Lightweight concreate or not?

Big Will

I talked to a concrete finisher today that has been doing pour overs using regular concrete. This is over a existing wood floor. I have never done it but always was under the impression that it was too heavy. He is saying that the concrete is not really that heavy and it works well. It sounds like it would be a bit more labor to level than the lightweight self leveling stuff. However the cost is dramatically lower. My understanding is the gypecreate is about 7 LB a square foot and regular is about 15 LB a square foot both at 1.5 inches thick. A friend that I work with is remodeling his kitchen and wants to try it. If my numbers are right a kitchen that is 30X18 would get 8100 lb of concrete. Spread out over the floor that is not that much pressure but all together that is a significant load on the foundation and framing. 

Paul Pollets

Lightweight vs. concrete

Lightweight gypsum weighs appx. 14lbs./SF at 11/2" depth. Lightweight concrete weighs more. Regular concrete weighs significantly more and is rarely used at pours under 3" because of its tendency to crack and shrink.  I wouldn't recommend any other product at an 11/2" overpour other than lightweight gypsum or concrete, or you can expect significant cracking and shrinkage. Ask to see the spec sheet from the manufacturer. You only want to do this once.

zacmobile

thin pour

I've done lots of thin regular concrete slabs; mostly 2" but sometimes 1-1/2". Cracking in regular concrete can be controlled by 25% deep cuts done no later than 24 hours after pour. Although I know of one place that used a super-plasticizer in the mix with very few control joints  and not a single crack in the entire place, and I mean nothing. They colored it jet black, which looks freaking amazing beside their fir trim.



PS: and I would recommend using a bond breaker like poly sheeting on the subfloor to accommodate slab movement & shrinkage.

Jamie Hall

Another

way to reduce if not completely eliminate cracking in thin pours (though I've never seen less that 2" done this way) is fiber reinforced -- glass or steel.  Stuff is amazing.

hot_rod

thin concrete

can be done with a pea gravel mix, Double the plastic fiber and add some admix products.



You want 1-1/2 times the aggregrate diameter over the tube. With 3/8 pea gravel that is possible in a 1-1/2 pour over 3/8 or 1/2 pex.



There are some lightweight products to mix with but you lose some psi strength.



The Romans invented concrete and had a lightweight mix for their domes that was blended with pumice.



Cracking depends a lot on how much the floor flexes, joist may need to be beefed up to tighten up the deflection number.



Most concrete cracks, some small shrinkage (road map) cracks may not be a problem, depending on what you are using for final coverings.



Shrinkage has a bit to do with how much water they add after the mix leaves the yard. Better to use a plasticizer to get the mix to flow, not additional water.



hr

Mark Eatherton

If you use concrete...

make CERTAIN that you use a slip sheet to break the potential bond between the concrete and the floor, or it WILL break into a bazillion pieces over time.



With gypcrete, you WANT it to bond to the substrate, in fact they use a glue to make certain it gets bonded.



The concrete guys don't know about slip sheets. Neither do some hydronic contractors... Can't tell you how many jigsaw puzzle looking floors I've run into over the years.



ME