huangyhg

questionable concrete
hello all,

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 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. waay better to just reject any questionable concrete in this case.

thanks for any advice or code citations (preferable).

will
starting and stopping the mixer will not likely have any effect on stopping hydration. if the pedestal is relatively small, the main effect of high slump will be increased shrinkage, and with a relatively small object this may not be an issue.
after all the concrete has been through, i'm a bit surprised the air content is as high as it is. for freeze thaw, it is likely ok.
i don't know how critical the application is, but if not critical, then things are likely ok.
dik
it wasn't stated in the original post, but i assume (hope) that the concrete achieved this high slump through the use of a high range water reducer (superplasticizer). typically, when using a hrwr, if your initial slump is higher than you designed for then your final slump will also be higher than designed. in my experience, when you start with a 3" slump and then add the hrwr (somewhere between 12 and 14 oz. per hundred weight of cement) the final slump is in the 6-8" range. if the initial is a 3 1/2" or 4" slump the addition of the chemical will produce a 9 or 10" slump. this 1" addition in slump should not have an adverse effect on the strength. however, concrete this wet will segregate,with the coarse aggregate "floating" to the surface. the time should not be a problem, since the hrwr also has a retarding effect.

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