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Modthyrth 11-10-2008 03:51 PM

Few Cracks in my Hearth
Hi guys,

After chasing around too many concrete companies who flaked out on us and delayed the project by nearly three months, we rented a mixer and poured our own hearth slab yesterday.

It's not pretty, but I'm fine with that as long as it works! However, a few cracks did develop last night. They're all very narrow, but a little more substantial than crazing. Here's a picture:

You'll also notice some discoloration and pockmarks from the rain we got yesterday afternoon (rain!! In Phoenix!! At this time of year!!). The slab is rebar-reinforced. Freeze/thaw cycles really aren't an issue here; we've only had one reasonable freeze in the eight years we've been here.

Anyone have any opinions about whether or not I can move forward? My forum searches turn up plenty of information about cracks, but not this type in the hearth. My google searches have been entirely unhelpful in answering whether or not this affects the necessary strength of the hearth slab. I throw myself upon the wisdom and mercy of the boards.

RTflorida 11-10-2008 04:28 PM

Re: Few Cracks in my Hearth
My first thought - are you keeping this covered (with a tarp or plastic NOT touching the surface) and damp (should give it a light spray daily for the first week)? Besides the freakish brief shower you had, it is usually very dry and very, very low humidity, yes?

Concrete needs to cure slowly with the moisture gradually evaporating away.....left uncovered, and not kept damp in low humidity would lead to premature drying, potential cracking and an overal reduction in its strength. I'm not a concrete expert so I may be falsely sounding an alarm...this is what I have been previously told and have always followed when pouring concrete.


Modthyrth 11-10-2008 04:35 PM

Re: Few Cracks in my Hearth
It's been uncovered the whole time, and we watered it once an hour for the first day, and periodically today. We topped out at about 70 degrees today, so it's not too hot, thankfully. But it's always pretty dry. Humidity was at 23% today.

Based on my research, it looks like the discoloration is a result of my husband overworking the surface while it was still a little too wet. It's not pretty, but the reading I've done suggests that it shouldn't affect the overall strength.

Wiley 11-10-2008 05:51 PM

Re: Few Cracks in my Hearth
Don't worry about any surface discolor if it were to be left to the elements it would all look very close to the same in 6 months or so. Since it will be covered with lots of stuff any discolor is a insignificant problem.

Do you have any pictures of your set rebar before the pour? If you put a reasonable amount of rebar in the form you should be OK. The cracks look like shrink cracks from having too much water in the mix. A common error, if one wants a loose more fluid mix one has to increase the cement to aggregate ratio otherwise one gets shrink cracks and weaker concrete. One can have "sloppy" concrete but in order for it to have strength when it cures it needs more cement than if it was poured with less water. And while we're on the topic of water: concrete only needs the necessary amount of water for the chemical reaction to take place when it's mixed, more water than that means a more porous final product that has less strength. Keeping poured concrete wet is simply a way to make sure the necessary water is present. The same result can be achieved by sealing the surface with a spray coating designed for that purpose or a plastic sheet.

Alot of people think because the final WFO weighs a couple-three thousand pounds it's heavy and it is if it's on your foot I'd agree but the PSI on these are low and the load is carried nearer the edges not point loaded in the center on the slab about which we are speaking. So if you didn't skimp on a few bucks worth of steel you are probably good to go. Don't worry, I suspect you are OK; not great, not the best you could have been but, it's just a WFO and you're not sleeping under it. It's a 6 instead of a 9 and a 4 will still get the job done. It just may not be the place you want to seek shelter in the event of an earthquake or hurricane.


Modthyrth 11-10-2008 06:13 PM

Re: Few Cracks in my Hearth
Thanks for the insight. Here's a picture of the #4 rebar that was embedded:

I tried to tell the guys that a drier mix was better, but it got progressively wetter as we went along. It was never too slushy and sloppy, but probably more than it ideally should have been. This picture shows the max. hydration level fairly well:

Les 11-10-2008 06:14 PM

Re: Few Cracks in my Hearth

I agree with the others - I don't think this is a huge problem. I had a few surface cracks around 1/16 in wide, and I was worried as well. They never grew and I saw absolutely none under hearth. When concrete chunks start dropping down - that would be a problem.


glennb 11-10-2008 07:00 PM

Re: Few Cracks in my Hearth
Cracks on top of your slab will be shinkage related, rather than structural, so no need to halt production. As you build the oven, and add weight to the top of the hearth slab, the top of the slab goes into compression and should actually close the cracks up.

Have a look under your slab to see if there is any cracking in the bottom as this would be more of an issue structurally.


Wiley 11-10-2008 07:34 PM

Re: Few Cracks in my Hearth
Thanks for the photos, IMHO you are good to go. And I'd upgrade that 6 to an 8

Modthyrth 11-11-2008 08:58 AM

Re: Few Cracks in my Hearth
Thanks, guys. I feel much better now. Onward to the dome!

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