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
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:
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.?
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.
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.
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:
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.
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.
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
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.
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
Now if I can just avoid freezing temperatures for another 11
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
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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