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SteveP 11-26-2008 05:59 PM

Possible hearth slab issues
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I poured my hearth slab today and ended up with a real rough texture. I used Sakrete brand baged concrete and once I was done, the surface texture felt like sandpaper. My previous concrete jobs (very small) left me with a creamy smooth surface after it was finished, but I could not get the same results with this slab. I had bleed water on the surface like I expected but when this water was reabsored into the concrete, I could not achieve a smooth texture. It's as if the sand in the mix is all at the top. I'm not worried about the texture but am concerned with the strength. Has anyone experienced this? I am thinking about using a patching compound to smooth it out. Any thoughts?

dmun 11-26-2008 07:47 PM

Re: Possible hearth slab issues
It shouldn't be a problem. It may be a little "over floated" which brought the small particles to the surface. I'm not the one to talk: I've never floated a smooth surface on a slab of any size.

Ken524 11-26-2008 08:00 PM

Re: Possible hearth slab issues

I wouldn't worry about it. You don't need a creamy smooth top. Most, if not all of the surface will be covered up (depending on your finish).

To assure your slab cures as strong as possible, make sure you keep it moist and covered with plastic. Give it a week like that then start building!

Your cantilever looks great. Keep us up to date with your progress!

Jed 11-26-2008 08:06 PM

Re: Possible hearth slab issues

That is the same product I have used, and mixed per factory instructions, and cured for a few days, the strength of my projects was fine (it worked..).

There are different sizes of sand that can be used in the concrete mix that will not generally effect strength. The volume of 'fine's' in the sand in a concrete mix can effect the finish.


nissanneill 11-27-2008 01:10 AM

Re: Possible hearth slab issues
I am not familiar with your premixed cement products over there, we also have them here but I often have my doubts as to the proportions of the ingredients used and often the quantity of portland for that quantity seems too little in my mind.
I like to get my own sand, gravel (metal) and portland, mix the portland with water in the mixer, add the gravel and then finally the sand.
I always get maximum strength concrete using the measured proportion as recommended on the bags of cement.
As the portland is put in the water, it is liquid and thoroughly coats the gravel which is the bulking agent in concrete thoroughly and then the sand is added which fills in the gaps. You might need to add a little more water to attain the optimum 'slump' (viscosity) for pouring.
The surface being a little rougher, will only improve the bond of your vermicrete layer later.


christo 11-27-2008 04:41 AM

Re: Possible hearth slab issues
My slabs were not so pretty, either. To hide my issues I used a broom finish as the final pass.

I have mixed a lot of the premix concrete stuff so far. First for the slab, hearth, and side foundations for the kitchen . Then the seating walls and columns. Wiley gave me some advise to add some portland to the mix and it would flow better into the columns. I added 2 to 3 shovel fulls to each batch (2 ea 80 lb bags) - I could really tell a difference in how creamy it was - especially when finishing off the exposed concrete at the top of the forms.

I believe I will continue this practice where cosmetics matter. Sounds like a little too much portland is not a terribly bad thing on the size of pours that we do to make a pizza oven stand.

I found this decription interesting when I read it a while back:

Concrete Basics | Portland Cement Association (PCA)


SteveP 11-27-2008 04:44 AM

Re: Possible hearth slab issues
The concrete for my foundataion was from a ready mix company and I did notice that the sand in the ready mix was finer than the sand in the bagged concrete and was easier to work. I did cover the hearth slab with a blanket and wet it down then covered it with a tarp to retain the moisture. I am planning on keeping it in this state for another week and a half.

christo 11-27-2008 04:48 AM

Re: Possible hearth slab issues
I don't think you'll have any strength issues at all (the rebar job looks very nice). I like the curved overhang on the hearth!! It's going to look really nice.


DrakeRemoray 11-27-2008 06:27 AM

Re: Possible hearth slab issues
You are putting insulation down on top of it anyway, doesn't matter what it looks like really. You ARE putting insulation down next right?


SteveP 11-27-2008 07:41 AM

Re: Possible hearth slab issues
I will be putting down some insulation. I am planning on using 2" board but haven't decided on what to use. I checked into SuperIsol boards but Armil hasn't gotten back with me yet. I'd like to use the FB board but the shipping is more than double the cost of the board last time I checked. I was really concerned with the strength of the slab since it has to bear a lot of weight, but everyone is reassuring me that it should be fine so I won't worry any longer about it.

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