New garage time!

[dodge man]

I agree, your going to want to make a few cuts across the slab.

 

[deezler]

I'm soaking it every hour with the hose, and it's supposed to rain for the next 6 days. Still figuring out what I'm going to do for the control joint.

Might as well cut one of the control joints along your seam between the front and rear pads. I know its not exactly in the middle of your building depth, but it shouldn't look too odd. Ovrszd spec'd the proper number of cuts for ya. Which puts them at about 12x12' I think, a little bit on the large side, but should be ok. You should cut them today before the slabs crack on their own.

Whatever depth you're sure your PEX is at, cut to 1/2" less. Ideally your control joint cuts are at least a 1/4 to 1/3 of the total slab depth. You can just use a masonry blade in a circular saw, but ideally a saw your dont care about much since it will become packed with dust. A rented saw with a 5HP engine that you walk behind is a lot easier to use. Then my preference is to re-fill the cleaned out cuts with sikaflex (self leveling concrete caulk). You can stuff a foam backer rod into the bottom of the cut first to save on sikaflex usage.

 

[EddieWalker]

I would not cut a control joint. The floor is two pads already. He has both rebar and wire mesh to hold the concrete together, and it's suspended inside the middle of the pad. Cutting just makes it easier for the concrete to crack, and the cut hides the crack.

 

[ovrszd]

His concrete will definitely crack. All. Concrete. Cracks.

The cuts will hopefully direct the cracks rather than them spider webbing across the floor.

 

[deezler]

Cutting just makes it easier for the concrete to crack, and the cut hides the crack.

Be-zactly! Otherwise you usually get a diagonal, erratic crack that looks ugly, and you can never hide (minus a full layer of epoxy over it, I suppose, but then the epoxy just cracks too eventually). So you cut a neat, square grid and fill the cuts in, and then never suffer ugly random cracking.

His rebar and wire mesh holds the concrete firmly in plane forever, but it does not prevent cracking.

 

[ovrszd]

Be-zactly! Otherwise you usually get a diagonal, erratic crack that looks ugly, and you can never hide (minus a full layer of epoxy over it, I suppose, but then the epoxy just cracks too eventually). So you cut a neat, square grid and fill the cuts in, and then never suffer ugly random cracking.

His rebar and wire mesh holds the concrete firmly in plane forever, but it does not prevent cracking.

Yep. All. Concrete. Cracks.

 

[buckeyefarmer]

They are called “ control joints” to control the cracking.

 

[Pixguy]

I always thought it was just a phenomenon in the frost states or when the pad wasn't prepped properly...... No?

 

[ovrszd]

I always thought it was just a phenomenon in the frost states or when the pad wasn't prepped properly...... No?

Cracks can appear due to pad deflection or movement. But that's different than the cracking being discussed here.

Cracking is caused by the moisture in the concrete evaporating as it cures. It begins immediately after pour. Causes the slab to shrink and crack. Cuts are an attempt to control the location of the crack. Not to eliminate it.

The drier the pour, less cracks. Wetter the pour, more cracks.

 

[sodamo]

His concrete will definitely crack. All. Concrete. Cracks.

The cuts will hopefully direct the cracks rather than them spider webbing across the floor.

My concrete guy (84 yo), who has done all my concrete last 17 years is always concerned about cracks and does the cuts morning after the pour. Some like the driveway just near equal segments, others like in curved areas maybe diagonals to look decent. Most memorable was an area between previous pours. About a month later we discovered a crack, paralleling the cut perfectly about an inch to one side. George was devastated, but we’ve laughed about it for years.

 

[pmsmechanic]

I would not cut a control joint. The floor is two pads already. He has both rebar and wire mesh to hold the concrete together, and it's suspended inside the middle of the pad. Cutting just makes it easier for the concrete to crack, and the cut hides the crack.

Amidst all the fluff on this thread here is a flag waving against the wind. Just out of curiosity why are you recommending not to cut a control joint. You've done lots of building and I'm sure this is not a lightly made recommendation.

 

[dragoneggs]

Amidst all the fluff on this thread here is a flag waving against the wind. Just out of curiosity why are you recommending not to cut a control joint. You've done lots of building and I'm sure this is not a lightly made recommendation.

Wondering as well. Great insurance. My experience says always. This slab is plenty large to risk 'local' movement.

The first house I built, I had a three car garage with the third bay extending back for a second vehicle or small shop. I argued with the concrete guy about putting a joint across from the inside corner. He insisted he knew what he was doing and asked me how many finished slabs I had poured. He assured me that with the compaction there was no need and I wouldn't like the seam in my garage. Five years later I wish I could have rubbed his nose in it.

Later, I had a first class 30' x 60' outdoor sport court poured. We took it down to hard pan creating a bit of a bowl on three sides removing about 400yards of dirt (was great for ball retrieval and became a mecca for the neighborhood ). But I digress. I really didn't want the seams for aesthetic reasons and true ball bounce (pickle ball), but the outfit said they would never pour a large slab without control joints no matter what I wanted.

 

[deezler]

I feel as though I've seen many slabs in the deeper south without cut control joints. An often discussed topic on the GarageJournal forum also.

The cracks we're worried about here are from contraction due to evaporation of moisture directly after pouring, not eventual stresses from frost heave, settling, etc (although control joints can mitigate those effects also). Why would that be any different in the south? I dunno.

 

[powersrp]

It will crack within a week, need those control joints, my basement was poured on a friday, builder was suppose to cut joints on sat am,didnt show, masons cut the joints on monday, it was already cracked or shortly there after. 50’x100’ slab cracked in 2 places across the 50’ and roughly in middle of slab.

 

[HomerGuy]

About 15 years ago I attended the World of Concrete expo in Las Vegas and went to a seminar on concrete flatwork. I remember the presenters were adamant about the need for control joints, with the spacing dependent on the thickness of the slab (slab thickness in inches X 1.5 = maximum control joint spacing in feet, IIRC that was a long time ago).

They also made a strong argument for early entry sawing (ie Soff cut saws) or at the very least ASAP for a conventional saw. Need to catch that shrinkage cracking as quickly as possible before it occurs.

 

[EddieWalker]

Amidst all the fluff on this thread here is a flag waving against the wind. Just out of curiosity why are you recommending not to cut a control joint. You've done lots of building and I'm sure this is not a lightly made recommendation.

In my opinion and experience, the reason to cut the slab is to create a place for the concrete to crack right after the pour because of too much water used in the mix. When the crew working the mud can get away with it, they add water to make it easier for them to work it. The mix needs a certain amount of water, and everything extra will evaporate, which leaves a void in the slab, and that results in the large ugly cracks that nobody wants. Cutting the slab while it's still green allows the slab to move and give you a clean crack that is mostly hidden by the cut itself.

If the concrete can pass a slump test, the cracking is so minimal that it becomes hairline and difficult to see.

If there is rebar in the middle of the slab, those hairline cracks become almost impossible to see.

If there is wire in the middle of the slab, for all basic discussion, those cracks really don't exist. They are there, concrete always cracks, but you'll never find them.

Almost every slab poured with wire has the wire walked on and pushed to the bottom of the slab, so the cracks become more noticeable. Same thing with rebar that is not supported by chairs. The biggest lie in concrete is that they will pull it up while spreading the mud, and that it will stay there in the middle of the slab while everyone is walking on top of it.

The other reason for cutting a slab is because concrete gets bigger when it gets warm, and contracts when it gets cold. The best way to deal with this is to pour concrete in sections and have something between each section to absorb this movement. I've read that just having 30 pound tar paper between two pads will accomplish this, but most people use 1x4's or a product specially designed for this. If it's one big monolithic slab that will be changing temperature on a daily basis, then cutting the slab has the advantage of releasing the stress of it's movement and allowing it to break up at the cut and move with the temperature changes. This is also why concrete highways are so bad, because of all the expansion joints needed to keep the concrete from destroying itself during heating and cooling cycles.

Inside a residential house, where the temperature is controlled, nobody that I have ever met, and no house that I have ever been in, has the concrete cut regardless of how big the house is. The garage and porches are done separately because those areas will heat and cool differently then inside the house, which allows them to move on their own. Unfortunately, that's where I see a lot of cracking from excessive water. So maybe more contractors should cut the slab on residential work to hide their bad concrete work? I can see that happening if the guy in charge of the pour knows that he's pouring with too much water. I just don't allow that to happen, so cutting is never needed to hide that problem.

In this case, the rebar and wire are on chairs strong enough to walk on, so they should be in the middle of the pad.
He has PEX inside the pad to keep the temperature of the pad constant, so it won't expand and contract.
He poured the slab in two pieces, so the slab can expand and contract without damaging itself.
Considering all the time and effort that he put into planning this, and making it happen, I'm assuming that he didn't add a lot of water to the mix.
Because I believe all of this to be true, there is no reason to cut the slab.

 

[KWentling]

His floor is nearly identical in size and construction as mine except my pex is deeper in the slab. Mine was also done in two pours. Mine has no control joints. It also has no cracks in 17
years. I don't care for the pvc rebar stands used in this pour. Looks to me like those would make weak spots. All the rest of my concrete has control joints cut and agree it is a good practice.

 

[ovrszd]

His floor is nearly identical in size and construction as mine except my pex is deeper in the slab. Mine was also done in two pours. Mine has no control joints. It also has no cracks in 17
years. I don't care for the pvc rebar stands used in this pour. Looks to me like those would make weak spots. All the rest of my concrete has control joints cut and agree it is a good practice.

I agree with Eddie concerning heat/cold. A heated floor is much less apt to excessively crack.

 

[buckeyefarmer]

My 24 x 30 garage floor has no control joints and has rebar. It cracked near middle, then splinters off each direction at each end of middle crack.

 

[ovrszd]

Logistically those cracks aren't a problem. The steel is obviously holding everything in place. If the wandering crack isn't a big deal to a person then cuts are a waste of time.

Friend of mine is building a slab house/garage. The concrete guy cut the garage, it cracked in the cut. Didn't cut the house, it cracked like yours.