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Re: Concrete Mix Design Variations

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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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