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Re: Free-Fall of Concrete [segregation]

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I beg to disagree.  Imagine the first bit of concrete falling 8' or 12' down into wall forms, hitting rebar on the way down, losing a little cement paste every time it touches a rebar, so that when it gets to the bottom of the form it's mostly aggregate, with much less paste.  I could show you a photo of the resultant wall, with nothing but aggregate (AKA rock pockets) at the bottom 6" or so of the wall.  Not good.

We solved that problem by first placing a several-inch-thick "bed" of grout (for lack of  better word), consisting of the regular concrete mix without the large aggregate, in the bottom of the forms.  This provided a good mix at the construction joint with the previous pour.

Ralph Hueston Kratz, S.E.
Richmond CA USA

In a message dated 4/30/08 12:45:20 PM, JCoombs(--nospam--at) writes:
If the larger aggregate gets "seggregated out" where does it go?  There is still the same amount of cement, sand, and aggregate getting dumped into the forms and dispersed.  (Seggregation at the joint of new-to-hardened concrete is always a problem no matter what method is used.)  It's difficult and time consuming to pull a 10 ft tremie out of 15 ft forms because the form ties are at 12'-18" centers.  That results is the workers dumping a WHOLE LOT in one spot so he can move the tremie over 10 ft. 
Let it drop down and mix itself up.  IMHO the bottom line is that proper mix, dispensing evenly, and proper consolidation (vibration) are more important than how far it falls.  The main stated problem with "believers" is that too much concrete hitting the tied rebar on the way down can displace said rebar.

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