Hi Everyone,
I’m kinda new to this but hoping for some sage advice.
I am having a 22×28′ cabin built by Mennonites in my valley.
The floating slab has been poured on 2″ of appropriate rigid foam insulation that sits on compacted gravel. On the advice of the builder I didn’t use reinforcing mesh, though there’s rebar in the footings.
Hopefully all that is okay, so my question is, should I ask for compression joints to be saw cut into the slab? I’m really getting various opinions regarding this. Out of curiousity, is the depth of the cut as deep as the slab, or just a score for it to follow?
By the way, I live in Canada, we have lots and lots of frost.
Thank you in advance for any advice
Replies
The joints should be approximately 10' apart, so two one way and one the other would leave you with 11' x 9'4" which will work.
Was there fibre in the concrete mix?
IanDG
I don't know if joints are a good or bad idea for your situation.
If done, sawn control joints should be at least (i.e. about) one quarter of the slab thickness. Generally they are sawn the day after the concrete is placed because it's soft and cuts well, and not all the shrinkage has occurred by that point.
If you cut, plan the cuts so they don't fall in the middle of a floor, especially a tile floor. Try for near the walls.
Do you need expansion joints on a floating slab as you've described? The answer is definately, it depends.
You will probably be happiest if you do. Depending on your wall layout, you may be able to hide most of them under walls. a 10' clearspan, like Ian mentioned, is a rule of thumb, but 14' is not too unreasonable if it allows you to hide the joints under walls (point loads=stress points.)
The MOST IMPORTANT thing you can do for your slab is to cover it with cotton or burlap material and KEEP IT WET for 7 days, 10 if the weather stays below 70*F much. Covering the wet material with visqueen sealed at the edges is better still, but KEEP IT WET.
The joints need to extend 1/3 of the depth of the slab.
SamT
Thank you Sam and the others who replied as well.
Building is such a learning experience isn't it? I knew concrete was to be wet and/or covered to slow down the drying time, but unfortunately the pour occured when I was away working in the city. The valley is very remote so theres no water to be had anyway.
But it sounds like the concencus is joints 10' or so apart, to a depth of 1/3 the slab. Perfect, thanks again.
Valleygirl, as an Ontarian, I can say that I've never seen control joints cut in a basement slab. Subdivision builders don't do it. I haven't seen a custom home builder do it.
I'm not saying you can't or shouldn't do it. I'm just saying it isn't something I'd worry about...but then a few random minor cracks in a basement floor is also pretty normal round these parts.
Regards,
Tim Ruttan
DanT hit all the points right on the head. The most important thing is to cure the concrete properly, keep it WET. This way the concrete is allowed to set at a slower rate and shrinkage cracks are reduced.
Control joints are normally placed at two to three times the slab thickness in feet for an unreinforced slab. In other words for a 4 inch slab the joints should be between 8 to 12 feet apart. Space them in this range to fit the overal slab plan dimensions. For a reinforced slab with wire mesh, you can put the joints further apart, I'd recommend no more than four times the slab thickness.
Normal joint depth is between one fourth to one third the overall thickness.
Cure it well, joint it right and you should have a good looking slab.
I am not much of a fan of wire mesh myself. I do believe in rebar though. Well cured slabs that have rebar grids in them are nearly indestructible. The reason that everyone is stressing the importance of keeping the slab wet is that concrete gets its strength by formation of crystalline structures. When the slab cures very slowly it forms large (long chain) crystals which give it great strength. When it cures more quickly (as it will if it begins to dry) it forms shorter crystals that have much less strength. Well it is too late now but I would have reccommended a rebar grid on about 16" centers. Cure it well and you should get good service anyway though ... I have a tendency toward overkill.