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Re: Concrete Mix Design Variations

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For the concrete, we spec'ed out a w/c of 0.45.  We
also had QA people at the batch plant and wash rack so
we use their truck ticket and observe their operation.

As far as limiting the mix to 90 degrees, the mix is
limited to 90 degrees, we typically exceed that
temperature in the summer.  We focus on the on the mix
temperatures and start casting concrete at midnight to
2:00 am to avoid the hottest parts of the day.

Paul.
Phoenix.

--- Jay Shilstone <j.s(--nospam--at)shilstone.com> wrote:

> Concrete Construction magazine
> (www.worldofconcrete.com) used to publish a 
> poster that showed the effects of the addition of
> one gallon of water to a 
> yard of concrete. Negative factors included
> shrinkage, permeability, 
> strength decline, decreased durability and so on.
> You can still purchase 
> the poster from their owners, Handley-Wood.
> 
> There is a big move afoot in the National Ready
> Mixed Concrete Assn. to try 
> to switch the concrete industry from prescriptive
> specifications to 
> performance specifications. There are advantages and
> disadvantages to both 
> methods.
> 
> There are problems on both sides of the fence - both
> design and 
> construction. You cite the problem with the
> construction side, but there is 
> an engineering problem as well.
> 
> You are in Phoenix. I don't know first hand, but if
> Phoenix is like other 
> places I have been in the U.S., there are engineers
> specifying a .35 
> water/cement ratio, sometimes with straight cement,
> for concrete cast in 
> the summer. They then wonder why their concrete is
> cracking, since a lower 
> water/cement ratio is supposed to be more durable
> than a higher 
> water/cement ratio.
> 
> In this case, the problem is that the engineers have
> forgotten that, while 
> low w/c concrete has less drying shrinkage, the
> higher cement factors 
> result in more heat of hydration and a higher
> initial thermal expansion. 
> The result is thermal shrinkage cracks.
> 
> I have seen all sides of the concrete process, from
> designers who don't 
> understand the material they specify, to concrete
> producers who admit to 
> lying about what they put in their concrete, to
> contractors who "give the 
> concrete a drink" when the lab isn't looking, to
> laboratories who don't 
> have the foggiest notion about the right way to run
> the tests they are 
> hired to perform. And don't forget owners who want
> the Taj Mahal for the 
> price of an outhouse.
> 
> There are two keys to the solution - education and
> accountability. If we 
> are not willing to make the effort and pay for both,
> we will continue to 
> get what we are getting right now.
> 
> Stepping off that soap-box and looking at your
> situation, here are a few 
> questions:
> 
> Did you specify just strength, or did you specify
> w/c (or w/(c+p)), 
> shrinkage, permeability, maximum water content,
> slump, air, unit weight or 
> anything else?
> 
> Are you experiencing cracking, dusting or any other
> substandard condition?
> 
> How do you know the w/c ratios as delivered are
> varying?
> 
> Have you tried to correlate w/c to strength or other
> characteristics?
> 
> Why is there a 90 degree limit on concrete
> temperature in Phoenix during 
> the summer? My father successfully worked with 110
> degree concrete in Saudi 
> Arabia.
> 
> Jay Shilstone
> 
> At 04:59 PM 8/20/2004, you wrote:
> >I have a concrete supplier that is less than
> accurate
> >when batching ready mix products.  The concrete
> >suppliers water-cement ratios vary considerably and
> he
> >loves to add water (beyond the ASTM tolerances) to
> >achieve his desired slump.  When non-conformances
> are
> >issued for the mix and the contractor goes ahead
> and
> >places the concrete at his own risk, the usual
> >performance measurement of 28 day strength is the
> >governing factor.
> >
> >Any thoughts on what other critical characteristics
> >are effected by higher water cement ratios.  I keep
> >discussing shrinkage cracks and perhaps other
> >durability issues but am trying to collect other
> >opinions.
> >
> >The subject building is a cast in place concrete
> >structure in the Phoenix, Arizona area (hot
> weather,
> >mixes approaching the 90 deg. temperature limit
> when
> >delivered).
> >
> >Appreciate your thoughts.
> >
> >Paul Blomberg
> >Phoenix, AZ
> >
> >
> >
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