Curing Concrete and Masonry

[Xabia Jim]

I'm a stong believer in keeping your concrete and masonry projects damp to allow it to "cure" and make sure it does not dry ( crack ). The other thing to remember is cement curing is an exothermic process giving off heat so you don't want to add heat too early to any oven/fireplace/chimney project.......and waiting to begin your curing fires is so hard to do

Here are two interesting excerpts from Wickipedia (and note the last line where it mentions that concrete is still curing and strengthening after over 70 years!) :

Curing is the process of keeping concrete under a specific environmental condition until hydration is relatively complete. Because the cement used in concrete requires time to fully hydrate before it acquires strength and hardness, concrete must be cured once it has been placed.

Good curing is typically considered to use a moist environment which promotes hydration, since increased hydration lowers permeability and increases strength, resulting in a higher quality material. Allowing the concrete surface to dry out excessively can result in tensile stresses, which the still-hydrating interior cannot withstand, causing the concrete to crack. Also, the amount of heat generated by the chemical process of hydration can be problematic for very large placements.

Allowing the concrete to freeze in cold climates before the curing is complete will interrupt the hydration process, reducing the concrete strength and leading to scaling and other damage or failure.

The effects of curing are primarily a function of specimen geometry, the permeability of the concrete, curing length, and curing history.

The second excerpt is about the Hoover Dam Project:

The first concrete was placed into the dam on June 6, 1933..... Since concrete heats up and contracts as it cures, uneven cooling and contraction of the concrete posed a serious problem....... The concrete is still curing and gaining in strength as time goes on.

It's really quite an interesting project.....

Hoover Dam - Wikipedia, the free encyclopedia

XJ

So while the waiting iis hard, the longer you can put off your first fires, the better it is for your oven/fireplace/chimney.....IMHO

[Acoma]

Re: Curing Concrete and Masonry

This does not mean we hurt ourselves by going the standard 7 days for curing? What then is your impression of the new and old curing standards?

[Xabia Jim]

Re: Curing Concrete and Masonry

Acoma, mine is a general statement and opinion....you can never wait too long but you can start too early on curing.

I cannot dispute the 7 day guideline but I'd take it as the minimum.

Conditions are very different in different parts of the country. Humidity, temperature.....

I'd defer the number of days to those who've done more with actual ovens and fireplaces than I have. I've done lots of masonry but only some woodstove floors/walls and my oven project.

[Xabia Jim]

Re: Curing Concrete and Masonry

Acoma, I don't know specifically what the actual "old and new" curing standards are but you know for sure I'd go for the longer period!

[edschmidt]

Re: Curing Concrete and Masonry

The curing strategy I have been most fastinated with lately is steam curing. AAC concrete is placed in an autoclave where steam/heat is added. After 4 hours they claim the same strength as a 28 day cure. If there was a way to heat the oven in the 250 range while maintaining the moisture I would think the results would be similar. Although you would still have to then slowly bring to temp to remove the moisture to avoid steam pockets.
eddie

[Acoma]

Re: Curing Concrete and Masonry

Very interesting info Eddy.

[Xabia Jim]

Re: Curing Concrete and Masonry

I've heard the strongest concrete is that poured under water.....

steaming....sounds like the Paella cure method

[John Fahle]

Re: Curing Concrete and Masonry

If there was a way to heat the oven in the 250 range while maintaining the moisture I would think the results would be similar.

The easy thing to forget, I guess, is that wood isn't the only way to heat a wfo. Stick a hot plate in there, and rig it to a thermostat sitting up high in the dome. You'd just have to make sure that the hotplate cord's electrical insulation could withstand 250F, but that's not very hot.

[james]

Re: Curing Concrete and Masonry

I've heard the strongest concrete is that poured under water.....

That reminds me of the Roman pozzolan concrete that sets under water. Smart guys those Romans.

Here's a quote from a web site on the construction of the Pantheon.

"These [foundation] rings are made of pozzolan concrete consisting of travertine pieces in layers held together by a mortar of lime and pozzolan. This will be discussed later in this work. Interestingly enough, the Jutland Society's investigation showed the foundation material had become "rock hard,"11 a case we might expect when we study the chemistry of pozzolanic reaction under these conditions."

And this from Wikipedia.

"The Roman port at Cosa was built of Pozzolana that was poured underwater, apparently using a long tube to carefully lay it up without allowing sea water to mix with it. The three piers are still visible today, with the underwater portions in generally excellent condition after 2100 years."

Cool.
James

[gjbingham]

Re: Curing Concrete and Masonry

Edschmidt,
Now that sounds like a revolution in oven curing. Sounds like it would be easy too. A tarp outside to hold in the water vapor, A 5 gallon pail of water inside the dome with a 250ish degree heat source - say for a day or two. Longer than the 4 hours you imply - what the heck. You lugged 35 lbs of water across the yard and hoisted it into the oven. Might as well let it work a while.
I think we need input from a structural or chemical engineer.

[Acoma]

Re: Curing Concrete and Masonry

I got the tarp and 5 gallong container Without confirmation, I will proceed with my curing method (Tuesday).

[Frances]

Re: Curing Concrete and Masonry

OK, I've got a question for this thread, too:

I put that layer of cement/lime - based render/mortar stuff on my oven on Saturday, got it well covered to stop drying out and was going to refrain from firing the oven for one or two weeks...

But now we've got an unexpected cold front approaching, should hit tomorrow night and go down to -2 C (about 28 F). So should I maybe light a small fire in my oven today in the hope that the heat will work through all the layers and keep the outside above freezing tomorrow night?

Or has the mortar had enough time to harden so a mildish frost won't hurt?

What's worse for curing in this situation, cold from the outside of heat from within?

Any advice will be very welcome!

[Acoma]

Re: Curing Concrete and Masonry

What is well covered. In my opinion, if you have warm tarps and blankets on it, you will be above freezing and fine.

[Xabia Jim]

Re: Curing Concrete and Masonry

Frances, you would probably only need to put a light bulb on inside your oven to maintain enough warmth to keep it above 32, particularly with the outside covered.

don't lose any sleep.....

[Frances]

Re: Curing Concrete and Masonry

A lightbulb on the inside will be enough to keep it above freezing on the outside through all the insulation and whatnot?? I wouldn't have thought the heat would penetrate that far. Whew, those little chaps are more effective than you'd ever guess!

Ah well, I'll put a light in and cover the oven with our picknick blanket (only a tarp so far, Acoma - not very warming, its more to keep miosture in right now). After all, its not going to be all that cold, and there's not much I can do at this stage anyway. It'll be ok. I guess.

Thank you guys!