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RE: Free-Fall of Concrete

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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.
JDC

>>> On 4/30/2008 at 1:00 PM, Harold Sprague <spraguehope(--nospam--at)hotmail.com> wrote:
Grout by definition either has limited coarse aggregate or no coarse aggregate.  The problem with dropping concrete and having it bang off of the rebar cage is segregation of the coarse aggregate from the mix.  
 
Another factor is that dropping concrete in caissons of 30 feet and further is fairly typical, but the maximum grout drop in CMU will be 24 feet maximum with lift heights not to exceed 5 feet. 
 
Drop chutes are inexpensive and easy to use by hand.  Placing caisson concrete with a tremie is much more difficult (although sometimes necessary) and requires a light crane to hold the tremie.  There are more than a few tremies that became permanent parts of caissons by accident.  This happens when the concrete elevation gets about 5 feet higher than the bottom of the tremie.  The friction forces on the buried portion of the tremie are very large. 

Regards,
Harold Sprague

> Date: Tue, 29 Apr 2008 21:04:20 -0400
> From: gtg740p(--nospam--at)gmail.com
> To: seaint(--nospam--at)seaint.org
> Subject: Re: Free-Fall of Concrete
>
> What makes high lift cmu grouting acceptable and greater than 5ft drop
> of concrete not?
>
> On Tue, Apr 29, 2008 at 12:35 PM, Garner, Robert
> <rgarner(--nospam--at)moffattnichol.com> wrote:
> >
> >
> > http://mail.google.com/mail/#inbox/1199a881a6f2cf83
> Gmail - Free-Fall of Concrete - gtg740p(--nospam--at)gmail.com
> >
> > This was back in the '70's so my memory is taxed, but as I remember, we had
> > a system of conveyor belts to reach the caissons, which were driven in the
> > Hudson River, just offshore. The concrete was free-fall from the end of the
> > conveyor. We had engineers working for us (I was just a draughtsman at the
> > time) and this shouldn't have happened. And we paid for it.
> >
> > Some day let me tell you about dredging too deep too close to shore on that
> > job, and creating a mud-wave which displaced most of the caissons. What a
> > great way to learn!
> >
> > ps: This is that sewage treatment plant in NYC with the State Park over it.
> >
> > pps: Even the mechanical had some learning processes - I don't believe the
> > odor problem has been solved to this day.
> >
> > Bob Garner, S.E.
> >
> >
> >
> > ________________________________
> >
> >
> > From: William.Sherman(--nospam--at)CH2M.com [mailto:William.Sherman(--nospam--at)CH2M.com]
> > Sent: Tuesday, April 29, 2008 8:54 AM
> >
> > To: seaint(--nospam--at)seaint.org
> > Subject: RE: Free-Fall of Concrete
> >
> >
> >
> >
> > Do you know how much care was taken in the upper part of the free fall?
> > I've seen articles that say that free fall works as long as the concrete
> > does not impact any obstructions (rebar, sides of the hole, etc).
> >
> >
> >
> > If the concrete was not controlled to drop vertically in the center of the
> > hole, segregation due to impact with the rebar or sides of the hole could
> > have occurred.
> >
> >
> >
> > Drop chutes are still the safest approach.
> >
> >
> >
> >
> > Bill Sherman
> >
> > CH2M HILL / DEN
> >
> > 720-286-2792
> >
> >
> >
> >
> >
> >
> > ________________________________
> >
> >
> > From: Garner, Robert [mailto:rgarner(--nospam--at)moffattnichol.com]
> > Sent: Tuesday, April 29, 2008 8:21 AM
> > To: seaint(--nospam--at)seaint.org
> > Subject: RE: Free-Fall of Concrete
> >
> > 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)carollo.com]
> > 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.
>
>
>
> --
> Will H.
>
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