Concrete Separation 1

[lmarciniwe]

What is the height limit concrete can be dropped (or poured) before separation of the granular occurs. I’ve heard a few differing opinions on this subject, and was just wondering what the consensus was.

Thanks,
Laura

 

[jdonville]

Laura,

The classic geotechnical application where this discussion arises is the construction of drilled shafts.

In FHWA IF-99-025 "Drilled Shafts - Construction Procedures and Design Methods", the O'Neil and Reese conclude that concrete may be placed by free fall for distances of

"up to about 24.4m (80 ft) without problem as long as the concrete does not strike the cage or the borehole wall." - p.200

This, according to the text, is based on a small number of test shafts where the concrete was sampled and tested after shaft construction. The deepest shaft tested was 80 ft deep.

I suspect that the distance may actually be greater than 80 ft, but not too many drilled shafts are constructed deeper than this.

The Portland Cement Association (PCA) or ADSC may have further/other information on this subject.

Hope this helps,

Jeff


Jeffrey T. Donville, PE
TTL Associates, Inc.

http://www.ttlassoc.com

 

[eric1037]

I have had projects with drilled shafts 100 feet deep that the free-fall method of concrete placement was used to no apparent ill effect.

 

[SlideRuleEra]

Agree with both jdonville & eric1037. Long ago I was talking with my father (engineer with 40 years in bridge & heavy construction) about this concept. He gave me excellent (practical) insight into why this is allowed. When concrete "hits bottom" after free falling inside a relative small diameter drilled shaft or column form it may have segregated, but there is nowhere (horizontally) for it to go. Almost immediately it is covered by more free falling concrete that has segregated too. The net result of these random impacts being a "reasonable" in-place concrete mix.

http://www.SlideRuleEra.net

 

[RCEJD]

Agree with the previous posts, and Dr Lymon Reese, out of Texas, is by far the leading expert in the field.

We caught a contractor placing concrete with heavy renforcement (bundles of #8 rebar, etc) in 4 foot diameter piles for a landslide mitigation where he dropped concrete 75 feet (against our recommendation of no more than 5 feet). We actually cored the middle of a couple piles and drilled a couple pilot holes where we cored laterally.

Much to our surprise we found no honeycombing, especially in the outer raches where we expected to see signs of voids or disaggregation.

Regardless we would never recommend such a practice with rebar cages, but it was interesting to see.