Replies

1. 
   Guest_ | Oct 05, 1999 05:43am | #1

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   Jeff:

   A good rule of thumb is: Concrete that is poured at moderate temperatures is at ~ 75% of it's full strength after 7 days and at ~95% of it's full strength after 28 days. To be very safe, wait 7 days.

   As far as 3000 PSI vs. 2500 PSI I don't know about the PSI 's effect on cure time but doubt that it is appreciable.

   If daytime temperatures are in the 90s, it is more important to ensure proper curing of a concrete slab. That would mean keeping it constantly damp for the first week (via some type of cover) or applying curing compound - the idea being to keep water from evaporating too rapidly from the slab.

2. 
   Sandie_C | Oct 06, 1999 07:39am | #2

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   As an ACI certified technician, I agree with Matt's rule of thumb on curing for 7 days. I would also suggest damp cover during the 7 day period with wet hay and black poly to prevent water migration from the concrete as is the case with clear poly.
   The strength of the concrete has no bearing on cure time. 3000 psi has more concrete than 2500 psi. I spec'd 3000 psi for my poured walls and basement slab for our new house because I know that the mix design for 3000 psi contains more aggregate and is generally a better quality mix. Mix temperature at the time of delivery is more critical to strength development with any psi concrete. The longer concrete is agitated, the hotter it becomes and the faster it cures initially. It will take longer than 28 days for the concrete to cure and will produce a weaker and easier to scar structure if it was placed hot. If possible, don't pour concrete that is over 60 minutes old (the time it was made is on the ticket) or 90 minutes if air temperature is between 70 to 80 (maybe 85 if overcast).
   My advice is to call several of your local suppliers (not just the one you plan to use) and ask them what they recommend and use what the majority of suppliers have recommended.

3. 
   Jeff_Walker | Oct 07, 1999 08:48am | #3

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   What is the recommended cure time for a concrete slab before beginning framing?
   Average highs in the low 90's, lows in the low 60's. Also, is there a significant difference between 2500 psi & 3000 psi concrete?

4. 
   Guest_ | Oct 07, 1999 08:48am | #4

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   Sandie:

   That's very interesting about the mix time. I'm going to get out some old concrete tickets and check for the mix time.

   By the way, what's a ACI certified technician?