How much concrete is needed to fill a core?
(M) I plan to fill every other core made of standard nominal 8"x8"x16" block.
(M) I have 4 courses and that brings me to Approx. 30-1/2"
(M) If I use 60 Lb. bag(s) of concrete, how many bag(s) are needed per core? ____
(M) If I use 90 Lb. bag(s) of concrete, how many bag(s) are needed per core? ____
(M) I plan to use 3/8th re-bar in each filled core. Is that 3/8th large enough? ___
They sell concrete in 60 and 80 pound bags around here. Portland cement in 94 pound bags. Make sure you use the right stuff.
Cement and concrete are not the same thing. Concrete is a mix of cement, sand and aggregate.
Cement is the silky raw stuff you can use in varying mixes of sand, perlite, rock, etc. to make concrete, mortar, stucco, the insulative concrete, and more...
Anyway, 3/8th should be fine. I can't tell you how much to use because I'm a paranoid dork in earthquake country, so I rebarred everything, filled every core and tried to vibrate it down. I also did a 20 foot long planter bed at the same time, so... ?
I filled the four corner cores, and one in the middle of the two side walls. Each core took (1) 60# bag of premixed concrete.
On the earthquake front, someone noted while we were building our latest stand that if an earthquake large enough to knock over my oven came along, we would all have bigger problems than a cracked pizza oven. :-) That said, you can pick your own level of security.
One 60 Lb. of concrete per core, and slab canteliver.
Dear Tarix, and James,
(M) Thank you both for your speedy replies. Per James reply, " Each core took (1) 60# bag of premixed concrete." I think that seems like a good plan.
(M) Robert, and Hope both employed an extension of the hearth slab. I like that look. Since I will pour my hearth slab on top of the 4th and last course, I planned to first surround the hearth stand with a doubled 2" x 6" frame for the hearth pour. That should give me an extra 1-1/2" all around overlap-cantilever.
(M) But for the front where the neck of the igloo will be located, I would like about a 6" - 8" extension, either softly curved like Robert's, or straight across like Hope's. My plan is to notch the supporting plywood for the 1/2 blocks (that's all I plan on) on either side of the wood storage entry. The plywood will then extand perhaps 10" beyond the Hearth stand in the front. Am I missing some critical consideration? ___
Actually, our entire neighborhood is looking forward to our oven and I've a friend with the nickname of madbaker who's eager to come to the Oven Warming party! He's planning on bringing his sourdough, so I can just leave the bread baking to him that weekend and concentrate on the pizza and roasting.
marcel, i'm not exactly sure i understand how the double 2 X 6 gives you an overhang. perhaps a drawing would help. in any event, below are some pictures which show how i did it. to get the plywood to completely wrap around the perimeter of the stand i had to, as you say, "notch" two pieces of plywood. i used shims under the 2 X 4's holding up the outer plywood to get everything level. rebar extended from the center to the edge of the overhang and i also ran rebar along the front of the overhang (see coke picture). more rebar went into the overhang when i poured the top layer. btw, forget the ash hole. its a waste of effort.
i ran into one problem which was that the vermiculite / cement mixture doesn't give a smooth surface upon curing; in fact, its downright ugly with gaps and crumbly parts. because of this i had to re-pour the front edge with repair mortar. in retrospect, i should have done a pour of plain concrete around the perimeter for a nice smooth effect and then poured the vermiculite. (or better yet, place an additional pieces of board around the perimeter, pour the center and then pull these boards out after the initial pour and then pour concerete into the channel left by the removal of the boards.) if you do this i would suggest letting the rebar extend thru into the perimeter area. everything should be poured on the same day so it all cures together.
keep in mind that i poured my hearth using "old" technology which was lower layer was vermiculite and upper layer was concrete. i think the new and better theory is concrete lower and vermiculite top with an island of firebrick.
make sure you remember to oil your forms. i didn't ...
i used wire supports for my 2 x 6 frame, which worked very well. simply screw the frame together very well on the corners. then drill 2 small holes in the middle of each 2 x 6 section. run wire through one hole, across to the opossite, side. loop it around through the adjacent hole, and back to the first side. twist the ends around a bolt or piece oc rebar, whatever. give it a twist or two to tighten, and you're done. when the forms come off, just snip the ends of the wire off.
also, i made my frame sit exactly outside of the hearth, using 2x scraps to support it from underneath. this left my hearth perfectly flush with the block stand, allowing for the veneering that will come eventually to sit on a continuous face. however, i don't think a 1/4- 1/2" differnece would affect anything, if the 2 x 6 form is just barely resting on the block stand, especially if you left the exact thickness if concrete backerboard as youi mentioned.
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