The slump test is a simple field indicator of the "workability" of the wet
concrete. The test is done before the main part of the load is poured.
However, the very first part of the discharge should not be tested as it's
not typical.
If it does not slump much the mix may not fill all the nooks and crannies in
the formwork and around the steel - there will be air voids - so your base
will not be as strong.
If the mix slumps excessively there is excess water in it and that will lead
directly to a weaker base.
The trick is for the lorry to arrive on site with a mix which is a little
too dry, and water can be added after the slump test and before the main
discharge of the mix.
Getting it just right is not always easy because the aggregate which forms
part of the concrete mix is often wet, either because it's stored in the
open, or because it's been washed.
I don't have the dimensions to hand, but a slump cone is about 12 inches
high, and a suitable slump is around 2 to 3 inches. In the UK anyway !
You could ask the driver to prove the slump on delivery ...........
John G4ZTR
-----Original Message-----
From: towertalk-bounces@contesting.com
[mailto:towertalk-bounces@contesting.com] On Behalf Of jim Jarvis
Sent: 07 July 2009 19:00
To: towertalk@contesting.com
Subject: Re: [TowerTalk] concrete set time
Julio Peralta wrote:
> I've always wondered how the average guy can tell the difference
> between
> 2500 and 5000 PSI concrete when it's delivered. It's my
> understanding that a
> test cylinder has to be poured and sent to a lab to determine the PSI
> strength. Couldn't a concrete company tell you its 5000 PSI and
> deliver
> something much less than that?
>
> Julio, W4HY
>
The only time I did this, we had a structural engineer on the team.
Before the
concrete was poured, he did a "slump" test. Poured a cone of the
stuff, and
measured how much it deflected downward. Evidently, that's an
indicator of
the eventual strength of the material.
n2ea
ps: I agree with Steve. By letting the concrete company know
you're testing,
they'll be careful to get the right stuff.
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