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-   -   How long for to cure a concrete footing? (http://www.diychatroom.com/f19/how-long-cure-concrete-footing-64160/)

Jeremy Hillary Boob, PhD 02-11-2010 12:48 PM

How long for to cure a concrete footing?
 
How long should a concrete footings cure before a retaining wall is poured on top of it? The temperature is in the 50 and it fairly humid. Thanks.

jlhaslip 02-11-2010 01:43 PM

the next day if you can form the walls that quickly. :P

Jeremy Hillary Boob, PhD 02-11-2010 10:38 PM

Quote:

Originally Posted by jlhaslip (Post 398157)
the next day if you can form the walls that quickly. :P

OK, thanks. It's going to be a few days...

Btw, how long should the forms stay on the wall after it's poured? Thanks.

jlhaslip 02-12-2010 06:28 AM

Where are you? And what is the weather like?
If it is above freezing, peel the forms 24 hours after the pour, remove them the next day following.
If it is below freezing, leave them in place for a few days with insulated tarps and heat as required based on weather conditions. The wall should not be without supplemental heat for a week.

Jeremy Hillary Boob, PhD 02-12-2010 11:24 AM

Quote:

Originally Posted by jlhaslip (Post 398460)
Where are you? And what is the weather like?
If it is above freezing, peel the forms 24 hours after the pour, remove them the next day following.
If it is below freezing, leave them in place for a few days with insulated tarps and heat as required based on weather conditions. The wall should not be without supplemental heat for a week.

I'm in California---definitely above freezing here. What do you mean by "peel the forms"? Thanks yet again.

jlhaslip 02-12-2010 12:02 PM

"peel the forms" means to loosen them off the concrete, but don't remove them yet.
Undo the whalers and pull the forms off the concrete so they can breathe. The pour will be too soft to pull the forms because usually the tools will leave marks on the concrete. At least that is how we usually do it here. Your mileage may vary. We also use High-Early in the mixture to stiffen them up faster.

wnabcptrNH 02-13-2010 08:00 AM

what does letting concrete "breath" mean?

Concrete cures based on a chemical reaction that does not require air so I am not sure exactly what that is supposed to mean. When taking concrete cylinders for test they are poured in and capped and then tested at whatever day (7, 14, 28 etc). So they don't get to "breath" persay and they achieve their strengths just fine.

Most concrete achieves 40-50% of its strength (1500 PSI if its 3000 mix and 2000 for a 4000 mix) within 48 hours.

The reason most concrete guys like to pull forms the following day is so they can move onto the next job with their forms and the concrete will continue to strengthen. Most concrete these days doesnt need 28 days to come to full strength and most achieve it in 14 or so days.

That being said people pour walls and within days are already building on it. The reason for this is because framing a house takes time and the full weight of a house wont bare on it for most of the time a month.

Now if you have a prefabbed house they suggest you wait till 21-28 days after the concrete if poured since full weight of the house is on it within a week.

concretemasonry 02-13-2010 11:58 AM

Retaining walls are different from homes.

For a retaining wall the wall should be well cured before backfilling. This is insure the steel and concrete are bonded together AND the wall and foundation are well bonded, especially the dowels from the footing to the wall.

This time delay and the cost are the reasons most similar retaining walls use SRW (segmenetal retaining wall block) since the excvations is less and no concrete footing is allowed. This method also allows almost immediate backfilling and less time for the excabation to fill with water if rains occur.

In a home the vertical loads on a block or concrete are really not much and wall thickness is increased to provide a wide sill plate and enough thickness for resisting the soil. Without the lateral load, the block have to be may 4" or 6" thick, although most codes require a 6" minimum. I have seen 22 story loadbearing block buildings that used 6" block with very light of little reinforcement and a minimum of grout. They usually build 1 story every day or two.

Dick

jomama45 02-13-2010 12:01 PM

Quote:

Originally Posted by wnabcptrNH (Post 399089)
what does letting concrete "breath" mean?

Concrete cures based on a chemical reaction that does not require air so I am not sure exactly what that is supposed to mean. When taking concrete cylinders for test they are poured in and capped and then tested at whatever day (7, 14, 28 etc). So they don't get to "breath" persay and they achieve their strengths just fine.

Actually, I believe ACI procedures call for submersing them in a room temp water bath. I think it actually increases the latter psi tests.

Most concrete achieves 40-50% of its strength (1500 PSI if its 3000 mix and 2000 for a 4000 mix) within 48 hours.

Right on.

The reason most concrete guys like to pull forms the following day is so they can move onto the next job with their forms and the concrete will continue to strengthen. Most concrete these days doesnt need 28 days to come to full strength and most achieve it in 14 or so days.

If they're wood forms, they often pull them to keep them from bondign too well also. Even form releases only go so far.

That being said people pour walls and within days are already building on it. The reason for this is because framing a house takes time and the full weight of a house wont bare on it for most of the time a month.

Now if you have a prefabbed house they suggest you wait till 21-28 days after the concrete if poured since full weight of the house is on it within a week.


Great post! :thumbup:

wnabcptrNH 02-13-2010 10:10 PM

Thanks!

To put a little background on this so people dont think I am full of it I have a degree in structural engineering with a good portion of that in concrete and I did many studies into concrete with alternate aggregates in it.

stadry 02-14-2010 11:58 AM

long story short, no one actually KNOWS when conc's finish'd curing,,, all we can do's test beams/cylinders to be sure its reached its design strength at certain intervals of time - 24hrs, 7d, & 4wks.

Daniel Holzman 02-14-2010 01:10 PM

Properly designed, mixed and placed concrete will continue to strengthen for many years after placement. I worked on a testing job for Lock and Dams 3, 4 5 and 5a on the Mississippi River, where we cored and tested concrete that had been placed 50 years ago at a design strength of 3500 psi. When tested, the majority of cylinders had a strength in excess of 7000 psi. Historical records clearly indicated that none of the original 28 day cylinders exceeded 4000 psi, so the concrete strength had nearly doubled over 50 years.

As previously noted, the "strength" of concrete is typically reported at 28 days after placement. You can typically pull forms one or two days after placement, as previously discussed. If the wall is going to act as a retaining wall, i.e. a basement wall, you need to be very careful when you backfill, since it is entirely possible to collapse a retaining wall if you backfill too soon, because the fill places a large lateral load on the concrete. With houses, you typically do not want to backfill the basement until the horizontal support elements (read first floor joists) are in place, as these provide substantial lateral resistance to the top of the wall, and minimize the chance of buckling or collapse.

jomama45 02-14-2010 01:31 PM

1 Attachment(s)
I agree Dan, except that around here we use "Foot-Lock" braces for backfilling before the floor system is in place. It seems no carpenters want to cap a foundation that isn't backfilled. I can't say I blame them a whole lot, it seems like it would take a lot longer & be less safe. Not a real good pic, but you can see the braces in place:

Attachment 17736

stadry 02-14-2010 04:09 PM

danny, tnx for catching my typo,,, you mention backfilling,,, typically, even w/footer toe drain in place, bldr's just push spoil back against the fnd wall [ no stone, no filter cloth ] after the code-rqd 3mil dampproofing,,, can't compact against a cmu wall due to lateral pressure - 2, its not rqd,,, &3, who'd measure the lifts & do compaction tests ? thankfully that's retirement $$$ for us waterproofers - we'll never finish long's the bldg code remains what it is :yes: naturally we didn't bld our own houses to meet code.

our motto is ' pay the bldr NOW or pay US LOTS more $$$ later on '
:thumbup:


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