Removing formwork after 24 hrs. 1

[SKIAK]

We designed a concrete utilidor that will be about 6'-0" underground. The walls are 10", the roof has 3" 16ga composite metal deck at 10" full depth with concrete, 8'-0" span, 4000 psi concrete. The contractor has requested that we allow them to pour the slab and a stub of wall in one pour, and the remaining walls and roof in the second pour. We said OK, added some more steel to take the shear instead of a key. The contractor will have formwork for the walls and shoring in the middle. Now the contractor and owner are in a rush to get this done so they have asked how soon the contractor can remove the formwork and shoring, they are hoping for 24 hours. The consensus around the office is "I think its OK, but boy... I'm not sure." I've done a lot of digging around the ACI website and other typical formwork sources and I haven't found anything that would lead me to believe this won't work. It doesn't seem like the low strength will pose any problems, but what you don't know can hurt you. Any bonding issues with this green of concrete? Or anything else?

 

[RHTPE]

Skiak,

Because you have composite deck, I would require some reasonable % of design f'c.

I have worked with a forming system that permits removal of everything but the shores the following day. But in this instance, the shores remain in contact with the slab, so the slab never need support itself beyond the 180cm shore spacing (each way). The shores remain until the slab reaches a strength appropriate for stripping.

Early stripping, or high loads early in the concrete's life, will increase the magnitude of long-term creep deflection of ordinary reinforced concrete structures.

I your case with composite metal deck I would be concerned about compromising the bond between the concrete and the deck by stripping too soon. Especially if the metal deck is your primary postive moment reinforcing.


Ralph
Structures Consulting
Northeast USA

 

[JAE]

Two things that the forms do for you while left in place:

1. They help the curing process by keeping the wall concrete away from the ambient air.

2. They keep the concrete "still" and away from damage due to impact or form removal.

Thus, ACI usually suggest removing forms only if the form removal will not damage the concrete. Also, if forms are removed earlier than 7 days, you should require some subsequent curing on the wall...either water fog/spray, plastic sheets, wet burlap, curing compound - all applied immediately after form removal.

Keep the curing in place for 7 days.

Make sure that the contractor understands that if the forms are removed earlier than 7 days and the concrete is damaged due to that form removal (prying, hammering, etc. to break forms loose), then they are responsible for all costs required to mitigate the damage.

 

[Ron]

In addition to the good advice from RHTPE and JAE, the concrete mix design will determine when you can removed the forms from a strength standpoint.

Make sure your mix does not included ground, granulated blast furnace slag cement...this will slow the strength gain significantly. Second, use Type III portland cement to gain high early strength. You can generally get about 40 to 50 percent of f'c in 24 hours with Type III cement; whereas it takes about 3 to 4 days to gain the same strength with Type I portland cement. Keep the water-cement ratio below 0.50 and use a high range water reducing admixture so that you can consolidate the concrete without adding water at the site.

Since this structure will be below grade, I assume you'll need to consider waterproofing. As JAE noted, curing will be extremely important...for strength gain, durability and to reduce the drying shrinkage cracking. Make sure wet curing is used...spray on curing compounds are not that great to begin, and you want true hydration of the cement to progress, so moist curing is best.

 

[dik]

For wall formwork, I generally permit the contractor to 'loosen' the forms after 24 hours, but to leave them firmly in place to minimise creep, to promote curing, and to permit strength gain. If the formwork is left without loosening, then it can be extremely difficult to remove.

If the steel deck is only used for formwork and is not capable of spanning, then I usually permit reshoring after 2 days, again to minimise creep.

Dik

 

[SKIAK]

Thanks for the suggestions.

When do you have to worry about rebar slipping or the bond to the deck? From a quick run down of the original numbers with a rough estimate of the concrete strength it seems OK. This is, of course, assuming that the concrete behaves the same just not as strong. Is that a resonable assumption?

 

[concretemasonry]

What kind of concrete mix are you dealing with? That is the variable.

Any fly ash or slag? - That slows the strength gain.

What was the concrete temp and what was the ambient conditions of the forms, adjacent materials and air temperature?

Where are you? - Things are different in a place like India than they are in parts of Canada.

When you are dealing in the short term this is very important.

 

[SKIAK]

It was a Type I, although I heard they are preparing to submit a mix design for a Type III. I don't know for sure where the Type III is going but I might ask them to put it in the walls they want to remove the formwork early on. No fly ash or slag.

They have not been poured yet, weather predictions are around the mid 70's or low 80's - low 50's at night.

This is being constructed in Alaska.

 

[RHTPE]

Speaking qualitatively, near full strength loads early in the concrete's age can adversely affect the concrete that the rebar's lugs bear against, thus leading to higher long-term creep deflection.

You do not say whether or not the composite metal deck is your primary positive moment reinforcing. If it is, I would be very cautious about the removal of the deck's supports in 24 hours.

It might be wise to obtain a number of additional test cylinders from the first placement, and determine a strength-gain curve for the mix being used - perhaps breaks at 24 hours, 3 days, 7 days, 14 days & 28 days. You could then compare against the strength necessary for the slab to support itself and any applied construction load (no doubt much less than your 6 feet of earth cover).

Check both shear and bending strength - your gain in shear strength is not as rapid as your gain in bending strength.

More importantly, how will the wall concrete be properly cured if the form are removed so quickly?


Ralph
Structures Consulting
Northeast USA

 

[SKIAK]

Sorry RHTPE, I thought I responded to that earlier, the steel deck is not the primary positive moment reinforcing and is much thicker than required for strength. There are #8 @ 12" O.C. running along the roof but are approx. 5.5" down from the top.

There were a host of changes made after the original design, the steel deck is sort of left over. My main concern is that although theoretically there is a lot of strength, I wanted to make sure this strength could be developed and utilized.

Right now my plan is to require a test cylinder at 24hrs (in addition to the normal required) with a minimum compressive strength of 1,200 psi which should be easily obtainable and provide enough strength not even considering the steel deck. I am asking the contractor, before they remove the forms, to respond with a plan to cure the concrete after the forms are removed.

This is fairly heavily designed, but with only the weight of concrete it is minimally loaded. This just helps me sleep better at night.

 

[hokie66]

I wouldn't worry about removing the props in 24 hours from a 10" thick deck which only spans 8'. But I would worry about what the contractor then does. If he plans to place 6' of backfill straight away, then I wouldn't allow it.

Another issue is placing the slab with part of the wall. You need to make sure the wall concrete has begun to harden before the slab is poured, otherwise you can get plastic settlement cracking at the slab to wall junction.