Is it okay to just let my slab crack as it may?
I’m planning a 4″ floating slab on grade between ICF walls. I’m coloring and sealing the concrete and using it as a finished floor. I’ll have some rebar in it because I need something to tie my hydronic heat tubes to (I want them suspended in the middle of the slab, not stapled to the foam at the bottom).
I don’t want to cut control joints. If and when the slab cracks, I’d rather it just crack in random patterns. To me, control joints make the floor look like an unfinished floor, while a natural pattern crack could be filled with a colored epoxy resin, and actually look pretty. Is there any reason to cut control joints besides that it (hopefully) makes your cracks happen in neat pre-determined lines? I read a comment from someone on another thread here that cracks in concrete can somehow cut or fracture the PEX tubing, but that sounds…farfetched. I don’t think I could cut PEX with jagged concrete no matter how hard I tried.
So please tell me if there are any reasons besides aesthetics to cut control joints, and also tell me if I’m the only person on earth who thinks natural cracks are prettier.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Zoe,
Maybe it's because I'm a builder, but to me a crack looks like an unintentional defect. I use the control joints as architectural features and fill them with contrasting coloured grout.
If you do go ahead and pour without joints, you can minimize the size and frequency of the cracks by paying meticulous attention to the compaction of the substrate, and adding fibre to the mix. Talk to your finisher first though - some fibres telegraph through to the finished surface.
>”I use the control joints as architectural features and fill them with contrasting coloured grout.”
Malcolm, that is genius! I will have to remember that idea of yours!
I would also add remesh to the slab, and BE SURE it is properly placed in the CENTER of the slab. Without supports, the remesh ends up on or near the bottom of the slab where it does little good.
Bill
Bill,
The main floor of my house is cut up into 3 ft squares. The concrete isn't stained but the green grout seems to lend it some colour. This is it after 25 years. No cracks anywhere. No rebar, mesh or fibre either.
That grout makes it look like a tile floor. Do you know if you’ve had any cracks in the control joints hidden by the grout? A slab with no cracks is good luck. Maybe count all those tiles and use those as your lucky lottery ticket numbers :-)
Bill
Thanks for the advice! I'll be sure to compact the subsoil as well as the gravel subbase!
You may very well be the only person who thinks cracks are prettier than control joints, but it's your house, so you can have all the cracks you want.
We didn't want cracks, so included control joints in a pattern that looks, and is, intentional. We also compacted the crap out of the stone base and, at the recommendation of the floor finisher, used 4000 psi concrete with pea stone aggregate. After five years, not a single crack.
Nice! What kind of steel reinforcement did you use (if any)?
The typical 6" wire mesh.
I have no control cuts in my house. We had a couple of early on cracks that were v cut and filled in prior to stain and look good and have held up fairly well after 12 years.
Zoe, the common assumption that concrete always cracks is not correct. If your slab has a well-prepared base, reinforcement designed to limit cracking, a well-designed concrete mix (and-importantly--actually mixed according to the recipe) and, most importantly, you allow the concrete to cure slowly, you should have little to no cracking. To allow it to cure slowly you keep it damp; for how long depends on the mix, but 7 days is typical for a standard concrete mix.
The larger the slab and the more corners and penetrations it has, the harder it is to prevent cracks. But the single biggest cause of cracks (in my experience) is that contractors want to work on it as soon as possible, so they let it dry before full hydration of the cement has occurred. Without full hydration--keeping the cement, the "glue" in the concrete damp--the chemical reaction that makes concrete strong can't occur. It will get hard, but it will be brittle and prone to cracking.
When control joints are used and concrete cracks, it does not always crack at the control joints. For residential work, control joints are not required by the IRC building code that most of the US uses as a reference. It's common for concrete contractors to say it's code-required, but it's not.