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Crazyson (Civil/Environmental) (OP)
5 May 06 9:36
Here is the situation:  Project to pave the entrance of an active truck terminal with concrete.  Current spec is 8” of 4000 PSI Concrete (No flyash permitted) with #4 Bars placed 16” OCEW.  I’d like to open the newly paced concrete to traffic after 3-4 days cure time.  

How should I spec the high early concrete?  4000 PSI in 3 Days?  3500 PSI in 3 Days?  Etc.  Do I even need high early concrete if I extend my cure time to a week?

I know there are some long term durability concerns regarding high early concrete, so I don’t want to spec something crazy like 4000 PSI in 24 hours.

Thanks for the help.
MichSt (Structural)
5 May 06 12:42
One option would be to spec a higher strength mix, say 6,000 psi, which could obtain your required 4,000 psi strength by the time you need to open the drive.  I would check your state DOT specs for concrete and concrete pavement patches for the specifics.
GeoPaveTraffic (Geotechnical)
5 May 06 13:50
I would, and often do, just spec. 4,000 psi in 3 days.  Assuming of course that you want 4,000 psi to open the pavement to traffic.  Then let the contractor and his supplier determine the best way to get that strength.  I suspect that they will use a 8.5 sack type I mix with a water reducer.
Crazyson (Civil/Environmental) (OP)
5 May 06 14:26
You're right....Spec'ing 4000 PSI in 3 days is the way to do it.  

I guess my question really should have been....At what strength can we safely drive trucks on the fresh concrete?  3000 PSI?  3500PSI?

Thanks.
dik (Structural)
5 May 06 17:15
Should delay loading as long as possible... loading at an early age can have long term effects.  I usually look at concrete strengths for slab design... using flexural tensile strength... and varies with sqrt of f'c... so, with twice the strength, you get a 40% increase in the flexural tensile strength.  It's like loading a beam or slab at an early strength; I can't demonstrate it, but, I suspect long term deflections are affected.

Dik
Ron (Structural)
5 May 06 21:21
Don't forget to specify the largest aggregate you can tolerate with the mix placement.  Smaller aggregate increases cement content and water demand, resulting in significantly higher shrinkage.
BigH (Geotechnical)
5 May 06 22:22
Most specs that I have seen for concrete pavement actually spec the tensile strength of the concrete (splitting strength) and not the compressive strength (or they spec both).  Typically, the tensile strength is something like 4 or 5 MPa at 7-days, if my grey cells are working this early in the morning.
Ron (Structural)
6 May 06 7:19
Pavement concrete is generally designed on its modulus of rupture (flexural strength).  For instance, a 4000 psi compressive strength concrete mix can have a widely varying modulus of rupture (anywhere from about 400 psi to 700 psi or so).  The difference comes from the aggregate/paste interaction, so particle size and shape, along with paste integrity are keys to achieving good flexural resistance.  Generally larger aggregate, angular works better.

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