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Concrete fall height

Concrete fall height

(OP)
Hello all...

I'm working as a special inspector on a project where there are 14' tall PCC walls, 12" thick, with double curtain rebar. The contractor plans on forming up the walls to full height and then placing concrete. There is nothing in the project plans that limit fall height for concrete and I didn't see anything in ACI 318 that specified a limit for concrete fall height. Am I missing something? Isn't there a potential for segregation of the aggregate material when pouring concrete from 14' high?

Thanks in advance for comments!

RE: Concrete fall height

The links below will take you to some position papers for your review. They deal manly with piles and, to some degree, reflect the biases of the authors.

Link

Link

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Concrete fall height

ACI has addressed this as well: Link

The only thing ACI will say is to deposit concrete as close to its final position as practical to prevent segregation. The link clarifies that that's from a lateral (horizontal) standpoint.

RE: Concrete fall height

(OP)
Thank You! VERY helpful links that add good perspective. I printed all of them for my reference collection. It looks like ACI is mainly concerned about segregation caused by lateral movement of concrete.

RE: Concrete fall height

Studies done for drilled shafts and caissons show no significant difference in quality of the free falling concrete. Long ago it was thought that free falling increased segregation; however, several studies have shown that significant segregation does not occur in confined vertical free fall. Lateral movement of plastic concrete can cause segregation.

RE: Concrete fall height

We have jobs we design where the walls are 26 ft and we limit vertical drop to 14 ft. This requires the contractor to use a snorkel pipe to place the concrete to prevent segregation of the aggregate from the cement. Vibration is an issue due to the height so they could use formwork vibrators, very common in the foundation end of the biz, or extensions of the vibrator used.

The other alternative is to specify SCC Self Consolidating Concrete. We have used this on a number of high rise shear wall and column jobs with great success.

RE: Concrete fall height

(OP)
I have seen the vibratory they are going to use. It is 20' long with a 3"(?) Head. Just or of curiosity, what is the diameter of those snorkel hoses? I think it would have to be pretty narrow to get down inside some of these walls with double curtain rebar.

RE: Concrete fall height

The are made of HD Plastic and are 12" but can be oval in the wall. They put a weight on the end to make sure they go down far enough. they can be cut to length and are considered a disposable item,. Large square funnel attachment goes on the top. I have attached a catalogue page from a local supplier showing the system and it's parts.
http://files.engineering.com/getfile.aspx?folder=5...

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