Casting Chimney Crown

smitty286

I have a brick chimney with a badly deteriorated mortar crown. I would like to replace it with a cast concrete crown, but I can't find any pre-made in the correct dimensions (it's an old-fashioned ornate chimney with a long, skinny rectangular cross-section). I've done a little concrete casting before, but only for aesthetic pieces (pavers and edging in a garden), so I have a couple questions before I attempt something like this which requires a little more rigidity:

1) What's the best "recipe" for concrete that will maintain a reasonable strength when cast only a few inches thick?

2) What's the best approach to reinforcing a piece like this? I had planned on making a 3-D "wire frame" to go inside the form, but I'm wondering if a single layer of wire mesh in the (vertical) center of the casting might be better because it's less likely to cause the outer faces of the piece to spall off.

3) I had planned on casting in four small metal tabs for mounting a cap on the chimney - bad idea?

I've tried to research this already, all the answers I can find seem geared more towards large structural concrete forms with heavy rebar and such, I'm not sure if that same information can be "scaled down" to a project like this. Any advice is most appreciated!

Ron6519

How big is this chimney and how easily is it accessed from the ground? Wouldn't it be easier to frame out the work up top and carry concrete up there in manageable loads?
Ron

yes.itsconcrete

frame it in place & pull up mixed conc in 5gal buckets as a pro would,,, your ground man can pull it for you using standard 10" pulley.

smitty286

Thanks Guys,

Sorry for dropping a question and disappearing, been grappling with some car problems these past few days. The chimney is about 13" by 28" and has a single 4" x 10" ceramic flue tile. It's actually somewhat difficult to access (for someone not used to walking around on steep roofs, anyway) - it's right at the peak of a 10/12 gable roof approximately 30 feet from the ground, with no access means other than straight up the ladder. I had considered a "poured in place" approach originally (a single casting would probably weigh a couple hundred pounds, even with some kind of rigging that's a hell of a thing to wrangle perched on a rooftop) but I had read that these were usually of inferior quality because it's difficult to control the environment up there and have the concrete set properly, and I sure as hell don't want to have to do this twice; if that's the way the pros do it, though, I think I'll follow the wisdom of their experience. Thanks again for taking the time to respond.

Termite

I'm no mason or chimney pro, but I cast my own in place. My old one was deteriorated to the point that there was no way to repair it, so I removed it in small pieces.

Mine is about 22' in the air, and was 24x48" approximately. I used drip edge to flash the outer perimeter, and for a clean look. I formed the cap's outer edge out of 2x4's, and screwed it to 2x4's that I had clamped to the outside of the chimney just beneath it. I used 5 gallon buckets to bring concrete up the ladder, and reinforced it with basic stucco mesh. I put a heck of a lot of slope on it and finished it very smooth so water would definately not stand on it. As I recall, I just used regular quickrete. It has survived a few Kansas winters and summers so far. It was a $30 project at the most. There's probably a better way, but this worked out pretty well for me.

It was definately a two person job. You'll need another person up there on another ladder just to help you set the forms if nothing else. At 30' up in the air, I'd want someone mixing concrete and tying it to a rope so I could lift it up as well.