free fall concrete

[gramconstruct]

I am working on a job that is all concrete using a tunnal form the walls are 6" and has one row of rebar in the center
the concrete is falling about 8' is there a code that would keep me for pouring concrete or is this within code perameters

 

[jike]

I know of no code requirement. There may be something in the written project specifications or the referenced specifications. You really should be asking the EOR.

The question is really can you pour the wall without segregation (paste at bottom and rocks on top)?

 

[Qshake]

ACI doesn't directly address this but has a Concrete Fundamentals publication CCS-0 which shows the various means of avoiding segregation.

I agree with jike that it should be noted in the provisions if it is a concern. Most state DOTs have a provision limiting to 5' freefall but that is not applicable here is my guess.



Regards,
Qshake

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[kslee1000]

I remember CRSI has something on concrete placement. Check it out.

 

[msquared48]

Personally, considering the steel placement (unless it is only on one side of a 10" wall), I don't know how you'd pour any 8 to 10 foot high retaining or basement wall without the initial drop.

And another thought... buying the concrete in the fall when it's free is much better than paying for it in the summer.

Mike McCann
MMC Engineering

 

[SlideRuleEra]

As Jike & Qshake have noted, codes issues should not be a problem. Also, for a reinforced, narrow (6" thick) wall you won't get significant segregation anyway. On impact with the bottom, the concrete can only "splash" down the length of the wall and the rebar cage is going to minimize that.

A greater concern is the length of time required for the placement, the upper part of the rebar cage will be well coated with concrete as it falls. You will want to complete the pour before this (thin) concrete coating on the upper rebar has time to set - degrading the final concrete/rebar bond.

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[hokie66]

I wouldn't worry about segregation in an 8' high wall, but 6" walls with central reinforcement are difficult to compact. You will need a small diameter vibrator, sometimes called a pencil vibrator.

 

[dik]

SRE:
Segregation can be a problem with the aggregate bouncing off the rebar.

Dik

 

[kslee1000]

For low slump concrete, the concrete will be difficult to place and compact, that will lead to the use of high slump concrete, while flow more easily, seperation, jamming of coarse aggregate, and layered (fine/coarse) wall are the likely results if placement is not well controlled. QA&QC hold the key.

 

[SlideRuleEra]

dik - I know, the coating on the upper rebar that I mentioned would be mostly fines from the mix. As a bridge contractor we had structures, such as columns, where lengthy free fall of concrete was allowed by the DOT (on a case-by-case basis). If the concrete pour is contained & placed quickly, segregation is random. The composition of the concrete averages out for the completed structure. I agree, however, high free fall is not desirable, but under the circumstances described is not as bad as it may seem at first glance.

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[DRC1]

Generally excessive free fall should be controlled by an elephant trunk, which is inexpensive and avoids excess segragation.

 

[kslee1000]

DRC1:

You are correct if there is adequate space. For this case (6" wall with rebars in center) would be almost impossible. The concrete placement might have to accomplished by making several lifts using sliding form or other methods.

 

[wesstallmann]

Ive heard that there are concrete mixes that have additional plasticisors that you can drop from heights exceeding 6' with no segregation issues. Ive placed concrete down a 10' high wall with a tremmie/elephant trunk and it does work but takes a LONG time to get the line into the wall, if you have alot to pour, using the trunk is tough.
Watch out with cowboying the pour, Ive seen the aggregates seperate with a high pour, especially when it hits rebar on the way down, get a good vibrator and us it ALOT if you attempt this.

 

[Qshake]

wesstallmann -

If you buy in on the FHWA drilled shaft concrete placement recommendations you can drop concrete 80' and not suffer ill effects.

One reason this is done is due to the amount of testing required for drilled shafts. Many state DOT require Crosshole Sonic Logging (CSL) and coring down into rock. Both the coring and CSL bear out that the concrete is placed as expected and consolidated fine.

So unless major testing is to be done for mid-height walls I don't recommend being so open as to allow extensive free fall especially with central reinforcing.

Regards,
Qshake

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