Cutting contraction/control joints on new pour

[logan1211]

Hi guys,

Having my 30x48 - 4" - 4000 psi - 6 bag mix slab poured on Thursday.

I'll be doing the cutting of the contraction joints. Amish crew doesn't do that apparently.

Probably a dumb sounding question... but when should I cut those? I understand within 24 hours. Is the slab pretty solid at that point? Enough to walk on?

And to follow up; what kind of saw? I saw the rental place has an early entry saw... but again, concerned about the hardness and walking on it, etc...

Any input or advice would be helpful! Thanks all!

 

[ItsNemo]

No problem walking on a slab after 24 hours...don't drag your feet and make sure your shoes are clean so you don't scratch it up, but it will be fine.

Just need a 12 or 14" saw, you only need to cut roughly 1" deep. Use something as a guide (a straight 2x4), don't try to free hand it. Plan out your cuts before you make them.

 

[Kaizen]

I as well will be doing this on mine soon. so interested in the responses. that depth is correct for a 4 inch slab. i'm either using my grinder with a mason blade or going to use a 30 dollar diamond blade in my circular saw.
from what I've read it should be as soon as it actually cuts and does not rip out material. i'll be out in the cold under lights doing mine. I've read waiting even a day it might be too late depending on the cure time.

 

[logan1211]

Wonder how you know when the "cut without ripping material" stage is without testing. Maybe I'm just over thinking this. I'm sure there's a lot of factors there...temp, moisture, etc.

 

[pstnbly]

I use a 7 1/4 circular saw with a dry diamond blade, no fancy tools needed.

 

[Kaizen]

Wonder how you know when the "cut without ripping material" stage is without testing. Maybe I'm just over thinking this. I'm sure there's a lot of factors there...temp, moisture, etc.

lol hoping you figure it out and let me know.



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[logan1211]

lol hoping you figure it out and let me know.



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ha. I'll report feedback this weekend

 

[Cyberbear]

Let the rest of us know how the saw cuts go and if it really controls any cracking. I'm really leery about weakening the slab by a deep 1" saw cut that supposedly helps prevent, or control, where the slab actually cracks as it dries, cures and shrinks. I've poured several slabs and none have ever cracked, and I can't understand why. I can only presume it was the amount of steel, concrete bag mix and the curing sealer used after the pour.

 

[OldNeons]

I cut my 16x18 vintage garage pad into four sections using my skilsaw, and I would definitely rent a walk behind wet saw for anything larger. Cutting will be so much faster and easier, and turn out better. The walk behind I used last time had a swing out pointer that followed your lines. You can use a string as long as you don't cut it. Otherwise chalk a line. At times with the skilsaw, I used a straight edge and a carpenters pencil to go over the chalk line as it was difficult to see.

As soon as you can get on it without damaging it, I'd start laying out and cutting. If possible you can do the measuring and marking on walls or forms so you have your layout ready to go....

Good luck

 

[GYPSY400]

Let the rest of us know how the saw cuts go and if it really controls any cracking. I'm really leery about weakening the slab by a deep 1" saw cut that supposedly helps prevent, or control, where the slab actually cracks as it dries, cures and shrinks. I've poured several slabs and none have ever cracked, and I can't understand why. I can only presume it was the amount of steel, concrete bag mix and the curing sealer used after the pour.

When they cut my slab it's only about 1/4 or 1/2" deep and 1/4" wide.. done as per the stamped drawing that my floor was built to. I have no cracking whatsoever other than a small hairline Crack in the control cut.
I figures if the engineer says cut the floor, then cut the floor.. and I wouldn't give my builder the final payment until it was done.
Mine was cut a month after the pour.. there is no rush to do it, as long as it's done before spring when there is major ground heaving.

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[matt_i]

I would cut the control joints the next morning after the pour. As said, 10-12foot squares, 1" deep should be plenty in a 4" slab. The walk-behind saw following snapped chalk lines will do a nicer job in my opinion.

I also recommend the curing sealer, it will be dry by then.

 

[larry4406]

In the houses my company builds, we approach our control joints in our basement slabs with a different approach than saw cuts.

We specify the use of Zip Strips. These are plastic strips which are inserted into the concrete slab as it sets up and is being finished. The flat piece is pulled off and then the cream tooled over it. After cure, the slabs typically develop a very fine straight line crack at the strip.
http://www.superiorprofiles.com/zip-strip.html

 

[ms fowler]

One of the tests I perform is to measure flatness and levelness of fresh slabs. The specs say to do it within 72 hours, but I like to do it as soon as possible which means I compete with the crews doing the cuts. As soon as they finish troweling, and you can walk on the surface with out marking it you can make the cuts. This is often only 3 hours or so after they finish placing the concrete--depends on mix, amount of water and temperature.

 

[Jlbc212]

Let the rest of us know how the saw cuts go and if it really controls any cracking. I'm really leery about weakening the slab by a deep 1" saw cut that supposedly helps prevent, or control, where the slab actually cracks as it dries, cures and shrinks. I've poured several slabs and none have ever cracked, and I can't understand why. I can only presume it was the amount of steel, concrete bag mix and the curing sealer used after the pour.

The 24 x 24 slab in my old garage doesn't have any control joints and there's only one small hairlne crack after 23 years. There's no steel or mesh in the slab, just fiber and a compacted base of 3/4" stone under the slab. I also cover the concrete the day after the pour with 4 or 6 mil plastic and leave it on for 30 days.

 

[MagKarl]

Diamond blade on my Skilsaw, piece of OSB for a straight edge, and the hose running worked great for me. Cut faster and easier than I thought it would.

 

[rburke65]

Before placing the concrete, I had pencil marks on side of the skirt board. As soon as you can walk on the floor....as said 3 hours or so....we sprayed curing solution. Hen that dried, we snapped chalk lines and cut the joints. You might want to consider a location for a possible lift if that is in your future. Keep the cuts away from the lift post area. Do not delay in cutting the floor. Good luck.

 

[larry_g]

In the houses my company builds, we approach our control joints in our basement slabs with a different approach than saw cuts.

We specify the use of Zip Strips. These are plastic strips which are inserted into the concrete slab as it sets up and is being finished. The flat piece is pulled off and then the cream tooled over it. After cure, the slabs typically develop a very fine straight line crack at the strip.
http://www.superiorprofiles.com/zip-strip.html

x2 on this. I've done this in both my buildings and been very happy. Read all the 'how do I fill in...' posts on here and I wonder why people cut ditches in a nice flat floor when the zip strips do the same with little to no obvious signs on hte surface.

lg
no neat sig line

 

[DougWil]

Saw cuts should be 1/4 of the slab thickness.

Good info here from the Portland Cement Association.
http://www.cement.org/for-concrete-...ntraction-control-joints-in-concrete-flatwork

 

[larry4406]

Saw cuts should be 1/4 of the slab thickness.

Good info here from the Portland Cement Association.
http://www.cement.org/for-concrete-...ntraction-control-joints-in-concrete-flatwork

Zip Strips can be purchased in 1", 1.5", and 2". So, these can meet the aforementioned 1/4 (25%) of most slab thicknesses.

 

[Supergumby5000]

The cuts definitely do not need to be that deep. For a 4" slab, a 1/2" cut is plenty.

The sooner you cut the better. 24 hours is a rule of thumb for standard grade concrete. once you get getting into plastisicers or VMAs that time goes up and down but for a standard slab I doubt you have any of that.

Other then that.... slap a diamond blade on a saw and have at 'er. If you want to get fancy at the edge of the cuts (where the saw hits the wall/curb before the blade does, hit it with a grinder.

 

[ForceFed70]

Lots of good advice here. As mentioned, the sooner the better. Next day is typical.

 

[machsnell]

Next morning for sure you always run the risk of an overnight Crack. Wet it down as soon as solid enough if you can to slow the cure. Or better yet cover with plastic after wetting down. Do this for as many days as you can and it will add psi to you mix by slowing your curing process.

I would have at least one control joint or zip strip in a slab that large at only 4 inch done with pour.

For the rest of the joimts a cut off saw is decent but hard for one man and you definitely need a straight edge for a guide. Also the blades are a little wide. Circular saw decent but slow.

Soffcut saw is best and reduces the risk of pulling off chips out of top edge of cut. It has a bottom plate that "holds" down the concrete. Also it has a narrower blade and 4 wheels to guide you very straight. I would put down chalk line at shoe edge but they cut straight.

Make sure your stone is watered down and wellllllll compacted. If no vapor barrier then wet the top of stone when concrete is placed so the moisture isn't sucked out of your concrete too fast.



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[machsnell]

BTW if you are pouring tomorrow morning then you can water down by midday or early afternoon.

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[joes169]

Is this a pole barn by chance? If so, I'd recommend sawing at the center of each post, as they contain 2 re-entrent points begging for a crack, and they tend to be spaced 8'-10' anyway.

Do you have a link to the early entry saw at the rental yard? If it's something like a Prowler 150, it's going to be very easy to use and give even a novice good results.

As for timing, I've poured and sawn more than 10,000 yards with my Soff-Cut saw, and there's just too many variables to give you actual timeframes. In 80+ degree weather, I've sawn in my socks (I now carry slippers in my truck) 1 hour after the last broom on a driveway. In cooler weather, I've had to stop sawing concrete we finished 8-10 hours before due to excessive spalling, and wait until the next morning.

All in all, if you live in a cooler climate, you should have no problem sawing at the 20-24 hour mark, after the concrete was initially placed. Trying to saw the same day, even with a Soff-Cut saw, might be pushing the limit.

 

[forAK]

The concrete truck left at 10 a.m and we cut the slab at 6 p.m. that same night with one of these. These don't need to have water used when cutting, even though we kept the slab wet. It is a dusty mess though and was done all the same day. Then it was just watering the slab. Its called a soff cut saw.

 

[logan1211]

Is this a pole barn by chance? If so, I'd recommend sawing at the center of each post, as they contain 2 re-entrent points begging for a crack, and they tend to be spaced 8'-10' anyway.

I'll see if I can dig up a link. On my 30' side, poles are 10'. The 48' section they're on 8' centers. Are 10x8 squares too small or would that look a little weird? I was going to shoot for 10x10. Marks are up all I need to do is drop a chalk line and cut away.

Thanks for your input!

 

[ms fowler]

Number one rule about concrete--Concrete will crack.
Sometimes- we can get it to crack where we want it to....sometimes. I have seen many slabs, sofcut immediately after finishing and some cracks--especially at the corners of square cutouts for columns.

 

[joes169]

I'll see if I can dig up a link. On my 30' side, poles are 10'. The 48' section they're on 8' centers. Are 10x8 squares too small or would that look a little weird? I was going to shoot for 10x10. Marks are up all I need to do is drop a chalk line and cut away.

Thanks for your input!

I would saw 8x10 sections, definately not too small. Respect the poles...........

 

[archy99]

When they cut my slab it's only about 1/4 or 1/2" deep and 1/4" wide.. done as per the stamped drawing that my floor was built to. I have no cracking whatsoever other than a small hairline Crack in the control cut.
I figures if the engineer says cut the floor, then cut the floor.. and I wouldn't give my builder the final payment until it was done.
Mine was cut a month after the pour.. there is no rush to do it, as long as it's done before spring when there is major ground heaving.

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Any joint is better than none, but I would disagree on the timing not being critical. Obviously there are many factors (weather, base material, vapor barrier, etc), a freshly poured slab will start to shrink immediately, and the control joints tell it where that should happen. Industry standard is to cut as soon after finishing as the slab will support the machine, and not generate tear out. On my 20x35, they cut it at 7pm, 12 hrs after starting the pour that morning. Based in the shape of the pour, it was divided into 12 panels.

As for the depth, what I am used to seeing on commercial work is 1/3 of slab thickness, and possibly as little as 1/4 of the thickness, no less.

I have seen slabs crack because the joints were not deep enough, or were too late. Make sure they hand cut the small bits next to the walls where the big blades of the walk behind machines cannot get to.

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[GYPSY400]

Any joint is better than none, but I would disagree on the timing not being critical. Obviously there are many factors (weather, base material, vapor barrier, etc), a freshly poured slab will start to shrink immediately, and the control joints tell it where that should happen. Industry standard is to cut as soon after finishing as the slab will support the machine, and not generate tear out. On my 20x35, they cut it at 7pm, 12 hrs after starting the pour that morning. Based in the shape of the pour, it was divided into 12 panels.

As for the depth, what I am used to seeing on commercial work is 1/3 of slab thickness, and possibly as little as 1/4 of the thickness, no less.

I have seen slabs crack because the joints were not deep enough, or were too late. Make sure they hand cut the small bits next to the walls where the big blades of the walk behind machines cannot get to.

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Yes I stand corrected after reading many posts on here about the timing of the cut..
Mine was cut late because my contractor really didn't want to do it as he was confident it wouldn't Crack... it still hasn't, aside from a hairline Crack within the cut.
I believe mine was cut shallow was because I have the pex piping installed for in floor heating, but nonetheless I still have a control cut.

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[logan1211]

For those following along at home, got too busy managing it all and then having to leave this week for work I didn't find much time to post.

Pour went great. Crew knew what they were doing, had the PERFECT amount...ended up ordering 20 yards and doing a little apron up front.

I was amazed at how quickly it firmed up. Poured starting at 8 AM, took the power trowel back to the rental place at 5 PM on the dot. Good weather too.

When I returned the trowel, I picked up one of these.

http://www.marshfieldrental.com/equipment.asp?action=category&category=5&key=000172

Woke up at 7AM the next day (after having the marks there already...I snapped chalk lines (did 10x10) and went to town with this thing. Amazing how well it worked. Was done in 30 minutes. 4 hour rental money well spent. One thing that irritated me though that I caught after the first 30' long cut was that the nut that stops the height adjustment just wasn't staying put and I ended up dropping to around 1 1/8" for the majority, and about 1.25" over the last 5 feet. Really hope that extra depth on the cut doesn't screw me too bad in the long run. Not sure what impact it has.

All I now is this is the first shop I've had...and the first shop with concrete... it really is a beautiful thing! Now to get the 200 amp installed and get some lights going. Will be starting insulation/framing office and bathroom over the winter. If I remember I'll take some pictures. Thanks for everyone's feedback and input....really helped.

 

[c39er]

I did zip strips... 10 years later still can hardly see them..

 

[Kaizen]

As so many are following this thread I figured I would stick my recent experience. My slab was poured yesterday and was still cooking well into the evening. I got a fifteen dollar diamond blade for my circular saw. I got a non toothed one. Timed cutting a thirty foot line at eleven minutes including readjusting my plywood straightedge. My concrete is 4K psi. Not sure if that is the reason but even a brand new blade is not going very fast. Sawed to 1.5 inches. I just hate renting anything so I paid for this with an hour of my time. You can see the fiber in the cut. Not much chipping.

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[roc_on_the_rocks]

Hi guys,

Having my 30x48 - 4" - 4000 psi - 6 bag mix slab poured on Thursday.

I'll be doing the cutting of the contraction joints. Amish crew doesn't do that apparently.

Probably a dumb sounding question... but when should I cut those? I understand within 24 hours. Is the slab pretty solid at that point? Enough to walk on?

And to follow up; what kind of saw? I saw the rental place has an early entry saw... but again, concerned about the hardness and walking on it, etc...

Any input or advice would be helpful! Thanks all!

No high heel shoes!

 

[revamped]

It wont help you now, but I used zip strips and they floated and hard trowled over them and it looks amazing! Consistent hairline cracks right where they need to be and you can barely see them

 

[PWC Repair]

We poured mine 17 days ago, no control joints or saw cuts. no cracks...............yet.