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.

 

[hardbutmild]

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?

 

[bootlegend]

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.

 

[ACtrafficengr]

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/

 

[skeletron]

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.

 

[Ron247]

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?

 

[BridgeSmith]

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.

 

[JoshPlumSE]

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.

 

[SlideRuleEra]

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?

http://www.SlideRuleEra.net

 

[bridgebuster]

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

 

[Hokie93]

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.

 

[AskTooMuch]

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

 

[Ron247]

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

 

[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.

http://www.SlideRuleEra.net

 

[AskTooMuch]

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

 

[Ron247]

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.

 

[MIStructE_IRE]

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

 

[bootlegend]

Thanks for all of the comments.

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.

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."

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.

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!

 

[SlideRuleEra]

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.

http://www.SlideRuleEra.net

 

[bones206]

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