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Heading: 1.2.1.2.1Concrete Pop outs
Message:
I noticed a problem here in Minnesota of concrete popout's in concrete. It happens after the first winter, when it appears the freeze thaw cycle has an effect on the coarse aggregate. It leaves a smooth rounded stone imprint, which leads me to believe it is a sand and gravel aggregate. I actually found one piece that looks to be a piece of shale disintegrating.

Question 1)

Is there a way to prove that pop outs do not have a detrimental effect on the concrete? I have been told by a group of readymix suppliers, that it does not have an effect on the concrete. I find this hard to believe.

Question 2)

I have been asked to come up with specifications for an international firm who uses concrete to build structures all over the world. What testing specifications should be used to identify aggregates susceptible to freeze - thaw in concrete? Any suggestions as to what specifications should be used?




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Jay Lukkarila


Jay,
Q1. If the odd pop-out occurs on the surface, provided cover to reo is not compromised there should not be a problem, however if subsequent cycles produce further deterioration then obviously there is a problem.

Q2. I'd suggest testing aggregate for "Sodium Sulphate Soundness", in this test repeated cycles of soaking in solution and drying (inducing crystaline growth within) mimics freeze thaw. Another simpler test would be aggregate absorbtion. If the aggregate doesn't have a high internal moisture content it should be stable.
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Kevin ..


These pop-outs are usually caused by very fine reactive particles in the fine aggregates, very similar to ASR reaction. The fine particles expand and pop off at the surface. Maybe a traetment with sodium silicate solution on the surface might help.

Oscar
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Oscar Tavares


This is a sticky question - I have seen it a lot in the Midwest where people pay more to "upgrade" from asphalt in their driveways and then get popouts.

It is covered in ASTM C33, Table 1, where limits are provided for coal and lignite (0.5% where "surface is appearance is of importance" and 1% for "all other concrete") and in the Table 3 limits for chert (3% for architectural surfaces, 5% for most other concrete) and sulfate soundness.

Two challenges exist - 1) ASTM C33 isn't used properly: most people don't specifically say that the "surface is important" limit apply, don't ever see the chert tests, and don't say what class designation applies; and 2) even at 0.5% coal or 3% chert, a significant number of popouts can occur with the concrete still "in spec."

The only recourse is to go more stringent than ASTM C33, leading to supplier angst and more $.

Matt Sherman, Simpson Gumpertz & Heger Inc.
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Matthew Sherman


I think it is funny that shale popouts in asphalt are much more acceptable than in concrete. Unfortunately what we are finding is that owners are never informed of the risk of shale popouts, or alternate aggregate upgrades until they complain about the popouts after the fact.
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Gregory Johnson
 
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