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Old 12-05-2009, 10:47 AM   #1
 
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concrete strength (psi 0?????????????


I have been in the concrete business doing formwork for a few years now. I poured my first slab, and it came out good. but iwould like to understand psi strength and when its a good time to pour certain concrete.

Last edited by bronze2273; 12-05-2009 at 12:16 PM. Reason: change heading
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Old 12-05-2009, 01:02 PM   #2
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PSI stands for pounds per square inch. Concrete strength is measured in psi. Typically, the required strength of concrete is specified by the architect or engineer, or if there is none, by the minimum required in the building code. Strength is typically specified at 28 days, meaning in theory the concrete is allowed to cure for 28 days, then its strength in psi is measured using a standard cylinder test.

In practice, the strength of concrete is rarely directly measured on single family home construction, rather the supplier warranties that the concrete they supply will meet the required strength (typically 3000 psi) after 28 days, and a simple slump test is performed when the concrete is delivered.

The last part of your question is confusing, I think you are asking how to determine the required strength for a given placement, and possibly how to design the mix. These are matters generally left to the design engineer, or the supplier. If you want to understand concrete, I recommend you purchase a book on concrete design and mix selection. Very large, complex subject, especially with the development of so many additives and special procedures.
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Old 12-05-2009, 09:14 PM   #3
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Enter different concrete subjects at the top search if you don't find the answers: http://books.google.com/books?as_brr...G=Search+Books
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Old 12-06-2009, 06:58 AM   #4
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no one knows when its a good time to place conc but, minimun, 40f & rising on a compact'd base w/no freezing temps w/i 24hrs,,, there're TONS more depending on owner, specs, & aci but, usually, we just look at the sky & keep on til we're run off the job by weather

as dan post'd, the main test for conc strength's compressive,,, another's flexural but, for most purposes, comp's enough,,, btw, there are many others, too.

since the only factor ' we ' can control in conc testing's the time period, we typically test at 24hrs, 7d, & 28d,,, also, since no one knows exactly ' when ' conc cures { indeed there are those who'll argue a yr or more under the right circumstances }, that's HOW we determine conc strengths - based on TIME.

far's mix design, etc, just order up the comp strength & slump you want in the trk,,, as mention'd, that's a whole 'nother ballgame


ps - there isn't ANY difference 'tween sidewalk, commercial, &/or residential conc mix's so don't fall for that 1
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Old 12-06-2009, 04:42 PM   #5
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Quote:
Originally Posted by bronze2273 View Post
I have been in the concrete business doing formwork for a few years now. I poured my first slab, and it came out good. but iwould like to understand psi strength and when its a good time to pour certain concrete.

A good place to start would be the guys you are forming for.

Another is books or an engineer.

There are many different concrete psi requirements especially on GOV funded jobs.

Runways need 5000 LB strength with the right slump.

Other jobs require 4000 LB strength.

Depends on the application.
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