On Fri, 22 Aug 2014 13:01:58 -0500, "Lloyd E. Sponenburgh"
lloydspinsidemindspring.com wrote:

Karl Townsend fired this volley in
:

I just finished pouring footings...

Turns out the only day the kid can help me with the slab is tommorrow
P.M. or 48 hours after the footings. I surfed the web to verify this
is enough cure time, tons of stuff about 1 week and longer vs. time,
could find nada about two days.

I wanted to find PSI compressive strength estimate at 48hours.

Anyone know?

The old manual rule of thumb is 6-6-6-6. Six bags per yard (90lb
Portland), six gallons of water per bag, six percent entrained air, and
six days' wet cure.

At that point, it has to dry enough for the surface to harden. The
compression strength is up by then, but marring of the surface can easily
occur until it's visibly dry. Of course, that's no issue with footers,
only the slab.

Lloyd

Lloyd

I don't know about that "rule of thumb," Lloyd, but concrete hardens
best when it's completely under water. You don't want it to dry at
all. And the surface should harden even when it's soaking wet, if the
mix wasn't too wet to begin with.

Once it dries, the hardening cure stops. It can't be restored. So,
whether it's a couple of days or a couple of years, that's the end of
the road for continued hardening.

Maximum strength is obtained in about three years of continuous
soaking, but the gain after 28 days (a standard for measuring maximum
strength) is very small.

When ferrocement boats were all the rage, back in the '70s, some of
the larger ones were finished and then intentionally sunk and kept
under water for a year. They were made with a standard sand mix, like
some kinds of mortar.

--
Ed Huntress