Concrete experts… take pity on me, read the whole thing in all it’s detail, and share your wisdom.
I have a new house slab poured within the stemwalls. The concrete was struck off exactly to the height of the stemwalls and on Day 1 it was flush everywhere, basically perfect.
The mud: 6 sack, 3″ slump with a water reducer, fiber, poured over a mat of #3 at 9″ OCEW (don’t ask), 4-1/2″ thick, all over 2″ XPS. The mat is on 1″ dobies, radiant tubing tied to it. There is an XPS thermal break at all edges, ripped at a 45* angle so that the edge of the slab has a big bevel on it, from the bottom, to zero.
Let me know if I should post a drawing…..
Weather has been warm, somewhat breezy, about 3 days of on/off rain in the ten days since the pour. I have diligently kept the slab wet, covered at times with plastic (i.e. over last weekend when we were not working on it). Two days ago, I stopped keeping it wet, figuring we had a week’s worth of damp cure and it should be OK.
Today I notice that the edges of the slab have crept up, and are 1/32 to 1/16″ above the walls in some places. It’s alive, it’s BREATHING!!! ARGHHHH!!!
Will this continue? Do I need to go back out there and pour hundreds and thousands of additional gallons of water on this thing for days and days to prevent this? I can live with a little, but not a lot–it interferes with laying a flat floor over it (bamboo, some tile).
In my shop… same thing happened. I paid a lot less attention to wetting the slab there, and I have as much as 1/4″ of curl in one place, mostly about 1/8″. Good thing it’s my building.
Replies
Here are some explanations I saw:
http://www.eng-tips.com/viewthread.cfm?qid=157140&page=1
http://www.metzgermcguire.com/yournew.htm
http://www.buildingscienceconsulting.com/resources/foundations/sand_layer_under_slab.htm
CH, I'm impressed with the depth of your resource base & the speed of your recall for an answer to David.
I took the liberty of checking out your suggested sites and they have now become some of my favourites.
Thank-you.
Sincerely,
STAINLESS
curl concrete is hard to detect, I think most people are seening the bull float pulling concrete to the edge. sloppy workman ship. 1/32, 1/16, 1/8 is nothing with concrete. i would not even think about 1/4.If a customer called me complaining about a quarter inch, I would hang up..Haga su trabajo de fricken
David, hello.
Cloud's ref. sites are all excellent sources of info. Slab curling is probably happening to more concrete than most realize, only because they have not checked for it. Your situation makes it easy to spot & you have perhaps been "sensitized" by your previous experience with your own shop floor.
One point that wasn't made clear enough in Cloud's ref's (to me at least!), however, is that even fully cured concrete can curl if differential moisture levels exist between top and bottom surfaces, ie: this effect can appear in old concrete if the right (wrong actually! ) conditions exist.
In cured concrete, (an ambiguous term for me since concrete can continue to cure for many years if a source of water continues to allow cement hydration to proceed), once the moisture differential goes away, the curl can correct itself, ( if indeed it was moisture induced curling to begin with). I include this remark as an awareness that curling can also be induced by temperature differentials. Although not likely in your case, if the top surface is hotter than the bottom surface, slab edges can actually curl downwards. (or upwards if the reverse happens.)
The engineered design and published strength of a concrete mix is reached in 28 days of wet curing, however, the fastest strength gains are made in the first hrs, & days of curing. What isn't readily obvious is that the strength gains can continue & provide more than 100% of the reported compressive strength should you be able to wait additional time (beyond 28 days) before allowing concrete to dry out.
In your case you may be lucky enough to have the curl correct itself once top and bottom reach the same moisture content. (You did take that important step to allow wet curing to develop much of the design strength of the mix.) This moisture equalization may take quite some time, however!
In a concrete pour of my own I needed a floor slab flat, right out to the very edges to secure structural steel wall angles onto. To reduce curling effects I continued my vertical wall rebar up about 18" above the foundation walls and bent them over at 90 deg. so they would be roughly above centre in my floor slab. I had added to the rebar in the top 1' of the walls so that my bent over bars were on 18" centers rather than stay with the much wider rebar schedule spec'd in the lower portion of the wall. I then poured the floor slab on top of the foundation walls right out to the exterior edge of the wall where I formed the exterior with wood.
The bent over rebar both tied the floor to the wall & helped reduce curling effects at the slab edge. (Even so, some curling still happened when I finally let the slab dry).
Being the retentive type I am, I used a 4-1/2" angle grinder & a diamond abrasive disk to grind the curled up portion back down so my steel could sit flat. With light passes (similar to using an automotive body grinder) you can feather the curl down to just what you need. A mask & goggles are strongly suggested and you might be pleased at just how fast concrete can be abraded with just a cheap Asian diamond disk.
If you can afford to wait, I would suggest that as a course of action (inaction?). Tape a 2' square of poly to the floor and seal the edges with the tape. If the concrete becomes darker under the poly, the slab is still losing moisture to the air. Once the poly test comes up light after a week of being down, check the curl again. If it is still curled it likely will stay that way.
That's the time to bring out the bad boy with the diamonds.
If you bring him in too early, you may just grind off too much so that when the slab moisture becomes consistent the edges will end up too low.
To answer the final questions in your post,
Will it continue? Probably! (until top and bottom surfaces reach similar moisture content & temperature.)
Do you need to pour all the water? I think you can now answer that for yourself, now that you have read Cloud's ref. sources. (Hint: can you keep it wet forever?)
Oh, & by the way, my appreciation for the details in your post as I didn't need to ask you further questions to frame my answer. (not that I would think for a moment of expecting anything less of someone who notices 1/32" in concrete work!) That's VERY FINE HOMEBUILDING!
Regards,
STAINLESS
Today I notice that the edges of the slab have crept up, and are 1/32 to 1/16" above the walls in some places.
You need to add more roughage to your diet.