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Concrete Base For Fireplace Hearth

bisquitjoint | Posted in General Discussion on October 24, 2007 10:30am

In a project to remodel our living room I built a new fireplace surround and mantle.  As the tile on the floor level hearth was cracked and would not match our new rug and walls I removed it and discovered the concrete base beneath was in poor shape…..cracked and quite crumbly.  I chipped it out, a very easy task, and now have a hole, 6′ by 18 inches.  This hole slants up from the sill (on the fireplace wall, about 8 inches deep) to the front of the hearth (about 4 inches deep.)  My question is this: What type of concrete should I use and should it be poured in one layer.  Also how long should I allow it to dry before tiling over it?

Thanks for any help or suggestions.

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  1. User avater
    ToolFreakBlue | Oct 24, 2007 11:07pm | #1

    http://www.maximizer.cc/index.htm
    not sure if it is available in your market.

    This stuff covers 1 cubic foot per 80 lb bag.

    also has a design strength of 5500 psi vs 3000 psi of regular sacked concrete.

    (you need 4.5 cf based on your dimensions)

    I would get 5 bags and start mixing one bag at a time and place a load, mix another, place it etc. don't go to lunch between bags 3 and 4 but keep mixing and placing. You won't have a cold joint if you keep working and don't take a break.

    Short of reading the bags of crete and your thinset I'd give it a few days at least to cure.

    TFB (Bill)
  2. Piffin | Oct 24, 2007 11:17pm | #2

    Any of the premixed bad products like Sakrete will work, but I like to add a shovelful of Portland to each, esp for something like this. Get more than enough to do it in one pour. If you run out and have to go back to the store, you get a cold joint and that is not good

    Are you replacing the hearth too or just the apron in front?

    Crete does not dry. it cures through a chemical reaction between the portland and the water. You can start the tile work the next morning - just don't try splitting firewood on it for a month -

    ;)

    Do not add too much water! this is a prime consideration here and the most common mistake of a novice. Excessive water will make it weaker and will encourage shrinkage cracks - just what you are trying to get away from. The mix should be stiff enough that you need to push into place, or even pack it some. If it flows, it is too wet for this job.

    If it is like cake batter, it is too wet. You want it like the icing on the cake.

    At the edges, you need a slip sheet to isolate the crete from the wood framing. Slips of tarpaper stapled in is a good way to do it. The idea is that when the wood moves from settlement or from normal expansion and contraction, the crete is not locked to it. That can encourage future cracks also.

     

     

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    1. bisquitjoint | Oct 24, 2007 11:57pm | #3

      Thanks to both of you for your advice.  I appreciate your taking the time to help.  I'll start mixing as soon as the rain stops.

      Ted

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