Time between concrete pours

[bhiggins]

Hi all!

I'm working on a small 2 story concrete podium project. The structure is a conventionally reinforced moment frame supported on piers on top of a slab on grade with pier caps. The piers have been poured roughly 3 weeks ago and now the slab/pier caps are ready to be poured. The owner is behind on schedule and would like to pour the columns ASAP. I told him the standard time is 7 days for the slab to cure before the columns can be poured unless an accelerator is used. I cannot seem to find any guidance in ACI318 on the matter.

Can concrete cylinders be tested and reach a target concrete strength (possibly 50% f'c) for an OK?

We specify f'c=4,000 psi for the elevated slab and columns, what would be the target strength to take off the shoring for the elevated slab? If the slab is only roughly 1000 sqft, how many test cylinders are appropriate? This is a pseudo commercial project so we are doing a lot of the inspections ourselves.

 

[JAE]

Some options:

1. Take additional concrete cylinders for compression tests. If the concrete achieves adequate strength for the imposed loads on the fresh concrete elements (you have to check this) then earlier construction can occur.
2. Have the concrete mix be adjusted to a higher strength to get higher, earlier strength results (still do item 1 above to verify)
3. Provide for a concrete accelerator admixture (similar to item 2)
4. Remember that the underlying concrete structure doesn't have to take full design loads (full wind, live, etc.) - only those loads imparted to them during their "young age".



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[SlideRuleEra]

...what would be the target strength to take off the shoring for the elevated slab?

For elevated highway bridge caps, decks, etc., a common requirement is that falsework can be removed when concrete reaches 75% of 28-day design strength. No superimposed loads until 90% of 28-day design strength is reached.

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[tenguy23]

Wearing the Code-Writer hat: I would assume that if this topic were in ACI, they would term it to the effect of, "capacity of structural systems not yet attaining f'c shall be analyzed using standard load combinations and with appropriate factors to resist subsequent temporary or permanent conditions not reviewed during the Engineer's initial design." I think it would be tricky for ACI to call out a catch-all percentage of f'c if they don't even know the tributary area of the column in question or the "boundary condition" of the partial column or footing in question.

Wearing the "formwork" hat: ACI 347 (Guide to formwork for concrete) in its section 3.8 has a section for hours or days between a pour and when you can remove the formwork. This doesn't address your question about min f'c, but here they explicitly cover themselves by saying you need to consider any shoring that may need to stay in place. Here, strength is not their concern: sag and creep is what they name drop. Therefore, consider whether strength is truly the underlying concern before you tackle the S.E. hat.

Wearing the S.E. Hat: I agree with JAE

 

[canwesteng]

Only thing I would add is to make sure you get 7 days of curing on the concrete regardless of if strength is reached - this is beneficial for durability and reduces the amount of shrinkage cracking.

 

[hokie66]

There are two parts to your question:

For columns, I see no reason to wait 7 days, or any specified time. You can really cast them as soon as the concrete hardens. It is normal practice to build the forms the next day, and cast columns the day after. So 2 days after the slab for the concrete placement.

For removal of slab formwork, that is normally done about 3 days after casting, and backpropping is kept in place as long as possible. If you had to wait 7 days for form removal, high rise buildings would never be built.

 

[BridgeSmith]

This may not be applicable to the situation being discussed, but hokie, if the forms are stripped in 2 or 3 days after pouring, are there steps that need to be taken to ensure proper curing (curing compound applied or wet curing) for the remainder of the 7 days? I was under the impression that one of the reasons the forms are left in place for 7 days is moisture retention.

 

[hokie66]

Curing of suspended slabs is almost always substandard. Wet curing is very messy on the top side, when work is going on below. And there are no curing compounds that are both effective and compatible with applied finishes. Plastic sheet gets in the way of further work. Yes, you can spray curing compounds on the soffit, but it rarely gets done, and is not very effective.

Slab strength is rarely critical, but slab maturity is, for deflection control. Therefore, diligent attention to backpropping is very important.

With columns, wrapping them in plastic after form stripping is good practice.

My comments are more in the nature of practicality/constructability than quality, and apply to work in other than severe environments.

 

[dik]

Hokie's got it... the only thing to concern yourself with is the 'pullout' of embedded things. Bearing is not usually an issue.

Dik

 

[bhiggins]

Thank you all for your responses. The foundation was poured this morning, it appears to be OK to begin setting forms and placing rebar as soon as tomorrow.

This is a small project so I believe most larger construction firms would have an in-house construction engineer that fields these types of questions?

 

[SlideRuleEra]

1) I believe most larger construction firms would have an in-house construction engineer...
2) ...that fields these types of questions?

You have really asked two questions:

1) Yes, a large construction company may have one, or more, in-house construction engineers.

2) No, The construction company engineer (CCE) is not there to answer those type questions. Answers remain the responsibility of the engineer of record (EOR).

The CCE efficiently implements the EOR's decisions... with the construction company's interest in mind.

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