screeding ultra thin layer of cement.


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Old 01-18-12, 04:02 PM
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screeding ultra thin layer of cement.

Can anyone give me any tips on screeding cement?

I'm trying to level out a slab in anticipation of laying down a wooden floor. Because there's a slight incline to the floor I can't use SLC, so I purchased some Sakrete Top N' Bond which is the suggested product for layers of cement from 1/2" down to feathering thickness. I was going to use bonding agent, but the manufacturer specifically states that I'm NOT to use it as the product already has some mixed in and using more could have a detrimental effect.

Note that I've got some thin strips fixed to the floor to act as my guides/rests while screeding.

So today I tried working with it, mixed in the appropriate amount of water, then found that it was a royal pain in the butt to work with, especially at ultra thin thicknesses. Most of the layer is in the neighbourhood of 1/4" to 3/8" , and though it's still a pain at that thickness I can manage it. But once it gets thinner the problems get worse - it either sticks to the lumber I'm using for the screed and comes straight off the floor in clumps, or it tears away as I try and move the screeding tool backwards and forwards.

I've tried making the cement thicker and thinner, but I'm not having much luck - adding more water is something I thought would make it easier to work with, but no, it clags and tears more readily.

The only thing I could finally manage was to forget the screed, trowel it down to a level where it was a little thicker than desired, wait for it to dry for 30 - 45 minutes, THEN screed it. This way it scraped the excess off at a gritty kind of consistency, with little tearing as before.

The way it keeps coming up off the floor so easily also has me concerned about how well the new layer is going to bond to the old. I keep putting down a scratch layer, mixing cement/water into a souply mix then spreading it around and letting it dry a little before placing the main cement down - but the results certainly didn't leave me feeling comfortable. Maybe it just bonds better as it settles and dries.

Any thoughts?
 
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Old 01-18-12, 09:28 PM
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I am no concrete expert but I always thought of that product as more of a concrete cosmetic surface dressing than much else. Something that you trowel on and finish off with a wet sponge for instance. More like a parging material. Did you consider laying a small strip down at the wall end of the incline and then pouring out some SLC letting it build up a little in the low areas?
 
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Old 01-18-12, 11:58 PM
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While I've never used the product you're using, I suspect part of the problem might be using "lumber" as the screeding mechanism. Why not try a magnesium float or darby to knock the stuff down? I've floated some very sticky stuff over the years (including latex modified and silica fume concrete), and none of them ever wanted to stick to the mag floats (hand and bull) I used. Or, you might even try a resin float, as they were always good at shedding the mud.
 
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Old 01-19-12, 05:32 AM
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Okay, what the heck's a resin float? I'll have to go and look it up. Thanks for the suggestion.

I checked the section I managed yesterday, and it seems to have come up okay. I don't know whether to trust it though for the job I'd planned, which was a glue-down floor. The rough texture I deliberately left will really help the glues adhere to the cement, but I'm just not sure how well this new layer of cement will hold to the old floor. Any kind of trick to test it after letting it dry out? As it's a sunken living area, I have the option of putting down ply and Tek-screwing it into the slab once I've finished leveling the floor, and nailing the wood flooring to the ply, but my preferred option would be glue.

@Equinox. No, it's the right product - I even phoned the company's online support to double check.

As for the SLC, nope, won't work. The idea you suggested is exactly why I went for an alternative. Working from the 'low end' as you suggested would only end up creating a bubble level floor, which is what we don't need - it would end up making some associated steps look crooked. Besides, the floor has some real issues thanks to the 'pro' we originally got in to do the job and who screwed it up royally. Not only is the floor very uneven now, but he cupped the whole thing - low in the center of the room, high at the walls.
 
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Old 01-19-12, 02:40 PM
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Okay, I think I solved it. I have a 6' aluminum level that's about 1 1/2" wide, and I found that if I troweled most of the cement to approximate the level I needed, used my fingers to sprinkle the surface with a little water to lubricate it, then finishing off by using the level worked perfectly. The surface stayed a little dappled from the water, but I just waited half an hour to let the cement dry out a bit, then went back over with the level to smooth everything one last time. So the key with cement laid down so thin is to make sure to lubricate the surface once you're ready to finish things off with the screed. At least that's what worked for an amateur like me!
 
 

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