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Concrete slump

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James,

1.  I've been told this as well, although I have never seen it documented
anywhere. The claim is that the slump decreases, and this is why the
contractors claim that they need to add more water to the batch before they
put it through the hose.  While this phenomenon may be somewhat true, I've
also found that the contractors yelling about this the loudest are often
trying to use a hose that is too small to pump concrete at the specified
slump to begin with.  
2.  See above.
3.  The slump test is field test to measure the consistency of the concrete.
While the slump can be affected by factors other than water content, the
slump is often the greatest indicator in determining whether the concrete
was batched as specified and/or whether the water content has been
increased.   The concrete truck drivers often add water to the mix from
their tanks on the way to the site, and this water is often not documented.
The batch tickets will only tell you how much water was in the mix when it
left the batch plant.  
4.  Exactly the right question.  At my former firm we speculated that, in
the case of light-weight concrete, it might be possible that the porous
light weight aggregate is absorbing some of the water when it is forced
through a hose at high pressure.  But this theory doesn't hold true for
normal concrete. Does the trip through the hose accelerate the setting of
the concrete? Higher temperatures accelerate the setting of the concrete, so
maybe this is true.  
5.  At my former firm  we always specified on our drawings what the concrete
samples for slump and cylinder samples should be taken "at the truck".  I
believe that  words to this effect are in an ACI document (it may be
somewhere else, but I no longer have access to the references).   The ASTM
C172(  Sampling Freshly Mixed Concrete) says "Sampling should normally be
performed as the concrete is delivered from the mixer to the conveying
vehicle used to transport the concrete to the forms; however, specifications
mya require other points of sampling, such as the discharge of a concrete
pump."  My opinion is that the water cannot be disappearing on it's trip
through the hose, and you want to know what the consistency of the concrete
is when it is freshly mixed because that is the slump for which the mix is
designed.

Hope this helps.
KE Casano, P.E.

From: James Allen <allen(--nospam--at)xyz.net>
To: "'Sturctural Eng List Server'" <seaint(--nospam--at)seaint.org>
Subject: Concrete slump

Does anyone have experience or know of technical resources that address =
the change of concrete slump during placement by pumping.

I have been told that the pumping process changes the concrete slump.
1. Is this correct?
2. If pumping does change the slump does the slump increase or decrease?
3. Without additives, i.e. plastizers, water reducers, etc. isn't  the =
slump test basically a measure of the relative water content in the mix?
4. If the slump changes where does the additional water come from if the =
slump increases or where does the water go to if the slump decreases?
5. If the pumping process changes the slump, what is the slump? Is it =
the slump of the design mix prior to pumping or after?

Thanks for your assistance.

James Allen, P.E
Homer, AK