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RE: Concrete Slump

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Although I've typically specified a 4" slump, I've often felt that the type
of wording that Jay has proposed makes more sense. Nevertheless, when I
proposed similar wording in the ACI 301 subcommittee for Mixing and Placing
concrete, I got resistance. There was objection to specifying a slump prior
to addition of the water reducing admixture, due to questions as to how that
slump would be verified in practice. Perhaps the initial mix design could be
based on this, but typical slump testing would not provide this information.
And requiring such testing in production may complicate concrete mixing
procedures. 

There is also resistance on the subcommittee with wording that the slump
requirement applies "at the point of placement". Slump testing is typically
taken at the point of delivery. The concrete suppliers want to keep it that
way, since their contract is to provide certain concrete mix properties at
the point of delivery, and they cannot control the mix after that point. And
testing personnel want to keep it that way, so they don't have to do slump
testing in difficult to reach placing areas. I have argued that the concrete
properties at the point of placement are what matter the most - there is at
least some wording in the Optional Requirements Checklist in ACI 301 that
discusses options to test at the point of placement. 

Some have concluded that the "slump test" is an outdated test procedure - it
probably made sense when concrete mixes did not have so many admixtures
included. Thus I tend to agree, but I have not found a suitable replacement
to verify general suitability of the concrete mix as delivered. I have
sometimes wondered if rather than an absolute slump value, perhaps the slump
should be required to be within a certain variation tolerance between
deliveries, regardless of the absolute slump value. This would verify that
mixes are reasonably consistent from delivery to delivery, which is one goal
of the slump test. But there are still potential problems, such as effects
on slump due to changes in temperature at time of delivery. 

Nothing is ever easy. 


William C. Sherman, PE 
(Bill Sherman) 
CDM, Denver, CO
Phone: 303-298-1311
Fax: 303-293-8236
email: shermanwc(--nospam--at)cdm.com
 

> -----Original Message-----
> From: Jay Shilstone [mailto:j.s(--nospam--at)shilstone.com] 
> Sent: Wednesday, August 25, 2004 11:31 AM
> To: seaint(--nospam--at)seaint.org
> Subject: RE: Concrete Slump
> 
> Concrete slump is actually a very imprecise indicator of 
> concrete quality. 
> ASTM actually has a disclaimer in their slump test 
> description that says that concrete strength may not 
> correlate to slump in the field.
> 
> Someone else sent you a message about specifying 3-5" slump. 
> This may or may not be appropriate for the SF area with your 
> high resteel congestion. I suggest the following:
> 
> "Concrete slump shall not exceed 5" maximum, except when an 
> ASTM C-494 Type F or G water reducer in which case the 
> initial slump prior to addition of Type F or G water reducer 
> shall not exceed 5" and after the addition of Type F or G 
> water reducer shall not exceed 7", as measured at the point 
> of placement."
> 
> Of course, the above statement is offered without any 
> knowledge of the project, and you should modify it for 
> extenuating circumstances on the project. (Canned disclaimer #105).
> 
> Jay Shilstone

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