concrete setting time

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 maximum strength.

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.

Good luck!

Karl Townsend fired this volley in news: snipped-for-privacy@4ax.com:

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 fired this volley in news: snipped-for-privacy@4ax.com:

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 throughout first.

Pay attention. You might learn something -- like how to read.

Lloyd

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.

Ed Huntress fired this volley in news: snipped-for-privacy@4ax.com:

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' quality finishes.

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 finishes.

Like I said... you don't read, even about what you 'study' for 20 years.

LLoyd

Ed Huntress on Fri, 22 Aug 2014 14:12:40

-0400 typed in rec.crafts.metalworking the following:

Ah, that explains why my dad covered the patio with burlap bags and kept wetting them down.

-- pyotr filipivich "With Age comes Wisdom. Although more often, Age travels alone."

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.

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.

Ed Huntress fired this volley in news: snipped-for-privacy@4ax.com:

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.

Lloyd

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.

karl

Eh, what else is new? d8-)

Lloyd doesn't usually start off like that. Maybe he had a bad day.

Ed Huntress fired this volley in news: snipped-for-privacy@4ax.com:

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 delivered.

Sorry for my outburst. That's not me, usually. I'm just generally pissed today; even at myself.

LLoyd

Oh, jeez! Bad day ain't the word for it. Sheesh.

Hey, Lloyd, if it makes you feel any better, you can call me some of the names that Klaus calls me. d8-)

Does this mean the stonework I'm doing

Plonk

Ed Huntress fired this volley in news: snipped-for-privacy@4ax.com:

Most lime mortars cure well wet or dry, but tend to effloresce if kept wet after curing, about like plasters do, but not nearly so badly. Do as block/brick/stone masons do. Put it on as wet as will sustain the load of the stone/brick/block, and just let it cure until cheezy enough to strike the joints -- then leave it be.

Lloyd

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.