Minimum Stripping Time for Slab Forwork

[ajk1]

Generally we specify to strip and re-shore when concrete in slab has reached 75% of its specified 28-day strength. The contractor for a 10 storey residential building is asking if we can reduce the stripping time if he uses high early strength concrete. My answer would be that he has to meet the 75% criterion but the thought occurs to me that perhaps there should also be a minimum age for stripping/reshoring even if the 75% criterion is met. For example, what if the 75% of specified strength is achieved at a very early age (say 24 hours, as determined by a reliable method such as Lok tests)...can the forms be stripped at very early age? Might that cause problems such as excessive deflection and creep, additional cracking, etc. if the modulus of elasticity does not develop as quickly as the strength developes? Any thoughts on that? Does ACI SP-4 say anything on that?

 

[KootK]

Might that cause problems such as excessive deflection and creep, additional cracking

These future serviceability issues would be precisely my primary concern. That said, high early strength alleviates much of this in my opinion by providing concrete that can be expected to have the same expected modulus of rupture at the same point in time. I think that the most important thing is not to have a slab that starts off life cracked up way more than you'd planned. Obviously, the stiffness of a cracked slab is drastically less than that of an uncracked slab and that can seriously affect deflections. While we've got a host of fancy long term deflection theories to apply nowadays, I still suspect that what saves our behinds most of the time is that many slabs remain substantially uncracked.

Whatever changes are made to the mix to obtain the high early strength, I'd make sure that those same changes don't significantly affect shrinkage. Otherwise, with some restraint thrown in, you'll still just wind up with a bunch more concrete cracking than your modelling probably assumed.

In the markets I've operated in, you'd get spanked if your design don't allow for at least two floor slabs per week. In that sense, there is a political imperative to be as cooperative as possible.

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.

 

[TehMightyEngineer]

All of our precast is poured with type III (high early) cement and I highly doubt you would have a significantly different modulus of elasticity or modulus of rupture while still meeting your strength requirements. Or at least I've not seen it happen with our concrete and we have poured a lot of thin precast slabs and such which 16 hours later were stripped and craned onto trucks and shipped down the road without having any service level cracking or deflection issues.

However, you're 100% right that high early strength cement will change the shrinkage characteristics. We've definitely had shrinkage cracking form even on moderate sized, simple precast pieces if we had some issues in our plant environment (hot day in the shop, sudden drop in humidity, not wet cured properly, etc). If the water isn't correct the cement will quickly dry out a slab so taking extra precautions to limit shrinkage cracking would be my biggest concern.

Professional and Structural Engineer (ME, NH, MA)
American Concrete Industries

http://www.americanconcrete.com

 

[ajk1]

ok, thanks for the very good points about shrinkage cracking.

to Kootk - 2 floors per week is an incredible rate...are you sure of that? What area of floor are you thinking in terms of? Are you talking about winter construction with enclosure and heating?

 

[KootK]

Repeating one day pour condo or office flat plates. Winter construction included. Let's call it five days rather than two a week. The point being that, if a slab is poured on a Monday, I need to be prepared to see another poured on the Friday or even Thursday of the same week.

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.

 

[KootK]

Bad asses in NY can do 2-3 on the high rises apparently: Link

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.

 

[Ingenuity]

The contractor can control-strip their floor forms and backprop, as required.

Do you usually specify max basic concrete shrinkage strain, at say 56 days, on your drawings/specs, so the concrete shrinkage properties can be compared?

 

[rapt]

The formwork essentially also provides curing for those surfaces. If you strip earlier then you would have to cure those surfaces of the concrete in some other way!

 

[NS4U]

at early ages concrete modulus is not proportional to strength gain. Modulus gain lags strength gain. So, beware of higher deflections if you remove forms early. May be higher strength concrete helps, but i'm not sure. There is a paper from years ago that talks about this, but i can't find it in my files for the life of me. But, i know there is research out there that talks about this issue.

 

[ajk1]

To NS4U : Thanks. That is what I had thought too, as stated in my original question. Would be nice though if we could locate a research paper confirming this one way or the other.

To Rapt: good point about curing. I suspect though that curing the bottom of the slab is usually overlooked (although it would be quite important in a corrosive environment.

To Kootk: I find that schedule hard to believe (especially when I see the relatively unimpressive pace I see on my annual vacation to Hallendale Florida; they were still building the same high rise when I returned again on vacation a year later) but you know more about it than I.

When they have done that aggressive schedule, can you tell me whether they end up with increased cracking and increased initial deflection and increased creep deflection?

Also can you tell me how long they leave the form in place before stripping and re-shoring, and how they determine that it is ready to strip? That would be very interesting to know, if you have that information.

 

[bookowski]

A 2 day cycle isn't uncommon in NYC, needs to be the right size plate - not too big or small. There is a paper on it by rossenwasser grossman, try googling 2 day cycle grossman.

 

[Ingenuity]

In the mid-1970's, using PT, flying forms, and advancing walls/columns (2 floors above floor construction), Honolulu often constructed 2 floors per 6-day work week, with a typical 3 day schedule of:

1. Place slab concrete on Monday morning
2. Stress tendons on Tuesday morning (using oversize anchors (4"x6") to permit stressing at 1,500 psi that was achieved in 24 hours)
3. Fly forms on Tuesday afternoon
4. Install tendons and rebar on Wednesday (placed through sleeves in walls and columns)
5. Place slab concrete on Thursday morning​

 

[racookpe1978]

Warm area though with Humid air in HI for that schedule.

 

[dik]

Typically loosened at 1 day and supported with formwork being removed at 3 days and re-shored for 14 days. Else you get added deflections and or cracking. If you don't loosen the formwork, it can get 'glued' to the concrete.

Dik

 

[manstrom]

Strip at 75%, but keep re-shored as long as possible. I request 30 days, but usually get less.

When I am working on a problem, I never think about beauty but when I have finished, if the solution is not beautiful, I know it is wrong.

-R. Buckminster Fuller

 

[ajk1]

To Bowkowski - thanks, I will google that. On that schedule, does the stuctural engineer provide resident site review of the work? Maybe set up a bed on site?

To Ingenuity for outlining the schedule. That helps me understand. But I don't see where you have allowed for time to form the columsn and walls, place wall rebar and pour the walls to support the floor above..or are you guys all talking about 2 pours a week per floor? Or some of you are and some are not.

Anyway, I think I am perhaps getting a little off-track. What I am interested in is the following:
Is reaching 75% of the strength sufficient to strip and reshore a non-prestressed floor, or should some minimum time period such a 2 days also be imposed? Does anyone specify that slab forms remain in place not less than say 2 days?

 

[bookowski]

The 2 day cycles that I have seen have at least 2 sets of formwork so they are not stripping after a day, just reshoring etc but not stripping until day 2.

Here is the grossman article, it describes the technical process even though it't a bit outdated

http://www.concreteconstruction.net/_view-object?id=00000153-8ba7-dbf3-a177-9fbff5df0000

and here is a more recent one that's a bit heavier on fluff and less technical but still interesting and describes some of the economics of the 2 day schedule

http://www.structuremag.org/?p=8954

Might just be me but I haven't seen as much involvement from the engineer as the articles imply. No bat phone.

 

[calvinandhobbes10]

Is reaching 75% of the strength sufficient to strip and reshore a non-prestressed floor, or should some minimum time period such a 2 days also be imposed? Does anyone specify that slab forms remain in place not less than say 2 days?

As a current industry standard--75% is what I see spec'd. Never seen minimum time. I think you will have a tough time convincing contractors to leave forms in place longer than the moment they receive the early cylinder breaks and the percentage is met. In my locality it is common for slabs (conv or PT) to be stripped 24 (6am next day) or 48 hours after pour, with a floor-per-week cycle as the rule of thumb. Although, all the concerns cited here about cracking/deflection certainly apply, even if not yet incorporated into standard practice.

I agree that the 75% specification should come from a modulus ratio, even though the 75% is based on strength. I.e., if your floor load is 100 self + 40 live, and your stripping load is 100 self, 100/140 = 0.71, roughly 75%. Here's something I found by AASHTO with a Ec / f'c chart. If your 75% is in the 3-4 ksi range I'd say you can work with ACI 318 formula.

I'd also suggest you can hedge/help yourself by enforcing curing procedure strictly (emphasize it in the pre-con), being diligent in your check of long-term deflection and your selection of concrete depth, and also considering a minimum shrinkage spec as Ingenuity asked, especially if your layout is such that shearwall restraint is an issue (10-story residence sounds like it could be a U-shape).

Your topic, as well as the general topic of concrete deflection, is one that I believe deserves much further research. We, not the least including myself, have much to learn.

 

[RHTPE]

Be cautious on the 75% requirement. It really depends on the ratio of structure weight to total design load. I have seen cases where something greater than 75% is required - typically with lighter superimposed loads and a relatively heavy slab. Roof slabs can sometimes require a higher % for f'c as the design superimposed load tends to be lighter than floors.

As others have pointed out, early stripping combined with the application of early-age construction loads on the slab can exacerbate long-term deflection performance. The loads imposed on the newly cast slab are often the heaviest loads that slab will ever see.

You might want to review ACI 347.2R-05.


Ralph
Structures Consulting
Northeast USA

 

[Retrograde]

My company generally does not specify a minimum age but I have seen others do that. I have a set of drawings in front of me that calls up a minimum of 7 days before stripping and backpropping.