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cracking concrete in 1945 fireplace

cracking concrete in 1945 fireplace

(OP)
See attached image. It shows a fireplace of river rock in a 1945 house with a concrete hearth of serious (6"?) thickness that has separated away from the cracked-up fireplace floor. I will be resupporting the hearth framing from underneath but am unsure what to do with the fireplace floor and the very large crack between the two. The chimney has been inspected and is in working order. The owners would like to have wood fires in it again. As best I could tell, the fireplace floor is approximately 2" thick with river rock stone below it. (That's a cleanout into the river rock wall below in the rear left corner of the fireplace floor.)

Any thoughts?

Please remember: we're not all guys!

RE: cracking concrete in 1945 fireplace

A Franklin stove (any kind of enclosed stove actually) will be much more energy efficient at burning wood, and at keeping excess draughts of outside cold air from going up the chimney uselessly, and at radiating warmth back into the wood. But, the two surfaces - the fireplace and the concrete stage in front of the fireplace have separated, and are never really going to act together again. For a fire, they don't really have to - as long as the burning (continuously hot) ashes and char are not going to get trapped in the crack and heat up the wood below the concrete. Just trapping cold (but dirty) ashes and soot or burned wood won't cause a problem other than dirt and char. Trapping flamable debris could built up a fire hazard over time. Both are another good reason to use a stove, not an open fire. Or place a good permanent screen between fire (wood) and the crack.

So you have to either close up the gap with something modestly flexible and 110% fireproof, or tie the two together with a fireproof link. Check the fireplace stores near you - they may have a repair mortar. Matching color will be hard. Caulk? Probably not flame-resistant enough.

Have you verified the slab is not going to move more? The slab is to catch sparks popping out of the fireplace onto the rug or wood floor. A thick but decoative metal tray could span the difference in height and do the same thing. Say, 1/4 inch thick, with two adjustable legs on the far end to hold up edges of the tray over the uneven surface. . Then add a decorative lip (2-3 inches high perhaps) to the tray to prevent spills of the trapped sparks..

RE: cracking concrete in 1945 fireplace

(OP)
They looked at a wood-burning stove but it stuck out too far into the room. Once I'm done with that slab, it better not move any more :D They didn't realize it but had used the hearth framing to also support the garage door track and to hang a 20' ladder. Both of those will be removed and the framing strengthened.

I'm considering removing all the cracked up flooring in the fireplace and then repouring, but yes, that crack moving more would be the main concern. I like that pan/lining idea a lot. Thanks!

Please remember: we're not all guys!

RE: cracking concrete in 1945 fireplace

More questions than answers.....

The left side of the firebox floor looks like it has heaved. The right side of the hearth is relatively flush, but has moved away from the chimney (apparently) more than the left side. How long did it take for this to happen?

Is the chimney on or near a slope?

Is the hearth on or near an opposite slope?

Is there any separation between the hearth and the wood structure?

Is there any settlement noted in other portions of the structure?

Is the structure supported on piers?

As for the floor of the firebox, it looks like it has been patched a few times, apparently with conventional portland cement concrete. I would concur that this needs to be removed and re-placed, but this time with refractory cement concrete.

RE: cracking concrete in 1945 fireplace

(OP)

Quote (Ron)

The left side of the firebox floor looks like it has heaved. The right side of the hearth is relatively flush, but has moved away from the chimney (apparently) more than the left side. How long did it take for this to happen?

Looks like heaved, but I'm not sure it actually did, or at least not much. It's a play on the eyes from the crack being so crazy wide. Here's another pic:



Quote (Ron)

Is the chimney on or near a slope?

Nope. There is a basement below the floor system but the chimney itself is part of the foundation. It seems rock solid (pun not intentional but quite appropriate).

Quote (Ron)

Is the hearth on or near an opposite slope?

Hearth is framed around by double wood joists with wood below, and concrete poured above. The garage door was supported on this. The framing in this area has sloped down and away towards the middle of the house, creating the crack. see:



Not the best pic, but look hard and you'll see the left double joist (with a ledger stuck on it so it looks like 3), the flat horiz boards under the hearth concrete, tracking for the garage door, the header between the double joists at the end of the conc hearth, the shortened joists, and the yellow hooks for the ladder. You can also just see the top of the cleanout in the river stone wall at the bottom left-middle of the pic.

Quote (Ron)

Is there any separation between the hearth and the wood structure?

None at all, that I can see.

Quote (Ron)

Is there any settlement noted in other portions of the structure?

There's a lot on the porch, which is the other side of this wall, but the porch is clearly falling away from the rest of the house and the chimney, which are strong.

Quote (Ron)

Is the structure supported on piers?

Nope. Whole house is river stone foundation.

Please remember: we're not all guys!

RE: cracking concrete in 1945 fireplace

SLTA...one more question....any chance this is related to creep in the wood supporting the hearth?

RE: cracking concrete in 1945 fireplace

(OP)
I'm not sure it's creep, but there's definitely settlement involved from the extra loads that were added.

Please remember: we're not all guys!

RE: cracking concrete in 1945 fireplace

The floor of the fireplace is the area that gets heated less than the back & walls above, refractory may not be necessary but if you're removing the floor, laying firebrick with refractory cement is good practice. The yellow bricks will soon blend with the stone if they actually use it for fires. You said there is a river rock foundation below so you should have support for a brick floor; the cracks are likely the result of differential thermal movement and movement of wood framing vs masonry. You have a solid masonry mass with a concrete shelf sticking out of it, the shelf can't cantilever because it doesn't form the fireplace floor and it is is vulnerable to all sorts of changes in temperature, humidity, loading of the floor system, settlement of other elements, etc. Remove the floor, drill some reinforcing into the back of the hearth & lay it into the mortar bed for the brick, & you should be okay.

RE: cracking concrete in 1945 fireplace

could you place triangle bracing to the stone wall foundation and jack the hearth back up to support it?

RE: cracking concrete in 1945 fireplace

If you haven't ruled it out, I'd suggest you check the condition of the joists at their bearing. In the first photograph, on the right, it looks like there is a gap between the baseboard the flooring. It may be that there is some rot at the joists where they sit on the stone and a subsequent drop in the joist elevation along this wall.

RE: cracking concrete in 1945 fireplace

First you need to check for any plumbing leakage around if non then Try to clean the cracks to get all loose material out then try injecting liquid mortar between the bulk Until you feel that the void is filled. Finally use thermal bricks on top of baseboard to protect the old base. Good luck.

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