How many days to allow contractor to take off forms from concrete wall

[Doublest]

Hi guys,

I have a question need your help. About how many days do you allow a contractor to take away the form from a poured concrete foundation wall? The wall has no external load at all except its own weight, the contractor wants to take away the form the next day after the concrete is poured. Will it cause any problem? Any suggestion?

Thanks.

 

[Ron]

Check the strength curve on the concrete and compare that to the stress caused by the self weight. When the strength exceeds the self weight stress, ok to remove.

 

[ToadJones]

We usually weight 5 days as our rule of thumb for walls that are seeing nothing but wind loads. Any significant loading (backfill or construction loads) and we break a 7 day cylinder to verify strength. We are usually at 75-80% of full design strength after 7 days.

 

[hokie66]

Strength is not normally a concern the next day, but I like to keep them on longer for curing. If the walls are going to be adequately cured by other means, I would allow the removal.

 

[Zambo]

I agree with hokie66, it is normal to remove the forms the next day, then move the form on to the next section. But then curing has to be carried out immediately.

The only time I require forms to be left on longer is when it is a mass concrete section and heat differential is a problem. In this case the form may help in reducing the difference between the heat of the core of the concrete and the outer surface.

 

[dik]

If you don't loosen them the next day... you can have an awful time pickin' the formply from the concrete wall... curing is addressed separately...

Dik

 

[MiketheEngineer]

There used to be a technique that would give the finished exterior walls a very nice look - like a rough stone. The contractor would pull the exterior forms within 24 hours - usually less and then hit the surface with a high pressure hose.

This "washed" away the cement and sand and left a real pretty aggregate finish. Often seen in lakeside settings - kind of fit in with nature?? Worked just fine.

 

[ToadJones]

I still see concrete finishers using that technique on drives and patios once in a while.

 

[doka1]

I wont quote anything from aci, so I may be off specifications a little. Speaking from a person in the field, it really varies by the service of the structure. There are differences between walls that are to be exposed and those that are simple frost walls that will be buried. You should already have a curing process for exposed walls that goes beyond specifying form removal times. Typically white poly and soaker hoses are required for a period(3-7 days). Its my experience that if the exposed wall is pretty thick (ie greater than 18")or if the wall has concrete greater than 4500 psi then you really should take measures to keep the temperature down to avoid shrinkage cracking. If the walls is not exposed or shrinkage cracking is not an issue, then stripping the next day is not a problem in the summer months.
You have to be very careful during the winter as the form removal process becomes more complicated, and maintaining the temperature of the concrete, while not "shocking" the concrete.
But I am sure someone on here will say that its not good practice to strip a footing or frost wall the next day in the summer...

 

[abusementpark]

Isn't it really the contractor's call on when to remove the forms as we are getting into means and methods of construction?

I suppose we could specify a minimum amount of time the forms must stay on the walls. Does anyone here put anything definitive in their specs?

 

[doka1]

yeah some engineers specify the required strength to remove shoring for slabs. But of course the contractor has to bear the responsibility and is technically required to have his own PE to specify stripping times.

I never really see it in the specs but do see the aci references in the specs, aci 347 comes to mind but i am not sure.