Got a lab report back for some concrete cylinders used in a foundation of a 2 story building. 28 day design strength was 3000 psi, one cylinder broke at 3160 and the other at 2850. It’s not often that I see numbers below design strength, so I’m curious how important this is.
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Replies
its not,they average out to 3005
The woman I knew who worked for the testing co here said they never saw under 3500 for 3000 spec concrete. Which is exactly what the readymix salesman told me, not that I believed him. Apparently they actually did always add that extra sack.
PAHS Designer/Builder- Bury it!
This is on a project I just took over, and I'm not entirely thrilled with the gc. On the other job I have going, different gc, the 3000 psi cylinders are testing over 3800, many over 4000."Put your creed in your deed." Emerson
"When asked if you can do something, tell'em "Why certainly I can", then get busy and find a way to do it." T. Roosevelt
it was poured wet, but it still met specification
I'm not sure it was poured wet. Looking at the test report:
admix - flyash
mix design - 3000 psi
air content - 1.5%
concrete temp - 43*F
water added - 0 gal
slump - 5.0
water withheld - 2 gal
Brownie - what is 'water withheld'"Put your creed in your deed." Emerson
"When asked if you can do something, tell'em "Why certainly I can", then get busy and find a way to do it." T. Roosevelt
water withheld is the allowable water they can put in, The temp was 43 on a flyash mix. This concrete is almost frozen so it will be a very slow mix, But and big but that nobody here will notice. The break met spefication. It was a 3000 mix, it broke 3000 psi, there is nothing wrong here. The concrete is fine, ok, nothing wrong with.a lot of mix will break high, that gravy, it a mix meet spefication that all that matter. since one cylinder went over spec. it metalso it a cylinder is under by 5% it still meet spec.
Edited 4/28/2007 10:57 pm by brownbagg
Did you see my response to the thread about checking to see if the CBU's had been filled?I looked back for it and can't find it.It should not have been delete or moved unless some one went off in the thread..
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A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.
I read that. I was hoping for something easy, like a hammer tap."Put your creed in your deed." Emerson
"When asked if you can do something, tell'em "Why certainly I can", then get busy and find a way to do it." T. Roosevelt
It's been a while since I had to have concrete tested, but the 3000psi tests came back between 3500-4000. You're probably OK anyway, unless the BI speaks up.
I have a friend who design runways for the government and he once told me that they now use much lower strength concrete and make it thicker because when concrete fails the problem is almost always the subgrade.When you consider the fact that concrete essentially has 0 tensile strength then its strength is in compression only does that little difference really matter all that much when one considers where most of ti goes? Recently in a lab they were able to produce 30 ksi concrete
It really depends upon the concrete use. Suspended slab...very important, footing...somewhat, but footing dimensions largely dependent upon soil strength which is far less than concrete, foundation wall...can be very important depending upon your backfill conditions, slab on grade...in cases where durability is critical, yes, but for your typical residential applications recent reports in the ACI suggest less important than once thought.
You should also remember that concrete tests really only give the maximum potential strength and may not really represent the actual strength of the concrete on site because when they take the cylinders back to the lab they are put into a water bath where they sit until the day they need to be tested. Concrete strength is dependent upon curing conditions, and the curing conditions in the lab are ideal whereas the site conditions rarely are.