rest of the mat. Also, I'm not sure of the strength of the block compared to the concrete. Assuming the compression block depth is below the deeper than the mat then you could have strength reduction there.

RE: New one to me

I'd expect stress concentrations at the corners and resultant cracking in the PCC.

My glass has a v/c ratio of 0.5

Maybe the tyranny of Murphy is the penalty for hubris. - http://xkcd.com/319/

RE: New one to me

I've only heard of that used. There's probably not a huge issue, but there will be segments of bars not fully enveloped by concrete. I also think you could get some abnormal cracking or stresses around the blocks.

I've seen the bent standee legs used where they put a 10M bent to stand the top mat. That at least seems less intrusive.


...but I can't recall if I have ever solved that problem yet.

RE: New one to me

Lack of concrete continuity at masonry blocks would be an issue with me. You will definitely have an allowed shear stress difference at the blocks.
I guess they are going to lower their blocks later? Rebar looks higher than the forms to me.
Why does he not use standees?

RE: New one to me

So long as the blocks get filled during the concrete pour, it would seem to be just a matter of having some isolated areas of vertical discontinuity within the core of the mat. Unless a small reduction in punching shear capacity is a problem, I don't it as an issue structurally.

Whether the discontinuities around the blocks will reflect through to the top of the slab and show up as rectangular-shaped crack patterns, is beyond my experience to make a guess about.

RE: New one to me

I don't like it for a few reasons. Some of which were already stated:
1) Strength of the block is probably lower than the concrete.
2) Increases the possibility of voids in those blocks.
3) Shear strength weakness at those locations. Seems like it would affect both 1-way and 2-way shear.
4) My guess is you will get some cracking at those locations. My guess is that this could be more problematic for the type of equipment you're talking about.

Could this just be providing a temporary work space that will be removed later? Someone made a point about the forms being significantly lower than the top rebar.

RE: New one to me

What does the project specification say about rebar support?

IMHO, hollow concrete blocks are totally unacceptable for rebar support. If you need to "find" (make up) some reasons:

I'm sure there is a requirement for concrete compressible strength. Has the Contractor submitted documents on compressive strength of concrete used to manufacture the blocks?

How will the Contractor ensure that concrete inside the block will be consolidated?

Placing and vibrating concrete is rarely a "gentle" process. How does to Contractor propose to keep blocks, simply stacked, from falling over during placement?

www.SlideRuleEra.net idea

RE: New one to me

I’ve been on projects where the specs permitted concrete “bricks”; never came across hollow blocks.

RE: New one to me

I have never seen or heard of hollow CMU blocks being used for this purpose. Is the project located in the United States? If so, have you reviewed ACI 301? I would be surprised if this practice satisfies the requirements of ACI 301.

RE: New one to me

I'm following this because this is sort of the "standard" (bricks) rebar support our contractors used.

RE: New one to me

I have seen small brick used on the lower mat. They are CMU brick usually, not regular clay brick.

RE: New one to me

While you are addressing rebar support with the Contractor, may as well point out that rebar stored on dirt is unacceptable, too. Should be supported by blocking.

www.SlideRuleEra.net idea

RE: New one to me

I reread OP, our contractors only use bricks at the bottom.

RE: New one to me

If the masonry block detail is Ok, so would me getting the small metal waste basket in our bedroom and using it for a support. Has similar features and it is steel. In fact, it might work better.

RE: New one to me

I really don’t like that.. I agree with all technical reasons outlined above so I won’t rehash the same stuff.

RE: New one to me

(OP)
Thanks for all of the comments.

Quote (JoshPlum)

Could this just be providing a temporary work space that will be removed later? Someone made a point about the forms being significantly lower than the top rebar.

They were originally meant to be temporary supports. My understanding is that the "standees" bars were arriving later, but now they are delayed and the pour is scheduled before they will arrive. I'm not sure about height but the intent was definitely to leave the blocks in and pour with or without the standees.

Quote (SlideRuleEra)

What does the project specification say about rebar support?

We have a generic note in our drawings that says "Furnish all accessories, chairs, supports, space bars, etc. necessary to secure reinforcement in place."

Quote (Hokie93)

Is the project located in the United States? If so, have you reviewed ACI 301? I would be surprised if this practice satisfies the requirements of ACI 301.

It is located in the US. I will look into ACI 301 and see if it specifically prohibits this.

Quote (SlideRuleEra)

While you are addressing rebar support with the Contractor, may as well point out that rebar stored on dirt is unacceptable, too. Should be supported by blocking.

Could have used some blocks for that!

RE: New one to me

bootlegend - Seems like that part of the spec won't help. As suggested by others, check what ACI has to say. Also, the Concrete Reinforcing Steel Institute (CRSI).

Yes, field storage of rebar on the hollow blocks would be good use of the blocks.

www.SlideRuleEra.net idea

RE: New one to me

They could get some #4 bar and field-fabricate the standees.

RE: New one to me

(OP)
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.

RE: New one to me

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.

RE: New one to me

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.

RE: New one to me

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

RE: New one to me

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.

RE: New one to me

(OP)

Quote (AskTooMuch)

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.

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