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RE: Concrete slump

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James, 

	I have been told that the pumping process changes the concrete
slump.
	1. Is this correct?
	Yes.  Depending on the mix, temperature, and pumping process.

	2. If pumping does change the slump does the slump increase or
decrease?
	Slump is generally decreased with pumping.

	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?
	It is only an indirect indication of w/c ratio.  You can not take
additives out of the picture.  I'll submit one example.  Your overhead
pumping a concrete slab.  The pumper in conjunction with the ready mix
company add air entraining agent, increase the fines, lower the max
aggregate, and add flyash in order to make it more pumpable. It will still
meet your specification.  The pumper charges the line with grout and starts
pumping the concrete.  The pump is laid out almost flat at the beginning of
the placement.  As you get toward the end the pump arcs up 25 feet in the
air.  The concrete goes up the hose, and then down.  It is on the downward
slide that air is lost and so is slump.  Slump is intended to indicate
workability.  You will have a different air content and slump from the
beginning of the placement to the end.  Your slump will decrease at the end
of the placement.

	Take the same mix.  Use a horizontal slick line.  Your concrete will
be the same from start to finish.  You still have to adjust the mix for
pumping.  

	That is why on large jobs you have a test placement of concrete
where you simulate all of the pumping and placement operations.

	You also want to use the largest diameter discharge line as
possible.

	There are some applications where you might be forced to bucket all
of the concrete.  You will get the best mix for performance, but you will
drive the cost up.  As a minimum you should require as large a diameter
discharge pipe as possible.  It is not as big a deal for lightweight
aggregate floor concrete.  It pumps easy.  But if you are doing a parking
garage in the chloride belt with large uniformly graded aggregate, low w/c
ratios, and critical air void ratios consider requiring bucketed concrete.

	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?
	Nowhere.  Pumping is generic.  You can slick line concrete as in
horizontal pumping, you can overhead pump concrete, you can pump it up, and
you can pump it down.  The water is the same.  What you are doing is loosing
air and working the concrete to a point where the cement is beginning to
hydrate.  It depends on temperature, cement content, cement type, and how
long it has been spinning in the truck.  Not all cements are the same, even
with the same cement supplier.  The only way you can assure consistent
cement is to use all the cement from the same manufacturer and lot.

	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?
	To be any use as for the primary purpose (workability), slump must
be measured at the hose discharge.  That is where it will be placed in
forms, on grade, on deck, etc.  By the same token, you want to measure both
air and slump at the tailgate and at the discharge so that the mix can be
adjusted to account for pumping losses.


Regards,
Harold Sprague


> -----Original Message-----
> From:	James Allen [SMTP:allen(--nospam--at)xyz.net]
> Sent:	Thursday, October 05, 2000 11:14 AM
> To:	'Sturctural Eng List Server'
> 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
>