Sequential Concrete Pour

[tclat]

Hi All,

I'm working on a project where we are casting a reinforced concrete frame against an existing building to reinforce it. The contractor is behind schedule and has proposed a somewhat odd methodology to speed up the programme. Currently all the reinforcement is hung on the building and they are proposing to cast two floors, beams and columns in one day. They plan to allow the columns/beams on the lower floor to set partially and continue pouring the second level columns and beams. The formwork for the beams are propped and supported by the existing structure so we will not be loading the beams. The columns on the lower level will have to take the weight of the plastic concrete of the upper floor. I have initially objected to the idea suggesting that loading the column that has only achieved a partial set will impact the long term strength of the column. Is there anything wrong with my objection?

How are very large/tall pours undertaken to prevent lower pours from setting before the pour is complete.

Thanks

 

[SlideRuleEra]

How are very large/tall pours undertaken to prevent lower pours from setting before the pour is complete.

I agree with your objections.

The tallest (fast) pours that I have been associated with were 3 ft. diameter columns for the piers of highway overpasses. The concrete quantity is very small, so it could be placed rapidly. The limiting factor is the fresh concrete's hydrostatic pressure that the forms can withstand. We limited height to 20 feet (3000 PSF). Manufactured steel forms were needed to withstand that pressure... and they were the ideal shape to do so - circular.

Even though the Contractor plans to let the lower level partially set, things do not always go as expected. Suggest you get the Contractor to address form design / construction - he may change his mind about very tall pours.

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[Ron]

Agree with SRE....further, the particular mix design of the concrete has a significant impact on the performance during construction. As an example, if the concrete contains fly ash or ground, granulated blast furnace slag (GGBFS), the strength gain of the concrete is greatly affected. If the contractor wants to do as proposed, he MUST prove that it will work. The results otherwise could be catastrophic and deadly.

 

[KootK]

I'm okay with this. Have them supply some rapid hardening cement concrete, if they're willing to pay for it (Link). You'll get 2500 psi in 90 minutes which ought to be plenty.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[tclat]

The design strength is 4000psi. What about increasing the concrete strength to 6000psi and then plan the next pour in 24hrs.

 

[KootK]

Just because the 28 day design strength is 4000 psi doesn't mean that you need anywhere near that to support the upper pour. I'm sure 500 psi would suffice.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[dcarr82775]

Why the partial set? Seems like the worst of two worlds. I would have them either: 1) pour it quickly all at once with something akin to cleanouts in the forms where needed, 2) add a retarding admixture to delay the set until they can complete the pour (again cleanouts), 3) place it in 2 pours allowing 24 hours for the concrete to set up between pours. As SRE noted, the forms will need to be able to resist the head of concrete which is significant.

 

[tclat]

Kootk....I'm not concerned about the strength required to support the load. What I'm not sure about is the impact on the 28day strength of this placing 10 feet of concrete over the previous pour that is only a few hours old.

dcarr....I agree with the two options but option 1 is too risky in my opinion. Prefer the two pours.

 

[KootK]

What I'm not sure about is the impact on the 28day strength of this placing 10 feet of concrete over the previous pour that is only a few hours o

I know. I contend that once you've achieved 2500 psi etc, it's a non-issue. Beams and slabs are poured on top of columns and walls at 2-3 day strengths that will be much less than this all the time. Heck, I spec most of my vertical concrete strengths at 56 day precisely because this is a non-issue.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[dicksewerrat]

Can the cylinders be cracked at 90 minutes?

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com

 

[tclat]

KootK.....beams and slabs at the top of columns (max 2 feet) is much different than 10 foot column poured over a lower beam/column that is only a couple hours old. To get to 2500psi I think we need atleast 24hrs of regular (5000-6000psi) concrete. To get 2500psi in a couple hours requires special concrete as you mentioned which the contractor is not able to get in a timely fashion.

Thanks

 

[KootK]

KootK.....beams and slabs at the top of columns (max 2 feet) is much different than 10 foot column poured over a lower beam/column that is only a couple hours old.

I don't think that it is much different tclat. If I understand your situation correctly, you will have the wet weight of only the upper columns loading your lower columns as the beam forms will remain in place longer. And that load will be pure compression. That's what, maybe 15 psi compression?

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.