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Removing formwork after 24 hrs.

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Removing formwork after 24 hrs.

Removing formwork after 24 hrs.

SKIAK

(Structural)

(OP)

17 Jul 09 21:50

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, 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?

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RE: Removing formwork after 24 hrs.

RHTPE

(Structural)

17 Jul 09 22:24

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.
 

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

RE: Removing formwork after 24 hrs.

JAE

(Structural)

17 Jul 09 23:55

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.

 

RE: Removing formwork after 24 hrs.

Ron

(Structural)

18 Jul 09 07:30

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.

RE: Removing formwork after 24 hrs.

dik

(Structural)

18 Jul 09 08:57

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

RE: Removing formwork after 24 hrs.

SKIAK

(Structural)

(OP)

20 Jul 09 12:54

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?

Additional resources:
Aluminum tubing: which to choose and where to find them

For more concrete slab formworkinformation, please contact us. We will provide professional answers.

RE: Removing formwork after 24 hrs.

concretemasonry

(Structural)

20 Jul 09 13:27

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.

RE: Removing formwork after 24 hrs.

SKIAK

(Structural)

(OP)

20 Jul 09 14:30

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.

RE: Removing formwork after 24 hrs.

RHTPE

(Structural)

20 Jul 09 15:57

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?
 

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

RE: Removing formwork after 24 hrs.

SKIAK

(Structural)

(OP)

20 Jul 09 18:18

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.

RE: Removing formwork after 24 hrs.

hokie66

(Structural)

20 Jul 09 18:33

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.

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News

When pouring concrete forms on a job site, time is of the essence. If you need to get those forms to another site, you might wonder if there&#;s an issue with leaving them on for too short a period of time and compromising the build. On the other hand, you might also wonder if there are any issues with leaving concrete forms on too long. While the short answer is that, no, leaving concrete forms on for too long will not be an issue, there are things you want to consider about the whole process.

Concrete forms create a framework that supports the curing process, and the longer the forms stay in place, the more concrete will simply continue the curing process. If you are looking at concrete forms for sale, here&#;s what you need to know about the curing process and the length of time necessary.

When Should You Remove Concrete Forms?

Concrete forms are designed to provide structure and framework for concrete to cure. The curing process refers to the time period when concrete dries enough to become and remain solid. Concrete forms can also help make sure the concrete retains the appropriate amount of moisture during the curing process. You might be wondering how long to leave concrete forms in place and when you should remove them.

Concrete forms should only be removed once the concrete has dried enough so that it can support its own weight. The typical curing process for concrete takes between 24 and 48 hours, depending on the size and scope of the project. The type of concrete mixture and the area&#;s climate can also have an impact on how long you should leave concrete forms in place. The ideal time to remove concrete forms will depend on a variety of factors and will inform whether or not you leave concrete forms on for too long or remove them too early.

What Happens if Concrete Forms Stay Too Long or Removed Too Early?

Removing concrete forms is also referred to as strike-off or stripping the formwork. The removal process for concrete formwork should only take place once the concrete has gained sufficient strength. The determining factor for whether the concrete has gained sufficient strength is that the concrete should be able to handle twice the amount of stress that it would be subjected to once the forms have been stripped. The concrete framework provides temporary stability while the concrete cures, so if it is removed too soon, it can result in unstable concrete forms.

Important factors for concrete forms include both self-load and live-load features. Self-load for concrete forms refers to the pressure the concrete must maintain, while live load refers to the additional weight the concrete will support during both the construction and the finished project. If the concrete does not have sufficient time to cure, it can deflect that strength onto other places and compromise the integrity of the structure. If concrete forms are removed too early, the concrete may not gain enough strength sufficient for handling the required loads. However, you can&#;t leave concrete forms on too long in a way that would compromise the integrity of the structure and load-bearing process. When concrete forms stay on past the ideal curing process timeline, this simply allows more time for the concrete to cure. It is important to note that as you continue to pour more concrete, the concrete forms do deflect some load-bearing qualities during the curing process, so you must time those pours appropriately.

How Long Should Concrete Cure Before Removing Forms?

The standard timeframe for concrete curing is between 24 and 48 hours. The timeline will vary due to a variety of factors. Contractors can estimate the hardening of concrete and strength of concrete before removing the formwork through a testing process. You may choose to test on concrete cubes or cylinders before carrying out the process to help estimate the strength of concrete. Concrete cubes or cylinders used in the testing process should be prepared with the same exact mix of concrete as what will be used for the structural members and undergo the exact same curing process.

A test on concrete cubes or cylinders should also take place in the same circumstances as the upcoming curing process as much as possible. This should include the same circumstances around climate, including temperature and moisture, that will impact the structural member. Formworks should never be removed from the test product or the final product to ensure the structural members gain sufficient strength to withstand the intended load. While you can remove the concrete forms too early and impact the overall design, the concrete formworks can be left on even longer than originally intended and continue to help in the curing process.

Factors That Affect Concrete Form Removal Time

The striking time or stripping of the framework will depend on the strength of the structural member. There are many factors that will affect concrete form removal time and impact the strength and development of the concrete. Here are six factors that may be impacted by the concrete formwork striking times:

Temperature

The temperature in the area will impact the timeline for concrete strengthening. In warmer weather and temperatures, the concrete is able to achieve higher strength in shorter periods of time. Conversely, in cold temperatures and winter months will prolong the strength gain time. Concrete forms help insulate the concrete from its surroundings, which can help support the rate of strength gain.

Moisture

Concrete forms also help insulate the concrete curing process so that less moisture gets loss during the heat of hydration and evaporation. In extreme heat, there is a risk of excessive moisture loss, which can lead to cracking and spalling. Leaving on concrete forms for a longer time period can help prevent excess evaporation by trapping moisture while the concrete continues to cure at its intended strength.

Concrete Mix

The concrete mix will also have a significant impact on the formwork process, including drying and curing times. While some fast-setting concrete mixes are designed for load-bearing within hours, these mixes are typically ideal for small-scale projects. However, standard concrete mix will take much longer to dry and cure so it can support the necessary loads for a construction project.

Concrete Grade

The grade of concrete will also impact when the concrete gains its strength and during what time frame. For example, the higher the concrete grade, the shorter period of time it will take for the concrete to strengthen and cure.

Cement Grade

The grade of cement impacts how much strength the cement can bear. The higher the cement grade, the shorter the time period it will take for the cement to achieve higher strength.

Concrete Size

The size of the concrete member will also impact the gain of strength over time. Larger concrete sections will actually gain strength in a shorter period of time compared to smaller sections of concrete.

Final Thoughts: Why Should You Leave Formwork On & When to Strip It Off?

The longer you leave on the concrete forms, the more time you give the concrete to develop the sufficient strength necessary to support all types of loads that will be placed upon it. The general timeline runs from 24-48 hours, though you should take into consideration the factors listed above to ensure a strong final product. For both new and renovated concrete forms for sale, visit Forming America for a large inventory that allows for competitive pricing and quality customer service. Forming America offers concrete forms for sale and rent, and we can manage large quantity orders to help you meet your project&#;s deadlines. Visit us to address all your concrete forming equipment needs for your next project.

The company is the world’s best formwork prop supplier. We are your one-stop shop for all needs. Our staff are highly-specialized and will help you find the product you need.