Footings Early Strength 3000 psi vs 4000/5000 1

[MauriceSports]

Hello -

I have a footing design requirement for a mezzanine (very long spans), that is calling for 3000 psi with 7 days cure before dead load and 28 days before live load. The engineer is on vacation now. I can't afford to wait the 28 days, so was trying to find out how long it would take 4000 psi or 5000 psi concrete to get to a 3000 psi strength, especially if we used high early strength.

I do not have the loads for the footings, but the dead load is 10 lbs/sq ft and live load of 75 lbs/sq ft.

So the question is:

With 4000 psi HES, how long would I need to wait for dead load and live load. What about 5000 psi HES?

I have uploaded the footing locations and engineer's notes.


Thanks

 

[Ron]

That depends on the mix design. Call your ready mix supplier and get a strength gain curve for the mix you want to use.

 

[msquared48]

It's not just the curves, it's also the actual 7 day break for the mix and how that falls within the data set.

Side comment... by not involving your engineer in this decision, you are assuming all the liability of any outcome here... good or bad. I strongly suggest you at least involve the local jurisdiction that approved the planset..

Mike McCann, PE, SE (WA)

 

[hokie66]

The requirement is for 3000 psi concrete. You should be able to get 3000 psi in 7 days with a 5000 psi mix.

Mike, you need to watch those little keys. Or predictive text...or something.

 

[cvg]

I don't see any leeway in those notes. they require 7 days cure - period. the curing period is not tied in any way to the design strength.

 

[msquared48]

I need larger buttons on my phone Hokie. Thanks...

Mike McCann, PE, SE (WA)

 

[hokie66]

cvg, I think there is plenty of leeway in the statement that "footers shall cure". What does that mean? I suspect someone clumsy enough with the language to call footings "footers" would not know the difference between strength and maturity, and in small pad footings there is not much difference anyway.

 

[cvg]

well, looking up the term "cure" in ACI, it is very clear what it means. And it doesn't mention any strength requirements. There is no 7-day strength requirement specified. With HES concrete, ACI may give you some wiggle room to reduce to 3 day cure.



5.11 — Curing
5.11.1 — Concrete (other than high-early-strength)shall be maintained above 50°F and in a moist condition for at least the first 7 days after placement,
except when cured in accordance with 5.11.3.
5.11.2 — High-early-strength concrete shall be maintained above 50°F and in a moist condition for at least the first 3 days, except when cured in accordance with 5.11.3.

 

[hokie66]

Are you then suggesting that the curing is to continue for 28 days? I think it is clear that the writer means the "footers" are to be 7 days old before dead load, and 28 days old before live load. And this logically means that he thinks 3000 psi is only going to be achieved at 28 days. There is no talk in these notes of testing requirements.

 

[cvg]

the note means exactly what it says. 7 days for dead loads, 28 days for live nothing more, nothing less

I have no idea if this engineer is logical or not. But according to ACI, it might be logical to assume that the concrete may reach approximately 70% of its 28 day strength after 7 days of moist curing (assuming Type I cement). More or less if different types of cement are used or if temperatures are high or low.

 

[BigH]

Point as Ron hinted - many mixes (at least the ones I use) have significant percentage of fly ash in them as a replacement to Type I cement and the old 70% (or 65% rule) doesn't really apply - the strength gain with time matters. Actually, most of my mixes, now, require compliance at 90 days. When using 50% fly ash on structural mixes we would get something like a 30% gain of strength from 28 days to 90 days. In one case where we left the crusher run fines in place (with up to, then, 15% fines in the aggregate) we had nearly a 50% increase from 28 days to 90 days.

Curing - normally 7 days and for fly ash mixes 10 to 14 days - and I've used up to 50% fly ash in mixes. I would have read the requirement as "concrete being cured and required strength to be 3000 psi at 7 days". And this is why words count; and clear precise and concise requirements need to be carefully spelled out.

 

[RHTPE]

Note #6 is perfectly clear to me - 7 days of cure before you can apply any dead load.

Does this make logical sense? Probably not.

But I have had EoR's take a very simple stand - "That's what my notes (or specs) require and that is what I expect. Period, end of conversation."

Would the mezzanine be 100% complete at 28 days after the footings have been cast? What kind of live load would be applied so early on in the structures life?



Ralph
Structures Consulting
Northeast USA

 

[hokie66]

The OP has not called in, but if he tests the concrete at 7 days, and it is at 3000 psi or better, I would tell him he is good to go with the rest of his work.

 

[MauriceSports]

Thanks everyone for the comments:

The EoR was concerned about strength at 28 days in initial conversations. So it would seem to me that an HES which reaches 3000 psi at 7 or 10 days would suffice. The mezzanine only will take about 10-12 days to put up, so that is what I am most concerned with. After that, conveyors will be installed, but would be several weeks to reach the full live load of the system. At 3-4 days, all I want to do is start erecting columns...we have a lot of them to do, but only footings on about a 3rd.

This is an indoor pour in GA, so no issue with temp or moisture.

Thanks again.