Concrete Placement

[WBUW]

Think I posted this in the wrong forum the first time...maybe someone here has some comments?

I had a question regarding the placement of some questionable concrete.

I was on a jobsite in the middle of a slough where the contractor was placing a pedestal that sits on top of a pile cap.

The concrete mix was delivered on site around 11:00. The specs stated the slump was to be 8" and the air content to be 6%. The tester slumped the mix out at about 10" the first time. They retested and were rather generous on how they conducted the test and measurement (my opinion) and it came to about 9.5" for the slump. The air pot had problems and it had to be replaced with another one. This whole time the contractor is arguing with the special inspector...

Why they didn't call the engineer at this point, I'm not sure. Seems like if it tests bad 2x the mix should be rejected or okay'd by the engineer...

Instead while the were waiting for the air pot to arrive the stopped the drum on the ready mix truck...I'm just guessing here but are they allowed to do that? Seems like it would key off some hydration of the cement possibly? Anyway that seemed to stiffen the mix up a little bit then they re-tested again...slumped at 9" and air content was around 7.5%. I *think* they then proceeded to call the engineer and tell them the results...I don't know if they told them about stopping the agitator on the truck, kind of doubt it. After getting the go ahead they proceeded to place the concrete in the form they were still going when I left, which was around 1...so the mix was probably 2-2.5 hours prior to being placed and consolidated in the forms.

I'm in a conference call tuesday 9/8 (I'm one of the owner's reps on this) and I'm wondering if ACI offers any guidance on this...I heard the 90 minute rule isn't in effect anymore, now they are supposed to be taking the temperature and it isn't supposed to go over 90 degrees...but if they stop turning the truck aren't they possibly starting and stopping hydration? Been a while since I took the civil materials for concrete construction class...but that seems like a possible issue.

Worries me a bit, we had to go through all kinds of permits to rebuild this line and if we had to go back in and replace some concrete in a few years that could be a major problem.

Thanks for any advice or code citations (preferable).

Will

 

[Ron]

Your project is a great example of a common concrete problem...the contractor just wants to place it without regard to what it does to the concrete or the structure.

While the 90 minute limit can be designed around and admixtures can extend this limit, it should not be violated unless the mix design was intended for such. The 90-minute limit comes from ASTM C94, a standard referenced in ACI 318, thus making it a code requirement. It also give the slump ranges based on the mix design.

The reason they stopped the transit mixer was to allow the concrete to stiffen. That's a bad practice.

You're right to raise these questions. The concrete should have been rejected.

 

[DRC1]

I would not be so hasty to reject the concrete. I do agree that stopping the truck is not a good practice as rotating it keeps it from setting.
Before we blame the contractor, it seems the main delay was the inspector did not have a proper air pot.
An 8" slump is a lot and I am assuming this was done with super p. Sometimes the super p can be fussy, especially in high slump concrete. If that is the case, I would not worry about slump, it really does not mean a lot. Check the batch ticket to see if the water is correct, and look at the mix to be sure that you do not have segrgation of the aggregate. If you don't have the air measurement, I would not hold up the truck. Check the batch ticket to the mix design, and against recent pours. If you have been adding 3.5 oz. per yard and it has been ok, I definately would not wait to get a pot. 5-7% is the traditionally acceptable range. Many modern oils only need 3% to be effective. Strength degredation begins I belive around 9-10%, but I would not risk delaying the pour unless the air has been a problem. The 90 min. rule is based on a 90 degree temp. Once you get that far, the concrete is begining to set and is lost. If it is cool, or has retarder, you may be able to go past 90 min., but I would be very careful, as 90 min/ 90 deg is pretty accurate. The stiffening may have been due tho set begining in the concrete, or it may have been some low range placticer dying. If it was setting, this is not good. If it is the low range ending it is not a big deal. If you know you have questions about the pour, at a minimum, take several extra cylinders so you can see early on how the strength gain looks. Note that the middle 4 yards in the truck allways looks better than the first or last 3 yards. I general, if youhave questions about a truck or can not accept it, reject it and order new. The load will cost around $1000 if you are wrong. If you pour it and it is no good, the $1,000 will look cheap.
Finally, if you are the owner's rep and you have questions about the concrete during the pour, call him, he is working for your firm. If there is a question, get him involved directly instead of working through the inspector or the contractor.

 

[dik]

Ron:
I've often allowed minor agitation of the mixer and for the concrete suppler to wait until the 90 minute limit prior to retesting for slump.

Then it fails properly.

Dik