FOUNDATION: less than 28 days to lower house?

Old 09-16-06, 03:03 PM
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FOUNDATION: less than 28 days to lower house?

i know concrete takes 28 days to reach full compressive strength.

what do you do in the scenario of a raised house with a newly poured foundation under it... must you wait the 28 days in order to avoid damage to the foundation before you lower the house on it?

some details:
the house is a 3000 sq ft duplex.
the basement level of the house is going to be rebuilt. - how long would you have to wait before building the basement framing and siding after you pour the concrete? can you do that right away because it's not so much weight?

thanks for your help and time.
Old 09-16-06, 04:09 PM
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FOUNDATION: less than 28 days to lower house?

The 28 days is the period of time that is used for the testing of the compresive strength of concrete in a laboratory situation. This is a parameter used in the initial design of a comcrete mix. This is at 68 degrees and not the higher temperatures encountered in the real sumer world, which accelerate the curing.

In the real world, a structure is rarely if ever subjected to the design loads the 3000 psi SPECIFIED for. Usually, the steel is the critical item.

Concrete gains strength very quickly in the first 4 or 5 days - maybe 75-90% of the untimate strength, so your project should have more than enough strength at this time. Your main concern might be the lateral loads on the wall since the vertical loads are so rediculously low (maybe 10% of max).

It really is up to your contractor (if you have one) whether you can drop the house.

The 28 day strength is totally blown out of perspective when it comes to putting loads on fresh concrete. High rise concrete building are routinely built one story per day (7 stories per week) without stopping or waiting for a 28 day test. There are relationships between 1 day, 2 day, 7 day, 14 day and 28 day strengths and that is why loads can be applied without waiting.

Slipformed curbs with 3000 psi 28 day concrete are stripped immediately and frequently are in use long before the 28 days (maybe 7 days).

Your 3000 psi concrete will probably test with an untimate strength of 4000 psi, since concrete continues to gain strength after 28 days. It is much cheaper for a supplier to get higher strengths than it is to take a chance on an error.

Bottom line is - ask your contractor or supplier. If you have a week behind you, you should be OK.


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