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Re: Concrete Mix Design Variations

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In hot weather and particularly if there is any wind, it is necessary to
have sufficient water in the mix to provide for evaporation at the
surface, and/or to provide a fine mist of water to the surface and a wind
fence to prevent drying shrinkage/cracking in the first few hours after
placement, before the concrete begins to gain strength to resist the
shrinkage. If this is not done fine cracks will develop and will be seen
the next day.

Stan Scholl, P.E.
Laguna Beach, CA  (ex LA Portland Cement Assn. rep.)

On Sat, 21 Aug 2004 16:45:25 -0400 Mark Gilligan
<MarkKGilligan(--nospam--at)compuserve.com> writes:
> The added cracking is because in order to obtain a 0.35 w/c ratio and 
> to be
> able to pour the concrete they add more cement and hence more water. 
>  The
> net result is you have more drying shrinkage even if you ignore 
> thermal
> issues.  If you want to limit drying shrinkage you want to limit the 
> water
> and not worry about the w/c ratio.  A low water cement ratio is 
> desirable
> in many servicability conditions but it is not always essential.
> 
> On the subject of performance specifications for concrete first we 
> need to
> be honest as to what characteristics we want from the in-place 
> concrete.  
> We then need to agree on a method to measure those characteristics 
> and then
> there needs to be some real accountability that will motivate the
> Contractor and his supplier to do what is necessary to achieve the 
> desired
> results.    Unless we can address these points there will be no
> improvement.
> 
> Mark Gilligan
> 
> 
> 
> Message text written by INTERNET:seaint(--nospam--at)seaint.org
> >
> To: <seaint(--nospam--at)seaint.org>
> From: Jay Shilstone <j.s(--nospam--at)shilstone.com>
> Subject: Re: Concrete Mix Design Variations
> 
> 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 
> /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:
> 
> 
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