Fixing a crumbling basement slab

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Old 05-06-14, 08:09 AM
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Fixing a crumbling basement slab

Hi,

I have until the end of the month to finish my basement and I have some questions about fixing the slab. I originally posted in the basement forum, but those questions have basically been resolved and I have new ones better suited to this forum, so I hope the mods don't mind me posting this here.

The house is from 1930 and has a slab that I believe was poured some time after that. They did a terrible job on it and I believe it is as thin as 1" in the area being finished. It had many holes and patches, and I've already removed the unsound patches, dug down 6" or so, and filled with hydraulic concrete. However, the slab is still in appalling condition - it is dusting and spalling, and some parts appear to have dirt mixed in with the concrete, and the whole thing has exposed aggregate. It's essentially impossible to clean.

My plan has been to level it with SilPro's SilFlo 230 self leveling compound, since it is the only SLC I could find that is nailable and I need to nail down sill plates. However, I am concerned about the bond between the SilFlo and the slab and the potential for delamination.

I've put a coat of SilPro C-21 Acrylic on it per the manufacturer's recommendations for a dusting floor. They also recommended putting down metal lath. Is that a good idea, and if so, what fasteners should I use? Does anyone have any other suggestions? The limited budget, urgency of the move-in date, and concern about damaging the concrete block walls during excavation basically precludes pouring a new slab.

Thanks
 
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Old 05-06-14, 01:05 PM
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Not a pro on this, but I will mention that a self leveling compound will adjust to being level, not necessarily to the floor. If the floor is basically level that will work, but if there is any slope you will fight the self leveling compound as it flows to the lower areas.

I'll let the pros answer the stated question.

Bud
 
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Old 05-06-14, 03:04 PM
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"Do not apply over dusting or unsound concrete" That is from the 230 spec sheets... It recommends lath over wood substrates.

You continually feel concerned about the fasteners for the wall. You can drill and tapcon any material, it does not have to have "nail holding power". The fastners are there for lateral strength to keep the wall in place so it doesn't kick out when you lean on it. I doubt you could put enough force on the wall to need solid protection against all sheer strength. Levelquik bent 50% of the nails I tried to run through some backerboard after leveling a bathroom, so strength is not the issue (that was at 3/4" thick as I recall).

You are over thinking your project. Go with your gameplan. SLC over properly primed concrete sticks pretty well. It will take more than you think, so don't get frustrated when the first bag barely covers anything. It says 24sf at 1/4" (12sf at 1/2" - 6 sf at 1") per bag. And, if the concrete is porous, your primer will absorb and you will go through a ton of that as well.

I laughed as the data sheet says no trowel necessary, yet the brochure picture shows 2 construction workers with floats applying the mixture. http://www.silpro.com/2010_datasheets/Silflo_230g.pdf
 
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Old 05-06-14, 08:26 PM
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I was calculating the cost given the area I have to fill... ouch.

Could I partially fill with another substance (something as cheapish as concrete), and finish with SLC?
 
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Old 05-06-14, 08:39 PM
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Also, are the typical concerns about concrete delamination and shrinkage really applicable for an indoor application that will be covered by a subfloor?
 
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Old 05-07-14, 05:08 PM
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.....Overthinking

Prime your concrete and do a first pour.

You will have more problems with adhesion if you try to add some form of fill. The SLC will stick better to a properly primed concrete base than other forms of concrete.
 
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