Building a Fireplace, Need Advice

Out with the old....took down the old fireplace and slab, and started filling that hole in.

 

Not a very exciting picture today, but it represents a lot of work. We made the wooden form and put it in, then filled it with 3 inches of marble rock chips and another 4 inches of concrete and re-mesh. It came to 1150 lbs of rock and around 1000 lbs of concrete.
There is plastic over it now to slow the curing process so that it won't be brittle when its all set up.
The chairs and other items are to keep the dogs from walking across it.
There is also a leftover piece of the re-mesh which looks like a piece of fence there on the right. It is basically really thin re-bar which is put into the middle of the concrete slab to make it more stable and to resist it breaking apart.

 

So somewhat on topic- I'm getting ready to lay a 7'x7' cement slab- Any recommendations for technique for forming it and getting it to set dead level?

PS- I'm really looking forward to see what the final product looks like here....

 

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A 7'x7' would be very easy because it is small. Just set your forms level and screed off them .. then float it out.

I have built 3 house from scratch ... can build almost anything except laying block or brick.

What loads will you put on the concrete ?
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SOSHeloPilot is correct (although I wouldn't call a 7' x 7' slab small by my standards).
Use a square tool or even a square cut piece of beam (which is what I used) in the inside corners of the form when you make it to make sure each corner is 90*. Then measure diagonally both ways from corner to corner and make sure its the same distance. You can hopefully tweak it a bit if needed.
Then after you place it in the hole, do what you have to in order to get it level in all directions.
After the concrete is poured and still fresh, use an old piece of 2x4 across the top of the form in a sawing motion to get the concrete all level with the form, and then use a concrete float to smooth it all out. I have to say though, getting concrete 'smooth' is a bit of a stretch because it has so many small rocks in it that you can't really get it super smooth in the same way that cement would be. At least I couldn't, but it will be good enough for underneath the fireplace where no one will ever see it again.

 

A 7'x7' prefab shed so the pad may actually be slightly larger...inches maybe. With garden and yard tools and some back yard stuff. It's not going to be holding houses up or anything.

Yeah...I'm not worried about it being super smooth- As long as it's level and going to hold itself together. I hadn't thought about gravel or anything under it-but with the wet nature of where I live down here...Probably will do that.

Thanks!!

 

Bruines , put a plastic vapour barrier under your concrete , that will keep ground moisture from saturating your slab and coming up into your shed

 

Also a good idea. I'm less worried about the moisture coming up into the shed (it's humid all the time no matter what and the shed is weather tight but not airtight) as I am about the ground run off subsiding the soil unevenly.
My house is about 3 feet above sea level...Most of the lot is 0 at best...OOPS.

 

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Since you will have the square forms out in the open ... you can measure the diagonals and that will show you square. Like Aero said .. squaring the corners is important.

OZ-T's idea on the vapor barrier is excellent because of "moisture wicking".

FWIW ... one catch 22 on oversizing a concrete pad for an out building ... depending on out building's design ... oversized pads can catch water in a hard rain storm and winds can channel & wick the water under the out building's edges and into the out building's contents ... even with a properly drained pad ... however, it would be great if you can "flash" over the concrete pad's sides to prevent this.

AeroCooper ... I like what you are doing and may copy some of your ideas ...
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Hopefully you're on somewhat level ground, if not you'll have to level it out with tightly packed sand and paver base.
Create a mold using 2x6x8's and screw them together square. If you go to a home center (lowe's) they have quikrete calculators to tell you how many bags you will need. May want to rent a gas powered cement mixer to keep it going while you pour and form.
Also, may want to add some decorative tile in the cement to spice it up a bit.
I can't find the picture of my in-laws hot tub slab I did for them

 

7x7x8" thick is about 1 1/4 yards of concrete , look into a delivery vs bags , sometimes it's suprising how comparable it is in cost .

PS , the volume I stated is an example , always measure your site conditions accurately

 

Luckily- This outbuilding has a floor-Hence the need to pour the pad to level the whole thing. Right now it's set up but the floor is kinda...well..wavy. Oops.

The yard is pretty level and once I dig out the spot for the pad I can fix the little differences. I thought about one of the powered mixers..cause the calculator I used already said about 21 80# bags of concrete. I might stop by the concrete place around the corner and see what it would cost to get a truck to stop by for 20 mins and dump it over the fence...

 

That's a good idea, a lot less hastle

 

::thumbs up:: and now...MORE FIREPLACE!!!!!

 

^^ Unfortunately, I am going to let the slab set up for about 2 weeks, so the progress is at a stand still until then.

As for mixers, I rented an electric one for $30/24 hrs and it was money well spent. Supposedly it could take 3 80# bags, but we went with 2 at a time to prevent sloshing out. That being said, we mixed 12 bags, smoothed it, covered it and returned the mixer within 2.5 hours of starting the first batch, so I was pretty psyched at how quickly it worked, and with minimal effort.

And although I checked with several different online concrete calculators and measured very carefully, I still ended up with 3 extra bags somehow. But I'll use them eventually, and they are cheap, $3.56 per bag if I recall. And I was planning on one extra bag anyway...they said 14 bags, I bought 15 to be safe. Better to have it and not need it than need it and not have it.

 

80 lb bags of premix ?

 

80# bags of dry, ready to mix, just add water.

 

Interesting , most pre mix bags in my area are smaller .

 

Here they have 40, 60 or 80 in most stores.