Brain teaser for concrete experts - the 28 day cure?

If concrete cures in about 28 days to about 90% of its ultimate strength, and the concrete has a rated strength when fully cured of 4000 psi, why is it necessary to wait two weeks after pouring to allow for enough strength to park a typical family sedan, weighing about 4000 lbs?
The contact area between the 4 tires and the concrete is maybe 100 square inches, and thus the car should exert no more than 40 psi at each tire. See:
http://auto.howstuffworks.com/question506.htm
It would appear from the strength maturity curve in this concrete reference that the concrete which is cured for 1 day should be very capable of supporting such a load:
http://www.ce.memphis.edu/1101/notes/concrete/section_3_properties.html
Am I missing something here or can I drive and park my 4000 pound sedan on a 3 day old driveway and garage floor, despite "conventional wisdom" which states that a 2 week wait is appropriate.?
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Show your calculations for the forces involved in dry steering on a fresh concrete surface. Please work your numbers two ways: in a perfect world (theoretical), and what the contractor actually did with the concrete.
R
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On 10/16/2010 1:27 PM, RicodJour wrote:

I think you have raised a really significant point. The static load of a vehicle sitting on a pad is quite different from the dynamic loads imposed by steering tires on a dry surface. The maturity curves tend to only look at compressive strength, but in a more dynamic situation, the tensile strength may also come into play. I'm not sure how to quantify these dynamic loads, but they may account for other forms of damage beyond strictly adding a load to the slab statically.
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I agree too that RicodJour is on to something. In the link Smarty provided to the characteristics of concrete, it says it's tensile strength is only 10 to 15% of the compressive rating. It would seem a tire turning might create enough sideways force to cause the concrete to start to rip out sideways if it were not cured enough.
Also, I'm not sure about the 2 week rule. Just doing a bit of googling I didn't find any consistent answer. Here's one example of what a concrete contractor recommends, and it's one week:
http://www.a-concrete.com/faqs.shtml
That seems consistent with the cure curve, which shows that at a week, the concrete is at about 60% of it;s final strength. I also saw where another contractor says that despite what he tells people, he's seen customers who drove on it after one day and there was no damage. If it were my driveway, given all the above, I'd be comfortable driving on it a 7 days. But, if I had a contractor do the job, I'd go with what he told me, just so if any problems do occur later, I can say I followed his directions and be honest.
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I'm not sure how to quantify them either - that's why I wanted you to show your work! ;) Actually, I could do the calcs, but why? It's an exercise in wild assed guesses. A person's driving technique and the driveway layout would have a lot to do with the shear forces on the surface, as would the amount of water in the mix, if the contractor wet down the surface to make it easier to work, etc, etc, etc.
R
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better safe than sorry... longer cure is always better....
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Bingo!
bit longer than necessary to minimize any risk of damage.<<<
and as RIco put forth.... why bother to run calcs that at best would involve more than a bit of guess work, estimates, etc
and as hallerb stated "better safe than sorry"
why attempt too shave a few days & wind up with damaged concrete that will remain that way for YEARS?
imo (& my professional experience) a week is probably safe, 10 days is highly likely to be safe, two weeks is certainly safe but there days with a "standard" concrete mix activates my "pucker factor"
btw your "analysis", though correct for bearing (local compressive) stress does not address bending stress in the slab at large, nor the shear / tensile / tearing stress imposed by turning a stationary tire. My wife's new car with low profile & sticky tires failed the surface of some very strong pavers when she turned the wheel while the car was not moving. I'll bet these bricks have strength in the same ball park as concrete.
cheers Bob
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On 10/16/2010 11:41 PM, DD_BobK wrote:

Thanks to all for clarifying this. I certainly intend to wait for the recommended 2 week period despite the inconvenience of parking at a neighbor's house a half a mile away. The other factors I had not considered explain why the compression strength does not tell the complete story.
Now if I can just avoid freezing temperatures for another 11 days...............
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On 10/16/2010 2:43 PM, Smarty wrote:

While having no practical experience, only thing I might guess at as is that curing may not be uniform, and surface for example is drier and may not cure as fast. It might hold up but you might mar the surface if not careful.
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On 10/16/2010 1:32 PM, Frank wrote:

No doubt you are correct that the curing will not happen uniformly throughout the slab, since water has a much easier time escaping at the outer surfaces. I intend to wait the recommended 2 week time, to avoid any issues with the contractor.
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Hmm, My understanding is concrete is always curing. Never cured. When I had my RV pad poured, I could park 7000 pound trailer and Ford F250 truck after 10 days w/o any problem.
Smarty wrote:

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On 10/16/2010 2:24 PM, Tony Hwang wrote:

My installer and other local "experts" seem to feel that 2 weeks is a reasonable time, but I am guessing that they are deliberately waiting a bit longer than necessary to minimize any risk of damage.

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