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RE: Free-Fall of Concrete

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There is no substitute for vibration unless you use SCC.  Voids are generally caused by a lack of consolidation.  Free fall bouncing off of rebar cages generally causes segregation. 
The Bureau of Reclamation has a good book called the "Concrete Manual". 

Harold Sprague

Subject: RE: Free-Fall of Concrete
Date: Tue, 29 Apr 2008 07:21:14 -0700
From: rgarner(--nospam--at)
To: seaint(--nospam--at)

I worked for the contractor in building New York City's West Side Treatment Plant.  We had 36" diameter steel pipe caissons that were driven to rock and then socketed into the rock.  There was rebar only in the top of the caissons.  We filled the caissons with concrete, using free-fall.  Everybody thought it was working until the Resident Engineer had us make an exploratory boring through the concrete in several of the caissons - DISASTER!!  Voids were found.  More borings had to be made.  More voids were found.  This one cost us large!

Bob Garner, S.E. 


From: Jerry Coombs [mailto:JCoombs(--nospam--at)]
Sent: Tuesday, April 29, 2008 7:10 AM
To: SEA ListServe
Subject: Free-Fall of Concrete


Howdy, Y'all.

I've seen many specs and General Notes that don't allow free-fall of concrete more than 5 ft (tremie or other means req'd).  There is nothing in any code documents that I'm aware of that nix free-fall.  Frankly, I don't see anything wrong w/ it and agree with the one article I've seen addressing it.  Maybe a very long drop and large aggregate may cause some of the aggregate to bounce off the concrete below separating it. 

I'd like to to hear what others are doing and any documented results or articles supporting either.

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