I have no experience with such short times. However, the research
shows that, at 50 hours and 70 deg. F, you have between 1/3 and 2/3 of
Most mixes show around 40% of maximum strength. Higher-strength
mixtures generally show a higher percentage of strength at 50 hours;
weaker mixes show a lower percentage.
Thanks Ed. I just did a rough calc of 400 lb/ft2 max force on the
footing for the final pour so I'm in great shape. I had guessed the
number to be three times that. i guess that's why we do engineering.
Sorry about the piss match below.
Lloyd had a real bad day... Customer to whom I'm delivering a $250K
machine Monday (this Monday coming) called to say that per the Army the
specs had changed. It'll take about $5K and three weeks to change.
They'll pay... in about five months... but in the meantime, I'm hanging
on the hook for salaries, overhead, and materials for the changes and the
time. And no pay for the machine itself until they 'approve' it, as
Sorry for my outburst. That's not me, usually. I'm just generally
pissed today; even at myself.
So long as you keep it wet, you can also hurry things along by getting
it hotter - so if the sun is shining for you, toss some plastic over it,
with a soaker hose under if needed. "Standard cure temp" is 70F - about
twice as slow at 50 F, about twice as fast at 90F, and darn fast if you
have an autoclave handy...
ie, 3-1/2 days at 90F and 14 days at 50F are both about the same as a
"standard" 7 day cure at 70F - so long as you keep it wet.
Cats, coffee, chocolate...vices to live by
Please don't feed the trolls. Killfile and ignore them so they will go away.
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.
On Fri, 22 Aug 2014 13:01:58 -0500, "Lloyd E. Sponenburgh"
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.
You don't read, do you, Ed?
What about "six days' WET cure" didn't you understand?
The surface will not become durable until it dries, but it has to cure
Pay attention. You might learn something -- like how to read.
On Fri, 22 Aug 2014 13:34:56 -0500, "Lloyd E. Sponenburgh"
Oh, I think I understand it pretty well, Lloyd
The surface is "durable" as soon as it cures. It has nothing to do
with "drying," unless you screwed up.
Ok, Mr. Over-the-Top. For almost 20 years, I studied, wrote about, and
experimented with post-tensioned concrete and ferrocement. I'm well
aware of how Portland cement concrete cures.
Your comments suggest that, like many concrete workers, you're used to
working with overly wet mixtures that are soft on top, NOT because
there was water on top, but because the mix was too wet to begin with
and settled out before it cured, it was poorly mixed, or it was hit
too hard with a hose as soon as it was poured. Portland cement does
NOT harden upon "drying."
Now, you want to challenge that? Go ahead.
It's a pity, that after those decades of 'study' you never learned
anything about surfacing and finishing.
Structural strength is the most important aspect, of course, but you
obviously haven't a clue about what it takes to make 'skating rink'
A wet portland surface marrs more easily than a dry one.
Even 'burnt' hard-trowled finishes must dry before friction traffic is
allowed on them.
No, I'm not one of those idiots who thinks "concrete has to dry to cure".
I know for a fact that a finished surface on a Portland slab has to dry
to achieve ultimate durability.
"Post-tensioning" and "ferro-cement" are affiliated with structures, not
Like I said... you don't read, even about what you 'study' for 20 years.
On Fri, 22 Aug 2014 13:56:45 -0500, "Lloyd E. Sponenburgh"
What you said, and I objected to, was this:
"At that point, it has to dry enough for the surface to harden."
The surface doesn't get any harder by drying.
"The compression strength is up by then, but marring of the surface
can easily occur until it's visibly dry."
That may be, but it's not because the surface is harder. It would be
because the surface is dryer.
Once concrete is cured, if any part is soft, it's because it wasn't
mixed or handled right. If it's wet, and it's soft, it will be powdery
or flaky when it's dry.
But it doesn't get any "harder." It just gets less wet. And if it was
wet and soft at the surface, because water floated to the top before
it was cured (too much water in the mix, or other handling issues), it
will be dry and equally soft when it dries.
"Durability" is ambiguous and I wouldn't have argued it. "Harden" is
not ambiguous. And it is not correct.
The hardness and behavior of Portland cement concrete doesn't care if
it's in a bridge or a garage floor.
I read Ok. You just don't write. You said the surface has to be dry
enough to "harden." That's what I objected to.
On Fri, 22 Aug 2014 12:55:52 -0700, pyotr filipivich
Yep, that's the common old method, and it's still a good one. You
don't want any water standing on it until it reaches an initial cure
(I forget the stages of cure). After that, keep it as wet as you can
for as long as you can, if you want maximum strength. If it dries out,
curing is done for.
It's _easy_ to tell when that 'initial cure' has occurred. When the
concrete sucks up all of its surface water, and becomes 'solid' (not dry)
to the touch, the initial cure has occurred. You can scratch it with a
fingernail, but not disturb the surface with the ball of your finger.
At that point, if you're not finishing, let it sit another couple of
hours to make sure the whole of the surface is at the same level of cure,
then gently flood it with water, and keep it that wet for six days.
It'll achieve about 75% of it's ultimate strength in (about) six days.
If you want to finish it, on the other hand, you have mere minutes to get
trowels on the surface before it becomes too hard to move the surface
material around. For 'hard troweling', you may have to do it in steps,
because when you move the cement (not the aggs) around, you break up the
gypsum crystals it's forming; it becomes motile again, and must re-cure
for a time between trowelings.
Does this mean the stonework I'm doing <wood stove pad of 2"+- Arkansas
field stone, then up the wall with same> has to be covered with wet burlap
for 6 days ? I sure don't want the concrete to be weak !
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